Abstract: Background: Adult T-cell leukemia/lymphoma (ATL) is a peripheral T-cell neoplasm caused by human T-cell leukemia virus type-1 (HTLV-1) retrovirus infection. As for its pathogenesis, viral products, such as Tax and HBZ, play indispensable roles and their oncogenic mechanisms have been extensively studied. Recently, we have performed an integrated genetic study of a large number of ATL cases and revealed the entire landscape of somatic mutations, copy number alterations, and gene fusions in ATL. However, the detailed analysis of HLTV-1 integration using next-generation sequencing has not been performed so far. In this study, combining whole-genome and RNA sequencing data, we delineated the effect of HTLV-I integration on viral and cellular transcription. Patients and Methods: We performed WGS and RNA-seq for 48 and 57 ATL cases, respectively. All the analyses of the sequencing data were performed using our in-house pipelines. We analyzed HTLV-1 proviral genomic structure and the effect of HTLV-1 integration on viral and cellular transcription. Results: A cardinal feature of ATL genome is HTLV-1 integration, which was precisely located in all the cases analyzed by WGS. Multiple proviral integration sites were detected in 12 cases (total, 62 HTLV-1 integrations sites). The provirus integration was clonal in the architecture inferred from somatic mutations, and apparently randomly integrated into the host genome as previously reported. Within the HTLV-1 genome, frequent 5' proviral segment (gag/pol/env loci) deletions and/or sense gene (gag/pol/env/tax/rex/p13/p30) mutations were observed, which seem to cause defective viral replication/production, whereas HBZ gene was maintained in all the cases. RNA-seq revealed that HTLV-1 integration in ATL cells was associated with aberrant transcription. In general, viral transcripts were predominantly derived from the antisense strand, whereas sense transcription was largely suppressed, leading to global silencing of the sense genes. Especially, in contrast to the ubiquitous HBZ expression (antisense strand), tax expression (sense strand) was almost completely lost in all but one case, which exceptionally exhibited high expression of both tax and HBZ. Strikingly, in most cases, the antisense transcripts were not terminated in 5'-long terminal repeat (LTR), but read through into the juxtaposed cellular genome, extending into up to 50 kb downstream therefrom (read-through transcript). Moreover, in 11 sites of intragenic proviral integration, aberrantly spliced fusion transcripts were observed between LTR and the affected gene, and more commonly associated with antisense (n = 9) than sense (n = 2) integration, accompanied by upregulated cellular gene expression. In other cases (n = 3), fusion transcripts were also generated between HBZ and an exon of highly expressed cellular gene adjacent to the integration site. These results indicate the potential significance of antisense transcription and aberrant fusion transcripts with host genome sequences during ATL development. Although the precise role of these novel aberrant antisense transcripts remains unknown, antisense transcripts containing the LTR region has been implicated in NF-κB activation, which is a hallmark of ATL pathogenesis. Conclusion: In summary, combining WGS and RNA-seq data, we demonstrated the global silencing of sense-oriented viral transcripts (including Tax) and the predominance of aberrant antisense-directed transcription, which often involved cellular gene expression, including aberrant fusion transcripts between host and viral genomes (read-through and aberrantly spliced fusion transcripts). These results suggest that antisense transcription and abnormal virus-host fusion transcripts play pivotal roles in the pathogenesis of ATL. Disclosures Tobinai: Gilead Sciences: Research Funding. Miyazaki:Kyowa-Kirin: Honoraria, Research Funding; Celgene Japan: Honoraria; Sumitomo Dainippon: Honoraria; Chugai: Honoraria, Research Funding; Shin-bio: Honoraria.

Abstract: Adult T-cell leukemia/lymphoma (ATL) is a peripheral T-cell neoplasm of largely unknown genetic basis, which is associated with human T-cell leukemia virus type-1 (HTLV-1) infection. To delineate a genetic landscape of somatic alterations in ATL, we have performed an integrated genetic study, in which whole-genome/exome (WGS/WES) and transcriptome sequencing (RNA-seq) was performed for a cohort of 83 paired ATL samples, followed by extensive validation using targeted sequencing of detected mutations in 370 follow-up samples. A striking feature of driver lesions in ATL was their strong enrichment in the components of T-cell receptor (TCR) / NF-κB pathway. Accounting for more than 90% of ATL cases, these lesions were characterized by the predominance of activating alterations, including hotspot missense mutations in PLCG1 (36%), PRKCB (33%), CARD11 (24%), VAV1 (18%), IRF4 (14%) and FYN (4%). Among these, most frequently mutated was PLCG1, which encodes phospholipase C γ1 (PLCγ1), a key regulator of the proximal TCR signaling. Besides the S345F and S520F mutations recently reported in cutaneous T-cell lymphoma, we identified an additional hotspot mutations (R48W, E1163K, and D1165H). The second most frequently mutated gene was PRKCB, encoding a member of the protein kinase C (PKC) family of proteins (PKCβ), a pivotal signaling molecule downstream of PLCγ. The frequent mutations of PKCβ were unexpected, because it is PKCθ that has been implicated in TCR signaling, whereas PKCβ has been more focused in the context of B-cell receptor signaling. Approximately 93% of the PRKCB mutations were confined to the catalytic domain with a prominent hotspot at D427, suggesting gain-of-function nature of these mutations. Consistent with this, when transduced with the D427N PKCβ mutant, HEK293T and/or Jurkat cells showed increased membrane translocation after PMA/Ionomycin-stimulation, enhanced IKK phosphorylation and p65 nuclear translocation, and augmented NF-κB transcription, compared to wild-type PKCβ-transduced cells. Thus, these PRKCB mutations are the first activating mutations of this family identified in human cancers. Downstream to PKC lies CARD11, a scaffolding protein required for antigen receptor-induced NF-κB activation. Although previously reported in B-cell lymphomas, CARD11 mutations were more common in ATL (24%). In B-cell lymphomas, mutations are largely limited to the coiled-coil (CC) domain, whereas in ATL, they were clustered not only within the CC domain, but also within the PKC-responsive inhibitory domain, showing a prominent mutational hotspot at E626. The inhibitory domain has been implicated in autoinhibition, whose deletion leads to constitutive activation of CARD11. Intriguingly, WGS identified small intragenic deletions confined to this domain (exons 14-17) in 4 cases (8%) without canonical mutations, and RNA-seq confirmed the skipping of the corresponding exons in these cases. Remarkably, CARD11 mutation significantly co-occurred with PRKCBmutations, suggesting potential functional synergism between these lesions. Actually, overexpression of wild-type CARD11 induced NF-κB activation, which was further augmented by E626K mutation. Similarly, when both CARD11 (E626K) and PRKCB (D427N) mutants were co-expressed, more enhanced NF-κB activation was observed. RNA-seq and follow-up RT-PCR screening also identified novel gene fusions in TCR / NF-κB pathway: five CTLA4-CD28 and three ICOS-CD28 fusions were observed in seven (7%) of the 105 cases examined, of whom one patient carried both chimeric fusions. WGS revealed tandem duplications of 2q33.2 segments containing CD28, CTLA4, and ICOS, compatible with the corresponding fusion transcripts. B7/CD28 co-signaling molecules, including CD28, CTLA4, and ICOS co-receptors, play pivotal roles in positive and negative regulations of TCR signaling. All the predicted chimeric proteins had the cytoplasmic part of CD28, and are expected to be expressed under the control of the regulatory element of CTLA4 or ICOS, likely leading to prolonged expression of CD28 co-stimulator. Our findings suggest that deregulated TCR / NF-κB pathway caused by genetic alterations is a hallmark of ATL pathogenesis. The predominance of gain-of-function mutations in this pathway offers good opportunities for exploiting these mutations for the targets of novel drugs to better manage patients. Disclosures Tobinai: Gilead Sciences: Research Funding. Miyazaki:Sumitomo Dainippon: Honoraria; Celgene Japan: Honoraria; Chugai: Honoraria, Research Funding; Shin-bio: Honoraria; Kyowa-Kirin: Honoraria, Research Funding. Watanabe:Daiichi Sankyo Co., Ltd.: Research Funding.

Abstract: Background: Allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative therapy for patients with myelodysplastic syndromes (MDS), whose benefit, however, is frequently offset by accompanying mortality and morbidity, underscoring the importance of accurate prognostication before the therapeutic choice. For this purpose, several systems, such as the International Prognostic Scoring System (IPSS), are being successfully applied to clinics, and recent genome profiling studies indicate that molecular diagnostics can further improve the prediction. Nevertheless, existing systems are based on the observation from those patients who were untreated or only supportively treated and therefore, may not successfully be applied to the prognostication of the patients who are actually treated by HSCT. Methods: We analyzed patients with MDS (N = 719) from a cohort of Japan Marrow Donor Program (JMDP) who were treated by unrelated HSCT between 2006 and 2013. Peripheral blood DNA was subjected to targeted deep sequencing in 68 major driver genes for the detection of both somatic mutations and copy number variations (CNVs) with accurate determination of their allelic burdens. Results: The median age at HSCT and observation period were 53 years old (20-66) and 372 days (2-3001), respectively. At the diagnosis, 63, 203, 163 and 65 patients have low, intermediate-1, intermediate-2 and high risk classified on the basis of IPSS, respectively (IPSS data was not available for 250 patients). The median time from diagnosis to HSCT was 274 (9-10900) days. Mutations were observed in 75% of the patients, of which TP53 was most frequently mutated (14.3%), followed by U2AF1 (13.2%), RUNX1 (12.2%), ASXL1 (11.0%) and DNMT3A (9.3%). The mean number of mutations was 2.1 per patient and the mean allelic burden was 23.4%. To evaluate karyotyping we combined metaphase cytogenetics and copy number variations using targeted sequencing data. Complex karyotype, chromosome 7 anomaly, deletion 5q, and deletion 20q were observed in 174 (24.4%), 173 (24.3%), 91 (12.8%), and 50 (7.0%) of the patients, respectively. Combined, 86.6% of the patients had one or more genetic lesions. Patients with one or more mutations or CNVs showed unfavorable overall survival (Hazard Ratio (HR) 2.46, P = 2.12 x 10-5). Univariate analysis for each gene identified mutations in TP53 (HR 2.85, P 〈 2.0 x 10-16), NRAS (HR 1.90, P = 5.4 x 10-4), ETV6 (HR 1.54, P = 0.029), CBL (HR 2.25, P = 5.3 x10-5), EZH2 (HR 1.74, P = 0.014), KRAS (HR 2.01, P = 2.0 x 10-3), U2AF2 (HR 1.97, P = 0.027), JARID2 (HR 2.09, P = 0.039), and RIT1 (HR 2.16, P = 0.023) as the unfavorable factors for the overall survival. Besides, mutations in PRPF8 had a favorable effect on overall survival (HR 0.50, P = 0.029). Then, we performed multivariate analysis with stepwise model selection of these significant mutations and clinical parameters. Mutations in TP53 (HR 2.31, P=0.015), and ETV6 (HR 2.57, P=0.015) remained significant together with complex karyotype (HR 2.15, P = 0.0063), grade of acute graft versus host disease (GVHD) (Grade I or II: HR 1.95, P = 0.011, Grade III or IV: HR 4.18, P = 7.94 x 10-5), and the number of red blood cell transfusion received before HSCT ( 〉 =10 times: HR 2.64, P = 0.027). Next, we analyzed the impact of mutations on relapse in cases who achieved complete response after HSCT (N = 423 (58.8%)). Patients with mutations in one or more genetic lesions showed unfavorable relapse free survival (HR 2.27, P = 1.65 x 10-4). Univariate analysis for each gene revealed mutations in TP53 (HR 3.09, P = 9.5 x 10-16), NRAS (HR 2.21, P = 0.0019), ETV6 (HR 1.90, P = 0.012), PRPF8 (HR 0.40, P = 0.046), and WT1 (HR 2.24, P = 0.013) were significant for the relapse free survival. Multivariate analysis and stepwise model selection identified ETV6 (HR 2.98, P = 0.011), WT1 (HR 4.01, P = 0.014), complex karyotype (HR 2.39, P = 0.0083), IPSS High (HR 6.22, P = 0.0053), and Grade III or IV acute GVHD (HR 2.91, P = 0.0071) as unfavorable factors. Conclusions: This large study of MDS cases treated by unrelated HSCT demonstrated that somatic mutations of several driver genes were novel prognostic factors for overall and relapse free survival. These genetic factors were independent of well-known prognostic makers, and therefore could be used to better guide therapy for MDS patients. Disclosures No relevant conflicts of interest to declare.

Abstract: TP53 and RAS-pathway mutations predict very poor survival, when seen with CK and MDS/MPNs, respectively. For patients with mutated TP53 or CK alone, long-term survival could be obtained with stem cell transplantation.

Abstract: DNA hypermethylation has long been implicated in the pathogenesis of myelodysplastic syndromes (MDS) and also highlighted by the frequent efficacy of demethylating agents to this disease. Meanwhile, recent genetic studies in MDS have revealed high frequency of somatic mutations involving epigenetic regulators, suggesting a causative link between gene mutations and epigenetic alterations in MDS. The accumulation of genetic and epigenetic alterations promotes tumorigenesis, hypomethylating agents such as Azacitidine exert their therapeutic effect through inhibition of DNA methylation. However, the relationship between patterns of epigenetic phenotypes and mutations, as well as their impact on therapy, has not been clarified. To address this issue, we performed genome-wide DNA methylation profiling (Infinium 450K) in combination with targeted-deep sequencing of 104 genes for somatic mutations in 291 patients with MDS. Beta-mixture quantile normalization was performed for correcting probe design bias in Illumina Infinium 450k DNA methylation data. Of the 〉 480,000 probes on the methylation chip, we selected probes using the following steps: (i) probes annotated with "Promotor_Associated" or "Promoter_Associated_Cell_type_specific; (ii) probes designed in "Island", "N_Shore" or "S_Shore"; (iii) removing probes designed on the X and Y chormosomes; (iv) removing probes with 〉 10% of missing value. Consensus clustering was performed utilizing the hierarchical clustering based on Ward and Pearson correlation algorithms with 1000 iterations on the top 0.5% (2,000) of probes showing high variation by median absolute deviation across the dataset using Bioconductor package Consensus cluster plus. The number of cluster was determined by relative change in area under cumulative distribution function curve by consensus clustering. Unsupervised clustering analysis of DNA methylation revealed 3 subtypes of MDS, M1-M3, showing discrete methylation profiles with characteristic gene mutations and cytogenetics. The M1 subtype (n=121) showed a high frequency of SF3B1 mutations, exhibiting the best clinical outcome, whereas the M2 subtype (n=106), characterized by frequent ASXL1, TP53 mutations and high-risk cytogenetics, showed the shortest overall survival with the hazard ratios of 3.4 (95% CI:1.9-6.0) and 2.2 (95% CI:1.2-4.0) compared to M1 and M3, respectively. Finally, the M3 subtype (n=64) was highly enriched (70% of cases) for biallelic alterations of TET2 and showed the highest level of CpG island methylation and showed an intermediate survival. In the current cohort, we had 47 patients who were treated with demethylating agents, including 11 responders and 36 non-responders. When DNA methylation status at diagnosis was evaluated in terms of response to demethylating agents, we identified 54 differentiated methylated genes showing 〉 20% difference in mean methylation levels between responders and non-responders (q 〈 0.1). Twenty-five genes more methylated in responders were enriched in functional pathways such as chemokine receptor and genes with EGF-like domain, whereas 29 less methylated gene in responders were in the gene set related to regulation of cell proliferation. Genetic alterations were also assessed how they affected treatment responses. In responders, TET2 mutated patients tended to more frequently respond (45% vs 34%), whereas patients with IDH1/2 and DNMT3A mutations were less frequently altered (0% vs 14%, 9% vs 14%) in responders, compared in non-responders. In conclusion, our combined genetic and methylation analysis unmasked previously unrecognized associations between gene mutations and DNA methylation, suggesting a causative link in between. We identified correlations between genetic/epigenetic profiles and the response to demethylating agents, which however, needs further investigation to clarify the mechanism of and predict response to demethylation agents in MDS. Disclosures Alpermann: MLL Munich Leukemia Laboratory: Employment. Nadarajah:MLL Munich Leukemia Laboratory: Employment. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Kiyoi:Taisho Toyama Pharmaceutical Co., Ltd.: Research Funding; Novartis Pharma K.k.: Research Funding; Pfizer Inc.: Research Funding; Takeda Pharmaceutical Co.,Ltd.: Research Funding; MSD K.K.: Research Funding; Sumitomo Dainippon Pharma Co., Ltd.: Research Funding; Alexion Pharmaceuticals.: Research Funding; Teijin Ltd.: Research Funding; Zenyaku Kogyo Company,Ltd.: Research Funding; FUJIFILM RI Pharma Co.,Ltd.: Patents & Royalties, Research Funding; Nippon Shinyaku Co.,Ltd.: Research Funding; Japan Blood Products Organization.: Research Funding; Eisai Co.,Ltd.: Research Funding; Yakult Honsha Co.,Ltd.: Research Funding; Astellas Pharma Inc.: Consultancy, Research Funding; Kyowa-Hakko Kirin Co.,Ltd.: Consultancy, Research Funding; Fujifilm Corporation.: Patents & Royalties, Research Funding; Nippon Boehringer Ingelheim Co., Ltd.: Research Funding; Bristol-Myers Squibb.: Research Funding; Chugai Pharmaceutical Co.,LTD.: Research Funding; Mochida Pharmaceutical Co.,Ltd.: Research Funding. Kobayashi:Gilead Sciences: Research Funding. Naoe:Toyama Chemical CO., LTD.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Nippon Boehringer Ingelheim Co., Ltd.: Research Funding; Kyowa Hakko Kirin Co., Ltd.: Patents & Royalties, Research Funding; Pfizer Inc.: Research Funding; Astellas Pharma Inc.: Research Funding; FUJIFILM Corporation: Patents & Royalties, Research Funding; Celgene K.K.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Patents & Royalties. Kern:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Miyazaki:Chugai: Honoraria, Research Funding; Shin-bio: Honoraria; Sumitomo Dainippon: Honoraria; Celgene Japan: Honoraria; Kyowa-Kirin: Honoraria, Research Funding.

Abstract: Adult T-cell leukemia/lymphoma (ATL) is a distinct form of peripheral T-cell lymphoma, which is etiologically associated with human T-cell leukemia virus type 1 (HTLV-1) infection during early infancy. Although HTLV-1 can effectively immortalize human T cells, there is a long latency period of ~50 years before the onset of ATL, suggesting that HTLV-1 infection alone may be insufficient for the development of ATL, but additional acquired genetic events that accumulate during the later life are essential for the development of ATL. However, such somatic alterations underlying the pathogenesis of ATL have not been fully elucidated. To obtain a complete registry of genetic alterations in ATL, we performed an integrated genetic study, in which whole-genome/exome and RNA sequencing (RNA-seq) was performed together with array-based methylation and genomic copy number analysis among a cohort of 50 paired ATL samples, followed by extensive validation using targeted deep sequencing of detected mutations in 〉 400 follow-up samples. Compared with other lymphoid malignancies, ATL cells carried higher numbers of mutations, copy number alterations, and rearrangements than in other lymphoid malignancies, suggesting the presence of global genomic instability in ATL. In addition to previously reported mutational targets in ATL (TP53,TCF8, and FAS) and known targets frequently mutated in other lymphoid malignancies (CARD11, GATA3, IRF4, POT1, and RHOA), we identified a variety of highly recurrent mutations affecting previously unknown mutational targets, many of which are involved in T-cell development, activation and migration, immunosurveillance, and transcriptional regulation. Molecular and functional analysis using human T-cell leukemia cell lines showed that some of these novel mutations actually augment T-cell receptor signaling, validating their biological significance in ATL. A comparison of mutations among disease subtypes revealed that several subtype-specific mutations, including TP53, CD58, IRF4 and TBL1XR1 mutations in acute and lymphoma types, and STAT3mutation in chronic and smoldering types, suggesting that different oncogenic mechanisms underlie different ATL subtypes. Furthermore, ATL cells had a distinct pattern of copy number changes and genomic rearrangements. Interestingly, their gene targets showed a significant overlap to mutational targets. Surprisingly, somatic focal deletions involving the 14q31.1 locus were observed in all the cases examined by whole-genome sequencing and therefore are thought to uniquely characterize ATL genomes, although their gene targets remained to be identified. Like other regions also frequently deleted in ATL, such as 7q31.1 and 1p21.3 loci, these deletions were thought to reflect high levels of genetic instability. Finally and conspicuously, pathway analysis revealed that multiple genes involved in the Tax interactome were systematically altered in ATL, although Tax itself underwent gene silencing in most cases. These data suggested that ATL cells can escape from cytotoxic T-lymphocytes by silencing immunogenic Tax expression, while developing alternative oncogenic mechanisms through acquiring somatic mutations or copy number alterations in the Tax-related pathway. Our findings suggest that deregulated T-cell functionalities caused by genetic alterations, especially those associated with HTLV-1 Tax oncoprotein, are central to ATL pathogenesis, and provide a novel clue to contrive new diagnostics and therapeutics for this intractable disease. Disclosures No relevant conflicts of interest to declare.

Abstract: Background: Studies on germline variants responsible for cancer predisposition provide an important clue to the understanding of genetic basis of cancer and also help better prediction and management of relevant cancers. As for myeloid neoplasms, only a handful of genes, including RUNX1, CEBPA, GATA2, ETV6, and ANKRD26, have been implicated in early onset familial acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), although they are rarely seen in sporadic cases. Recently, using whole exome sequencing of familial AML/MDS, we have reported novel AML/MDS predisposing gene, DDX41, an encoding dead-box helicase gene. Conspicuously, the onset of AML/MDS was over 60 in most of the affected cases, raising a possibility that the genetic predisposition might be obscured and many cases could be diagnosed with sporadic AML/MDS. In this study, we investigated germline DDX41 mutations in sporadic cases with AML/MDS and the incidence and mutation types were compared between Asian and Western patients. Patients and Methods: We performed targeted sequencing of DDX41 in patients from Asian (N = 239) cohort of AML/MDS, where the origin of the detected variations was determined by using matched germline DNA. The result was compared to those obtained from the Western cohort (N = 1,034) in terms of frequency and type of mutation. The effect size of the mutations was estimated by calculating odds ratios of each variant for AML/MDS development using the data for DDX41 variants in Asian and Western population from the ExAC (Exome Aggregation Consortium) database (http://exac.broadinstitute.org) as controls. Results: Germline and somatic DDX41 mutations were found in 12 (5.0%) and 10 (4.7%) of sporadic cases with AML/MDS from the Asian cohort, as compared to 8 (0.8%) and 10 (1.0%) from the Western cohort. All the patients with germline variants were aged over 40 year-old with a median of 68.5, confirming the late onset of the disease also in the sporadic cases with germline variants. Eight of the 12 germline variants (67%) in the Asian cohort were accompanied by an additional somatic mutation, as compared to 2 of the 8 (25%) in the Western cohort. Biallelic involvement was demonstrated in selected cases (N = 2). In total, 8 and 3 germline variants were observed in the Asian and the Western cohorts, respectively, without no common variants between both cohorts, of which the predominant variants included p.A500fs (n=5; 42%) and p.E7X (n=2; 17%) in the Asian cohort and p.F140fs (n=6; 75%) in Western cohort. In contrast, a prominent hotspot mutation involving a highly conserved amino-acid within the helicase domain (p.R525H) was commonly observed in both cohorts, accounting for 55% of all the somatic mutations. These germline variants as a whole showed significant enrichment in AML/MDS cases compared to the respective control population (OR 〉 171, 95% confidence interval (CI): 51-730 for the Asian variants and more than 21.7, 95%CI: 8.4-50 for the Western variants), although the enrichment of individual variants showed substantial variations, suggesting different effect size among these variants: the odds ratio was 19.5 (p 〈 0.001) for p.F140fs, and 92.4 (p 〈 0.001) for p.A500fs. p.E7X was detected in 2 out of 239 cases with MDS/AML, whereas not in the control Asian population. Conclusion: We demonstrated frequent germline variants of DDX41 among sporadic cases with AML/MDS from different ethnic populations. Having common ancestral origins in different ethnic populations, these alleles are found in the general population at very low frequencies ( 〈 1 in 4000), accounting for the largest congenital risk for the development of sporadic AML/MDS therein (3-5% of all sporadic AML/MDS). The onset was typically over 40 years of age and frequently accompanied by an additional somatic mutation most likely in the unaffected allele, showing a prominent hotspot at p.R525. The germline variants seem to be dominant and caused premature truncation of the protein, leading to loss-of-function in most cases, whereas somatic mutations were typically missense variants not totally abrogating protein function, suggesting the importance of less than haploinsufficiency but more than null function for leukemogenesis. At the meeting, an extended result from more than 1000 Asian cases will be presented. Disclosures Kiyoi: Kyowa-Hakko Kirin Co., Ltd.: Consultancy, Research Funding; Pfizer Inc.: Research Funding; Novartis Pharma K.k.: Research Funding; Mochida Pharmaceutical Co., Ltd.: Research Funding; Taisho Toyama Pharmaceutical Co., Ltd.: Research Funding; Eisai Co., Ltd.: Research Funding; Zenyaku Kogyo Company, Ltd.: Research Funding; FUJIFILM RI Pharma Co.,Ltd.: Patents & Royalties, Research Funding; Chugai Pharmaceutical Co., LTD.: Research Funding; Fujifilm Corporation.: Patents & Royalties, Research Funding; Nippon Boehringer Ingelheim Co., Ltd.: Research Funding; Bristol-Myers Squibb.: Research Funding; Alexion Pharmaceuticals.: Research Funding; Sumitomo Dainippon Pharma Co., Ltd.: Research Funding; Takeda Pharmaceutical Co., Ltd.: Research Funding; Yakult Honsha Co., Ltd.: Research Funding; Astellas Pharma Inc.: Consultancy, Research Funding; Teijin Ltd.: Research Funding; Japan Blood Products Organization.: Research Funding; Nippon Shinyaku Co.,Ltd.: Research Funding; MSD K.K.: Research Funding. Miyazaki:Shin-bio: Honoraria; Sumitomo Dainippon: Honoraria; Chugai: Honoraria, Research Funding; Celgene Japan: Honoraria; Kyowa-Kirin: Honoraria, Research Funding. Naoe:Kyowa Hakko Kirin Co., Ltd.: Patents & Royalties, Research Funding; Celgene K.K.: Research Funding; FUJIFILM Corporation: Patents & Royalties, Research Funding; Astellas Pharma Inc.: Research Funding; Toyama Chemical CO., LTD.: Research Funding; Nippon Boehringer Ingelheim Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Pfizer Inc.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Patents & Royalties. Usuki:Boehringer Ingelheim: Other: personal fees, Research Funding; Shionogi: Other: personal fees; Fujimoto Pharmaceutical: Research Funding; Takeda Pharmaceutical: Research Funding; SymBio Pharmaceutical: Other: personal fees, Research Funding; Eisai: Research Funding; Otsuka Pharmaceutical: Research Funding; Kyowa Hakko Kirin: Other: personal fees, Research Funding; Shire: Research Funding; Nippon Shinyaku: Other: personal fees, Research Funding; Novartis: Other: personal fees, Research Funding; Sanofi: Other: personal fees, Research Funding; MSD: Other: personal fees, Research Funding; Celgene: Other: personal fees, Research Funding; Sumitomo Dainippon Pharma: Other: personal fees, Research Funding; Taiho Pharmaceutical: Other: personal fees, Research Funding; Fuji Film RI Pharma: Other: personal fees; Chugai Pharmaceutical: Other: personal fees; GlaxoSmithKline: Other: personal fees, Research Funding; Bristol-Myers Squibb: Other; Astellas: Research Funding. Miyawaki:Astellas Pharma Inc.: Consultancy, Other: personal fees; Ohtsuka Pharma Co, LTD.: Other: Safety Data Committee.

Abstract: Background: Acute erythroid leukemia (AEL) is a rare subtype of AML characterized by erythroid predominant proliferation and classified into two subtypes with pure erythroid (PEL) and myeloid/erythroid (MEL) phenotypes. Although gene mutations in AEL have been described in several reports, genotype phenotype correlations are not fully understood with little knowledge about the feasible molecular targets for therapy. Methods: To understand the mechanism of the erythroid dominant phenotype of AEL and identify potential therapeutic targets for AEL, we analyzed a total of 105 AEL cases with the median age of 60 (23-86), using targeted-capture sequencing of commonly mutated genes in myeloid neoplasms, together with 1,279 SNPs for copy number measurements. Among these 105 cases, 13 were also analyzed by RNA sequencing. Genetic profiles of these 105 AEL cases were compared to those of 775 cases with non-erythroid AML (NEL) including 561 cases from The Cancer Genome Atlas and Beat AML study. An immature erythroid cell line (TF1) and three patient-derived xenografts (PDX) established from AEL with JAK2 and/or EPOR amplification. Cell line and samples from patients were inoculated into immune-deficient mice and tested for their response to JAK1/2 inhibitor. Results: According to unique genetic alterations, AEL was classified into 4 subgroups (A-D). Characterized by TP53 mutations and complex karyotype, Group A was the most common subtype and showed very poor prognosis. Remarkably, all PEL cases were categorized into Group A. Conspicuously, 80% of PEL cases had amplifications of JAK2 (6/10; 60%), EPOR (7/10;70%), and ERG (6/10;60%) loci on chromosomes 9p, 19q, and 21q, respectively, frequently in combination, although they were rarely seen in NEL cases. All cases in Group B (n=19, 18%), another prevalent form of AEL, had STAG2 mutations and classified in MEL. To further characterize this subgroup, we compared genetic profiles of STAG2-mutated AEL and NEL. Prominently, 70% (14/20) of STAG2-mutated cases in AEL had KMT2A-PTD, whereas it was found only in 8.8% (3/34) of NEL. CEBPA mutations were also more common in AEL (6/21; 29%) than NEL (4/34; 12%). While Group C was characterized by frequent NPM1 mutations, in contrast to the frequent co-mutation of FLT3 in the corresponding subgroup of NPM1-mutated cases in NEL, NPM1-mutated patents in this subgroup lacked FLT3 mutations but had frequent PTPN11 mutations (8/16; 50%), which were much less common in NEL (25/209; 12%). The remaining cases were categorized into Group D, which was enriched for mutations in ASXL1, BCOR, PHF6, U2AF1 and KMT2C. Recurrent loss-of-function mutations in USP9X were unique to this subtype, although USP9X mutations have been reported in ALL with upregulation of JAK-STAT pathway. In RNA sequencing analysis, AEL cases exhibited gene expression profiles implicated in an upregulated STAT5 signaling pathway, which was seen not only those cases with JAK2 or EPOR amplification, but also those without, suggesting that aberrantly upregulated STAT5 activation might represent a common defect in AEL. Based on this finding, we evaluated the effect of a JAK inhibitior, ruxolitinib, on an AEL-derived cell line and three PDX models established from AEL having TP53 mutations and JAK2 and EPOR mutation/amplification. Of interest, ruxolitinib significantly suppressed cell growth and prolonged overall survival in mice engrafted with TF1 and 2 PDX models with STAT5 downregulation, although the other model was resistant to JAK2 inhibition with persistent STAT5 activation. Conclusion: AEL is a heterogeneous group of AML, of which PEL is characterized by frequent amplifications/mutations in JAK2, EPOR and/or ERG. Frequent involvement of EPOR/JAK/STAT pathway is a common feature of AEL, in which a role of JAK inhibition was suggested. Disclosures Yoda: Chordia Therapeutics Inc.: Research Funding. Shih:Novartis: Research Funding; Celgene: Research Funding; PharmaEssentia: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Consultancy, Membership on an entity's Board of Directors or advisory committees. Ishiyama:Alexion: Research Funding; Novartis: Honoraria. Miyazaki:Astellas Pharma Inc.: Honoraria; Sumitomo Dainippon Pharma Co., Ltd.: Honoraria; NIPPON SHINYAKU CO.,LTD.: Honoraria; Celgene: Honoraria; Otsuka Pharmaceutical: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria; Novartis Pharma KK: Honoraria; Kyowa Kirin Co., Ltd.: Honoraria. Nakagawa:Sumitomo Dainippon Pharma Co., Ltd.: Research Funding. Takaori-Kondo:Celgene: Honoraria, Research Funding; Ono Pharmaceutical: Research Funding; Thyas Co. Ltd.: Research Funding; Takeda: Research Funding; CHUGAI: Research Funding; OHARA Pharmaceutical: Research Funding; Sanofi: Research Funding; Novartis Pharma: Honoraria; Bristol-Myers Squibb: Honoraria, Research Funding; Pfizer: Research Funding; Otsuka Pharmaceutical: Research Funding; Eisai: Research Funding; Astellas Pharma: Honoraria, Research Funding; Kyowa Kirin: Honoraria, Research Funding; Nippon Shinyaku: Research Funding; MSD: Honoraria. Kataoka:Asahi Genomics: Current equity holder in private company; Otsuka Pharmaceutical: Research Funding; Takeda Pharmaceutical Company: Research Funding; CHUGAI PHARMACEUTICAL CO., LTD.: Research Funding. Usuki:Alexion: Research Funding, Speakers Bureau; Apellis: Research Funding; Novartis: Research Funding, Speakers Bureau; Chugai: Research Funding. Maciejewski:Novartis, Roche: Consultancy, Honoraria; Alexion, BMS: Speakers Bureau. Ganser:Novartis: Consultancy; Celgene: Consultancy. Thol:Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Ogawa:Sumitomo Dainippon Pharma Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Asahi Genomics Co., Ltd.: Current equity holder in private company; Eisai Co., Ltd.: Research Funding; Chordia Therapeutics, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; KAN Research Institute, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding. OffLabel Disclosure: Ruxolitinib is used for drug efficacy test using patient-derived xenografts established from acute erythroid leukemia.