HUMAN SODIUM-CHANNEL GATING DEFECTS CAUSED BY MISSENSE MUTATIONS IN S6 SEGMENTS ASSOCIATED WITH MYOTONIA - S804F AND V1293I

1. Missense mutations in the alpha-subunit of the human skeletal muscl e sodium channel (hSkM1) have been detected In some heritable forms or myotonia. By recording Na+ currents from cells transfected with cDNA encoding either wild-type or mutant hSkM1, we characterized the functi onal consequences of two myotonia-associated mutations that lie at the cytoplasmic end of the sixth transmembrane segment in domain II (S804 F) or domain III (V1293I). 2. Both mutations caused modest, but unequi vocal, alterations in the voltage-dependent gating behaviour of hSkM1. For S804F, the abnormalities were limited to fast inactivation: the p ersistent Na+ cnrrent at the end of a 50 ms depolarization was increas ed 3-fold, the rate of inactivation from the open state was slowed 2-f old, and the voltage dependence of fast inactivation (h(infinity)) was shifted by +3 mV. V1293I also disrupted fast inactivation, as evidenc ed by a 3-fold faster rate of recovery at hyperpolarized potentials (l ess than or equal to -70 mV). Activation was altered as well for V1293 I: the voltage dependence was shifted by -6 mV (hyperpolarized). 3. Sl ow inactivation was not altered by S804F or V1293I. 4. We conclude tha t S804F and V1293I are not benign polymorphisms. Either mutation cause s detectable alterations in channel gating and, in model simulations, the magnitude of the defects is sufficient to produce runs of myotonic discharges.