Somatic mutations in the thyrotropin receptor gene cause hyperfunctioning thyroid adenomas

Abstract

The pituitary hormone thyrotropin stimulates the function, expression of differentiation and growth of thyrocytes by cyclic AMP-dependent mechanisms. Tissue hyperplasia and hyperthyroidism are therefore expected to result when activation of the adenylyl cyclase-cAMP cascade is unregulated. This is observed in several situations, including when somatic mutations impair the GTPase activity of the G protein Gsa (ref 6, 7). Such a mechanism is probably responsible for the development of a minority of monoclonal hyperfunctioning thyroid adenomas. Here we identify somatic mutations in the carboxy-terminal portion of the third cytoplasmic loop of the thyrotropin receptor in three out of eleven hyperfunctioning thyroid adenomas. These mutations are restricted to tumour tissue and involve two different residues (aspartic acid at position 619 to glycine in two cases, and alanine at position 623 to isoleucine in one case). The mutant receptors confer constitutive activation of adenylyl cyclase when tested by transfection in COS cells. This shows that G-protein-coupled receptors are susceptible to constitutive activation by spontaneous somatic mutations and may thus behave as proto-oncogenes.

Mutations of thyrotropin receptor isolated from thyroid autonomous functioning adenomas confer TSH-independent growth to thyroid cells

“…To date, the biological activity of these mutants has been analysed by transient transfection in COS-7 or 3T3 cells (Parma et al, 1993Porcellini et al, 1994Porcellini et al, , 1995Paschke et al, 1994). Under these conditions the potency of the receptors was measured as activation of endogenous adenylyl cyclase.…”

“…Under these conditions the potency of the receptors was measured as activation of endogenous adenylyl cyclase. Since cAMP stimulates the growth and the di erentiation of thyroid cells, it was concluded that TSH-independent growth of thyroid autonomous adenomas was caused by these somatic activating mutations, which increased the basal cAMP levels (Parma et al, 1993Porcellini et al, 1994Porcellini et al, , 1995Kopp et al, 1995;Paschke et al, 1994). This is the ®rst demonstration that these mutations of the receptor confer TSH independent growth when expressed in thyroid cells.…”

“…A molecular basis for the TSH-independent growth and function of these tumours has been recently found: some of these tumours have somatic mutations in the alpha subunit of heterotrimeric G-protein (G s a) coupled to the TSH receptor, which stimulates the adenylyl cyclase (Lyons et al, 1990;O'Sullivan et al, 1991;Suarez et al, 1991). In other cases of thyroid hyperfunctioning adenomas activating mutations of the TSH receptor have been found in the III8 intracellular loop of the receptor (Parma et al, 1993).…”

Regulatory mechanisms that modulate signalling by G-protein-coupled receptors

“…Defects in the attenuation mechanisms result in the uncontrolled stimulation of cells, which may lead to disease. For example, abnormalities in the 5-hydroxytryptamine transporter may be associated with chronic depression [233], and point mutations of the receptors for thyrotropin hormone and luteinizing hormone which result in constitutive activity of the receptors in the absence of ligand are associated with thyroid adenoma and a form of male precocious puberty respectively [234,235]. From a therapeutic standpoint, over half of all medicines used today exert their effects through signalling pathways that involve G-proteins.…”

Cyclic AMP activates Ras

“…Mutations that give rise to constitutive activity of cAMPmediated signaling pathways have been identi®ed in human tumors. Mutations in Gas (Lyons et al, 1990;Suarez et al, 1991) and in the TSH receptor (Parma et al, 1993) that result in constitutive activation of cAMP-mediated signaling are prevalent in human thyroid tumors. Despite this, very little is known regarding how cAMP elicits proliferation in some cells and impairs it in others.…”