Gabra2 is a genetic modifier of Dravet syndrome in mice

Abstract

Pathogenic 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 of Scn1a (Scn1a+/-) model Dravet syndrome, a severe epilepsy most often caused by SCN1A haploinsufficiency. Scn1a+/- heterozygous mice recapitulate key features of Dravet syndrome, including febrile and afebrile spontaneous seizures, SUDEP, and cognitive and behavioral deficits. The Scn1a+/- 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 [B6x129]F1.Scn1a+/- mice. In previous work, we identified Dravet Survival Modifier loci (Dsm1-Dsm5) responsible for strain-dependent differences in survival. Gabra2, encoding the GABAA 2 subunit, was nominated as the top candidate modifier at the Dsm1 locus on chromosome 5. Direct measurement of GABAA receptors found lower abundance of 2-containing receptors in hippocampal synapses of B6 mice relative to 129. We also identified a B6-specific single nucleotide intronic deletion within Gabra2 that lowers mRNA and protein by nearly 50%. Repair of this de novo deletion reestablished normal levels of Gabra2 transcript and protein expression. In the current study, we used B6 mice with the repaired Gabra2 allele to validate it as a modifier of phenotype severity in Scn1a+/- mice. Repair of Gabra2 restored transcript and protein expression, increased abundance of 2-containing GABAA receptors in hippocampal synapses, and improved seizure and survival phenotypes of Scn1a+/- mice. These findings validate Gabra2 as a genetic modifier of Dravet syndrome.