Interrogating the role of H3K4 & H3K27 methylation in hematopoiesis with novel histone tools

About This Grant

SUMMARY Developmental gene expression is tightly regulated by the dynamic interplay of H3K4 methylation (H3K4me) and H3K27 methylation (H3K27me) associated with active and repressed genes, respectively. However, our understanding of the individual and combinatorial roles these histone modifications play in adult physiological contexts remains incomplete. To overcome these limitations, we have recently generated histone mutant transgenic tools to uncover a previously unappreciated role for H3K4me in adult hematopoiesis. Adult mice globally depleted for all forms of H3K4me via expression of an inducible histone H3 lysine-4-to-methionine (H3K4M) mutant allele succumbed to a severe loss of all major mature blood cell types. Unexpectedly however, H3K4M-expressing hematopoietic stem cells (HSCs) and most committed progenitors were present at normal numbers and persisted upon transplantation into recipient mice, suggesting that H3K4me is dispensable for the maintenance and early commitment of HSCs and progenitors but essential for the terminal maturation of progenitors. Mechanistically, we showed that H3K4me opposes the deposition of repressive H3K27me at differentiation-associated genes bivalently marked by H3K4me3 and H3K27me3 in HSCs or progenitors. Indeed, by concomitantly suppressing H3K27me in H3K4me-depleted mice with an H3K27M transgene, we could rescue the acute lethality, hematopoietic failure and gene dysregulation. Thus, our results reveal that H3K4me guides hematopoiesis by opposing repressive H3K27me at fate-instructive bivalent genes, providing the first evidence for the functional interaction between these crucial chromatin marks in mammalian tissue homeostasis. These preliminary data raise fundamental questions with clinical relevance that will be addressed in 3 complementary aims. In Aim 1, we will further define the consequences of H3K4me loss on the function of HSCs and progenitors using self-renewal and differentiation assays. Additionally, we will assess whether any observed defects are reversible upon restoration of H3K4me. In Aim 2, we will identify epigenetic regulators that mediate the H3K4M- dependent arrest and the H3K27M-dependent rescue by purifying proteins associated with H3K4M and H3K27M; measuring changes to all major histone modifications; and testing select candidates for their ability to phenocopy the effects of H3K4M and H3K27M. In Aim 3, we will dissect the molecular basis by which H3K4me/H3K27me safeguard hematopoiesis with a focus on fate-instructive cytokine receptors and transcription factors dysregulated in H3K4M mice but normalized in H3K4M/H3K27M mice. Moreover, we will investigate the contribution of other epigenetic marks to the H3K4M phenotype using DNA methylation inhibitors and a novel histone mutant library. Collectively, this proposal will leverage novel tools to probe the direct, physiological impact of two antagonizing chromatin marks on hematopoiesis. As arrested differentiation and disrupted H3K4me/H3K27me have been implicated in diverse hematological conditions, our results will elucidate the underlying mechanisms and may pave the way for novel therapeutic interventions.