Fine Mapping of a Dravet Syndrome Modifier Locus on Mouse Chromosome 5 and Candidate Gene Analysis by RNA-Seq

<div><p>A substantial number of mutations have been identified in voltage-gated sodium channel genes that result in various forms of human epilepsy. <i>SCN1A</i> mutations result in a spectrum of severity ranging from mild febrile seizures to Dravet syndrome, an infant-onset epileptic encephalopathy. Dravet syndrome patients experience multiple seizures types that are often refractory to treatment, developmental delays, and elevated risk for SUDEP. The same sodium channel mutation can produce epilepsy phenotypes of varying clinical severity. This suggests that other factors, including genetic, modify the primary mutation and change disease severity. Mouse models provide a useful tool in studying the genetic basis of epilepsy. The mouse strain background can alter phenotype severity, supporting a contribution of genetic modifiers in epilepsy. The <i>Scn1a</i><sup><i>+/-</i></sup> mouse model has a strain-dependent epilepsy phenotype. <i>Scn1a</i><sup><i>+/-</i></sup> mice on the 129S6/SvEvTac (129) strain have a normal phenotype and lifespan, while [129xC57BL/6J]F1-<i>Scn1a</i><sup><i>+/-</i></sup> mice experience spontaneous seizures, hyperthermia-induced seizures and high rates of premature death. We hypothesize the phenotypic differences are due to strain-specific genetic modifiers that influence expressivity of the <i>Scn1a</i><sup><i>+/-</i></sup> phenotype. Low resolution mapping of <i>Scn1a</i><sup><i>+/-</i></sup> identified several Dravet syndrome modifier (<i>Dsm</i>) loci responsible for the strain-dependent difference in survival. One locus of interest, <i>Dsm1</i> located on chromosome 5, was fine mapped to a 9 Mb region using interval specific congenics. RNA-Seq was then utilized to identify candidate modifier genes within this narrowed region. Three genes with significant total gene expression differences between 129S6/SvEvTac and [129xC57BL/6J]F1 were identified, including the GABA<sub>A</sub> receptor subunit, <i>Gabra2</i>. Further analysis of <i>Gabra2</i> demonstrated allele-specific expression. Pharmological manipulation by clobazam, a common anticonvulsant with preferential affinity for the GABRA2 receptor, revealed dose-dependent protection against hyperthermia-induced seizures in <i>Scn1a</i><sup><i>+/-</i></sup> mice. These findings support <i>Gabra2</i> as a genetic modifier of the <i>Scn1a</i><sup><i>+/-</i></sup> mouse model of Dravet syndrome.</p></div>