Functional characteristics of three new germline mutations of the thyrotropin receptor gene causing autosomal dominant toxic thyroid hyperplasia.

Abstract

We report three unrelated families in which hyperthyroidism associated with thyroid hyperplasia was transmitted in an autosomal dominant fashion, in the absence of signs of autoimmunity. Exon 10 of the TSH receptor gene was directly sequenced after PCR amplification from DNA of peripheral leukocytes. In one family, a C to A transversion resulted in an S505R substitution in the third transmembrane segment; in the second, an A to T transversion caused a N650Y substitution in the sixth transmembrane segment; and in the third family, an A to G transition resulted in an N670S substitution in the seventh transmembrane segment. When expressed by transfection in COS-7 cells, each mutated receptor displayed an increase in constitutive stimulation of cAMP production; no effect on basal accumulation of inositol phosphates (IP) could be detected. In binding studies, cells transfected with wild-type or mutated receptors showed similar levels of expression, with the mutated receptors displaying similar or slightly increased affinity for bovine TSH (bTSH) binding. Cells transfected with S505R and N650Y mutants showed a similar cAMP maximal TSH-stimulated accumulation over the cells transfected with the wild type, whereas N670S transfectants showed a blunted response with an increase in EC50. A higher IP response to 100 mU/mL bTSH over that obtained with the wild-type receptor was obtained in cells transfected with N650Y; in contrast, cells transfected with S505R showed a blunted IP production (50% less), and the N670S mutant completely lost the ability to stimulate IP accumulation in response to bTSH. The differential effects of individual mutations on stimulation by bTSH of cAMP or IP accumulation suggest that individual mutant receptors may achieve different active conformations with selective abilities to couple to Gs alpha and to Gq alpha.

Congenital hyperthyroidism caused by a solitary toxic adenoma harboring a novel somatic mutation (serine281-->isoleucine) in the extracellular domain of the thyrotropin receptor.

“…To date, naturally occuring activating mutations have been reported only in exon 10 of the TSH receptor. They were documented in all extracellular loops, the second and third intracellular loops, and all but the first transmembrane domain, and the still-growing list contains almost twenty different residues (1,14,16, and our unpublished results). Supporting our observation, the very same amino acid, serine281, has also been found to be mutated in three toxic adenomas (Gilbert Vassart, personal communication).…”

“…In the thyroid, somatic mutations in the TSH receptor gene were first demonstrated to be a major molecular cause of hyperfunctioning thyroid adenomas (4). If present in the germline, they lead to nonautoimmune hereditary hyperthyroidism (13,14), and de novo germline mutations have been found in children with severe congenital hyperthyroidism (15)(16)(17).…”

“…The prevalence of these mutations in toxic adenomas varies between 2.5 and 80% (7, 10), a finding which might, in part, be influenced by differences in iodine intake, sampling technique, and methodological approach (11,12). Germline TSH receptor mutations have also been found in nonautoimmune familial hyperthyroidism (13,14), and de novo germline mutations can cause sporadic congenital hyperthyroidism (15)(16)(17).…”

A Conserved Asn in Transmembrane Helix 7 Is an On/Off Switch in the Activation of the Thyrotropin Receptor

“…6B). Increase in apparent affinity with no significant modification of hormone potency has been observed in most constitutive mutants of the TSH receptor (40). Interestingly, mutant N7.49A binds TSH with high affinity, even slightly higher than the wt receptor ( Fig.…”

Oncogenic potential of a mutant human thyrotropin receptor expressed in FRTL-5 cells

“…Mutations in genes of this pathway are often observed in thyroid tumors independent of TSH in their secretory function and growth. Mutations in the TSHR gene have been observed in human adenomas (Russo et al, 1995a;Paschke et al, 1994;Parma et al, 1993) and in congenital hyperthyroidism (Duprez et al, 1994;Kopp et al, 1995;De Roux et al, 1996;Tonacchera et al, 1996). These mutated TSHR permanently activate adenylyl cyclase; some of them also activate the phosphatidylinositide pathway .…”