SCN5A Missense Mutation from a Patient with Complex Cardiac Rhythm and Conduction Disorder Requires the Common Polymorphism H558R on the Same Allele for Arrhythmogenic Biophysical Phenotype

Abstract

Background: Mutations in SCN5A that decrease peak INa cause several arrhythmogenic syndromes such as type 1 Brugada syndrome (BrS1), cardiac conduction disease (CCD), and congenital sick sinus syndrome (SSS). Here, we report a novel missense mutation (V240M) in SCN5A, in the presence of the common polymorphism H558R that perturbs cardiac rhythm and conduction. Methods and results: A 10-year-old boy had atrial fibrillation and sinus pauses up to 6 seconds during 24h Holter monitoring. Nine months after pacemaker implantation, monomorphic ventricular tachycardia (VT) was recorded during exercise. He was diagnosed clinically with SSS, CCD and VT. Comprehensive open reading frame/splice site mutational analysis of SCN5A was performed using DHPLC and DNA sequencing. A missense mutation (V240M), localized between the DI-S4 and DI-S5 region of the sodium channel, and the common polymorphism H558R were found on the same allele. The double mutation was engineered by site direct mutagenesis and expressed in HEK cells for voltage clamp study. After 24h of transfection, the current densities of SCN5A-V240M were reduced compared with WT channels (-175 ± 27 pA/pF for V240M and -417 ± 100 pA/pF for WT), after 48h of incubation, the current densities of SCN5A-V240M were comparable to WT levels. However, SCN5A-V240M/H558R had current densities dramatically reduced (-34 ± 17 pA/pF). In addition, gating kinetic analysis showed a 10 mV negative shift of inactivation and slower time constants of recovery, all of which would tend to reduce peak INa. Conclusion: The profound biophysical phenotype with loss of function could account for the severity of the clinical phenotype. The requirement for H558R represents another example, and a dramatic one, of phenotype modification by this common polymorphism.