A novel SCN5A mutation manifests as a malignant form of long QT syndrome with perinatal onset of tachycardia/bradycardia

Objective: Congenital long QT syndrome (LQTS) with in utero onset of the rhythm disturbances is associated with a poor prognosis. in this study we investigated a newborn patient with fetal bradycardia, 2:1 atrioventricular block and ventricular tachycardia soon after birth. Methods: Mutational analysis and DNA sequencing were conducted in a newborn. The 2:1 atrioventricular block improved to 1:1 conduction only after intravenous lidocaine infusion or a high dose of mexiletine, which also controlled the ventricular tachycardia. Results: A novel, spontaneous LQTS-3 mutation was identified in the transmembrane segment 6 of domain IV of the Na(v)1.5 cardiac sodium channel, with a G-A substitution at codon 1763, which changed a valine (GTG) to a methionine (ATG). The proband was heterozygous but the mutation was absent in the parents and the sister. Expression of this mutant channel in tsA201 mammalian cells by site-directed mutagenesis revealed a persistent tetrodotoxin-sensitive but lidocaine-resistant current that was associated with a positive shift of the steady-state inactivation curve, steeper activation curve and faster recovery from inactivation. We also found a similar electrophysiological profile for the neighboring V1764M mutant. But, the other neighboring I1762A mutant had no persistent current and was still associated with a positive shift of inactivation. Conclusions: These findings suggest that the Na(v)1.5/V1763M channel dysfunction and possible neighboring mutants contribute to a 4 persistent inward current due to altered inactivation kinetics and clinically congenital LQTS with perinatal onset of arrhythmias that responded to lidocaine and mexiletine. (C) 2004 European Society of Cardiology Published by Elsevier B.V. All rights reserved.