Regulation of Hematopoietic Stem and Progenitor Cell Fate Determination By the Dpy30 Subunit of Set1/Mll Complexes

Many hematological diseases result from aberrant chromatin and gene regulation in the maintenance, proliferation and differentiation of hematopoietic stem and progenitor cells (HSPCs). As the major H3K4 methylation enzymes in mammals, the SET1/MLL family complexes occupy a crucial position in developmental biology, and are considered potential drug targets for epigenetic therapeutics due to the intimate connection of H3K4 methylation with gene expression as well as the extensive association of several subunits in these complexes with many diseases including multiple blood cancers. The SET1/MLL complexes comprise either SET1A, SET1B, MLL1, MLL2, MLL3, or MLL4 as the catalytic subunit, and WDR5, RBBP5, ASH2L, and DPY30 as integral core subunits necessary for the full methylation activity. However, it remains unclear how the enzymatic activity (H3K4 methylation) of these complexes regulates normal and abnormal hematopoiesis, potentially through regulating target genes critically involved in HSPC fate determination. Our previous finding of the direct and important activity of Dpy30 in facilitating genome-wide H3K4 methylation (Jiang et al., Cell 144:513-525, 2011) allows an effective interrogation of the functional role of H3K4 methylation through genetic studies of Dpy30.

We have previously shown that Dpy30 is crucial for efficient differentiation of embryonic stem cells by facilitating the induction of many bivalently marked developmental genes (Jiang et al., Cell, 2011). We then demonstrated an important role of human DPY30 in ex vivo proliferation and differentiation of mobilized hematopoietic progenitors, as well as in zebrafish hematopoiesis (Yang et al., Blood, accepted). To further determine a role for Dpy30 and its associated H3K4 methylation in regulating HSPC maintenance and differentiation, we have generated a Dpy30 conditional knockout (KO) mouse model. Hematopoietic KO of Dpy30 in mice resulted in marked reduction of cellular H3K4 methylation and severe pancytopenia. Surprisingly, in contrast to the rapid HSC depletion upon hematopoietic loss of Mll, we detected a massive accumulation of phenotypic early HSPCs at the expense of more downstream hematopoietic cells in the Dpy30 KO mouse bone marrow (BM), despite little effects on cell proliferation and apoptosis. Competitive transplantation assays revealed a profound defect of the Dpy30 KO BM in multilineage hematopoietic reconstitution. These results are most consistent with a defect in hematopoietic differentiation. We have also started mixed BM chimera assays to further investigate the role of Dpy30 in HSPC fate determination. Our early time point results strongly support a defect in hematopoietic differentiation following Dpy30 loss, and demonstrate a role of Dpy30 in the efficient induction of many lineage regulatory genes during the transitions of the hematopoietic cell fate. Our data from later time points in the transplantation assays will allow us to discover if Dpy30 loss has a potential effect on HSC self-renewal, and will be presented and discussed. By revealing a previously unrecognized role of the H3K4 methylation activity of the Set1/Mll complexes in regulating HSPC fate determination, our studies may have important implications for developing therapeutic strategies against certain HSPC-based hematological diseases.