Abstracts

Rationale: Mutations in the voltage-gated sodium channels SCN1A and SCN2A are responsible for several types of human epilepsy. Family members with inherited sodium channel mutations often exhibit variable expressivity of the clinical phenotype, suggesting that genetic modifiers may influence clinical severity. The mouse model Scn2a-Q54 has an epilepsy phenotype caused by a mutation in Scn2a that slows channel inactivation. Clinical severity of the Scn2a-Q54 phenotype is influenced by genetic background. C57BL/6J.Q54 mice exhibit low spontaneous seizure incidence, delayed onset, and increased survival, compared with (C57BL/6JxSJL/J)F1.Q54 mice. This indicates that strain SJL/J carries dominant modifier alleles that influence the severity of the epilepsy phenotype. We identified two modifier loci responsible for the strain difference in seizure susceptibility on Chromosomes 11 and 19, designated Moe1 (modifier of epilepsy 1) and Moe2 (Bergren et al, Mamm Genome 16:683, 2005). Methods: We crossed interval specific congenic (ISC) mice that carry an SJL/J-derived Chromosome 19 segment on the C57BL/6J background with Scn2a-Q54 mice. Offspring were assessed for spontaneous behavioral seizures by video-taped observations at 3, 4.5 and 6 weeks of age. We evaluated positional candidate genes for coding sequence variants by DNA sequencing and transcript level variation by quantitative RT-PCR. To test the putative modifier Kcnv2, we generated transgenic mice carrying an SJL-derived Kcnv2 transgene in the B6 background. Kcnv2 transgenic mice were crossed with Scn2a-Q54 mice and phenotyped for spontaneous behavioral seizures. Results: We localized the Moe2 minimal interval to a 4 Mb region using interval specific congenic mice that carry an SJL/J-derived Chromosome 19 segment on the C57BL/6J background and confer the modified phenotype. The Moe2 region contains 43 known and predicted genes, of which 15 have confirmed brain expression. We analyzed the coding sequence and relative transcript expression levels of positional candidate genes and identified Kcnv2 as a putative modifier. We observed 2 non-synonymous coding variants and a difference in Kcnv2 transcript levels between the B6 and SJL strains. Kcnv2 encodes the potassium channel subunit Kv8.2 which forms heterotetramers with Kv2 family members and alters their activity. Thus Kcnv2 is a strong functional candidate. In order to directly test Kcnv2 as a modifier, we generated transgenic mice carrying a Kcnv2-SJL transgene in the B6 background. B6.Q54 mice carrying the Kcnv2-SJL transgene exhibit increased seizure susceptibility that correlates with the level of transgene expression. Functional analysis of the non-synonymous coding variants is in progress. Conclusions: Identification of modifier genes that influence susceptibility and disease progression will provide insight into the molecular events of epileptogenesis, and may identify novel therapeutic targets for treatment of human patients.