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 >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<.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 (> 2 fold up or downregulated; p<.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 (> 2 fold up or downregulated; p<.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