14q31.1

CHNG1,LGR3,hTSHR-I

Thyroid adenoma

Somatic activating TSHR mutations are a major cause of hyperfunctioning thyroid adenomas, benign neoplasms of thyroid follicular cells:
- 10-80% of hyperfunctioning or hot adenomas (i.e. ademomas with active radioiodide uptake and thyroid hormone synthesis) harbor somatic activating mutations (Parma et al., 1993; Russo et al., 1996; Führer et al., 2007). The highly variable frequency of mutations may reflect geographic differences.
- Activating mutations are largely located in exon 10 encoding the TSHR β-subunit, and are particularly frequent in the sixth transmembrane segment (TM6) important for coupling to G proteins.
- Mutations result in the constitutive activation of adenylate cyclase, stimulating thyrocyte proliferation and thyroid hormone synthesis. As a consequence, hyperfunctioning nodules provoke hyperthyroidism and thyrotoxicosis.
- Nonfunctioning or cold adenomas (i.e. not able to accumulate radioiodide or synthesize thyroid hormones) do not harbor TSHR mutations.

Toxic multinodular goiter (Plummers disease)

Somatic activating TSHR mutations are a major cause of Toxic Multinodular Goiter (TMNG), a hyperplastic thyroid enlargement with multiple bilateral nodules:
- 70-80% of hyperfunctioning nodules of TMNG harbor somatic activating mutations, many of which are shared with hyperfunctioning thyroid adenoma suggesting a common pathogenic event (Tonacchera et al., 1998; Tonacchera et al., 2000).
- Different hyperfunctioning nodules within a goiter harbor may distinct TSHR activating mutations, confirming the polyclonal etiology of TMNG. In contrast, nonfunctioning nodules within the same goiter do not harbor mutations.
- TMNG is most common in iodine-deficient areas, and this may reflect the increased mutagenic load associated with chronic TSH stimulation and thyrocyte proliferation.
- A germline polymorphism of codon 727 of TSHR was reported to be associated with TMNG (Gabriel et al., 1999), however a subsequent study failed to substantiate this finding in a European population (Mulhberg et al., 2000).

Thyroid carcinoma

TSHR mRNA in thyroid tumours: Reduced or absent TSHR mRNA in thyroid tumours is a negative prognostic marker, indicative of reduced effectiveness of radioiodine therapy. TSHR expression in thyroid carcinoma correlates with the state of differentiation of tumours, with loss of differentiation resulting in a loss of mRNA expression. Thus, well-differentiated carcinomas (papillary and follicular) show variable TSHR mRNA levels ranging from normal to markedly reduced, whereas undifferentiated anaplastic carcinoma shows absent TSHR mRNA (Brabant et al., 2001; Brönnegård et al., 1994; Shiels et al., 1999). TSHR regulates the thyroid expression of the iodide transporter NIS, and decreased TSHR expression impairs the iodine-concentrating capacity of thyroid tumours.
Circulating TSHR mRNA: TSHR mRNA in peripheral blood has been suggested as a potential molecular marker of circulating thyroid carcinoma cells to aid in the differential diagnosis of malignant and benign thyroid disease preoperatively, and to detect residual, recurrent or metastatic thyroid cancer (Barzon et al., 2004; Chinnappa et al., 2004).
TSHR promoter methylation: TSHR promoter methylation may be a marker for malignancy in thyroid carcinoma (Xing et al., 2003).

TSHR mutations: Mutagenic screening of thyroid carcinomas reveals a low incidence of somatic TSHR mutations ranging from 0-22%, suggesting a limited role for TSHR mutations in the pathogenesis of thyroid carcinoma (Matsuo et al., 1993; Russo et al., 1995; Ohno et al., 1995; Spambalg et al., 1996; Esapa et al., 1997; Cetani et al., 1999).
- Thr632Ile in thyroid hormone-producing follicular carcinoma (Spambalg et al., 1996).
Rare cases of TSHR mutations in nonfunctioning thyroid carcinoma have also been reported:
- Ile486Phe in an autonomously functioning follicular carcinoma presenting a hot nodule causing hyperthyroidism (Camacho et al., 2000).
- Leu512Arg in autonomously functioning papillary carcinoma with hot nodule (Gozu et al., 2004).
- Leu677Val in autonomously functioning Hürthle cell thyroid carcinoma causing thyrotoxicosis (Russo et al., 1999).
- Phe631Ile and Asp633Tyr in toxic metastasizing follicular thyroid carcinoma; no TSHR mutations in non-functioning lung metastases (Führer et al., 2003).
- Asp633His in aggressive insular thyroid carcinoma (poorly-differentiated carcinoma) presenting as an autonomously functioning thyroid nodule and causing severe thyrotoxicosis; TSHR mutation also present in lymph node metastasis (Russo et al., 1997).
- Thr632Ala in nonfunctioning follicular carcinoma (Spambalg et al., 1996).
- Ala623Ser activating mutation in papillary carcinoma with cold nodule, in patient with past history of Graves disease (De Cross et al., 2008).
The role of activating TSHR mutations in neoplastic transformation is unclear. Although TSHR activation stimulates thyrocyte proliferation, hyperfunctioning nodules (adenoma or toxic multinodular goiter) are rarely malignant, suggesting that constitutive activation of the cAMP cascade alone is insufficient for the malignant transformation of thyroid follicular cells.
Epigenetics: The TSHR gene promoter is frequently hypermethylated in thyroid carcinoma, with preferential methylation in undifferentiated carcinoma (Xing et al., 2003; Schagdarsurengin et al., 2006). In contrast, TSHR gene promoter is unmethylated in the normal thyroid and in benign tumours (thyroid adenoma). Hypermethylation results in TSHR gene silencing and reduced TSHR expression in some malignant thyroid tumours. In thyroid cancer cell lines, demethylating agents partially restore TSHR expression.

Congenital nongoitrous hypothyroidism-1 (CHNG1)

Germline inactivation TSHR mutations are a cause of hereditary congenital nongoitrous hypothyroidism. Inactivating mutations result in variable degrees of TSH resistance, with clinical consequences ranging from euthyroid hyperthyrotropinemia to mild or severe hypothyroidism. CHNG1 is inherited in an autosomal recessive manner, manifesting in homozygotes or compound heterozygotes. However, subclinical hypothyroidism may also manifest in heterozygotes, suggesting autosomal dominant inheritance.

Hereditary nonautoimmune hyperthyroidism

Germline activating TSHR mutations are a cause of hereditary nonautoimmune congenital hyperthyroidism (autosomal dominant inheritance). Sporadic cases of nonautoimmune congenital hyperthyroidism result from de novo germinal mutations. Activating germline mutations result in constitutive activation of the cAMP pathway in thyrocytes. Mutations are predominantly in exon 10 encoding the transmembrane domains. One activating mutation (Arg183Lys) increases sensitivity towards hCG and manifests in women during pregnancy (familial gestational hyperthyroidism).