The role of WDR5 in silencing human fetal globin gene expression

Background Histone H3 lysine 4 (K4) methylation has been linked with transcriptional activity in mammalian cells. The WD40-repeat protein, WDR5, is an essential component of the MLL complex that induces histone H3 K4 methylation, but the role of WDR5 in human globin gene regulation has not yet been established. Design and Methods To study the role of WDR5 in human globin gene regulation, we performed knockdown experiments in both K562 cells and primary human bone marrow erythroid progenitor cells (BMC). The effects of WDR5 knockdown on gamma-globin gene expression were determined. Biochemical approaches were also employed to investigate WDR5 interaction molecules. Chromosomal marks in the globin locus were analyzed by ChIP. Results We found that WDR5 interacted with protein arginine methyltransferase 5 (PRMT5), a known repressor of gamma-globin gene expression, and was essential for generating tri-methylated H3K4 (H3K4me3) at the gamma-globin promoter in K562 cells. Enforced expression of WDR5 in K562 cells reduced gamma-globin gene expression, whereas knockdown of WDR5 increased gamma-globin gene expression in both K562 cells and primary human bone marrow erythroid progenitor cells. Consistent with this, both histone H3 and H4 acetylation at the gamma-globin promoter were increased, while histone H4R3 and H3K9 methylation were decreased, in WDR5 knockdown cells compared to controls. We found that WDR5 interacted with HDAC1 and a PHD domain-containing protein, ING2 (inhibitor of growth), an H3K4me3 mark reader, to enhance gamma-globin gene transcriptional repression. In human BMC, levels of WDR5 were highly enriched on the gamma-promoter relative to levels on other globin promoters and compared to the gamma-promoter in cord blood erythroid progenitors, suggesting that WDR5 is important in the developmental globin gene expression program. Conclusions Our data are consistent with a model in which WDR5 binds the gamma-globin promoter in a PRMT5-dependent manner; H3K4me3 induced at the gamma-globin promoter by WDR5 may result in the recruitment of the ING2-associated HDAC1 component and consequent silencing of gamma-globin gene expression.