Two Human Epilepsy Mutations Cause Developmentally Dependant Changes in Seizure Phenotype and GABAA Receptor Expression in Genetically Modified Mice

Abstract

Epilepsy is a disease characterized by two or more unprovoked seizures. Two mutations, S326fs328X and A322D, in the α1 subunit of GABAA receptor (Gabra1), are associated with childhood absence epilepsy and juvenile myoclonic epilepsy, respectively. In vitro, S326fs328X caused complete, while A322D caused an 88% reduction in α1 subunit expression. Here, we showed that both Gabra1 knock-out (KO) and A322D knock-in (KI) mutations cause absence seizures in post-natal day 33-37 (P30s) genetically modified mice that persist at least until P120. In addition, at P120, both Gabra1 KO and A322D KI mice also had atypical polyspike discharges on electroencephalogram recordings that were occasionally associated with subtle, but visible, myoclonic-like jerks. Western blot experiments showed that the expression of α1 subunit decreased, while α3 subunit increased, in the cortex of both Gabra1 KO and A322D KI mice. Surprisingly, at P120, wild type mice had reduced α3 subunit expression, but it remained high in the mutants. Confocal fluorescent microscopy showed, that at P120, gephyrin, a marker of inhibitory synapses, co-localized with α3 subunit more in the mutants compared to wild types, indicating that there was substantially more synaptic α3 subunit in the mutants. These findings demonstrate the α1 subunit haploinsufficiency causes absence epilepsy and developmentally-dependent myoclonic epilepsy. In addition, because α3 expressing GABAA receptors have reduced GABA sensitivity and prolonged activation times, our results suggest that reduced GABAergic inhibition may contribute to the epilepsy phenotype in these mutant mice at this age.