Gabra2is a genetic modifier of Dravet syndrome in mice

AbstractPathogenic variants in epilepsy genes result in a spectrum of clinical presentation, ranging from benign phenotypes to intractable epilepsies with significant co-morbidities and increased risk of sudden unexpected death in epilepsy (SUDEP). One source of this phenotypic heterogeneity is modifier genes that affect penetrance, dominance or expressivity of a primary pathogenic variant. Mouse models of epilepsy also display varying degrees of clinical severity on different genetic backgrounds. Mice with heterozygous deletion ofScn1a(Scn1a+/−) model Dravet syndrome, a severe epilepsy most often caused bySCN1Ahaploinsufficiency.Scn1a+/−heterozygous mice recapitulate key features of Dravet syndrome, including febrile and afebrile spontaneous seizures, SUDEP, and cognitive and behavioral deficits. TheScn1a+/−mouse model also exhibits strain-dependent phenotype severity.Scn1a+/−mice maintained on the 129S6/SvEvTac (129) strain have normal lifespan and no overt seizures. In contrast, admixture with C57BL/6J (B6) results in severe epilepsy and premature lethality in [B6×129]F1.Scn1a+/−mice. In previous work, we identified Dravet Survival Modifier loci (Dsm1-Dsm5) responsible for strain-dependent differences in survival.Gabra2, encoding the GABAAα2 subunit, was nominated as the top candidate modifier at theDsm1locus on chromosome 5. Direct measurement of GABAAreceptors found lower abundance of α2-containing receptors in hippocampal synapses of B6 mice relative to 129. We also identified a B6-specific single nucleotide intronic deletion withinGabra2that lowers mRNA and protein by nearly 50%. Repair of thisde novodeletion reestablished normal levels ofGabra2transcript and protein expression. In the current study, we used B6 mice with the repairedGabra2allele to validate it as a modifier of phenotype severity inScn1a+/−mice. Repair ofGabra2restored transcript and protein expression, increased abundance of α2-containing GABAAreceptors in hippocampal synapses, and improved seizure and survival phenotypes ofScn1a+/−mice. These findings validateGabra2as a genetic modifier of Dravet syndrome.