Authors :
Presenting Author: Nathan Absalom, Dr – Western Sydney University
Susan Lin, PhD Student – University of Sydney; Angelo Keramidas, PhD – University of Queensland; Vivian Liao, PhD – University of Sydney; Rikke Moller, PhD – Danish Epilepsy Centre, Filadelfia; Mary Chebib, PhD – University of Sydney; Philip Ahring, PhD – University of Sydney
Rationale:
Genetic variants associated with developmental and epileptic encephalopathies have been identified in the
GABRB3 gene that encodes the β3 subunit of GABA
A receptors. Typically, variants alter receptor sensitivity to GABA resulting in either gain or loss of function, which correlates with patient phenotypes. However, it is unclear how another important receptor property, desensitization, contributes to the greater clinical severity of gain of-function variants.
Methods:
Desensitization properties of 20 gain-of-function
GABRB3 variant receptors were evaluated using two-electrode voltage-clamp electrophysiology. The parameters measured included current decay rates and steady-state currents. Selected variants with increased or reduced desensitization were also evaluated using whole-cell electrophysiology in transfected mammalian cell lines.
Results:
Of the 20 gain-of-function variants assessed, thirteen were found to alter receptor desensitization properties. Seven variants reduced desensitization at equilibrium, which acts to worsen gain-of-function traits. Six variants accelerated current decay kinetics, which limits gain-of-function traits. All affected patients displayed severe clinical phenotypes with intellectual disability and difficult-to-treat epilepsy. Nevertheless, variants that reduced desensitization at equilibrium were associated with more severe clinical outcomes. This included younger age of first seizure onset (median 0.5 months), movement disorders (dystonia and dyskinesia), epilepsy of infancy with migrating focal seizures (EIMFS) and risk of early mortality. Variants that accelerated current decay kinetics were associated with slightly milder phenotypes with later seizure onset (median four months), unclassifiable developmental and epileptic encephalopathies or Lennox-Gastaut syndrome and no movement disorders.
Conclusions:
Our study reveals that gain-of-function
GABRB3 variants can increase or decrease receptor desensitization properties and that there is a correlation with the degree of disease severity.
It is anticipated that this knowledge will allow more accurate diagnosis of the likely progression of developmental and epileptic encephalopathies associated with gain-of-function GABRB3 variants. Funding:
This work was funded by the Australian National Health & Medical Research Council grant APP1185122 (P.A., N.A., M.C.), the Australian Research Training Program Stipend scholarship (S.L.), the Novo Nordisk Foundation NNF19OC0058749 (R.M.) and The Lundbeck Foundation R383-2022-276 (to R.M. and P.A.).