Use of small molecules to rescue mutated MCT8 as a potential future treatment for Allan-Herndon-Dudley syndrome

Allan-Herndon-Dudley syndrome is a recessive X-linked rare disease induced by alterations of the SLC16A2 gene (Xq13.2) that encodes the monocarboxylate transporter 8 (MCT8). MCT8 is a membrane protein responsible for the uptake of T3, T4 and rT3 in cells of the central nervous system and play a major role, also, in the crossing of the brain blood barrier. The lack of thyroid hormones in the brain and their accumulation in tissues that, even though expressing low level of the protein, are MCT8-independent, result in an hypothyroidic phenotype in the cerebral area, leading to developmental mental retardation, and in a thyrotoxic state at a peripheral level, culminating in motor impairment, low body weight and tachycardia. The absence/reduction/lack of function of the mutated MCT8 on the plasma membrane is due to different alterations on the SLC16A2 gene that can lead to impaired trafficking, correct translocation of a completely non-functional protein or defective folding, causing premature degradation through the ERAD pathway (Endoplasmic-reticulum-associated protein degradation). The objective of this thesis is the assessment of the ability of small molecules, known as CFTR correctors, to help the folding capacity of the mutated MCT8 protein and, consequently, the correct translocation of the transporter on the plasma membrane. The considered compounds are protein folding correctors initially developed for cystic fibrosis, a disease that share some similarities in the pathological mechanism with AHDS, and that have been successfully tested in different conditions like sarcoglycanopathies. We performed in vitro experiments using Madin-Darby canine kidney cells (MDCK) constitutively expressing the mutated (L568P) MCT8 cDNA to investigate the protein recovery, upon the treatment with several correctors and focusing particularly at the plasma membrane through biotinylation analysis. Moreover, we analysed the cytotoxicity of two of such compounds (C17 and VX661), to evaluate and compare their potential detrimental effect on the viability of cells.