MacroH2A histone variants maintain nuclear organization and heterochromatin architecture.

Genetic loss-of-function studies in 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 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 repeat sequences when depleted of macroH2A are disorganized, expanded and fragmented and mildly re-expressed. On 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.