Project Summary/ Abstract While substantial progress has been made to delineate the somatic mutations that drive myeloproliferative neoplasms (MPNs), epidemiologic studies demonstrate a significant heritable risk for disease acquisition. We have recently conducted the largest genome-wide association study of MPNs to define inherited risk variants that promote the acquisition of this disease. Our initial analyses suggest a role for modulation of HSC self- renewal and function by these inherited risk variants. However, we lack a complete mechanistic understanding of how the selective pressures that arise from these inherited variants can promote MPN acquisition. Here, in this R01, we specifically want to understand how inherited risk variants can provide selective pressures to enable MPN initiation. In the first aim of this grant, we seek to use rigorous mouse models to define how a variant- containing HSC-selective enhancer of Gfi1b can promote MPN acquisition. We will study how this enhancer plays a role in native hematopoiesis to promote MPN initiation, as well as how this enhancer can cooperate with somatic mutations in this process. In the second aim of this grant, we seek to understand how loss-of-function of CHEK2 can promote MPN acquisition and clonal hematopoiesis by coupling genome editing of primary human hematopoietic stem/ progenitor cells and whole genome sequencing to assess how such germline genetic mutations can promote oncogenesis. Finally, in the third aim, we seek to more holistically define how variant- harboring regulatory elements and target genes may play a role in HSC self-renewal and function through the use of a multiplexed guide-swap Cas9 genome editing approach in primary human hematopoietic stem and progenitor cells. Collectively, the studies we propose in this grant will provide new insights into how inherited risk variants can provide selection pressures that enable MPNs to arise.