[Leukemia research in Germany: the Competence Network Acute and Chronic Leukemias]

The Competence Network "Acute and Chronic Leukemias" was founded in 1997 by the consolidation of the leading leukemia study groups in Germany. Key results are the development of new trials and cooperative studies, the setup of patient registries and biobanking facilities, as well as the improvement of study infrastructure. In 2003, the concept of the competence network contributed to the foundation of the European LeukemiaNet (ELN). Synergy with the ELN resulted in cooperation on a European and international level, standardization of diagnostics and treatment, and recommendations for each leukemia and interdisciplinary specialty. The ultimate goal of the network is the cure of leukemia through cooperative research.

A 4-gene expression score associated with high levels of Wilms Tumor-1 (WT1) expression is an adverse prognostic factor in acute myeloid leukaemia

Wilms Tumor-1 (WT1) expression level is implicated in the prognosis of acute myeloid leukaemia (AML). We hypothesized that a gene expression profile associated with WT1 expression levels might be a good surrogate marker. We identified high WT1 gene sets by comparing the gene expression profiles in the highest and lowest quartiles of WT1 expression in two large AML studies. Two high WT1 gene sets were found to be highly correlated in terms of the altered genes and expression profiles. We identified a 17-probe set signature of the high WT1 set as the optimal prognostic predictor in the first AML set, and showed that it was able to predict prognosis in the second AML series after adjustment for European LeukaemiaNet genetic groups. The gene signature also proved to be of prognostic value in a third AML series of 163 samples assessed by RNA sequencing, demonstrating its cross-platform consistency. This led us to derive a 4-gene expression score, which faithfully predicted adverse outcome. In conclusion, a short gene signature associated with high WT1 expression levels and the resultant 4-gene expression score were found to be predictive of adverse prognosis in AML. This study provides new clues to the molecular pathways underlying high WT1 states in leukaemia.

Abstract B146: Next-generation dendritic cell vaccination in postremission therapy of AML: Results of a clinical phase I trial

Postremission therapy for acute myeloid leukemia (AML) is critical for elimination of minimal residual disease (MRD). In patients not eligible for allogeneic stem cell transplantation, alternative treatment options are needed. Therapeutic vaccination with autologous dendritic cells (DCs) loaded with leukemia-associated antigens (LAAs) is a promising treatment strategy to induce anti-leukemic immune responses and to eradicate chemorefractory cells. We have developed a GMP-compliant 3-day protocol including a TLR7/8 agonist to differentiate monocytes of intensively pretreated AML patients into next-generation DCs.

A phase I proof-of-concept study (n=6) was completed using next-generation DCs as postremission therapy of AML patients with a non-favorable genetic risk profile in CR after intensive induction therapy (NCT01734304), and we have already started enrollment into a phase II trial building on these results. DCs are loaded with ivt-RNA encoding the LAAs WT1 and PRAME as well as CMVpp65 as adjuvant and surrogate antigen. Patients are vaccinated intradermally with 5x106 DCs of each antigen species up to 10 times within 26 weeks. The primary endpoint of the phase I trial is feasibility and safety of the vaccination. Secondary endpoints are immunological responses and disease control.

In total, 10 patients have been enrolled into the phase I/II trial. With two screening failures, DCs of sufficient number and quality were generated from leukapheresis in 7/8 cases. DC analysis demonstrated a positive costimulatory profile, secretion of IL-12p70, migration towards a chemokine gradient, antigen processing and presentation and activation of specific T cells in vitro. 5 patients have completed the vaccination schedule; the 6th patient has received 4/10 vaccinations. We observed delayed-type hypersensitivity (DTH) responses at the vaccination site in 5/5 patients, accompanied by slight erythema and indurations at the injection site, but no grade III/IV toxicities. Multimer analysis revealed the induction of antigen-specific T cell responses in 3/3 patients tested. We detected an increase of WT1-specific T cells in one patient and strong inductions of CMVpp65-specific T cells in two CMV-seronegative patients. TCR repertoire analysis by next-generation sequencing revealed an enrichment of particular clonotypes at DTH sites. In an individual treatment attempt, we treated one patient with impending relapse with a combination of DC vaccination and 5-azacytidine, resulting in MRD conversion.

From the results of our phase I trial, we conclude that vaccination with next-generation DCs in AML is feasible and safe and induces leukemia-specific immune responses in vivo. The protocol is continued to be studied in an ongoing phase II trial.

Citation Format: Felix S. Lichtenegger, Frauke M. Schnorfeil, Thomas Köhnke, Torben Altmann, Veit Bücklein, Andreas Moosmann, Monika Brüggemann, Beate Wagner, Wolfgang Hiddemann, Iris Bigalke, Gunnar Kvalheim, Marion Subklewe. Next-generation dendritic cell vaccination in postremission therapy of AML: Results of a clinical phase I trial. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B146.

Abstract B070: Characterization of covariables modulating CD33/CD3 BITE® antibody construct mediated cytotoxicity against primary AML cells

Antibody-based immunotherapy represents a promising strategy to target and eliminate chemoresistant leukemic cells in acute myeloid leukemia (AML). In our previous work, we were able to show effective elimination of primary AML cells by CD33/CD3 BiTE® antibody construct (AMG 330)-activated and -expanded residual autologous T cells. The goal of the present study was to characterize parameters that interfere with AMG 330-mediated lysis of AML cells. A long-term culture system supporting the growth of primary AML cells for up to 36 days ex vivo was used to determine covariables of AMG 330 cytolytic activity. One dominant variable of cytotoxicity was the expression level of the target antigen CD33 on primary AML cells. After 4 days of culture, a significantly higher lysis of target cells with CD33BRIGHT surface expression levels (n=38, p=0.01) was detected. However, after 12–15 days of culture, we could demonstrate similar lysis between samples with CD33DIM and CD33BRIGHT surface expression level, indicating that AMG 330 is also active at low antigen density (n=38, p=0.7). The CD33 expression level might not only be relevant in inter-patient comparison, but also in intra-patient comparison of AML subpopulations. In particular for CD34+/CD38- leukemia initiating cells (LICs), we have shown a significantly lower expression level of CD33 compared to AML bulk cells from the same donor {Krupka et al, Blood 2014}. To test if AMG 330 can also eliminate LICs, we performed in vivo engraftment experiments in NOD/SCID gamma null (NSG) mice. Patient-derived AML cells were co-cultured with healthy donor T-cells and either AMG 330 or control BiTE® antibody construct for 7 days. Residual CD3- cells were injected into NSG mice and monitored for AML outgrowth. Mice from the control group rapidly developed leukemia, and had to be sacrificed starting from 47 days post injection. In contrast, residual cells from AMG 330 treated cultures did not initiate leukemia in NSG mice in the entire observation period (6/6 mice, p=0.0003; n=2).

Another covariable in response to treatment with AMG 330 might be the induction of immune escape mechanisms due to continuous T-cell activation. The latter induces a proinflammatory environment, favoring the induction of immune checkpoint molecules on cancer cells. We therefore tested the relevance of immune checkpoint molecules in AMG 330 mediated lysis of primary AML cells. Although not constitutively expressed on primary AML cells (n=123), PD-L1 was strongly upregulated upon the addition of AMG 330 to ex vivo cytotoxicity experiments (p<0.0001; n=27). This phenomenon was cytokine-mediated as the sole addition of IFN-γ and TNF-α also induced expression (n=6). Furthermore, we observed a significant upregulation of PD-1 on activated T cells (p=0.0002, n=18).

In ex vivo cytotoxicity experiments blocking of the PD-1/PD-L1 interaction significantly enhanced AMG 330 mediated lysis efficacy (n=9, p=0.03) which was accompanied by a significant increase in T cell proliferation (n=9, p=0.01) and a markedly increase in IFN-γ secretion (n=8, p=0.008).

In summary we demonstrate that CD33DIM cells are lysed with an equal efficacy compared to CD33BRIGHT cells through a prolongation in culture time. The data suggest that the relevance of the CD33 expression level can be compensated by an increased duration of AMG 330 exposure.

Our work further provides evidence that cytokine mediated PD-L1 upregulation is a relevant mechanism of primary AML cells to escape T cell driven immune responses. We demonstrate that AMG 330 mediated cytotoxicity is enhanced by blockade of inhibitory receptors on AML cells. Our results support the use of combinatorial approaches of BiTE® antibody constructs with immune checkpoint blockade.

Citation Format: Christina Krupka, Peter Kufer, Roman Kischel, Gerhard Zugmaier, Thomas Köhnke, Felix Lichtenegger, Torben Altmann, Karsten Spiekermann, Binje Vick, Irmela Jeremias, Wolfgang Hiddemann, Gert Riethmüller, Marion Subklewe. Characterization of covariables modulating CD33/CD3 BITE® antibody construct mediated cytotoxicity against primary AML cells. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B070.

Abstract B002: Automated target antigen characterization on residual leukemic cells by flow cytometry

Introduction: In acute myeloid leukemia (AML), the prognostic relevance of minimal residual disease (MRD) detection has increased within the past decade and currently, clinical trials are evaluating the value of MRD-guided therapy. Immunotherapy represents an exciting prospect to eliminate residual chemorefractory leukemic cells. Development and implementation of these therapies rely on effective and highly sensitive methods to detect and characterize target antigen expression on residual cells.

Methods: To test the feasibility of the recently proposed viSNE (visualization of t-distributed stochastic neighbor embedding) algorithm to detect and characterize rare, aberrant populations we analyzed patient samples acquired within 3 months prior to hematological relapse, which were MRD-negative by traditional, manual gating. 400.000 events were acquired using an 8 color, 10 parameter panel on a Navios® flow cytometer (Beckman Coulter, Brea, CA, USA). For the bioinformatic analysis, the patient's sample and a healthy donor sample measured on the same machine were combined digitally and viSNE clustering was applied to the resulting dataset. As previously suggested, MRD positivity was defined as the presence of a distinct cluster of >100 cells which consisted of >90% patient cells. Then, expression intensity of the target antigens CD33 and CD123 was compared to primary diagnosis and relapse.

Results: From our database, we identified 12 patients with comprehensive MRD flow assessment available within 3 months prior to hematological relapse who were in CR by cytomorphology at the time of MRD assessment. 9 of these patients were MRD-positive by traditional LAIP-gating (>0.1%). This first analysis therefor focused on the remaining 3 patients with MRD-levels at 0.01%, 0.03% and 0.07% at 21, 42 and 37 days prior to hematological relapse. Using viSNE, distinct clusters of 5058, 225 and 1043 events consisting of 95.5%, 92.6% and 93.8% patient cells were identified, consistent with MRD positivity as defined above. In all three cases, the cells identified by the viSNE algorithm showed the immunophenotype seen at relapse, while there were immunophenotypical shifts compared to primary diagnosis, possibly explaining false-negative MRD-assessment by traditional gating. Specifically, there was a decrease in CD34-expression in one case, and an increase in CD33-expression in 2 cases. Concerning the target antigens CD33 and CD123, both antigens were present on residual cells, showing stable (CD123) expression or an increased (CD33) expression compared to primary diagnosis. Target antigen expression was identical compared to the immunophenotype seen at relapse.

Conclusion: viSNE clustering detected minor, aberrant populations in 3 patients with subsequent hematological relapse, which were not detected by traditional LAIP-gating strategy. viSNE might therefor aid in the analysis of high-dimensional flow cytometry data to detect small, aberrant populations. Target antigen expression could then be determined on these residual cells indicating feasibility of this approach to characterize these rare events for the development of personalized immunotherapeutic strategies as well as response assessment during such therapies. Further validation is ongoing.

Citation Format: Thomas Köhnke, Sandra Rechkemmer, Veit Leonhard Bücklein, Wolfgang Hiddemann, Marion Subklewe. Automated target antigen characterization on residual leukemic cells by flow cytometry. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B002.

RNA and protein expression of herpesvirus entry mediator (HVEM) is associated with molecular markers, immunity-related pathways and relapse-free survival of patients with AML

Immune checkpoint molecules are highly relevant as potential prognostic markers and therapeutic targets in malignant diseases. HVEM belongs to the TNF receptor family and provides stimulatory as well as inhibitory signals depending on the ligand. Abnormal HVEM expression has been described in various malignancies, but the role in AML is unknown. Here we report extensive data on HVEM surface protein expression analyzed by flow cytometry on bone marrow leukemic cells of 169 AML patients at diagnosis. An independent cohort of 512 AML patients was analyzed for HVEM mRNA expression in bone marrow samples by Affymetrix microarrays. Consistently for both cohorts and methods, we show that HVEM was differentially expressed and that expression levels were associated with defined genetic markers. HVEM expression was lower in cases with FLT3-ITD (p = 0.001, p < 0.001), with mutations in NPM1 (p = 0.001, p < 0.001) or with the combination of NPM1 mutation and FLT3 wild type (p = 0.049, p = 0.050), while a biallelic mutation in CEBPA correlated positively with higher HVEM expression (p = 0.015, p < 0.001). In a differential gene expression analysis, we found 13 genes including HOXA9, MEIS1 and MN1 that were closely associated with HVEM expression. Besides, four gene sets closely linked to immunity were enriched in HVEM (high) samples. Finally, high expression of HVEM was associated with a trend toward longer relapse-free survival. The results of this study provide new information on the potential significance of HVEM in AML.

Similar clinical features in follicular lymphomas with and without breaks in the BCL2 locus

Approximately 15% of follicular lymphomas (FLs) lack breaks in the BCL2 locus. The aim of this study was to better define molecular and clinical features of BCL2-breakpoint/t(14;18)-negative FLs. We studied the presence of BCL2, BCL6 and MYC breaks by fluorescence in situ hybridization and the expression of BCL2, MUM1, CD10, P53 and Ki67 in large clinical trial cohorts of 540 advanced-stage FL cases and 116 early-stage disease FL patients treated with chemotherapy regimens and radiation, respectively. A total of 86% and 53% of advanced- and early-stage FLs were BCL2-breakpoint-positive, respectively. BCL2 was expressed in almost all FLs with BCL2 break and also in 86% and 69% of BCL2-breakpoint-negative advanced- and early-stage FLs, respectively. CD10 expression was significantly reduced in BCL2-breakpoint-negative FLs of all stages and MUM1 and Ki67 expression were significantly increased in BCL2-break-negative early-stage FLs. Patient characteristics did not differ between FLs with and without BCL2 breaks and neither did survival times in advanced-stage FLs. These results suggest that the molecular profile differs to some extent between FLs with and without BCL2 breaks and support the notion that FLs with and without BCL2 breaks belong to the same lymphoma entity.

Preclinical efficacy of maternal embryonic leucine-zipper kinase (MELK) inhibition in acute myeloid leukemia

Maternal embryonic leucine-zipper kinase (MELK), which was reported to be frequently up-regulated in various types of solid cancer, plays critical roles in formation and maintenance of cancer stem cells. However, little is known about the relevance of this kinase in hematologic malignancies. Here we report characterization of possible roles of MELK in acute myeloid leukemia (AML). MELK is expressed in AML cell lines and AML blasts with higher levels in less differentiated cells. MELK is frequently upregulated in AML with complex karyotypes and is associated with worse clinical outcome. MELK knockdown resulted in growth inhibition and apoptosis of leukemic cells. Hence, we investigated the potent anti-leukemia activity of OTS167, a small molecule MELK kinase inhibitor, in AML, and found that the compound induced cell differentiation and apoptosis as well as decreased migration of AML cells. MELK expression was positively correlated with the expression of FOXM1 as well as its downstream target genes. Furthermore, MELK inhibition resulted in downregulation of FOXM1 activity and the expression of its downstream targets. Taken together, and given that OTS167 is undergoing a phase I clinical trial in solid cancer, our study warrants clinical evaluation of this compound as a novel targeted therapy for AML patients.

Integration of gene mutations in risk prognostication for patients receiving first-line immunochemotherapy for follicular lymphoma: a retrospective analysis of a prospective clinical trial and validation in a population-based registry

BACKGROUND

Follicular lymphoma is a clinically and genetically heterogeneous disease, but the prognostic value of somatic mutations has not been systematically assessed. We aimed to improve risk stratification of patients receiving first-line immunochemotherapy by integrating gene mutations into a prognostic model.

METHODS

We did DNA deep sequencing to retrospectively analyse the mutation status of 74 genes in 151 follicular lymphoma biopsy specimens that were obtained from patients within 1 year before beginning immunochemotherapy consisting of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP). These patients were recruited between May 4, 2000, and Oct 20, 2010, as part of a phase 3 trial (GLSG2000). Eligible patients had symptomatic, advanced stage follicular lymphoma and were previously untreated. The primary endpoints were failure-free survival (defined as less than a partial remission at the end of induction, relapse, progression, or death) and overall survival calculated from date of treatment initiation. Median follow-up was 7·7 years (IQR 5·5-9·3). Mutations and clinical factors were incorporated into a risk model for failure-free survival using multivariable L1-penalised Cox regression. We validated the risk model in an independent population-based cohort of 107 patients with symptomatic follicular lymphoma considered ineligible for curative irradiation. Pretreatment biopsies were taken between Feb 24, 2004, and Nov 24, 2009, within 1 year before beginning first-line immunochemotherapy consisting of rituximab, cyclophosphamide, vincristine, and prednisone (R-CVP). Median follow-up was 6·7 years (IQR 5·7-7·6).

FINDINGS

We established a clinicogenetic risk model (termed m7-FLIPI) that included the mutation status of seven genes (EZH2, ARID1A, MEF2B, EP300, FOXO1, CREBBP, and CARD11), the Follicular Lymphoma International Prognostic Index (FLIPI), and Eastern Cooperative Oncology Group (ECOG) performance status. In the training cohort, m7-FLIPI defined a high-risk group (28%, 43/151) with 5-year failure-free survival of 38·29% (95% CI 25·31-57·95) versus 77·21% (95% CI 69·21-86·14) for the low-risk group (hazard ratio [HR] 4·14, 95% CI 2·47-6·93; p<0·0001; bootstrap-corrected HR 2·02), and outperformed a prognostic model of only gene mutations (HR 3·76, 95% CI 2·10-6·74; p<0·0001; bootstrap-corrected HR 1·57). The positive predictive value and negative predictive value for 5-year failure-free survival were 64% and 78%, respectively, with a C-index of 0·80 (95% CI 0·71-0·89). In the validation cohort, m7-FLIPI again defined a high-risk group (22%, 24/107) with 5-year failure-free survival of 25·00% (95% CI 12·50-49·99) versus 68·24% (58·84-79·15) in the low-risk group (HR 3·58, 95% CI 2·00-6·42; p<0.0001). The positive predictive value for 5-year failure-free survival was 72% and 68% for negative predictive value, with a C-index of 0·79 (95% CI 0·69-0·89). In the validation cohort, risk stratification by m7-FLIPI outperformed FLIPI alone (HR 2·18, 95% CI 1·21-3·92), and FLIPI combined with ECOG performance status (HR 2·03, 95% CI 1·12-3·67).

INTERPRETATION

Integration of the mutational status of seven genes with clinical risk factors improves prognostication for patients with follicular lymphoma receiving first-line immunochemotherapy and is a promising approach to identify the subset at highest risk of treatment failure.

FUNDING

Deutsche Krebshilfe, Terry Fox Research Institute.

Blockade of the PD-1/PD-L1 axis augments lysis of AML cells by the CD33/CD3 BiTE antibody construct AMG 330: reversing a T-cell-induced immune escape mechanism

Bispecific T-cell engagers (BiTEs) are very effective in recruiting and activating T cells. We tested the cytotoxicity of the CD33/CD3 BiTE antibody construct AMG 330 on primary acute myeloid leukemia (AML) cells ex vivo and characterized parameters contributing to antileukemic cytolytic activity. The E:T ratio and the CD33 expression level significantly influenced lysis kinetics in long-term cultures of primary AML cells (n=38). AMG 330 induced T-cell-mediated proinflammatory conditions, favoring the upregulation of immune checkpoints on target and effector cells. Although not constitutively expressed at the time of primary diagnosis (n=123), PD-L1 was strongly upregulated on primary AML cells upon AMG 330 addition to ex vivo cultures (n=27, P<0.0001). This phenomenon was cytokine-driven as the sole addition of interferon (IFN)-γ and tumor necrosis factor-α also induced expression. Through blockade of the PD-1/PD-L1 interaction, AMG 330-mediated lysis (n=9, P=0.03), T-cell proliferation (n=9, P=0.01) and IFN-γ secretion (n=8, P=0.008) were significantly enhanced. The combinatorial approach was most beneficial in settings of protracted AML cell lysis. Taken together, we have characterized a critical resistance mechanism employed by primary AML cells under AMG 330-mediated proinflammatory conditions. Our results support the evaluation of checkpoint molecules in upcoming clinical trials with AMG 330 to enhance BiTE antibody construct-mediated cytotoxicity.

Temsirolimus inhibits cell growth in combination with inhibitors of the B-cell receptor pathway

Lately, mTOR inhibitors have gained clinical relevance in malignant lymphoma. Still, rapamycin derivatives may activate a pro-survival feedback loop through PI3K-Akt. In this current study, temsirolimus effectively reduced cell growth in GCB and ABC diffuse large cell B-cell lymphoma (GCB=30-66%, ABC=45-57%). Combination treatment with the PI3K-δ inhibitor idelalisib additively effected ABC and GCB lymphoma (GCB=16-38%, ABC=25-50%). Since Bruton's Tyrosine Kinase (BTK) plays a significant role for the survival of ABC lymphoma, this study also combined the BTK inhibitor ibrutinib with temsirolimus, which resulted in additive cell growth reduction (ibrutinib 50%, temsirolimus 44%, combination 25%) in ABC lymphoma. In contrast, bortezomib, which has been shown previously to be efficient in ABC lymphoma, revealed an antagonistic effect with temsirolimus in some GCB lymphoma (temsirolimus 53%, temsirolimus+bortezomib 63%). Western blot analysis identified the increase of phosphorylated pro-survival kinases Akt and PDK as a possible underlying mechanism of this interaction.

T cells are functionally not impaired in AML: increased PD-1 expression is only seen at time of relapse and correlates with a shift towards the memory T cell compartment

Background

T cell function is crucial for the success of several novel immunotherapeutic strategies for the treatment of acute myeloid leukemia (AML). However, changes in phenotype and function of T cells have been described in various hematologic malignancies, mimicking T cell exhaustion known from chronic viral infections. Detailed knowledge about phenotype and function of T cells in AML patients at different stages of the disease is indispensable for optimal development and application of immunotherapeutic strategies for this disease.

Methods

We used flow cytometry-based assays to characterize T cell phenotype and function in peripheral blood and bone marrow of AML patients at diagnosis, at relapse after intensive chemotherapy, and at relapse after allogeneic stem cell transplantation (SCT). Surface expression of CD244, PD-1, CD160, and TIM-3 was determined, and proliferation and production of IFN-γ, TNF-α, and IL-2 were measured.

Results

We detected similar expression of inhibitory molecules on T cells from patients at diagnosis and from age-matched healthy controls. At relapse after SCT, however, PD-1 expression was significantly increased compared to diagnosis, both on CD4⁺ and CD8⁺ T cells. This pattern was not associated with age and cytomegalovirus (CMV) status but with a shift towards effector memory cells in relapsed AML patients. Proliferation and cytokine production assays did not reveal functional defects in T cells of AML patients, neither at diagnosis nor at relapse.

Conclusion

We thus conclude that T cell exhaustion does not play a major role in AML. Immunotherapeutic strategies targeting autologous T cells thus have particularly good prospects in the setting of AML.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0189-2) contains supplementary material, which is available to authorized users.

Identification of the Adapter Molecule MTSS1 as a Potential Oncogene-Specific Tumor Suppressor in Acute Myeloid Leukemia

The adapter protein metastasis suppressor 1 (MTSS1) is implicated as a tumor suppressor or tumor promoter, depending on the type of solid cancer. Here, we identified Mtss1 expression to be increased in AML subsets with favorable outcome, while suppressed in high risk AML patients. High expression of MTSS1 predicted better clinical outcome of patients with normal-karyotype AML. Mechanistically, MTSS1 expression was negatively regulated by FLT3-ITD signaling but enhanced by the AML1-ETO fusion protein. DNMT3B, a negative regulator of MTSS1, showed strong binding to the MTSS1 promoter in PML-RARA positive but not AML1-ETO positive cells, suggesting that AML1-ETO leads to derepression of MTSS1. Pharmacological treatment of AML cell lines carrying the FLT3-ITD mutation with the specific FLT3 inhibitor PKC-412 caused upregulation of MTSS1. Moreover, treatment of acute promyelocytic cells (APL) with all-trans retinoic acid (ATRA) increased MTSS1 mRNA levels. Taken together, our findings suggest that MTSS1 might have a context-dependent function and could act as a tumor suppressor, which is pharmacologically targetable in AML patients.

RUNX1/ETO blocks selectin-mediated adhesion via epigenetic silencing of PSGL-1

RUNX1/ETO (RE), the t(8;21)-derived leukemic transcription factor associated with acute myeloid leukemia (AML) development, deregulates genes involved in differentiation, self-renewal and proliferation. In addition, these cells show differences in cellular adhesion behavior whose molecular basis is not well understood. Here, we demonstrate that RE epigenetically silences the gene encoding P-Selectin Glycoprotein Ligand-1 (PSGL-1) and downregulates PSGL-1 expression in human CD34+ and murine lin− hematopoietic progenitor cells. Levels of PSGL-1 inversely and dose-dependently correlate with RE oncogene levels. However, a DNA-binding defective mutant fails to downregulate PSGL-1. We show by ChIP experiments that the PSGL-1 promoter is a direct target of RE and binding is accompanied by high levels of the repressive chromatin mark histone H3K27me3. In t(8;21)+ Kasumi-1 cells, PSGL-1 expression is completely restored at both the mRNA and cell surface protein levels following RE downregulation with short hairpin RNA (shRNA) or RE inhibition with tetramerization-blocking peptides, and at the promoter H3K27me3 is replaced by the activating chromatin mark H3K9ac as well as by RNA polymerase II. Upregulation of PSGL-1 restores the binding of cells to P- and E-selectin and re-establishes myeloid-specific cellular adhesion while it fails to bind to lymphocyte-specific L-selectin. Overall, our data suggest that the RE oncoprotein epigenetically represses PSGL-1 via binding to its promoter region and thus affects the adhesive behavior of t(8;21)+ AML cells.

An advanced preclinical mouse model for acute myeloid leukemia using patients' cells of various genetic subgroups and in vivo bioluminescence imaging

Acute myeloid leukemia (AML) is a clinically and molecularly heterogeneous disease with poor outcome. Adequate model systems are required for preclinical studies to improve understanding of AML biology and to develop novel, rational treatment approaches. Xenografts in immunodeficient mice allow performing functional studies on patient-derived AML cells. We have established an improved model system that integrates serial retransplantation of patient-derived xenograft (PDX) cells in mice, genetic manipulation by lentiviral transduction, and essential quality controls by immunophenotyping and targeted resequencing of driver genes. 17/29 samples showed primary engraftment, 10/17 samples could be retransplanted and some of them allowed virtually indefinite serial transplantation. 5/6 samples were successfully transduced using lentiviruses. Neither serial transplantation nor genetic engineering markedly altered sample characteristics analyzed. Transgene expression was stable in PDX AML cells. Example given, recombinant luciferase enabled bioluminescence in vivo imaging and highly sensitive and reliable disease monitoring; imaging visualized minimal disease at 1 PDX cell in 10000 mouse bone marrow cells and facilitated quantifying leukemia initiating cells. We conclude that serial expansion, genetic engineering and imaging represent valuable tools to improve the individualized xenograft mouse model of AML. Prospectively, these advancements enable repetitive, clinically relevant studies on AML biology and preclinical treatment trials on genetically defined and heterogeneous subgroups.