Objective: Mutations in the gene encoding the thyroid hormone transport protein, monocarboxylate transporter 8 (MCT8), underlie severe mental retardation. In vitro expression of mutant transporters was performed to understand phenotypical differences.
Methods: We established cell lines stably expressing 16 MCT8 variants in JEG1 and MDCK cells. Several of these mutants have never been analysed before. The cell lines were characterized according to MCT8 mRNA and protein expression, T3 transport activity, substrate KM characteristics, surface expression and responsiveness to treatments aiming at rescuing transporter function.
Results: We could clearly demonstrate that functional activities of MCT8 mutants depend on the cell type in which they are expressed (e.g. S194F, V235M, ins235V, ΔF230, R271H, L434W, L512P, L568P). These mutants exhibited considerable transport activity when present at the cell surface as demonstrated by surface biotinylation. All mutations found in patients with milder impairments are partially active in at least one cell type in vitro, whereas other mutants are functionally inactive even if present at the cell surface (ins189I, A224V). G418 treatment of the non-sense mutants did not induce read through to yield full-length MCT8 irrespective of dose incubation time.
Conclusions: The finding that the cell type determines surface expression and T3 transport activity of missense mutants in MCT8 is important to understand phenotypic variability among carriers of different mutations. Moreover, the clinical observation that the severity of derangements of thyroid hormone levels does not correlate with mental impairments of the patients, may be based on different residual activity in different cell types e.g. pituitary thyrotrophs and central neurons. Our results indicate that patients may benefit from treatment strategies that enhance surface expression of mutated MCT8.