Abstracts

Rationale: More than 1200 mutations in the neuronal voltage-gated sodium channels (VGSCs) SCN1A and SCN2A have been identified in patients with several epilepsy syndromes, including Dravet syndrome. A common feature of genetic epilepsies is variable expressivity in individuals with the same mutation, suggesting an influence of genetic modifier loci on phenotype severity. Mouse models with mutations in VGSCs Scn1a and Scn2a exhibit striking strain-dependent epilepsy phenotypes, in support of our hypothesis that modifier genes influence epilepsy penetrance and severity. Cacna1g, encoding the Cav3.1 voltage-gated calcium channel subunit, modifies epilepsy severity in both the Scn1aKO/+ Dravet model and the transgenic Scn2aQ54/+ epilepsy model. Based on these findings, we hypothesized that pharmacologic inhibition of Cav3.x T-type calcium channels would modify seizure susceptibility in the Scn1aKO/+ and Scn2aQ54/+ genetic epilepsy models. Methods: Mice were treated acutely by intraperitoneal injection of either vehicle (saline) or one of two well-characterized T-type calcium channel inhibitors: (1) trimethadione (TMO), an oxazolidinedione anticonvulsant or (2) ethosuximide (ETX), a succinimide anticonvulsant. For the Scn2aQ54/+ epilepsy model, spontaneous seizure frequency was evaluated during a 30 min period prior to treatment to establish a baseline and a 30 min post-treatment period coinciding with predicted peak brain levels. For the Scn1aKO/+ Dravet model, we evaluated the susceptibility to hyperthermia-induced myoclonic jerks (MJs) and generalized tonic-clonic seizures (GTCSs). Results: We observed a reduction in spontaneous focal motor seizure (FMS) frequency in Scn2aQ54/+ mice treated with 400 mg/kg (mpk) TMO (-74.8±6.4 % change; n=10) relative to vehicle-treated littermate controls (5.3±13.3 % change; n=11) (p < 0.0001; Student’s t test with Welch’s correction). A similar reduction in FMS was observed in Scn2aQ54/+ mice treated with 200 mpk ETX (-75.0±12.0 % change; n=6) relative to vehicle-treated controls (12.4±29.5 % change; n=9) (p < 0.05; Student’s t test with Welch’s correction). TMO treatment resulted in a dose-dependent decrease in susceptibility to hyperthermia-induced MJs and GTCSs in Scn1aKO/+ mice (200-400 mpk; n=9-11 per group). Each TMO dose tested displayed anticonvulsant activity for MJs, while a minimum of 300 mpk TMO was required to affect GTCS susceptibility (Mantel-Cox logrank; p < 0.05 considered statistically significant). 200 mpk ETX treatment decreased susceptibility to hyperthermia-induced MJs and GTCSs (200 mpk; n=9-10 per group; p < 0.05). Conclusions: We observed anticonvulsant activity using two independent T-type calcium channel inhibitors (TMO and ETX) in two independent genetic mouse epilepsy models resulting from VGSC mutation. While T-type calcium channel inhibitors are classically utilized to control absence seizures, our data suggest these inhibitors may have broader utility for the treatment of non-absence epilepsy. Funding: This work was supported by National Institutes of Health grants R01-NS053792 (J.A.K.) and R01-NS084959 (J.A.K.).