C121W Implicated in GEFS+ is a Thermosensitive Sodium Channel Mutation

Abstract

Genetic epilepsy with febrile seizures plus (GEFS+) is a multifaceted pediatric epilepsy syndrome of which febrile seizures (FS) are one of the most common symptoms. A cysteine to tryptophan substitution at position 121 (C121W) in β1 subunit of NaV is one of the many mutations related to GEFS+. The underlying mechanism of this disorder is currently unknown, but it has been proposed that βC121W causes the FS phenotype by altering the temperature sensitivity of NaV. To uncover the biophysical mechanisms of temperature dependent changes, we performed whole cell voltage clamp experiments at 22° and 34°C on CHO cells stably expressing the α subunit of neuronal isoform NaV1.2, transiently transfected with either wild type or mutant β1. Our results suggest that the βC121W mutant alters the gating of NaV1.2 compared to wild type β1, and that increased temperature exaggerates these disparities. Specifically, we focused on temperature- and mutation-induced changes to steady state slow inactivation (SSI). This type of inactivation determines the proportion of channels available to open after extended (over 1 min) or repetitive membrane depolarizations, and thus has a profound impact on cellular and tissue excitability. At 22°, we saw no apparent differences in V1/2 of SSI between wild type and mutant channels. Increased temperature, however, unmasked a depolarizing shift in V1/2 of SSI for α+βC121W in comparison to α+β1. A depolarizing shift in SSI is pro-excitatory, and we predict that the differential response to temperature between wild type and mutant subunits contributes to neuronal destabilization and epileptogenisis during febrile states. (Supported by an NSERC Discovery Grant and a CFI Infrastructure Grant to PCR.)