| MacroH2A1 (mH2A1, encoded by MACROH2A1/H2AFY) is a replication-independent H2A histone variant with a tripartite structure comprising a conventional histone H2A-like fold, an unstructured linker, and a globular macrodomain, and it replaces canonical H2A in defined chromatin regions where it modulates nucleosome stability, chromatin accessibility and long-term transcriptional programs. The histone domain harbors dispersed sequence elements that are each sufficient to direct enrichment on the inactive X chromosome, explaining how macroH2A1 becomes a hallmark of Xi heterochromatin and contributes to maintenance of X-linked gene silencing, while the macrodomain and docking region confer isoform-specific ligand binding and chromatin targeting that distinguish macroH2A1.1 and macroH2A1.2. Structural and biochemical work shows that macroH2A1 localizes to two major chromatin subtypes marked either by PRC2-dependent H3K27me3 or by H2B acetylations; recruitment to H2B-acetylated chromatin requires its docking domain and acetylation of H2B lysine 20, defining a mechanism by which histone cross-talk and macroH2A1 structure specify its chromatin occupancy at active or poised regulatory regions. The macroH2A1.1 isoform contains a metabolite-binding macrodomain that recognizes ADP-ribose–related ligands, including those produced by the NAD⁺-dependent deacetylase SIRT1, and this ligand binding shapes transcriptional control of genes involved in lipid metabolism and oxidative stress, such that macroH2A1.1 protects hepatocytes against lipid accumulation and restrains lipogenic gene expression in models of non-alcoholic fatty liver disease. In contrast, macroH2A1.2 lacks the metabolite-binding capacity of macroH2A1.1 and is more closely associated with activation of lipogenic genes and lipid uptake: hepatic overexpression of macroH2A1.2 increases lipid uptake, triglyceride content and expression of lipogenic genes, while macroH2A1.1-enriched liver chromatin is associated with minimal steatosis under high-fat diet, indicating opposing roles of the two splice isoforms in hepatocyte lipid handling and steatosis progression. Genetic ablation of macroH2A1 in mice confers protection against diet-induced obesity and metabolic derangements, with knockout animals showing reduced fat mass, improved glucose tolerance and increased energy expenditure under high-fat feeding, linked to elevated thermogenic gene expression in brown adipose tissue and reduced adipogenic gene expression in visceral fat, supporting macroH2A1 as an epigenetic regulator that senses nutrient excess and tunes energy balance and adipogenesis. |
