A distinct molecular mechanism by which phenytoin rescues a novel long QT 3 variant.

Background: Genetic variants in SCN5A can result in channelopathies such as the long QT syndrome type 3 (LQT3), but the therapeutic response to Na ⁺ channel blockers can vary. We previously reported a case of an infant with malignant LQT3 and a missense Q1475P SCN5A variant, who was effectively treated with phenytoin, but only partially with mexiletine. Here, we functionally characterized this variant and investigated possible mechanisms for the differential drug actions.
Methods: Wild-type or mutant Na _v 1.5 cDNAs were examined in transfected HEK293 cells with patch clamping and biochemical assays. We used computational modeling to provide insights into altered channel kinetics and to predict effects on the action potential.
Results: The Q1475P variant in Na _v 1.5 reduced the current density and channel surface expression, characteristic of a trafficking defect. The variant also led to positive shifts in the voltage dependence of steady-state activation and inactivation, faster inactivation and recovery from inactivation, and increased the "late" Na ⁺ current. Simulations of Na _v 1.5 gating with a 9-state Markov model suggested that transitions from inactivated to closed states were accelerated in Q1475P channels, leading to accumulation of channels in non-inactivated closed states. Simulations with a human ventricular myocyte model predicted action potential prolongation with Q1475P, compared with wild type, channels. Patch clamp data showed that mexiletine and phenytoin similarly rescued some of the gating defects. Chronic incubation with mexiletine, but not phenytoin, rescued the Na _v 1.5-Q1475P trafficking defect, thus increasing mutant channel expression.
Conclusions: The gain-of-function effects of Na _v 1.5-Q1475P predominate to cause a malignant long QT phenotype. Phenytoin partially corrects the gating defect without restoring surface expression of the mutant channel, whereas mexiletine restores surface expression of the mutant channel, which may explain the lack of efficacy of mexiletine when compared to phenytoin. Our data makes a case for experimental studies before embarking on a one-for-all therapy of arrhythmias.
Competing Interests: Declaration of Competing Interest None of the authors have any significant affiliations or relationships with commercial enterprise or any other potential conflicts of interest.
(Copyright © 2020 Elsevier Ltd. All rights reserved.)