H2A.Z : a molecular rheostat for gene regulation in embryonic stem cells

Abstract

Chromatin regulation is a key mechanism for controlling gene expression patterns during development and differentiation. The histone H2A variant H2A.Z is highly conserved among eukaryotes and is of particular interest because it has an essential, yet unknown role in early development. H2A.Z is enriched at the promoter regions of most genes that harbor H3K4me3 in mouse embryonic stem cells (mESCs) including both active and silent, poised genes, marked additionally by polycomb-mediated H3K27me3 and compromising a large cohort of developmental regulators. How H2A.Z mediates these contrasting gene expression states is not known. H2A.Z displays homology to canonical H2A throughout the histone fold domain, however considerable divergence exists outside of this domain, suggesting specialized functions. Here we developed a quantitative chromatin immunoprecipitation followed by mass spectrometry approach to identify downstream effectors of H2A.Z. We identified BET (bromodomain and extraterminal) transcriptional regulator proteins including Brd2 as highly enriched in H2A.Z chromatin. We demonstrate by ChIP-seq that Brd2 significantly overlap H2A.Z at the promoter region of active genes. Conversely, PRC1 -dependent H2A.Z ubiquitination prevents Brd2 occupancy at poised, bivalent genes. Loss of H2A.Z ubiquitination of by mutation of Cterminal lysines results in a Brd2 recruitment and de-repression of bivalent genes. Moreover, inhibition of Brd2 by small molecule inhibition or siRNA-mediated depletion restores repression and leads to a recruitment of PRC2. In contrast, siRNA inhibition of another BET family member Brd4, does not restore repression suggesting that Brd2 and Brd4 play distinct roles in ESCs. This thesis provides novel insights into how H2A.Z acts as a molecular rheostat to regulate the balance between active and silent genes in ESCs, and more broadly a model for its role in responsive systems including development and cancer.