Supplementary Material for: The Protein Translocation Defect of MCT8L291R Is Rescued by Sodium Phenylbutyrate

Introduction: The monocarboxylate transporter 8 (MCT8; SLC16A2) is a specific transporter for thyroid hormones. MCT8 deficiency, formerly known as the Allan-Herndon-Dudley syndrome, is a rare genetic disease that leads to neurological impairments and muscle weakness. Current experimental treatment options rely on thyromimetic agonists that do not depend on MCT8 for cellular uptake. Another approach comes from studies with the chemical chaperone sodium phenylbutyrate (NaPB), which was able to stabilize MCT8 mutants having protein folding defects in vitro. In addition, NaPB is known as a compound that assists with plasma membrane translocation. Objective: The pathogenic MCT8L291R leads to the same severe neurological impairments found for other MCT8-deficient patients but, unexpectedly, lacks alterations in plasma 3,3′,5-triiodothyronine (T3) levels. Here we tried to unravel the underlying mechanism of MCT8 deficiency and tested whether the pathogenic MCT8L291R mutant responds to NaPB treatment. Therefore, we overexpressed the mutant in Madin-Darby canine kidney cells in the human choriocarcinoma cell line JEG1 and in COS7 cells of African green monkey origin. Results: In our recent study we describe that the MCT8L291R mutation most likely leads to a translocation defect. The pathogenic mutant is not located at the plasma membrane, but shows overlapping expression with a marker protein of the lysosome. Mutation of the corresponding amino acid in murine Mct8 (Mct8L223R) displays a similar effect on cell surface expression and transport function as seen before for MCT8L291R. NaPB was able to correct the translocation defect of MCT8L291R/Mct8L223R and restored protein function by increasing T3 transport activity. Furthermore, we detected enhanced mRNA levels of wild-type and mutant MCT8/Mct8 after NaPB treatment. The increase in mRNA levels could be an explanation for the positive effect on protein expression and function detected for wild-type MCT8. Conclusion: NaPB is not only suitable for the treatment of mutations leading to misfolding and protein degradation, but also for a mutant wrongly sorted inside a cell which is otherwise functional.