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

Abstract

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 torsade 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 in SCN5A (L409P) and a homozygous common variant (R558). In vitro electrophysiological studies demonstrated that the mutation in combination with R558 caused significant depolarized shifts in voltage-dependence of inactivation and activation, faster recovery from inactivation and a 7-fold greater 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, 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.

In Utero Diagnosis of Long QT Syndrome by Magnetocardiography

“…The most unusual tracing was seen in the 21 week fetus with a de novo SCN5A L409P mutation (subject #16; Figure 5B). 11 In this tracing, the T-wave is not well defined and instead, repolarization appears to occur continuously throughout the cardiac cycle. In contrast to adult TWA, the episodes of fetal TWA were typically brief and transient, often lasting 10 s or less.…”

Arrhythmia Phenotype During Fetal Life Suggests Long-QT Syndrome Genotype

“…For example, studies of SCN5A-R1623Q, noted in sporadic LQTS cases with severe perinatal arrhythmia 2, 4, 12 , identified a novel LQTS mechanism characterized by early channel re-openings and increased probability of long openings 12 . On the other hand, sporadic occurrence of SCN5A-L409P in combination with H558R caused significant depolarized shifts in voltage-dependence of inactivation and activation, faster recovery from inactivation and a greater level of persistent current potentiated by a developmentally regulated alternative splicing event in SCN5A 3 . In contrast, KCNH2-G628S, a mutation in the pore of the KCNH2 -encoded Kv11.1 potassium channel, was reported to have a dominant negative effect on wild type I Kr 13,14 .…”

“…Two subjects had uncharacterized mutations (Table 1). De novo mutations were found in 7 subjects: 6 in Group 1 (4 with SCN5A-R1623Q, 1 each with SCN5A-L409P 3 and KCNH2-G628S 10 ) and 1 in Group 2 with CALM2 9 . The portion of subjects with a familial/inherited mutation varied: Group 1 (14%), Group 2 (75%), Group 3 (92%).…”

Dominant negative consequences of a hERG 1b-specific mutation associated with intrauterine fetal death

“…Developmentally-dependent disease phenotypes have been described for cardiac sodium channels, where the LQTS mutation L409P/H558R confers a dramatically more deleterious phenotype in the fetal splice variant compared to the adult variant (Murphy et al, 2012). Interestingly, the ratio of hERG 1b/1a transcript is greater in fetal compared to adult heart, suggesting that 1b-specific mutations may be more deleterious early in development (Crotti et al, 2013).…”