Abstract 16964: SCN5A Splice Variant Potentiates Dysfunction of a Novel LQT3 Mutation Associated with Life-threatening Arrhythmia in a 19 Week Fetus

Abstract

BACKGROUND: Inherited arrhythmia susceptibility such as the congenital long-QT syndrome (LQTS) can contribute to infant mortality, but the biological basis for LQTS presenting in utero is poorly understood. The purpose of this study was to elucidate the molecular genetic and pathophysiologic basis for LQTS and torsade de pointes in a fetus presenting at 19-weeks gestation, the earliest known presentation of this disease. METHODS: Molecular genetic studies of an affected fetus demonstrated a de novo SCN5A mutation. To elucidate the functional consequences of the mutation, in vitro electrophysiological studies were conducted on wild type (WT) and mutant human cardiac sodium channels in the canonical channel and in an alternatively spliced form expressed in fetal heart. RESULTS: A fetus presented at 19-6/7 weeks with episodes of ventricular ectopy that progressed to incessant ventricular tachycardia and hydrops fetalis. Family history did not suggest inherited arrhythmia susceptibility, but fetal magnetocardiography demonstrated torsade de pointes and a prolonged rate-corrected QT interval (604 ms). Genetic analysis following pregnancy termination disclosed a novel, de novo heterozygous SCN5A mutation (L409P) and homozygosity for the common SCN5A variant H558R. In vitro studies demonstrated that the mutation in combination with R558 causes significant depolarized shifts in voltage-dependence of steady-state inactivation and activation, accelerated recovery from inactivation and a 7-fold increase in persistent current (1.4% of peak current vs 0.2% for WT). However, when the mutation was expressed in the background of a fetal-expressed SCN5A transcript, channel dysfunction was potentiated with a marked increase in persistent current (11% of peak current) and greater shifts in voltage-dependence. Also, R558 alone in the fetal splice variant evoked a large increased persistent current (3.7% of peak current), hence both alleles were dysfunctional. CONCLUSION: Fetal LQTS may present as early as 19 weeks gestation. The functional effects of certain SCN5A mutations and variants may be magnified by alternative splicing events during fetal development helping to explain the unusual clinical severity of LQT3 in this setting.