A novel mutation (E333D) in the thyroid hormone β receptor causing resistance to thyroid hormone syndrome

Resistance to thyroid hormone (RTH) is an inherited syndrome characterized by elevated serum thyroid hormones (TH), failure to suppress pituitary thyroid stimulating hormone (TSH) secretion, and variable peripheral tissue responsiveness to TH. The disorder is associated with diverse mutations in the thyroid hormone beta receptor (TR beta). Here, we report a novel natural RTH mutation (E333D) located in the large carboxy-terminal ligand binding domain of TR beta. The mutation was identified in a 22-year-old French woman coming to medical attention because of an increasing overweight. Biochemical tests showed elevated free thyroxine (T4: 20.8 pg/ml (normal, 8.5-18)) and triiodothyronine (T3: 5.7 pg/ml (normal, 1.4-4)) in the serum, together with an inappropriately nonsuppressed TSH level of 4.7 mU/ml (normal, 0.4-4). Her father and her brother's serum tests also showed biochemical abnormalities consistent with RTH. Direct sequencing of the TR beta gene revealed a heterozygous transition 1284A > C in exon 9 resulting in substitution of glutamic acid 333 by aspartic acid residue (E333D). Further functional analyses of the novel TRP mutant were conducted. We found that the E333D mutation neither significantly affected the affinity of the receptor for T3 nor modified heterodimer formation with retinoid X receptor (RXR) when bound to DNA. However, in transient transfection assays, the E333D TR beta mutant exhibited impaired transcriptional regulation on two distinct positively regulated thyroid response elements (F2- and DR4-TREs) as well as on the negatively regulated human TSH alpha promoter. Moreover, a dominant inhibition of the wild-type TR beta counterpart transactivation function was observed on both a positive (F2-TRE) and a negative (TSH alpha) promoter. These results strongly suggest that the E333D TR beta mutation is responsible for the RTH phenotype in the proposita's family.