Hematopoiesis impact in germline RUNX1 mutation carriers: of inflammation and the bone marrow niche PROJECT SUMMARY/ABSTRACT Germline variants that confer risk for leukemia have been recognized increasingly with 15-20% of acute leukemia patients having at least 1 first-degree relative afflicted with leukemia. Germline variants are now included for the first time in the revised leukemia classification scheme outlined by the World Health Organization, and new clinical guidelines now include testing for inherited susceptibility as a critical element of patient diagnostics. It is devastating for these patients and their family members to live with the knowledge of having increased risk for developing cancer in their lifetimes. Thus, for these patients, early cancer detection, disease monitoring, and prevention would be life-changing, but require a comprehensive understanding of molecular changes that occur prior to overt leukemia. The long-term goal of this study is to identify the mechanisms that regulate hematopoiesis at the premalignant stage in patients with germline RUNX1 mutations [also called familial platelet disorder (FPD)]. These individuals have life-long thrombocytopenia, qualitative platelet aggregation defects, and a risk of developing a variety of hematopoietic malignancies. The acquisition of secondary mutations occurs over time in FPD patients leading to leukemia development. However, the intrinsic and/or extrinsic factors that render pre-leukemic cells vulnerable to acquire secondary mutations are unknown. To fill this knowledge gap, we performed single-cell transcriptome profiling of primary FPD bone marrow samples and identified unique transcriptional changes in FPD progenitors compared to healthy controls. Consistent these changes, FPD progenitors have impaired megakaryocytic but enhanced myeloid differentiation. Cytokine profiling of bone marrow hematopoietic and mesenchymal stromal cells (MSCs) show that there is increased cytokine production within the bone marrow, leading to chronic inflammatory stress, which may confer a growth advantage to FPD progenitors. Moreover, FPD stromal cells also show defective differentiation, clonogenic capacity, and deregulated gene expression that may further promote an inflammatory state. Thus, we hypothesize that early transcriptomic changes in FPD stem/progenitors cooperate with inflammatory microenvironmental signals to provide a growth advantage to these pre-leukemic cells and alter their differentiation. Wewill test our hypothesis using the following aims: 1)Identify how early transcriptomic changes alter hematopoiesis of FPD cells. 2) Determine the impact of inflammatory cytokines and their blockade in FPD evolution. 3) Assess the role of the FPD stromal microenvironment on growth and differentiation of FPD and healthy stem/progenitors. To achieve our goals, we have assembled several tools, including primary stem/progenitor/and stromal cells, iPSCs, xenograft and transgenic mouse models,...