Mutation‐Specific Effects of Polymorphism H558R in SCN5A‐Related Sick Sinus Syndrome

Effects of H558R on SSS‐Related Mutant Channels. Introduction: Mutations in SCN5A, the gene encoding α subunit of cardiac type sodium channel, Na_v1.5, lead to familial sick sinus syndrome (SSS). Although several molecular mechanisms for this genetic condition have been explored, the underlying mechanisms for the variable genotype–phenotype relationships have not been well addressed. One of the important contributors to such relationships is the genetic background such as single‐nucleotide polymorphisms. Methods and Results: To clarify the effects of a common polymorphism in SCN5A gene, H558R, on SCN5A‐related SSS phenotype, we investigated the electrophysiological properties of all of the 13 known SSS‐related hNa_v1.5 mutant channels on both H558 and R558 background. Electrophysiological properties of hNa_v1.5 mutant channels were investigated by the whole‐cell patch clamp technique in HEK293 cells. When peak currents were affected by the mutation, cell surface biotinylation was performed to quantify the fraction of correctly cell membrane‐targeted mutant channels. Loss‐of‐function defect of D1275N in SCN5A was rescued by R558 through enhancing cell surface targeting and improving steady‐state activation of the mutant channels. In contrast, the defects of mutants E161K, P1298L, and R1632H were aggravated in the R558 background, mainly due to the reduced steady‐state availability of mutant channels. The electrophysiological properties of the remaining SSS‐related hNa_v1.5 mutants including the missense mutants (L212P, T220I, DelF1617, T187I, R878C, G1408R), and the truncated mutants (W1421X, K1578fs/52, R1623X) were not significantly affected by H558R. Conclusion: We conclude that polymorphism H558R has mutation‐specific effects on SCN5A‐related SSS. Our data highlight the importance of common genetic variants in modulating phenotypes of genetic diseases. (J Cardiovasc Electrophysiol, Vol. 21, pp. 564‐573, May 2010)