Molecular phenotypes segregate missense mutations in SLC13A5 Epilepsy

The sodium-coupled citrate transporter (NaCT, SLC13A5) mediates citrate uptake across the plasma membrane via an inward Na ⁺ gradient. Mutations in SLC13A5 cause early infantile epileptic encephalopathy type-25 (EIEE25, SLC13A5 Epilepsy) due to impaired citrate uptake in neurons. Despite clinical identification of disease-causing mutations, underlying mechanisms and cures remain elusive. We mechanistically classify the molecular phenotypes of six mutations. C50R, T142M, and T227M exhibit impaired citrate transport despite normal expression at the cell surface. G219R, S427L, and L488P are hampered by low protein expression, ER retention, and reduced transport. Mutants’ mRNA levels resemble wildtype, suggesting post-translational defects. Class II mutations display immature core-glycosylation and shortened half-lives, indicating protein folding defects. These experiments provide a comprehensive understanding of the mutation’s defects in SLC13A5 Epilepsy at the biochemical and molecular level and shed light into the trafficking pathway(s) of NaCT. The two classes of mutations will require fundamentally different treatment approaches to either restore transport function, or enable correction of protein folding defects.