We have studied the molecular mechanisms of variants in solute carrier Family 6 Member 1 associated with
neurodevelopmental disorders, including various
epilepsy syndromes,
autism and
intellectual disability. Based on functional assays of solute carrier Family 6 Member 1 variants, we conclude that partial or complete loss of γ-amino
butyric acid uptake due to reduced membrane γ-amino
butyric acid transporter 1 trafficking is the primary aetiology. Importantly, we identified common patterns of the mutant γ-amino
butyric acid transporter 1 protein trafficking from biogenesis, oligomerization, glycosylation and translocation to the cell membrane across variants in different cell types such as astrocytes and neurons. We hypothesize that therapeutic approaches to facilitate membrane trafficking would increase γ-amino
butyric acid transporter 1
protein membrane expression and function.
4-Phenylbutyrate is a Food and Drug Administration-approved
drug for paediatric use and is orally bioavailable.
4-Phenylbutyrate shows promise in the treatment of
cystic fibrosis. The common cellular mechanisms shared by the mutant γ-amino
butyric acid transporter 1 and
cystic fibrosis transmembrane conductance regulator led us to hypothesize that
4-phenylbutyrate could be a potential treatment option for solute carrier Family 6 Member 1 mutations. We examined the impact of
4-phenylbutyrate across a library of variants in cell and knockin mouse models. Because γ-amino
butyric acid transporter 1 is expressed in both neurons and astrocytes, and γ-amino
butyric acid transporter 1 deficiency in astrocytes has been hypothesized to underlie seizure generation, we tested the effect of
4-phenylbutyrate in both neurons and astrocytes with a focus on astrocytes. We demonstrated existence of the mutant γ-amino
butyric acid transporter 1 retaining wildtype γ-amino
butyric acid transporter 1, suggesting the
mutant protein causes aberrant
protein oligomerization and trafficking.
4-Phenylbutyrate increased γ-amino
butyric acid uptake in both mouse and human astrocytes and neurons bearing the variants. Importantly,
4-phenylbutyrate alone increased γ-amino
butyric acid transporter 1 expression and suppressed spike wave discharges in heterozygous knockin mice. Although the mechanisms of action for
4-phenylbutyrate are still unclear, with multiple possibly being involved, it is likely that
4-phenylbutyrate can facilitate the forward trafficking of the wildtype γ-amino
butyric acid transporter 1 regardless of rescuing the mutant γ-amino
butyric acid transporter 1, thus increasing γ-amino
butyric acid uptake. All patients with solute carrier Family 6 Member 1 variants are heterozygous and carry one wildtype allele, suggesting a great opportunity for treatment development leveraging wildtype protein trafficking. The study opens a novel avenue of treatment development for genetic
epilepsy via
drug repurposing.