FUNCTIONAL EXPRESSION OF THE ILE693THR NA-CONGENITA IN A HUMAN CELL-LINE( CHANNEL MUTATION ASSOCIATED WITH PARAMYOTONIA)

1. The Ile693Thr mutation of the skeletal muscle Na+ channel alpha-sub unit is associated with an unusual phenotype of paramyotonia congenita characterized by cold-induced muscle weakness but no stiffness. This mutation occurs in the S4-S5 linker of domain II, a region that has no t been previously implicated in paramyotonia congenita. 2. The Ile693T hr mutation was introduced into the human skeletal muscle Na+ gene for functional expression in human embryonic kidney (HEK) cells. The curr ents expressed were recorded with the whole-cell voltage-clamp techniq ue. 3. In comparison with wild-type currents, Ile693Thr mutant current s showed a clear shift of about -9 mV in the voltage dependence of act ivation. 4. In contrast to other mutations of the Na+ channel known to cause paramyotonia congenita, the Ile693Thr mutation did not induce a ny significant change in the kinetics, nor in the voltage dependence, of fast inactivation. 5. In conclusion, this study provides further ev idence of the involvement of the S4-S5 linker in the voltage dependenc e of Na+ channel activation. The negative shift in the voltage depende nce found in this mutation must be associated to other defects, plausi bly an impairment of the slow inactivation, to account for the long pe riods of muscle weakness experienced by the patients.