Biophysical characterization of M1476I, a sodium channel founder mutation associated with cold-induced myotonia in French Canadians

M1476I, a French Canadian founder mutation of Na+ channel Na(v)1.4, causes potassium-aggravated myotonia, with cold-induced myotonia as the most distinctive clinical feature. Mexiletine, a class 1B local anaesthetic, relieves the myotonic symptoms of patients carrying the M1476I mutation. We used the patch-clamp method to investigate the functional characteristics of thismutation by heterologous expression in tsA201 cells. TheM1476Imutation caused an increased persistent Na+ current, a 2-to 3-fold slower fast inactivation, a 6.4mV depolarizing shift in the midpoint of steady-state inactivation, and an accelerated recovery from fast inactivation compared to the wild-type (WT) channel. Cooling slowed the kinetics of both channel types and increased the amplitude of the persistent current in M1476I channels. Mexiletine suppressed the persistentNa+ current generated by the M1476Imutation and blocked both WT and M1476I channels in a use-dependent manner. The inactivation-deficient M1476I channelswere less susceptible tomexiletine during repetitive pulses. The decreased use-dependent block of M1476I channels might have resulted from the slower onset of mexiletine block, and/ or the faster recovery from mexiletine block, given that the affinity of mexiletine for the inactivated state of the WT and mutant channels was similar. Increased extracellular concentrations of potassium had no effect on either M1476I or WT currents. These results indicated that cooling can augment the disruption of the voltage dependence of fast inactivation by M1476I channels. The therapeutic efficacy of mexiletine in M1476I carriers may be partly due to the open-channel block targeting the persistent Na+ currents generated by M1476I channels.