δ subunit susceptibility variants E177A and R220H associated with complex epilepsy alter channel gating and surface expression of α4β2δ GABAA receptors

Most human idiopathic generalized epilepsies (IGEs) are polygenic, but virtually nothing is known of the molecular basis for any of the complex epilepsies. Recently, two GABA(A) receptor delta subunit variants (E177A, R220H) were proposed as susceptibility alleles for generalized epilepsy with febrile seizures plus and juvenile myoclonic epilepsy. In human embryonic kidney 293T cells, recombinant h alpha 1 beta 2 delta(E177A) and h alpha 1 beta 2 delta(R220H) receptor currents were reduced, but the basis for the current reduction was not determined. We examined the mechanistic basis for the current reduction produced by these variants using the h alpha 4 beta 2 delta receptor, an isoform more physiologically relevant and linked to epileptogenesis, by characterizing the effects of these variants on receptor cell surface expression and single-channel gating properties. Expression of variant alpha 4 beta 2 delta(R220H) receptors resulted in a decrease in surface receptor proteins, and a smaller, but significant, reduction was observed for variant alpha 4 beta 2 delta(E177A) receptors. For both variants, no significant alterations of surface expression were observed for mixed population of wild-type and variant receptors. The mean open durations of alpha 4 beta 2 delta(E177A) and alpha 4 beta 2 delta(R220H) receptor single-channel currents were both significantly decreased compared to wild-type receptors. These data suggest that both delta(E177A) and delta(R220H) variants may result in disinhibition in IGEs by similar cellular and molecular mechanisms, and in heterozygously affected individuals, a reduction in channel open duration of delta subunit-containing GABA(A) receptors may be the major contributor to the