Genetic Analysis of Cardiac Growth

描述（由申请人提供）：在发育过程中，细胞分裂（增殖或增生）与细胞团的积累（肥大）紧密耦合，以确保肌细胞大小恒定；然而，在成年心肌细胞（ACMs）中，尽管许多相同的信号通路被激活，但相似的生长信号主要诱导肥厚性生长而非增殖。在分子水平上，虽然增生性生长与一系列由转录因子E2F家族调控的细胞周期基因的表达有关，但这些基因在肥厚性肌细胞中并没有上调。尽管大量的描述性研究描述了ACMs在各种刺激下退出G1期或分裂的能力有限，但几乎没有数据可以解释为什么大多数ACMs在受到刺激时不进入S期。我们已经确定了在ACMs中沉默G2M/细胞分裂基因的新机制；即Rb-E2F的组蛋白甲基化调节细胞周期基因。我们发现，与稳定基因沉默相关的两种主要组蛋白修饰在ACMs中上调，并靶向e2f依赖性细胞周期基因。我们建议在体内测试这些表观遗传标记的重要性，以及它们是否被Rb家族成员靶向e2f依赖性细胞周期基因。转基因小鼠细胞周期基因的基因再激活与特异性组蛋白去甲基化酶的再表达有关，这通常只在增殖的胎儿心肌细胞中看到，而不是在肥大的胎儿心肌细胞中。有趣的是，这些表观遗传变化可能是可逆的这一事实表明，这可能是一种“重塑”或“重编程”ACMs以恢复其增殖潜力的治疗途径。我们将通过确定逆转H3K9和H3K27组蛋白甲基化是否将成年心肌细胞的肥厚反应转化为增生（Aim 1），确定ACMs中靶向组蛋白甲基化的因素及其在沉默细胞周期基因和防止增殖中的作用（Aim2），以及确定组蛋白甲基化重塑如何在ACMs中发生及其生理意义（Aim 3）来探索组蛋白甲基化在限制ACM增殖中的重要性。

DESCRIPTION (provided by applicant): During development, cell division (proliferation or hyperplasia) is tightly coupled to the accumulation of cell mass (hypertrophy) to ensure that myocyte size is constant; however, in adult cardiac myocytes (ACMs), similar growth signals primarily induce hypertrophic growth without proliferation even though many of the same signaling pathways are activated. At a molecular level, while hyperplastic growth is associated with the expression of a panel of cell cycle genes regulated by the E2F family of transcription factors, these genes are not upregulated in hypertrophic myocytes. Despite numerous descriptive studies characterizing the limited ability of ACMs to exit G1 or divide in response to various stimuli, almost no data exists to explain why the majority of ACMs do not enter S phase when stimulated. We have identified a novel mechanism for silencing G2M/cytokinesis genes in ACMs; namely, histone methylation of Rb-E2F regulated cell cycle genes. We show that the two major histone modifications associated with stable gene silencing are upregulated in ACMs and targeted to E2F-dependent cell cycle genes. We propose to test if the importance of these epigenetic marks and if they are targeted to E2F-dependent cell cycle genes by Rb family members in vivo. Genetically reactivating cell cycle genes in transgenic mice is associated with the reexpression of specific histone demethylases, something normally seen only in proliferating fetal cardiac myocytes not hypertrophy. Interestingly, the fact that these epigenetic changes might be reversible suggests that this might be a therapeutic avenue to "remodel" or "reprogram" ACMs to restore their proliferative potential. We will explore the importance of histone methylation in limiting ACM proliferation by determining if reversing H3K9 and H3K27 histone methylation converts a hypertrophic response to hyperplasia in adult cardiac myocytes (Aim 1), determining the factors that target histone methylations in ACMs and their role in silencing cell cycle genes and preventing proliferation (Aim2) and determining how histone methylation remodeling occurs in ACMs and its physiologic significance (Aim 3). PUBLIC HEALTH RELEVANCE: Myocardial regeneration to restore cardiac muscle mass after injury has been proposed as a means to prevent the development of congestive heart failure for decades. Developing strategies that promote dedifferentiation and proliferation of the endogenous cardiac myocytes holds great promise as a therapeutic strategy. The studies in this application will address critical deficiencies in our current knowledge of cardiac growth and will identify specific molecular pathways amenable to directed therapies.