Crispr-Mediated Expression Of Fetal Scn5a Splice Variant Results In Arrhythmias And Conduction Defects

Voltage-gated sodium channel type 1.5 (Na v 1.5), encoded by the gene SCN5A , is the predominant sodium channel in the heart. Mutations in SCN5A are associated with diseases including Brugada and long QT syndrome, which contribute to sudden unexplained death among other deleterious effects. Current research on SCN5A continues to primarily focus on mutations; however, the role of SCN5A splice variants in disease remains largely unexplored. It has been shown that SCN5A undergoes a developmentally-regulated splicing switch from the inclusion of exon 6A during the fetal stages of heart development to the inclusion of exon 6B in adults. This change from the fetal exon 6A to adult exon 6B results in a different Na v 1.5 isoform with different electrophysiological properties. An example that implicates the significance of this splicing event in disease is found in myotonic dystrophy type 1 (DM1). Arrhythmias and conduction defects are the second leading cause of mortality in DM1 and recent studies have shown SCN5A to be mis-spliced toward the fetal isoform in adults affected by this disease. With the aim of determining whether mis-spliced fetal Na v 1.5 expression contributes to arrhythmias and conduction defects, we created a CRISPR knockout of the Scn5a adult exon 6B in FVB mice thereby shifting the expression toward fetal exon 6A. Surface electrocardiography (ECG), echocardiography, and intracardiac electrophysiology (EP) studies were performed on homozygous (n=6) and heterozygous (n=8) knockout mice in addition to wild type littermates (n=8) at 2-3 months old. ECG showed decreased heart rate (p=0.0006) with prolonged QRS (p=0.0078) and PR (p=0.0196) intervals in the knockout compared to wild type. EP studies revealed prolonged sinus node recovery time (SNRT, p=0.0017) and atrioventricular effective refractory period (AVERP, p=0.0186) in addition to arrhythmias including sinus pause and premature atrial contractions. No significant differences were found in the echocardiogram. At 6 months of age, conduction defects were exacerbated in both heterozygous and homozygous knockouts compared to wild type. Thus, our findings suggest that the change in Scn5a splicing pattern significantly contributes to heart dysfunction and in an age-dependent manner.