Troponin T mutations and Sudden Cardiac Death

描述(申请人提供)：家族性肥厚性心肌病(FHC)是一种常染色体显性疾病，由心脏收缩蛋白编码基因突变引起，是导致心脏性猝死(SCD)的重要原因。基因/表型相关性研究表明，大多数突变的预后意义与心肌肥大和纤维化的程度有关。几个肌钙蛋白T(TnT)突变是一个例外，即使在没有明显的心肌肥大和纤维化的情况下，这些突变也会增加SCD的风险。体外研究表明，这些TnT突变都增加了肌丝对钙的敏感性。为了研究钙敏感性增加本身是否可能导致心律失常，没有混杂的纤维化和/或肥大的动物模型将是理想的。我们最近通过转基因表达突变型人TNT(179N)建立了这样的模型，其结果是肌丝对钙的敏感性增加，心肌肥大和纤维化消失。我们在单细胞上的初步实验显示，对β-肾上腺素能受体激动剂的反应，钙瞬变减少，衰变速度较慢，舒张期(钙)i升高。孤立的灌流心脏表现出较高的室性心律失常发生率，特别是在较高的(Ca)o时，与后除极和心室动作电位波形改变相关。因此，我们推测，肌丝对钙的敏感性增加改变了细胞内的钙信号，并独立地参与了后除极和室性心律失常的发展，导致心源性猝死。另一种假说认为，肌丝钙敏感性以外的其他因素(即，钙信号的改变而不改变肌丝对钙的敏感性)与TnT突变相关的心律失常有关。为了解决另一种假设，我们将检查一个表达TNT突变(R278C)的转基因模型，该突变不会改变肌丝对钙的敏感性。我们将使用TG-179N和TG-R278C转基因模型从分子水平到器官水平验证这些假说，每个特定的AIM将独立地测试肌丝钙敏感性变化、细胞钙信号变化以及可能导致室性心律失常的全心电生理变化之间的潜在机制联系。如果被本研究证实，所提出的心律失常发生机制可能适用于其他FHC突变和增加肌丝钙敏感性的疾病。

DESCRIPTION (provided by applicant): Familial Hypertrophic Cardiomyopathy (FHC) is an autosomal-dominant disease resulting from mutations in genes encoding cardiac contractile proteins, and an important cause of Sudden Cardiac Death (SCD). Genotype/phenotype correlation studies suggest that the prognostic significance of most mutations is related to the degree of cardiac hypertrophy and fibrosis. An exception are several Troponin T (TnT) mutations, which confer a high risk of SCD even in absence of significant cardiac hypertrophy and fibrosis.In vitro studies suggest that these TnT mutations all increase myofilament Ca 2+ sensitivity. To investigate if an increased Ca 2+ sensitivity itself could be pro-arrhythmic, an animal model without the confounding fibrosis and/or hypertrophy would be ideal. We recently generated such a model by transgenic expression of mutant human TnT (179N), which resulted in increased myofilament Ca 2+ sensitivity and lack of cardiac hypertrophy and fibrosis. Our preliminary experiments on single cells show decreased Ca 2+ transients with slower decay rates, and elevated diastolic (Ca2+)i in response to beta-adrenergic receptor agonists. Isolated perfused hearts show a high incidence of ventricular arrhythmias, particularly at higher (Ca)o, associated with afterdepolarizations and altered ventricular action potential waveform. Thus we hypothesize that increased myofilament Ca 2+ sensitivity changes intracellular Ca 2+ signaling and contributes independently to development of afterdepolarizations and ventricular arrhythmias, leading to sudden cardiac death. The alternate hypothesis is that other factors than myofilament Ca 2+ sensitivity (i.e., alterations of Ca 2 + signaling without changes in myofilament Ca 2+ sensitivity) contribute to arrhythmias related to TnT mutations. To address the alternate hypothesis, we will examine a transgenic model that expresses a TnT mutation (R278C), which does NOT change myofilament Ca 2+ sensitivity. We will use the Tg-179N and Tg-R278C transgenic models to test these hypotheses from the molecular to the organ level, Each SPECIFIC AIM will independently test potential mechanistic links between changes in myofilament Ca 2+ sensitivity, in cellular Ca 2+ signaling, and in whole heart electrophysiology that may lead to ventricular arrhythmias. If validated by this research, the proposed mechanism of arrhythmogenesis may be applicable to other FHC mutations and diseases that increase myofilament Ca 2+ sensitivity.