Thyroid cancer originating from follicular epithelial cells accounts for approximately 1% of all new case of cancer each year and its incidence has increased significantly over the last two decades (Hodgson et al., 2004; Davies and Welch, 2006). Papillary thyroid carcinoma (PTC) accounts for approximately 85% of all cases, and it is responsible for the overall increase in incidence of thyroid cancer. Mortality in PTC is low and the majority of patients can be considered cured after thyroidectomy followed by ablation of thyroid remnant by 131-iodine (Cooper et al., 2009).
Molecular studies performed in the last decades, have elucidated in part the molecular mechanisms underlying thyroid cancer initiation and progression. Specific genetic alterations are associated to this thyroid tumor histotype: RET/PTC and TRK rearrangements and BRAF and RAS mutations.
The first genetic alteration discovered in PTC and also the most specific was the RET/PTCrearrangement (Fusco et al., 1987). RET/PTC is a chimeric gene generated by the fusion of the tyrosine kinase domain of the rearranged during transfection gene (RET) to the 5′terminal region of genes that are constitutively expressed in thyroid follicular cells (Pierotti et al., 1992; Santoro et al., 1992, 2006; Nikiforov, 2002). The chimeric proteins generated dimerize in a ligand-independent manner and result in a cytoplasmatic constitutively active tyrosine kinase. The higher frequency of PTC observed in the population exposed to the Chernobyl accident supports a role for the external radiations in the chromosome rearrangements observed in this tumor (Nikiforov, 2006). It has been proposed that the spatial proximity of translocation-prone gene loci may favor gene rearrangements. Indeed, proximity between RET and H4, and NTRK1 and TPR has been reported in interphase thyroid nuclei. Thus, in this simplified model, radiations induce chromosome rearrangements and generation of RET/PTC or TRK oncogenes that will be initiator of thyroid carcinogenesis. The role of RET/PTC in thyroid carcinogenesis is supported by experimental evidences generated in cells in culture and in animal models. PCCl3, a differentiated rat thyroid cell line, stably transfected with a RET/PTC3 expressing plasmid undergoes morphological alterations and is no longer TSH dependent for growth (Santoro et al., 1993). Thyroid-specific expression of the RET/PTC1 or RET/PTC3 in transgenic mice induces thyroid tumors with features resembling those of human PTC. These tumors are characterized by nuclear grooves and ground glass cells, continuous slow growth rate, and loss of iodide uptake (Jhiang et al., 1996; Santoro et al., 1996). However, some evidence suggest that RET/PTC alone is not sufficient to develop thyroid carcinoma, and other molecular events are needed. Thyroid cancer occurs only after a long latency period and only in a fraction of RET/PTC transgenic animals. At beginning, the majority of studies excluded the occurrence of RET/PTC in benign thyroid nodules. In following studies, RET rearrangements have been demonstrated in nodules diagnosed as benign at histology. Ishizaka et al. (1991) have been the first to detected RET/PTC in 21% of follicular adenomas. The use of highly sensitive detection methods contributed to definitively demonstrate that RET rearrangements occurs in a significant fraction of both radiation-induced and sporadic benign nodules (Bounacer et al., 1997; Cinti et al., 2000; Guerra et al., 2011; Marotta et al., 2011a; Sapio et al., 2011). Its presence in benign nodules, raised some queries about the role of RET/PTC in thyroid carcinogenesis. Doubts on the primary role of RET/PTC in thyroid carcinogenesis are also supported by the evidence that some irradiated PTC are composed of a mixture of cells with and without RET rearrangements. In sporadic microcarcinomas and post-Chernobyl PTC interphase fluorescence in situ hybridization (FISH) analysis demonstrated that RET/PTC rearrangements can occur only in a fraction of the cells, indicating that PTC can be composed of a mixture of cells with and without RET rearrangements (Corvi et al., 2001; Unger et al., 2004). These evidences are in favor of a secondary role of RET/PTC which would not be the initiating event in thyroid carcinogenesis.
BRAF is a protein-serine/threonine kinases that participate in the mitogen-activated protein kinase (MAPK) cascade (Wellbrock et al., 2004). By modulating the MAPK cascade, BRAF plays a pivotal role in many aspects of cell biology in nearly every cell type. More than 65 different mis-sense BRAF mutations have been detected in human cancer so far (Davies et al., 2002). The BRAFV600E mutation, resulting from the BRAFT1799AG transversion, is nearly the only mutation of this kinase found in thyroid cancer and the most common genetic mutation in PTC, being detected in about 50% of cases (Kimura et al., 2003; Xing, 2005; Marotta et al., 2011b). This mutation occurring within the activation segment, disrupts the hydrophobic interaction between the glycine-rich loop of the N-terminal region and the activation segment of the kinase domain, and transforms BRAF in a constitutively activated kinase (Davies, et al., 2002; Brummer et al., 2006; Moretti et al., 2009). In the thyroid, this oncogene is restricted to papillary-patterned cancer and it does not occur in Hashimoto’s thyroiditis, benign colloid nodules, thyroid adenomas, or other types of thyroid tumor (Xing, 2007). Its restricted occurrence makes BRAFV600E of clinical diagnostic utility (Xing, 2007; Zatelli et al., 2009). Its carcinogenetic potential has been demonstrated in several different cell types and in animal models. Thyroid-specific expression of BRAFV600E obtained in transgenic mice by the bovine thyroglobulin promoter provided us with important information on the tumorigenic potential of this oncogene. By 12–22 weeks age, transgenic mice revealed a large goiter, well differentiated thyroid cancer foci, and poorly differentiated foci in some animals, depending on the level of expression of the BRAFV600E mRNA. These tumors displayed a phenotype similar to that one of spontaneous human PTC, supporting a key role for this oncogene in the tumor initiation of this type of cancer and in the progression to poorly differentiated carcinomas (Knauf et al., 2005). In a more recent animal model, expression of BRAFV600E was obtained in adult mice in already developed thyroid glands. After 1 month of induced expression of the oncogene, mice developed an hypercellular thyroid, up to 10 times larger in size than controls, whilst nodules of tumor cells displaying a characteristic papillary structure were readily apparent 6 months after BRAFV600E expression (Charles et al., 2011). These experimental animal models demonstrate that BRAFV600E can promote the transformation process of the thyroid follicular cell, however they do not demonstrate that this oncogene is the initiating event in spontaneous human PTC. Very recently a more detailed analysis of BRAFV600E expression by means of a quantitative assay, demonstrated the heterogeneous intratumoral nature of spontaneous PTC. The analysis of the percentage of mutant BRAF demonstrated that clonal BRAFV600E is a rare occurrence in PTC, while more frequently this cancer consists of a mixture of tumor cells with wild-type and mutant BRAF. This result demonstrates that BRAF mutation in PTC is a secondary subclonal event (Guerra et al., 2012a).
Thus, the original idea that a normal thyroid cell, under the effect of ionizing radiations or other mutagenic factors, acquires the RET/PTC rearrangement or BRAFV600E mutation and consequently is transformed by these oncogenes in what we call a PTC cell, should be revised. Although heterogeneity is the rule in cancer, the identification of the genetic initiating event is important, not only to understand the molecular mechanisms of tumorigenesis, but also for practical purposes. A high percentage of BRAFV600E alleles is associated with a higher frequency of recurrence (Guerra et al., 2012b). This makes a quantitative assessment necessary to use BRAFV600E in clinical practice as a predictor of recurrence in PTC. Also targeted therapeutic interventions must take into account of this heterogeneity. The recent development of novel small-molecule inhibitors targeting one or more of these oncogenes may provide selective advantages for the treatment of advanced thyroid cancer harboring these mutations (Salerno et al., 2010; Nucera et al., 2011). Many of these promising drugs are currently being evaluated in clinical trials and the presence of target-negative subpopulations should be considered.
In conclusion, we have to revise our simplistic vision of thyroid carcinogenesis. Oncogenes known so far may play important role in the fate of a PTC, conferring specific biological and clinical features, but the genetic event initiating thyroid cancer is still to be identified.
References
Bounacer, A., Wicker, R., Caillou, B., Cailleux, A. F., Sarasin, A., Schlumberger, M., and Suarez, H. G. (1997). High prevalence of activating ret proto-oncogene rearrangements, in thyroid tumors from patients who had received external radiation. Oncogene 15, 1263–1273.
Brummer, T., Martin, P., Herzog, S., Misawa, Y., Daly, R. J., and Reth, M. (2006). Functional analysis of the regulatory requirements of B-Raf and the B-Raf(V600E) oncoprotein. Oncogene 25, 6262–6276.
Charles, R. P., Iezza, G., Amendola, E., Dankort, D., and McMahon, M. (2011). Mutationally activated BRAF(V600E) elicits papillary thyroid cancer in the adult mouse. Cancer Res. 71, 3863–3871.
Cinti, R., Yin, L., Ilc, K., Berger, N., Basolo, F., Cuccato, S., Giannini, R., Torre, G., Miccoli, P., Amati, P., Romeo, G., and Corvi, R. (2000). RET rearrangements in papillary thyroid carcinomas and adenomas detected by interphase FISH. Cytogenet. Cell Genet. 88, 56–61.
Cooper, D. S., Doherty, G. M., Haugen, B. R., Kloos, R. T., Lee, S. L., Mandel, S. J., Mazzaferri, E. L., McIver, B., Pacini, F., Schlumberger, M., Sherman, S. I., Steward, D. L., and Tuttle, R. M. (2009). Revised American Thyroid Association management guidelines for patients with thyroid nodules and differentiated thyroid cancer. Thyroid 19, 1167–1214.
Corvi, R., Martinez-Alfaro, M., Harach, H. R., Zini, M., Papotti, M., and Romeo, G. (2001). Frequent RET rearrangements in thyroid papillary microcarcinoma detected by interphase fluorescence in situ hybridization. Lab. Invest. 81, 1639–1645.
Davies, H., Bignell, G. R., Cox, C., Stephens, P., Edkins, S., Clegg, S., Teague, J., Woffendin, H., Garnett, M. J., Bottomley, W., Davis, N., Dicks, E., Ewing, R., Floyd, Y., Gray, K., Hall, S., Hawes, R., Hughes, J., Kosmidou, V., Menzies, A., Mould, C., Parker, A., Stevens, C., Watt, S., Hooper, S., Wilson, R., Jayatilake, H., Gusterson, B. A., Cooper, C., Shipley, J., Hargrave, D., Pritchard-Jones, K., Maitland, N., Chenevix-Trench, G., Riggins, G. J., Bigner, D. D., Palmieri, G., Cossu, A., Flanagan, A., Nicholson, A., Ho, J. W., Leung, S. Y., Yuen, S. T., Weber, B. L., Seigler, H. F., Darrow, T. L., Paterson, H., Marais, R., Marshall, C. J., Wooster, R., Stratton, M. R., and Futreal, P. A. (2002). Mutations of the BRAF gene in human cancer. Nature 417, 949–954.
Davies, L., and Welch, H. G. (2006). Increasing incidence of thyroid cancer in the United States, 1973-2002. JAMA 295, 2164–2167.
Fusco, A., Grieco, M., Santoro, M., Berlingieri, M. T., Pilotti, S., Pierotti, M. A., Della Porta, G., and Vecchio, G. (1987). A new oncogene in human thyroid papillary carcinomas and their lymph-nodal metastases. Nature 328, 170–172.
Guerra, A., Sapio, M. R., Marotta, V., Campanile, E., Rossi, S., Forno, I., Fugazzola, L., Budillon, A., Moccia, T., Fenzi, G., and Vitale, M. (2012a). The primary occurrence of BRAFV600E is a rare clonal event in papillary thyroid carcinoma. J. Clin. Endocrinol. Metab. 97, 517–524.
Guerra, A., Fugazzola, L., Marotta, V., Cirillo, M., Rossi, S., Cirello, V., Forno, I., Moccia, T., Budillon, A., and Vitale, M. (2012b). A high percentage of BRAFV600E alleles in papillary thyroid carcinoma predicts a poorer outcome. J. Clin. Endocrinol. Metab. (in press)
Guerra, A., Sapio, M. R., Marotta, V., Campanile, E., Moretti, M. I., Deandrea, M., Motta, M., Limone, P. P., Fenzi, G., Rossi, G., and Vitale, M. (2011). Prevalence of RET/PTC rearrangement in benign and malignant thyroid nodules and its clinical application. Endocr. J. 58, 31–38.
Hodgson, N. C., Button, J., and Solorzano, C. C. (2004). Thyroid cancer: is the incidence still increasing? Ann. Surg. Oncol. 11, 1093–1097.
Ishizaka, Y., Kobayashi, S., Ushijima, T., Hirohashi, S., Sugimura, T., and Nagao, M. (1991). Detection of retTPC/PTC transcripts in thyroid adenomas and adenomatous goiter by an RT-PCR method. Oncogene 6, 1667–1672.
Jhiang, S. M., Sagartz, J. E., Tong, Q., Parker-Thornburg, J., Capen, C. C., Cho, J. Y., Xing, S., and Ledent, C. (1996). Targeted expression of the ret/PTC1 oncogene induces papillary thyroid carcinomas. Endocrinology 137, 375–378.
Kimura, E. T., Nikiforova, M. N., Zhu, Z., Knauf, J. A., Nikiforov, Y. E., and Fagin, J. A. (2003). High prevalence of BRAF mutations in thyroid cancer: genetic evidence for constitutive activation of the RET/PTC-RAS-BRAF signaling pathway in papillary thyroid carcinoma. Cancer Res. 63, 1454–1457.
Knauf, J. A., Ma, X., Smith, E. P., Zhang, L., Mitsutake, N., Liao, X. H., Refetoff, S., Nikiforov, Y. E., and Fagin, J. A. (2005). Targeted expression of BRAFV600E in thyroid cells of transgenic mice results in papillary thyroid cancers that undergo dedifferentiation. Cancer Res. 65, 4238–4245.
Marotta, V., Guerra, A., Sapio, M. R., and Vitale, M. (2011a). RET/PTC rearrangement in benign and malignant thyroid diseases: a clinical standpoint. Eur. J. Endocrinol. 165, 499–507.
Marotta, V., Sapio, M. R., Guerra, A., and Vitale, M. (2011b). BRAF mutation in cytology samples as a diagnostic tool for papillary thyroid carcinoma. Expert Opin. Med. Diagn. 5, 277–290.
Moretti, S., De Falco, V., Tamburrino, A., Barbi, F., Tavano, M., Avenia, N., Santeusanio, F., Santoro, M., Macchiarulo, A., and Puxeddu, E. (2009). Insights into the molecular function of the inactivating mutations of B-Raf involving the DFG motif. Biochim. Biophys. Acta 1793, 1634–1645.
Nikiforov, Y. E. (2006). Radiation-induced thyroid cancer: what we have learned from chernobyl. Endocr. Pathol. 17, 307–317.
Nucera, C., Nehs, M. A., Nagarkatti, S. S., Sadow, P. M., Mekel, M., Fischer, A. H., Lin, P. S., Bollag, G. E., Lawler, J., Hodin, R. A., and Parangi, S. (2011). Targeting BRAFV600E with PLX4720 displays potent antimigratory and anti-invasive activity in preclinical models of human thyroid cancer. Oncologist 16, 296–309.
Pierotti, M. A., Santoro, M., Jenkins, R. B., Sozzi, G., Bongarzone, I., Grieco, M., Monzini, N., Miozzo, M., Herrmann, M. A., and Fusco, A. (1992). Characterization of an inversion on the long arm of chromosome 10 juxtaposing D10S170 and RET and creating the oncogenic sequence RET/PTC. Proc. Natl. Acad. Sci. U.S.A. 89, 1616–1620.
Salerno, P., De Falco, V., Tamburrino, A., Nappi, T. C., Vecchio, G., Schweppe, R. E., Bollag, G., Santoro, M., and Salvatore, G. (2010). Cytostatic activity of adenosine triphosphate-competitive kinase inhibitors in BRAF mutant thyroid carcinoma cells. J. Clin. Endocrinol. Metab. 95, 450–455.
Santoro, M., Carlomagno, F., Hay, I. D., Herrmann, M. A., Grieco, M., Melillo, R., Pierotti, M. A., Bongarzone, I., Della Porta, G., and Berger, N. (1992). Ret oncogene activation in human thyroid neoplasms is restricted to the papillary cancer subtype. J. Clin. Invest. 89, 1517–1522.
Santoro, M., Chiappetta, G., Cerrato, A., Salvatore, D., Zhang, L., Manzo, G., Picone, A., Portella, G., Santelli, G., Vecchio, G., and Fusco, A. (1996). Development of thyroid papillary carcinomas secondary to tissue-specific expression of the RET/PTC1 oncogene in transgenic mice. Oncogene 12, 1821–1826.
Santoro, M., Melillo, R. M., and Fusco, A. (2006). RET/PTC activation in papillary thyroid carcinoma: European journal of endocrinology prize lecture. Eur. J. Endocrinol. 155, 645–653.
Santoro, M., Melillo, R. M., Grieco, M., Berlingieri, M. T., Vecchio, G., and Fusco, A. (1993). The TRK and RET tyrosine kinase oncogenes cooperate with ras in the neoplastic transformation of a rat thyroid epithelial cell line. Cell Growth Differ. 4, 77–84.
Sapio, M. R., Guerra, A., Marotta, V., Campanile, E., Formisano, R., Deandrea, M., Motta, M., Limone, P. P., Fenzi, G., Rossi, G., and Vitale, M. (2011). High growth rate of benign thyroid nodules bearing RET/PTC rearrangements. J. Clin. Endocrinol. Metab. 96, E916–E919.
Unger, K., Zitzelsberger, H., Salvatore, G., Santoro, M., Bogdanova, T., Braselmann, H., Kastner, P., Zurnadzhy, L., Tronko, N., Hutzler, P., and Thomas, G. (2004). Heterogeneity in the distribution of RET/PTC rearrangements within individual post-chernobyl papillary thyroid carcinomas. J. Clin. Endocrinol. Metab. 89, 4272–4279.
Wellbrock, C., Karasarides, M., and Marais, R. (2004). The RAF proteins take centre stage. Nat. Rev. Mol. Cell Biol. 5, 875–885.
Xing, M. (2007). BRAF mutation in papillary thyroid cancer: pathogenic role, molecular bases, and clinical implications. Endocr. Rev. 28, 742–762.
Zatelli, M. C., Trasforini, G., Leoni, S., Frigato, G., Buratto, M., Tagliati, F., Rossi, R., Cavazzini, L., Roti, E., and degli Uberti, E. C. (2009). BRAF V600E mutation analysis increases diagnostic accuracy for papillary thyroid carcinoma in fine-needle aspiration biopsies. Eur. J. Endocrinol. 161, 467–473.
Citation: Vitale M (2012) Rethinking the role of oncogenes in papillary thyroid cancer initiation. Front. Endocrin. 3:83. doi: 10.3389/fendo.2012.00083
Received: 29 May 2012; Accepted: 08 June 2012;
Published online: 26 June 2012.
Edited by:
Carmelo Nucera, Beth Israel Deaconess Medical Center, USAReviewed by:
Maria Chiara Zatelli, University of Ferrara, ItalyCopyright: © 2012 Vitale. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.
*Correspondence: mavitale@unisa.it