Phenylbutyrate-Responsive SLC6A1-Related Neurodevelopmental Disorder Associated With a Familial Variant.

SLC6A1-related neurodevelopmental disorder is a synaptopathy characterized by developmental delay, epilepsy, and neurobehavioral manifestations with marked phenotypic variability. Variants impair γ-aminobutyric acid (GABA) transporter-1 (GAT-1) folding and trafficking, reducing inhibitory neurotransmission and promoting hyperexcitability. Pharmacologic chaperones such as 4-phenylbutyrate (4-PBA) may restore GAT-1 function. We report a 3-generation family harboring a heterozygous SLC6A1 variant with segregating neurodevelopmental and epileptic phenotypes. The proband presented with drug-resistant developmental and epileptic encephalopathy, multiple seizure types, diffuse epileptiform abnormalities, and global developmental delay. Segregation analysis demonstrated co-segregation of the variant with epilepsy and neurodevelopmental features across affected relatives. Because of persistent seizures despite antiseizure medications, glycerol phenylbutyrate (GPB), a prodrug of 4-PBA, was initiated, resulting in complete seizure freedom and reduction of epileptiform discharges on follow-up electroencephalography. These findings highlight the potential role of genotype-informed precision therapy in SLC6A1-related disorders and underscore the importance of careful variant interpretation in familial cases.