Reduced cortical inhibition in a mouse model of familial childhood absence epilepsy

Mutations in the GABA_A receptor γ2 subunit are associated with childhood absence epilepsy and febrile seizures. To understand better the molecular basis of absence epilepsy in man, we developed a mouse model harboring a γ2 subunit point mutation (R43Q) found in a large Australian family. Mice heterozygous for the mutation demonstrated behavioral arrest associated with 6-to 7-Hz spike-and-wave discharges, which are blocked by ethosuximide, a first-line treatment for absence epilepsy in man. Seizures in the mouse showed an abrupt onset at around age 20 days corresponding to the childhood nature of this disease. Reduced cell surface expression of γ2(R43Q) was seen in heterozygous mice in the absence of any change in α1 subunit surface expression, ruling out a dominant-negative effect. GABA_A-mediated synaptic currents recorded from cortical pyramidal neurons revealed a small but significant reduction that was not seen in the reticular or ventrobasal thalamic nuclei. We hypothesize that a subtle reduction in cortical inhibition underlies childhood absence epilepsy seen in humans harboring the R43Q mutation.