Role of hSET1 Complex in Epigenetic Controls of HoxB4 Expression and Development of Hematopoietic Stem Cells

Abstract 212

Hox genes play an important role in embryonic development and are sequentially activated in a temporal and spatial fashion. Among them, HoxB4 regulates the self-renewal ability of adult and embryonic hematopoietic stem cells. It was shown that USF proteins positively regulate the transcription of HoxB4 gene. However, the epigenetic mechanism specifically controlling HoxB4 transcription during HSC formation remains unknown. In this report, we found that USF1 interacts with histone H3K4 methylatransferase SET1 complex, but not MLL proteins. The interaction is mediated by SET1/MLL core component, ASH2L. Both USF1 and hSET1 are colocalized at the HoxB4 promoter in hematopoietic progenitors, which is accompanied by H3K4 trimethylation and transcriptional activation. siRNA-mediated knockdown of both SET1 and HCF1 specifically impaired H3K4 trimetylation and the HoxB4 promoter-driven reporter gene activity. Overexpression of AUSF1, a dominant negative mutant of USF1 in which the basic DNA recognition domain is deleted and thereby, inhibits endogenous USF DNA binding activity, leads to dramatically decrease in HoxB4 transcription, disruption of embryonic body formation and inhibition of hematopoietic stem cell population in mouse ES (mES) cells and the AUSF1 transgenic mice. The suppression of USF activity drastically reduces SET1 recruitment, H3K4 trimethylation, and pol II binding at the HoxB4 promoter. Furthermore, knock-down of SET1, an enzymatic subunit of the SET1 complex, also resulted in reduction of HoxB4 transcription and impairment of hematopoietic differentiation capacity of mES cells that are accompanied by reducing the HoxB4 promoter H3K4 methylation and recruitment of RNA PolII machinery. Thus, our data reveal that recruitment of hSET1 complex and its H3K4 methyltransferase activity by USF1 is essential for HoxB4 transcription and expansion of hematopoietic stem cells.