Abstracts

Rationale: Several types of human epilepsies can be attributed to mutations in voltage-gated sodium channel genes. Mutations in SCN1A produce a spectrum of phenotypes ranging from simple febrile seizures to Dravet syndrome, an infant-onset epileptic encephalopathy defined by various seizure types with associated psychomotor and cognitive development delays. Genetic epilepsies caused by the same sodium channel mutation often result in heterogeneous phenotypes. This suggests that other factors, possibly genetic, modify the primary mutation and alter disease severity. The genetic basis of epilepsy can be studied using mouse models. Frequently, the strain background can change the disease phenotype, supporting a contribution of genetic modifiers in epilepsy. The Scn1aKO/+ mouse model has a strain-dependent epilepsy phenotype. Scn1aKO/+ on the 129S6/SvEvTac (129) strain have a normal phenotype and lifespan while [129xC57BL/6J]F1-Scn1aKO/+ mice experience spontaneous generalized tonic-clonic seizures and a 50% mortality rate by 1 month of age. We hypothesize that this difference results from genetic modifier alleles that influence expressivity of the Scn1aKO/+ phenotype. Low resolution mapping of Scn1aKO/+ identified several Dravet syndrome modifier (Dsm) loci responsible for the strain-dependent survival difference. One locus of interest, Dsm1 located on chromosome 5, was fine-mapped using interval specific congenic strains (ISCs) and candidate modifier genes were identified by RNA-seq transcriptome analysis.Methods: To refine the map interval, we generated ISCs carrying 129-derived chromosome 5 alleles on the C57BL/6J background. ISC mice were crossed with 129.Scn1aKO/+ mice and offspring were monitored for survival. Total brain RNAs from the C57BL/6J and 129S6/SvEvTac strains were sequenced on the Illumina HiSeq platform and analyzed using the Tuxedo Tools on the Galaxy web platform.Results: Using ISCs, we refined the map position of the Dsm1 locus to a 9 Mb interval of mouse chromosome 5. The interval contains 109 known and predicted genes, of which 39 are expressed in brain. We used RNA-seq for candidate gene analysis in the modifier region and identified a number of genes with brain transcript expression differences and coding sequence differences. Additional consideration of gene expression pattern and function suggested several candidate genes, including a GABAA receptor subunit.Conclusions: Fine mapping and RNA-seq analysis will identify potential modifier genes at the Dsm1 locus that contribute to strain-dependent variability of the Scn1aKO/+ phenotype. Identification of modifier genes may suggest new therapeutic targets for improved treatment of epilepsy and aid in predicting the clinical course of epilepsy due to a sodium channel mutation.