Mutation of Cardiac Nav1.5 in an Hisian-Type Arrhythmia, Associated with Dilated Cardiomayopathy

Abstract

Cardiac sodium channels are complexes including alpha and beta1 subunits allowing sodium influx during the depolarization phase of the ventricular action potential. The pore-forming alpha subunit, Nav1.5, is encoded by SCN5A. Using a candidate-gene approach, we detected a variant of SCN5A, leading to the R222Q substitution by screening one family with cardiac arrhythmia resulting in frequent premature ventricular contractions, non sustained ventricular tachycardia and dilated cardiomyopathy. Arrhythmia mechanisms involved ectopic foci originating from the proximal part of the His-Purkinje system. To evaluate the incidence of this substitution on Nav1.5 function, whole-cell patch-clamp experiments were performed on COS-7 cells transfected with the human alpha and beta1 subunits. The presence of the mutation at the heterozygous or homozygous state did not modify the sodium current density. In contrast, the activation curve was shifted toward more negative potentials (V1/2act, WT: −30.6±2.1 mV, n=11; R222Q: −42.3±1 mV, n=11, p<0.001; heterozygous: −37.2±1.6 mV, n=9; p<0.05) and the slope was changed in the heterozygous condition only (WT: 5.7±0.3 mV, R222Q: 6.5±0.4 mV, heterozygous: 7.1±0.3 mV; p<0.01). Activation kinetics were also accelerated in mutant homozygous condition only (p<0.001, versus WT). Inactivation voltage sensitivity was also changed (V1/2inact, WT: −79.6±0.7 mV, n=10; R222Q: −84.6±0.7 mV, n=8, p<0.001; heterozygous: −82.2±1 mV, n=9; p<0.05), its kinetics accelerated (p<0.001 versus WT for both mutant and heterozygous conditions) and the slope was changed in the mutant homozygous condition only (WT: 5.6±0.2 mV; R222Q: 4.8±0.2 mV; p<0.01; heterozygous: 5.3±0.1 mV). Finally, recovery from inactivation was not modified by the R222Q mutation. We studied the impact of the current biophysical changes in cellular models of the Purkinje and ventricular action potentials. The premature ventricular contractions are explained by the appearance of electrical abnormalities rather in Purkinje fibers than in ventricular cardiomyocytes.