From Department of Cardiovascular Medicine, Humanitas Research Hospital, Rozzano, Milan, Italy (R.P., S.S., C.P., F.R., P.C., N.S., M. Miragoli, G.C.); Genetic and Biomedical Research Institute, National Research Council of Italy, Rozzano, Milan, Italy (R.P., F.R., P.C., N.S., G.C.); Humanitas University, Rozzano, Milan, Italy (M. Mazzola, G.C.); School of Medicine, University of Verona, Italy (M. Mazzola); and Department of Medicine and Surgery, University of Parma, Italy (M. Miragoli).
Circulation. 2017 Sep 26;136(13):1233-1246. doi: 10.1161/CIRCULATIONAHA.117.028561. Epub 2017 Aug 4.
Correct gene expression programming of the cardiomyocyte underlies the normal functioning of the heart. Alterations to this can lead to the loss of cardiac homeostasis, triggering heart dysfunction. Although the role of some histone methyltransferases in establishing the transcriptional program of postnatal cardiomyocytes during heart development has been shown, the function of this class of epigenetic enzymes is largely unexplored in the adult heart. In this study, we investigated the role of G9a/Ehmt2, a histone methyltransferase that defines a repressive epigenetic signature, in defining the transcriptional program for cardiomyocyte homeostasis and cardiac hypertrophy.
We investigated the function of G9a in normal and stressed cardiomyocytes with the use of a conditional, cardiac-specific G9a knockout mouse, a specific G9a inhibitor, and high-throughput approaches for the study of the epigenome (chromatin immunoprecipitation sequencing) and transcriptome (RNA sequencing); traditional methods were used to assess cardiac function and cardiovascular disease.
We found that G9a is required for cardiomyocyte homeostasis in the adult heart by mediating the repression of key genes regulating cardiomyocyte function via dimethylation of H3 lysine 9 and interaction with enhancer of zeste homolog 2, the catalytic subunit of polycomb repressive complex 2, and MEF2C-dependent gene expression by forming a complex with this transcription factor. The G9a-MEF2C complex was found to be required also for the maintenance of heterochromatin needed for the silencing of developmental genes in the adult heart. Moreover, G9a promoted cardiac hypertrophy by repressing antihypertrophic genes.
Taken together, our findings demonstrate that G9a orchestrates critical epigenetic changes in cardiomyocytes in physiological and pathological conditions, thereby providing novel therapeutic avenues for cardiac pathologies associated with dysregulation of these mechanisms.
心肌细胞中正确的基因表达编程是心脏正常功能的基础。这种变化会导致心脏内稳态的丧失，触发心脏功能障碍。虽然一些组蛋白甲基转移酶在心脏发育过程中对成体心肌细胞的转录程序的建立作用已被证实，但这一类表观遗传酶在成年心脏中的功能在很大程度上仍未被探索。在这项研究中，我们研究了 G9a/Ehmt2 的作用，G9a/Ehmt2 是一种定义抑制性表观遗传特征的组蛋白甲基转移酶，它在定义心肌细胞内稳态和心脏肥大的转录程序中起作用。
我们使用条件性、心脏特异性 G9a 敲除小鼠、特异性 G9a 抑制剂以及用于研究表观基因组（染色质免疫沉淀测序）和转录组（RNA 测序）的高通量方法，研究了 G9a 在正常和应激心肌细胞中的功能；传统方法用于评估心脏功能和心血管疾病。
我们发现 G9a 通过介导关键基因的抑制，通过 H3 赖氨酸 9 的二甲基化和与增强子结合抑制复合物 2 的催化亚基增强子结合抑制同源物 2（enhancer of zeste homolog 2，Ehmt2）的相互作用，调节心肌细胞功能，从而在成年心脏中维持心肌细胞内稳态是必需的，并且与 MEF2C 形成复合物，通过与该转录因子形成复合物，调节 MEF2C 依赖性基因表达。还发现 G9a-MEF2C 复合物对于维持成年心脏中发育基因沉默所需的异染色质也是必需的。此外，G9a 通过抑制抗肥大基因促进心脏肥大。
总之，我们的研究结果表明，G9a 在生理和病理条件下协调心肌细胞中的关键表观遗传变化，从而为与这些机制失调相关的心脏病理提供了新的治疗途径。
