Congenital long QT syndrome (LQTS) is an inherited channelopathy associated with life-threatening arrhythmias. LQTS type 2 (LQT2) is caused by mutations in KCNH2, which encodes the potassium channel hERG. We hypothesized that modifier genes are partly responsible for the variable phenotype severity observed in some LQT2 families. Here, we identified contributors to variable expressivity in an LQT2 family by using induced pluripotent stem cell–derived cardiomyocytes (iPSC-CMs) and whole exome sequencing in a synergistic manner. We found that iPSC-CMs recapitulated the clinical genotype-phenotype discordance in vitro. Importantly, iPSC-CMs derived from the severely affected LQT2 patients displayed prolonged action potentials compared with cells from mildly affected first-degree relatives. The iPSC-CMs derived from all patients with hERG R752W mutation displayed lower I_Kr amplitude. Interestingly, iPSC-CMs from severely affected mutation-positive individuals exhibited greater L-type Ca²⁺ current. Whole exome sequencing identified variants of KCNK17 and the GTP-binding protein REM2, providing biologically plausible explanations for this variable expressivity. Genome editing to correct a REM2 variant reversed the enhanced L-type Ca²⁺ current and prolonged action potential observed in iPSC-CMs from severely affected individuals. Thus, our findings showcase the power of combining complementary physiological and genomic analyses to identify genetic modifiers and potential therapeutic targets of a monogenic disorder. Furthermore, we propose that this strategy can be deployed to unravel myriad confounding pathologies displaying variable expressivity.

中文翻译：

---

生理基因组学确定长QT综合征2型严重程度的遗传修饰因子

先天性长QT综合征（LQTS）是与威胁生命的心律不齐相关的遗传性通道病。LQTS 2型（LQT2）是由KCNH2中的突变引起的，它编码钾通道hERG。我们假设修饰基因部分负责某些LQT2家族中观察到的可变表型的严重性。在这里，我们通过诱导多能干细胞衍生的心肌细胞（iPSC-CM）和整个外显子组序列的协同作用，确定了LQT2家族中可变表达的贡献者。我们发现，iPSC-CMs在体外概括了临床基因型-表型不一致。重要的是，与受到轻度影响的一级亲属的细胞相比，受到严重影响的LQT2患者的iPSC-CM表现出了更长的动作电位。来自所有具有hERG R752W突变的患者的iPSC-CM显示出较低的I _Kr振幅。有趣的是，来自严重受影响的突变阳性个体的iPSC-CMs表现出更大的L型Ca ²⁺电流。整个外显子组测序确定了KCNK17和GTP结合蛋白REM2的变异体，为这种可变表达提供了生物学上合理的解释。基因组编辑以纠正REM2变异逆转了增强的L型Ca ²⁺受严重影响的人在iPSC-CM中观察到当前和长期的动作电位。因此，我们的研究结果展示了结合互补的生理和基因组分析来鉴定单基因疾病的遗传修饰因子和潜在治疗靶点的力量。此外，我们建议可以部署此策略来解开显示可变表达的无数混杂病理。