Abstract：
Paramyotonia congenita (PMC) is a rare skeletal muscle disorder characterized by muscle stiffness upon repetitive exercise and cold exposure. PMC was reported to be caused by dominant mutations in the SCN4A gene encoding the α subunit of the Nav1.4 channel. Recently, we identified two missense mutations of the SCN4A gene, p.V781I and p.A1737T, in two PMC families. To evaluate the changes in electrophysiological properties caused by the mutations, both mutant and wild-type (WT) SCN4A genes were expressed in CHO-K1 and HEK-293T cells. Then, whole-cell patch-clamp recording was employed to study the altered gating of mutant channels. The activation curve of transient current showed a hyperpolarizing shift in both mutant Nav1.4 channels as compared to the WT channel, whereas there was a depolarizing shift in the fast inactivation curve. These changes confer to an increase in window current in the mutant channels. Further investigations demonstrated that the mutated channel proteins generate significantly larger resurgent currents as compared to the WT channel and take longer to attain the peak of resurgent current than the WT channel. In conclusion, the current study demonstrates that p.V781I and p.A1737T mutations in the Nav1.4 channel increase both the sustained and the resurgent Na+ current, leading to membrane hyperexcitability with a lower firing threshold, which may influence the clinical phenotype.
摘要：
先天性副肌强直症（PMC）是一种罕见的骨骼肌疾病，其特征是重复运动和寒冷暴露后的肌肉僵硬。据报道，PMC是由编码Nav1.4通道α亚基的SCN4A基因中的显性突变引起的。最近,我们确定了SCN4A基因的两个错义突变，p.V781I和p.A1737T,在两个PMC家族中。为了评估由突变引起的电生理特性的变化，突变型和野生型(WT)SCN4A基因均在CHO-K1和HEK-293T细胞中表达。然后，全细胞膜片钳记录用于研究突变通道的门控改变.与WT通道相比，瞬态电流的激活曲线显示两个突变体Nav1.4通道中的超极化位移，而快速失活曲线出现去极化偏移。这些变化赋予突变通道中窗口电流的增加。进一步的研究表明，与WT通道相比，突变的通道蛋白产生明显更大的复苏电流，并且比WT通道需要更长的时间才能达到复苏电流的峰值。总之,当前的研究表明，Nav1.4通道中的p.V781I和p.A1737T突变增加了持续和复苏的Na电流，导致膜过度兴奋，具有较低的射击阈值，这可能会影响临床表型。
