Role Of Dpy30/MLL Complexes In Regulating Hematopoietic Progenitor Cell Maintenance and Differentiation

The stability and plasticity of cell identity is ultimately controlled at the level of gene expression, which is closely tied to the global and local chromatin and epigenetic status of the cell. Although the general association/correlation of histone H3K4 methylation and gene activation is well-established, the functional roles of this modification in gene expression involved in animal development are far from clear. In mammals, the most notable H3K4 methyltransferases are the SET1/MLL family complexes (hereafter MLL complexes), which contain WDR5, RbBP5, Ash2L, and Dpy30 as integral core subunits and either Set1A, Set1B, MLL1, MLL2, MLL3, or MLL4 as the catalytic subunit. Our previous work has shown that mammalian Dpy30 in the MLL complexes facilitates H3K4 methylation throughout the genome. We have also established a crucial role for Dpy30 in mediating the transition of the pluripotent state of embryonic stem cells to differentiated state by priming the H3K4 methylation of the bivalently marked developmental genes. However, it remains unclear what roles Dpy30 plays in the maintenance and differentiation of somatic stem cells and lineage progenitor cells, including the hematopoietic stem and progenitor cells.

In this work, we have studied roles of Dpy30/MLL complexes in regulating the stability and plasticity of hematopoietic cells. We have found that Dpy30 is important for ex vivo expansion of human CD34+ stem/progenitor cells (HSPCs), as indicated by the markedly reduced proliferation and colony formation after Dpy30 knockdown in these cells. Furthermore, we found that Dpy30 was critical in the expression of many important cell-cycle regulators in HSPCs. Interestingly, while Dpy30 knockdown impaired the efficient differentiation of HSPCs to the myeloid lineage, it enhanced their differentiation to the erythroid lineage, especially at the later stage of the differentiation. These results suggest important roles for Dpy30 in the expansion of HSPCs as well as in their correct differentiation to appropriate lineages. We are currently studying the molecular mechanisms by which Dpy30 and its associated H3K4 methylation regulate the hematopoietic lineage differentiation. Our preliminary results have pointed toward a physical and functional collaboration of the MLL complexes with certain master regulators of the hematopoietic lineage maintenance and differentiation.

Moving toward a more physiologically relevant system, we have shown that the Dpy30 homologue is required for the normal development of the hematopoietic system in zebrafish. Morpholino-mediated Dpy30 knockdown resulted in striking defects in the development of zebrafish hematopoiesis, which can be rescued by co-injection of Dpy30 mRNA. Based on the wide expression of Dpy30, these phenotypes, which appeared to be most obvious in the blood system, suggest that the correct levels of Dpy30 and its associated H3K4 methylation might be particularly crucial to hematopoiesis. For a more definitive understanding of the role of Dpy30 in hematopoiesis, we have established a mouse model in which Dpy30 is specifically ablated in the hematopoietic system in an inducible fashion. The results of the ongoing studies using this mouse model will be presented and discussed.