Abstracts

Rationale: Mutations in voltage-gated sodium channels are responsible for several types of human epilepsy. Variable expressivity and penetrance among family members carrying the same primary mutation is a common feature of inherited epilepsy. This suggests that genetic modifiers may influence clinical severity. The mouse model Scn2a^Q54 has an epilepsy phenotype caused by a mutation in Scn2a. The mutation results in persistent sodium current in hippocampal neurons from Scn2a^Q54 mice (Kearney et al, Neuroscience, 102:307, 2001). The level of persistent current measured in hippocampal neurons is comparable to that produced by SCN1A mutations from GEFS+ families (Lossin et al, Neuron, 34:877, 2002). The Scn2a^Q54 phenotype is influenced by genetic background. On the C57BL/6J strain background, Scn2a^Q54 mice (B6.Q54) have low spontaneous seizure frequency with delayed onset and increased survival, compared to (C57BL/6JxSJL/J)F1.Q54 mice. This indicates that strain specific modifier alleles 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: To further refine the map position of the Moe1 locus on Chromosome 11, we generated an interval specific congenic (ISC) strain that carries B6 alleles in the Moe1 interval (D11Mit188-D11Mit360) on an SJL background. Recombinant animals with smaller B6-derived intervals were used to establish 3 subcongenic lines that subdivide the interval. B6.Q54 mice were crossed with the ISC heterozygotes to generate Q54 transgenic mice carrying heterozygous or homozygous B6 alleles in the Moe1 interval. Transgenic mice were video-taped for 30 minute sessions at 3, 4.5 and 6 weeks age for offline analysis of visible spontaneous seizures. We also tested the ISC lines for susceptibility to induced seizures independent of the Q54 transgene. Thresholds to seizures induced by flurothyl were determined in ISC backcross and intercross offspring. Results: For Q54 transgenic mice carrying heterozygous or homozygous B6 alleles in the Moe1 interval, the number of spontaneous partial motor seizures was quantitated and compared between genotype groups. Our data supports that a modifier locus influencing seizure frequency was captured within the B6-introgressed interval between D11Mit36-D11Mit360 (84-103 Mb). Further refinement of the Moe1 interval is in progress. Analysis of flurothyl seizure thresholds in ISC intercross and backcross mice suggests the presence of multiple seizure susceptibility genes on Chromosome 11 between D11Mit188 (45 Mb) and D11Mit360 (103 Mb). Conclusions: The Moe 1 interval on mouse Chromosome 11 contains interesting candidates, including many genes encoding ion channel subunits. Identification of genes that influence epilepsy susceptibility and disease progression will provide insight into the molecular events underlying epileptogenesis, and may identify novel therapeutic targets for treatment of human patients.