RET oncogene activation in papillary thyroid carcinoma.

The RET proto-oncogene encodes a cell membrane tyrosine-kinase receptor protein whose ligands belong to the glial cell line-derived neurotrophic factor. RET functions as a multicompetent receptor complex that includes alphaGFRs and RET. Somatic rearrangements of RET designated as RET/PTC (from papillary thyroid carcinoma) were identified in papillary thyroid carcinoma before RET was recognized as the susceptibility gene for MEN2. There are now at least at least 15 types of RET/PTC rearrangements involving RET and 10 different genes. RET/PTC1 and RET/PTC3 are by far the most common rearrangements. All of the rearrangements are due to DNA damage and result in the fusion of the RET tyrosine-kinase (RET-TK) domain to the 5'-terminal region of heterologous genes. RET/PTC rearrangements are very common in radiation-induced tumors but have been detected in variable proportions of sporadic (i.e., non-radiation associated) papillary carcinomas. It is estimated that up to approximately half the papillary thyroid carcinomas in the United States and Canada harbor RET/PTC rearrangements, most commonly RET/PTC-1, followed by RET/PTC-3 and occasionally RET/PTC-2. The cause of these rearrangements in sporadic papillary carcinomas is not known, but the close association between their presence and the papillary carcinoma phenotype indicates that they play a causative role in tumor development. The proposed mechanisms of RET/PTC-induced tumorigenesis and the clinical and pathologic implications of RET/PTC activation are discussed.     

A subset of the follicular variant of papillary thyroid carcinoma harbors the PAX8-PPARgamma translocation

The occurrence of the PAX8-PPARgamma fusion gene is thought to be restricted to follicular tumors (adenomas and carcinomas) of the thyroid (FTA and FTC). Using interphase fluorescent in situ hybridization (FISH), together with recombinant tissue-type polymerase chain reaction (RT-PCR) and immunohistochemistry, we detected the PAX8-PPARgamma translocation in 4 of 8 cases of the follicular variant of papillary thyroid carcinoma (FVPTC) exclusively or almost exclusively (>95%) composed of follicles. The 4 tumors with the translocation were larger and apparently more invasive than the remaining tumors, but the series is too small to allow a statistically meaningful comparison of the data. Our findings show that follicular thyroid carcinoma (PTC) may also harbor the PAX8-PPARgamma fusion gene and indicate that a subset of FVPTC shares some molecular features of FTA and FTC.     

RET/PTC rearrangement in thyroid tumors

Rearrangement of the RET gene, also known as RET/PTC rearrangement, is the most common genetic alteration identified to date in thyroid papillary carcinomas. The prevalence of RET/PTC in papillary carcinomas shows significant geographic variation and is approx 35% in North America. RET/PTC is more common in tumors in children and young adults, and in papillary carcinomas associated with radiation exposure. There are at least 10 different types of RET/PTC, all resulting from the fusion of the tyrosine kinase domain of RET to the 5′ portion of different genes. RET/PTC1 and RET/PTC3 are the most common types, accounting for >90% of all rearrangements. There is some evidence that different types of RET/PTC may be associated with distinct biologic properties of papillary carcinomas. RET/PTC1 tends to be more common in tumors with typical papillary growth and microcarcinomas and to have a more benign clinical course, whereas RET/PTC3 in some populations shows a strong correlation with the solid variant of papillary carcinoma and more aggressive tumor behavior. RET/PTC has recently been found in hyalinizing trabecular adenomas of the thyroid gland, providing molecular evidence in favor of this tumor to be a variant of papillary carcinoma. The occurrence of RET/PTC in Hashimoto thyroiditis and thyroid follicular adenomas and hyperplastic nodules reported in several studies has not been confirmed in other observations and remains controversial.     

Understanding the genotype of follicular thyroid tumors

Tumors of the thyroid with a follicular growth pattern are controversial and can be diagnostically challenging for the pathologist. This group of tumors includes both follicular-derived lesions (adenomas and carcinomas) and papillary carcinoma (follicular variant of papillary carcinoma). H&E morphology has classically been the gold standard for diagnosis. In the past several decades, however, several important molecular markers have been identified that may be unique to different types of thyroid carcinomas. These include the translocations RET/PTC and PAX8-PPARγ and point mutations in the BRAF and RAS genes. Other molecular events in tumor suppressor genes may be useful for diagnosis of these tumors as well. None of the mutational markers are very sensitive, and there is some question regarding specificity for malignancy, because mutations have also been described in histologically benign tumors. However, with increasing availability of molecular testing for the general pathologist, a molecular testing panel used in conjunction with the H&E morphology and immunohistochemical stains may become useful in the clinical setting for the diagnosis of thyroid tumors.     

Hyalinizing Trabecular Tumor of the Thyroid: An Update

Hyalinizing trabecular tumor (HTT) is a rare thyroid tumor of follicular cell origin with a trabecular pattern of growth and marked intratrabecular hyalinization. This tumor is known to share morphological and architectural similarities with paraganglioma and medullary thyroid carcinoma, as well as the nuclear features and RET/PTC1 translocations of papillary thyroid carcinoma. These tumors are not associated with RAS or BRAF mutations. Whether the presence of RET alterations in HTT are sufficient molecular proof of its relationship with papillary thyroid carcinoma (PTC) is still to be defined. Of great interest is the characteristic strong peripheral cytoplasmic and membranous staining of the tumor cells with MIB1 immunostain, not seen in any other thyroid neoplasm. Although cases of malignant HTT have been recorded, HTT should be considered a benign neoplasm or, at most, a neoplasm of extremely low malignant potential.     

Molecular pathology of thyroid tumours of follicular cells: a review of genetic alterations and their clinicopathological relevance

Thyroid cancer is the most common endocrine malignancy. Knowledge of the molecular pathology of thyroid tumours originating from follicular cells has greatly advanced in the past several years. Common molecular alterations, such as BRAF p.V600E, RAS point mutations, and fusion oncogenes (RET–PTC being the prototypical example), have been, respectively, associated with conventional papillary carcinoma, follicular‐patterned tumours (follicular adenoma, follicular carcinoma, and the follicular variant of papillary carcinoma/non‐invasive follicular thyroid neoplasm with papillary‐like nuclear features), and with papillary carcinomas from young patients and arising after exposure to ionising radiation, respectively. The remarkable correlation between genotype and phenotype shows how specific, mutually exclusive molecular changes can promote tumour development and initiate a multistep tumorigenic process that is characterised by aberrant activation of mitogen‐activated protein kinase and phosphoinositide 3‐kinase–PTEN–AKT signalling. Molecular alterations are becoming useful biomarkers for diagnosis and risk stratification, and as potential treatment targets for aggressive forms of thyroid carcinoma. What follows is a review of the principal genetic alterations of thyroid tumours originating from follicular cells and of their clinicopathological relevance.     

Overexpression of wild-type c-RET and zero prevalence of RET/PTC rearrangements are associated with papillary thyroid cancer (PTC) in Kuwait

Background

Papillary thyroid carcinoma (PTC) is common in Kuwait. The activation of the RET oncogene by DNA rearrangement (RET/PTC) is known to have an important role in PTC carcinogenesis. However, the real frequency of the RET/PTC expression in PTC is variable between different studies. This study seeks to determine the prevalence of RET/PTC and to analyze the RET oncogene expression associated with PTC in Kuwait.

Methods

RET expression and DNA rearrangements (RET/PTC 1, RET/PTC 2 and RET/PTC 3) were studied by RT–PCR in different thyroid diseases. Results were confirmed by the Southern blot and by immunohistochemistry. Quantitative real-time PCR was used to determine the level of RET mRNA expression in PTCs.

Results

Wild-type (nonrearranged) c-RET oncogene was overexpressed in 60% of PTC cases and absent in follicular thyroid carcinoma (FTC), anaplastic thyroid carcinoma (ATC), follicular adenomas (FA) or normal thyroid. No RET/PTC rearrangement was detected in any sample. The c-RET expression in Hashimoto's thyroiditis and multinodular goiter was limited to follicular cells with PTC-like nuclear changes.

Conclusions

The overexpression of wild-type c-RET is a characteristic molecular event of PTCs in Kuwait. The prevalence of RET/PTC is zero and among the lowest recorded in the world.     

An evaluation and recommendation of the optimal methodologies to detect RET gene rearrangements in papillary thyroid carcinoma

To recommend a reliable and clinically realistic RET/PTC rearrangement detection assay for papillary thyroid carcinoma (PTC), we compared multiplex quantitative polymerase chain reaction (qPCR), fluorescence in situ hybridization (FISH), and immunohistochemistry (IHC). RET/PTC rearrangement was detected using either RET break‐apart FISH followed by multicolor FISH to confirm CCDC6/RET or NCOA4/RET fusions, or by multiplex qPCR to detect 14 RET/PTC subtypes with simultaneous RET mRNA expression. RET protein expression was detected by IHC. The specificity and sensitivity of multiplex qPCR and IHC were calculated using break‐apart FISH as a reference. Among 73 PTC patients with sufficient tissue available for FISH and multiplex qPCR, 10 cases were defined as RET/PTC positive by both assays, including eight CCDC6/RET and two NCOA4/RET fusions with relatively high RET mRNA. In addition, multiplex qPCR identified another two CCDC6/RET fusion positive cases, but with low RET mRNA expression. IHC staining identified 11 RET positive cases among 39 patients with available samples. In comparison to FISH, multiplex qPCR displayed 100% sensitivity and 97% specificity to detect RET/PTC fusions, while IHC was neither sensitive nor specific. Our data reveal that both multiplex qPCR and FISH assays are equally applicable for detection of RET/PTC rearrangements. Break‐apart FISH methodology is highly recommended for the wider screening of RET rearrangements (regardless of partner genes), while multiplex qPCR is preferred to identify all known fusion types using one assay, provided mRNA expression is also measured. IHC analysis could potentially provide an additional method of fusion detection dependent on further optimization of assay conditions and scoring cutoffs. © 2014 Wiley Periodicals, Inc.     

Molecular profile and clinical-pathologic features of the follicular variant of papillary thyroid carcinoma. An unusually high prevalence of ras mutations

The follicular variant (FV) of papillary thyroid carcinoma is characterized by a follicular growth pattern and cytologic features of papillary carcinoma. ret/PTC rearrangements are common in classic papillary thyroid carcinoma (PTC) and PAX8-PPAR gamma and ras mutations in follicular thyroid carcinoma. Their prevalence in FV has not been established. We studied these genetic alterations and clinical-pathologic features in 30 FV cases and compared those with 46 non-FV papillary carcinomas. FV cases revealed 1 ret/PTC rearrangement (3%) and 13 ras mutations (43%). Non-FV cases harbored 13 ret/PTC (28%) (P = .006) and no ras mutations (P = .0002). No PAX8-PPAR gamma was found in either group. FV cases demonstrated a significantly higher prevalence of tumor encapsulation, angiovascular invasion, and poorly differentiated areas and a lower rate of lymph node metastases. These data indicate that the FV of papillary carcinoma has a distinct set of molecular alterations and is characterized by a high frequency of ras point mutations.     

The RET proto-oncogene in medullary and papillary thyroid carcinoma

 The evolution of cancer is a multistep phenomenon, and multiple cellular genetic lesions are involved in the emergence of the malignant neoplasm. Several early events have been implicated in the neoplastic transformation of thyrocytes, and recent reports have described the involvement of specific genetic alterations in different types of thyroid neoplasms: ras point mutations are frequently observed in tumours with follicular histology, gsp – the mutated form of the alpha subunit of the Gs-protein – is encountered in up to 73% of papillary or follicular thyroid carcinomas, and a high prevalence of p53 point mutations has been found in anaplastic thyroid carcinomas but not in differentiated follicular tumours. More recent studies revealed that the RET proto-oncogene is involved in the oncogenesis of medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma (PTC) by activation of its tyrosine kinase either by point mutation or rearrangement. In this review the most important recently published data on alterations of the RET proto-oncogene in heritable and sporadic MTCs and in PTCs will be summarized. Emphasis will be directed to the pathophysiological mechanisms of tumour initiation, the indications and limitations of DNA testing, and the clinical implications of identified RET defects in thyroid lesions. Received: 9 January 1997 / Accepted: 17 February 1997     

Papillary thyroid carcinoma with aggressive fused follicular and solid growth pattern: A unique histological subtype with high-grade malignancy?

Papillary thyroid carcinoma (PTC) is usually indolent; however, some rare subtypes of PTCs, such as columnar cell and hobnail subtypes, carry poor prognosis as an intermediate malignancy between differentiated carcinoma and anaplastic carcinoma. We present the case of a 56-year-old Japanese woman having PTC with aggressive behavior showing characteristic histological features of a predominantly fused follicular and focally solid (FFS) pattern. The fused follicular pattern is cribriform-like without intermingled vessels. This PTC with FFS pattern included frequent mitotic figures, necrosis, lymphovascular invasion, and metastases with high clinical stage. The tumor cells were broadly positive for antibodies to TTF-1, PAX8, and bcl-2, and negative for cyclin D1. Ki-67 labeling index was approximately 10%, and there was occasional positivity of p53. Targeted next generation sequencing analysis only detected a NRAS mutation (Q61K); there was no mutation and no translocation of other genes including BRAF and RET/PTC. To our knowledge, this is first report that PTC shows aggressive FFS growth pattern. The tumor is possibly included in the new category of differentiated high-grade thyroid carcinoma in the World Health Organization 2022 classification, or in a novel subtype of PTC owing to its characteristic histological feature and intermediate malignancy between differentiated carcinoma and anaplastic carcinoma.     

Genetic alterations involved in the transition from well-differentiated to poorly differentiated and anaplastic thyroid carcinomas

Recent molecular studies have provided new insights into thyroid carcinogenesis. In thyroid papillary carcinomas at least three initiating events may occur, which are point mutations in the BRAF and RAS genes and RET/PTC rearrangements. Tumors harboring mutant BRAF and RAS are prone to progression to poorly differentiated and anaplastic carcinoma, but most likely require additional mutations to trigger this process. In thyroid follicular carcinomas, two known initiating events are RAS mutations and PAX8-PPARγ rearrangements, and RAS predisposes to dedifferentiation of follicular carcinomas. p53 and β-catenin mutations, found with increasing incidence in poorly differentiated and anaplastic carcinomas but not in well-differentiated tumors, may serve as a direct molecular trigger of tumor dedifferentiation. Additional evidence for progression from a preexisting well-differentiated carcinoma to poorly differentiated and anaplastic carcinoma comes from the studies of loss of heterozygosity and comparative genomic hybridization. Molecular studies, although limited by the lack of uniform histologic criteria for poorly differentiated carcinomas, revealed no genetic mutations or chromosomal abnormalities that are unique for poorly differentiated carcinoma and not present in well-differentiated or anaplastic carcinomas. This suggests that poorly differentiated carcinoma, as a group, represents a distinct step in the evolution from well-differentiated to anaplastic thyroid carcinoma, rather than an entirely separate type of thyroid malignancy.     

Potent mitogenicity of the RET/PTC3 oncogene correlates with its prevalence in tall-cell variant of papillary thyroid carcinoma.

The tall-cell variant (TCV) of papillary thyroid carcinoma (PTC), characterized by tall cells bearing an oxyphilic cytoplasm, is more clinically aggressive than conventional PTC. RET tyrosine kinase rearrangements, which represent the most frequent genetic alteration in PTC, lead to the recombination of RET with heterologous genes to generate chimeric RET/PTC oncogenes. RET/PTC1 and RET/PTC3 are the most prevalent variants. We have found RET rearrangements in 35.8% of TCV (14 of 39 cases). Whereas the prevalences of RET/PTC1 and RET/PTC3 were almost equal in classic and follicular PTC, all of the TCV-positive cases expressed the RET/PTC3 rearrangement. These findings prompted us to compare RET/PTC3 and RET/PTC1 in an in vitro thyroid model system. We have expressed the two oncogenes in PC Cl 3 rat thyroid epithelial cells and found that RET/PTC3 is endowed with a strikingly more potent mitogenic effect than RET/PTC1. Mechanistically, this difference correlated with an increased signaling activity of RET/PTC3. In conclusion, we postulate that the correlation between the RET/PTC rearrangement type and the aggressiveness of human PTC is related to the efficiency with which the oncogene subtype delivers mitogenic signals to thyroid cells.     

Alterations of the BRAF gene in thyroid tumors

BRAF belongs to the RAF family of protein kinases that are important components of the MAPK signaling pathway mediating cell growth, differentiation and survival. Activating point mutation of the BRAF gene resulting in V600E (previously designated as V599E) is a common event in thyroid papillary carcinoma, being found in approx 40% of this tumor. It has strong association with classical papillary carcinoma and tall cell and possibly Warthin-like variants. This mutation also occurs in thyroid poorly differentiated and anaplastic carcinomas, usually those containing areas of papillary carcinoma. Alterations in the BRAF gene do not overlap with RAS mutations and RET/PTC rearrangement, indicating that activation of one of the effectors of the MAPK pathway is sufficient for papillary thyroid carcinogenesis. Recently, another mechanism of BRAF activation has been identified, which involves chromosome 7q inversion that results in the AKAP9-BRAF fusion. It is rare in sporadic papillary carcinomas and is more common in tumors associated with radiation exposure. Yet another mechanism of BRAF activation may involve copy number gain, which is seen in a significant portion of thyroid follicular tumors of both conventional and oncocytic (Hürthle cell) types.     

Fusion of a novel gene,ELKS, to RET due to translocation t(10;12)(q11;p13) in a papillary thyroid carcinoma

In papillary thyroid carcinomas, the genes for receptor‐type tyrosine kinase, RET or TRKA, are sometimes rearranged, resulting in fusion of its tyrosine kinase domain to 5′ portions of several activating genes. In a papillary thyroid carcinoma, we identified a novel gene (ELKS), the 5′ portion of which is fused to the RET gene by gene rearrangement due to the translocation t(10;12)(q11;p13). Subsequent cloning of the ELKS cDNA revealed that ELKS encodes a novel 948 amino acid peptide and is expressed ubiquitously in human tissues. The presence of multiple coiled‐coil domains in the ELKS product suggests that the ELKS protein forms dimers. Since the tyrosine kinase of RET is activated by dimerization that occurs when its ligands bind to the receptor, fusion of RET with the 5′ dimerization domains of ELKS would activate its cytoplasmic tyrosine kinase constitutively in papillary thyroid carcinomas. Genes Chromosomes Cancer 25:97–103, 1999. © 1999 Wiley‐Liss, Inc.     

Metallothionein 1G acts as an oncosupressor in papillary thyroid carcinoma

The molecular pathogenesis of tumors arising from the thyroid follicular epithelial cells, including papillary (PTC) and follicular thyroid carcinoma (FTC), is only partially understood, and the role of tumor suppressor genes has not yet been assessed. The metallothionein (MT) gene family encodes a class of metal-binding proteins involved in several cellular processes, and their expression is often deregulated in human tumors. Recently, downregulation of MT gene expression in PTC has been reported, suggesting a possible oncosuppressor role of this gene family in the pathogenesis of thyroid tumors. To further explore this possibility, we performed expression and functional studies. Analysis of microarray data of thyroid tumors of different histologic types showed that several MT genes were downregulated with respect to normal tissue. The microarray data were corroborated by quantitative PCR experiments, showing downregulation of MTs in PTC and FTC, but to a greater extent in papillary carcinoma. The expression of MTs was also investigated at the protein level by immunohistochemistry; the results were consistent with the microarray data, showing general downregulation in tumor samples, which was more evident in PTC. The functional consequence of MT downregulation was addressed employing an experimental model made of the PTC-derived K1 cell line in which MT1G expression is repressed by promoter methylation. Restoration of MT1G expression by cDNA transfection affected growth rate and in vivo tumorigenicity of K1 cells, indicating an oncosuppressor role for MT1G in thyroid papillary tumorigenesis.     

RET/PTC rearrangement is prevalent in follicular Hürthle cell carcinomas

de Vries M M, Celestino R, Castro P, Eloy C, Máximo V, van der Wal J E, Plukker J T M, Links T P, Hofstra R M W, Sobrinho‐Simões M & Soares P
(2012) Histopathology  61, 833–843 RET/PTC rearrangement is prevalent in follicular Hürthle cell carcinomas Aims: The molecular alterations underlying follicular Hürthle cell carcinomas (FHCCs) are largely unknown. In an attempt to clarify this issue, we analysed a series of Hürthle cell tumours for the presence of RET/PTC and PAX8/PPARG rearrangements and BRAF, HRAS and NRAS mutations. Methods and results: We investigated a series of 20 follicular Hürthle cell tumours [17 FHCCs and three follicular Hürthle cell adenomas (FHCAs)]. RET/PTC rearrangements were found in 33% of FHCAs and in 38% of FHCCs. All RET/PTC‐positive FHCCs had a solid pattern of growth. PAX8/PPARG rearrangement was present in 27% of the FHCCs which displayed, in most cases, a follicular architecture. NRAS mutation was detected in one FHCC. An FHCC with a solid/microfollicular growth pattern scored positive for both RET/PTC and PAX8/PPARG rearrangement. Conclusions: Our study has shown a significant association between RET/PTC rearrangements and FHCCs with a solid growth pattern, thus raising the possibility of using tyrosine kinase inhibitors for the treatment of patients with FHCCs, which are often refractory to radioiodine treatment.