A1152D mutation of the Na+ channel causes paramyotonia congenita and emphasizes the role of DIII/S4–S5 linker in fast inactivation

Missense mutations in the human skeletal muscle Na⁺ channel α subunit (hSkM1) are responsible for a number of muscle excitability disorders. Among them, paramyotonia congenita (PC) is characterized by episodes of muscle stiffness induced by cold and aggravated by exercise. We have identified a new PC-associated mutation, which substitutes aspartic acid for a conserved alanine in the S4–S5 linker of domain III (A1152D). This residue is of particular interest since its homologue in the rat brain type II Na⁺ channel has been suggested as an essential receptor site for the fast inactivation particle. To identify the biophysical changes induced by the A1152D mutation, we stably expressed hSkM1 mutant or wild-type (WT) channels in HEK293 (human embryonic kidney) cells, and recorded whole-cell Na⁺ currents with the patch-clamp technique. Experiments were performed both at 21 and 11°C to better understand the sensitivity to cold of paramyotonia. The A1152D mutation disrupted channel fast inactivation. In comparison to the WT, mutant channels inactivated with slower kinetics and displayed a 5 mV depolarizing shift in the voltage dependence of the steady-state. The other noticeable defect of A1152D mutant channels was an accelerated rate of deactivation from the inactivated state. Decreasing temperature by 10°C amplified the differences in channel gating kinetics between mutant and WT, and unveiled differences in both the sustained current and channel deactivation from the open state. Overall, cold-exacerbated mutant defects may result in a sufficient excess of Na⁺ influx to produce repetitive firing and myotonia. In the light of previous reports, our data point to functional as well as phenotypic differences between mutations of conserved S4–S5 residues in domains II and III of the human skeletal muscle Na⁺ channel.