The Genetic and Phenotypic Spectrum of GABRB1-related Disorders.

Pathogenic variants in GABAA receptor subunit genes (GABR*) are important contributors to rare and common genetic epilepsies. Here, we present a comprehensive analysis of variants in GABRB1, which encodes the GABAA receptor β1 subunit, by revealing their functional implications, establishing genotype-phenotype correlations and evaluating treatment response. Clinical information on individuals carrying a GABRB1 variant was obtained through an international collaboration and literature review. Our cohort included 19 individuals (7 males, 12 females) from 15 families harbouring 13 different GABRB1 variants (11 missense, 1 indel, 1 stop). Functional analysis was performed using two-electrode voltage-clamp recordings in Xenopus laevis oocytes. For all 11 missense variants, α1β1γ2 GABAA receptors with a single mutant β1 subunit were used. Four missense variants were selected for further functional analysis using α5β1γ2 GABAA receptors with two mutant β1 subunits. Gain-of-function (GoF) effects, characterized by increased GABA-sensitivity, were observed for eight missense variants. Loss-of-function (LoF) effects were observed for one variant and no functional effects for two variants. Clinically, GoF variants were only observed in individuals with severe early-onset disease, including profound intellectual disability, hypotonia and early mortality. Additionally, cortical visual impairment, dysmorphisms and cortical atrophy were exclusive to this cohort. By integrating previously reported clinical data for variants in other GABR* genes, we validated that these features were associated with GoF variants more broadly. The only LoF variant was identified in a nuclear family with the relatively milder syndrome of genetic epilepsy with febrile seizures plus. Seizures were therapy-resistant in all individuals with GoF variants and a single individual with a LoF variant. The GABAergic anti-seizure medication (ASM) vigabatrin caused life-threatening side-effects in two individuals with GoF variants, while the sodium-channel blocker (SCB) lamotrigine exacerbated seizures in a single individual carrying a LoF variant. By integrating data from literature on all GABR* variants, we observed a potential dichotomy in treatment responses: GABAergic and broad-spectrum ASMs, such as valproate and levetiracetam, were more effective for individuals with LoF variants in GABR* genes, while SCBs showed greater benefit for GoF variants. Additionally, there is an increased risk of adverse effects of SCBs in LoF and vigabatrin in GoF variants. Our results highlight the importance of functional characterization of variants and clinical predictors in guiding treatment strategies for individuals with GABRB1 and other GABR* variants, although larger prospective studies are needed to confirm these observations.