The functional role of DDX41 in myelodysplastic syndromes

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Project Summary Myelodysplastic syndromes (MDS) are a group of hematologic malignancies characterized by clonal hematopoiesis, one or more cytopenias, and increased risks to transform into acute myeloid leukemia (AML). Multiple mutations have been identified to be associated with MDS. One of these mutations occurs on DEAD- box helicase 4 (DDX41) gene. At least 70 DDX41 germline or somatic mutations 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. Recent studies in zebrafish showed that knockout of DDX41 induces increased R-loop accumulation and aberrant hematopoiesis. Another research using conditional DDX41 knockout mice reported that DDX41 biallelic mutations disrupt snoRNA biogenesis and lead to apoptosis in hematopoietic cells. These and other studies started to shed lights on the mechanisms of DDX41 mutated pathogenesis of MDS. However, it still remains unknown how loss of DDX41 in hematopoietic cells causes apoptosis and what lineages are most sensitive to its deficiency. Therefore, it is essential to dissect the molecular and cellular mechanisms of DDX41’s functions, which could be helpful to develop targeted therapy for DDX41 mutated MDS. G-quadruplexes (G4) are four-stranded, noncanonical secondary DNA structures formed in guanine-rich sequences. We showed in our preliminary study that loss of DDX41 led to aberrant upregulation of G4 in the hematopoietic system. We further revealed that G4 is enriched during erythropoiesis. G4 accumulation due to DDX41 deficiency severely disrupted erythropoiesis with less effects in other lineages in vitro and ex vivo. Importantly, we validated these in vitro findings and revealed that the lethality of hematopoietic-specific DDX41 knockout mice is likely due to the defects in erythropoiesis. Our preliminary mechanistic studies also revealed that DDX41 deficiency-mediated G4 upregulation in erythroid cells compromised the expression of various ribosome proteins that led to p53-dependent apoptosis in erythroid cells. Based on these data, we hypothesize that DDX41 deficiency in MDS leads to G4 accumulation and defects in ribosome biogenesis, which induces p53-mediated cell death specifically 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 how DDX41 regulates the accumulation of G4 in erythropoiesis. Aim 3 will focus on the mechanistic studies on DDX41-G4-ribosomal biogenesis-p53 pathway. Successful completion of our proposed research will provide novel mechanisms of DDX41’s functional role in normal and abnormal hematopoiesis, which will be impactful for the development of novel therapies for DDX41 mutation-related hematologic malignancies.