Phenotypic Severity of SCN5A -Related Bradycardia Is Independent of Dominant-Negative and Coupled Gating Effects

Abstract Background Pathogenic SCN5A variants are associated with inherited arrhythmias such as long QT syndrome, Brugada syndrome, and sick sinus syndrome (SSS). While Na v 1.5, an α-subunit of the cardiac sodium channel encoded by SCN5A , has been considered to function as a monomer, recent studies reveal that a reduction of sodium current in wild-type (WT) Na v 1.5 can be caused by dimerization with loss-of-function (LOF) mutated Na v 1.5 through dominant-negative (DN) effects. However, the clinical significance of the DN effect remains unclear. Method We genetically screened a family who presented with SSS and sudden cardiac death (SCD). Whole-cell patch-clamp study using HEK293 cells co-expressing WT- and variant SCN5A was performed. Channel dimerization was assessed by co-immunoprecipitation and proximity ligation assays. Also, the effects of difopein, a high affinity inhibitor of Na v 1.5 interaction via 14-3-3 proteins, were evaluated. Results The proband carried compound heterozygous variants p.T1396P and p.G833R. The whole-cell mode patch-clamp techniques demonstrated that the p.T1396P showed a DN effect on the peak sodium currents (37% decrease in I Na ) and altered gating properties (+5.6 mV shift in steady-state inactivation) when expressed with WT SCN5A . These effects were abolished by difopein. p.G833R showed no DN or coupled gating effect but still formed dimers. The proband developed earlier and more severe bradycardia than her parent, who only carries p.T1396P, suggesting that loss of coupled gating effect contributed to the severe phenotype. Conclusion Our findings suggest that coupled gating may be physiologically important for normal Na v 1.5 function, and that its loss can exacerbate disease severity.