Arrhythmogenic Biophysical Phenotype for SCN5A Mutation S1787N Depends upon Splice Variant Background and Low pH

Background: SCN5A encodes the voltage-dependent sodium channel α-subunit hNav1.5 that is responsible for the peak inward sodium current (INa) that underlies excitability and conduction in the heart and the late INa that influences repolarization and refractoriness. Mutations in SCN5A that pathologically increase late INa cause type 3 long QT syndrome (LQT3). INa 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 results: We 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 which contains Q1077, and expressed them in HEK-293 cells for voltage clamp study. After 24h transfection, S1787N in Q1077 background had WT-like INa including peak density, activation, inactivation and late INa in both pHi 7.4 and pHi 6.7. However, with S1787N in the Q1077del background, the percentage of INa late/peak was increased 2.1 fold compared to WT in pHi 7.4 (n= 7-9, p<0.05) and was increased 2.9 fold compared to WT in pHi 6.7 (n= 6-8, p<0.03). In adult rat ventricular myocytes infected with an adenoviral recombinant of S1787N/Q1077del, action potential duration was prolonged in normal pH.Conclusion: An LQT3-like biophysical phenotype for S1787N is both SCN5A isoform and intracellular pH dependent. These findings provide further evidence that the splice variant and environmental factors affect the molecular phenotype with implications for the clinical phenotype and may provide insight into acidosis-induced arrhythmia mechanisms.