The Functional Effect of R1648H, a Sodium Channel Mutation that Causes Generalized Epilepsy with Febrile Seizures Plus in Splice Variants of SCN1A

SCN1A, the gene that encodes the alpha subunit of the voltage-gated sodium channel Nav1.1, is alternatively spliced at exon 5. SCN1A contains two copies of exon 5, denoted 5N and 5A (for ‘Neonatal’ and ‘Adult’ according to their developmental expression). There are 3 amino acid substitutions between the splice variants, all within the D1:S3/S4 extracellular linker. It is unknown how exons 5N and 5A alter channel function. Because patients with Generalized Epilepsy with Febrile Seizures plus (GEFS+) frequently exhibit age-dependent changes in seizure frequency and severity, we have asked whether the GEFS+-associated SCN1A mutation R1648H differentially affects Nav1.1-5N and Nav1.1-5A.We examined brain tissue obtained from patients undergoing epilepsy surgery to examine the relative proportion of SCN1A transcripts containing exons 5A and 5N. A significantly greater proportion of Nav1.1 mRNA in epilepsy tissue contain exon 5N than in control brain tissue. We expressed either splice variant of SCN1A in HEK293 cells, and recorded whole-cell currents with a CsCl-based pipette solution. Nav1.1-5N demonstrated a leftward shift of both activation (Nav1.1-5N: V50= −18.3±-0.6 mV ; Nav1.1-5A: −15.3±-0.5 mV; P<0.05) and inactivation (Nav1.1-5N: V50= −60.0±-1.0 mV; Nav1.1-5A: −54.0±-1.1 mV). The GEFS+ mutation R1648H, did not affect activation or current density for either variant. The mutation also failed to increase the size of the persistent current evoked by prolonged depolarising steps. Instead, a hyperpolarizing shift in inactivation was observed when the mutation was expressed in Nav1.1-5A but not Nav1.1-5N channels (mutant: V50 inactivation = −60.9±-1.0 mV; wild-type: −54±-1.1 mV). This suggests that R1648H leads to a net loss of function in adult neurons. This effect may lead to an impairment of recruitment of GABAergic interneurons that preferentially express Nav1.1.