Methylation of H3K9 by G9a/GLP protects against pathological cardiac hypertrophy

The heart adapts to increased workload through hypertrophic growth of cardiomyocytes. Although beneficial when induced physiologically by exercise, pathological cues including hypertension cause reexpression of fetal genes and dysfunctional hypertrophy, with lasting consequences for cardiac health. We hypothesised that these differences are driven by changes in chromatin-encoded cellular memory. We generated genome-wide maps of transcription and of two stable epigenetic marks, H3K9me2 and H3K27me3, specifically in hypertrophied cardiomyocytes, by selectively flow-sorting their nuclei. This demonstrated a pervasive loss of euchromatic H3K9me2 specifically upon pathological but not physiological hypertrophy, derepressing genes associated with pathological hypertrophy. Levels of the H3K9 methyltransferases, G9a and GLP, were correspondingly reduced. Importantly, pharmacological or genetic inactivation of these enzymes was sufficient to induce pathological hypertrophy and the dedifferentiation associated with it. These findings suggest novel therapeutic opportunities by defining an epigenetic state of cardiomyocytes, acquired during maturation, which is required for maintaining cardiac health.