Sodium Channel Variants Associated with Atrial Fibrillation Exhibit Abnormal Fast and Slow Inactivation

Abstract

Mutations and rare genetic variants in SCN5A, the gene encoding the cardiac voltage-gated sodium channel NaV1.5, have been associated with inherited predisposition to ventricular arrhythmia. More recently, SCN5A variants have been identified in families segregating atrial fibrillation. We evaluated the biophysical properties of seven novel SCN5A variants associated with atrial fibrillation identified by our previous genetic study to elucidate potential molecular mechanisms underlying this common arrhythmia. Functional properties of E428K, H445D, N470K, E655K, T1131I, R1826C and V1951M were assessed by whole-cell patch clamp recording of recombinant mutant channels heterologously expressed with the human β1 subunit in tsA201 cells. One variant (R1826C) did not exhibit substantial differences in biophysical properties of activation or fast inactivation, and another variant (E655K) only exhibited minor differences in recovery from inactivation as compared with wildtype (WT) channels. However, two mutants (H445D, T1131I) exhibited significant shifts in the voltage-dependence of activation toward more negative potentials (p < 0.005), and four other mutant channels (E428K, H445D, N470K, V1951M) exhibited significant shifts in the voltage-dependence of steady-state inactivation toward more positive potentials as compared with WT channels. Further, H445D and V1951M exhibited more rapid onset, impaired recovery and enhancement of slow inactivation evoked by 1000 ms depolarizing prepulses as compared with WT channels. For the five variants with either hyperpolarized activation voltage-dependence or depolarized steady-state inactivation, we predict increased window current as defined by the overlap of these two curves. Increased window current and enhanced slow inactivation of some variants is further predicted to alter excitability and/or conduction in myocardial tissue and is a plausible mechanism by which SCN5A variants may increase vulnerability to atrial fibrillation.