SCN5A Rare Variants in Familial Dilated Cardiomyopathy Decrease Peak Sodium Current Depending on the Common Polymorphism H558R and Common Splice Variant Q1077del

Obtaining functional data with newly identified rare variants increases certainty that the variant identified is relevant for dilated cardiomyopathy (DCM) causation. Two novel SCN5A rare variants, R222Q and I1835T, segregated with DCM in two families with affected individuals homozygous or heterozygous for the common SCN5A polymorphism H558R. cDNAs with each rare variant were constructed in the common Q1077del or Q1077 splice variant backgrounds with and without the H558R polymorphism and expressed in HEK293 cells. Sodium current (I(Na) ) was studied for each using whole-cell voltage clamp. In the Q1077del background I(Na) densities of R222Q and I1835T were not different from wild type, but the combined variants of R222Q/H558R, I1835T/H558R caused approximately 35% and approximately 30% reduction, respectively, and each showed slower recovery from inactivation. In the Q1077del background R222Q and R222Q/H558R also exhibited a significant negative shift in both activation and inactivation while I1835T/H558R showed a significant negative shift in inactivation that tended to decrease window current. In contrast, expression in the Q1077 background showed no changes in peak I(Na) densities, decay, or recovery from inactivation for R222Q/H558R and I1835T/H558R. We conclude that the biophysical findings, dependent upon common SCN5A variants, provide further evidence that these novel SCN5A rare variants are relevant for DCM.