Deletion of HP1γ in cardiac myocytes affects H4K20me3 levels but does not impact cardiac growth

Background: Heterochromatin, which is formed when tri-methyl lysine 9 of histone H3 (H3K9me3) is bound by heterochromatin 1 proteins (HP1s), plays an important role in differentiation and senescence by silencing cell cycle genes. Cardiac myocytes (CMs) accumulate heterochromatin during differentiation and demethylation of H3K9me3 inhibits cell cycle gene silencing and cell cycle exit in CMs; however, it is unclear if this process is mediated by HP1s. In this study, we created a conditional CM-specific HP1 gamma (HP1 gamma) knockout (KO) mouse model and tested whether HP1 gamma is required for cell cycle gene silencing and cardiac growth. Results: HP1 gamma KO mice were generated by crossing HP1 gamma floxed mice (fl) with mice expressing Cre recombinase driven by the Nkx2.5 (cardiac progenitor gene) promoter (Cre). We confirmed that deletion of critical exons of HP1 gamma led to undetectable levels of HP1 gamma protein in HP1 gamma KO (Cre;fl/fl) CMs. Analysis of cardiac size and function by echo revealed no significant differences between HP1 gamma KO and control (WT, Cre,fl/fl) mice. No significant difference in expression of cell cycle genes or cardiac-specific genes was observed. Global transcriptome analysis demonstrated a very moderate effect of HP1 gamma deletion on global gene expression, with only 51 genes differentially expressed in HP1 gamma KO CMs. We found that HP1 beta protein, but not HP1 alpha, was significantly upregulated and that subnuclear localization of HP1 beta to perinuclear heterochromatin was increased in HP1 gamma KO CMs. Although HP1 gamma KO had no effect on H3K9me3 levels, we found a significant reduction in another major heterochromatin mark, tri-methylated lysine 20 of histone H4 (H4K20me3). Conclusions: These data indicate that HP1 gamma is dispensable for cell cycle exit and normal cardiac growth but has a significant role in maintaining H4K20me3 and regulating a limited number of genes in CMs.