AUTHOR=Bielopolski Noa , Stawarski Michal , Roitman Ilana , Fridman Karen , Wald-Altman Shane , Früh Simon , Bettler Bernhard , Nissenkorn Andreea 

TITLE=Characterization of a de novo GABBR2 variant linked to autism spectrum disorder

JOURNAL=Frontiers in Molecular Neuroscience

VOLUME=16

YEAR=2023

URL=https://www.frontiersin.org/journals/molecular-neuroscience/articles/10.3389/fnmol.2023.1267343

DOI=10.3389/fnmol.2023.1267343

ISSN=1662-5099

ABSTRACT=<p>GABA<sub>B</sub> receptors (GABA<sub>B</sub>Rs) are G protein-coupled receptors for γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system. Pathogenic variants in the <italic>GABBR1</italic> and <italic>GABBR2</italic> genes, which encode the GB1 and GB2 subunits of GABA<sub>B</sub>Rs, are implicated in several neurological and developmental disorders, including epilepsy and autism. Here we present a 7-year-old boy with Level 3 Autism Spectrum Disorder who carries a <italic>de novo</italic> heterozygous missense <italic>GABBR2</italic> p.Arg212Gln variant. This variant was identified through whole exome sequencing and classified as variant of unknown significance (VUS). Treatment with the GABA<sub>B</sub>R agonist baclofen showed no clinical improvement, raising the question whether this VUS is responsible for the patient’s phenotype. We conducted a study to investigate the impact of the <italic>GABBR2</italic> p.Arg212Gln and the previously reported <italic>GABBR2</italic> p.Arg212Trp variants on protein structure and receptor activity. This study utilized a combination of molecular dynamics (MD) simulations, and <italic>in vitro</italic> experiments. Our simulations demonstrate that both amino acid substitutions locally alter amino acid interactions in the extracellular domain of GB2. Most importantly, the substitutions influence the positioning of transmembrane helices, shifting the conformation towards an active state with <italic>GABBR2</italic> p.Arg212Gln and an inactive state with <italic>GABBR2</italic> p.Arg212Trp. Functional assays confirmed the MD predictions, as evidenced by increased constitutive activity and enhanced potency of GABA for <italic>GABBR2</italic> p.Arg212Gln, and a decreased constitutive activity with a loss of GABA potency for <italic>GABBR2</italic> p.Arg212Trp. Our findings demonstrate the utility of MD simulations in predicting the functional consequences of VUS. Clarifying the pathogenic mechanisms associated with gene variants will aid in the identification of personalized treatment approaches.</p>