Distinct Functional Alterations and Therapeutic Options of Two Pathological De Novo Variants of the T292 Residue of GABRA1 Identified in Children with Epileptic Encephalopathy and Neurodevelopmental Disorders

Mutations of GABA(A)R have reportedly led to epileptic encephalopathy and neurodevelopmental disorders. We have identified a novel de novo T292S missense variant of GABRA1 from a pediatric patient with grievous global developmental delay but without obvious epileptic activity. This mutation coincidentally occurs at the same residue as that of a previously reported GABRA1 variant T292I identified from a pediatric patient with severe epilepsy. The distinct phenotypes of these two patients prompted us to compare the impacts of the two mutants on the receptor function and to search for suitable therapeutics. In this study, we used biochemical techniques and patch-clamp recordings in HEK293 cells overexpressing either wild-type or mutated rat recombinant GABA(A)Rs. We found that the alpha 1T292S variant significantly increased GABA-evoked whole-cell currents, shifting the dose-response curve to the left without altering the maximal response. In contrast, the alpha 1T292I variant significantly reduced GABA-evoked currents, shifting the dose-response curve to the right with a severely diminished maximum response. Single-channel recordings further revealed that the alpha 1T292S variant increased, while the alpha 1T292I variant decreased the GABA(A)R single-channel open time and open probability. Importantly, we found that the T292S mutation-induced increase in GABA(A)R function could be fully normalized by the negative GABA(A)R modulator thiocolchicoside, whereas the T292I mutation-induced impairment of GABA(A)R function was largely rescued with a combination of the GABA(A)R positive modulators diazepam and verapamil. Our study demonstrated that alpha 1T292 is a critical residue for controlling GABA(A)R channel gating, and mutations at this residue may produce opposite impacts on the function of the receptors. Thus, the present work highlights the importance of functionally characterizing each individual GABA(A)R mutation for ensuring precision medicine.