A Critical Role of RUNX1 in Governing Megakaryocyte-Primed Hematopoietic Stem Cell Differentiation.

As a member of the RUNT domain family core-binding transcription factors, RUNX1 is crucial for multiple stages of hematopoiesis and its mutation can cause familial platelet disorder with predisposition to acute myeloid leukemia (FPD/AML). Previous work has established that RUNX1 is involved in the maturation of megakaryocytes and the production of platelets. Recently studies have shown that there exists a subpopulation of hematopoietic stem cells (HSCs) with relatively high expression of vWF and CD41 at the apex of the HSC hierarchy, termed MK-HSCs, which can give rise to megakaryocytes without going through the traditional differentiation trajectory from HSC via MPP and MEP. Here, by using Runx1F/FMx1-Cre mouse model we discovered that the MK-HSC to megakaryocyte direct differentiation can happen within one cell division and RUNX1 is an important regulator in the process. Runx1 knockout results in a drastic decrease in platelet counts and a severe defect in the differentiation from MK-HSCs to megakaryocytes. Single cell RNA sequencing (RNAseq) analysis shows that MK-HSCs have a distinct gene expression signature compared with non-MK-HSCs, and Runx1 deletion alters the platelet and megakaryocyte related gene expression in MK-HSCs. Further bulk RNAseq and Cut & Run analyses show that RUNX1 binds to multiple essential megakaryocyte/platelet developmental genes such as Spi1, Selp and Itga2b and regulates their expressions in MK-HSCs. Thus, by modulating the expression of megakaryocyte-related genes, RUNX1 governs the direct differentiation of MK-HSCs to megakaryocytes and platelets.