European Journal of Endocrinology (2006) 155 645–653 ISSN 0804-4643 INVITED REVIEW RET/PTC activation in papillary thyroid carcinoma: European Journal of Endocrinology Prize Lecture Massimo Santoro1, Rosa Marina Melillo1 and Alfredo Fusco1,2 1Istituto di Endocrinologia ed Oncologia Sperimentale del CNR ‘G. Salvatore’, c/o Dipartimento di Biologia e Patologia Cellulare e Molecolare, University ‘Federico II’, Via S. Pansini, 5, 80131 Naples, Italy and 2NOGEC (Naples Oncogenomic Center)–CEINGE, Biotecnologie Avanzate & SEMM, European School of Molecular Medicine, Naples, Italy (Correspondence should be addressed to M Santoro; Email: [email protected]) Abstract Papillary thyroid carcinoma (PTC) is frequently associated with RET gene rearrangements that generate the so-called RET/PTC oncogenes. In this review, we examine the data about the mechanisms of thyroid cell transformation, activation of downstream signal transduction pathways and modulation of gene expression induced by RET/PTC. These findings have advanced our understanding of the processes underlying PTC formation and provide the basis for novel therapeutic approaches to this disease. European Journal of Endocrinology 155 645–653 RET/PTC rearrangements in papillary growth factor, have been described in a fraction of PTC thyroid carcinoma patients (7). As illustrated in figure 1, many different genes have been found to be rearranged with RET in The rearranged during tansfection (RET) proto-onco- individual PTC patients. RET/PTC1 and 3 account for gene, located on chromosome 10q11.2, was isolated in more than 90% of all rearrangements and are hence, by 1985 and shown to be activated by a DNA rearrange- far, the most frequent variants (8–11). They result from ment (rearranged during transfection) (1).As the fusion of RET to the coiled-coil domain containing illustrated in Fig. 1, it encodes a single-pass trans- gene 6 (CCDC6, formerly called H4/D10S170) or to the membrane tyrosine kinase that functions as the nuclear receptor co-activator gene 4 (NcoA4, formerly receptor (receptor tyrosine kinase, RTK) for the growth called RET fused gene (RFG)/ELE1/androgen receptor factors of the glial cell line-derived neurotropic factor activator 70(ARA70)) (8–11). family (2). RET is essential for the development of the sympathetic, parasympathetic, and enteric nervous systems, the kidney and the testis (2). Germline point Prevalence of RET/PTC rearrangements mutations in RET lead to multiple endocrine neoplasia type 2 syndromes thereby predisposing to thyroid C-cell- For a detailed discussion of the prevalence of RET/PTC derived medullary thyroid carcinoma (3). rearrangements and their correlation with clinico- With an incidence ranging between 0.5 and 10 cases pathological features of thyroid carcinomas, we refer per 100 000 population, thyroid cancer is the most the reader to three excellent reviews (8–10). The frequent endocrine malignancy (4). Papillary thyroid prevalence of RET/PTC rearrangements in thyroid carcinoma (PTC), which accounts for approximately cancer varies widely among studies. In an adult population from the United States, the prevalence of 80% of cases, is the most frequent of all thyroid rearrangements was approximately 35% (9). Preva- malignancies (4). In PTC, genomic rearrangements lences between 3 and 85% have been reported for other juxtapose the RET kinase and COOH-terminus encoding regions (8–10). This wide range of values probably domains (exons 11–21) to unrelated genes, thereby reflects not only the geographic variation but also the creating dominantly transforming oncogenes called different procedures used to identify RET/PTC RET/PTC (5, 6). Similar rearrangements of another rearrangements, i.e. various reverse transcriptase neurotropic RTK, namely the neurotropic tyrosine (RT)-PCR methods, Southern blot, and fluorescence receptor kinase type 1 (NTRK1), the receptor for nerve in situ hybridization. An exhaustive study by Zhu and co-workers (12) demonstrated that the method used has Presented at the 7th European Congress of Endocrinology, a striking effect on the efficacy of RET/PTC detection, Gothenburg, Sweden, 2005. and hence on the prevalence reported. Tumor q 2006 Society of the European Journal of Endocrinology DOI: 10.1530/eje.1.02289 Online version via www.eje-online.org Downloaded from Bioscientifica.com at 09/24/2021 07:34:47PM via free access 646 M Santoro and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2006) 155 Figure 1 Schematic drawing of the RET protein with the four extracellular cadherin-like domains, the cysteine-rich box adjacent to the plasma membrane, the juxtamembrane domain and the split tyrosine kinase domain (TK). In PTCs, RET is rearranged with diverse genes, encoding protein dimerization motifs (highlighted) that mediate ligand-independent RET dimerization. The arrowheads indicate RET breakpoints. heterogeneity is another factor that can affect the the presence of multiple RET/PTC variants in individ- evaluation of RET/PTC prevalence. In addition to PTC ual PTC samples (14). samples with ‘clonal’ RET/PTC rearrangements (those RET/PTC rearrangements have so far been identified affecting the majority of tumor cells), there are samples only in thyroid lesions, and in particular in PTC (8–10). with ‘non-clonal’ rearrangements (those affecting a Most studies concur that RET/PTC rearrangements are small portion of tumor cells) (12, 13). Whereas RET/ rare or absent in benign follicular adenomas, and absent PTC is likely to be important for tumor formation in in follicular and medullary carcinomas (8–10). Various samples with clonal rearrangements, RET/PTC could PTC histological variants have been identified, namely not be a ‘driver’ mutation in PTC samples with non- classic, follicular, diffuse-sclerosing, columnar-cell, clonal rearrangements (13). This distinction has Hurthle-cell, cribriform, solid and tall-cell variants (4). important implications for the stratification of patients Classic (w45%) and follicular (w18%) variants are the who could potentially benefit from novel therapeutic most prevalent (4). RET/PTCs are more frequent in approaches based on the use of RET kinase inhibitors tumors that have a classic architecture (15) and in (see below). It is still controversial the presence of RET/ microcarcinomas (!1 cm) (16). They are rare in the PTC rearrangements in non-neoplastic cells in Hashi- follicular variant of PTC (15). RET/PTCs have been moto’s thyroiditis. It is possible that a heterogeneous reported in the cribriform variant, which is typically presence of the rearrangement may account, at least in associated with familial adenomatous polyposis (17),in part, for such a controversy (8,9,13).Tumor the Hurthle-cell variant (18, 19), and in hyalinizing heterogeneity and multiclonality is also indicated by trabecular adenoma, a rare tumor that can be www.eje-online.org Downloaded from Bioscientifica.com at 09/24/2021 07:34:47PM via free access EUROPEAN JOURNAL OF ENDOCRINOLOGY (2006) 155 RET/PTC in thyroid cancer 647 morphologically similar to PTC (20, 21). The solid of childhood PTC was reported in contaminated areas. variant, which is an aggressive PTC subtype, is closely Post-Chernobyl PTC featured a high prevalence (over associated with the RET/PTC3 oncogene, whereas the 60%) of RET/PTC rearrangements (22, 36–39). classic variant is associated with RET/PTC1 (22). RET/PTC3 was more prevalent among short latency Clinical, epidemiologic, and pathologic evidence solid PTCs, whereas RET/PTC1 was more frequently supports the possibility of a stepwise progression from found after a long latency period (9, 22, 36–39). Rapid well-differentiated carcinoma, including PTC, to poorly thyroid cell proliferation may account for the particu- differentiated and anaplastic carcinoma (23). Accor- larly high sensitivity to radiation-induced RET/PTC dingly, some anaplastic carcinomas are associated with rearrangements among children (40). However, also in genetic lesions (rat sarcoma viral oncogene homolog cases of PTC in non-exposed populations, the prevalence (RAS) and BRAF mutations) that overlap those present of RET/PTC was higher in children than in adults, in PTC and in other well-differentiated carcinomas (23). which suggests that in general RET/PTC recombina- Conflicting results have been reported about the tions occur more frequently in young thyroids (41, 42). presence of RET/PTC in poorly-differentiated and RET/PTC rearrangements are not the only intrachro- anaplastic carcinomas and, therefore about whether mosomal aberration found in radiation-associated PTC. or not RET/PTC predisposes to thyroid tumor pro- Also NTRK1 rearrangements, caused by paracentric gression (23–28). Again, methodological differences inversions of chromosome 1q, have been reported in and tumor heterogeneity may account for these post-Chernobyl PTC (7, 38). Moreover, in 11% of the post- controversies. A large multicenter study in which all Chernobyl PTCs there was a paracentric inversion of centers use the same methodology should resolve this chromosome 7q that resulted in the A-kinase anchor issue. It is also important to study the different RET/PTC protein (AKAP)9–BRAF fusion oncogene (43). Differently, variants individually, because, as mentioned above, they point mutations in BRAF, which are very frequent in could confer different degrees of aggressiveness on PTC. sporadic PTC (see below) are rare in Chernobyl cancers It should be noted that intercross of RET/PTC transgenic affecting children (44). This suggests that young age and mice with p53null mice induced