Functional properties of a novel mutant thyroid hormone receptor in a family with generalized thyroid hormone resistance syndrome.

OBJECTIVE: We wished to ascertain whether a mutation in the thyroid hormone receptor beta gene was present in a family with generalized thyroid hormone resistance syndrome and to characterize the functional properties of this mutant receptor.
DESIGN: Blood samples were obtained from family members for hormone assays and genomic DNA was isolated from leucocytes for genetic analyses. PATIENTS: Three members (B,C,E) of a family with possible thyroid hormone resistance and two normal family members (A,D) were studied. MEASUREMENTS: Basal thyroid function tests together with serum sex hormone binding globulin (SHBG) levels were measured. The thyroid hormone receptor beta gene was amplified using the polymerase chain reaction and the receptor mutation identified by sequence analysis. The ability of mutant receptor to bind T3, interact with a specific DNA sequence and to modulate target gene expression was tested. The effects of mutant receptor on co-expressed wild type receptor action were determined.
RESULTS: Patients with resistance had raised levels of T4 and T3 together with inappropriately normal serum TSH and SHBG whereas unaffected individuals had a normal hormone profile. A single nucleotide substitution corresponding to a glycine to serine mutation at codon 340 (G340S) in the hormone binding domain was identified in one of the two beta receptor gene alleles in patients with resistance, but not in the normal family members. When expressed in vitro, this receptor protein (G340S), as well as a related (G340R) mutant identified in another family, retained the ability to bind to a specific DNA sequence but were unable to bind ligand or to activate or repress target gene expression. In addition both receptor mutants were capable of inhibiting the function of wild type thyroid hormone receptor in a co-expression assay but differed in their inhibitory potential.
CONCLUSIONS: We report a second type of mutation (Gly to Ser) in codon 340 of hTR beta in a family with generalized thyroid hormone resistance. Mutations at this site eliminate T3 binding, causing a loss of hormone-stimulated receptor function. However, the mutant receptors retain the ability to block normal receptor action. The occurrence of different mutations at the same site suggests that alterations in this region of the receptor may be important for generating the clinical phenotype of this disorder.