ret/PTC-1, -2, and -3 oncogene rearrangements in human thyroid carcinomas: implications for metastatic potential?

RET EXPRESSION AND ITS CORRELATION WITH CLINICOPATHOLOGIC DATA IN PAPILLARY THYROID CARCINOMA

SUMMARY – The purpose of this study was to analyze the possible prognostic value of RET mutation in papillary thyroid carcinoma and its incidence in the past few decades in our population, due to the increasing incidence of papillary thyroid carcinoma. The present study included 180 patients operated for papillary thyroid carcinoma. The clinical and histopathologic characteristics were analyzed. Paraffin sections of the selected histologic slides were cut again and immunohistochemically stained by the Clone 3F8 P (HIER) from Novocastra (Vision Bio Systems Europe, Newcastle upon Tyne, UK) monoclonal antibody to RET oncoprotein. Univariate analysis indicated sex (p=0.01), histologic subtype (p=0.075) and capsular invasion (p=0.010) to be statistically significant predictors of lymph node metastases, whereas age (p=0.796), tumor size (p=0.556) and intraglandular dissemination (p=0.131) showed no such correlation. The presence of RET mutation (p=0.704) was not a statistically significant predictor of the tumor metastasizing potential. RET mutation (p=0.500) showed no statistically significant correlation with papillary thyroid carcinoma classifed into prognostic groups according to clinicopathologic features either. RET mutation was detected in 30% of 180 papillary thyroid carcinomas. This is the first large study demonstrating that RET mutation incidence in papillary thyroid carcinoma in Croatian population is consistent with the classic distribution of sporadic cases, despite the increased prevalence of papillary thyroid carcinoma in the past few decades.

Molecular markers in well-differentiated thyroid cancer

PURPOSE

Thyroid nodules are of common occurrence in the general population. About a fourth of these nodules are indeterminate on aspiration cytology placing many a patient at risk of unwanted surgery. The purpose of this review is to discuss various molecular markers described to date and place their role in proper perspective. This review covers the fundamental role of the signaling pathways and genetic changes involved in thyroid carcinogenesis. The current literature on the prognostic significance of these markers is also described.

METHODS

PubMed was used to search relevant articles. The key terms "thyroid nodules", "thyroid cancer papillary", "carcinoma papillary follicular", "carcinoma papillary", "adenocarcinoma follicular" were searched in MeSH, and "molecular markers", "molecular testing", mutation, BRAF, RAS, RET/PTC, PAX 8, miRNA, NIFTP in title and abstract fields. Multiple combinations were done and a group of experts in the subject from the International Head and Neck Scientific Group extracted the relevant articles and formulated the review.

RESULTS

There has been considerable progress in the understanding of thyroid carcinogenesis and the emergence of numerous molecular markers in the recent years with potential to be used in the diagnostic algorithm of these nodules. However, their precise role in routine clinical practice continues to be a contentious issue. Majority of the studies in this context are retrospective and impact of these mutations is not independent of other prognostic factors making the interpretation difficult.

CONCLUSION

The prevalence of these mutations in thyroid nodule is high and it is a continuously evolving field. Clinicians should stay informed as recommendation on the use of these markers is expected to evolve.

A comprehensive overview of the role of the RET proto-oncogene in thyroid carcinoma

The rearranged during transfection (RET) proto-oncogene was identified in 1985 and, very soon thereafter, a rearrangement named RET/PTC was discovered in papillary thyroid carcinoma (PTC). After this discovery, other RET rearrangements were found in PTCs, particularly in those induced by radiation. For many years, it was thought that these genetic alterations only occurred in PTC, but, in the past couple of years, some RET/PTC rearrangements have been found in other human tumours. 5 years after the discovery of RET/PTC rearrangements in PTC, activating point mutations in the RET proto-oncogene were discovered in both hereditary and sporadic forms of medullary thyroid carcinoma (MTC). In contrast to the alterations found in PTC, the activation of RET in MTC is mainly due to activating point mutations. Interestingly, in the past year, RET rearrangements that were different to those described in PTC were observed in sporadic MTC. The identification of RET mutations is relevant to the early diagnosis of hereditary MTC and the prognosis of sporadic MTC. The diagnostic and prognostic role of the RET/PTC rearrangements in PTC is less relevant but still important in patient management, particularly for deciding if a targeted therapy should be initiated. In this Review, we discuss the pathogenic, diagnostic and prognostic roles of the RET proto-oncogene in both PTC and MTC.

Prognosis and prognostic factors of papillary thyroid carcinoma in patients under 20 years

Age is an important prognostic factor of papillary thyroid carcinoma (PTC). In this study, we investigated the prognosis and prognostic factors of PTC in patients younger than 20 years. We enrolled 110 patients who underwent initial surgery at Kuma Hospital between 1987 and 2008. Tumor size > 4 cm, metastatic node ≥ 3 cm, and significant extrathyroid extension were more frequently detected in 8 patients with distant metastasis at diagnosis than in 102 patients without distant metastasis. Ten- and 20-year lymph node recurrence-free survival (LN-RFS) and distant recurrence-free survival (DRFS) rates were 84 and 80%, and 95 and 89%, respectively. Metastatic node ≥ 3 cm, age ≤ 16 years, tumor size > 4 cm, and male gender affected LN-RFS, and the former two had an independent prognostic value in multivariate analysis. Metastastic node ≥ 3 cm, significant extrathyroid extension, age ≤ 16 years, tumor size > 4 cm, and a male gender predicted a poor DRFS, and the former two were independent prognostic factors. To date, only 2 patients have died of PTC. These findings suggest that, in the subset of PTC patients younger than 20 years, metastatic node ≥ 3 cm, significant extension, and age ≤ 16 were important signs of aggressiveness of carcinoma, and careful treatment is necessary for patients with these characteristics, although the cause-specific survival was excellent.

Intracellular signal transduction and modification of the tumor microenvironment induced by RET/PTCs in papillary thyroid carcinoma

RET gene rearrangements (RET/PTCs) represent together with BRAF point mutations the two major groups of mutations involved in papillary thyroid carcinoma (PTC) initiation and progression. In this review, we will examine the mechanisms involved in RET/PTC-induced thyroid cell transformation. In detail, we will summarize the data on the molecular mechanisms involved in RET/PTC formation and in its function as a dominant oncogene, on the activated signal transduction pathways and on the induced gene expression modifications. Moreover, we will report on the effects of RET/PTCs on the tumor microenvironment. Finally, a short review of the literature on RET/PTC prognostic significance will be presented.

RET/PTC and CK19 expression in papillary thyroid carcinoma and its clinicopathologic correlation

Recently, the rearrangement of RET proto-oncogene has been reported to be the most common genetic change in papillary thyroid carcinoma (PTC). However, its prevalence has been reported variably and its relation to clinical outcome has been controversial. The characteristic nuclear features of PTC usually render the diagnosis, but problem arises with equivocal cytologic features that are present focally. Although there remains some controversy, CK19 has been reported to be a useful ancillary tool for diagnosis of PTC. To evaluate the expression rate of RET/PTC rearrangement and CK19 in PTCs in a Korean population, we studied 115 papillary thyroid carcinomas in 3 mm-core tissue microarray based immunohistochemical analysis. The prevalence of Ret protein expression was 62.6% and the CK19 immunoreactivity was 80.9%. There was no statistically significant association between the Ret positivity and CK19 immunoreactivity, although the percent agreement of the two was relatively high. The clinicopathological variables did not correlate with the expression of Ret. In conclusion, the prevalence of Ret protein expression and its clinicopathological implications in a Korean population are not much different from those reported in previous studies. However, its detection via immunohistochemistry can be a useful diagnostic tool for diagnosing papillary thyroid carcinoma in conjunction with CK19.

The role of immunohistochemical markers in the diagnosis of follicular-patterned lesions of the thyroid

Thyroid nodules are extremely common in the general population. The differential diagnosis includes numerous entities, non-neoplastic and neoplastic, benign and malignant. However, the diagnosis of follicular-patterned lesions remains an area fraught with controversy and diagnostic criteria are highly variable. It is, therefore, a field in need of objective, scientific markers that better characterize these lesions than has been possible by classical morphology. A number of candidates have been proposed. No single marker can identify all malignant follicular-patterned lesions, however, various combinations have been proposed. They include HBME-1, high molecular weight cytokeratins and ret, galectin-3 and TPO, galectin-3, fibronectin-1, CITED-1, HBME-1, and CK19. Advances in our understanding of the molecular basis of thyroid cancer will allow the identification of new markers and more accurate characterization of specific subtypes of neoplasia and malignancy. As new markers are characterized and validated, directed by molecular profiling of thyroid lesions with characteristic morphology, behavior, and outcome, they will become available as routine immunohistochemical markers that will provide a more accurate, scientific, and clinically relevant consultation report from the pathologist for cytology and surgical pathology procedures. Application of these markers will enhance the diagnosis of thyroid nodules and better guide the management of patients with these lesions.

Controversies in papillary microcarcinoma of the thyroid

Papillary thyroid carcinomas that are smaller than 1 cm are classified as papillary microcarcinomas (PMC). These lesions are frequently detected as incidental findings on autopsy or in surgical specimens. They are often multifocal. The relationship between PMC and clinical papillary thyroid carcinoma (PTC) is not clear. In patients with clinical thyroid cancer, PMC may represent intrathyroidal metastases; they may be the earliest form of future large lesions. These uncertainties raise questions about appropriate clinical management of patients with these lesions. Review of the literature substantiates the argument that clinically evident PTC may be distinctly different from solitary or multifocal PMC in terms of etiology and biologic behavior, supporting a conservative approach to management.

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.

Expression of Ret/PTC1, -2, -3, -delta3 and -4 in German papillary thyroid carcinoma

Ret/PTC oncogene has been described with a frequency of 2.5-30% in papillary thyroid carcinomas. We examined the expression of ret/PTC in 99 German papillary thyroid carcinomas, including two recently described new variants of ret/PTC3 and identified eight ret/PTC-positive tumours (8%) but none with the new variants.