Clonal origin of toxic thyroid nodules with constitutively activating thyrotropin receptor mutations

Constitutively activating TSH receptor mutations have recently been detected in toxic nodules. In vitro studies suggest that mutated receptor signaling constitutively elevates cAMP, which causes hyperfunction and proliferation of thyrocytes. Therefore, toxic nodules with constitutively activating somatic TSH receptor mutations should result from clonal expansion of a single mutated cell. To test this hypothesis, we studied the clonal origin of 27 toxic nodules. In 13 of 27 nodules, a somatic mutation in the TSH receptor was identified. A PCR-based clonality assay that analyzes X-chromosome inactivation was used. The assay amplifies a polymorphism located in the androgen receptor gene. Of 27 toxic nodules studied, 23 (85%) were informative for the polymorphism. In the group that contains a somatic mutation in the TSH receptor, 10 of 11 cases showed nonrandom X inactivation, indicating clonal expansion. In only one toxic nodule with a TSH receptor mutation was random X inactivation detected. In the group without detectable mutations in exons 9 and 10 of the TSH receptor and exons 7-10 of the G(s) alpha protein, only 6 of 12 toxic nodules show nonrandom X-chromosome inactivation. Therefore, the majority of toxic nodules with constitutively activating TSH receptor mutations are of clonal origin. This finding supports the hypothesis that toxic nodules arise from aberrant growth of a single cell. It is widely accepted that somatic mutations might initiate monoclonal growth. The TSH receptor mutations in these toxic nodules together with G(s) alpha mutations in others are the most likely candidates for the initiation of this thyroid tumor. The clonal origin of toxic nodules in the group without detected mutations in the TSH receptor or the G(s) alpha protein suggests somatic mutations in genes that are unknown to date.