Abstract: Background: Breast cancer risk models are used to predict the risk of carrying a variant, for one of the most common breast cancer susceptibility genes such as BRCA1 and BRCA2, and the lifetime risk of developing breast cancer. The prediction of harboring a germline variant of the BRCA gene and the development of breast or ovarian cancer over time affects the decision-making for undergoing genetic testing and screening using imaging techniques as the common practice. For instance, the American Cancer Society and the National Comprehensive Cancer Network (NCCN) recommends screening using MRI in women with 20% or greater lifetime risk of having breast cancer. We aimed to investigate the prediction of these risks in Japanese women, particularly on the relationship between the presence of pathogenic germline variants and breast cancer susceptibility genes, using a cohort of 1016 primary breast cancer patients. Patients and Methods: We analyzed a cohort of Japanese patients with primary breast cancer who were treated at the Kyoto University Hospital and the related institutions or hospitals from the period of 2011 to 2016. The germline variants were examined for a set of 13 breast cancer susceptibility genes, using targeted-capture sequencing of pooled DNA, and it was found that 66 out of 1016 patients had pathogenic variants. These included 11 functionally well-established genes (BRCA1, BRCA2, TP53, PTEN, CDH1, STK11, NF1, PALB2, ATM, CHEK2, and NBN) and two additional genes (BARD1 and BRIP1), which are recommended for the screening of high-risk patients with hereditary breast cancer in the NCCN guidelines. Using this cohort, we studied the association of the calculated risk of carrying a germline variant of BRCA1/ BRCA2, using the Tyrer-Cuzick model Breast Cancer Risk Evaluation Tool, within the laboratory germline test results. Results: Pathogenic germline variants of BRCA1/ BRCA2 were carried by 54 (5.3%) out of the 1016 patients (12 cases of BRCA1 and 42 cases of BRCA2). According to the NCCN guidelines, which focus on Genetic/ Familial High-Risk Assessment: Breast and Ovarian, it was found that 500 out of 1016 (49.2%)patients were categorized for considering germline testing. In fact, 38 (7.6%) of the 500 patients, harbored a pathogenic germline variant of BRCA1/ BRCA2. In the remaining 516 patients, 16 (3.1%) harbored the pathogenic germline variant of BRCA1/ BRCA2. The predictive risks of the Tyrer-Cuzick model Breast Cancer Risk Evaluation Tool were recorded as follows: Area under the ROC curve, BRCA1 (area 0.750, 95% CI- 0.581-0.919), BRCA2 (area 0.741, 95% CI- 0.661-0.820), BRCA1 or BRCA2 (Area 0.749, 95% CI: 0.675-0.822), suggesting that the Tyrer-Cuzick model may be useful for the Japanese population. In the mammography breast density analysis, 484 patients showed almost entirely fat or scattered fibroglandular breast tissue, and 362 cases had heterogeneous or extreme fibroglandular breast tissue. In this study, the correlations of breast tissue density with age and breast or ovarian cancer familial history have been reported in greater detail. Discussion and Conclusions: In a retrospective cohort of 1016 Japanese patients with primary breast cancer, 5.3% had pathogenic germline variants of BRCA1/ BRCA2. In a group recommended by NCCN guidelines for considering genetic testing, the BRCA1/ BRCA2 variant rate was 7.6%. Predictive risks calculated by the Tyrer-Cuzick model similar with the known data. Further data are reported. Citation Format: Noriko Senda, Nobuko Kawaguchi-Sakita, Masahiro Kawashima, Yukiko Inagaki-Kawata, Kenichi Yoshida, Tomomi Nishimura, Masahiro Takada, Eiji Suzuki, Yuki Kataoka, Fumiaki Sato, Yoshiaki Matsumoto, Masae Torii, Hiroshi Yoshibayashi, Kazuhiro Yamagami, Shigeru Tsuyuki, Akira Yamauchi, Nobuhiko Shinkura, Hironori Kato, Yoshio Moriguchi, Ryuji Okamura, Norimichi Kan, Hirofumi Suwa, Shingo Sakata, Susumu Mashima, Fumiaki Yotsumoto, Tsuyoshi Tachibana, Mitsuru Tanaka, Takashi Inamoto, Masahiro Sugimoto, Seishi Ogawa, Masakazu Toi. Relationship between predicted risks of carrying breast cancer susceptibility genes and the presence of germline variants in Japanese patients with primary breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-10-12.

Abstract: RHOA mutations are common in ATLL and show a unique distribution compared with other T-cell lymphomas. Depending on patients, functionally distinct RHOA mutations are clonally selected and involved in the pathogenesis of ATLL.

Abstract: [Introduction] Proliferative lesions in the breast have been implicated in the development of invasive breast cancer. Previous studies showed that adjacent atypical proliferative lesions and breast cancers shared common genetic alterations, suggesting that these evolved from the same ancestral cell. However, the clonal structure of atypical proliferative lesions and their clonal dynamics during progression to cancer are poorly understood. In this study, we compared genetic profiles of proliferative lesions and cancers from the same patients to illustrate the clonal evolution of cancer from a non-malignant epithelial cell. [Methods] Multiple samples were collected from various lesions with different disease-level located in the cancer-borne breast, using micro-dissection from formalin-fixed, paraffin-embedded surgical specimens. Somatic mutations and copy number alterations (CNAs) were evaluated by whole exome sequencing. [Results] A total of 39 samples from 6 premenopausal females carrying estrogen receptor-positive cancers were analyzed, where the samples were obtained from non-atypical (N = 1) and atypical (N = 8) proliferative lesions, and non-invasive (N = 21) and invasive (N = 4) cancers. The number of somatic mutations per sample increased with disease progression (ranging from 2 to 311). Two cases with bilateral cancers had a pathogenic germline mutation of either BRCA2 or TP53, where no somatic mutations or CNAs were shared by individual proliferative lesions, suggesting multifocal independent cancerous evolutions. By contrast, in the remaining four unilateral cases, no pathogenic germline mutations were detected, but all proliferative lesions, which were separated by a distance of 7 - 70 mm, shared one or more driver alterations, such as an AKT1 mutation, a GATA3 mutation, a CBFB mutation and concurrent 1q gain and 16q loss, while harboring private mutations/CNAs of their own. The analysis of phylogenic trees based on the number of shared mutations predicted an early origin of these founder mutations, which frequently predated decades before the onset of cancer. [Conclusions] Our results suggest that early breast cancer development is shaped by the evolution of multiple precancerous clones. In hereditary cases, this process is thought to be substantially promoted multi-focally within the entire breasts, due to innate genomic instability in each mammary epithelial cell for pathogenic germline mutations. By contrast, in sporadic cases, the ancestral cell which has acquired a founder genetic alteration expands macroscopically long before the onset of cancer, most likely in early adolescent. These expanded clones work as the niche predisposed to additional mutations, in which branching evolution occurs multi-focally and raises up multiple proliferative lesions, from which invasive cancer finally evolves. Our findings provide unique insight into the early development of breast cancer. Citation Format: Tomomi Nishimura, Nobuyuki Kakiuchi, Kenichi Yoshida, Yasuhide Takeuchi, Hirona Maeda, Yusuke Shiozawa, Masahiro M. Nakagawa, Yotaro Ochi, Yukiko Kawata, Kosuke Aoki, Masahiro Hirata, Tatsuki R. Kataoka, Takaki Sakurai, Satoko Baba, Yuichi Shiraishi, Kenichi Chiba, Kengo Takeuchi, Hironori Haga, Satoru Miyano, Masakazu Toi, Seishi Ogawa. Clonal evolution of proliferative lesions into breast cancers [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4902.

Abstract: Introduction: Proliferative lesions in the breast have been implicated in the development of invasive breast cancer. Previous studies showed that adjacent atypical proliferative lesions and breast cancers shared common genetic alterations, suggesting that these evolved from the same ancestral cell. However, the clonal structure of atypical proliferative lesions and their clonal dynamics during progression to cancer are poorly understood. In this study, we compared genetic profiles of proliferative lesions and cancers from the same patients to illustrate the clonal evolution of cancer from a non-malignant epithelial cell. Methods: Multiple samples were collected from various lesions with different disease-level located in the cancer-borne breast, using micro-dissection from formalin-fixed, paraffin-embedded surgical specimens. Somatic mutations and copy number alterations (CNAs) were evaluated by whole exome sequencing. Results: A total of 39 samples from 7 premenopausal females carrying estrogen receptor-positive cancers were analyzed, where the samples were obtained from proliferative lesions (without/with atypia) (N = 3 and 10, respectively), and non-invasive (N = 22) and invasive (N = 4) cancers. The number of somatic mutations per sample increased with disease progression (ranging from 7 to 311). Two cases with bilateral cancers had a pathogenic germline mutation of either BRCA2 or TP53, where no somatic mutations or CNAs were shared by individual proliferative lesions, suggesting multifocal independent cancerous evolutions. By contrast, in the remaining five unilateral cases, no pathogenic germline mutations were detected, but all proliferative lesions, which were separated by a distance of 7 - 70 mm, shared one or more driver alterations, such as an AKT1 mutation, a GATA3 mutation, a CBFB mutation, a PTEN mutation and concurrent 1q gain and 16q loss, while harboring private mutations/CNAs of their own. The analysis of phylogenic trees based on the number of shared mutations predicted an early origin of these founder mutations, which frequently predated decades before the onset of cancer. Conclusions: Our results suggest that early breast cancer development is shaped by the evolution of multiple precancerous clones. In hereditary cases, this process is thought to be substantially promoted multi-focally within the entire breasts, due to innate genomic instability in each mammary epithelial cell for pathogenic germline mutations. By contrast, in sporadic cases, the ancestral cell which has acquired a founder genetic alteration expands macroscopically long before the onset of cancer. These expanded clones work as the niche predisposed to additional mutations, in which branching evolution occurs multi-focally and raises up multiple proliferative lesions, from which invasive cancer finally evolves. Our findings provide unique insight into the early development of breast cancer. Citation Format: Tomomi Nishimura, Nobuyuki Kakiuchi, Kenichi Yoshida, Yasuhide Takeuchi, Hirona Maeda, Yusuke Shiozawa, Masahiro M. Nakagawa, Yotaro Ochi, Yukiko Kawata, Kosuke Aoki, Masahiro Hirata, Tatsuki R. Kataoka, Takaki Sakurai, Satoko Baba, Yuichi Shiraishi, Kenichi Chiba, Kengo Takeuchi, Hironori Haga, Satoru Miyano, Masakazu Toi, Seishi Ogawa. Clonal evolution of proliferative lesions into breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 92.

Abstract: DDX41 is a newly identified leukemia predisposition gene encoding an RNA helicase, whose germline mutations are tightly associated with late-onset myeloid malignancies. Importantly, germline DDX41 mutations were also found in as many as ~7 % of sporadic cases of high-risk MDS, conferring the largest germline risk for myeloid malignancies. In typical cases, a germline loss-of-function allele (most commonly p.A500fs or p.D140fs, depending on the ethnicity) is compounded by a somatic missense mutation affecting the helicase domain in the remaining allele (p.R525H). However, the molecular mechanism by which DDX41 mutations lead to myeloid neoplasms have not been elucidated. To clarify the role of these distinct DDX41 alleles, we generated mice models carrying either or both of conditional/constitutive Ddx41 knock-out (KO) and conditional R525H knock-in (KI) alleles. Vav1-Cre mediated homozygous deletion of Ddx41 resulted in embryonic lethality, suggesting that Ddx41 is indispensable for normal hematopoiesis. Next, by crossing these mice and further breeding with Rosa26-CreERT2 transgenic mice, we engineered mice that were wild-type for Ddx41 (Ddx41+/+), heterozygous Ddx41 KO (Ddx41+/-), heterozygous for the Ddx41 R525H mutation (Ddx41R525H/+), or hemizygous for the Ddx41 R525H mutation (Ddx41R525H/-), in which expression of the mutant allele was induced by tamoxifen administration. First, we assessed cell intrinsic effects of these Ddx41 alleles, using noncompetitive transplantation experiments. Shortly after tamoxifen administration, most of the recipient mice that were reconstituted with BM from Ddx41R525H/- mice died within a month after CreERT2 induction due to severe BM failure (BMF) with no development of myeloid neoplasms. However, about 20% of mice transplanted with BM derived from Ddx41R525H/- mice survived longer without showing BMF. These mice exhibited macrocytic anemia and increased platelet counts four months after tamoxifen-induction. In contrast, mice transplanted with BM from Ddx41+/- and Ddx41R525H/+ animals showed increased white blood cell counts compared to those with BM from Ddx41+/+ mice. In flow cytometry, Ddx41R525H/--derived BM-transplanted mice showed a significant increase in the number of long-term and short-term hematopoietic stem cells (HSCs), common myeloid progenitors (CMPs) and granulocyte/macrophage lineage-restricted progenitors (GMPs), compared to those transplanted with BM from Ddx41+/+, Ddx41+/- or Ddx41R525H/+ mice. Single cell RNA-seq of lineage negative cell fractions from these mice also revealed expanded stem cell fractions in mice transplanted with BM from Ddx41R525H/- mice, even though there was impaired formation of mature peripheral blood cells, which was suggestive of impaired HSPC differentiation. We also assessed the reconstitution capacity of whole BM cells from different Ddx41 mutant mice in competitive transplantation experiments. The donor chimerism of Ddx41R525H/- mice-derived cells in PB was reduced compared to that of cells derived from Ddx41+/+, Ddx41+/- or Ddx41R525H/+ mice. Transcriptome analysis of stem cells (Kit+Sca-1-Linlow cells) from different Ddx41 mutant mice revealed significant changes in gene expression and splicing patterns in many genes in stem cells from all the mutant mice, with larger changes for Ddx41R525H/- than Ddx41+/- or Ddx41 R525H/+ cells. Notably, Ddx41R525H/- cells exhibited a significant upregulation of genes involved in innate immunity, whereas there was a downregulation of genes related to RNA metabolism and ribosome biogenesis. Proteomics analysis confirmed the significant downregulation of ribosomal proteins in hematopoietic cells derived from Ddx41R525H/- mice. In summary, our results revealed an essential role of Ddx41 in normal hematopoiesis. While both heterozygous Ddx41 KO and heterozygous R525H knock-in did not develop myeloid neoplasm, compound biallelic loss-of function and R525 alleles led to a compromised function of hematopoietic stem cells, which was evident from reduced competitive repopulation capacity and impaired hematopoietic differentiation, where activated innate immunity and impaired ribosome functions may play important roles. Their roles in myeloid neoplasms need further evaluation. Disclosures Nakagawa: Sumitomo Dainippon Pharma Co., Ltd.: Research Funding. Inagaki:Sumitomo Dainippon Pharma Co., Ltd.: Current Employment. Kataoka:Takeda Pharmaceutical Company: Research Funding; Asahi Genomics: Current equity holder in private company; CHUGAI PHARMACEUTICAL CO., LTD.: Research Funding; Otsuka Pharmaceutical: Research Funding. Ogawa:KAN Research Institute, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Sumitomo Dainippon Pharma Co., Ltd.: Research Funding; Chordia Therapeutics, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Eisai Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Asahi Genomics Co., Ltd.: Current equity holder in private company.

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: Recent genetic studies have revealed frequent and specific pathway mutations involving multiple components of the RNA splicing machinery in myelodysplasia. Among these, U2AF1 mutations are more prevalent in MDS without increased ring sideroblasts and AML with myelodysplasia-related changes and are associated with a poor prognosis. Also found in approximately 4% of lung adenocarcinoma, U2AF1 mutations exclusively involved two highly conserved amino acid positions (S34 or Q157) within the amino- and the carboxyl-terminal zinc finger motifs flanking the U2AF homology motif (UHM) domain. Comprehensive analysis in a large cohort of MDS showed that U2AF1 mutations showed a significant trend to coexist with ASXL1. The molecular mechanism by which U2AF1 mutations lead to myelodysplasia have not fully been elucidated. To elucidate the role of U2AF1 mutations in the development of myelodysplasia, we generated heterozygous conditional knock-in mice for the U2af1 S34F mutation, which were crossed them with Vav1-Cre transgenic mice. Vav1-Cre mediated U2af1 S34F knock-in mice exhibited severe leukopenia and macrocytic anemia at 8-20 weeks after birth. Although there was no significant difference in blood cell morphology between wild-type and mutant mice in bone marrow (BM) and peripheral blood (PB) cells, there was strong myeloid skewing in lineage composition both in U2af1 mutant BM and PB cells compared to wild-type controls. Flow cytometry of U2af1 S34F BM cells showed a significant decrease in the number of hematopoietic stem cells (HSCs), common myeloid progenitors (CMPs) and megakaryocyte/erythrocyte lineage-restricted progenitors (MEPs), and an increase of the granulocyte/macrophage lineage-restricted progenitors (GMPs) compared to wild-type BM cells. These observations suggest that heterozygous U2af1 mutation leads to differentiation defects of HSCs, which however, is not sufficient for the development of MDS. We next assessed the phenotype of U2af1-mutated BM cells in transplantation settings to evaluate the effect of increasing replicative stress, which has been shown to substantially affect the behavior of normal and abnormal stem cells. In PB, progressive leukopenia, macrocytic anemia and decreased platelet counts were observed in mutant mice in transplantation settings. Surprisingly, all of the U2af1 mutant-transplanted mice died within two months after transplantation due to severe bone marrow failure. Cytological analysis of BM cells revealed morphological abnormalities in U2af1 mutant-transplanted mice, including hypersegmentation in neutrophils and erythroid dysplasia. Flow cytometrical analysis revealed decreased numbers of HSCs, CMPs and MEPs, and increased number of GMPs. These observations suggest that the U2af1 mutation leads to ineffective hematopoiesis and morphological abnormalities, which seems to recapitulate the phenotype of MDS in transplantation settings. Subsequently, we assessed the reconstitution capacity of whole BM cells from U2af1 mutant mice in competitive transplantation experiments. The donor chimerism of U2af1 S34F-derived cells in PB was remarkably reduced compared to that of wild-type cells. At four months post transplantation, the chimerism of U2af1 S34F-derived cells was markedly lower than that of wild-type cells in the fractions of HSCs, CMPs, MEPs, GMPs and common lymphoid progenitors (CLPs) in BM. RNA sequencing analysis of HSCs defined as Kit+Sca-1+Linlow (KSL) cells and CMPs from the mutant mice showed significant changes in alternative splicing and expression levels in many genes, including several potential targets implicated in the pathogenesis of hematopoietic malignancies. In summary, our results demonstrated that heterozygous U2af1 S34F mutation led to impaired HSC functions that was evident from reduced competitive repopulation and deregulated hematopoietic differentiation, which were augmented in transplantation settings. Our mice model provides a valuable tool to understand the molecular pathogenesis of U2af1-mutated myeloid neoplasms. Disclosures Nakagawa: Sumitomo Dainippon Pharma Co., Ltd.: Research Funding.

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.

Abstract: Blood-specific expression of the Srsf2 P95H mutant results in decreased stem/progenitor cell numbers and a reduced repopulation capacity. Srsf2 P95H mutation by itself is not sufficient to develop MDS but contributes to the MDS phenotype in transplantation settings.