IDENTIFICATION AND CHARACTERIZATION OF A NOVEL DE-NOVO MUTATION (L346V) IN THE THYROID-HORMONE RECEPTOR-BETA GENE IN A FAMILY WITH GENERALIZED THYROID-HORMONE RESISTANCE

We have investigated an Italian family with generalized resistance to thyroid hormone (RTH), consisting of two individuals with elevated ser um thyroid hormones (TH) and a non-suppressed TSH, together with unaff ected family members, for a mutation in the thyroid hormone receptor b eta gene (hTR beta). We have identified a single nucleotide substituti on (1321 CTT to GTT) corresponding to a leucine to valine substitution at codon 346 (L346V) in the predicted protein. The index case and her affected child are heterozygous for the receptor defect, with normal sequence in unaffected family members. Furthermore, both parents of th e index case were unaffected, suggesting that the mutation had arisen de novo. When expressed in vitro, the L346V mutant receptor showed a m arked reduction in its affinity for tri-iodothyronine (T-3), impaired ligand-dependent transactivation and potent dominant negative activity . Its functional impairment could not be alleviated, even at supraphys iological concentrations of T-3, suggesting that the mutation might in terfere with the intrinsic ligand-dependent transactivation function ( AF-2) located in the hormone binding domain of hTR beta. Finally, the presence of the L346V mutation in the son of the propositus, who died from complications associated with congenital heart disease, raises th e possibility that RTH might have contributed to the pathogenesis or s everity of the latter.