4-Phenylbutyrate restored GABA uptake, mitigated seizures in SLC6A1 and SLC6A11 microdeletions/3p- syndrome: From cellular models to human patients.

BACKGROUND: Haploinsufficient deletions of GABA transporter 1 (GAT-1)- encoding SLC6A1, and GABA transporter 3 (GAT-3)-encoding SLC6A11 are implicated in epileptic syndromes. Despite their significance, the impact of these deletions has not been characterized. Our previous work on SLC6A1 missense mutations prompted a clinical trial for Ravicti (NCT04937062), a glycerol formulation of 4-phenylbutyrate (PBA), for treatment-resistant epilepsy. We observed phenotypic overlap between trial-eligible SLC6A1 mutation patients and 3p- syndrome patients carrying deletions of SLC6A1 and SLC6A11. This study characterizes the functional impact of these deletions and assesses the urgent question of whether 3p- syndrome patients could benefit from this treatment. METHODS: Chromosomal microarray analysis identified a deletion affecting one allele of both SLC6A1 and SLC6A11 in two pediatric patients with 3p- syndrome. Clinical phenotyping included electroencephalogram (EEG) recordings and neurodevelopmental assessments. Functional characterization was conducted using (3)H-labeled GABA uptake assays and Western blotting in HEK293T cells, comparing haploinsufficient and missense variant models. RESULTS: The haploinsufficient GAT-1 and GAT-3 conditions demonstrated reduced GABA uptake and protein expression, comparable to known SLC6A1 missense variants. Post-treatment EEGs showed a moderate reduction in epileptiform discharges following PBA administration, and patients exhibited improved motor function. However, varying degrees of cognitive impairments persisted. CONCLUSIONS: Haploinsufficiency of SLC6A1 and SLC6A11 contributes to the epileptic phenotypes observed in 3p- syndrome, marking this as the first study to biochemically characterize the functional impact of these deletions. Treatment with PBA may provide therapeutic benefits, particularly for addressing seizures and motor deficits, though further exploration of PBA's long-term effects in patients with 3p- syndrome is warranted.