Abstracts

Rationale: Over 800 mutations have been identified in the voltage-gated sodium channel genes SCN1A and SCN2A in human epilepsies, including Genetic Epilepsy with Febrile Seizures Plus (GEFS+), Dravet Syndrome and Benign Familial Neonatal-Infantile Seizures. Family members with the same mutation frequently display variation in the clinical severity of the disease, suggesting that other factors modify the primary mutation. Several mouse models have been generated to study the genetic basis of epilepsy. A common feature of these epilepsy models is that genetic strain background alters the disease phenotype, suggestive of genetic modifiers in epilepsy. The Scn2aQ54 transgenic mouse model has an epilepsy phenotype that varies depending on the genetic strain background. Scn2aQ54 mice congenic on the C57Bl/6J (B6.Q54) strain exhibit delayed seizure onset and improved survival compared to F1(B6xSJL/J).Q54 mice. Two dominant modifier loci of Scn2aQ54 seizure susceptibility were mapped and designated Moe1 and Moe2. We previously fine mapped the Moe1 locus using interval-specific congenic mouse strains and identified candidate modifier genes by RNA-Seq analysis. Consideration of gene function and RNA-Seq expression data identified Cacna1g (α1G), encoding Cav3.1, as a strong candidate modifier gene. We are using a transgenic transfer approach to assess the modifier potential of α1G. Alteration of the Scn2aQ54 phenotype by α1G in double mutant mice will support its involvement as an epilepsy modifier gene. Methods: α1G BAC transgenic mice were generated by microinjection of BAC RP23-65I14 into SJL/J oocytes. α1G BAC transgenic hemizygotes were crossed with B6.Q54 mice to generate double transgenic offspring and Scn2aQ54 single transgenic littermate controls. Spontaneous seizure frequency was measured during 30 minute observations at 3 and 6 weeks of age. Seizure severity was assessed with a modified Racine scale and survival was monitored until 12 weeks of age. Results: The α1G BAC transgenic mice carry 8-10 copies of the transgene and overexpress α1G transcript and protein. Double transgenic α1G;Q54 male mice exhibited more spontaneous seizures than Q54 single transgenic littermates (p< 0.0048). There was no significant difference in female seizure frequency (p< 0.62). Double transgenic α1G;Q54 mice of both sexes exhibited increased seizure severity, as assessed by the Racine scale, compared to littermate controls. Survival analysis of double transgenic α1G;Q54 mice and littermate controls is in progress. Conclusions: These experiments evaluate a potential modifier gene that may influence the clinical course of epilepsy caused by a sodium channel mutation. The interplay between various ion channels contributes to overall neuronal excitability. Understanding gene interactions that contribute to epilepsy penetrance and severity will improve the utility of molecular diagnostics and may suggest novel therapeutic strategies.