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Myelodysplastic syndromes (MDS) are age-related myeloid neoplasms with Increased risk of progression to acute myeloid leukemia (AML). The mechanisms of transformation of MDS to AML are poorly understood, especially in relation to the aging microenvironment. We previously established an mDia1/miR-146a double knockout (DKO) mouse model phenocopying MDS. These mice develop age-related pancytopenia with oversecretion of proinflammatory cytokines. Here, we found that most of the DKO mice underwent leukemic transformation at 12-14 months of age. These mice showed myeloblast replacement of fibrotic bone marrow and widespread leukemic infiltration. Strikingly, depletion of IL-6 in these mice largely rescued the leukemic transformation and markedly extended survival. Single-cell RNA sequencing analyses revealed that DKO leukemic mice had increased monocytic blasts that were reduced with IL-6 knockout. We further revealed that the levels of surface and soluble IL-6 receptor (IL-6R) in the bone marrow were significantly increased in high-risk MDS patients. Similarly, IL-6R was also highly expressed in older DKO mice. Blocking of IL-6 signaling significantly ameliorated AML progression in the DKO model and clonogenicity of CD34-positive cells from MDS patients. Our study establishes a mouse model of progression of age-related MDS to AML and indicates the clinical significance of targeting IL-6 signaling in treating high-risk MDS., Introduction Myelodysplastic syndromes (MDS) are age-related clonal myeloid neoplasms characterized by ineffective hematopoiesis. Patients with high-risk MDS have a significantly increased incidence of progression into acute myeloid leukemia (AML). The [...]

mDia formin proteins regulate the dynamics and organization of the cytoskeleton through their linear actin nucleation and polymerization activities. We previously showed that mDia1 deficiency leads to aberrant innate immune activation and induces myelodysplasia in a mouse model, and mDia2 regulates enucleation and cytokinesis of erythroblasts and the engraftment of hematopoietic stem and progenitor cells (HSPCs). However, whether and how mDia formins interplay and regulate hematopoiesis under physiological and stress conditions remains unknown. Here, we found that both mDia1 and mDia2 are required for HSPC regeneration under stress, such as serial plating, aging, and reconstitution after myeloid ablation. We showed that mDia1 and mDia2 form hetero-oligomers through the interactions between mDia1 GBD-DID and mDia2 DAD domains. Double knockout of mDia1 and mDia2 in hematopoietic cells synergistically impaired the filamentous actin network and serum response factor-involved transcriptional signaling, which led to declined HSPCs, severe anemia, and significant mortality in neonates and newborn mice. Our data demonstrate the potential roles of mDia hetero-oligomerization and their non-rodent functions in the regulation of HSPCs activity and orchestration of hematopoiesis. Author summary: Blood production and generation entirely depend on the hematopoietic stem progenitor cells (HSPCs) that have to be tightly controlled. mDia formin proteins are critical regulators for linear-actin synthesis that are involved in actin filament formation and cytoskeleton organization. Previous studies have revealed the important roles of mDia1 or mDia2 in regulating the functions of T cells, neutrophils, or red blood cells, but there is a lack of evidence for the non-redundant roles of these two factors in vivo. Here we combined genetic manipulation of the mDia1 and mDia2 double deficient mouse with biochemical analysis. We find that mDia1 interacts directly with mDia2 to form hetero-oligomers in vitro and in vivo, and both mDia1 and mDia2 are required to maintain HSPCs hemostasis during stress in mice. Dual deficiency of mDia1 and mDia2 in mice leads to compromised serum response factor (SRF) signaling, with declined HSPC, severe anemia, and increased animal mortality. Our work not only provides a previously unrecognized working model for the mDia1-mDia2 complex but also highlights their cooperative roles in hematopoiesis. [ABSTRACT FROM AUTHOR]