Beneficial effects of chronic mexiletine treatment in a human model of SCN5A overlap syndrome.

Aims SCN5A mutations are associated with various cardiac phenotypes, including long QT syndrome type 3 (LQT3), Brugada syndrome (BrS), and cardiac conduction disease (CCD). Certain mutations, such as SCN5A -1795insD, lead to an overlap syndrome, with patients exhibiting both features of BrS/CCD [decreased sodium current (I _Na)] and LQT3 (increased late I _Na). The sodium channel blocker mexiletine may acutely decrease LQT3-associated late I _Na and chronically increase peak I _Na associated with SCN5A loss-of-function mutations. However, most studies have so far employed heterologous expression systems and high mexiletine concentrations. We here investigated the effects of a therapeutic dose of mexiletine on the mixed phenotype associated with the SCN5A -1795insD mutation in HEK293A cells and human-induced pluripotent stem cell–derived cardiomyocytes (hiPSC-CMs). Methods and results To assess only the chronic effects on trafficking, HEK293A cells transfected with wild-type (WT) SCN5A or SCN5A -1795insD were incubated for 48 h with 10 µ m mexiletine followed by wash-out, which resulted in an increased peak I _Na for both SCN5A -WT and SCN5A -1795insD and an increased late I _Na for SCN5A -1795insD. Acute re-exposure of HEK293A cells to 10 µ m mexiletine did not impact on peak I _Na but significantly decreased SCN5A -1795insD late I _Na. Chronic incubation of SCN5A -1795insD hiPSC-CMs with mexiletine followed by wash-out increased peak I _Na, action potential (AP) upstroke velocity, and AP duration. Acute re-exposure did not impact on peak I _Na or AP upstroke velocity, but significantly decreased AP duration. Conclusion These findings demonstrate for the first time the therapeutic benefit of mexiletine in a human cardiomyocyte model of SCN5A overlap syndrome. [ABSTRACT FROM AUTHOR]