Biophysical Characterization of Two NaV1.4 Mutations Making a Clinical Overlap between the Myotonia-Hyperkalemic and Hypokalemic Periodic Paralysis Clusters of Disorders

Abstract

We report here the functional characterization of two dominant SCN4A missense mutations targeting the R1451 residue located in transmembrane segment S4 of DIV. The R1451L is a novel mutation found in two unrelated patients. The first patient was diagnosed with SMC, and the second with a mixed hyperPP-hypoPP phenotypes. The R1451C mutation was previously reported by our group in one patient with a single attack of PP induced by glucocorticoids. To elucidate the mechanism underlying the phenotypes caused by the R1451C/L mutations and their roles in different types of sodium channelopathies, we used the whole-cell patch-clamp technique to study tsA201 cells expressing WT, R1451C or R1451L channels. Our results showed that both mutations shifted the steady-state inactivation to hyperpolarized potentials, reduced the current density, slowed the recovery from slow inactivation and exhibited a slow of the overall kinetics of fast inactivation. Cooling further enhances the abnormalities of fast inactivation kinetics in R1451L channels. Homology modeling revealed a dissimilar disruption of hydrogen bonds in the VSD of R1451C/L mutant channels.The altered biophysical properties of R1451C/L channels account for the clinical phenotypes seen in our patients but that additional factors are likely to play a role in the diversity of symptoms.