Missense variants in GABAA receptor beta2 subunit disrupt receptor biogenesis and cause loss of function

Abstract

Gamma-aminobutyric acid type A receptors (GABAARs) are the major inhibitory neurotransmitter-gated channel in the mammalian central nervous system. GABAARs function as heteropentamers, typically composed of two 1, two {beta}2, and one {gamma}2 subunits. Protein homeostasis between GABAAR folding, trafficking, assembly, and degradation is critical to ensure normal physiological functions. Variants in genes encoded for GABAARs lead to numerous neurological disorders, such as genetic epilepsy with or without neurodevelopmental delay. While these variants are associated with severe clinical presentations of epilepsy, the molecular mechanisms driving the disease remain to be elucidated. In this study, we focused on four missense epilepsy-associated variants (EAVs) in the GABRB2 gene: Q209F210delinsH (c. 627_629del), R240T (c. 719G>C), I246T (c. 737T>C), and I299S (c. 896T>G). HEK293T cells exogenously expressing these {beta}2 variants exhibited significantly reduced GABA-induced peak chloride current, indicating their loss of function. However, the four {beta}2 EAVs differed in the degree of proteostasis deficiencies, including increased ER retention, compromised assembly, decreased protein stability, and reduced trafficking and surface expression, with Q209F210delinsH and R240T variants leading to the most severe degradation. Collectively, these results indicate that these epilepsy-linked variants have debilitating effects on the early biogenesis of the {beta}2 subunit, causing misfolding, aggregation, and rapid degradation before it can be assembled with other subunits and transported to the plasma membrane. Overall, our work offers crucial mechanistic insight into how specific {beta}2 missense variants impact the proteostasis maintenance of GABAARs, which could facilitate the development of effective therapeutics for genetic epilepsy by targeting trafficking-deficient GABAAR variants.