Targeted knockout of GABA-A receptor gamma 2 subunit provokes transient light-induced reflex seizures in zebrafish larvae

U. Kundap¹, M. Liao1, D.R.W. Burrows², M.P. Meyer², B.O. Amar Bencheikh¹, R.E. Rosch², E. Samarut¹

The University of Montreal Hospital Research Center (CRCHUM), University of Montreal, Neuroscience, Montreal, Canada¹, MRC Centre for Neurodevelopmental Disorders-Institute of Psychiatry- Psychology and Neuroscience- King’s College London, Department for Developmental Neurobiology, London, United Kingdom²

Epilepsy is a widespread neurological disorder characterized by recurrent seizures that are neuronal discharges arising within specific networks. It affects more than 50 million individuals worldwide causing mortality, disability, social and behavioural stigma. Epilepsies can arise from a range of causes associated with different prognoses and treatment outcomes, including metabolic disorders, brain lesions, and autoimmune origins, as well as ‘idiopathic’ cases. Mutations in the GABA-A receptor gamma 2 subunit gene (GABRG2), for example, have been associated with absence epilepsy and febrile seizures in humans. Several rodent models of GABRG2 loss of function depict clinical features of the disease; however, alternative genetic models more amenable for the study of ictogenesis and for high-throughput screening purposes were still needed. Since numerous mutations in the GABRG2 gene, encoding a GABA receptor subunit, have been reported to cause epilepsy, we launched a project aiming at modelling GABRG2-dependent epilepsies in zebrafish. To do so, we established and characterized a gabrg2 knockout (KO) zebrafish model (using CRISPR/CAS9) and showed that mutant fish display reflex seizures upon photic stimulation. Through in vivo calcium imaging of the brain, it was confirmed that this phenotype is associated with a widespread increase in neuronal activity. Moreover, these seizures only occur at the larval stages but disappear after 1 week of age. Interestingly, our whole-transcriptome analysis showed that gabrg2 KO does not alter the expression of genes in the larval brain. Thus, the gabrg2−/− zebrafish is a novel in vivo genetic model of early epilepsies that opens new doors to investigate ictogenesis and for further drug-screening assays.