The Physiology and Pathophysiology of a Fetal Splice Variant of the Cardiac Sodium Channel

Abstract

Mutations in SCN5A encoding the cardiac voltage-gated sodium channel (NaV1.5) can result in severe life-threatening cardiac arrhythmias such as long QT syndrome (LQTS). However, the molecular basis for arrhythmia susceptibility in early developmental stages remains unclear. Our lab has shown prominent expression of a fetal-expressed splice variant of the cardiac sodium channel (fetal NaV1.5) in human fetal and infant hearts. We hypothesized that mutations in SCN5A expressed in the fetal NaV1.5 may result in more severe functional effects vs expression in adult NaV1.5. Electrophysiological studies were conducted on heterologously expressed WT or mutant adult or fetal NaV1.5. We compared the functional effects of SCN5A mutations associated with early-onset LQTS with that of a mutation associated with typical onset LQTS (delKPQ) expressed in the context of fetal NaV1.5. We have shown that early onset LQT3 mutations exhibit equal or more severe functional consequences such as persistent sodium current (INa) in the fetal NaV1.5 vs expression in the adult NaV1.5. Typical onset mutation, delKPQ, demonstrated an attenuated gain of function in fetal NaV1.5. In addition to these findings, we elucidated the molecular mechanism of calmodulin (CaM) mutations associated with neonatal LQTS on NaV1.5. We hypothesized that mutant CaM would evoke an increased persistent INa associated with LQTS. Electrophysiological studies were conducted on WT adult or fetal NaV1.5 co-expressed with WT or mutant calmodulin. We observed that CaM-D130G co-expressed with fetal NaV1.5 exhibits a significantly greater persistent INa vs co-expression with WT CaM. This increase in persistent INa was not observed for CaM-D130G co-expressed with the canonical NaV1.5 and required the presence of high calcium. We also show that CaM mutations caused slowing of calcium-dependent inactivation of L-type calcium channels. We conclude that developmentally regulated alternative splicing of SCN5A contributes to the genetic risk for prenatal life-threatening cardiac arrhythmias. We also conclude that the voltage-gated cardiac sodium channel is not consistently involved in the molecular mechanism of LQTS associated with mutations in calmodulin.