Abstract: Mutations of CCAAT/enhancer–binding protein alpha (CEBPAmu) are found in 10% to 15% of de novo acute myeloid leukemia (AML) cases. Double-mutated CEBPA (CEBPAdm) is associated with a favorable prognosis; however, single-mutated CEBPA (CEBPAsm) does not seem to improve prognosis. We investigated CEBPAmu for prognosis in 1028 patients with AML, registered in the Multi-center Collaborative Program for Gene Sequencing of Japanese AML. It was found that CEBPAmu in the basic leucine zipper domain (bZIP) was strongly associated with a favorable prognosis, but CEBPAmu out of the bZIP domain was not. The presence of CEBPAmu in bZIP was a strong indicator of a higher chance of achieving complete remission (P & lt; .001), better overall survival (OS; P & lt; .001) and a lower risk of relapse (P & lt; .001). The prognostic significance of CEBPAmu in bZIP was also observed in the subgroup with CEBPAsm (all patients: OS, P = .008; the cumulative incidence of relapse, P = .063; patients aged ≤70 years and with intermediate-risk karyotype: OS, P = .008; cumulative incidence of relapse, P = .026). Multivariate analysis of 744 patients aged ≤70 years showed that CEBPAmu in bZIP was the most potent predictor of OS (hazard ratio, 0.3287; P & lt; .001). CEBPAdm was validated as a cofounding factor, which was overlapping with CEBPAmu in bZIP. In summary, these findings indicate that CEBPAmu in bZIP is a potent marker for AML prognosis. It holds potential in the refinement of treatment stratification and the development of targeted therapeutic approaches in CEBPA-mutated AML.

Abstract: Background Azacitidine (AZA) is one of the hypomethylating agents (HMAs) proven to significantly prolong overall survival (OS) for patients with higher-risk myelodysplastic syndromes (h-MDS) via a prospective phase 3 trial. However, the optimal treatment schedule of AZA for h-MDS has not yet been prospectively determined. As a 7-day treatment schedule includes weekends, a 5-day administration has also been applied with h-MDS, though solid evidence has not been established. Based on this background, we planned to compare AZA treatment on 7-day and 5-day schedules. Patients Patients diagnosed with de novo or treatment-related MDS (FAB-defined RAEB and RAEB-t), aged 16 years or older were eligible for this study, if they showed adequate performance status (ECOG PS 0-2) and no history of HMA treatment or chemotherapy. Candidates for allogeneic stem-cell transplantation were excluded. Study design This was a multicenter, randomized, open-label, phase 3 trial to compare the efficacy of AZA treatment for 7-days (AZA-7) to 5-days (AZA-5). The primary endpoint was 2-year OS rate (2y OS). AZA was given subcutaneously at 75 mg/m² every 28 days. To test the non-inferiority of AZA-5 to AZA-7, 2y OS of AZA-7 was estimated at 30%, and the delta was defined as 11%, for which 410 patients were needed to complete the trial. However, because of the poor recruitment, this study closed prematurely, and the protocol-planned interim analysis (when 200 patients were registered) was performed. This protocol was approved by the IRB at each trial center. All patients provided written informed consent, and the trial was conducted in accordance with the Declaration of Helsinki. The efficacy of AZA was evaluated using IWG2006 criteria, and treatment was continued until study completion, or relapse, unacceptable toxicity, or disease progression was observed. Efficacy was analyzed by intention-to-treat. Secondary endpoints were the hematological response rate, 2-year leukemia-free survival (2y LFS) rate, the cytogenetic response, and occurrence of adverse events. Results Between January 2013 and Jun 2018, 201 patients were randomly assigned to AZA-7 (n=99) or AZA-5 group (n=102). The median age of all patients at the enrollment was 73.5 years old (range, 48 to 91). Between the two groups, there was no significant difference in the baseline characteristics, such as sex, type of MDS (de novo or treatment-related), percent of BM blasts, FAB and WHO classification, and IPSS and IPSS-R risk. After excluding 5 and 6 patients from AZA-7 and AZA-5, respectively (poor data entry, change in diagnosis), at the time of last follow-up, 59 and 67 patients died in AZA-7 and AZA-5, respectively. The Kaplan-Meier estimates of the 2y OS and 2y LFS were 35.1% (95% CI, 23.8 to 46.7) and 27.3% (95% CI, 17.3 to 38.3) for AZA-7, and 22.4% (95% CI, 13.1 to 33.3) and 20.5% (95% CI, 11.6 to 31.2) for AZA-5, respectively. There was no significant difference in the frequency of erythroid, platelet or neutrophil improvement, or hematological response (complete or partial response, or any hematological improvement) between the two groups. The most common non-hematological adverse event of grade 3 to 4 (in ≥20% of the patients in either group) was febrile neutropenia (33.7% for AZA-7, and 30.5% for AZA-5). Conclusion and Discussion Because of the premature termination of this trial, statistical analysis for the primary endpoint (2y OS of 35.1% for AZA-7, and 22.4% for AZA-5) could not provide any solid evidence. However, considering the difference of more than 10% in 2y OS, and no major differences in the safety profiles, we favor AZA-7 to AZA-5 for MDS patients with RAEB and RAEB-t. Figure Disclosures Kiguchi: Tejin Co., Ltd.: Research Funding; Taiho Pharmaceutical Co., Ltd.: Research Funding; SymBio Pharmaceutical Co., Ltd.: Research Funding; Celgene Co., Ltd.: Research Funding; Janssen Pharmaceutical Co., Ltd.: Research Funding; Sumitomo Dainippon Pharmaceutical Co., Ltd.: Research Funding; Novartis Pharmaceutical Co., Ltd.: Research Funding; Nippon Shinyaku Co., Ltd.: Research Funding; Astellas Pharmaceutical Co., Ltd.: Research Funding; MSD CO., Ltd.: Research Funding; Kyowa Hakko Kirin Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Bristol-Myeres Squibb Co., Ltd.: Research Funding; Sanofi K.K., Ltd.: Research Funding; Celltrion, Inc.: Research Funding. Usuki:Daiichi Sankyo Co., Ltd.: Research Funding, Speakers Bureau; Astellas Pharma Inc: Research Funding, Speakers Bureau. Suzuki:kyowa Hakko Kirin: Consultancy, Honoraria; MSD k.k.: Research Funding; Nippon Shinyaku: Honoraria; Novartis: Honoraria; takeda: Research Funding; Incyte and Pfizer: Research Funding; astellas: Research Funding; Celgene: Honoraria; Chugai: Honoraria, Research Funding; daiichi sankyo: Research Funding; eisai: Research Funding. Tomita:Chugai Pharmaceutical Co., Ltd.: Honoraria, Research Funding; Kyowa Kirin: Research Funding; Taiho Pharma: Research Funding. Handa:Ono: Research Funding. Maeda:Janssen Pharmaceutical K.K..: Honoraria; Nippon Shinyaku Co., Ltd.: Honoraria. Iyama:Otsuka Pharmaceutical Co., Ltd.: Honoraria; Otsuka Pharmaceutical Factory: Honoraria; Astellas Pharma: Honoraria; Daiichi Sankyo: Honoraria; Allexion Pharma: Honoraria; CSL Behring: Honoraria. Yamauchi:Astellas, AbbVie: Consultancy; Pfizer, Chugai, Teijin, Solasia: Research Funding. Kiyoi:Daiichi Sankyo Co., Ltd: Research Funding; Bristol-Myers Squibb: Research Funding; Pfizer Japan Inc.: Honoraria; Nippon Shinyaku Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co.,Ltd.: Research Funding; Perseus Proteomics Inc.: Research Funding; FUJIFILM Corporation: Research Funding; Astellas Pharma Inc.: Honoraria, Research Funding; Zenyaku Kogyo Co., Ltd.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Research Funding; Sumitomo Dainippon Pharma Co., Ltd.: Research Funding; Eisai Co., Ltd.: Research Funding; Kyowa Hakko Kirin Co., Ltd.: Research Funding; Takeda Pharmaceutical Co., Ltd.: Research Funding. Miyazaki:Chugai: Research Funding; Otsuka: Honoraria; Novartis: Honoraria; Nippon-Shinyaku: Honoraria; Dainippon-Sumitomo: Honoraria; Kyowa-Kirin: Honoraria.

Abstract: Monocyte-derived fibrocytes recently garnered attention because the novel pathogenesis of myelofibrosis (MF), and suppression of fibrocyte differentiation by serum amyloid P remarkably improved MF. We previously revealed that human fibrocytes highly expressed signaling lymphocytic activation molecule F7 (SLAMF7) compared with macrophages and that SLAMF7high monocytes in the peripheral blood (PB) of MF patients were significantly elevated relative to those in healthy controls (HCs). In this study, we evaluated SLAMF7high monocyte percentage in the PB of HCs, myeloproliferative neoplasm (MPN) patients with MF, and MPN patients without MF by using a cross-sectional approach. We found that MPN patients with MF who harbored JAK2V617F had a significantly elevated SLAMF7high monocyte percentage, which correlated positively with the JAK2V617F allele burden. In addition, the serum concentration of interleukin-1ra (IL-1ra) was significantly correlated with the SLAMF7high monocyte percentage and JAK2V617F allele burden. These findings suggest that both SLAMF7high monocytes and IL-1ra could be useful noninvasive markers of MF onset. Furthermore, the JAK2V617F allele burden of SLAMF7high monocytes was significantly higher than that of SLAMF7low monocytes and could be a potential target of elotuzumab (Elo), an anti-SLAMF7 antibody used for treating multiple myeloma. Elo independently inhibited differentiation of fibrocytes derived not only from HCs but also from MF patients in vitro. Elo also ameliorated MF and splenomegaly induced by romiplostim administration in humanized NOG mice. In conclusion, an increase of SLAMF7high monocytes with higher JAK2V617F allele burden was associated with the onset of MF in MPN patients harboring JAK2V617F, and Elo could be a therapeutic agent for MPN patients with MF who harbor JAK2V617F.

Abstract: Myeloproliferative neoplasms (MPN) have an inherent tendency toward leukemic transformation, but its mechanisms remain largely unknown. Recently, TP53 mutation is reported to be frequently found in cases with post-MPN leukemia. Here, to address the contribution of p53 loss to leukemic transformation from MPN in vivo, we retrovirally transduced c-kit+ bone marrow (BM) cells from p53 knockout (p53-/-) and littermate mice (p53+/+) with either wild-type Jak2 (Jak2WT) or Jak2V617F respectively, and transplanted them into lethally irradiated mice. At 3 weeks after transplantation, both recipients of Jak2V617F/p53-/- and Jak2V617F/p53+/+ cells developed a polycythemia vera-like disease characterized by high WBC count and elevated hemoglobin (Hb) level. Jak2V617F/p53+/+ mice survived and continued to have elevated Hb level, whereas 5 weeks after transplantation, Jak2V617F/p53-/- recipients developed cachexia, and their Hb level declined. Eventually, these mice developed fatal leukemia with a median survival of 46.5 days after transplantation, suggesting loss of p53 cooperates with Jak2V617F mutation to promote leukemic transformation from MPN. To characterize these leukemias, we analyzed leukemic tissues from moribund Jak2V617F/p53-/- mice. Peripheral blood smears and BM specimen from Jak2V617F/p53-/- recipients showed a marked increase of erythroid precursors with dysplastic features, leading to suppression of normal hematopoiesis. Notably, Jak2V617F/p53-/- mice displayed marked hepatosplenomegaly and extensive pulmonary hemorrhage. Consistent with the histopathologic findings, Jak2V617F/p53-/- animals exhibited a remarkable accumulation of erythroid precursors (CD71+), and especially more immature progenitors (Ter119-/CD71+) in the BM and spleen, compared with Jak2V617F/p53+/+ animals. These data suggest Jak2V617F/p53-/- recipients developed infiltrative disease with accumulation of immature erythroid cells, fulfilling the Bethesda Criteria of erythroleukemia in mice. To assess the transplantability of Jak2V617F/p53-/- leukemia, we injected unfractionated BM cells from Jak2V617F/p53-/- mice into lethally irradiated mice. In all cases, lethal leukemia developed earlier than in primary recipients. Moreover, there was a significant increase in erythroid progenitors in secondary recipients, suggesting the erythroid component is the predominant lineage involved in this leukemia model. As Jak2V617F/p53-/- leukemic tissues contained three major populations: CD71+ erythroid progenitors, Mac1+ mature myeloid cells, and lineage-negative (CD71-/Mac1-) primitive leukemic cells, we purified and transplanted these subfractions into secondary recipients to evaluate their leukemia-initiating potential. As a result, both lineage-negative (CD71-/Mac1-) cells and CD71+ erythroid progenitors possessed leukemia- initiating capacity, but Mac1+ myeloid cells could not reconstitute the disease. In addition, these two fractions had different capacities to induce leukemias; recipients of CD71+ cells rapidly developed erythroleukemia, whereas lineage-negative cells caused lethal leukemia after the polycythemic state. Moreover, hematopoietic tissues in recipients transplanted with CD71+ cells mainly consisted of erythroid lineages, whereas lineage-negative cells produced both erythroid and myeloid lineages, suggesting lineage-negative cells are more immature than CD71+ erythroid precursors. Furthermore, subsequent fractionation of lineage-negative cells revealed leukemia-initiating cells were enriched in Lin-/Sca-1+/c-kit+ (LSK) cells. To further characterize two types of leukemia-initiating cells in Jak2V617F/p53-/- leukemia, we assessed their sensitivity to a JAK2 inhibitor, INCB18424, in vitro. Interestingly, INCB18424 treatment significantly reduced CD71+ cell proliferation, whereas LSK cells were able to expand in the presence of INCB18424, indicating different leukemia-initiating cells existing in post-MPN leukemia have different responsiveness to JAK2 inhibiton. In summary, these results demonstrate p53 loss is sufficient for inducing leukemic transformation in JAK2V617F-postive MPN and offers an in vivo model to assess novel therapeutic approaches for post-MPN leukemia. In addition, we revealed leukemia-initiating cells at different differentiation stages could exist in post-MPN leukemia. Disclosures: Kurokawa: Novartis: Consultancy, Research Funding; Bristol-Myers Squibb: Research Funding; Celgene: Consultancy, Research Funding.

Abstract: Azacitidine is a mainstay of therapy for myelodysplastic syndrome (MDS)–related diseases. The purpose of our study is to elucidate the effect of gene mutations on hematological response and overall survival (OS), particularly focusing on their posttreatment clone size. We enrolled a total of 449 patients with MDS or related myeloid neoplasms. They were analyzed for gene mutations in pretreatment (n = 449) and posttreatment (n = 289) bone marrow samples using targeted-capture sequencing to assess the impact of gene mutations and their posttreatment clone size on treatment outcomes. In Cox proportional hazard modeling, multihit TP53 mutation (hazard ratio [HR], 2.03; 95% confidence interval [CI] , 1.42-2.91; P  & lt; .001), EZH2 mutation (HR, 1.71; 95% CI, 1.14-2.54; P = .009), and DDX41 mutation (HR, 0.33; 95% CI, 0.17-0.62; P  & lt; .001), together with age, high-risk karyotypes, low platelets, and high blast counts, independently predicted OS. Posttreatment clone size accounting for all drivers significantly correlated with International Working Group (IWG) response (P  & lt; .001, using trend test), except for that of DDX41-mutated clones, which did not predict IWG response. Combined, IWG response and posttreatment clone size further improved the prediction of the original model and even that of a recently proposed molecular prediction model, the molecular International Prognostic Scoring System (IPSS-M; c-index, 0.653 vs 0.688; P  & lt; .001, using likelihood ratio test). In conclusion, evaluation of posttreatment clone size, together with the pretreatment mutational profile as well as the IWG response play a role in better prognostication of azacitidine-treated patients with myelodysplasia.

Abstract: Introduction Myelofibrosis (MF) occurrence can be attributed to various pathogenic mechanisms. A recent study showed that the neoplastic clone of fibrocytes (spindle shaped, fibroblast-like blood cells derived from monocyte lineage) was essential in primary MF pathogenesis; moreover, serum amyloid P (PRM-151), which suppresses fibrocyte differentiation, markedly improved survival and MF in a murine xenograft model (J Exp Med 2016; 213: 1723). Using a romiplostim-induced murine MF model, direct induction of fibrocyte differentiation using TPO receptor activation, leading to MF progression, was demonstrated (Leukemia 2017; 31: 2709). Using DNA microarray analysis, we previously showed that compared with macrophages, human fibrocytes highly express SLAMF7 (J Immunol 2015; 195: 4341). Elotuzumab (Elo) is an anti-SLAMF7 antibody recently developed for treating multiple myeloma. We showed that Elo inhibits human fibrocyte differentiation in vitro and relieves MF using the mouse model in vivo, identifying Elo as a potential therapeutic agent (manuscript in preparation). Additionally, monocytes with high SLAMF7 expression and negative CD16 expression were significantly elevated in the peripheral blood (PB) of MF patients compared with that of healthy controls (HCs). We hypothesized that this monocyte subset reflects fibrocyte activation and is the target of Elo. For the clinical study of Elo, we evaluated SLAMF7high CD16− monocyte percentage in PB of HCs, myeloproliferative neoplasm (MPN) patients with or without MF, and non-MPN patients with MF in a cross-sectional manner. Methods Six HCs, 12 non-MPN patients with MF, and 44 MPN patients (18 without and 26 with MF) were enrolled; their blood samples were collected. All MPN patients underwent bone marrow biopsy in 2016-2018 and were diagnosed by the 2016 WHO classification and diagnostic criteria. Non-MPN patients underwent bone marrow biopsy in 2015-2018, and MF was confirmed over MF-1 according to the European Consensus Criteria. SLAMF7high CD16- monocyte percentage in PB monocytes of HCs and patients was calculated using flow cytometry. Further, all patients were tested for JAK2V617F, CALR, or MPL genetic mutations. The allele burden of JAK2V617F was measured. Moreover, fibrocytes were differentiated from PB mononuclear cells of patients with genetic mutation and were verified for genetic mutation in their fibrocytes. Results Patients were classified in three groups (MPN without MF, MPN with MF, and non-MPN with MF). The median percentage of SLAMF7high CD16− monocytes in PB of HCs, MPN patients without MF, MPN patients with MF, and non-MPN patients with MF were 1.66%, 2.48%, 27.4%, and 19.8%, respectively. Compared with HCs and MPN patients without MF, SLAMF7high CD16− monocyte percentage of MPN and non-MPN patients with MF significantly increased (p 〈 0.05 and p 〈 0.01, respectively) (Figure A). MPN patients with MF harboring JAK2V617F (n = 17) had a significantly higher SLAMF7high CD16− monocyte percentage than those without MF harboring JAK2V617F (n = 8) (median: 43.70% vs. 7.00%, p 〈 0.001). While a similar trend was observed in patients not harboring JAK2V617F (median: 1.15% vs. 3.26%, p = 0.05), the contrast between patients with and without MF patients was significant for those with JAK2V617F (Figure B). More than 25% of SLAMF7high CD16− monocytes in PB indicates MF in MPN patients harboring JAK2V617F (sensitivity: 82.4%, specificity: 87.5%). In addition, the JAK2V617F allele burden of fibrocytes was strongly correlated with the SLAMF7high CD16− monocyte percentage in PB (p = 0.0003) (Figure C). In all patients, fibrocytes and PB harbored the same genetic mutation. Conclusion The percentage of SLAMF7high CD16− monocytes in PB elevated in MF patients regardless of MPN, whereas it was not elevated in MPN patients without MF. In MPN patients, those with MF harboring JAK2V617F presented a high SLAMF7high CD16-monocyte percentage in PB, positively correlating with the JAK2V617F allele burden of fibrocytes. In conclusion, SLAMF7high CD16- monocyte levels are closely related with MF and neoplastic clones of fibrocytes harboring JAK2V617F, indicating fibrocyte activation and representing a non-invasive marker of MF progression and a potential target for Elo treatment. Figure. Figure. Disclosures Usuki: Sumitomo Dainippon Pharma: Research Funding, Speakers Bureau; Celgene Corporation: Research Funding, Speakers Bureau; Daiichi Sankyo: Research Funding; Boehringer-Ingelheim Japan: Research Funding; Sanofi K.K.: Research Funding; Shire Japan: Research Funding; SymBio Pharmaceuticals Limited.: Research Funding; Chugai Pharmaceutical: Speakers Bureau; Takeda Pharmaceutical: Speakers Bureau; Ono Pharmaceutical: Speakers Bureau; Novartis: Speakers Bureau; Janssen Pharmaceutical K.K: Research Funding; Pfizer Japan: Research Funding, Speakers Bureau; GlaxoSmithKline K.K.: Research Funding; Kyowa Hakko Kirin Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Astellas Pharma Inc.: Research Funding; Nippon Shinyaku: Speakers Bureau; Mochida Pharmaceutical: Speakers Bureau; MSD K.K.: Speakers Bureau.

Abstract: Whole genome sequencing has revealed DNMT3A mutation is present in over 20% of cytogenetically normal acute myeloid leukemia (CN-AML) and R882 is the most frequent and recurrent mutated site. Cumulating clinical data have emphasized the importance of the mutation as a poor prognostic factor of AML. Since the functional role of DNMT3A mutation in leukemogenesis remains largely unknown, we aimed to elucidate the impact of DNMT3A mutation on the development and maintenance of AML. To investigate the effect of exogenous expression of DNMT3A R882 mutant (Mut) in hematopoiesis, we transplanted 5-FU primed mouse bone marrow cells transduced with empty vector (EV), DNMT3A wild type (WT), or DNMT3A Mut to lethally irradiated mice. Recipients transplanted with DNMT3A Mut-transduced cells exhibited hematopoietic stem cell (CD150+CD48-Lin-Sca1+c-Kit+) accumulation and enhanced repopulating capacity compared with EV and DNMT3A WT recipients. To identify the downstream target genes of DNMT3A Mut that evoked hematopoietic stem cell accumulation, we sorted vector-transduced LSK cells from transplanted mice and conducted quantitative PCR (Q-PCR) of various hematopoiesis-related genes. Q-PCR revealed that Hoxb cluster expression was up-regulated and differentiation-associated genes, such as PU.1 and C/ebpa, were down-regulated in DNMT3A Mut-transduced LSK cells. Targeted bisulfite sequencing showed hypomethylation of the Hoxb2 promoter-associated CpG island in DNMT3A Mut-transduced cells compared with EV-transduced cells, which suggests dominant-negative effect of DNMT3A R882 mutation. DNMT3A Mut caused no change in methylation status of PU.1 promoter-associated CpG island, indicating that DNA methylation-independent mechanism underlies PU.1 downregulation. Given that DNMT3A interacts with several histone modifiers to regulate target gene transcription, we performed co-immunoprecipitation to investigate whether these interactions are altered by DNMT3A mutation. We found that DNMT3A Mut has the emhanced capacity to interact with polycomb repressive complex 1 (PRC1), which is thought to be a potential mechanism of the DNMT3A Mut-induced differentiation defect. Co-immunoprecipitation experiments showed that DNMT3A R882H and R882C mutant exhibited augmented interaction with BMI1 and MEL18, respectively. In addition, RING1B, an essential component of PRC1, co-localized with DNMT3A Mut more frequently than WT, irrespective of the type of amino acid substitution. Furthermore, heterozygosity of Bmi1 restored the PU.1 mRNA to the normal level and canceled the effect of stem cell accumulation in mice transplanted with DNMT3A Mut bone marrow cells. Chromatin immunoprecipitation in AML cell lines showed that BMI1 and RING1B were more efficiently recruited to the upstream regulatory element of PU.1 upon expression of DNMT3A Mut than WT, while the amount of DNMT3A recruited were comparable between DNMT3A WT and Mut. In the murine transplantation model, we found that exogenous PU.1 expression impaired repopulating capacity in both EV and R882H-transduced cells to the similar level. Exogenous expression of DNMT3A WT inhibited proliferation and induced terminal myeloid differentiation, whereas DNMT3A Mut-transduced cells remained immature in AML cell lines. DNMT3A Mut-transduced cells were resistant to ATRA-induced differentiation compared to EV-transduced cells. Furthermore, R882 mutation promoted blastic transformation of murine c-Kit+ bone marrow cells in vitro in combination with HOXA9 which is highly expressed in clinical cases harboring DNMT3A mutation. Morphological and surface marker analysis revealed these cells were F4/80+ monocytic blasts, consistent with clinical observation that DNMT3A mutation is found frequently in FAB M4/M5 leukemia. These results indicate a distinct role for DNMT3A Mut as well as a potential collaboration between DNMT3A Mut and HOXA9 in malignant transformation of hematopoietic cells. Interestingly, Bmi1 heterozygosity impaired this monoblastic transformation of R882H and HOXA9 co-transduced progenitors. Taken together, our results highlight the functional role of DNMT3A mutation in differentiation block of hematopoietic stem cells and in promoting leukemic transformation via aberrant recruitment of Bmi1 and other PRC1 components. Disclosures: Kurokawa: Celgene: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Bristol-Myers Squibb: Research Funding.

Abstract: Adult T-cell leukemia-lymphoma (ATL) is an aggressive hematological malignancy of CD4+ T cells transformed by human T-cell lymphotropic virus-1 (HTLV-1). Most HTLV-1–infected individuals are asymptomatic, and only 3% to 5% of carriers develop ATL. Here, we describe the contribution of aberrant DNA methylation to ATL leukemogenesis. HTLV-1–infected T-cells and their uninfected counterparts were separately isolated based on CADM1 and CD7 expression status, and differentially methylated positions (DMPs) specific to HTLV–infected T cells were identified through genome-wide DNA methylation profiling. Accumulation of DNA methylation at hypermethylated DMPs correlated strongly with ATL development and progression. In addition, we identified 22 genes downregulated because of promoter hypermethylation in HTLV-1–infected T cells, including THEMIS, LAIR1, and RNF130, which negatively regulate T-cell receptor (TCR) signaling. Phosphorylation of ZAP-70, a transducer of TCR signaling, was dysregulated in HTLV-1–infected cell lines but was normalized by reexpression of THEMIS. Therefore, we hypothesized that DNA hypermethylation contributes to growth advantages in HTLV-1–infected cells during ATL leukemogenesis. To test this idea, we investigated the anti-ATL activities of OR-1200 and OR-2100 (OR21), novel decitabine (DAC) prodrugs with enhanced oral bioavailability. Both DAC and OR21 inhibited cell growth, accompanied by global DNA hypomethylation, in xenograft tumors established by implantation of HTLV-1–infected cells. OR21 was less hematotoxic than DAC, whereas tumor growth inhibition was almost identical between the 2 compounds, making it suitable for long-term treatment of ATL patient-derived xenograft mice. Our results demonstrate that regional DNA hypermethylation is functionally important for ATL leukemogenesis and an effective therapeutic target.