Epileptic Encephalopathy De Novo GABRB Mutations Impair γ-Aminobutyric Acid Type A Receptor Function

Objective: The Epi4K Consortium recently identified 4 de novo mutations in the gamma-aminobutyric acid type A (GABA(A)) receptor beta 3 subunit gene GABRB3 and 1 in the beta 1 subunit gene GABRB1 in children with one of the epileptic encephalopathies (EEs) Lennox-Gastaut syndrome (LGS) and infantile spasms (IS). Because the etiology of EEs is often unknown, we determined the impact of GABRB mutations on GABA(A) receptor function and biogenesis. Methods: GABA(A) receptor alpha 1 and gamma 2L subunits were coexpressed with wild-type and/or mutant beta 3 or beta 1 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 GABA(A) receptors were explored using structural modeling. Results: LGS-associated GABRB3(D120N, E180G, Y302C) mutations located at beta(+) 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 beta(-) 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 5 de novo GABRB mutations impaired GABA(A) 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, whereas 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.