Department of Biology, York University, Toronto, Ontario, Canada.
Electrophysiology and Heart Modeling Institute (LIRYC), Bordeaux University Foundation, Pessac, France.
PLoS One. 2018 May 23;13(5):e0197273. doi: 10.1371/journal.pone.0197273. eCollection 2018.
The SCN5A mutation, P1332L, is linked to a malignant form of congenital long QT syndrome, type 3 (LQT3), and affected patients are highly responsive to the Na+ channel blocking drug, mexiletine. In contrast, A647D is an atypical SCN5A mutation causing Brugada syndrome. An asymptomatic male with both P1332L and A647D presented with varying P wave/QRS aberrancy and mild QTc prolongation which did not shorten measurably with mexiletine.
We characterized the biophysical properties of P1332L, A647D and wild-type (WT) Na+ channels as well as their combinations in order to understand our proband's phenotype and to guide mexilitine therapy.
Na+ channel biophysics and mexilitine-binding kinetics were assessed using heterologous expression studies in CHO-K1 cells and human ventricular myocyte modeling.
Compared to WT, P1332L channels displayed a hyperpolarizing shift in inactivation, slower inactivation and prominent late Na+ currents (INa). While A647D had no effect on the biophysical properties of INa, it reduced peak and late INa density when co-expressed with either WT or P1332L. Additionally, while P1332L channels had greater sensitivity to block by mexiletine compared to WT, this was reduced in the presence of A647D. Modelling studies revealed that mixing P1332L with A647D channels, action potential durations were shortened compared to P1332L, while peak INa was reduced compared to either A647D coexpressing with WT or WT alone.
While A647D mitigates the lethal LQT3 phenotype seen with P1332L, it also reduces mexilitine sensitivity and decreases INa density. These results explain our proband's mild repolarization abnormality and prominent conduction defect in the atria and ventricles, but also suggest that expression of P1332L with A647D yields a novel disease phenotype for which mexiletine pharmacotherapy is no longer suitable.
SCN5A 突变 P1332L 与恶性先天性长 QT 综合征 3 型（LQT3）有关，受影响的患者对钠离子通道阻断药物美西律高度敏感。相比之下，A647D 是导致 Brugada 综合征的非典型 SCN5A 突变。一名患有 P1332L 和 A647D 突变的无症状男性表现出不同的 P 波/QRS 异常和轻度 QTc 延长，美西律不能显著缩短。
我们描述了 P1332L、A647D 和野生型（WT）Na+通道的生物物理特性及其组合，以了解我们的先证者的表型并指导美西律治疗。
使用 CHO-K1 细胞和人心室肌细胞建模的异源表达研究评估 Na+通道生物物理特性和美西律结合动力学。
与 WT 相比，P1332L 通道表现出失活的超极化移位、较慢的失活和明显的晚期 Na+电流（INa）。虽然 A647D 对 INa 的生物物理特性没有影响，但当与 WT 或 P1332L 共表达时，它会降低峰值和晚期 INa 密度。此外，虽然 P1332L 通道对美西律的阻断比 WT 更敏感，但在存在 A647D 的情况下，这种敏感性降低。模型研究表明，与 P1332L 相比，混合 P1332L 和 A647D 通道会缩短动作电位持续时间，而与 A647D 与 WT 共表达或 WT 单独表达相比，峰值 INa 降低。
虽然 A647D 减轻了 P1332L 所见的致命 LQT3 表型，但它也降低了美西律的敏感性并降低了 INa 密度。这些结果解释了我们先证者的轻微复极异常和心房和心室的明显传导缺陷，但也表明 P1332L 与 A647D 的表达产生了一种新的疾病表型，美西律药物治疗不再适用。
