Syndromes of thyroid hormone resistance due to mutations in the T3[beta] receptor: progress in our understanding

Thyroid hormone resistance (RTH) is a rare autosomal dominant disorder, characterized clinically by goiter and biochemically by elevated circulating free thyroid hormone levels in the presence of measurable serum thyroid-stimulating hormone (TSH) concentrations. About 85% of patients with RTH harbor mutations in thyroid hormone receptor β (TRβ). These mutations cluster in three different 'hot spots' in the ligand (T3) binding domain of the receptor. When mapped to their homologous residues in the thyroid hormone receptor (TR) crystal structure, these three clusters of mutations border the T3-binding pocket. As a consequence, most TRβ mutations impair the hormone binding to the receptor and interfere with the mechanisms of corepressor release and consequent recruitment of coactivators. Thus, the remodeling of chromatin structure throughout the process of histone acetylation is prevented and the transcriptional activity of the mutant receptor on both positively and negatively regulated genes is severely disrupted. The lack of interaction with coactivators appears to be an additional mechanism for the dominant negative effects of mutant TRβ on the transcriptional activity of the normal (wild-type) TRs.