The mutation L69P in the PAS domain of the hERG potassium channel results in LQTS by trafficking deficiency

The congenital long QT syndrome (LQTS) is a cardiac disorder characterized by a prolonged QT interval on the electrocardiogram and an increased susceptibility to ventricular arrhythmias and sudden cardiac death. A frequent cause for LQTS is mutations in the <i>KCNH2</i> gene (also known as the <i>human ether-a-go-go-related gene</i> or <i>hERG</i>), which reduce or modulate the potassium current I_Kr and hence alter cardiac repolarization. In a patient with a clinically diagnosed LQTS, we identified the mutation L69P in the N-terminal PAS (Per-Arnt-Sim) domain of hERG. Functional expression in HEK293 cells shows that a homotetrameric hERG channel reconstituted with only mutant subunits exhibits a drastically reduced surface expression of the channel protein thus leading to a diminished hERG current. Unlike many other mutations in the hERG-PAS domain the negative impact of the L69P substitution cannot be rescued by facilitated protein folding at a lower incubation temperature. Further, co-expression of wt and mutant monomers does not restore either wt like surface expression or the full hERG current. These results indicate L69P is a dominant negative mutation, with deficits which most likely occurs at the level of protein folding and subsequently inhibits trafficking to the plasma membrane. The functional deficits of the mutant channel support the clinical diagnosis of a LQTS.

先天性长QT综合征（congenital long QT syndrome, LQTS）是一类以心电图QT间期延长、心室心律失常易感性升高及心源性猝死风险增加为典型特征的心脏疾病。LQTS的常见致病原因之一为<i>KCNH2</i>基因（又称人类ether-a-go-go相关基因（human ether-a-go-go-related gene，简称hERG））发生突变，此类突变可削弱或调控钾电流I_Kr，进而干扰心脏复极过程。本研究在1例临床确诊为LQTS的患者中，检出hERG的N端PAS（Per-Arnt-Sim）结构域存在L69P突变。在HEK293细胞中开展的功能表达实验结果显示，仅由突变亚基重构得到的同源四聚体hERG通道，其通道蛋白的表面表达量大幅降低，进而导致hERG电流减弱。与hERG-PAS结构域的多数其他突变不同，L69P突变的负面效应无法通过在较低孵育温度下促进蛋白折叠来挽救。此外，野生型与突变型单体的共表达，既无法恢复至野生型水平的表面表达量，也无法重建完整的hERG电流。上述实验结果证实，L69P属于显性负突变（dominant negative mutation），其功能缺陷极大概率源于蛋白折叠障碍，并随后抑制了通道蛋白向质膜的转运过程。该突变通道的功能缺陷，进一步佐证了该患者LQTS的临床诊断。