R222Q Nav1.5 Mutation Associated with a New SCN5A-Related Cardiac Arrhythmia

Abstract

Using a candidate-gene approach, we detected a variant of SCN5A, encoding the cardiac Na+ channel Nav1.5, by screening a family with cardiac arrhythmia resulting in frequent premature ventricular contractions (PVCs) and non-sustained ventricular tachycardia. Arrhythmia mechanism involved ectopic foci originating from the His-Purkinje system. The same mutation, leading to the R222Q substitution, was present in two additional unrelated families with the same associated cardiac phenotype. Exercise or hydroquinidine dramatically decreased the number of PVCs. To evaluate the functional incidence of this substitution, whole-cell patch-clamp experiments were performed on transfected COS-7 cells. The activation and inactivation curves were negatively shifted in the presence of the mutation (V1/2act, WT: −30.6±2.1 mV, n=9; heterozygous: −37.2±1.6 mV, n=9; p<0.05; V1/2inact, WT: −79.6±0.7 mV, n=10; heterozygous: −82.2±1 mV, n=9; p<0.05) whereas the current density was unchanged. The use of depolarizing-voltage ramp confirmed the increase and negative shift of the TTX-sensitive window current (potential of gmax, WT: −42.8±0.5 mV, n=12; homozygous: −58.6±1.1 mV, n=, p<0.001). WT and R222Q peak INa were similarly half-reduced by 30 μM quinidine (p<0.001 vs control) as well as the window current (WT: from −1.87±0.42 to −0.74±0.16 pA/pF in quinidine, n=7; homozygous: −2.54±0.41 to −1.17±0.12 pA/pF in quinidine, n=6; p<0.001 vs. control).We carried out computer simulations in single-cell models of human Purkinje fibers and ventricle action potentials (AP). In heterozygous conditions, incomplete repolarization occurred in Purkinje cells only. We also built a multicellular model incorporating both cell models. In the heterozygous conditions, incomplete repolarization in the Purkinje fibers triggered premature APs propagating into the ventricle. This was normalized at higher pacing frequency or when quinidine was ‘added’.From all these studies, the premature ventricular contractions are explained by the appearance of electrical abnormalities rather in Purkinje fibers than in ventricular cardiomyocytes.