Epileptic encephalopathy de novo GABRB mutations impair γ‐aminobutyric acid type A receptor function

Abstract

Objective The Epi4K consortium recently identified four de novo mutations in the γ-aminobutyric acid type A (GABAA) receptor β3 subunit gene GABRB3 and one in the β1 subunit gene GABRB1 in children with epileptic encephalopathies (EEs) Lennox-Gastaut syndrome (LGS) or infantile spasms (IS). Since the etiology of EEs is often unknown, we determined the impact of GABRB mutations on GABAA receptor function and biogenesis. Methods GABAA receptor α1 and γ2L subunits were co-expressed with wild-type and/or mutant β3 or β1 subunits in HEK 293T cells. Currents were measured using whole cell and single channel patch clamp techniques. Surface and total expression levels were measured using flow cytometry. Potential structural perturbations in mutant GABAA receptors were explored using structural modeling. Results LGS-associated GABRB3(D120N, E180G, Y302C) mutations located at β+ subunit interfaces reduced whole cell currents by decreasing single channel open probability without loss of surface receptors. In contrast, IS-associated GABRB3(N110D) and GABRB1(F246S) mutations at β-subunit interfaces produced minor changes in whole cell current peak amplitude but altered current deactivation by decreasing or increasing single channel burst duration, respectively. GABRB3(E180G) and GABRB1(F246S) mutations also produced spontaneous channel openings. Interpretation All five de novo GABRB mutations impaired GABAA receptor function by rearranging conserved structural domains, supporting their role in EEs. The primary effect of LGS-associated mutations was reduced GABA-evoked peak current amplitudes while the major impact of IS-associated mutations was on current kinetic properties. Despite lack of association with epilepsy syndromes, our results suggest GABRB1 as a candidate human epilepsy gene.

Critical roles of αII spectrin in brain development and epileptic encephalopathy

“…Gephyrin is required for the organization, stability, and clustering of GABA A Rs (37,38). Genetic mutations in both GABA A Rs and gephyrin have been identified in different epilepsy syndromes, including severe epileptic encephalopathies (39)(40)(41)(42)(43)(44). Our data suggest that defective gephyrin clustering and GABAergic innervation of cortical pyramidal neurons contribute to the pathogenesis of EIEE5, providing further evidence that inhibitory transmission plays critical roles in EIEEs.…”

Inhibitory synapse deficits caused by familial α1 GABAA receptor mutations in epilepsy

“…The -aminobutyric acid (type A) receptor (GABA A R) is emerging as a major causal agent in some forms of genetic epilepsy. Heritable mutations to the GABA A R , ,  and  subunits have been identified in family pedigrees of epilepsy (6) and new mutations continue to be identified (7,8). Studies of thalamo-cortical connections of the brain have demonstrated that epileptic states are preceded by the synchronisation of excitatory activity in neuronal populations.…”

Mutations in GABRB3

“…This is corroborated in a recent study in which 4 EE-associated GABRB3 mutations showed loss of function with a possible genotype-phenotype correlation. 26 Future studies will clarify whether these findings may extend to a number of novel mutations reported in our publication.…”