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Loss-of-function mutations in the SCN5A gene, encoding the cardiac Nav1.5 sodium channel, have been previously associated with Brugada syndrome (BrS). Despite the low prevalence of the disease, we identified a patient carrying two SCN5A mutations. We aimed at establishing a correlation between genotype, clinical phenotype and in vitro sodium current. A 3-year-old boy presented with right bundle branch block and ST-segment elevation. Genetic analysis and electrophysiology studies in transfected HEK293 cells were performed to identify possibly disease-causing variants and assess their effect on sodium channel function. Two SCN5A variants were identified: a new frameshift deletion causing premature truncation of the putative protein (c.3258_3261del4) and a missense substitution (p.F1293S). In vitro studies revealed that the truncated mutant did not produce functional channels and decreased total sodium current when co-expressed with p.F1293S channels compared to p.F1293S alone. In addition, p.F1293S channels presented with a steep slope of steady-state activation voltagedependency, which was shifted towards more positive potentials by the co-expression with the truncated channel. p.F1293S channels also showed shift towards more positive potentials of the steady-state inactivation both alone and co-expressed with the deletion mutant. Our data identified a severe reduction of sodium channel current associated with two distinct SCN5A changes. However, all mutation carriers were asymptomatic and BrS electrocardiogram was observed only transiently in the compound heterozygous subject. These observations underline the difficulty of genotype/ phenotype correlations in BrS patients and support the idea of a polygenic disorder, where different mutations and variants can contribute to the clinical phenotype.
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