Project Summary N6-methyladenosine (m6A) is the most abundant RNA modification and plays key roles in RNA metabolism. m6A writers and readers are highly expressed in hematopoietic stem and progenitor cells and dysregulated in myelodysplasia and leukemia. We show that loss of Mettl3 in hematopoietic stem and progenitor cells (HSPCs) resulted in upregulation of an anti-viral innate immune response signature as well as marked proliferation and differentiation defects. In particular, we are the first to report that loss of m6A RNA modification resulted in aberrant dsRNA formation, leading to activation of the MDA5-RIG-I, PKR-eIF2A, and OAS-RNAse L pathways. By differential immunoprecipitation of dsRNAs from Mettl3 KO versus WT cells, we identified a specific subset of long, normally highly m6A modified transcripts with low folding energies, signifying propensity to form extensive RNA secondary structures. Disruption of innate immune signaling via deletion of Mavs or knockdown of Rnasel in part rescued colony formation in vitro and engraftment in competitive transplantation assays in vivo. These data suggest that one of the roles of m6A RNA modifications is to maintain single-strandedness of endogenous RNAs whereby they mark RNAs as “self” and distinguish them from exogenous pathogen-derived dsRNAs. In this SHINE II application we propose to dissect the role of m6A RNA modification in HSPCs specifically as it pertains to its role as suppressor of aberrant innate immune signaling. We will determine the mechanism by which METTL3-mediated m6A RNA modification prevents abnormal dsRNA formation and consequent induction of interferon signaling, we will dissect how m6A loss results in a deleterious immune response that disrupts HSPC function, and we will assess the universality this phenomenon across hematopoietic lineages.