Genotype-Phenotype Discordance in Long QT Syndrome

描述（由申请人提供）：心源性猝死（SCD）是一种灾难性事件，在美国每年导致多达45万人死亡。SCD的高危患者是那些有遗传性心律失常的患者。长QT综合征（LQTS）是一组产生心膜电流缺陷的遗传性心律失常的一个例子。作为直接后果，LQTS与心电图QT间期延长、室性心律失常和SCD发生率增加有关。在LQTS2中，在编码hERG的KCNH2基因中发现了超过200个错义突变，其中绝大多数被认为是蛋白质加工和运输缺陷，导致钾电流急剧减少。然而，正如在许多常染色体显性心脏通道病中普遍观察到的那样，这些患者的遗传模式和临床表型是复杂的，并且经常显示不完全外显，其中致病突变携带者是无症状的。这种可变临床表达的原因尚不清楚，但在目前的研究计划中，我们将通过测试修饰基因导致可变临床表达的假设来研究这个问题。我们在MetroHealth和凯斯西储大学的多学科小组在临床和体外研究了一个携带hERG突变R752W的大型“Cleveland”LQT2家族。在研究的101名家庭成员中，26人携带hERG R752W突变。然而，症状性LQTS仅出现在5个受遗传影响的家庭成员中，从而说明该疾病的不完全外显性。我们假设疾病修饰基因的存在可以解释在LQT2家族中观察到的基因型-表型不一致。在这一提议中，我们将通过外显子组测序和从患者来源的诱导多能干细胞（iPS）分化的心肌细胞来阐明LQT2家族不完全外显的机制。我们假设患者来源的iPS分化心肌细胞（iPS- cm）忠实地再现了心律失常病理，并且外显子组测序显示的迄今未知的候选基因解释了可变的表型外显率。本建议的目的是：1。确定LQT2家族中导致不完全外显的候选修饰基因。2. 阐明来自LQT2家族成员的人心肌细胞的电生理变异性。3. 确定候选疾病修饰基因变异的表型。我们将通过研究密切相关的LQT2 hERG R752W携带者对（即父亲/儿子和兄弟姐妹对）来实现这些目标，这些携带者对表现出不一致的临床表型。我们相信，目前的建议将提供一个基本的，机制的解释，基因型-表型不一致可能出现在一个大的LQT2家族。这对个性化临床管理具有潜在的重要影响，并将为个性化药物干预提供新的靶点，旨在调节心脏功能失调的离子通道。

DESCRIPTION (provided by applicant): Sudden cardiac death (SCD) is a catastrophic event that accounts for up to 450,000 deaths each year in the US. Among patients at high risk for SCD are those with inherited cardiac arrhythmias. Long QT syndrome (LQTS) is one example of a group of inherited cardiac arrhythmias that produces defects in cardiac membrane currents. As a direct consequence, LQTS has been associated with prolongation of the QT interval on the ECG, ventricular arrhythmias, and an increased incidence of SCD. In LQTS2 well over two hundred missense mutations have been identified in the KCNH2 gene encoding hERG with the overwhelming majority thought to be characterized by protein processing and trafficking defects leading to a drastic reduction in potassium currents. However, as commonly observed in many autosomal dominant cardiac channelopathies the pattern of inheritance and clinical phenotypes of these patients are complex and often display incomplete penetrance, where disease-causing mutation carriers are asymptomatic. The causes for this variable clinical expressivity are not well understood but in the present research proposal, we will investigate this question by testing the hypothesis that modifier genes contribute to the variable clinical expressivity. Our multidisciplinary group at MetroHealth and Case Western Reserve University has studied clinically as well as in vitro a large 'Cleveland' LQT2 family carrying the hERG mutation R752W. Out of the 101 family members studied, 26 individuals carried the hERG R752W mutation. However, symptomatic LQTS was present in only 5 of the genetically affected family members thus illustrating incomplete penetrance of the disease. We hypothesize that the presence of disease modifying genes can explain the genotype-phenotype discordance observed in this LQT2 family. In this proposal, we will elucidate the mechanisms of incomplete penetrance in this LQT2 family using exome sequencing and cardiomyocytes differentiated from patient derived induced pluripotent stem cells (iPS). We hypothesize that patient-derived iPS differentiated cardiomyocytes (iPS-CM) faithfully recapitulate the arrythmogenic pathology and that heretofore unknown candidate genes revealed by exome sequencing account for variable phenotypic penetrance. The aims of this proposal are: 1. Identify candidate modifier genes responsible for incomplete penetrance in a LQT2 family. 2. Elucidate electrophysiological variability of human cardiomyocytes derived from LQT2 family members. 3. Determine the phenotype of candidate disease modifying gene variants. We will perform these aims by studying closely related LQT2 hERG R752W carrier pairs (i.e. father/son and sib pair) that display discordant clinical phenotype. We believe that the current proposal will offer a fundamental, mechanistic explanation by which genotype-phenotype discordance can arise in a large LQT2 family. This holds potentially significant ramifications for personalized clinical management and will offer novel targets for personalized pharmacologic intervention aimed at the modulation of dysfunctional ion channels in the heart.