Thyroid Cancer

Receptor Tyrosine Kinases (RTKs)

Physiologically, RET is a transmembrane RTK with expression typically restricted to certain neuronal and neuroendocrine cells, the collecting duct of the kidneys, and spermatogonial stem cells, but is not normally expressed in thyroid follicular cells. In cooperation with co-receptors, RET binds ligands of the glial cell-derived neurotrophic growth factor (GDNF) family, which leads to receptor dimerization and autophosphorylation of key intracellular tyrosine residues that subsequently activate downstream signaling in multiple pathways.

Somatic chromosomal rearrangements of the RET gene were identified in papillary thyroid carcinomas more than 20 years ago that cause constitutive expression and activation of a RET fusion protein denoted RET/PTC. To date, more than a dozen RET/PTC translocations have been reported, although RET/PTC1 and RET/PTC3 account for more than 90% of the tumors associated with the mutation. In each case, the promoter and N-terminal domain of a heterologous gene fuse with a C-terminal domain of the RET gene containing the tyrosine kinase functions, permitting inappropriate expression of RET kinase in thyroid follicular cells under control of the heterologous promoter. The resultant fusion protein resides in the cytosol rather than the cell membrane and is capable of ligand-independent homodimerization, thus triggering downstream signaling activation. A further mechanism to promote tumorigenesis is through cooperation between RET/PTC and another RTK, the epidermal growth factor receptor (EGFR), which is found to colocate with RET/PTC and jointly signal downstream through RAS. RET/PTC mutations are commonly seen in about 10% to 20% of PTC (particularly the classic, solid, and almost all oxyphilic variants but less commonly in follicular and tall cell variants), and rarely in PDTC or FTC. The oncogenic role of RET/PTC mutations is supported by a high rate of papillary carcinomas in transgenic mice expressing either RET/PTC1 or RET/PTC3 , and the mutation is frequently found in occult microcarcinomas.

A second group of chromosomal translocations associated with PTC comprise mutations in the NTRK gene that lead to formation of various TRK oncogenes. Similar to RET/PTC, these fusion TRK proteins combine the N terminus of one of several genes normally expressed in thyroid follicular cells with the tyrosine kinase domain of the receptor for nerve growth factor. These mutations are seen in about 5% of PTC.

These chromosomal rearrangements involving tyrosine receptor genes are commonly seen following exposure to ionizing radiation and may be the leading mechanism of thyroid oncogenesis in this setting. This has been particularly true following the unfortunate experiences of the Chernobyl nuclear power plant accident as well as atomic bomb explosions in Japan. Spatial proximity of RET or NTRK genes to the heterologous donors of the N-terminal promoter regions specific to chromosomal folding in thyroid follicular cells may permit a single radiation event to cause double-strand breaks in each gene, thus permitting the recombination event.

Other RTKs have been found to be overexpressed in DTC cells and may be relevant to disease biology. In addition to EGFR, these include platelet-derived growth factor receptor (PDGFR) α and β; vascular endothelial growth factor receptors (VEGFR) 1 and 2; fibroblast growth factor receptors (FGFR); and hepatocyte growth factor receptor (MET). Genetic abnormalities are rarely observed, and epigenetic alterations may be contributory; however, the exact mechanisms leading to overexpression are generally not known.

BRAF

Mutations of the serine-threonine kinase BRAF are the most common oncogenic abnormality reported in PTC. Activated frequently in other cancers as well, BRAF has high affinity for binding and phosphorylating MEK isoforms in the MAPK pathway. Although point mutations in multiple codons have been reported to cause constitutive activation of BRAF, the most studied and most frequent is a valine-to-glutamine substitution at amino acid residue 600 (denoted V600E mutation). By destabilizing the inactive conformation of the kinase, the V600E BRAF mutation causes the protein to remain in a catalytically competent conformation that allows continuous phosphorylation of MEK. BRAF mutations are virtually never seen in benign thyroid lesions, and thus the presence of a BRAF mutation can be pathognomonic of a malignancy if detected in a cytologically suspicious biopsy specimen. Of note, a rare translocation mutation of BRAF (causing fusion of the AKAp9 gene with BRAF) has been reported to cause PTC after radiation exposure.

BRAF mutations occur in 40% to 50% of cases of PTC (especially the classical and tall-cell variants) and are also frequent in PDTC and ATC. Multiple studies describe a more aggressive phenotype associated with these mutations, including higher rates of lymph node metastases, extrathyroidal extension, poor radioiodine uptake and response to therapy, and advanced stage at presentation. Prognostically, BRAF mutations are associated with higher rates of recurrence and worse survival. In a model of conditional activation of the V600E BRAF mutant in thyroid follicular cells, mice develop rapidly growing poorly differentiated tumors with negligible expression of thyroid-specific genes such as the sodium-iodide symporter as well as loss of iodine incorporation; these changes are reversible on inhibition of BRAF or MEK kinase functions. Other downstream effects of mutant BRAF include alterations in DNA methylation and increased expression of genes associated with invasive and metastatic disease such as matrix metalloproteinases.

RAS

Point mutations in RAS genes are among the most common oncogenic abnormalities in all cancers, and DTC is no different. Mutations in the RAS protein lead to constitutive activation through alterations in the binding affinity of the kinase for GTP or through inactivation of its intrinsic GTPase activity. Thus, mutant RAS can signal downstream through both the MAPK and PI3K/Akt pathways without upstream activation derived from ligand-bound RTK. All three RAS genes ( H-RAS, K-RAS, and N-RAS ) are implicated in thyroid tumor formation from follicular cells, including 20% to 40% of benign follicular adenomas, 40% to 50% of FTC (including 15% to 20% of oxyphilic variants), 10% to 20% of PTC (almost exclusively follicular variants of PTC), and 25% of PDTC. The presence of a RAS mutation may portend more aggressive disease with worse outcomes, but this has not been extensively examined. Each of these histologies has also been observed in transgenic mice expressing RAS mutations, although the presence of mutant RAS proteins alone is likely insufficient to cause tumor formation.