TSH receptor mutation V509A causes familial hyperthyroidism by release of interhelical constraints between transmembrane helices TMH3 and TMH5

Mutations of the human thyrotrophin receptor (TSH-R) are a cause of thyroid adenomas and hyperthyroidism. Here we study mechanisms of receptor activation in a genomic TSH-R variant V509A located in transmembrane helix (TMH) 3, which we identify in a family with congenital hyperthyroidism, multiple adenomas and follicular thyroid cancer. Using molecular modelling and dynamic simulation, we predicted the release of amino acid residue A593 (located opposite in domain TMH5) from a tight ‘knob-and-hole’ interaction with TMH3, physiologically constrained in the native receptor state by the bulky side chain of V509. To experimentally validate this concept, we generated mutant TSH-R expression constructs for functional in vitro studies. TSH-R mutant V509A showed a 2.8-fold increase in basal cAMP production, confirming constitutive TSH-R activation. The addition of a second site suppressor mutant A593V to TSH-R V509A resulted in the normalization of basal cAMP release, and the dose-responsiveness to TSH ligand was maintained. These data thus demonstrate that TSH-R V509A activation is caused by the release of TMH3–TMH5 interhelical constraints, while the native TSH-R conformation is re-stabilized by the introduction of a spacious valine residue at position 593. In conclusion, we delineate a novel mechanism of constitutive TSH-R activation, leading to thyroid hyperfunction and neoplasia.

Detection of an Activating Mutation of the Thyrotropin Receptor in a Case of an Autonomously Hyperfunctioning Thyroid Insular Carcinoma1

Thyroid carcinomas, even when well differentiated, usually appear as hypofunctioning at scintigraphy. We report a case of an aggressive insular thyroid carcinoma presenting as an autonomously functioning thyroid nodule and causing severe thyrotoxicosis. The tumor was metastatic to a cervical lymph node and both lungs. An activating mutation of the TSH receptor gene in both the primary tumor and the lymph node metastasis was found, due to a base substitution at codon 633 (normal guanine at position 1896 replaced by cytosine CAC for GAC causing aspartic acid substitution by histidine). Other known oncogenes (gsp, ras, PTC/ret, trk, met, and p53) were not involved. This is the first description of an activating TSH receptor mutation in a thyroid hyperfunctioning carcinoma in which an aggressive malignant phenotype coexisted with activation of the cAMP cascade and differentiated thyroid functions.