We are interested in the epigenetic mechanisms that control gene expression in haematopoietic stem cells (HSCs). Our focus is on Polycomb Repressive Complex 2 (PRC2), that has been shown to establish H3K27me3 and repress key developmental regulators in ES cells. In addition to the core PRC2 members Ezh2/1, Suz12 and Eed, several accessory factors have been shown to modulate PRC2 activity and direct its binding to target genes in ES cells. We have previously shown that reduction in the level of core PRC2 components results in enhanced HSC repopulating capacity, but the role of PRC2 accessory factors in HSCs has not been described. Using shRNA-mediated knockdown, we examined the function of six known PRC2 accessory factors in HSCs by testing the capacity of transduced murine foetal liver cells to competitively reconstitute irradiated recipients. Depletion of the enzymatically inactive histone methyltransferase Jarid2 enhanced contribution to all blood cell lineages compared to cells containing non-silencing control constructs, similar to the phenotype observed upon depletion of PRC2 core components. Our data suggest that the enhanced activity of Jarid2 depleted cells is due to an increase in HSC number post Jarid2 knockdown, but that this role for Jarid2 is restricted to foetal HSCs. Interestingly, mutations have been identified in PRC2 core components in haematopoietic malignancy, and recently mutations and deletions in JARID2 have also been reported.
In addition to molecular studies of the Jarid2 depleted HSCs, we are now using our bespoke shRNA library targeting over 250 enzymatic epigenetic modifiers to perform in vivo and in vitro screens designed to identify additional factors that, similar to PRC2, restrict HSC function and may cooperate with PRC2 to regulate HSC activity.