4-phenylbutyrate restored GABA uptake and reduced seizures in SLC6A1 variants-mediated disorders
We have previously studied the molecular mechanisms of solute carrier family 6 member 1 (SLC6A1) associated with a continuum of neurodevelopmental disorders, including various epilepsy syndromes, autism, and intellectual disability. Based on functional assays of variants in a large cohort with heterogenous clinical phenotypes, we conclude that partial or complete loss of GABA uptake function in the mutant GAT-1 is the primary etiology as identified in GABAA receptor mutation-mediated epilepsy and in cystic fibrosis. Importantly, we identified that there are common patterns of the mutant protein trafficking from biogenesis, oligomerization, glycosylation, and translocation to the cell membrane across variants with the conservation of this process across cell types. Conversely any approach to facilitate membrane trafficking would increase presence of the functional protein in the targeted destination in all involved cells. PBA is an FDA-approved drug for pediatric use and is orally bioavailable so it can be quickly translated to patient use. It has been demonstrated that PBA can correct protein misfolding, reduce ER stress, and attenuate unfolded protein response in neurodegenerative diseases, it has also showed promise in treatment of cystic fibrosis. The common cellular mechanisms shared by the mutant GAT-1 and the mutant cystic fibrosis transmembrane conductance regulator led us to test if PBA and other pharmaco-chaperones could be a potential treatment option for SLC6A1 mutations. We examined the impact of PBA and other small molecules in a library of variants and in cell and knockin mouse models. Because of the critical role of astrocytic GAT-1 deficit in seizures, we focused on astrocytes, and demonstrated that the existence of the mutant GAT-1 retained the wildtype GAT-1, suggesting aberrant protein oligomerization and trafficking caused by the mutant GAT-1. PBA increased GABA uptake in both mouse and human astrocytes bearing the mutations. Importantly, PBA increased GAT-1 expression and suppressed spike wave discharges (SWDS) in the heterozygous knockin mice. Although the detailed mechanisms of action for PBA are ambiguous, it is likely that PBA can facilitate the forward trafficking of the wildtype GAT-1 favoring over the mutant GAT-1, thus increasing GABA uptake. Since all patients with SLC6A1 mutations are heterozygous and carry one wildtype functional allele, this suggests a great opportunity for treatment development by leveraging the endogenous protein trafficking pathway to promote forward trafficking of the wildtype in combination with enhancing the disposal of the mutant allele as treatment mode. The study opens a novel avenue of treatment development for genetic epilepsy via drug repurposing.