Introduction: Individual mutations in the SCN5A-encoding cardiac sodium channel α-subunit usually cause a single cardiac arrhythmia disorder, some cause mixed biophysical or clinical phenotypes. Here we report an infant, female patient harboring a N406K mutation in SCN5A with a marked and mixed biophysical phenotype and assess pathogenic mechanisms. Methods and Results: A patient suffered from recurrent seizures during sleep and torsades de pointes with a QTc of 530 ms. Mutational analysis identified a N406K mutation in SCN5A. The mutation was engineered by site-directed mutagenesis and heterologously expressed in HEK293 cells. After 48 hours incubation with and without mexiletine, macroscopic voltage-gated sodium current (INa) was measured with standard whole-cell patch clamp techniques. SCN5A-N406K elicited both a significantly decreased peak INa and a significantly increased late INa compared to wide-type (WT) channels. Furthermore, mexiletine both restored the decreased peak INa of the mutant channel and inhibited the increased late INa of the mutant channel. Conclusion: SCN5A-N406K channel displays both “gain-of-function” in late INa and “loss-of-function” in peak INa density contributing to a mixed biophysical phenotype. Moreover, our finding may provide the first example that mexiletine exerts a dual rescue of both “gain-of-function” and “loss-of-function” of the mutant sodium channel.

Background SCN5A is a susceptibility gene for type 3 long QT syndrome, Brugada syndrome, and sudden infant death syndrome. I Na dysfunction from mutated SCN5A can depend upon the splice variant background in which it is expressed and also upon environmental factors such as acidosis. S1787N was reported previously as a LQT3-associated mutation and has also been observed in 1 of 295 healthy white controls. Here, we determined the in vitro biophysical phenotype of SCN5A-S1787N in an effort to further assess its possible pathogenicity.Methods and ResultsWe engineered S1787N in the two most common alternatively spliced SCN5A isoforms, the major isoform lacking a glutamine at position 1077 (Q1077del) and the minor isoform containing Q1077, and expressed these two engineered constructs in HEK293 cells for electrophysiological study. Macroscopic voltage-gated I Na was measured 24 hours after transfection with standard whole-cell patch clamp techniques. We applied intracellular solutions with pH7.4 or pH6.7. S1787N in the Q1077 background had WT-like I Na including peak I Na density, activation and inactivation parameters, and late I Na amplitude in both pH 7.4 and pH 6.7. However, with S1787N in the Q1077del background, the percentages of I Na late/peak were increased by 2.1 fold in pH 7.4 and by 2.9 fold in pH 6.7 when compared to WT.ConclusionThe LQT3-like biophysical phenotype for S1787N depends on both the SCN5A splice variant and on the intracellular pH. These findings provide further evidence that the splice variant and environmental factors affect the molecular phenotype of cardiac SCN5A-encoded sodium channel (Nav1.5), has implications for the clinical phenotype, and may provide insight into acidosis-induced arrhythmia mechanisms.