“We report the in vitro functional characterization of the p.L689F mutation in the human Nav1.4 sodium channel, associated with severe myotonia. The proband is the second of two siblings from non-consanguineous parents and the sole affected by myotonia. Since early infancy, he showed bilateral talus feet, convergent strabismus, generalized muscular hypertrophy, and neck stiffness. A permanent myotonia progressively appeared in the four limbs and jaws. EMG showed abundant myotonic discharges and type III pattern, suggesting sodium channel myotonia. He currently obtains partial symptom relief with carbamazepine. A trial of mexiletine was interrupted due to limited efficacy. The c.2065C>T (p.L689F) mutation in SCN4A was found in the proband, whereas absent in relatives.
Whole-cell sodium currents were recorded using patch-clamp technique in HEK293 cells transfected with either L689F or wild-type (WT) hNav1.4 cDNA.
The L689F and WT sodium currents were apparently similar with no difference in current decay. Yet, the I-V relationship of L689F was shifted toward more negative voltages, because of significant shifts of activation (about -13 mV) and fast inactivation (about -6 mV). The window current (open probability) generated by L689F was consequently greatly increased and shifted toward negative voltages. Such effects likely account for myotonia. In addition, the voltage dependence of slow inactivation was significantly shifted by -20 mV, which might partly explain the absence of flaccid paralysis.
In conclusion, the functional in vitro characterization confirms the genotype/phenotype relationship, allowing precision diagnosis, while undergoing in vitro pharmacological studies may help to define a more effective therapy.”