Developmentally regulated SCN5A splice variant potentiates dysfunction of a novel mutation associated with severe fetal arrhythmia

Background

Congenital long-QT syndrome (LQTS) may present during fetal development and can be life-threatening. The molecular mechanism for the unusual early onset of LQTS during fetal development is unknown.

Objective

We sought to elucidate the molecular basis for severe fetal LQTS presenting at 19 weeks' gestation, the earliest known presentation of this disease.

Methods

Fetal magnetocardiography was used to demonstrated torsades de pointes and a prolonged rate-corrected QT interval. In vitro electrophysiological studies were performed to determine functional consequences of a novel SCN5A mutation found in the fetus.

Results

The fetus presented with episodes of ventricular ectopy progressing to incessant ventricular tachycardia and hydrops fetalis. Genetic analysis disclosed a novel, de novo heterozygous mutation (L409P) and a homozygous common variant (R558 in SCN5A). In vitro electrophysiological studies demonstrated that the mutation in combination with R558 caused significant depolarized shifts in the voltage dependence of inactivation and activation, faster recovery from inactivation, and a 7-fold higher level of persistent current. When the mutation was engineered in a fetal-expressed SCN5A splice isoform, channel dysfunction was markedly potentiated. Also, R558 alone in the fetal splice isoform evoked a large persistent current, and hence both alleles were dysfunctional.

Conclusion

We report the earliest confirmed diagnosis of symptomatic LQTS and present evidence that mutant cardiac sodium channel dysfunction is potentiated by a developmentally regulated alternative splicing event in SCN5A. Our findings provide a plausible mechanism for the unusual severity and early onset of cardiac arrhythmia in fetal LQTS.

Introduction

Congenital long-QT syndrome (LQTS) refers to a group of disorders with the primary impairment of myocardial repolarization predisposing to life-threatening cardiac arrhythmias especially torsades de pointes (TdP) that are caused by genetic mutations in cardiac ion channels or channel-modulating proteins.¹ The disease is typically recognized in late childhood or early adolescence, but extreme cases may present during infancy or in the perinatal period.2, 3, 4, 5 Clinical signs suggestive of fetal LQTS include ventricular tachycardia, second-degree atrioventricular (AV) block, and, most commonly, sinus bradycardia,6, 7 but such findings may go undetected owing to the lack of routine electrocardiographic testing of fetuses. Evidence for Mendelian inheritance is not always apparent in cases of fetal LQTS because of de novo mutations or germ line mosaicism.8, 9 Certain SCN5A mutations, many of which are de novo,2, 4, 5, 10, 11, 12, 13, 14 present with earlier onset and more severe congenital arrhythmia syndromes than is typical for LQTS. The reason for greater severity and lethality of certain genetic variants during early life is unknown.

Here we report the clinical, electrocardiographic, and genetic diagnosis of LQTS in a fetus at 19 weeks' gestation presenting with ventricular tachycardia and severe hydrops fetalis. To our knowledge, this is the earliest gestational age at which a diagnosis of LQTS has been made after being suspected on the basis of clinical presentation. A novel, de novo SCN5A mutation combined with a common genetic variant was discovered in the proband, and we demonstrated a plausible molecular basis for arrhythmia presentation. Specifically, we determined that the mutation and common variant both conferred severe functional disturbances when expressed in the context of a cardiac sodium channel isoform generated by a developmentally regulated SCN5A alternative splicing event. Our findings implicate dysfunction of a fetal-expressed sodium channel splice isoform as a predisposition to intrauterine mortality in LQTS. These results may have relevance to perinatal and neonatal deaths in other clinical settings.