1 PROJECT SUMMARY 2 3 Hematopoietic stem and progenitor cells (HSPCs) are long-lived immature cells that retain the potential to 4 generate all blood cell lineages including erythroid, myeloid, and lymphoid. HSPC dysregulation is a major driver 5 of myeloid malignancies including myelodysplastic syndromes (MDS). This disorder is a significant health 6 problem with approximately 10,000 people per year diagnosed with MDS in the United States. The ATPase 7 DEAD-box Helicase 41 (DDX41) gene is mutated in an inherited form of adult-onset MDS, providing a compelling 8 model to investigate the mechanism of MDS progression. These mutations are heterozygous and loss-of- 9 function, indicating that diminished DDX41 levels contribute to MDS etiology. Individuals with DDX41 mutations 10 take years to develop MDS, suggesting that features of aging contribute to the asymptomatic-to-smoldering 11 malignancy transition. Human and zebrafish DDX41 are highly conserved. Like humans, ddx41 heterozygous 12 zebrafish begin to display altered hematopoietic features such as elevated HSPC frequency, shifts in myeloid 13 cell composition, and dysplasia. These findings implicate aspects of aging contribute to disease evolution in our 14 model similar to humans. I hypothesize that environmental alterations synergize with genotype-specific factors 15 to promote HSPC dysfunction. Previously, we showed that HSPC expansion in homozygous ddx41 zebrafish 16 mutants during embryogenesis was driven by cGAS-STING activation. This pathway is associated with age- 17 driven inflammation; thus, we will test the hypothesis that aging and cGAS-STING activation could play a role in 18 age-associated HSPC expansion in ddx41 heterozygous animals. Neutrophils are also key regulators of HSPC 19 function. Aging and mutations in MDS-associated factors are linked with neutrophil dysfunction. We will test the 20 model that ddx41 heterozygosity alters neutrophil functions and that this dysfunction contributes to age 21 associated HSPC expansion. My findings have the potential to uncover new ways to prevent MDS progression 22 and will also provide valuable insight into fundamental mechanisms controlling HSPC homeostasis.