Electrobehavioral phenotype and seizure pharmacosensitivity in a novel mouse model of patient-derived SLC6A1 S295L mutation-associated neurodevelopmental epilepsy

Abstract

Solute carrier family 6 member 1 (SLC6A1) gene encodes GAT-1, a GABA transporter expressed on glia and presynaptic terminals of inhibitory neurons. Mutations in SLC6A1 are associated with myoclonic atonic epilepsy, absence epilepsy, autism, and intellectual disability. However, the mechanisms leading to these defects are unknown. Here, we used a novel mouse model harboring a point mutation (S295L) recently identified in the human SLC6A1 gene that results in impaired membrane trafficking of the GAT-1 protein. We performed chronic wireless telemetry recordings of heterozygous (GAT-1S295L/+) mice, and of mice lacking one or both copies of the gene (GAT-1+/- and GAT-1-/-). We assessed their behaviors and pharmacosensitivity, and investigated the relationship between seizure burden and behavioral performance. GAT-1S295L/+ mice exhibited frequent spikewave discharges (SWDs) associated with behavioral arrest, and there was a dose-effect relationship between GAT-1 gene copy number and the severity of electrocorticogram (ECoG) abnormalities. Seizure burden was inversely correlated with behavioral performance. Forelimb grip strength was reduced in female mice. Acute administration of GAT-1 antagonist NO-711 induced SWDs in wildtype mice, exacerbated the phenotype in GAT-1S295L/+ and GAT-1+/- mice, and had no effect on GAT-1-/- mice lacking the drug target. By contrast, ethosuximide normalized the ECoG in GAT-1S295L/+ and GAT-1+/- mice. In conclusion, GAT-1S295L/+ mice show haploinsufficiency with evidence of GAT-1 hypofunction. This mouse model reconstitutes major aspects of human disease and thus provides a useful preclinical model for drug screening and gene therapy. Significance StatementThe SLC6A1 gene encodes for GAT-1, a major GABA transporter. Mutations in SLC6A1 lead to a spectrum of neurodevelopmental disorders and epilepsies collectively referred to as SLC6A1-related disorders (SRD). A critical contributor to disability in SRD patients is the burden of seizures and their sequelae. There is an urgent need to understand the mechanisms of SRD as they will inform therapeutic interventions. Mouse models can provide critical information allowing both the assessment of candidate therapies and the design of next generations therapies. Here we used behavioral assessments and wireless electrophysiology in a new mouse model of SRD to understand the disease pathogenesis and the association between seizure burden and behavioral deficits.