Biophysical and structural insights into the SCN4A E452K variant linked to myotonia and paramyotonia congenita

Myotonia and paramyotonia congenita (PC) are rare neuromuscular disorders characterized by muscle stiffness that intensifies in cold environments. These disorders are associated with variants in the SCN4A gene, that encodes the alpha subunit of the voltage-gated sodium channel Nav1.4. We report here the case of a 36-year-old female who experiences diverse neurological symptoms, including myotonia, cold induced myotonia, resulting in muscle stiffness, and tightness. A whole exome sequencing revealed a missense variant in the SCN4A gene at position c.1354G > A, named p.E452K. We characterized the biophysical properties of this SCN4A variant by overexpressing the wild-type (WT) and mutant channels with the β1 regulatory subunit in HEK293 cells by transfection. Sodium currents were recorded at different temperatures and different extracellular potassium concentrations using the patch-clamp technique. Functional studies of the E452K variant revealed both loss and gain of function phenotypes at different temperatures, which were characterized by a decrease in current density and an increase in the window current. This was related to the shift of inactivation toward more depolarized voltages at both 22 °C and 10 °C and a slower slope factor of activation at 22 °C. A further gain-of-function effect was also observed, which was characterized by a faster onset and recovery from slow inactivation. MD simulation of the alpha subunit in a lipid bilayer suggested that the charge reversal destabilized a native salt bridge (E452-K249). We concluded that the observed enhanced functionality facilitates the activation process, leading to enhanced muscle excitability.