Functional Effects of Two Voltage-Gated Sodium Channel Mutations That Cause Generalized Epilepsy with Febrile Seizures Plus Type 2

Abstract

Two mutations that cause generalized epilepsy with febrile seizures plus (GEFS+) have been identified previously in the SCN1A gene encoding the alpha subunit of the Na(v)1.1 voltage-gated sodium channel (Escayg et al., 2000). Both mutations change conserved residues in putative voltage-sensing S4 segments, T875M in domain II and R1648H in domain IV. Each mutation was cloned into the orthologous rat channel rNa(v)1.1, and the properties of the mutant channels were determined in the absence and presence of the beta1 subunit in Xenopus oocytes. Neither mutation significantly altered the voltage dependence of either activation or inactivation in the presence of the beta1 subunit. The most prominent effect of the T875M mutation was to enhance slow inactivation in the presence of beta1, with small effects on the kinetics of recovery from inactivation and use-dependent activity of the channel in both the presence and absence of the beta1 subunit. The most prominent effects of the R1648H mutation were to accelerate recovery from inactivation and decrease the use dependence of channel activity with and without the beta1 subunit. The DIV mutation would cause a phenotype of sodium channel hyperexcitability, whereas the DII mutation would cause a phenotype of sodium channel hypoexcitability, suggesting that either an increase or decrease in sodium channel activity can result in seizures.

A Novel Epilepsy Mutation in the Sodium ChannelSCN1AIdentifies a Cytoplasmic Domain for β Subunit Interaction

“…Separate disease-causing mutations in Na v 1.4 and Na v 1.5, as well as our previously published data for the GEFSϩ mutation R1648H in Na v 1.1, demonstrate that more rapid recovery from inactivation can result in sodium channel-mediated hyperexcitability (Hayward et al, 1996; Q. Chen et al, 1998;Spampanato et al, 2001). We therefore analyzed the effects of the D1866Y mutation on recovery from inactivation in the absence and presence of the ␤1 subunit (Fig.…”

“…Second, the ␤1 subunit has a clear and easily quantifiable effect on the properties of ␣ subunit sodium channels in oocytes, so that we could reliably determine whether the ␣ subunit mutation altered modulation by ␤1. Third, the data could be directly compared with our previous results for three other GEFSϩ mutations with respect to both the effects on sodium channel properties and the predicted effects on neuronal firing using a computational model (Spampanato et al, 2001(Spampanato et al, , 2003(Spampanato et al, , 2004. It is important to note that although the oocyte expression system provides a powerful means to determine biophysical differences between mutant and wild-type channels, there have been functional differences as well as similarities between observations in oocytes and in mammalian-transfected cells for some mutants (Spampanato et al, 2001(Spampanato et al, , 2003Lossin et al, 2002).…”

“…Most of these are located within or adjacent to transmembrane segments or in the pore region of the channel. The biophysical characteristics of eight mutations have been examined, revealing an array of phenotypic variations in channel function (Spampanato et al, 2001(Spampanato et al, , 2003Lossin et al, 2002Lossin et al, , 2003Cossette et al, 2003). Functional characterization of additional disease alleles of SCN1A can further our understanding of the structure-function relationships of the channel protein.…”

Sodium channels SCN1A, SCN2A and SCN3A in familial autism

“…21 SCN1A mutations responsible for GEFS þ cause only minor changes in channel kinetics. 23,32 Polymorphic sodium channel variants, such as those described here, may contribute to the range of cognitive and emotional function in normal individuals. The combined effect of variants in multiple ion channel genes could also predispose to psychiatric disorders such as autism.…”

Identification of Epilepsy Genes in Human and Mouse

“…Both mutations changed evolutionarily invariant residues located in the voltage-sensing S4 segments of the protein. Introduction of these mutations into the SCN1A channel and examination of the kinetic properties in Xenopus oocytes demonstrated that the R1648H mutation accelerated the recovery from inactivation and decreased the use dependence of channel activity (66). This accelerated recovery could lead to rapid firing patterns and neuronal hyperexcitability.…”