Abstracts

Mutations in GABA[sub]A[/sub] receptor [gamma]2 and [alpha]1 subunits have been linked to human generalized epilepsies. However, these monogenic mutations only account for a minority of the generalized epilepsies, suggesting that most of the generalized epilepsies are polygenic. Recently a rare variant of GABA[sub]A[/sub] receptor [delta] subunit (E177A) was found in a small family heterozygously associated with generalized epilepsy with febrile seizures plus (GEFS+), and a polymorphic allele (R220H) was present in a family with juvenile myoclonic epilepsy as well as in general population. Recombinant h[alpha]1[beta]2S[delta] receptors containing heterozygous and homozygous [delta]E177A or [delta]R220H subunit variants had significantly reduced GABA[sub]A[/sub] receptor currents. The variants E177A and R220H were proposed to be susceptibility genes for the generalized epilepsies (Dibbens et al., Human Molecular Genetics, in press). Studies found that the [delta] subunit preferably coassembles with the [alpha]4 subunit. The function and gating properties of [alpha]4[beta]2S[delta] receptors containing E177A or R220H variant are unknown. HEK 293T cells were transfected with human [alpha]4, [beta]2S and wild type as well as variant [delta] subunit cDNAs and the expressed receptors were studied using the whole cell and single channel patch clamp technique. Compared to wild type [alpha]4[beta]2S[delta] receptor whole cell currents, saturating GABA-evoked currents were significantly smaller for heterozygous or homozygous [alpha]4[beta]2S[delta](E177A) receptors as well as for heterozygous or homozygous [alpha]4[beta]2S[delta](R220H) receptors. No significant changes in kinetic properties were observed with [alpha]4[beta]2S[delta](R220H) receptors. However, desensitization was significantly decreased for heterozygous and homozygous [alpha]4[beta]2S[delta](E177A) receptors, and deactivation was significantly prolonged for homozygous [alpha]4[beta]2S[delta](E177A) receptors. The mean open durations of both homozygous [alpha]4[beta]2S[delta](E177A) (1.3 [plusmn] 0.2 ms) and [alpha]4[beta]2S[delta](R220H) (1.6 [plusmn] 0.1 ms) receptor single channel currents were significantly decreased as compared to wild type [alpha]4[beta]2S[delta] receptors (2.6 [plusmn] 0.2 ms). These data suggest that these [delta] subunit variants may decrease surface receptor expression and produce defective channel gating, resulting in GABA[sub]A[/sub] receptor current reduction. (Supported by NIH NS 33300 to RLM and an Epilepsy Foundation postdoctorate fellowship to HJF)