Overlap Syndrome of Cardiac Sodium Channel Disease in Mice Carrying the Equivalent Mutation of Human SCN5A -1795insD

Abstract

Patients carrying the cardiac sodium channel (SCN5A) mutation 1795insD show sudden nocturnal death and signs of multiple arrhythmia syndromes including bradycardia, conduction delay, QT prolongation, and right precordial ST-elevation. We investigated the electrophysiological characteristics of a transgenic model of the murine equivalent mutation 1798insD. On 24-hour continuous telemetry and surface ECG recordings, Scn5a(1798insD/+) heterozygous mice showed significantly lower heart rates, more bradycardic episodes (pauses > or = 500 ms), and increased PQ interval, QRS duration, and QTc interval compared with wild-type mice. The sodium channel blocker flecainide induced marked sinus bradycardia and/or sinus arrest in the majority of Scn5a(1798insD/+) mice, but not in wild-type mice. Epicardial mapping using a multielectrode grid on excised, Langendorff-perfused hearts showed preferential conduction slowing in the right ventricle of Scn5a(1798insD/+) hearts. On whole-cell patch-clamp analysis, ventricular myocytes isolated from Scn5a(1798insD/+) hearts displayed action potential prolongation, a 39% reduction in peak sodium current density and a similar reduction in action potential upstroke velocity. Scn5a(1798insD/+) myocytes displayed a slower time course of sodium current decay without significant differences in voltage-dependence of activation and steady-state inactivation, slow inactivation, or recovery from inactivation. Furthermore, Scn5a(1798insD/+) myocytes showed a larger tetrodotoxin-sensitive persistent inward current compared with wild-type myocytes. Mice carrying the murine equivalent of the SCN5A-1795insD mutation display bradycardia, right ventricular conduction slowing, and QT prolongation, similar to the human phenotype. These results demonstrate that the presence of a single SCN5A mutation is indeed sufficient to cause an overlap syndrome of cardiac sodium channel disease.