Interfering with histone demethylases in HSCs.

S tem cells are governed by epigenetic mechanisms that can either help maintain the necessary gene expression programs to preserve the stem cell state or ensure the establishment of new cell-specific expression programs during differentiation. One important level of epigenetic control originates from histone modifications such as methylation or acetylation. These modifications can be associated with either gene activation or gene repression. Histone methylation was originally believed to be a relatively permanent modification but the discovery of enzymes that can demethylate histones, about a decade ago, changed this perception entirely. It is now evident that histone methylation is both highly dynamic and reversible. Since the first histone demethylase, LSD1, was described in 2004, around 30 additional histone demethylating enzymes have been identified, the vast majority belonging to the evolutionary conserved Jumonji C (JmjC) family. Functional studies of JmjC proteins have shown that they modulate the expression of specific gene sets associated with fundamental cellular functions such as self-renewal, proliferation, differentiation, and malignant progression. Many JmjC demethylases show distinct substrate specificity and have nonoverlapping functions. Therefore, understanding how each one of these enzymes function and how they may coordinately act together to influence gene expression poses a great challenge. In this context, Cellot et al now report on an RNA interference (RNAi) screening strategy to functionally dissect the role of nearly all (23 of 27) known JmjC histone demethylases in HSCs. The Sauvageau laboratory had previously developed RNAi screening paradigms for other gene categories in mouse HSCs. In this new screen, Cellot et al applied a scoring system based on gene expression profile (the genes preferentially expressed in HSCs got a higher score) and the ability of short hairpin RNAs to alter the in vivo reconstitution kinetics of transplanted HSCs. From this analysis and subsequent validation assays, Cellot et al identified Jarid1b as a negative regulator and Jhdm1f as a putative positive regulator of HSC activity. They focused their subsequent work primarily on Jarid1b and could show that Jarid1b knockdown decreased hematopoietic differentiation in vitro while it promoted HSC activity and renewal both in vitro and in vivo. The findings clearly implicate Jarid1b as a novel and crucial mediator of HSC function. Proposedmodel for JARID1Bactivity in HSC regulation. The box of letters depicts the amino-terminal tail of histoneH3.By counteracting the trimethylated status of lysine 4 (H3K4), JARID1B represses “stemness” loci and expression of genes associated with multipotency (brown shaded area). Conversely, knockdown of JARID1B shifts the balance toward the methylated status induced by the MLL/WDR5 complex to sustain multipotency (blue shaded area). Figure adapted from Figure 7 in the article by Cellot et al that begins on page 1545. Professional illustration by Alice Y. Chen.