Abstract 18946: A Complex Cardiac Rhythm and Conduction Abnormalities in Three Families Carrying a SCN5A Missense Mutation May Involve Single Nucleotide Polymorphisms in SCN5A and RYR2 genes

Abstract

Background: Mutations in the gene encoding the cardiac sodium channel (SCN5A) have been implicated in arrhythmia syndromes including Brugada (BrS) and long-QT type 3 syndromes. Also, wide spectrums of mixed phenotypes, known as “overlap syndromes”, have been associated to SCN5A mutations. The contributions of single nucleotide polymorphisms (SNPs) add more complexity to the disease phenotype. Here we report a missense mutation in SCN5A (T1708N) in three Hispanic families with history of sudden cardiac death (SCD) and extensive variability and severity of symptoms, including syncope, atrial and ventricular tachycardia, and conduction abnormalities. Methods and Results: We used the Ion Torrent platform and AmpliSeq panel to sequence 19 genes associated with arrhythmia syndromes. In addition to SCN5A-T1708N mutation, the patients carry two common SNPs, SCN5A-H558R and the cardiac ryanodine receptor RYR2-Q2958R, which we previously reported to be highly prevalent (~30%) in Hispanics and to increase the risk of SCD in homozygous individuals. To study the functional properties of SCN5A-T1708N, the mutation was engineered by site-directed mutagenesis into SCN5A with or without the SCN5A-H558R and expressed in HEK cells for voltage clamp experiments. SCN5A-T1708N showed a significant increase of late INa compared to WT. Also, the kinetic properties of SCN5A-T1708N showed slower time constants of recovery and increase in slow inactivation, which would tend to reduce INa; these effects were not modified by the presence of SCN5A-H558R. On the other hand, INa density for SCN5A-T1708N was reduced compared to WT (289± 63 and 387± 62 pA/pF, respectively), yet, for the double mutant H558R-T1708N, INa was more severely depressed (145± 32 pA/pF). Conclusion: The profound biophysical phenotype of T1708N with loss and gain of function could account for the variability and the severity of the clinical phenotype in these families. The RYR2-Q2958R polymorphism may function as a pro-arrhythmic genetic modifier and it is conceivable that it may predispose to SCD in some members of these families. The study of these polymorphisms in large cohorts may identify common haplotypes associated to cardiac arrhythmias and facilitate identification of patients at high risk for SCD.