Abstracts

Rationale: Mutations in the SCN1A voltage-gated sodium channel gene have previously been shown to cause Generalized Epilepsy with Febrile Seizures Plus Type 2 (GEFS+2). The SCN1A gene encodes the pore forming α subunit of the voltage-gated Na_v1.1 sodium channel in the CNS, which is expressed in the dendrites and soma of neurons. GEFS+2 is an autosomal dominant disorder that results in febrile seizures in children and afebrile seizures in adults. One of the mutations that cause this disease is R1648H, which substitutes a histidine for an evolutionarily conserved arginine in the voltage sensor region of domain IV. Previous studies using heterologous expression systems have reported inactivation defects due to the R1648H mutation in Na_v1.1 channels, but there are no data concerning the effects of the mutation in native cells.Methods: We constructed transgenic mice expressing R1648H Na_v1.1 to study the properties of mutant channels in neurons both in isolation and in the context of the other CNS sodium channel isoforms. The mice were constructed using a Bacterial Artificial Chromosome to express the channel using its endogenous promoter. The channels also included the E945Q mutation, which makes them resistant to saxitoxin (STX). Endogenous sodium channels are blocked by nanomolar concentrations of STX, so that the mutant channels could be studied in isolation in the presence of STX.Results: Based on the percentage of STX-resistant current in different populations of cortical neurons, we estimated that Na_v1.1 channel expression is 3 times greater in cortical bipolar neurons compared to pyramidal neurons in newborn mice. We characterized the properties of the R1648H mutant channels in both types of cells. In bipolar neurons, the R1648H channels demonstrated 20% more of a decrease in current during rapid depolarizations (use-dependence) and slower recovery from fast inactivation compared to wild-type channels. In contrast, R1648H channels in pyramidal neurons demonstrated no change in use-dependence but a 7 mV hyperpolarized shift in the voltage-dependence of inactivation.Conclusions: These results suggest that decreased sodium channel function in inhibitory bipolar neurons contributes to the seizure phenotype in GEFS+. (Research support provided by NIH, the McKnight Endowment Fund for Neuroscience, the Epilepsy Foundation and Citizens United for Research in Epilepsy.)