Two rare TSH receptor amino acid substitutions in toxic thyroid adenomas

Locally invasive classical papillary thyroid carcinoma with TSH receptor I568T mutation: case report

Summary

Autonomous thyroid adenomas are caused by activating mutations in the genes encoding the thyroid-stimulating hormone receptor (TSHR) or mutations in the Gas subunit of the TSHR. Nodules with suspicious sonographic features should be submitted to fine-needle aspiration. Additional molecular testing may be performed to characterize the thyroid nodule's malignant potential further. We present a patient who underwent whole-transcriptome RNA-sequencing that indicated a TSHR I568T mutation after an ultrasound showed suspicious sonographic features and fine-needle aspiration was 'suspicious for malignancy'. The patient underwent thyroid resection and was found to have a locally invasive classical papillary thyroid carcinoma. Most reports of TSHR I568T mutation have been seen in patients with benign thyroid conditions. While there is insufficient data to suggest that the TSHR I568T mutation causes aggressive thyroid malignancy, we believe clinicians who identify the presence of this mutation on genome sequencing should be cautious about the possibility of locally invasive thyroid malignancy, especially when associated with Bethesda V cytopathology.

Learning points

Germline and somatic activating mutations in the genes coding for the thyroid-stimulating hormone receptor (TSHR) have been frequently reported in familial and sporadic autonomous thyroid adenomas and non-autoimmune hyperthyroidism. Most reports of TSHR I568T mutation have been detected in patients with benign thyroid conditions. We present a patient who underwent whole-transcriptome RNA-sequencing that indicated a TSHR I568T mutation and subsequently underwent thyroid resection and was found to have a locally invasive classical papillary thyroid carcinoma. Clinicians who identify the presence of TSHR I568T mutation on genome sequencing should be cautious about the possibility of locally invasive thyroid malignancy, especially when associated with Bethesda V cytopathology.

Clinical Significance of Thyroid-Stimulating Hormone Receptor Gene Mutations and/or Sodium-Iodine Symporter Gene Overexpression in Indeterminate Thyroid Fine Needle Biopsies

Objectives: To examine the prevalence of genetic alterations of thyroid-stimulating hormone receptor (TSHR) gene and sodium-iodine symporter (NIS) in a series of thyroid fine needle biopsy (FNB) specimens with indeterminate cytology, and to assess the correlation of the type of genetic changes with clinical features and follow-up results in the target thyroid nodule. Methods: Between February 2015 and September 2017, 388 consecutive FNBs with indeterminate cytology were evaluated for TSHR mutations and NIS gene overexpression using ThyroSeqV.2 next-generation sequencing (NGS) panel. Medical records were reviewed for target nodules. Results: Among 388 indeterminate FNBs, TSHR mutations and/or NIS overexpression were detected in 25 (6.4%) nodules. Ten nodules (2.6%) harbored TSHR mutations only, 7 nodules (1.8%) over-expressed NIS gene only, and 8 nodules (2.1%) had both alterations. The TSHR mutations were located between codons 281 and 640, with codon 453 being the most frequently affected. The allelic frequency of the mutated TSHR ranged from 6 to 36%. One nodule with NIS overexpression was simultaneously detected EIF1AX mutation and GNAS mutation. Nodules with TSHR mutations and/or NIS overexpression presented hyperfunctioning (n = 4), hypofunctioning (n = 5), and isofunctioning (n = 3) on the available thyroid scintigraphies. Eight cases accompanied with hyperthyroidism in which only 1 was caused by the target nodule. Evidence of co-existing autoimmune thyroid disease (AITD) and multinodular goiter were found in 52% and 52% of cases, respectively. Seven nodules underwent surgeries and all were benign on final pathology. None of 9 nodules with follow-up by ultrasound (3~33 mon, median 12 mon) showed grow in size. Conclusions: TSHR mutations and/or NIS overexpression can be detected in pre-operative FNB specimens using the NGS approach. These genetic alterations occurred in 6.4% thyroid nodules in this consecutive series with indeterminate cytology. They present not only in hyperfunctioning nodules but also in hypo- or iso-functional nodules, indicating their prevalence may be higher than previously expected. Co-existing AITD was common in cases with these molecular alterations. None of our patients with TSHR mutations and/or NIS overexpression manifested malignant outcomes. How to use these two molecular markers in thyroid FNBs to guide our clinical practice warrants further investigation.

Autonomous adenomas caused by somatic mutations of the thyroid-stimulating hormone receptor in children

In adults, autonomous adenomas of the thyroid causing hyperthyroidism are relatively common and are most often due to somatic mutations that increase the constitutive activity of the thyroid-stimulating hormone receptor (TSHR). By contrast, autonomous adenomas in hyperthyroid children are exceptional and reports of their clinical and molecular characteristics are few. We reviewed papers describing 16 autonomous adenomas due to a somatic mutation activating the TSHR and diagnosed in patients younger than 18 years, to which we added two of our own unpublished observations in a 4- and 8-year-old with the same TSHR mutation (c.CAG>CAC; p.Asp633His). This revealed that (a) autonomous adenomas occur more often in the right lobe (11 of 14 with available information) and the associated hyperthyroidism tends to be more severe, possibly reflecting the richer vascular supply of the right thyroid lobe, and (b) mutations found in benign adenomas in children have been associated with cancer in adults, suggesting that malignancy requires a second 'hit' at a later age.

Somatic mutations in 33 benign and malignant hot thyroid nodules in children and adolescents

Hot thyroid nodules (HTNs) in children are rare. Their reported malignancy rate is higher than in adults. However molecular data are rare. We present clinical and molecular data for 33 consecutive (29 benign and 4 malignant) HTNs. 17/29 Benign HTNs (59%) harbored somatic TSHR mutations. The most commonly observed mutation was M453T (in 8/29 samples). T632I and D633Y mutations were each detected twice. All other TSHR mutations were each found in one sample, including the new A538T mutation. One NRAS mutation was detected in a benign HTN with a M453T mutation. A PAX8/PPARG rearrangement was found in one malignant HTN. A T632I mutation was detected in one hot papillary thyroid carcinoma. The percentage of TSHR mutation positive HTNs in children and adolescents is within the range observed in adults. Contrary to adults, the M453T mutation is the predominant TSHR mutation in HTNs of children and adolescents. The increased malignancy rate of HTNs of children does not appear to be associated with RAS, BRAF, PAX8/PPARG and RET/PTC mutations.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.