TSLP as a Potential Therapy in the Treatment of CRLF2 B Cell Acute Lymphoblastic Leukemia

Cytokine receptor-like factor 2 B-cell acute lymphoblastic leukemia (CRLF2 B-ALL) is a high-risk subtype characterized by CRLF2 overexpression with poor survival rates in children and adults. CRLF2 and interleukin-7 receptor alpha (IL-7Rα) form a receptor for the cytokine thymic stromal lymphopoietin (TSLP), which induces JAK/STAT and PI3K/AKT/mTOR pathway signals. Previous studies from our group showed that low TSLP doses increased STAT5, AKT, and S6 phosphorylation and contributed to CRLF2 B-ALL cell survival. Here we investigated the role of TSLP in the survival and proliferation of CRLF2 B-ALL cells in vitro and in vivo. We hypothesized that high doses of TSLP increase CRLF2 signals and contribute to increased proliferation of CRLF2 B-ALL cells in vitro and in vivo. Interestingly, we observed the opposite effect. Specifically, high doses of TSLP induced apoptosis in human CRLF2 B-ALL cell lines in vitro, prevented engraftment of CRLF2 B-ALL cells, and prolonged the survival of +TSLP patient-derived-xenograft mice. Mechanistically, we showed that high doses of TSLP induced loss of its receptor and loss of CRLF2 signals in vitro. These results suggest that high doses of TSLP could be further investigated as a potential therapy for the treatment of CRLF2 B-ALL.

Abstract 1528: Molecular mechanisms of TSLP as a therapy for CRLF2 B-Cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (B-ALL) is the most common type of leukemia in children. B-ALL characterized by cytokine receptor-like factor 2 (CRLF2) overexpression (CRLF2 B-ALL) has a survival rate of <30% and is the highest risk sub-group of B-ALL in both adults and children. CRLF2 is a receptor component for the cytokine thymic stromal lymphopoietin (TSLP). TSLP plays a role in the survival and proliferation of B-cell precursors, thus explaining the oncogenic role of increased CRLF2 signaling in CRLF2 B-ALL. To our surprise, we found that high-levels of TSLP eliminated leukemia cells in patient-derived xenograft (PDX) models of CRLF2 B-ALL. CRLF2 and IL-7 receptor-alpha (IL-7Ra) form the heterodimer type-I cytokine receptor for TSLP cytokine. Binding of TSLP to its CRLF2 receptor complex induces JAK-STAT5 and PI3K-AKT pathway signals. TSLP shares the IL-7Ra with Interleukin 7 (IL-7) which has a heterodimer receptor consisting of IL-7Ra and the common gamma chain. High-levels of IL-7 (50 ng/ml) have been shown to induce IL-7Ra internalization and degradation in T-cells. We hypothesize that high-level TSLP induces internalization and degradation of IL-7Ra leading to CRLF2 signal inhibition, death of CRLF2 B-ALL cells and the anti-leukemia effects that we have observed in PDX mice. To test this hypothesis, we treated CRLF2 B-ALL cell lines with different TSLP concentrations to observe the effect of TSLP on its receptor and CRLF2 signaling. Flow cytometry data showed that continuous or a pulse of high-dose TSLP induced a loss of surface IL-7Ra expression for up to 24 hours. Phosphorylation assays showed that cells cultured with high-dose TSLP were unresponsive to subsequent TSLP-induced phosphorylation events (pSTAT5 and pRPS6), indicating CRLF2 signal inhibition. In conclusion, high-dose TSLP induces loss of (IL-7Ra) and inhibition of CRLF2 signaling. These results suggest that TSLP exerts its anti-leukemia effects by shutting down CRLF2-mediated signals possibly via the loss of the IL-7Ra receptor component.

Citation Format: Hossam Alkashgari, Cornelia Stoian, Caleb Ruiz-Jimenez, Jacqueline Coats, Carlos A. Casiano, Sinisa Dovat, Kimberly J. Payne. Molecular mechanisms of TSLP as a therapy for CRLF2 B-Cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1528.

Dual targeting of MTOR as a novel therapeutic approach for high-risk B-cell acute lymphoblastic leukemia

Children of Hispanic/Latino ancestry have increased incidence of high-risk B-cell acute lymphoblastic leukemia (HR B-ALL) with poor prognosis. This leukemia is characterized by a single-copy deletion of the IKZF1 (IKAROS) tumor suppressor and increased activation of the PI3K/AKT/mTOR pathway. This identifies mTOR as an attractive therapeutic target in HR B-ALL. Here, we report that IKAROS represses MTOR transcription and IKAROS' ability to repress MTOR in leukemia is impaired by oncogenic CK2 kinase. Treatment with the CK2 inhibitor, CX-4945, enhances IKAROS activity as a repressor of MTOR, resulting in reduced expression of MTOR in HR B-ALL. Thus, we designed a novel therapeutic approach that implements dual targeting of mTOR: direct inhibition of the mTOR protein (with rapamycin), in combination with IKAROS-mediated transcriptional repression of the MTOR gene (using the CK2 inhibitor, CX-4945). Combination treatment with rapamycin and CX-4945 shows synergistic therapeutic effects in vitro and in patient-derived xenografts from Hispanic/Latino children with HR B-ALL. These data suggest that such therapy has the potential to reduce the health disparity in HR B-ALL among Hispanic/Latino children. The dual targeting of oncogene transcription, combined with inhibition of the corresponding oncoprotein provides a paradigm for a novel precision medicine approach for treating hematological malignancies.

Platelet and neutrophil responses to Porphyromonas gingivalis in human whole blood

Porphyromonas gingivalis is a causative agent for periodontal disease. Binding of platelets to this gram-negative anaerobe can regulate host hemostatic (thrombus forming) and immune (neutrophil interacting) responses during bacterial infection. Additionally, in response to bacterial pathogens neutrophils can release their DNA, forming highly prothrombotic neutrophil extracellular traps (NETs), which then further enhance platelet responses. This study evaluates the role of P. gingivalis on platelet expression of CD62P, platelet-neutrophil interactions, and labeled neutrophil-associated DNA. Human whole blood was preincubated with varying P. gingivalis concentrations, with or without subsequent addition of adenosine diphosphate (ADP). Flow cytometry was employed to measure platelet expression of CD62P using PerCP-anti-CD61 and PE-anti-CD62P, platelet-neutrophil interactions using PerCP-anti-CD61 and FITC-anti-CD16, and the release of neutrophil DNA using FITC-anti-CD16 and Sytox Blue labeling. Preincubation with a high (6.25 × 10⁶  CFU/mL) level of P. gingivalis significantly increased platelet expression of CD62P in ADP treated and untreated whole blood. In addition, platelet-neutrophil interactions were significantly increased after ADP stimulation, following 5-22 min preincubation of blood with high P. gingivalis CFU. However, in the absence of added ADP, platelet-neutrophil interactions increased in a manner dependent on the preincubation time with P. gingivalis. Moreover, after ADP addition, 16 min preincubation of whole blood with P. gingivalis led to increased labeling of neutrophil-associated DNA. Taken together, the results suggest that the presence of P. gingivalis alters platelet and neutrophil responses to increase platelet activation, platelet interactions with neutrophils, and the level of neutrophil antimicrobial NETs.

Abstract 2927: Targeted combination treatment for B-cell acute lymphoblastic leukemia

The CCND3 gene encodes Cyclin D3–a protein that is essential for G1/S cell cycle progression and proliferation of malignant cells. Targeting Cyclin D3 is used to treat B-cell acute lymphoblastic leukemia (B-ALL); however, the mechanisms that regulate expression of the CCND3 gene in B-ALL are largely unknown. Here, we present evidence that oncogenic Casein Kinase II (CK2) regulates expression of CCND3 and G1/S cell cycle progression via direct phosphorylation of Ikaros, a transcription regulator and tumor suppressor protein. Global genome-wide DNA-binding analysis using ChIP-seq, show that Ikaros binds the promoter of CCND3 in primary human B-ALL. The role of Ikaros in regulating CCND3 expression in B-ALL was tested using gain-of-function and loss-of-function experiments. Ikaros knock-down with shRNA results in increased transcription of CCND3 in B-ALL. Overexpression of Ikaros in human B-ALL was associated with reduced expression of CCND3. Since Ikaros activity in leukemia is regulated by pro-oncogenic Casein Kinase II (CK2), we tested whether CK2 regulates expression of CCND3 in B-ALL. Increased expression of CK2 in B-ALL results in increased expression of the CCND3 gene. This was associated with a loss of Ikaros binding to the promoter of the CCND3 gene. Inhibition of CK2 with shRNA, and/or a specific CK2 inhibitor, CX-4945, resulted in increased Ikaros binding to the CCND3 promoter and reduced expression of Cyclin D3 in B-ALL. Treatment with CX-4945 showed strong therapeutic activity in preclinical models of B-ALL. A combination treatment with CX-4945 and dexamethasone that targets G1/S cell cycle progression shows a strong synergistic effect on B-ALL cells. In conclusion, presented data show that CK2 and Ikaros regulate G1/S cell cycle progression via transcriptional regulation of the CCND3 gene in B-ALL and that CK2 inhibition represses CCND3 expression by enhancing Ikaros tumor suppressor function. Results demonstrate the synergistic efficacy of a combination treatment with CK2 inhibitor and dexamethasone and provide a rationale for the use of this combination treatment as a novel therapeutic approach for B-cell acute lymphoblastic leukemia.

Citation Format: Shriya Kane, Yali Ding, Chandrika Gowda, Jonathon Lee Payne, Soumya Iyer, Pavan K. Dhanyamraju, Chunhua Song, Dhimant Desai, Arati Sharma, Kimberly J. Payne, Sinisa Dovat. Targeted combination treatment for B-cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2927.

Developing a novel biologic for the treatment of high-risk B cell acute lymphoblastic leukemia

The subtype of B cell acute lymphoblastic leukemia (B-ALL) at highest risk for relapse overexpresses surface CRLF2 (CRLF2 B-ALL) and disproportionately affects Hispanic children and adults. CRLF2 partners with the IL-7Ra to form a receptor complex activated by the cytokine, Thymic Stromal Lymphopoietin (TSLP). TSLP activates the JAK/STAT and PI3/AKT/mTOR pathways which are known to promote cell survival and proliferation. Our in vitro studies of CRLF2 B-ALL cells showed that supra-physiological levels of TSLP (1 ng/ml) induced a complete shutdown of TSLP receptor signaling, following the initial activation event. This effect was dose dependent and did not occur at physiological levels (20 pg/ml) of TSLP. This signal shutdown was correlated with the loss of surface IL-7Ra. Using a novel patient-derived xenograft (PDX) model that expresses human TSLP (hTSLP, mouse TSLP does not activate the human TSLP receptor) created in our lab, we showed that high dose hTSLP virtually eliminated human CRLF2 B-ALL cells in vivo. The goal of work presented here was to test whether treatment of CRLF2 B-ALL PDX with high doses of recombinant hTSLP (rhTSLP) could re-capitulate anti-tumor effects of hTSLP observed in our engineered mice and to evaluate its effects on prolonged survival. After 24 days of treatment, rhTSLP reduced the leukemia cell burden by 90% in treated PDX as compared to control PDX that received physiological levels of rhTSLP. Similarly, survival was prolonged in PDX mice receiving high-dose rhTSLP as compared to controls: all 11 control mice died before any of the 11 treated mice died. Taken together, these data suggest that TSLP has promise as a biologic for the treatment of CRLF2 B-ALL.

Belimumab alters transitional B-cell subset proportions in patients with stable systemic lupus erythematosus

OBJECTIVE

We evaluated the effects of the B-cell activating factor (BAFF)-targeting antibody Belimumab on human nonmemory B-cell pools. Human B-cell pools were identified using surface markers adapted from mouse studies that specifically assessed reductions in immature B cells due to BAFF depletion. Patients with systemic lupus erythematosus (SLE) have high levels of both BAFF and immature B cells. Mechanistic mouse studies provide a framework for understanding human responses to therapies that target B cells.

METHODS

Peripheral blood mononuclear cells were isolated from healthy donors and SLE patients on Belimumab or standard-of-care therapy (SCT). Cells were stained for flow cytometry to identify B-cell subsets based on CD21/CD24. Differences in subset proportions were determined by one-way ANOVA and Tukey's post hoc test.

RESULTS

Patients treated with Belimumab show alterations in the nonmemory B-cell pool characterized by a decrease in the Transitional 2 (T2) subset (p = 0.002), and an increase in the proportion of Transitional 1 (T1) cells (p = 0.005) as compared with healthy donors and SCT patients. The naïve B-cell compartment showed no significant differences between the groups (p = 0.293).

CONCLUSION

Using a translational approach, we show that Belimumab-mediated BAFF depletion reduces the T2 subset in patients, similar to observations in mouse models with BAFF depletion.

Abstract 2620: Global epigenetic regulation of gene expression and tumor suppression in T-cell leukemia by Ikaros

Ikaros encodes a DNA-binding zinc finger protein that functions as a tumor suppressor. Loss of Ikaros function results in the development of high-risk B-cell and T-cell acute lymphoblastic leukemia. Previous studies suggested that Ikaros controls expression of its target genes via chromatin remodeling, and by forming complexes with the histone deacetylase, HDAC1. However, the mechanisms through which Ikaros regulates the global epigenetic signature in leukemia is unknown. We studied the role of Ikaros in regulating tumor suppression in Ikaros-defficient T-cell acute lymphoblastic leukemia (T-ALL) cells. The re-introduction of Ikaros at physiological levels via retroviral transduction into T-ALL cells, results in a cessation of cellular proliferation and cell cycle arrest within 4 days. To determine the dynamics of Ikaros-mediated epigenetic regulation of gene expression in T-ALL, we performed ChIP-seq of Ikaros, HDAC1, and histone modifications, along with microarray and ATAC-seq in wild-type T-ALL (that lacks Ikaros), and during each of 3 days following the re-introduction of Ikaros. Our results demonstrated that Ikaros directly regulates the global epigenomic landscape including genome wide de novo formation of enhancers in T-ALL. Further analysis showed that Ikaros induces the re-distribution of HDAC1 via direct recruitment of HDAC1 to promoter and enhancer regions of different target genes resulting in their transcriptional repression. Expression analysis identified a large number of novel signaling pathways that are directly regulated by Ikaros and HDAC1, and are responsible for the cessation of proliferation and the cell cycle arrest of T-ALL cells. Our results identified novel functions of Ikaros in the epigenetic regulation of gene expression and tumor suppression in T-ALL.

Citation Format: Yali Ding, Bo Zhang, Jonathon L. Payne, Kimberly J. Payne, Chunhua Song, Chandrika Gowda, Soumya Iyer, Pavan K. Dhanyamraju, Dhimant Desai, Suming Huang, Feng Yue, Sinisa Dovat. Global epigenetic regulation of gene expression and tumor suppression in T-cell leukemia by Ikaros [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2620.

Abstract 871: Regulation of CDC25a expression by the ikaros and casein kinase II (CK2) in T-cell acute lymphoblastic leukemia (T-ALL)

CDC25A is a member of the CDC25 family of phosphatases that plays a major role in cell cycle progression. Here, we present evidence that expression of CDC25a in T-ALL is regulated at the transcriptional level by oncogenic Casein Kinase II (CK2) via direct phosphorylation of Ikaros, a transcription factor and tumor suppressor protein. Global chromatin immunoprecipitation coupled with next-generation sequencing (ChIP-seq) studies in both primary human acute lymphoblastic leukemia cells and cell lines, demonstrated that Ikaros binds to the promoter of the CDC25a gene. Ikaros functions as a tumor suppressor protein and deletion of which is associated with development of T-ALL. Ikaros binding to CDC25a promoter was confirmed by quantitative chromatin immunoprecipitation (qChIP) in primary T-ALL cells. Ikaros knock-down with shRNA results in increased transcription of CDC25a in T-ALL. In mice, T-ALL cells that were derived from Ikaros-knockout mice express high levels of CDC25a. Transduction of these cells with Ikaros-containing retrovirus results in sharp reduction of CDC25a expression. Overexpression of CK2 via retroviral transduction resulted in increased transcription of the CDC25a gene, as measured by qRT-PCR, as well as increased overall expression of CDC25a, as measured by Western blot. Increased expression of CK2 was associated with a loss of Ikaros binding to the CDC25a gene promoter. Molecular inhibition of CK2 using shRNA, as well as pharmacological inhibition with a specific CK2 inhibitor resulted in reduced expression of CDC25a in primary human T-ALL. CK2 inhibition was also associated with strong reduction in AKT phosphorylation, emphasizing that CK2 inhibition downregulates CDC25a and other cell cycle progression genes. Inhibition of CK2 was associated with increased Ikaros binding at the promoter of CDC25a. Ikaros knock-down restored high expression of CDC25a in T-ALL cells that were treated with CK2 inhibitors. These data showed that CK2 and Ikaros are major transcriptional regulators of CDC25a transcription in T-ALL and that CK2 inhibition represses CDC25a transcription via Ikaros-mediated repression. In conclusion, these results indicate that expression of the CDC25a oncogene in T-ALL is regulated by the CK2 which modulates Ikaros activity. Presented data revealed a novel mechanism of therapeutic action of CK2 inhibitors - repression of CDC25a expression via Ikaros. Results provide a rationale for the use of novel CK2 inhibitors in T-ALL.

Citation Format: Soumya C. Iyer, Shriya Kane, Chandrika Gowda, Chunhua Song, Yali Ding, Jon Payne, Pavan Kumar Dhanyam Raju, Bihua Tan, Mary McGrath, Yevgeniya Bamme, Mario Solimon, Nathalia Moreno Cury, Dhimant Desai, Arati Sharma, Kimberly J. Payne, Sinisa Dovat. Regulation of CDC25a expression by the ikaros and casein kinase II (CK2) in T-cell acute lymphoblastic leukemia (T-ALL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 871.

Abstract 286: Synergistic efficacy of CK2 inhibitor with common chemotherapy drugs by restoring Ikaros function in high-risk ALL

Objective: IKZF1 gene-coding protein, Ikaros functions as a leukemia suppressor. Casein Kinase II activity is overexpressed in acute lymphoblastic leukemia (ALL) and CK2-mediated-dysfunction of Ikaros is one of the key reason for high-risk ALL and CK2 inhibitor -CX4945 treatment shows high therapeutic efficacy on high-risk ALL. The anti-apoptotic factors are highly expressed in leukemia and the commonly-used 1st-line chemotherapy drugs exerts the anti-tumor effect by suppression of anti-apoptosis signaling. Ikaros binding peaks was identified in the promoter of anti-apoptotic genes by ChIP-seq, suggesting Ikaros regulation on their expression. These observations also suggest the synergistic effect of restoring Ikaros function with common chemotherapy durgs in ALL.

Methods: The ChIP-seq and qChIP assays were performed to determine the enrichment of Ikaros and H3K4me3 in promotor of the genes. Lentiviral Ikaros or IKZF1 shRNA were used for functional analysis. WST-1 cell proliferation assay, Annexin-V staining plus flow cytometry and Patients-derived xenograft mouse (PDX) model were used for observing the anti-tumor effect in vitro and in vivo, respectively.

Results: ChIP-seq and qChIP assays identified Ikaros binding peaks in the promoter of anti-apoptotic genes in cell-lines and patients’ samples. Ikaros overexpression suppresses but IKZF1 knockdown promotes the gene expression. CX-4945 suppresses the expression of the genes by decreasing the H3k27me3 enrichment in an Ikaros and HDAC1-dependent manner in B-ALL cells. The anti-apoptotic gene is significantly up-regulated in ALL patients. CX-4945+chemoterhapy drugs significantly induces the cell proliferation arrest and apoptosis compared to single drugs in vitro and also show the synergistic effect analyzed by CalcuSyn software. CX-4945+chemotherapy drugs significantly reduced the total leukemia cells and % leukemic cells in the three high-risk B-ALL Patient Derived Xenograft (PDX) mice model compared to that of single drugs, which indicated that their synergistic therapeutic efficacy on leukemia development.

Conclusion: Ikaros suppressed anti-apoptotic gene expression through histone modification in ALL. CK2 inhibitor, CX-4945 by restoring Ikaros function have synergistic efficacy with common chemotherapy drugs on high-risk B-ALL.

Citation Format: chunhua song, Zheng Ge, Chandrika Gowda, Yali Ding, Jonathon Payne, Bihua Tan, Nathalia M. Cury, Elanora Dovat, Zhijun Zhao, Xiaoguang Lyu, Mary McGrath, Dhimant Desai, Soumya lyer, Pavan K. DhanyamRaju, Kimberly J. Payne, Sinisa Dovat. Synergistic efficacy of CK2 inhibitor with common chemotherapy drugs by restoring Ikaros function in high-risk ALL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 286.

Mechanism of biologic therapy for pre B-cell acute lymphoblastic leukemia with CRLF2 overexpression

7035

Background: Acute lymphoblastic leukemia (ALL) is the most common childhood cancer, of which pre B-cell ALL with CRLF2 overexpression is a high risk subtype. CRLF2 B-ALL is associated with poor outcome, increased rate of relapse, and health disparities in Hispanic children. Together with IL-7 receptor alpha (IL-7Rα), CRLF2 makes up a receptor complex that is activated by the cytokine, thymic stromal lymphopoetin (TSLP). Activation of the receptor complex leads to JAK2/STAT5 and PI3/AKT/mTOR signals that promote leukemia cell survival and proliferation. To study the role of TSLP in CRLF2 B-ALL, we developed a patient-derived xenograft (PDX) model of CRLF2 B-ALL that allows us to alter circulating levels of human TSLP (hTSLP). We generated PDX from CRLF2 B-ALL cells harvested from patient samples and compared leukemia burden in mice with varying levels of hTSLP. In PDX models with hTSLP levels at or below physiological levels in pediatric cancer patients, CRLF2 B-ALL cells grew robustly. However, in PDX with elevated levels of hTSLP, leukemia cells were essentially eliminated. Our objective is to elaborate on the mechanism of high dose hTSLP’s antileukemic effect. Methods: We performed TSLP dose response studies and used flow cytometry to evaluate the effect of TSLP on SOCS protein expression, CRLF2 signaling shutdown, and loss of TSLP receptor components. Results: CRLF2 B-ALL cells cultured with hTSLP showed a dose-dependent loss in the ability to induce STAT5 and S6 phosphorylation following hTSLP stimulation. This loss was correlated with the loss of IL-7Rα, and maintained for 24-48 hours following a pulse of high-dose, but not low-dose, hTSLP. The loss of signaling and surface IL-7Rα could be prolonged if high-dose hTSLP levels were maintained. Flow cytometry analysis showed that high-dose hTSLP upregulated SOCS1 and SOCS3 proteins in CRLF2 B-ALL cells. Whole genome microarray showed that SOCS1, SOCS2, SOCS3 and CISH mRNA were upregulated in primary CRLF2 B-ALL cells cultured with high dose hTSLP. Conclusions: These data support the hypothesis that TSLP exerts its anti-leukemia effects through shutdown of CRLF2-mediated signals and that these effects are at least partially mediated by the loss of the IL-7Rα component, and potentially through SOCS family proteins. These studies elucidate the mechanism of the human TSLP cytokine as a potential biologic therapy to treat CRLF2 B-ALL. Supported in part by 1R01CA209829 (KJP), 1R43CA224723 (KJP), ASH HONORS Award 2018-2019 (WBW), and Alpha Omega Alpha Carolyn L. Kuckein Student Research Fellowship 2018-2019 (WBW).

Ikaros tumor suppressor function includes induction of active enhancers and super-enhancers along with pioneering activity

Ikaros encodes a transcription factor that functions as a tumor suppressor in T-cell acute lymphoblastic leukemia (T-ALL). The mechanisms through which Ikaros regulates gene expression and cellular proliferation in T-ALL are unknown. Re-introduction of Ikaros into Ikaros-null T-ALL cells resulted in cessation of cellular proliferation and induction of T-cell differentiation. We performed dynamic, global, epigenomic and gene expression analyses to determine the mechanisms of Ikaros tumor suppressor activity. Our results identified novel Ikaros functions in the epigenetic regulation of gene expression: Ikaros directly regulates de novo formation and depletion of enhancers, de novo formation of active enhancers and activation of poised enhancers; Ikaros directly induces the formation of super-enhancers; and Ikaros demonstrates pioneering activity by directly regulating chromatin accessibility. Dynamic analyses demonstrate the long-lasting effects of Ikaros DNA binding on enhancer activation, de novo formation of enhancers and super-enhancers, and chromatin accessibility. Our results establish that Ikaros’ tumor suppressor function occurs via global regulation of the enhancer and super-enhancer landscape and through pioneering activity. Expression analysis identified a large number of novel signaling pathways that are directly regulated by Ikaros and Ikaros-induced enhancers, and that are responsible for the cessation of proliferation and induction of T-cell differentiation in T-ALL cells.

Platelet-Neutrophil Interactions Are Lower in Cord Blood of Premature Newborns

OBJECTIVE

To quantify platelet-neutrophil interaction by flow cytometry, in newborn cord blood, as a function of gestational age.

RATIONALE

Little is known about platelet function markers in the newborn, and developmental variations in these markers are not well described.

METHODS

Cord blood samples were obtained from 64 newborns between 23 and 40 weeks' gestation. The neonates were grouped into three categories: preterm (< 34 weeks' gestation, n = 21), late preterm (34 to < 37 weeks' gestation, n = 22), and term (≥37 weeks' gestation, n = 21). We monitored the expression of P-selectin and the formation of platelet-neutrophil aggregates (PNAs) by flow cytometry while using adenosine 5'-diphosphate (ADP) or thrombin receptor-activating peptide (TRAP) as agonists.

RESULTS

PNAs were significantly lower in preterm compared to term neonates after TRAP or ADP stimulations (11.5 ± 5.2% vs. 19.9 ± 9.1%, p < 0.001, or 24.0 ± 10.1% vs. 39.1 ± 18.2%, p = 0.008, respectively). The expression of P-selectin also tended to be lower in preterm neonates, with significant positive correlations between P-selectin expression and PNA formation.

CONCLUSIONS

The potential formation of PNAs correlates with gestational age. This suggests that the development of functional competencies of platelets and neutrophils continues throughout gestation, progressively enabling interactions between them.

Aberrant ARID5B expression and its association with Ikaros dysfunction in acute lymphoblastic leukemia

Mutations and single nucleotide polymorphisms of AT-rich interactive domain-containing protein 5B (ARID5B) are involved in the oncogenesis of acute lymphoblastic leukemia (ALL) and treatment outcomes. However, ARID5B expression and clinical significance in ALL remain unclear. We found ARID5B is significantly down-regulated in ALL compared to healthy bone marrow controls. ARID5B also interacts with PHD finger protein 2 (PHF2). Low expression of ARID5B (ARID5B^low) or ARID5B and PHF2 (ARID5B^lowPHF2^low) is correlated with the markers of cell proliferation and poor prognosis in ALL patients. Ikaros directly regulates ARID5B expression in ALL. Restoring Ikaros function by Casein Kinase II inhibition also promotes ARID5B expression through recruitment of trimethylation of lysine 4 on histone H3 (H3K4me3) at its promoter region. In summary, our data show that aberrant expression of ARID5B and PHF2 is related to leukemic cell proliferation and several poor prognostic markers. Our data indicate ARID5B^low expression, particularly ARID5B^lowPHF2^low expression, is linked to Ikaros dysfunction and involved in the oncogenic effect of high-risk ALL, which may represent a high-risk subgroup of ALL.

Abstract 1513: Epigenetic regulation of CD117 expression in B-cell acute lymphoblastic leukemia by Ikaros and histone deacetylase HDAC1

The type III receptor tyrosine kinase, CD117, functions as a receptor for stem cell factor (SCF) and is encoded by the c-kit gene. During hematopoiesis, CD117 is normally expressed in hematopoietic stem cells, multipotent progenitors, common lymphoid progenitors, and early-stage thymocytes. Overexpression and/or activating mutations of c-kit have been demonstrated in acute myeloid leukemia (AML), early T-cell precursor acute lymphoblastic leukemia (ETP-ALL), and B-cell acute lymphoblastic leukemia (B-ALL). It has been suggested that increased expression of CD117 is associated with stem-like phenotype and worse clinical outcomes in AML and T-ALL. The regulation of expression of c-kit in leukemia is still largely unknown. Here we report that transcription of c-kit in B-ALL is regulated by the Ikaros tumor suppressor protein and histone deacetylase HDAC1. Global genome-wide binding studies using ChIP-seq, demonstrate the occupancy of both Ikaros and HDAC1 at the promoter of the c-kit gene in B-ALL cells. Ikaros and HDAC1 binding to the c-kit promoter was confirmed by quantitative chromatin immunoprecipitation (qChIP). Overexpression of Ikaros via retroviral transduction results in reduced transcription of c-kit in B-ALL cells. Consistent with this, Ikaros knock-down with shRNA results in increased transcription of c-kit in B-ALL. These data suggest that Ikaros represses transcription of c-kit. Ikaros overexpression was associated with increased HDAC1 occupancy while Ikaros knock-down resulted in reduced HDAC1 binding to the promoter of the c-kit gene. We tested whether Ikaros-mediated transcriptional repression of c-kit requires HDAC1 activity. Results showed that inhibition of HDAC1 activity with a pan-histone deacetylase inhibitor, trichostatin (TSA), or a specific HDAC1 inhibitor, MS-275, abolishes Ikaros' ability to repress c-kit transcription in luciferase reporter assays. Molecular inhibition of HDAC1 with shRNA confirmed that HDAC1 activity is essential for Ikaros-mediated transcriptional repression of c-kit. A serial qChIP assay spanning the c-kit promoter was used to analyze the epigenetic changes that are associated with Ikaros and HDAC1 binding at the c-kit promoter. Results showed that increased Ikaros and HDAC1 occupancy at the c-kit promoter in B-ALL cells results in enrichment for the markers of the repressive chromatin, H3K9me3 and H3K27me3, as well as reduced occupancy of H3K9ac, a marker of active chromatin. In conclusion, the presented results show that the expression of c-kit in B-ALL is regulated at the transcriptional level by Ikaros and HDAC1 via chromatin remodeling. These data provide a novel insight into the role of Ikaros in both tumor suppression and transcriptional regulation of gene expression in B-cell acute lymphoblastic leukemia.

Citation Format: Shriya Kane, Jonathon L. Payne, Mario Soliman, Chandrika Gowda, Meixan Xiang, Chunhua Song, Kimberly J. Payne, Sinisa Dovat. Epigenetic regulation of CD117 expression in B-cell acute lymphoblastic leukemia by Ikaros and histone deacetylase HDAC1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1513.

Abstract 5442: Dose-dependent toxicity of FTY720, a sphingosine-1-phosphate receptor agonist, on double hit lymphoma cells via PP2A-mediated dephosphorylation of c-myc and bcl-2

High grade B cell lymphoma with translocations involving MYC and rearrangements involving BCL2 and/or BCL6 define an aggressive subtype of diffuse large B cell lymphoma (DLBCL) with poor prognosis and high rates of relapse. The standard treatment for DLBCL is not considered effective and targeted molecular therapy has yet to be identified for this disease. The mechanisms that make this type of lymphoma particularly aggressive and refractory to treatment include cell growth signals from the c-myc transcriptional regulator working together with the anti-apoptotic activity of the bcl-2 protein.

Protein Phosphatase 2A (PP2A) is a ubiquitous cellular enzyme that acts as a tumor suppressor through dephosphorylation of specific cellular phosphoproteins. In fact, PP2A has been shown to dephosphorylate bcl-2 at ser70 leading to inhibition of its function. Further, PP2A-mediated dephosphorylation of c-myc at ser62 promotes its degradation. In many cancers, PP2A activity is inhibited due to overexpression of binding proteins such as SET and CIP2A. Small molecules can act as PP2A activators by releasing PP2A from these inhibitory binding proteins. In the present study we investigated the efficacy of FTY720, a high-affinity agonist of the sphingosine 1-phosphate receptor-1 and PP2A activator, on the double hit lymphoma derived cell line, WSU DLCL2.

WSU DLCL2 cells were treated with various concentrations of FTY720, a derivative of myriocin, a fungal metabolite of the Chinese herb Iscaria sinclarii known to promote PP2A activation. Cell viability and PP2A activity were measured 24 hours after FTY720 treatment. In addition, cell lysates were assayed by Western blot using antibodies to detect PP2A subunits, as well as total and phosphorylated c-myc and bcl-2 proteins.

A dose-dependent decrease in cell viability was observed over the range of FTY720 concentrations from 1 to 10 micromolar with concomitant increase in in vitro PP2A activity. Western blots showed decreases in c-myc protein levels with FTY720 treatment as well as decreases in ser70 phosphorylation levels of bcl-2 and ser62 phosphorylation levels of c-myc.

The results indicate that PP2A activation via FTY720, a small molecule activator of PP2A, leads to a dose-dependent induction of cell death in double hit lymphoma. The data are consistent with an underlying cell death mechanism that involves the targeted dephosphorylation of c-myc and bcl2. Taken together, these data suggest that FTY720 may act as a targeted treatment with therapeutic potential as a single agent or as a combination with other therapies in the treatment of this subtype of lymphoma. There is also the potential of a therapeutic role in other lymphoma subtypes with poor prognosis including transformed follicular lymphoma and relapsed burkitt's lymphoma that rely individually on bcl-2 and c-myc, respectively, for their growth.

Citation Format: Geoffrey P. Shouse, Rosalia de Necochea-Campion, Saied Mirshahidi, Chien-Shing Chen, Kimberly J. Payne. Dose-dependent toxicity of FTY720, a sphingosine-1-phosphate receptor agonist, on double hit lymphoma cells via PP2A-mediated dephosphorylation of c-myc and bcl-2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5442.

Abstract C44: Epigenetic regulation of CRLF2 oncogene expression by Casein Kinase II (CK2) signaling in B-cell acute lymphoblastic leukemia that occurs at high frequency in Hispanic children

B-cell acute lymphoblastic leukemia (B-ALL) occurs more frequently in Hispanic children as compared to non-Hispanic whites. The specific subtype of B-ALL that is caused by overexpression of CRLF2 (CRLF2 B-ALL), occurs 5 times more frequently in Hispanic children as compared to the others. Since this type of B-ALL is associated with poor prognosis, the death rate of B-ALL is 39% higher in Hispanic children than in non-Hispanic whites. Thus, understanding the molecular mechanisms that regulate CRLF2 expression in CRLF2 B-ALL is essential for the development of targeted therapy for this disease. Our previous work determined that transcription of CRLF2 is negatively regulated by a tumor-suppressor protein, Ikaros. Ikaros deregulation is a feature of over 80% of CRLF2 B-ALL. Here we present evidence that Ikaros-mediated repression of CRLF2 transcription in B-ALL in Hispanic children is regulated by Casein Kinase II (CK2). CK2 is an oncogenic kinase that is overexpressed in B-ALL. We have previously shown that CK2 can directly phosphorylate Ikaros and that phosphorylation by CK2 can impair Ikaros function as transcriptional regulator. We tested whether inhibition of CK2 affects the ability of Ikaros to regulate CRLF2 transcription. Molecular inhibition by shRNA that targets the catalytic subunit of CK2, as well as pharmacologic inhibition of CK2 by the specific inhibitor, CX-4945, resulted in reduced expression of CRLF2 as measured by qRT-PCR. This was associated with increased Ikaros binding to the CRLF2 promoter as measured by quantitative chromatin immunoprecipitation (qChIP). To determine whether Ikaros function is essential for CRLF2 repression following CK2 inhibition, we compared expression of CRLF2 in cells that have Ikaros knocked-down by shRNA vs. cells with control shRNA, following treatment with CX-4945. Results showed that Ikaros knockdown abolished the ability of CK2 inhibitors to repress transcription of CRLF2 in B-ALL. These results demonstrate that Ikaros is an essential component of CK2 signaling that regulates CRLF2 expression. Analysis of the epigenetic signature at the CRLF2 promoter performed by serial qChIP assays showed that increased Ikaros binding to the CRLF2 promoter, following CK2 inhibition, is associated with enrichment for the H3K9me3 histone modification, which is a marker of repressive chromatin. In conclusion, we demonstrate that expression of the CRLF2 oncogene in acute leukemia that disproportionally occurs in Hispanic children is epigenetically regulated by the CK2-Ikaros axis. In CRLF2 B-ALL, Ikaros-mediated repression of CRLF2 is impaired due to overexpression of CK2. Treatment of CRLF2 B-ALL with CK2 inhibitors restores Ikaros tumor suppressor function, resulting in CRLF2 repression. Results identified a signaling network that regulates CRLF2 expression and suggest that restoration of Ikaros activity with CK2 inhibitors can be a therapeutic approach for CRLF2 B-ALL to reduce the health disparity for Hispanic children with ALL. Supported by 1R01CA209829.

Citation Format: Chunhua Song, Zheng Ge, Kimberly J. Payne, Sinisa Dovat. Epigenetic regulation of CRLF2 oncogene expression by Casein Kinase II (CK2) signaling in B-cell acute lymphoblastic leukemia that occurs at high frequency in Hispanic children [abstract]. In: Proceedings of the Tenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2017 Sep 25-28; Atlanta, GA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2018;27(7 Suppl):Abstract nr C44.

Abstract 1512: Regulation of the CDC42 signaling pathway by IKZF1 in T-cell acute lymphoblastic leukemia

The IKZF1 gene encodes Ikaros—a DNA-binding, Kruppel-like zinc finger protein that functions as a tumor suppressor in T-cell acute lymphoblastic leukemia (T-ALL). Ikaros regulates transcription of a large number of genes that are involved in control of cellular proliferation. However, the molecular mechanisms through which Ikaros regulates proliferation of T-ALL cells are still unknown. The use of quantitative chromatin immunoprecipitation (qChIP) showed that Ikaros binds to the promoter regions of CDC42 in vivo in primary T-ALL patient samples as well as in T-ALL cell lines. Ikaros overexpression in B-ALL cells via retroviral transduction results in decreased transcription of CDC42, as evidenced by qRT-PCR and Western blot. The luciferase reporter assay showed that Ikaros represses CDC42 transcription by directly binding to its promoter. The shRNA-mediated knock-down of Ikaros in T-ALL resulted in increased expression of CDC42. These results suggest that Ikaros functions as a transcriptional repressor of CDC42 in T-ALL. Next, we studied the upstream signaling pathways that regulate Ikaros-mediated control of CDC42 transcription in T-ALL. Because Ikaros activity in leukemia is regulated via direct phosphorylation by Casein Kinase II (CK2), we tested the effect of CK2 inhibition on Ikaros-mediated regulation of CDC42 expression. Treatment of T-ALL cells with a specific CK2 inhibitor, CX-4945, resulted in reduced expression of CDC42 in T-ALL cells. This was associated with an increase in DNA-binding of Ikaros to the promoter of the CDC42 gene. Ikaros knock-down restored high expression of CDC42 in T-ALL cells that were treated with CK2 inhibitors. These data suggest that transcriptional repression of the CDC42 gene by Ikaros is impaired in T-ALL due to increased CK2 activity and reduced Ikaros DNA-binding affinity toward the CDC42 gene promoter. Inhibition of CK2 restores Ikaros function and results in transcriptional repression of the CDC42 gene. In conclusion, our results suggest that Ikaros regulates CDC42 signaling pathway via transcriptional repression of the small GTPase, CDC42, in T-ALL. Ikaros-mediated repression of CDC42 in T-ALL is negatively regulated by CK2. These results suggest that regulation of CDC42 expression is one of the mechanisms through which Ikaros regulates cellular proliferation in T-ALL.

Citation Format: Mario Soliman, Chunhua Song, Jonathon L. Payne, Zheng Ge, Chandrika Gowda, Yali Ding, Kimberly J. Payne, Sinisa Dovat. Regulation of the CDC42 signaling pathway by IKZF1 in T-cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1512.

Abstract 3169: Mechanisms of novel cytokine therapy for Ph-like B-cell acute lymphoblastic leukemia with overexpression of CRLF2

B-cell acute lymphoblastic leukemia with overexpression of CRLF2 (CRLF2 B-ALL) comprises ~50% of Ph-like B-ALL, a leukemia that is associated with poor outcomes, high relapse rates and leukemia health disparities in Hispanic children. CRLF2, together with the IL-7 receptor alpha (IL-7Ra), comprises a receptor complex that is activated by the cytokine, TSLP. Receptor activation induces JAK2/STAT5 and PI3/AKT/mTOR signals that are believed to contribute to survival and proliferation of leukemia cells. To study the role of TSLP in CRLF2 B-ALL, we developed a novel patient-derived xenograft (PDX) model of CRLF2 B-ALL that allows us to vary circulating levels of human TSLP (hTSLP). Primary CRLF2 B-ALL cells injected into PDX mice without hTSLP or with circulating hTSLP levels similar to pediatric leukemia patients (~4-10 pg/ml) showed engraftment and expansion of leukemia cells. In contrast, CRLF2 B-ALL cells were essentially eliminated in PDX with elevated physiologic levels of hTSLP (40-140 pg/mL). We observed these results in 5 independent experiments produced using primary CRLF2 B-ALL cells from two different Hispanic pediatric patients with CRLF2 B-ALL (N= 40 PDX). We hypothesize that the observed antileukemia effects are mediated via TSLP-induced upregulation of the Suppressor of Cytokine Signaling (SOCS) genes. SOCS genes encode a family of proteins (SOCS1-7 and CISH) that regulate cytokine signaling via negative feedback through multiple mechanisms including ubiquitin-mediated cytokine receptor degradation. Consistent with our hypothesis, SOCS1, SOCS2, SOCS3 and CISH mRNA were upregulated in primary CRLF2 B-ALL cells cultured with high-dose hTSLP. To gain mechanistic insights we evaluated the CRLF2 B-ALL cell lines, MUTZ5 and CALL4, following culture with and without hTSLP. Flow cytometry analysis showed that high-dose hTSLP upregulated SOCS1 and SOCS3 proteins in both CRLF2 B-ALL cell lines. We found that CRLF2 B-ALL cells cultured with hTSLP for 3 days showed a dose-dependent loss in the ability to induce STAT5 and S6 phosphorylation following hTSLP stimulation. This loss was correlated with the loss of IL-7Ra, and maintained for 24-48 hours following a pulse of high-dose (but not low-dose) hTSLP. The loss of signaling and surface IL-7Ra could be prolonged if high-dose hTSLP levels were maintained. These data provide evidence that TSLP exerts its antileukemia effects by shutting down CRLF2-mediated signals and suggest that these effects are at least partially mediated by the loss of the IL-7Ra component, and potentially through SOCS family proteins. These studies identify the human TSLP cytokine as a potential biologic therapy to treat CRLF2 B-ALL and reduce cancer health disparities for Hispanic children with CRLF2 B-ALL. (Supported by 1R01CA209829.)

Citation Format: Cornelia Stoian, Jacqueline S. Coats, Hossam Alkashgari, Veriah Vidales, Ineavely Baez, Juliette Personius, Hannah Choi, WayAnne Watson, Brandon Ng, Benjamin Becerra, Rishikesh Chavan, Muhammad Kamal, Shadi Farzin Gohar, Sinisa Dovat, Kimberly J. Payne. Mechanisms of novel cytokine therapy for Ph-like B-cell acute lymphoblastic leukemia with overexpression of CRLF2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3169.

High CRLF2 expression associates with IKZF1 dysfunction in adult acute lymphoblastic leukemia without CRLF2 rearrangement

Overexpression of cytokine receptor-like factor 2 (CRLF2) due to chromosomal rearrangement has been observed in acute lymphoblastic leukemia (ALL) and reported to contribute to oncogenesis and unfavorable outcome in ALL. We studied B-ALL and T-ALL patients without CRLF2 rearrangement and observed that CRLF2 is significantly increased in a subset of these patients. Our study shows that high CRLF2expression correlates with high-risk ALL markers, as well as poor survival. We found that the IKZF1-encoded protein, Ikaros, directly binds to the CRLF2 promoter and regulates CRLF2 expression in leukemia cells. CK2 inhibitor, which can increase Ikaros activity, significantly increases Ikaros binding in ALL cells and suppresses CRLF2 expression in an Ikaros-dependent manner. CRLF2 expression is significantly higher in patients with IKZF1 deletion as compared to patients without IKZF1 deletion. Treatment with CK2 inhibitor also results in an increase in IKZF1 binding to the CRLF2 promoter and suppression of CRLF2 expression in primary ALL cells. We further observed that CK2 inhibitor induces increased H3K9me³ histone modifications in the CRLF2 promoter in ALL cell lines and primary cells. Taken together, our results demonstrate that high expression of CRLF2 correlates with high-risk ALL and short survival in patients without CRLF2 rearrangement. Our results are the first to demonstrate that the IKZF1-encoded Ikaros protein directly suppresses CRLF2 expression through enrichment of H3K9me³ in its promoter region. Our data also suggest that high CRLF2 expression works with the IKZF1 deletion to drive oncogenesis of ALL and has significance in an integrated prognostic model for adult high-risk ALL.

Co-existence of IL7R high and SH2B3 low expression distinguishes a novel high-risk acute lymphoblastic leukemia with Ikaros dysfunction

Acute lymphoblastic leukemia (ALL) remains the leading cause of cancer-related death in children and young adults. Compared to ALL in children, adult ALL has a much lower cure rate. Therefore, it is important to understand the molecular mechanisms underlying high-risk ALL and to develop therapeutic strategies that specifically target genes or pathways in ALL. Here, we explored the IL7R and SH2B3 expression in adult ALL and found that IL7R is significantly higher and Sh2B3 lower expressed in B-ALL compared to normal bone marrow control, and the IL7R^highSH2B3^low is associated with high-risk factors, and with high relapse rate and low disease-free survival rate in the patients. We also found that Ikaros deletion was associated with the IL7R^highSH2B3^low expression pattern and Ikaros directly binds the IL7R and SH2B3 promoter, and suppresses IL7R and promotes SH2B3 expression. On the other hand, casein kinase inhibitor, which increases Ikaros function, inhibits IL7R and stimulates SH2B3 expression in an Ikaros dependent manner. Our data indicate that IL7R^highSH2B3^low expression distinguishes a novel subset of high-risk B-ALL associated with Ikaros dysfunction, and also suggest the therapeutic potential for treatment that combines casein kinase inhibitor, as an Ikaros activator, with drugs that target the IL7R signaling pathway.

Abstract 5829: Targeting TSLP-induce upregulation of Mcl-1 for the treatment of Ph-like ALL with CRLF2 alterations

B cell precursor acute lymphoblastic leukemia (B-ALL) is the most common childhood cancer. The subset of pediatric B-ALL patients at greatest risk for relapse and death have a gene expression profile similar to Philadelphia chromosome positive ALL. Approximately half of these Ph-like B-ALL are defined by genetic alterations that result in overexpression of the cytokine receptor, CRLF2. Stimulation of the CRLF2 receptor by the cytokine, TSLP, causes downstream activation of the JAK/STAT5 and PI3/AKT/MTOR pathways. Activation of these pathways has been associated with oncogenesis and chemoresistance. A gene target of activated STAT5 in B cell precursors is Mcl-1, a Bcl2 family pro-survival molecule. Further, Mcl-1 protein levels are increased through post-transcriptional mechanisms induced by activation of the mTOR pathway. We hypothesized that circulating TSLP cytokine contributes to chemoresistance by increasing CRLF2 activation leading to increased Mcl-1 expression and that targeting Mcl-1 could be an effective strategy for treating Ph-like B-ALL with overexpression of CRLF2 (CRLF2 B-ALL). To test this hypothesis we cultured human CRLF2 B-ALL cell lines (MUTZ5 and CALL4) with and without TSLP for 3 days and evaluated expression of Mcl-1 by flow cytometry. We found that TSLP induced significant increases in Mcl-1 proteins in both cell lines. To determine if these results are reflective of what happens in patients, primary CRLF2 B-ALL cells from pediatric patients were cultured with physiological levels of TSLP (~20 pg/ml) and similarly evaluated. Physiological TSLP significantly increased Mcl-1 protein in primary CRLF2 B-ALL cells, including those with activating JAK mutations. Our next question was whether TSLP-induced increases in Mcl-1 could be effectively targeted with Mcl-1 inhibitors (MIM-1 or Maritcolax). CRLF2 B-ALL cells were incubated with and without TSLP and treated with increasing doses of Mcl-1 inhibitor. Dose responses were evaluated by flow cytometry after 2 or 3 days. Mcl-1 inhibitors induced dose-dependent decreases in cell count and increases in caspase-3 activation and apoptosis (Annexin V/7-AAD). These corresponded with dose-dependent decreases in Mcl-1 protein, suggesting that both inhibitors target Mcl-1 for degradation. MIM-1 and Maritoclax showed efficacy against both CRLF2 B-ALL cell lines and primary patient samples, including those with activating JAK mutations, although cells cultured with TSLP typically required twice the dose of Mcl-1 inhibitor to achieve the same effect observed without TSLP. These data provide evidence that TSLP can contribute to leukemia cell survival and identify Mcl-1 inhibitor as a candidate therapy for CRLF2 B-ALL. Ongoing studies are evaluating the efficacy of the Mcl-1 inhibitor, Maritoclax, in novel patient-derived xenograft models of CRLF2 B-ALL that provide physiological levels of human TSLP.

Citation Format: Cornelia Stoian, Nathaniel George Mambo, Pierce McCarthy, Veriah Vidales, Jacqueline S. Coats, Ineavely Baez, Sinisa Dovat, Shadi Farzin Gohar, Dhimant Desai, Muhammad Kamal, Kimberly J. Payne. Targeting TSLP-induce upregulation of Mcl-1 for the treatment of Ph-like ALL with CRLF2 alterations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5829. doi:10.1158/1538-7445.AM2017-5829

Abstract 5533: Signaling pathways that regulate LMO2 oncogene expression in pediatric high risk T cell acute lymphoblastic leukemia

T-cell Acute Lymphoblastic Leukemia (T-ALL) accounts for about 15% of pediatric ALL and is characterized as a high-risk disease with frequent relapse, chemotherapy resistance, and a poorer prognosis. LIM domain only protein 2 (LMO2) is a regulator of hematopoiesis and an oncogene that is overexpressed in about 10% of T-ALL. The IKZF1 gene encodes a zinc finger protein called Ikaros which is a master regulator of lymphoid development and a tumor suppressor. In pediatric ALL, Ikaros alteration is considered independent prognostic marker for poor outcome. Pro oncogenic Casein Kinase II (CK2) is overexpressed in various malignancies including leukemia. CK2 directly phosphorylates Ikaros in vivo and inhibits its function as transcriptional regulator. Inhibition of CK2 restores Ikaros tumor suppressor function and results in anti-leukemic effect. Objective of this study is to understand the mechanisms of transcriptional regulation of LMO2 in T-cell ALL. Global chromatin immunoprecipitation (ChIP) coupled with the next-generation sequencing (ChIP-seq) studies in primary human ALL cells and in cell lines, demonstrated Ikaros occupancy of the promoter of LMO2 gene. We hypothesize that Ikaros negatively regulates expression of LMO2 at transcriptional level and CK2 impairs Ikaros mediated repression of LMO2 in T-cell ALL.

Results: qChIP in primary leukemia cells confirmed that Ikaros binds to the promoter region of LMO2. Using gain-of-function and loss-of-function experiments we dissected the role of Ikaros in regulation of LMO2 transcription in T-ALL. Ikaros silencing using shRNA transfection revealed increase in LMO2 expression as measured by qRT PCR. Conversely, overexpression of Ikaros was associated with strongly reduced transcription of LMO2. T-ALL cells that are derived from Ikaros-knockout mouse models express high level of LMO2. Retroviral transduction of these cells with Ikaros, resulted in significant reduction of LMO2 expression. Next we investigated how CK2 affects the regulatory functions of Ikaros towards LMO2. Molecular and pharmacological inhibition of CK2 resulted in reduced expression of LMO2 in primary human T-ALL. Ikaros binding at promoter of LMO2 was noted to be significantly increased following CK2 inhibition. This effect was not seen when cell were subjected to CK2 inhibition after Ikaros silencing. Further, we analyzed changes in the histone markers at the heterochromatin at the LMO2 promoter following Increased Ikaros binding such as reduced histone H3K9ac and H3K4me3 markers. This suggests that Ikaros regulates LMO2 transcription via chromatin remodeling.

Conclusion: This data reveals new regulatory mechanism for oncogene LMO2 in pediatric T-ALL. New evidence suggests that repression of LMO2 expression in T cell ALL via Ikaros can be potentiated using CK2 inhibitors. Findings provide the rationale for the use of CK2 inhibitors in T-ALL with LMO2 overexpression.

Citation Format: Chandrika S. Gowda, Yali Ding, Chunhua Song, Malika Kapadia, Kimberly J. Payne, Sinisa Dovat. Signaling pathways that regulate LMO2 oncogene expression in pediatric high risk T cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5533. doi:10.1158/1538-7445.AM2017-5533

Expression of Exogenous Cytokine in Patient-derived Xenografts via Injection with a Cytokine-transduced Stromal Cell Line

Patient-derived xenograft (PDX) mice are produced by transplanting human cells into immune deficient mice. These models are an important tool for studying the mechanisms of normal and malignant hematopoiesis and are the gold standard for identifying effective chemotherapies for many malignancies. PDX models are possible because many of the mouse cytokines also act on human cells. However, this is not the case for all cytokines, including many that are critical for studying normal and malignant hematopoiesis in human cells. Techniques that engineer mice to produce human cytokines (transgenic and knock-in models) require significant expense before the usefulness of the model has been demonstrated. Other techniques are labor intensive (injection of recombinant cytokine or lentivirus) and in some cases require high levels of technical expertise (hydrodynamic injection of DNA). This report describes a simple method for generating PDX mice that have exogenous human cytokine (TSLP, thymic stromal lymphopoietin) via weekly intraperitoneal injection of stroma that have been transduced to overexpress this cytokine. Use of this method provides an in vivo source of continuous cytokine production that achieves physiological levels of circulating human cytokine in the mouse. Plasma levels of human cytokine can be varied based on the number of stromal cells injected, and cytokine production can be initiated at any point in the experiment. This method also includes cytokine-negative control mice that are similarly produced, but through intraperitoneal injection of stroma transduced with a control vector. We have previously demonstrated that leukemia cells harvested from TSLP-expressing PDX, as compared to control PDX, exhibit a gene expression pattern more like the original patient sample. Together the cytokine-producing and cytokine-negative PDX mice produced by this method provide a model system that we have used successfully to study the role of TSLP in normal and malignant hematopoiesis.

Palmitic acid is a toll-like receptor 4 ligand that induces human dendritic cell secretion of IL-1β

Palmitic acid (PA) and other saturated fatty acids are known to stimulate pro-inflammatory responses in human immune cells via Toll-like receptor 4 (TLR4). However, the molecular mechanism responsible for fatty acid stimulation of TLR4 remains unknown. Here, we demonstrate that PA functions as a ligand for TLR4 on human monocyte derived dendritic cells (MoDCs). Hydrophobicity protein modeling indicated PA can associate with the hydrophobic binding pocket of TLR4 adaptor protein MD-2. Isothermal titration calorimetry quantified heat absorption that occurred during PA titration into TLR4/MD2, indicating that PA binds to TLR4/MD2. Treatment of human MoDCs with PA resulted in endocytosis of TLR4, further supporting the function of PA as a TLR4 agonist. In addition, PA stimulated DC maturation and activation based on the upregulation of DC costimulatory factors CD86 and CD83. Further experiments showed that PA induced TLR4 dependent secretion of the pro-inflammatory cytokine IL-1β. Lastly, our experimental data show that PA stimulation of NF-κB canonical pathway activation is regulated by TLR4 signaling and that reactive oxygen species may be important in upregulating this pro-inflammatory response. Our experiments demonstrate for the first time that PA activation of TLR4 occurs in response to direct molecular interactions between PA and MD-2. In summary, our findings suggest a likely molecular mechanism for PA induction of pro-inflammatory immune responses in human dendritic cells expressing TLR4.

Abstract B45: Regulation of CRLF2 oncogene expression by the Ikaros tumor suppressor in B cell acute lymphoblastic leukemia that occurs at high frequency in Hispanic children

Hispanic children are 1.24 times more likely to develop acute lymphoblastic leukemia (ALL) than non-Hispanic whites and when they do their death rate is 39% higher. B-cell precursor ALL caused by overexpression of CRLF2 (CRLF2 B-ALL) occurs 5 times more frequently in Hispanic children than others. In addition, over 80% of CRLF2 B-ALL has deletion or inactivation of one allele of the Ikaros (IKZF1) tumor suppressor, which is associated with a 12-fold increase in relapse rate as compared to standard-risk leukemia. Thus, the high rate of CRLF2 overexpression that occurs in combination with the Ikaros defect is likely a major contributor to the increased death rate in Hispanic children with ALL. Currently, these two genetic evens are thought to arise separately, to affect different signaling pathways and to be largely functionally disconnected. Here, we present evidence that expression of CRLF2 is transcriptionally regulated by Ikaros in B-ALL, thus providing evidence to link the two signaling pathways. Our data from chromatin immunoprecipitation coupled with next generation sequencing (ChIP-seq) show that Ikaros strongly binds to the CRLF2 promoter in vivo. This was confirmed by qChIP in different B-ALL cell lines and in primary B-ALL cells. Using Ikaros gain-of-function and loss-of-function experiments we studied how Ikaros binding at the CRLF2 promoter affects transcription of CRLF2 in B-ALL. Overexpression of Ikaros via lentiviral transduction resulted in reduced transcription of CRLF2 as measured by qRT-PCR. As a different approach, we used the luciferase reporter assay to show that transcription of the luciferase reporter gene under the control of the CRLF2 promoter was reduced after Ikaros transfection as compared to the negative control. Targeting Ikaros transcription with Ikaros-specific shRNA in B-ALL cells resulted in increased transcription of CRLF2. In conclusion, the presented data demonstrate that the Ikaros tumor suppressor directly represses transcription of CRLF2 in B-ALL cells and that reduced expression of Ikaros results in increased CRLF2 transcription. These results suggest that the two primary genetic alterations that characterize the high-risk CRLF2 B-ALL that occurs at increased frequency in Hispanic children are interconnected and that impaired Ikaros function affects the CRLF2 signaling pathway. These data underscore the importance of impaired Ikaros function in the pathogenesis of high-risk CRLF2 B-ALL that disproportionately affects Hispanic children. They also suggest that restoring normal Ikaros function could be a crucial step for achieving a therapeutic effect for this type of leukemia and for reducing the health disparity for Hispanic children with ALL.

Citation Format: Sinisa Dovat, Zheng Ge, Chunhua Song, Kimberly J. Payne. Regulation of CRLF2 oncogene expression by the Ikaros tumor suppressor in B cell acute lymphoblastic leukemia that occurs at high frequency in Hispanic children. [abstract]. In: Proceedings of the Ninth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2016 Sep 25-28; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2017;26(2 Suppl):Abstract nr B45.

Abstract B46: A novel patient-derived xenograft model for evaluating therapies that target the CRLF2 signaling pathway to reduce health disparities for Hispanic children with leukemia

The purpose of the studies described here was to identify drug targets and develop a preclinical model for testing therapies that can reduce health disparities for Hispanic children with high-risk acute lymphoblastic leukemia (ALL). Hispanic children are 1.24 times more likely to develop ALL than non-Hispanic whites and that number rises to 2.09 by adolescence and early adulthood. A major contributor to this health disparity is a type high-risk B-cell ALL called CRLF2 B-ALL. CRLF2 B-ALL occurs 5 times more often in Hispanic children than others, is prevalent in adolescents and young adults, and is associated with a high relapse rate and poor prognosis. CRLF2 B-ALL is caused by genetic alterations that result in over expression of the cytokine receptor, CRLF2. The CRLF2 receptor is activated by the cytokine, TSLP, causing downstream activation of the JAK/STAT5 and PI3/AKT/MTOR pathways. A gene target of activated STAT5 in B cell precursors is Mcl-1, a Bcl2 family pro-survival molecule. In addition, Mcl-1 protein levels are known to be increased through post-transcriptional mechanisms by activation of the mTOR pathway. We hypothesized that the normal level of circulating TSLP cytokine could induce CRLF2 activation leading to increased Mcl-1 expression in CRLF2 B-ALL cells. Our data show that TSLP increases phosphorylation of STAT5, as well as AKT and S6 (downstream of mTOR) in primary CRLF2 B-ALL cells from Hispanic pediatric patients, even when activating JAK mutations are present. When CRLF2 B-ALL cells from Hispanic pediatric patients were cultured for 3 days with and without physiological levels of TSLP, flow cytometry showed that expression of the Mcl-1 protein was significantly increased in cultures with TSLP as compared to cultures without TSLP. CRLF2 B-ALL cells treated in vitro with Mcl-1 inhibitor showed dose-dependent increases in caspase 3 activation and apoptosis as indicated by flow cytometry. These data provide evidence that TSLP can contribute to leukemia cell survival and identify Mcl-1 inhibitor as a candidate therapy for CRLF2 B-ALL. Our next step was to develop a preclinical model for testing therapies that target genes, such as Mcl-1, that are regulated by TSLP-induced CRLF2 signals in this disease. Patient-derived xenograft (PDX) models produced by transplanting leukemia cells from patients into immune deficient mice provide an in vivo model of disease that includes contributions of the background genetic landscape that can influence disease progression or treatment outcome in health disparities diseases. PDX models are possible because most cytokines produced in the mouse are active on human cells, however mouse TSLP is species-specific. Thus classic PDX models do not provide TSLP that can activate the CRLF2 receptor that is overexpressed in CRLF2 B-ALL. To address this issue we engineered PDX mice to express physiological levels of human TSLP (+T PDX mice) and control -T mice that lacked human TSLP. In vivo TSLP activity was validated and +T PDX were successfully generated using leukemia cells from two Hispanic pediatric patients with CRLF2 B-ALL. To determine whether +T PDX mice provide a preclinical model of B-ALL that more closely mirrors patients than -T PDX mice, we compared RNAseq gene expression profiles of leukemia cells isolated from +T PDX and -T PDX mice to that from the original patient sample. The gene expression pattern in leukemia cells from +T mice was significantly closer to primary patient sample than that from -T mice. The +T PDX mice described here provide a novel in vivo preclinical model for evaluating efficacy of drugs, such as Mcl-1 inhibitor, in context of the background genetic landscape and physiological human TSLP present in patients.

Citation Format: Kimberly J. Payne, Cornelia Stoian, Jacqueline S. Coats, Olivia Francis, Terry-Ann M. Milford, Ineavely Baez, Pierce J. McCarthy, George Mambo, Anna V.C. White, Mariah M.Z. Jackson, Juliette M. Personius, Veriah Vidales, Muhammad Omair Kamal, Shadi Farzin Gohar, Sinisa Dovat. A novel patient-derived xenograft model for evaluating therapies that target the CRLF2 signaling pathway to reduce health disparities for Hispanic children with leukemia. [abstract]. In: Proceedings of the Ninth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2016 Sep 25-28; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2017;26(2 Suppl):Abstract nr B46.

Abstract A21: Epigenetic regulation of cell cycle progression at the G2/M transition and mitosis in high-risk leukemia

High-risk acute lymphoblastic leukemia (ALL) is a clinical challenge due to drug resistance and poor prognosis. A characteristic molecular defect of most high-risk ALL is the deletion or inactivating mutation of one allele of the IKZF1 (Ikaros) tumor suppressor. Ikaros encodes a DNA-binding protein that regulates transcription of its target genes via chromatin remodeling. The mechanisms through which Ikaros regulates cellular proliferation in high-risk leukemia, are unknown. Using a systems biology approach, we determined that Ikaros regulates transcription of genes that are critical in the control of G2/M transition (CDC2) and mitotic progression (ANAPC1 and ANAPC7) in leukemia. Gain- and loss-of-function experiments demonstrate that Ikaros represses the transcription of CDC2, ANAPC1 and ANAPC7. Overexpression of Ikaros in leukemia also results in cell cycle arrest. We studied the mechanism through which Ikaros represses CDC2, ANAPC1 ad ANAPC7. The use of serial quantitative chromatin immunoprecipitation (qChIP) analyses spanning the promoters of Ikaros target genes demonstrated that Ikaros can repress transcription of its target genes by two different mechanisms: 1) via recruitment of histone deacetylase 1 (HDAC1), which is associated with the formation of repressive chromatin characterized by H3K27me3 and loss of H3K9ac (for ANAPC1 and CDC2); and 2) via an HDAC1-independent mechanism which is associated with the formation of repressive chromatin characterized by H3K9me3, along with the loss of H3K9ac (for ANAPC7). In high-risk ALL that is characterized by deletion of one Ikaros allele, the function of Ikaros as a transcriptional regulator is impaired due to reduced binding to promoters of Ikaros target genes. We showed previously that Ikaros DNA-binding affinity is regulated via direct phosphorylation by pro-oncogenic Casein Kinase II (CK2). CK2 is overexpressed in high-risk B-ALL as compared to normal B-cell precursors, which further reduces Ikaros function in this disease. In vivo CK2 inhibition with the CK2 specific inhibitor, CX-4945, results in a strong therapeutic effect in primary high-risk ALL xenografts. Analysis of primary high-risk B-ALL (that have deletion of one Ikaros allele) showed that treatment with CX-4945, restored Ikaros function as a transcriptional regulator of CDC2, ANAPC1 and ANAPC7, and was associated with cell cycle arrest. Epigenetic analysis of promoters of CDC2, ANAPC1 and ANAPC7 genes revealed that restoration of Ikaros binding to the promoters of these genes is associated with epigenetic alterations that are consistent with Ikaros overexpression and formation of repressive heterochromatin. In conclusion, our results reveal that: 1) Ikaros functions as a tumor suppressor by repressing transcription of genes that are critical for G/M transition (CDC2) and mitotic progression (ANAPC1 and ANAPC7); 2) Ikaros represses transcription by inducing two distinct epigenetic alterations at promoters of its target genes and 3) CK2 inhibition with CX-4945 restores Ikaros function as a transcriptional regulator of CDC2, ANAPC1 and ANAPC7 in high-risk leukemia. These results provide novel insights into the control of cell cycle progression in high-risk leukemia and the mechanisms by which CK2 inhibitors exert their therapeutic effects. Supported by the National Institutes of Health R01 HL095120, and the Four Diamonds Fund Endowment.

Citation Format: Chunhua Song, Chandrika Gowda, Yali Ding, Kimberly J. Payne, Sinisa Dovat. Epigenetic regulation of cell cycle progression at the G2/M transition and mitosis in high-risk leukemia. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr A21.

Abstract 2444: TSLP regulates expression of Bcl2 family proteins in Ph-like ALL with CRLF2 alterations

B cell precursor acute lymphoblastic leukemia (B-ALL) is the most common childhood malignancy. A subset of children with B-ALL are at high risk for relapse and death. Gene expression profiles in these high-risk B-ALLs is similar to that of Philadelphia chromosome positive ALL. Approximately half of these Ph-like B-ALL are characterized by genetic alterations that result in overexpression of CRLF2. CRLF2, together with the IL-7 receptor α chain, forms a receptor complex for the cytokine, TSLP. When TSLP binds, the receptor initiates downstream JAK2/STAT5 and PI3/AKT/mTOR pathway activation. The activating JAK mutations found in some CRLF2 B-ALL led to speculation that TSLP stimulation is not a factor in this disease. However, we find that TSLP increases phosphorylation of STAT5, AKT and S6 (downstream of mTOR) in CRLF2 B-ALL cells, including those with JAK defects. Activation of these pathways has been associated with oncogenesis and chemoresistance and their downstream targets include members of the Bcl2 family. The Bcl2 family pro-survival molecule Bcl-XL is a down stream target of STAT5 in Ph+ B-ALL. Mcl-1, another BCL2 family pro-survival molecule is known to be upregulated by mTOR activation via post-translational mechanisms in B cell lymphoma. We hypothesized that TSLP-induced JAK2/STAT5 and PI3/AKT/mTOR pathway activation contribute to chemoresistance in high risk CRLF2 B-ALL by upregulating the expression of Bcl-XL and Mcl-1. To test this hypothesis we cultured human CRLF2 B-ALL cell lines (MUTZ5 and CALL4) with and without TSLP and evaluated expression of the Bcl2 family pro-survival proteins, Bcl-XL, Mcl-1, and Bcl2. We found that TSLP induced significant increases in Bcl-XL and Mcl-1 proteins, but not Bcl2 in CRLF2 B-ALL cells. These cell lines have activating Jak mutations and thus reflect the ability of TSLP to increase expression of the Bcl2 family proteins in cases where activating JAK mutations are present. Next we evaluated the effect of Mcl-1 inhibitor on MUTZ5 and CALL4 cells. Preliminary data from these experiments show that cell counts in cultures treated with Mcl-1 inhibitor are reduce by &gt;90% and this reduction is maintained in the presence of TSLP. These data provide evidence that TSLP-induced CRLF2 signals increase expression of Bcl2 pro-survival proteins, even in CRLF2 B-ALL cells with activating JAK mutations. These data also suggest that Mcl-1 inhibitors could be an effective treatment for this disease. Ongoing studies will evaluate the effect of TSLP and Mcl-1 inhibitors in primary CRLF2 B-ALL samples.

Citation Format: Cornelia Stoian, Muhammad Omair Kamal, Olivia Francis, Rhaya Johnson, Simone Montgomery, Jacqueline Coats, Hannah Choi, Shania Aponte-Paris, Micheal Reed, Shanalee Martinez, Karina Mayagoitia, Evgeny Chirshev, Chunhua Song, Sinisa Dovat, Kimberly J. Payne. TSLP regulates expression of Bcl2 family proteins in Ph-like ALL with CRLF2 alterations. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2444.

Abstract 4463: Epigenetic regulation of gene expression in high-risk B-cell acute lymphoblastic leukemia by Casein Kinase II

Acute lymphoblastic leukemia (ALL) is the most common malignancy in childhood. Although advances in the treatment of ALL have resulted in a high cure rate for this disease, high-risk ALL is characterized by both resistance to conventional chemotherapy and a poor prognosis. The pathogenesis of high-risk ALL is still not understood. Casein Kinase II (CK2) is an oncogenic kinase that is overexpressed in both B-cell ALL (B-ALL) and T-cell ALL (T-ALL) and is associated with poor outcome. Inhibition of CK2 results in a strong therapeutic effect in a preclinical model of leukemia. However, the mechanism by which CK2 promotes oncogenesis in leukemia is unknown. Here, we studied how CK2 regulates expression of histone demethylase KDM5B in ALL. The KDM5B gene encodes a histone demethylase that regulates levels of histone modification H3K4me3 in leukemia. Molecular inhibition of CK2 using shRNA that targets the CK2 catalytic subunit resulted in transcriptional repression of KDM5B in ALL as evidenced by qRT-PCR. A similar effect was observed when leukemia cells were treated with the CK2 inhibitor CX-4945. Inhibition of CK2 resulted in reduced expression of KDM5B with an increase in the global cellular level of H3K4me3 as evidenced by Western blot. The use of quantitative chromatin immunoprecipitation (qChIP) showed that CK2 inhibition enhances DNA binding of the Ikaros tumor suppressor to the promoter of the KDM5B gene. Ikaros is a DNA-binding protein that regulates transcription of its target genes via chromatin remodeling. Loss of Ikaros function results in high-risk ALL. Serial qChIP analysis demonstrated that the increased Ikaros binding to the KDM5B promoter following CK2 inhibition is associated with an alteration of the epigenetic signature at the DNA region that surrounds the Ikaros binding site. Specifically, enhanced Ikaros binding results in increased occupancy of the H3K27me3 histone modification, along with a reduced occupancy of the H3K9ac histone modification at the KDM5B promoter. These results are consistent with the formation of heterochromatin and transcriptional repression. We tested the effect of CK2 inhibitors on Ikaros-mediated repression of KDM5B in primary, high-risk B-ALL cells that have a deletion of one Ikaros allele. Results showed that CK2 inhibition in high-risk B-ALL restores Ikaros binding to KDM5B promoter and represses KDM5B transcription. These data suggest that the inhibition of CK2 controls expression of KDM5B and the global H3K4me3 level in ALL by regulating the function of Ikaros as a transcriptional repressor of KDM5B. This presented data demonstrates the role of the CK2-Ikaros signaling axis in the regulation of both gene expression and the global epigenetic signature in ALL, and provide a mechanistic insight into the role of CK2 in the pathogenesis of ALL.

Citation Format: Morgann Loaec, Jonathon Payne, Elanora Dovat, Chunhua Song, Kimberly J. Payne, Sinisa Dovat. Epigenetic regulation of gene expression in high-risk B-cell acute lymphoblastic leukemia by Casein Kinase II. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4463.

TSLP or IL-7 provide an IL-7Rα signal that is critical for human B lymphopoiesis

Thymic stromal lymphopoietin (TSLP) and IL-7 are cytokines that signal via the IL-7 receptor alpha (IL-7Rα) to exert both overlapping and unique functions during early stages of mouse B-cell development. In human B lymphopoiesis, the requirement for IL-7Rα signaling is controversial and the roles of IL-7 and TSLP are less clear. Here, we evaluated human B-cell production using novel in vitro and xenograft models of human B-cell development that provide selective IL-7 and human TSLP (hTSLP) stimulation. We show that in vitro human B-cell production is almost completely blocked in the absence of IL-7Rα stimulation, and that either TSLP or IL-7 can provide a signal critical for the production and proliferation of human CD19(+) PAX5(+) pro-B cells. Analysis of primary human bone marrow stromal cells shows that they express both IL-7 and TSLP, providing an in vivo source of these cytokines. We further show that the in vivo production of human pro-B cells under the influence of mouse IL-7 in a xenograft scenario is reduced by anti-IL-7 neutralizing antibodies, and that this loss can be restored by hTSLP at physiological levels. These data establish the importance of IL-7Rα mediated signals for normal human B-cell production.

Abstract PR09: CK2 inhibition exerts a therapeutic effect in high-risk ALL by restoring IKZF1-mediated repression of cell cycle progression and the PI3K pathway

The IKZF1 (Ikaros) gene encodes a DNA-binding protein that acts as a tumor suppressor in leukemia. Deletion of one Ikaros allele results in B-cell acute lymphoblastic leukemia (B-ALL) with a high rate of relapse and poor outcome. The mechanisms through which Ikaros suppresses leukemogenesis and that regulate Ikaros activity as a tumor suppressor in leukemia are unknown. Using a systems biology approach, we determined that Ikaros regulates transcription of genes that control two pathways crucial for proliferation of leukemia cells: 1) cell cycle progression and 2) the phosphatidylinositol 3-kinase (PI3K) pathway. Using gain-of-function and loss-of-function experiments we demonstrate that Ikaros transcriptionally represses genes that promote cell cycle progression and the PI3K pathway and activates transcription of a gene that suppresses the PI3K pathway. In high-risk B-ALL with deletion of one Ikaros allele, we show that the function of Ikaros as a transcriptional regulator is impaired due to reduced binding at promoters of its target genes. Previous work shows that Ikaros DNA-binding affinity is regulated via direct phosphorylation by the pro-oncogenic kinase, CK2 (Casein Kinase II). We show that CK2 is overexpressed in high-risk B-ALL as compared to normal B-cell precursors, further reducing Ikaros function. Treatment of primary high-risk B-ALL (with deletion of one IKZF1allele) using the CK2 specific inhibitor, CX-4945, restored Ikaros function as a transcriptional regulator of genes that control cell cycle progression and the PI3K pathway. Treatment with CK2 inhibitor was also associated with cell cycle arrest and reduced phosphorylation of the AKT kinase, a downstream PI3K pathway target. Using serial quantitative chromatin immunoprecipitation (qChIP) analyses spanning the promoters of Ikaros target genes, we demonstrated that Ikaros can repress transcription of its target genes through two distinct mechanisms: 1) via recruitment of histone deacetylase 1 (HDAC1), which is associated with the formation of repressive chromatin characterized by H3K27me3 and the loss of H3K9ac; and 2) by an HDAC1-independent mechanism that is associated with the formation of repressive chromatin characterized by H3K9me3, along with the loss of H3K9ac. The therapeutic efficacy of CK2 inhibition using CX-4945 against high-risk B-ALL was demonstrated in vivo using 4 different xenografts: 3 different high-risk primary pre-B-ALL patient-derived xenografts and Nalm6 xenografts. CX-4945 showed strong therapeutic effects in all 4 xenografts, as evidence by reduced leukemia cell numbers in bone marrow and in spleen, together with prolonged survival of treated xenograft animals. Expression analysis of Ikaros target genes in leukemia cells treated in vivo with CX-4945 revealed an expression pattern cell cycle regulatory and PI3K pathway genes that was highly similar to that observed with Ikaros overexpression. These data suggest that CK2 inhibition in vivo exerts its therapeutic effect on high-risk B-ALL by restoring Ikaros function as a transcriptional regulator of genes that promote cell cycle progression and the PI3K pathway. In summary, our results reveal that: 1) Ikaros functions as a tumor suppressor by suppressing cell cycle progression and the PI3K pathway; 2) Ikaros regulates transcription by inducing two distinct epigenetic alterations at promoters of its target genes and 3) CK2 inhibition with CX-4945 restores Ikaros function as a transcriptional regulator in vivo, and has a strong therapeutic effect in primary xenografts of high-risk B-ALL. These results provide support for the use of CK2 inhibitors in clinical trials for high-risk B-ALL.

Supported by the National Institutes of Health R01 HL095120, and the Four Diamonds Fund Endowment.

This abstract is also presented as Poster B10.

Citation Format: Chunhua Song, Chandrika Gowda, XiaoKang Pan, Kimberly J. Payne, Sinisa Dovat. CK2 inhibition exerts a therapeutic effect in high-risk ALL by restoring IKZF1-mediated repression of cell cycle progression and the PI3K pathway. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr PR09.

Abstract B32: A novel preclinical model and targeted therapy to reduce pediatric leukemia health disparities

Data presented here identify a targeted therapy and a novel preclinical model for evaluating therapeutic efficacy to reduce pediatric cancer health disparities. Hispanic children are 1.24 times more likely to develop acute lymphoblastic leukemia (ALL) than non-Hispanic whites and that number rises to 2.09 by adolescence and early adulthood. A major contributor to this health disparity is a subtype of high-risk B cell precursor ALL called CRLF2 B-ALL. CRLF2 B-ALL occurs 5 times more often in Hispanic and Native American children than others and is also prevalent in adolescents and young adults. CRLF2 B-ALL is caused by genetic alterations leading to over expression of the cytokine receptor, CRLF2. CRLF2 activation stimulates proliferation of B cell precursors. Approximately 80% of CRLF2 B-ALL cases also have inactivating deletions or mutations in one allele of the (Ikaros) IKZF1 tumor suppressor gene. Ikaros normally represses genes responsible for cellular proliferation and Ikaros mutations are highly associated with relapse. Thus, our strategy is to identify therapies that target both the Ikaros and CRLF2 genetic alterations that are responsible for high-risk CRLF2 B-ALL and the health disparities caused by this disease. Our previous work has shown that Ikaros proteins can be inactivated Casein Kinase II (CK2) and that CK2 inhibitors can be used to restore normal levels of Ikaros activity from the remaining normal Ikaros allele in B-ALL. Patient-derived xenografts produced by transplanting leukemia cells from patients into immune deficient mice provide an in vivo model of disease that includes contributions of the background genetic landscape to health disparities diseases. Here we evaluated the in vivo therapeutic efficacy of the CK2-specific inhibitor, CX-4945, using patient-derived xenografts (PDX) produced by transplanting immune deficient mice with B-ALL cells from 2 different Hispanic pediatric patients with high-risk CRLF2 B-ALL. Survival was significantly prolonged (p&lt;.0002) in treated as compared to untreated PDX produced from both patients. Flow cytometry analysis also showed reduced leukemia cell number in bone marrow (BM) and in spleen of treated as compared to untreated PDX. These data provide evidence that CK2-specific inhibitors can be an effective therapy for targeting the Ikaros defect to restore Ikaros tumor suppressor activity in CRLF2 B-ALL in Hispanic patients. Our next step was the development of a preclinical model that would allow us to target the pathway activated by overexpressed CRLF2 in this disease. Biologically, CRLF2 acts as a receptor component for the cytokine, TSLP, which induces JAK2-STAT5 and PI3/AKT/mTOR pathway activation leading to increased production of B cell precursors. PDX models are possible because most cytokines produced in the mouse are active on human cells, however mouse TSLP is species-specific. Thus classic PDX models do not provide TSLP that can activate the CRLF2 receptor that is overexpressed in CRLF2 B-ALL. To address this hurdle we engineered PDX mice to express normal serum levels (12-32 pg/ml) of human TSLP (+T PDX mice). In vivo TSLP activity was validated and +T PDX were successfully generated using leukemia cells from two Hispanic pediatric patients with CRLF2 B-ALL. We are using this model in ongoing studies evaluate therapies that target the CRLF2 pathway. Future studies will be aimed at using the +T Hispanic Patient PDX preclinical model to evaluate the efficacy of therapies that target the CRLF2 pathway in combination with CK2 inhibitors to restore Ikaros tumor suppressor activity.

Citation Format: Kimberly J. Payne, Sinisa Dovat. A novel preclinical model and targeted therapy to reduce pediatric leukemia health disparities. [abstract]. In: Proceedings of the Eighth AACR Conference on The Science of Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; Nov 13-16, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2016;25(3 Suppl):Abstract nr B32.

Abstract A07: A novel patient-derived xenograft model to define the role of TSLP-induced CRLF2 signals and identify therapies for Ph-like B-ALL

A subset of high-risk B cell acute lymphoblastic leukemia (ALL) shows a gene expression profile similar to Philadelphia chromosome positive (Ph+) ALL and has been described as Ph-like ALL. Approximately 50% of Ph-like B-ALL is characterized by genetic alterations leading to overexpression of CRLF2 (CRLF2 B-ALL). CRLF2 B-ALL occurs 5 times more often in Hispanic and Native American children than others and is prevalent in adolescents and young adults. Biologically, CRLF2 acts as a receptor component for the cytokine, TSLP, which induces JAK2-STAT5 and PI3/AKT/mTOR pathway activation downstream of binding to CRLF2. While activating JAK mutations are associated with CRLF2 B-ALL, over half of CRLF2 B-ALL lack such mutations. Our data show that primary human bone marrow (BM) stromal cells express TSLP. Thus TSLP is present in the tumor microenvironment to provide TSLP-induced CRLF2 signals that could play a role in the initiation, maintenance and/or progression of CRLF2 B-ALL. Consistent with this, TSLP has been reported to increase in vitro production of human fetal B cell precursors. However studies of TSLP in B lymphopoiesis have been conducted almost exclusively in mice which show low homology (~40%) with respect to human TSLP and CRLF2. Further, phospho flow cytometry assays show that human, but not mouse TSLP activates CRLF2 signals in primary human CRLF2 B-ALL cells and cell lines as indicated by increased pSTAT5, pAKT and pS6. These data indicate that the mouse TSLP present in classic patient derived xenograft models (PDX) does not produce the TSLP-induced CRLF2 signals present in the patient. To address this challenge we engineered PDX mice to produce human TSLP (hTSLP) by transplanting them with stromal cells transduced to express hTSLP (+T mice). Control (T) mice were produced by transplantation with stroma transduced with a control vector. Supernatant from engineered +T stroma, but not T stroma, induced JAK/STAT5 and PI3K/AKT/mTOR pathway activation in human CRLF2 B-ALL cells. ELISA assays showed that serum levels of hTSLP in mice was proportional to numbers of stromal cells injected at weekly time points. Normal human serum levels of hTSLP (12-32 pg/ml) could be achieved in +T mice, while hTSLP was undetectable in T mice. Because TSLP has been shown to increase in vitro production of human B cell precursors, we evaluated the in vivo functionality of our model by comparing the production of normal B cell precursors in the BM of +T and T PDX mice generated with human umbilical cord blood CD34+ cells. Data from 3 different cord blood donors showed that production of B cell precursors is 3-5 fold increased in +T as compared to T mice. TSLP-induced increases were specific to B lineage cells, initiated in the earliest CD19+ B cell precursors, and maintained through later stages of B cell development. Next we evaluate the in vivo functionality of our model using primary CRLF2 B-ALL leukemia cells. Human CRLF2 B-ALL cells were isolated from the BM of PDX mice and whole genome microarray was performed. Evaluation of microarray data by Gene Set Enrichment Analysis (GSEA) and Ingenuity Pathway Analysis showed that genes downstream of mTOR pathway activation were upregulated in +T as compared to T PDX mice, confirming hTSLP activity in the +T PDX mice. To determine whether +T PDX mice provide a preclinical model of B-ALL that more closely mirrors patients than T PDX mice, we compared RNAseq gene expression profiles of leukemia cells from +T and T PDX mice to that from original patient sample. The gene expression pattern in +T mice was significantly closer to primary patient sample than that from T mice. The +T and T PDX mice described here provide a novel preclinical model for studying the role of TSLP in the initiation, progression and maintenance of CRLF2 B-ALL and for evaluating drug efficacy in an in vivo model that more closely mirrors the in vivo environment present in patients.

Citation Format: Olivia L. Francis, Terry-Ann Milford, Ineavely Baez, Jacqueline S. Coats, Christopher L. Morris, Ross Fisher, Ben Van Handel, Ruijun Su, Batul Suterwala, Muhammad Kamal, Shadi Farzin Gohar, Sinisa Dovat, Kimberly J. Payne. A novel patient-derived xenograft model to define the role of TSLP-induced CRLF2 signals and identify therapies for Ph-like B-ALL. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr A07.

Fine-tuning patient-derived xenograft models for precision medicine approaches in leukemia

Many leukemias are characterized by well-known mutations that drive oncogenesis. Mice engineered with these mutations provide a foundation for understanding leukemogenesis and identifying therapies. However, data from whole genome studies provide evidence that malignancies are characterized by multiple genetic alterations that vary between patients, as well as inherited genetic variation that can also contribute to oncogenesis. Improved outcomes will require precision medicine approaches-targeted therapies tailored to malignancies in each patient. Preclinical models that reflect the range of mutations and the genetic background present in patient populations are required to develop and test the combinations of therapies that will be used to provide precision medicine therapeutic strategies. Patient-derived xenografts (PDX) produced by transplanting leukemia cells from patients into immune deficient mice provide preclinical models where disease mechanisms and therapeutic efficacy can be studied in vivo in context of the genetic variability present in patient tumors. PDX models are possible because many elements in the bone marrow microenvironment show cross-species activity between mice and humans. However, several cytokines likely to impact leukemia cells are species-specific with limited activity on transplanted human leukemia cells. In this review we discuss the importance of PDX models for developing precision medicine approaches to leukemia treatment. We illustrate how PDX models can be optimized to overcome a lack of cross-species cytokine activity by reviewing a recent strategy developed for use with a high-risk form of B-cell acute lymphoblastic leukemia (B-ALL) that is characterized by overexpression of CRLF2, a receptor component for the cytokine, TSLP.

Transcriptional Regulation of JARID1B/KDM5B Histone Demethylase by Ikaros, Histone Deacetylase 1 (HDAC1), and Casein Kinase 2 (CK2) in B-cell Acute Lymphoblastic Leukemia

Impaired function of the Ikaros (IKZF1) protein is associated with the development of high-risk B-cell precursor acute lymphoblastic leukemia (B-ALL). The mechanisms of Ikaros tumor suppressor activity in leukemia are unknown. Ikaros binds to the upstream regulatory elements of its target genes and regulates their transcription via chromatin remodeling. Here, we report that Ikaros represses transcription of the histone H3K4 demethylase, JARID1B (KDM5B). Transcriptional repression of JARID1B is associated with increased global levels of H3K4 trimethylation. Ikaros-mediated repression of JARID1B is dependent on the activity of the histone deacetylase, HDAC1, which binds to the upstream regulatory element of JARID1B in complex with Ikaros. In leukemia, JARID1B is overexpressed, and its inhibition results in cellular growth arrest. Ikaros-mediated repression of JARID1B in leukemia is impaired by pro-oncogenic casein kinase 2 (CK2). Inhibition of CK2 results in increased binding of the Ikaros-HDAC1 complex to the promoter of JARID1B, with increased formation of trimethylated histone H3 lysine 27 and decreased histone H3 Lys-9 acetylation. In cases of high-risk B-ALL that carry deletion of one Ikaros (IKZF1) allele, targeted inhibition of CK2 restores Ikaros binding to the JARID1B promoter and repression of JARID1B. In summary, the presented data suggest a mechanism through which Ikaros and HDAC1 regulate the epigenetic signature in leukemia: via regulation of JARID1B transcription. The presented data identify JARID1B as a novel therapeutic target in B-ALL and provide a rationale for the use of CK2 inhibitors in the treatment of high-risk B-ALL.

A novel xenograft model to study the role of TSLP-induced CRLF2 signals in normal and malignant human B lymphopoiesis

Thymic stromal lymphopoietin (TSLP) stimulates in-vitro proliferation of human fetal B-cell precursors. However, its in-vivo role during normal human B lymphopoiesis is unknown. Genetic alterations that cause overexpression of its receptor component, cytokine receptor-like factor 2 (CRLF2), lead to high-risk B-cell acute lymphoblastic leukemia implicating this signaling pathway in leukemogenesis. We show that mouse thymic stromal lymphopoietin does not stimulate the downstream pathways (JAK/STAT5 and PI3K/AKT/mTOR) activated by the human cytokine in primary high-risk leukemia with overexpression of the receptor component. Thus, the utility of classic patient-derived xenografts for in-vivo studies of this pathway is limited. We engineered xenograft mice to produce human thymic stromal lymphopoietin (+T mice) by injection with stromal cells transduced to express the cytokine. Control (-T) mice were produced using stroma transduced with control vector. Normal levels of human thymic stromal lymphopoietin were achieved in sera of +T mice, but were undetectable in -T mice. Patient-derived xenografts generated from +T as compared to -T mice showed a 3-6-fold increase in normal human B-cell precursors that was maintained through later stages of B-cell development. Gene expression profiles in high-risk B-cell acute lymphoblastic leukemia expanded in +T mice indicate increased mTOR pathway activation and are more similar to the original patient sample than those from -T mice. +T/-T xenografts provide a novel pre-clinical model for understanding this pathway in B lymphopoiesis and identifying treatments for high-risk B-cell acute lymphoblastic leukemia with overexpression of cytokine-like factor receptor 2.

Targeting casein kinase II restores Ikaros tumor suppressor activity and demonstrates therapeutic efficacy in high-risk leukemia

Ikaros (IKZF1) is a tumor suppressor that binds DNA and regulates expression of its target genes. The mechanism of Ikaros activity as a tumor suppressor and the regulation of Ikaros function in leukemia are unknown. Here, we demonstrate that Ikaros controls cellular proliferation by repressing expression of genes that promote cell cycle progression and the phosphatidylinositol-3 kinase (PI3K) pathway. We show that Ikaros function is impaired by the pro-oncogenic casein kinase II (CK2), and that CK2 is overexpressed in leukemia. CK2 inhibition restores Ikaros function as transcriptional repressor of cell cycle and PI3K pathway genes, resulting in an antileukemia effect. In high-risk leukemia where one IKZF1 allele has been deleted, CK2 inhibition restores the transcriptional repressor function of the remaining wild-type IKZF1 allele. CK2 inhibition demonstrated a potent therapeutic effect in a panel of patient-derived primary high-risk B-cell acute lymphoblastic leukemia xenografts as indicated by prolonged survival and a reduction of leukemia burden. We demonstrate the efficacy of a novel therapeutic approach for high-risk leukemia: restoration of Ikaros tumor suppressor activity via inhibition of CK2. These results provide a rationale for the use of CK2 inhibitors in clinical trials for high-risk leukemia, including cases with deletion of one IKZF1 allele.

Cutting Edge: Hematopoietic Stem Cell Expansion and Common Lymphoid Progenitor Depletion Require Hematopoietic-Derived, Cell-Autonomous TLR4 in a Model of Chronic Endotoxin

Hematopoietic stem and progenitors cells (HSPCs) are activated through TLR4 in vitro. However, it remains unclear whether in vivo TLR4 sensing by HSPCs occurs directly or via other cell intermediates. In this study, we examined the cellular mechanisms underlying murine hematopoietic stem cell (HSC) expansion and common lymphoid progenitor (CLP) depletion in a model of chronic low-dose LPS. Using adoptive-transfer approaches, we show that HSC and CLP sensitivity to chronic LPS depends on hematopoietic-derived, cell subset-autonomous TLR4. Like murine progenitors, human HSPCs are activated by TLR4 in vitro. Using humanized mice, a preclinical model relevant to human physiology, we show that persistent endotoxin increases the frequency of Ki-67(+) HSCs and severely depletes CLPs and B precursors. Together, our findings show that murine HSPCs directly respond to endotoxin in vivo and that persistent LPS, a feature of several diseases of global health significance, impairs human lymphopoiesis.

Abstract 3295: A novel patient-derived xenograft model for evaluating the role of TSLP in CRLF2 B-ALL

B-cell acute lymphoblastic leukemia (B-ALL) with genetic alterations leading to overexpression of CRLF2 (CRLF2 B-ALL) is associated with poor outcomes. CRLF2 B-ALL is 5 times more common in Hispanic children than others making it a significant biological component of pediatric cancer health disparities. CRLF2 is a component of the receptor complex that is activated by the cytokine, TSLP. Receptor signaling induces Jak/STAT5 and PI3/AKT/mTOR pathway activation and plays a role in the proliferation and differentiation B cell precursors. We found that primary human bone marrow (BM) stroma express TSLP providing an in vivo source of TSLP to stimulate CRLF2 B-ALL cells. Our goal was to develop patient-derived xenograft (PDX) models of CRLF2 B-ALL for studies to understand disease mechanisms and identify therapies to treat CRLF2 B-ALL and reduce the health disparities for Hispanic children with this disease. PDX models are possible because many cytokines produced in the mouse show cross species activity on human cells. However, available data suggests that mouse TSLP does not activate human CRLF2-mediated signals. Using phospho flow cytometry we show that mouse TSLP was unable to induce increases in pSTAT5, pAKT and pS6 observed in CRLF2 B-ALL cells stimulated with human TSLP. We developed a human TSLP +/- PDX model system by transplanting immune deficient NSG mice with HS-27 stroma transduced to express human hTSLP (hTSLP+ mice) or with control vector (hTSLP- mice). Human TSLP was present at normal human serum levels in hTSLP+ mice but undetectable in hTSLP- mice. To identify genes targeted by TSLP in CRLF2 B-ALL and verify pathway activation, we transplanted primary leukemia cells from a Hispanic patient into hTSLP+ and hTSLP- mice. Whole genome microarray was performed on CRLF2 B-ALL cells isolated from the BM of the hTSLP+ and hTSLP- PDX mice. Microarray identified 280 genes that are upregulated and 281 genes that are downregulated in vivo in leukemia cells from hTSLP+ as compared to hTSLP- PDX mice. Evaluation of microarray data by Gene Set Enrichment Analysis (GSEA) and Ingenuity Pathway Analysis showed that genes downstream of mTOR pathway activation were upregulated in hTSLP+ as compared to hTSLP- mice, confirming hTSLP activity in the hTSLP+ PDX mice. Our next question was whether cells expanded in hTSLP+ vs. hTSLP- mice would exhibit changes in their ability to respond to TSLP. When we subjected PDX-expanded primary CRLF2 B-ALL cells to ex vivo TSLP stimulation ∼1/3 fewer gene targets were up- and downregulated in the leukemia cells expanded in hTSLP- mice as compared to cells from hTSLP+ mice. This suggests that CRLF2 B-ALL cells expanded in xenograft without TSLP lose some of their ability to respond to TSLP. The hTSLP+ CRLF2 B-ALL PDX mice described here provide a novel preclinical model for studying disease mechanisms and identifying therapies to target signaling pathways activated by TSLP in CRLF2 B-ALL and reduce cancer health disparities for this disease.

Citation Format: Olivia L. Francis, Parveen Shiraz, Terry-Ann Milford, Ineavely Baez, Jacqueline S. Coats, Karina Mayagoitia, Elizabeth Ginelli, Katherine R. Salcedo-Concepcion, Shannalee Martinez, Xiaobing Zhang, Valeri Filippov, Ruijun J. Su, Ross Fisher, Christopher L. Morris, Sinisa Dovat, Kimberly J. Payne. A novel patient-derived xenograft model for evaluating the role of TSLP in CRLF2 B-ALL. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3295. doi:10.1158/1538-7445.AM2015-3295

Abstract 2159: Ikaros and Casein kinase II (CK2) regulate PI3K pathway in pediatric leukemia

Ikaros (IKZF1) is a tumor suppressor whose function is impaired in high-risk pediatric B-cell acute lymphoblastic leukemia (B-ALL). IKZF1 encodes a DNA-binding, zinc finger protein that regulates expression of genes involved in important biological pathways. Using chromatin immunoprecipitation coupled with next-generation sequencing (ChIP-SEQ) we found that Ikaros binds to the upstream regulatory regions of multiple genes that regulate the phosphatidylinositol-3-Kinase (PI3K) pathway. Ikaros target genes include PIK3C2B and PI3KFYVE. We used gain-of-function and loss-of-function experiments to determine how Ikaros regulates transcription of its target genes. Overexpression of Ikaros by retroviral transduction in Nalm6 leukemia cells results in reduced transcription of PIK3C2B and PI3KFYVE as evidenced by qRT-PCR. Luciferase reporter assays with PIK3C2B and PI3KFYVE promoters showed that Ikaros can function as a transcriptional repressor of these genes. Transfection of Nalm6 cells with Ikaros shRNA resulted in increased expression of PIK3C2B and PI3KFYVE genes. These results suggest that Ikaros functions as a transcriptional repressor of PIK3C2B and PI3KFYVE genes in leukemia. Next, we studied signaling pathways that regulate the ability of Ikaros to transcriptionally repress the PIK3C2B and PI3KFYVE genes. We have previously shown that a pro-oncogenic Casein Kinase II (CK2) can directly phosphorylate Ikaros in vivo and that CK2-mediated phosphorylation impairs Ikaros function. We tested whether inhibition of CK2 activity affects Ikaros ability to regulate PIK3C2B and PI3KFYVE transcription in leukemia. Results show that molecular and pharmacological inhibition of CK2 have a very similar effect on transcription of Ikaros target genes and they result in transcriptional repression of both PIK3C2B and PI3KFYVE genes. Treatment of leukemia cell lines, as well as primary B-ALL cells, with different CK2 inhibitors resulted in enhanced Ikaros binding to its target genes, as evidenced by quantitative chromatin immunoprecipitation (qChIP). In summary, the presented data provide evidence that Ikaros and CK2 regulate the PI3K pathway via transcriptional regulation of the PIK3C2B and PI3KFYVE genes. Our results demonstrate that CK2 inhibition enhances Ikaros activity as a transcriptional repressor of genes that promote the PI3K pathway in primary B-ALL cells, and identify CK2 inhibitors as candidate drugs to therapeutically restore Ikaros function in B-ALL.

Supported by the National Institutes of Health R01 HL095120, and the Four Diamonds Fund Endowment.

Citation Format: Chandrika Gowda, Chunhua Song, Yali Ding, Sunil Muthusami, Xiaokang Pan, Dhimant Desai, Shantu G. Amin, Kimberly J. Payne, Sinisa Dovat. Ikaros and Casein kinase II (CK2) regulate PI3K pathway in pediatric leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2159. doi:10.1158/1538-7445.AM2015-2159

Surface APRIL Is Elevated on Myeloid Cells and Is Associated with Disease Activity in Patients with Rheumatoid Arthritis

OBJECTIVE

To assess surface APRIL (a proliferation-inducing ligand; CD256) expression by circulating myeloid cells in rheumatoid arthritis (RA) and to determine its relationship to disease activity.

METHODS

Peripheral blood mononuclear cells (PBMC) and plasma were obtained from patients with RA and healthy donors. PBMC were stained for flow cytometry to detect surface APRIL and blood cell markers to identify circulating myeloid cell subsets. Based on CD14 and CD16 phenotypes, monocyte subsets described as classical (CD14+CD16-), intermediate (CD14+CD16+), and nonclassical (CD14loCD16+) were identified. Levels of surface APRIL expression were measured by flow cytometry and median fluorescence intensity was used for comparisons. Levels of soluble APRIL in the plasma were determined by ELISA. Disease activity was measured by the Disease Activity Score in 28 joints.

RESULTS

In patients with RA, total myeloid cells showed expression of surface APRIL that correlated with disease activity and with plasma APRIL levels observed in these patients. In healthy donors, classical monocytes were composed of > 80% of circulating monocytes. However, in patients with RA, the intermediate and nonclassical subsets were elevated and made up the majority of circulating monocytes. In contrast to healthy donors, where high levels of surface APRIL were only observed in nonclassical monocytes, patients with RA showed high levels of surface APRIL expression by all circulating monocyte subsets.

CONCLUSION

Surface APRIL is elevated in circulating myeloid cells in patients with RA where it is highly correlated with disease activity. Patients with RA also showed skewing of monocytes toward subsets associated with secretion of tumor necrosis factor-α and/or interleukin 1β.

Protein phosphatase 1 (PP1) and Casein Kinase II (CK2) regulate Ikaros-mediated repression of TdT in thymocytes and T-cell leukemia

BACKGROUND

Ikaros is a DNA-binding protein that acts as master-regulator of hematopoiesis and a tumor suppressor. In thymocytes and T-cell leukemia, Ikaros negatively regulates transcription of terminal deoxynucleotide transferase (TdT), a key protein in lymphocyte differentiation. The signaling pathways that regulate Ikaros-mediated repression of TdT are unknown. Our previous work identified Casein Kinase II (CK2) and Protein Phosphatase 1 (PP1) as regulators of Ikaros DNA binding activity. Here, we investigated the role of PP1 and CK2 in regulating Ikaros-mediated control of TdT expression.

PROCEDURES

Ikaros phosphomimetic and phosphoresistant mutants and specific CK2 and PP1 inhibitors were used in combination with quantitative chromatin immunoprecipitation (qChIP) and quantitative reverse transcriptase-PCR (q RT-PCR) assays to evaluate the role of CK2 and PP1 in regulating the ability of Ikaros to bind the TdT promoter and to regulate TdT expression.

RESULTS

We demonstrate that phosphorylation of Ikaros by pro-oncogenic CK2 decreases Ikaros binding to the promoter of the TdT gene and reduces the ability of Ikaros to repress TdT expression during thymocyte differentiation. CK2 inhibition and PP1 activity restore Ikaros DNA-binding affinity toward the TdT promoter, as well as Ikaros-mediated transcriptional repression of TdT in primary thymocytes and in leukemia.

CONCLUSION

These data establish that PP1 and CK2 signal transduction pathways regulate Ikaros-mediated repression of TdT in thymocytes and leukemia. These findings reveal that PP1 and CK2 have opposing effects on Ikaros-mediated repression of TdT and establish novel roles for PP1 and CK2 signaling in thymocyte differentiation and leukemia.

Abstract B25: A human-mouse xenograft model to evaluate therapies and study the role of TSLP-induced signals in Ph-like ALL

While the overall survival rate for children with B cell precursor acute lymphoblastic leukemia (B-ALL) is high, a subset of children with this disease are at high risk for relapse and death. Genome-wide analysis has shown that gene expression profiles in these high-risk B-ALLs is similar to that of Philadelphia chromosome–positive ALL and these are designated Ph-like ALL. Approximately half of Ph-like ALL are characterized by genetic defects resulting in overexpression of CRLF2. CRLF2, together with the IL-7Rα, forms a receptor complex that is activated by the cytokine, TSLP. The JAK-STAT5 pathway is phosphorylated downstream of this receptor complex activation. The activating JAK mutations found in some CRLF2 B-ALL have led to speculation that TSLP stimulation is not a factor in CRLF B-ALL. In preliminary studies to address this question we evaluated the effect of TSLP on a CRLF2 B-ALL cell lines with JAK defects and which have been reported to exhibit constitutive JAK-STAT5 activation. Our data show that TSLP increases STAT5 phosphorylation in these cell lines and also in primary CRLF2 B-ALL cells. Our next step was to evaluate the role of TSLP-CRLF2 interactions in vivo in the human-mouse xenograft model. However, mouse TSLP is different from most other cytokines produced in the xenograft in that it is species-specific and does not activate the human TSLP receptor complex that includes CRLF2. Thus, traditional xenograft models do not provide the TSLP-CRLF2 interactions that we believe to be a major factor in CRLF2 B-ALL. To overcome this obstacle we engineered immune-deficient NOD/SCID IL-2Rγ null (NSG) mice to express human TSLP (hTSLP+ mice) as well as control mice that lack the TSLP cytokine (hTSLP– mice). ELISA assays show serum hTSLP levels in the hTSLP+ mice that approximate the normal range in human serum. We used this hTSLP+/- xenograft model system to study the in vivo effects of TSLP on mice transplanted with a CRLF2 B-ALL. We used this hTSLP+/– xenograft model system to evaluate the in vivo effects of TSLP on survival and proliferation of transplanted CRLF2 B-ALL cells harboring a JAK defect (MUTZ5 cell line). Mice were euthanized at 5 weeks and BM was harvested. Evaluation of BM disease by flow cytometry showed that the percentage of viable human leukemia cells in hTSLP+ mice was twice that observed in hTSLP– mice. Evaluation of cell cycle progression in human CRLF2 B-ALL cells isolated from xenograft BM showed that the percentage of cycling cells in hTSLP+ mice was 2.5 fold higher than in hTSLP– mice. When primary Ph-like ALL cells were transplanted to produce hTSLP+/– xenografts, the viable pre-B ALL cells present in the BM of hTSLP+ mice showed higher expression levels of the TSLPR components (CRLF2 and IL-7Rα) than those in the hTSLP- mice. These data provide evidence that the TSLP produced in this model is active and that it impacts primary pre-B ALL cells. Preliminary data obtained from this model suggests that TSLP provides a signal that promotes in vivo survival of CRLF2 B-ALL cells and that it may play a role in selection of leukemia clones during in vivo leukemogenesis. Microarray analysis comparing gene expression in primary CRLF2 B-ALL cells isolated from hTSLP+ and hTSLP– xenograft mice identified 565 that genes are differentially regulated (&gt; 2 fold up or downregulated; p&lt;.05). Ingenuity Pathway Analysis is currently underway to identify the signaling pathways that are regulated by hTSLP in CRLF2 B-ALL in vivo in the hTSLP+/– xenograft model. The identification of genes downstream of TSLP-CRLF2 signaling has the potential of providing drug targets for combination therapy to effectively treat Ph-like B-ALL. The hTSLP+/– xenograft model provides an important tool for evaluating the in vivo efficacy of these and other drugs to treat CRLF2 B-ALL.

Citation Format: Ruijun Su, Francis L. Olivia, Shannalee R. Martinez, Ineavely Baez, Terry Ann Milford, Terrence Bennett, Ross Fisher, Christopher L. Morris, Sinisa Dovat, Kimberly J. Payne. A human-mouse xenograft model to evaluate therapies and study the role of TSLP-induced signals in Ph-like ALL. [abstract]. In: Proceedings of the AACR Special Conference: The Translational Impact of Model Organisms in Cancer; Nov 5-8, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(11 Suppl):Abstract nr B25.

Abstract B21: A human-mouse xenograft model to evaluate therapies and study the role of TSLP-induced signals in Ph-like ALL

While the overall survival rate for children with B cell precursor acute lymphoblastic leukemia (B-ALL) is high, a subset of children with this disease are at high risk for relapse and death. Genome-wide analysis has shown that gene expression profiles in these high-risk B-ALLs is similar to that of Philadelphia chromosome–positive ALL and these are designated Ph-like ALL. Approximately half of Ph-like ALL are characterized by genetic defects resulting in overexpression of CRLF2. CRLF2, together with the IL-7Rα, forms a receptor complex that is activated by the cytokine, TSLP. The JAK-STAT5 pathway is phosphorylated downstream of this receptor complex activation. The activating JAK mutations found in some CRLF2 B-ALL have led to speculation that TSLP stimulation is not a factor in CRLF B-ALL. In preliminary studies to address this question we evaluated the effect of TSLP on a CRLF2 B-ALL cell lines with JAK defects and which have been reported to exhibit constitutive JAK-STAT5 activation. Our data show that TSLP increases STAT5 phosphorylation in these cell lines and also in primary CRLF2 B-ALL cells. Our next step was to evaluate the role of TSLP-CRLF2 interactions in vivo in the human-mouse xenograft model. However, mouse TSLP is different from most other cytokines produced in the xenograft in that it is species-specific and does not activate the human TSLP receptor complex that includes CRLF2. Thus, traditional xenograft models do not provide the TSLP-CRLF2 interactions that we believe to be a major factor in CRLF2 B-ALL. To overcome this obstacle we engineered immune-deficient NOD/SCID IL-2Rγ null (NSG) mice to express human TSLP (hTSLP+ mice) as well as control mice that lack the TSLP cytokine (hTSLP– mice). ELISA assays show serum hTSLP levels in the hTSLP+ mice that approximate the normal range in human serum. We used this hTSLP+/- xenograft model system to study the in vivo effects of TSLP on mice transplanted with Ph-like B-ALL. First, we used the hTSLP+/– xenograft model system to evaluate the in vivo effects of TSLP on survival and proliferation of transplanted CRLF2 B-ALL cells harboring a JAK defect (MUTZ5 cell line). Mice were euthanized at 5 weeks and BM was harvested. Evaluation of BM disease by flow cytometry showed that the percentage of viable human leukemia cells in hTSLP+ mice was twice that observed in hTSLP– mice. Evaluation of cell cycle progression in human CRLF2 B-ALL cells isolated from xenograft BM showed that the percentage of cycling cells in hTSLP+ mice was 2.5 fold higher than in hTSLP– mice. When primary Ph-like ALL cells were transplanted to produce hTSLP+/– xenografts, the viable pre-B ALL cells present in the BM of hTSLP+ mice showed higher expression levels of the TSLPR components (CRLF2 and IL-7Rα) than those in the hTSLP- mice. These data provide evidence that the TSLP produced in this model is active and that it impacts primary pre-B ALL cells. Preliminary data obtained from this model suggests that TSLP provides a signal that promotes in vivo survival of CRLF2 B-ALL cells and that it may play a role in selection of leukemia clones during in vivo leukemogenesis. Microarray analysis comparing gene expression in primary CRLF2 B-ALL cells isolated from hTSLP+ and hTSLP– xenograft mice identified 565 that genes are differentially regulated (&gt; 2 fold up or downregulated; p&lt;.05). Ingenuity Pathway Analysis is currently underway to identify the signaling pathways that are regulated by hTSLP in CRLF2 B-ALL in vivo in the hTSLP+/– xenograft model. The identification of genes downstream of TSLP-CRLF2 signaling has the potential of providing drug targets for combination therapy to effectively treat Ph-like B-ALL. The hTSLP+/– xenograft model will provide an important tool for evaluating the in vivo efficacy of these and other drugs to treat CRLF2 B-ALL.

Citation Format: Olivia Francis, Ruijun Su, Shannalee Martinez, Ineavely Baez, Terry-Ann Milford, Terrence Bennett, Ross Fisher, Christopher L. Morris, Sinisa Dovat, Kimberly J. Payne. A human-mouse xenograft model to evaluate therapies and study the role of TSLP-induced signals in Ph-like ALL. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr B21.

Abstract 409: Regulation of chromatin remodeling in leukemia by Ikzf1 and Casein Kinase II

Ikzf1 (Ikaros) encodes a zinc finger protein that binds DNA and regulates gene expression via chromatin remodeling. The loss of Ikaros activity due to genetic or functional inactivation results in leukemia with a poor prognosis. The goal of our project is to determine the mechanism by which Ikaros regulates chromatin remodeling in human leukemia. Previous studies showed that Ikaros function in leukemia is controlled through its direct phosphorylation by Casein Kinase II (CK2). Treatment of leukemia cells with CK2 inhibitors results in enhanced Ikaros activity, which leads to cessation of cell growth. We have studied the mechanism by which inhibition of CK2 regulates Ikaros-induced epigenetic changes in leukemia. The human Nalm6 pre-B cell leukemia was treated with CK2 inhibitor, and the epigenetic signature of the histone modifications H3K9ac and H3K9me3 were determined using chromatin immunoprecipitation coupled with next generation sequencing (ChIP-SEQ). H3K9ac histone modification is associated with positive regulation of gene expression, while H3K9me3 is associated with the formation of heterochromatin and repression. The enrichment of particular histone modifications was confirmed by quantitative chromatin immunoprecipitation (qChIP). The results demonstrated that the inhibition of CK2 activity in leukemia results in a marked alteration in the epigenetic signature of both H3K9ac and H3K9me3 compared to untreated cells. This is associated with altered Ikaros binding to its target genes following CK2 inhibition. Current bioinformatics analysis is directed toward establishing a link between epigenetic modifications and Ikaros binding in leukemia. These results suggest that CK2 and Ikaros regulate gene transcription via epigenetic modifications and chromatin remodeling in leukemia.

Citation Format: Jonathon L. Payne, Carlos M. Casiano, Kimberly J. Payne, Justin Sloane, Elanora Dovat, Chunhua Song, Sinisa Dovat. Regulation of chromatin remodeling in leukemia by Ikzf1 and Casein Kinase II. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 409. doi:10.1158/1538-7445.AM2014-409

Abstract 3504: Regulation of cell cycle progression by Ikaros in leukemia

Control of cell cycle progression is achieved by the coordinated function of a large set of genes that are highly conserved in eukaryotic organisms. Malignant cells have impaired regulation of cell cycle progression which results in uncontrolled cellular proliferation. Thus, understanding the regulation of cell cycle progression in malignant cells is essential to advance our knowledge of the process of malignant transformation and for designing novel treatments. Ikaros is a zinc finger protein that acts as a tumor suppressor in leukemia. The loss of Ikaros activity due to deletion or mutation has been associated with the development of high-risk B-cell acute lymphoblastic leukemia (B-ALL), as well as with T-cell ALL and acute myelogenous leukemia (AML). Ikaros binds DNA and regulates transcription of its target genes via chromatin remodeling. The mechanism of Ikaros tumor suppressor activity is largely unknown. Here, we present evidence that Ikaros regulates cell cycle progression in leukemia. Using quantitative Chromatin Immunoprecipitation assay (qChIP), we demonstrate that Ikaros binds in vivo to promoter regions of several genes that regulate cell cycle progression in B-ALL cell lines and in primary cells from patients with B-ALL. To study how Ikaros regulates transcription of these genes, luciferase reporter assays were performed. The promoter regions of three Ikaros target genes were cloned into luciferase reporter constructs. Each of these constructs has been co-transfected with Ikaros or an empty vector (as a negative control) into HEK 293T cells. Results showed that Ikaros represses transcription of the three genes that promote cell cycle progression. Overexpression of Ikaros in leukemia cells by retroviral transduction results in reduced transcription of the cell cycle promoting genes, as evidenced by quantitative real-time PCR (qRT-PCR), as well as cell cycle arrest. These data suggest that Ikaros regulates cell cycle progression in leukemia by direct repression of the transcription of the genes that promote cell cycle progression, and identifies one mechanism of Ikaros function as a tumor suppressor in leukemia.

Citation Format: Elanora Dovat, Jonathon Payne, Carlos M. Casiano, Justin Sloane, Chandrika Gowda, Kimberly J. Payne, Sinisa Dovat, Chunhua Song. Regulation of cell cycle progression by Ikaros in leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3504. doi:10.1158/1538-7445.AM2014-3504