ALBERT

Description: Introduction The current standard to assess chemotherapy tolerability relies on patient self-reporting. However, as the sole mechanism of managing symptom burden, this may be inconsistent and fraught with bias. Mobile wearable health devices have the ability to monitor and aggregate objective activity and sleep data over long periods of time, but have not been systematically used in the oncology clinic. The aim of the study was to assess whether the use of mobile wearable technology establishes patterns of "sleep" and "wake" states in newly diagnosed Multiple Myeloma (NDMM) patients receiving therapy, and whether these patterns differ over time. Methods Patients presenting to the myeloma clinic at Memorial Sloan Kettering Cancer Center (MSKCC) with a new diagnosis of Multiple Myeloma and smart phone or tablet (iOS or Android) compatible with the Garmin Vivofit device were offered to participate in a mobile wearable bio-monitoring study. All eligible participants were required to receive primary chemotherapy treatment at a MSKCC facility. Treatment was determined by physician. NDMM patients were assigned to one of two cohorts (20 in each; Cohort A - patients

Description: Introduction Pediatric chronic myeloid leukemia (CML) accounts for 10 to 15% of children with myeloid leukemia and 2 to 9% of all pediatric leukemias. Prior to the discovery of tyrosine kinase inhibitors (TKI) such as imatinib, stem cell transplantation was the only curative treatment for both adults and children with CML. However, due to the small numbers of patients, standardized treatment approaches for pediatric CML have not been established. There are several unique characteristics of CML diagnosed in children and adolescents, and young adults (AYA; 16-29 years), compared to adults. Children and AYA with CML present with a higher white blood count and have larger spleens, higher peripheral blast counts, and lower hemoglobin levels, suggesting that the biology of pediatric CML is different than adult CML. In addition, potential side effects of TKIs unique to pediatric CML patients include impaired bone growth, fertility and immune function, however none have been extensively studied. We hypothesize that the differences in clinical presentation of pediatric CML patients are due to unique molecular characteristics that are absent in adult CML patients. To test this hypothesis, we studied the transcriptomic signature of pediatric CD34+ CML cells compared to adult CML and normal age-matched bone marrow CD34+ cells. Methods CD34+ cells were isolated from pediatric CML (n=7), adult CML (n=8), pediatric normal (n=2) and adult normal (n=3) bone marrow samples. Total RNA was isolated from cells, and then cDNA libraries were generated. Prepared libraries were sequenced on the Illumina HiSeq 4000 instrument. We aligned reads using the HISAT2 alignment software, and mapped to genes with HT-Seq. We removed genes that had zero reads across all the samples, resulting in a set of 4,696 genes that were detected in one or more samples. In case of technical replicates, we used mean of replicates. We performed three differential expression comparisons with edgeR: (1) Pediatric CML vs Adult CML, (2) Adult CML vs Adult Normal, and (3) Pediatric CML vs Pediatric Normal. We used a False Discovery Rate (FDR) of £ 20% and absolute log2 fold-change ³ 1 for selecting differentially expressed genes in each comparison. We used Fisher's exact test to identify significant KEGG pathways for the differentially expressed genes in each comparison. Results Pediatric CML vs Adult CML We found 24 differentially expressed genes (15 over- and 9 under-expressed). Though no pathway was found to be significant at the false discovery rate (FDR) £ 20%, we identified a number of sub-pathways that are relevant. For example, the Chemokine Signaling pathway shows at the top of the list (ordered by raw p-value) because of two genes, XCR1 and HCK, associated with VEGF and MAPK pathways involved in cell proliferation, angiogenesis, DNA repair, and cancer pathogenesis. Adult CML vs Adult Normal We found 60 genes (30 over- and 30 under-expressed) differentially expressed when comparing adult CML patients to normal adults. Ten genes overlapped with 24 genes we identified when comparing pediatric and adult CML patients. We found 11 pathways as significant at FDR £ 10%. Multiple pathways, including Cell adhesion, allograft rejection, Graft versus Host Disease, and Type I diabetes pathways, showed downregulation of MHC, with subsequent downstream reduction in expression of apoptosis-related genes. The IL-17 pathway makes sense, as MAPK, well-known to be associated with various cancers, is down-regulated. Lastly, in the NK pathway the gene DAP12 is up-regulated. This gene is known as a tyrosine kinase binding protein, and although tyrosine kinase inhibitors are the standard treatment for CML, the role of DAP12 in relation to leukemia has not yet been described. Pediatric CML vs Pediatric Normal We found 509 genes (350 over- and 159 under-expressed) differentially expressed in pediatric CML patients compared to normal. Interestingly, transcriptional regulators are differentially enriched in the hematopoietic stem cell differentiation function group including GATA1, GATA2, KLF1 and KLF2. RFC is down-regulated. RFC is a mismatch repair gene known to be involved in colorectal cancer. Many of the significant pathways are involved in glucose and fatty acid metabolism. Our pilot study identified novel molecular features of pediatric CML bone marrow stem cells, providing new insights into the novel biomarkers and pathogenesis of pediatric CML. Disclosures Gotlib: Blueprint Medicines: Consultancy, Honoraria, Research Funding; Promedior: Research Funding; Deciphera: Consultancy, Honoraria, Research Funding; Incyte: Consultancy, Honoraria, Research Funding; Kartos: Consultancy; Celgene: Consultancy, Honoraria, Research Funding; Gilead: Consultancy, Research Funding; Novartis: Consultancy, Honoraria, Research Funding.

Description: Introduction: The optimal clinical setting and cell product characteristics for chimeric antigen receptor (CAR) T cell therapy in multiple myeloma (MM) are uncertain. In CLL patients treated with anti-CD19 CAR T cells (CART19), prevalence of an early memory (early-mem) T cell phenotype (CD27+ CD45RO- CD8+) at time of leukapheresis was predictive of clinical response independently of other patient- or disease-specific factors and was associated with enhanced capacity for in vitro T cell expansion and CD19-responsive activation (Fraietta et al. Nat Med 2018). T cell fitness is therefore a major determinant of response to CAR T cell therapy. In an accompanying abstract (Cohen et al.), we report that higher percentage of early-mem T cells and CD4/CD8 ratio within the leukapheresis product are associated with favorable clinical response to anti-BCMA CAR T cells (CART-BCMA) in relapsed/refractory MM. Here, we compare leukapheresis samples from MM patients obtained at completion of induction therapy (post-ind) with those obtained in relapsed/refractory (rel/ref) patients for frequency of early-mem T cells, CD4/CD8 ratio, and in vitro T cell expansion. Methods: Cryopreserved leukapheresis samples were analyzed for the percentage of early-mem T cells and CD4/CD8 ratio by flow cytometry and in vitro expansion kinetics during anti-CD3/anti-CD28 bead stimulation. Post-ind samples were obtained between 2007 and 2014 from previously reported MM trials in which ex-vivo-expanded autologous T cells were infused post-ASCT to facilitate immune reconstitution (NCT01245673, NCT01426828, NCT00046852); rel/ref samples were from MM patients treated in a phase-one study of CART-BCMA (NCT02546167). Results: The post-ind cohort includes 38 patients with median age 55y (range 41-68) and prior exposure to lenalidomide (22), bortezomib (21), dexamethasone (38), cyclophosphamide (8), vincristine (2), thalidomide (8), and doxorubicin (4); median time from first systemic therapy to leukapheresis was 152 days (range 53-1886) with a median of 1 prior line of therapy (range 1-4). The rel/ref cohort included 25 patients with median age 58y (range 44-75), median 7 prior lines of therapy (range 3-13), and previously exposed to lenalidomide (25), bortezomib (25), pomalidomide (23), carfilzomib/oprozomib (24), daratumumab (19), cyclophosphamide (25), autologous SCT (23), allogeneic SCT (1), and anti-PD1 (7). Median marrow plasma cell content at leukapheresis was lower in the post-ind cohort (12.5%, range 0-80, n=37) compared to the rel/ref cohort (65%, range 0-95%). Percentage of early-mem T cells was higher in the post-ind vs rel/ref cohort (median 43.9% vs 29.0%, p=0.001, left figure). Likewise, CD4/CD8 ratio was higher in the post-ind vs rel/ref cohort (median 2.6 vs 0.87, p2 lines of therapy prior to apheresis (n=3) compared to the rest of the cohort (n=35). Conclusion: In MM patients, frequency of the early-mem T cell phenotype, a functionally validated biomarker of fitness for CAR T cell manufacturing, was significantly higher in leukapheresis products obtained after induction therapy compared to the relapsed/refractory setting, as was CD4/CD8 ratio and magnitude of in vitro T cell expansion. This result suggests that CAR T cells for MM would yield better clinical responses at early points in the disease course, at periods of relatively low disease burden and before exposure to multiple lines of therapy. Figure. Figure. Disclosures Garfall: Novartis: Research Funding; Kite Pharma: Consultancy; Amgen: Research Funding; Bioinvent: Research Funding. Cohen:GlaxoSmithKline: Consultancy, Research Funding; Kite Pharma: Consultancy; Oncopeptides: Consultancy; Celgene: Consultancy; Novartis: Research Funding; Poseida Therapeutics, Inc.: Research Funding; Bristol Meyers Squibb: Consultancy, Research Funding; Janssen: Consultancy; Seattle Genetics: Consultancy. Fraietta:Novartis: Patents & Royalties: WO/2015/157252, WO/2016/164580, WO/2017/049166. Davis:Novartis Institutes for Biomedical Research, Inc.: Patents & Royalties. Levine:CRC Oncology: Consultancy; Brammer Bio: Consultancy; Cure Genetics: Consultancy; Incysus: Consultancy; Novartis: Consultancy, Patents & Royalties, Research Funding; Tmunity Therapeutics: Equity Ownership, Research Funding. Siegel:Novartis: Research Funding. Stadtmauer:Janssen: Consultancy; Amgen: Consultancy; Takeda: Consultancy; Celgene: Consultancy; AbbVie, Inc: Research Funding. Vogl:Karyopharm Therapeutics: Consultancy. Milone:Novartis: Patents & Royalties. June:Tmunity Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding; Tmunity Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding; Immune Design: Membership on an entity's Board of Directors or advisory committees; Novartis Pharmaceutical Corporation: Patents & Royalties, Research Funding; Celldex: Consultancy, Membership on an entity's Board of Directors or advisory committees; Immune Design: Membership on an entity's Board of Directors or advisory committees; Novartis Pharmaceutical Corporation: Patents & Royalties, Research Funding. Melenhorst:Novartis: Patents & Royalties, Research Funding; Incyte: Research Funding; Tmunity: Research Funding; Shanghai UNICAR Therapy, Inc: Consultancy; CASI Pharmaceuticals: Consultancy.

Description: Introduction: Although proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and PI+IMiD combinations have become the standard of care in multiple myeloma (MM), they are not considered curative. Most patients (pts) eventually relapse or become refractory to treatment, and outcomes are poor after treatment failure. A median overall survival (OS) of 13 mo was reported in pts with relapsed/refractory MM (RRMM) who were double-refractory to a PI and IMiD and received ≥3 prior therapies (Kumar et al, Leukemia 2017). Recently developed therapies with novel mechanisms of action (MoAs), including the monoclonal antibodies (mAbs) elotuzumab and daratumumab, are associated with prolonged progression-free survival (PFS) in clinical trials of RRMM (Dimopoulos et al, Cancer in press; NEJM 2016). However, there is little up-to-date information on real-world survival outcomes. Herein we report survival patterns across key milestone timepoints in pts with RRMM in a real-world clinical setting. Methods: PREAMBLE (NCT01838512) is an ongoing international, prospective, observational cohort study. Adults with RRMM who were not participating in a clinical trial at the time of enrollment, had ≥1 prior therapy and documented disease progression, and began treatment with a PI, IMiD, PI+IMiD combination, or newer agent (those with newer MoAs, such as mAbs, histone deacetylase inhibitors, and novel combinations) ≤90 d before to 30 d after informed consent, were included and followed for up to 3 years. During the observational period, data were collected at each healthcare visit, and vital status was assessed every 6 mo throughout the follow-up period. Clinical efficacy and patient characteristic data were collected by healthcare providers using electronic case report forms. Descriptive statistics were used to describe pt characteristics and Kaplan-Meier plots were generated for time-to-event data. Multivariate Cox regression, with stepwise variable selection to select independent variables, was used to assess the association between baseline characteristics and PFS or OS. Results: At database lock (Nov 30, 2017), 1159 pts with RRMM were enrolled from the USA (31%) and Europe (67%), and included in this analysis. After a median (IQR) follow-up of 12.4 (6.3-24.9) mo, 39% of pts were still on study and 34% had died; most deaths (67%) were due to MM progression. At enrollment, median (range) age was 69 (34-92) y; 77% of pts had relapsed disease and 21% refractory disease. Among pts with a medical history, the most common comorbidities were vascular disorders (49%), metabolism and nutrition disorders (34%), and musculoskeletal/connective tissue disorders (29%). Pts received a median of 2 prior therapies before enrollment, with 50% having received an IMiD, 61% a PI, 21% a PI+IMiD, and 2% a newer agent. At enrollment, 45% of pts were receiving an IMiD, 41% a PI, 10% a PI+IMiD, and 4% a newer agent. Since enrollment, a total of 58% of pts received an IMiD, 49% a PI, 16% a PI+IMiD, and 10% a newer agent (including regimen received at enrollment). Median (95% CI) PFS was 11.4 (10.5-12.2) mo and OS was 33.7 (28.5-37.5) mo. Survival rates at milestone timepoints are shown in Table 1. Among pts who died due to MM progression (n=266), MM-related mortality was 56% at 12 mo, 85% at 24 mo, and 97% at 36 mo. In multivariate analysis, risk of disease progression/death was lower in pts with relapsed vs refractory disease at baseline (hazard ratio [HR] 0.82; 95% CI 0.68-0.99; p=0.0425) and higher in pts who had received 2-3 (HR 1.37; 95% CI 1.15-1.64; p=0.0004) or ≥4 (HR 1.54; 95% CI 1.22-1.95; p=0.0003) vs 1 prior therapy. Multivariate analysis for OS identified younger age, female sex, non-white race, relapsed disease, early-stage disease, fewer prior therapies, and the absence of baseline cardiac or gastrointestinal disorders as independent predictors of longer OS (Table 2). Conclusions: Real-world data suggest that over the last decade, survival has improved for pts with RRMM; however, OS remained low (48%) after 3 years in this analysis and most deaths were attributed to MM. Baseline comorbidities may also increase the risk of death and should be carefully considered when selecting treatments. Newer treatment options are still needed to further extend survival in pts with RRMM. Study support: Bristol-Myers Squibb (BMS). Writing support: Janice Zhou, Caudex, funded by BMS. Disclosures Cook: Bristol-Myers Squibb: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Takeda: Consultancy, Honoraria, Research Funding, Speakers Bureau; Seattle Genetics: Honoraria; Celgene Corporation: Consultancy, Honoraria, Research Funding, Speakers Bureau; Amgen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Sanofi: Consultancy, Honoraria, Speakers Bureau; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Glycomimetics: Consultancy, Honoraria. Cella:Novartis: Consultancy, Research Funding; Janssen Global services: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding. Chen:Bristol-Myers Squibb: Employment. Davis:Bristol-Myers Squibb: Employment. Durie:Amgen: Consultancy; Celgene: Consultancy; Takeda: Consultancy; Janssen: Consultancy. Goldschmidt:Novartis: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Research Funding; Chugai: Honoraria, Research Funding; Mundipharma: Research Funding; Sanofi: Consultancy, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Research Funding; Adaptive Biotechnology: Consultancy; ArtTempi: Honoraria. Moreau:Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Vij:Karyopharma: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Jazz Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Jansson: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Description: Key Points The genetic cause of SCID impacts on survival and immune reconstitution and should be considered in tailoring HCT for individual patients. Total and naive CD4+ cell counts in SCID patients 6 and 12 months post-HCT predict long-term survival and sustained immune reconstitution.

Description: The Thrombopoietin (THPO)/ thrombopoietin receptor (MPL) signaling axis is not only critical for the generation of platelets and megakaryocytes, but also for the maintenance of hematopoietic stem cells (HSC) and the bone marrow niche. MPL is expressed on primitive HSC, HSC progenitors, megakaryocytes, platelets, osteoblasts and osteoclasts, clonal hematopoietic stem cells and many leukemias. THPO production is constitutive but is also increased by inflammatory cytokines. Sustained exposure to high levels of THPO not only enhances platelet production, but also has a profound effect on HSC and the bone marrow microenvironment. Excess THPO/MPL signaling, whether driven by inflammatory cytokines, or due to mutations in THPO, JAK2, MPL or CALR, is associated with HSC expansion, megakaryocyte hypertrophy and increased platelet count, excess release of megakaryocyte and platelet-based cytokines such as TGF-beta and PDGF-alpha, and the development of stromal myofibroblasts that drive tissue fibrosis and anemia. We developed a robust and clinically validated RNAi therapeutics platform for the delivery of siRNAs to the liver using trivalent N-acetylgalactosamine (GalNAc) conjugates, enabling specific silencing of hepatocyte-expressed genes following subcutaneous injection. Since liver is the major source of THPO expression, we utilized GalNAc-siRNA technology to develop siRNAs targeting THPO for evaluation in wild type mice and murine models of myeloproliferative neoplasms (MPN). Active siRNAs were identified by in vitro screening in primary mouse hepatocytes and the 12 best siRNAs were evaluated in vivo in normal mice to select the most potent siRNA. THPO liver mRNA levels were reduced by up to 80% after a single subcutaneous THPO siRNA dose, with no effect on THPO mRNA expression in other organs (kidney, and bone marrow, both of which had marginal THPO expression compared to liver). Circulating TPO levels were reduced by 80% by day 7 and were suppressed for up to 28 days post a single dose treatment. Platelet counts were reduced to 60% of baseline by day 14, and a further reduction to more than 70% of baseline was observed with every other week dosing. No changes in red blood cell or white blood cell subsets were observed. Platelet reduction was accompanied by a reduction in megakaryocyte mass, as evidenced by a 50% decrease in the number of bone marrow megakaryocytes in THPO siRNA treated mice compared to controls. Mice treated every other week with TPO siRNA for three months demonstrated sustained circulating THPO protein and platelet count reductions, and a significant reduction in bone marrow HSC, Lin-Sca1+Kit- (LSK) and multipotent progenitor (MPP) frequency. Evaluation of impact THPO silencing on MPN phenotypes in transgenic JAK2V617F mice is ongoing. THPO silencing is a potential novel targeted therapeutic approach that may be beneficial in benign and malignant conditions in which deregulated THPO/MPL signal transduction drives disease pathology. Disclosures No relevant conflicts of interest to declare.

Description: Background: Financial Toxicity (FT) is increasingly recognized as a major contributor to morbidity and mortality in a variety of cancers. Treatment of acute leukemia is associated with heavy healthcare utilization and high costs. The purpose of this study was to define rates, risk factors, and mortality implications for FT in patients with acute leukemia using patient reported data. Methods: All patients seen at the Levine Cancer Institute, a tertiary hospital-based leukemia practice, were surveyed prior to each visit over a six-month period. All patients were aged ≥18 years and were diagnosed with acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL). The survey consisted of the PROMIS Global-10 measure and two questions from the COST measure. FT was defined as scoring 4 or less (maximum: 10) in agreement with the COST questions: "I know that I have enough money in savings, retirement, or assets to cover the costs of my treatment" and "I am satisfied with my current financial situation." Demographic data and disease characteristics were abstracted from the medical record. Model selection was carried out using logistic regression to identify factors impacting the incidence of financial toxicity. Correlation of numerical financial toxicity scores with PROMIS scores and with mortality data was assessed using linear regression. Results: Of the 106 patients, 58 (54%) met the definition of exhibiting FT. The factors associated with incidence of FT included: age, race, and insurance type. The odds of FT in those patients 4 compared to patients with FT scores

Description: Introduction: Despite therapeutic advances and the increasing number of available regimens, multiple myeloma (MM) remains largely incurable, with a decreased durability of response with successive therapies (Majithia et al, Leukemia 2016). The monoclonal antibodies (mAbs) daratumumab (dara) and elotuzumab (elo), targeting CD38 and SLAMF7, respectively, have demonstrated significant efficacy in relapsed/refractory (RR) MM and may lead to a paradigm change in the treatment of RRMM. In the USA, dara is indicated for the treatment of RRMM when given in combination with dexamethasone (dex) plus either lenalidomide (len; Ld) or bortezomib for patients (pts) with ≥1 prior line of therapy (LoT). Dara in combination with pomalidomide/dex (pom; Pd) was also recently approved for pts with RRMM after ≥2 prior LoTs, including len and a proteasome inhibitor (PI), on the basis of data from a single-arm trial that demonstrated a median progression-free survival of 9.9 mo (Facon et al, Blood 2017), and as a monotherapy in pts with ≥3 prior LoTs. Elo, combined with Ld, is approved for treatment of RRMM after 1-3 LoTs. In ELOQUENT-3 (NCT02654132), an ongoing phase 2 randomized study in pts with RRMM after failure of len and a PI, elo plus Pd was associated with a 46% reduction in risk of progression or death vs Pd alone (Dimopoulos et al. EHA 2018 [LB2606]). There is a lack of current data on the array of treatments received by pts with RRMM after failing mAb therapy. This study evaluated treatment sequences among pts with RRMM after failure of dara-based therapy. Methods: Pts from the USA aged ≥18 y with RRMM, who received dara in their second to sixth LoT from November 2015 onward, were identified from PREAMBLE (NCT01838512), an ongoing, prospective, observational study, and the McKesson electronic medical record (EMR) database. Pts who had received a prior mAb were excluded. Pts were followed until database lock (PREAMBLE, April 2018; McKesson EMR, May 2018). Baseline demographics and clinical characteristics were assessed using descriptive analysis, and statistical comparisons were made using t or Mann-Whitney U tests (continuous variables) and chi-square tests (categorical variables). Kaplan-Meier analyses were used to estimate duration of therapy (DoT). Results: In total, 1016 pts received dara as their first mAb in their second to sixth LoT. Baseline characteristics are shown in Table 1A. The most common dara-based regimen was dara plus an immunomodulatory drug (IMiD; 37% of pts, of whom 50% received dara plus pom), followed by dara plus dex and/or chemotherapy (32%), dara plus a PI (27%), dara plus a PI and an IMiD (4%), and dara plus panobinostat (

Description: Myelodysplastic syndromes (MDS) and leukemias require the acquisition of multiple mutations during disease development resulting in clonal diversity and different responses. Splicing factors, transcription factors, epigenetic regulators, and cell signaling proteins are the common molecular events mutated during disease evolution and those events rarely occur alone. However, it remains unclear how the combinations of mutations in different categories may have cooperative effects in gene regulation and disease etiology. Mutations in the splicing factor SRSF2 and the transcription factor RUNX1 are closely associated in MDS patients, and their co-existence is linked to poor prognosis. To understand the functional contribution of the coexistence in vivo, we utilized Mx1-Cre based conditional knock-in Srsf2-P95H mutation (P95H/+) mice, and Mx1-Cre based Runx1 conditional knockout mice (Runx1 f/f). We crossed these two strains to establish a new mouse model with inducible double mutations (Srsf2 P95H/+ Runx1Δ/Δ). Double mutant mice showed pancytopenia with MDS features including severe leukopenia in multiple lineages, macrocytic anemia, thrombocytopenia, and dysplastic morphology in peripheral blood. Double mutant mice also displayed more dramatic skewing toward the myeloid lineage at the expense of the B cell lineage when compared to single mutant mice. In competitive bone marrow transplantation assays, SRSF2 P95H cooperated with RUNX1 deficiency to confer a competitive disadvantage in vivo. To investigate the mechanistic basis of this cooperation, differential splicing and gene expression were assessed by RNA sequencing of Lineage- c-kit+ cells isolated from WT, SRSF2 P95H, RUNX1 KO, and Double mutant bone marrow cells. Interestingly, deletion of the Runx1 gene alone resulted in significant changes to RNA splicing in 1120 genes, while the SRSF2 P95H mutation itself induced splicing changes in 935 genes. Furthermore, 2468 splice junctions in 1677 genes showed splicing changes in double mutant samples compared to wildtype controls. Among these altered splicing events, intriguingly, exon skipping was the major alteration in single and double mutants. Furthermore, the double mutant demonstrated increased aberrant splicing events when compared to the single mutants alone. We performed pathway analysis using the differentially spliced genes identified in double mutant cells. Pathways in cancer, DNA replication/repair, cell death and survival, hematological disease and inflammatory response were enriched. Splicing changes were detected in genes recurrently mutated in blood malignancies, including Fanca, Fance, Fancl, Ezh2, Atm, Gnas, Braf, Bcor, Fyn, and Wsb1 as well as in genes critical for splicing regulation, such as Srsf6, Fus, Hnrnpa2b1, and Srrm2. Gene expression analysis revealed 869 significantly differentially expressed genes in double mutant cells. Within the events in the double mutant population, 60% of the differentially expressed genes were also observed in RUNX1 single mutant cells, while only 2% of the differentially expressed genes were observed in SRSF2 single mutant cells, and 38% of the differentially expressed genes were uniquely presented in the double mutant cells. These results suggest that the gene expression program is heavily affected by loss of RUNX1 and the coexistence of an SRSF2 mutation contributes to certain synergistic effects in transcriptional regulation. Furthermore, we identified 101 genes that showed both differential splicing and expression, including Jak3, Jag2, Csf3r, Fcer1g, CD244 which are important in hematologic disorders. Together, these results suggest that the deficiency of compound RUNX1 and SRSF2 P95H mutations impairs multi-lineage hematopoiesis and exacerbates the disease phenotypes caused by single mutations alone. At the genome-wide level, loss of the transcription factor RUNX1 itself dysregulates splicing outcomes and cooperates with the splicing factor SRSF2 P95H mutation to further perturb the expression and splicing of key regulators involved in hematopoietic stem/progenitor cell development, inflammatory responses, DNA damage, and RNA splicing. Disclosures No relevant conflicts of interest to declare.