Project Summary Increased evidence started to show that many germline mutations predispose individuals to myeloid neoplasms, including myelodysplastic syndromes (MDS). One of these mutations occurs on DEAD-box helicase 41 (DDX41) gene. More than 80 distinct DDX41 variants have been reported, making DDX41 one of the most frequently mutated MDS predisposition genes. Germline DDX41 mutations lead to its loss of function, whereas somatic mutations often produce hypomorphic changes. Our understanding of the roles of DDX41 in the hematopoietic system and how DDX41 mutations predispose myeloid neoplasms is still in its infancy. It is essential to dissect the molecular and cellular mechanisms of DDX41’s functions, which is critical to developing targeted therapy for DDX41 mutated MDS. G-quadruplexes (G4) are noncanonical secondary nucleic acid structures formed in guanine-rich sequences. DDX family proteins have been shown to resolve DNA G4 structures. We found in our preliminary study that loss of DDX41 led to aberrant upregulation of G4 in the hematopoietic cells, especially the erythroid lineage. Aberrant G4 accumulation due to DDX41 deficiency severely disrupted erythropoiesis with fewer effects in other lineages in vitro and ex vivo. Importantly, we confirmed these findings in vivo using lineage- specific Ddx41 knockout (KO) mice, which showed that the lethality of hematopoietic-specific Ddx41 KO mice is likely due to the defects in erythropoiesis. Our preliminary mechanistic studies indicate that DDX41 directly binds to and dissolves G4. Ddx41 deficiency-mediated G4 upregulation compromised erythroid genomic integrity and ribosomal biogenesis, which upregulates p53 and activates the cGAS-STING pathway. Our additional in vivo preliminary experiments reveal that the knockout of cGas, but not p53, rescued the lethality of hematopoietic- specific Ddx41 KO mice. Based on these data, we hypothesize that DDX41 loss of function in MDS causes aberrant G4 accumulation, which leads to genomic instability and cGAS-mediated cell death predominantly in erythroid cells. To test our hypothesis, we propose three specific aims. Aim 1 will focus on the study of the functions of DDX41 in various lineages of hematopoietic cells by generating lineage-specific Ddx41 knockout mice. In Aim 2, we will investigate the functions of DDX41 in G4 homeostasis in erythropoiesis. Aim 3 will focus on the mechanism of DDX41 deficiency-induced ineffective erythropoiesis and MDS pathogenesis. Successful completion of our proposed research will provide novel insights into DDX41’s functional role in normal and abnormal hematopoiesis, which will be impactful for the development of novel therapies for DDX41 mutation- related hematologic diseases.