MacroH2A histone variants maintain nuclear organization and heterochromatin architecture

Abstract

Genetic loss-of-function studies on development, cancer and somatic cell reprogramming have suggested that the group of macroH2A histone variants might function through stabilizing the differentiated state by a yet unknown mechanism. Here, we present results demonstrating that macroH2A variants have a major function in maintaining nuclear organization and heterochromatin architecture. Specifically, we find that a substantial amount of macroH2A is associated with heterochromatic repeat sequences. We further identify macroH2A on sites of interstitial heterochromatin decorated by histone H3 trimethylated on K9 (H3K9me3). Loss of macroH2A leads to major defects in nuclear organization, including reduced nuclear circularity, disruption of nucleoli and a global loss of dense heterochromatin. Domains formed by DNA repeat sequences are disorganized, expanded and fragmented, and mildly re-expressed when depleted of macroH2A. At the molecular level, we find that macroH2A is required for the interaction of repeat sequences with the nucleostructural protein lamin B1. Taken together, our results argue that a major function of macroH2A histone variants is to link nucleosome composition to higher-order chromatin architecture.

Induction of cancer cell stemness by depletion of macrohistone H2A1 in hepatocellular carcinoma

“…C). We also confirmed the reported loss of nuclear heterochromatin, which is most likely replaced by euchromatin. These findings indicate that depletion of macroH2A1 in HCC cells induces profound morphological alterations at the subcellular level, consistent with the increased tumorigenesis seen in vivo .…”

“…We detected an increase of expression of stem surface marker CD44, associated with aggressive HCC, by KD of macroH2A1 in HCC cells; this increase could be rescued by OE of macroH2A1 isoforms. HCC cells KD for macroH2A1 display a striking loss of heterochromatin visible in electron microscopy, confirming a function for this histone variant as a linker between nucleosome composition and higher‐order chromatin architecture, as we have recently demonstrated: this might be due to reduced compaction of repeats and a reduced interaction with the nuclear periphery protein lamin B1, and it is consistent with the increased sensitivity to micrococcal nuclease digestion in macroH2A1 KD cells . MacroH2A1‐depleted HepG2 cells showed structures whose morphology resembled autophagosomes, in lipid droplets.…”

“…A recent report showing a role for macroH2A1 in stemness of bladder tumor cell lines screened a panel of known CSC‐associated genes and suggested that the observed CSC phenotype was driven by the RNA‐binding protein LIN28B; in this tumor type, mRNA alternative splicing leads to macroH2A1.1, but not macroH2A1.2, down‐regulation, whereas in HCC alternative splicing of macroH2A1 seems to be irrelevant, this study. Confining the genomic action of macroH2A1 to the promoter of one gene (i.e., LIN28B) could be a reductive approach because genome occupancy of macroH2A1 covers large genomic domains in HepG2 cells and is not restricted to promoters . Moreover, previous studies suggested indirect oncogenic transcriptional programs independent of changes in macroH2A1 genomic occupancy .…”

MacroH2A histone variants limit chromatin plasticity through two distinct mechanisms

“…Taken together, these and other studies support the notion that macroH2A proteins promote the maintenance and stability of the epigenome in differentiated cells, acting as barriers to reprogramming and malignant transformation. The molecular basis for this function is not known, but may be related to a major role of macroH2A in regulating chromatin architecture and nuclear organization . Knockdown of macroH2A proteins leads to defects in nuclear circularity, disruption of nucleoli, and a global loss of dense heterochromatin.…”

Regulation of Breast Cancer-Induced Osteoclastogenesis by MacroH2A1.2 Involving EZH2-Mediated H3K27me3

“…Several recent studies, including ours, suggested that mH2A incorporation into nucleosomes influences histone modification patterns associated with transcriptional silencing or activation (Chen et al, 2014; Douet et al, 2017; Gamble et al, 2010; Gaspar-Maia et al, 2013; Kim et al, 2013). Thus, we asked whether any histone modifications are altered in mH2A1.2 nucleosomes and play a role, if any, mH2A1.2-induced gene silencing in MDA-MB-468 breast cancer cells.…”