GABAA Receptor α1 Subunit Mutation A322D Associated with Autosomal Dominant Juvenile Myoclonic Epilepsy Reduces the Expression and Alters the Composition of Wild Type GABAA Receptors

A GABA(A) receptor (GABA(A)R) alpha 1 subunit mutation, A322D (AD), causes an autosomal dominant form of juvenile myoclonic epilepsy (ADJME). Previous studies demonstrated that the mutation caused alpha 1(AD) subunit misfolding and rapid degradation, reducing its total and surface expression substantially. Here, we determined the effects of the residual alpha 1(AD) subunit expression on wild type GABA(A)R expression to determine whether the AD mutation conferred a dominant negative effect. We found that although the alpha 1(AD) subunit did not substitute for wild type alpha 1 subunits on the cell surface, it reduced the surface expression of alpha 1 beta 2 gamma 2 and alpha 3 beta 2 gamma 2 receptors by associating with the wild type subunits within the endoplasmic reticulum and preventing them from trafficking to the cell surface. The alpha 1(AD) subunit reduced surface expression of alpha 3 beta 2 gamma 2 receptors by a greater amount than alpha 1 beta 2 gamma 2 receptors, thus altering cell surface GABAAR composition. When transfected into cultured cortical neurons, the alpha 1(AD) subunit altered the time course of miniature inhibitory postsynaptic current kinetics and reduced miniature inhibitory postsynaptic current amplitudes. These findings demonstrated that, in addition to causing a heterozygous loss of function of alpha 1(AD) subunits, this epilepsy mutation also elicited a modest dominant negative effect that likely shapes the epilepsy phenotype.