Polycomb Repressive Complex 2 Regulates Normal Development of the Mouse Heart

Abstract

Rationale: Epigenetic marks are crucial for organogenesis, but their role in heart development is poorly understood. Polycomb repressive complex 2 (PRC2) trimethylates histone H3 at lysine 27, which establishes H3K27me3 repressive epigenetic marks that promote tissue-specific differentiation by silencing ectopic gene programs. Objective: We studied the function of PRC2 in murine heart development using a tissue-restricted conditional inactivation strategy. Methods and Results: Inactivation of the PRC2 subunit Ezh2 by Nkx2–5 Cre (Ezh2 NK ) caused lethal congenital heart malformations, namely, compact myocardial hypoplasia, hypertrabeculation, and ventricular septal defect. Candidate and genome-wide RNA expression profiling and chromatin immunoprecipitation analyses of Ezh2 NK heart identified genes directly repressed by EZH2. Among these were the potent cell cycle inhibitors Ink4a/b (inhibitors of cyclin-dependent kinase 4 A and B), the upregulation of which was associated with decreased cardiomyocyte proliferation in Ezh2 NK . EZH2-repressed genes were enriched for transcriptional regulators of noncardiomyocyte expression programs such as Pax6 , Isl1 , and Six1 . EZH2 was also required for proper spatiotemporal regulation of cardiac gene expression, because Hcn4 , Mlc2a , and Bmp10 were inappropriately upregulated in ventricular RNA. PRC2 was also required later in heart development, as indicated by cardiomyocyte-restricted TNT-Cre inactivation of the PRC2 subunit Eed . However, Ezh2 inactivation by TNT-Cre did not cause an overt phenotype, likely because of functional redundancy with Ezh1 . Thus, early Ezh2 inactivation by Nk2–5 Cre caused later disruption of cardiomyocyte gene expression and heart development. Conclusions: Our study reveals a previously undescribed role of EZH2 in regulating heart formation and shows that perturbation of the epigenetic landscape early in cardiogenesis has sustained disruptive effects at later developmental stages.