Molecular analysis of new subtypes of ELE/RET rearrangements, their reciprocal transcripts and breakpoints in papillary thyroid carcinomas of children after Chernobyl

Predictive molecular pathology in metastatic thyroid cancer: the role of RET fusions

BACKGROUND

Rearranged during transfection (RET) gene fusions are detected in 10-20% of thyroid cancer patients. Recently, RET fusion-positive metastatic thyroid cancers have attracted much attention owing to the FDA approval of two highly selective anti-RET tyrosine kinase inhibitors, namely, selpercatinib, and pralsetinib.

AREAS COVERED

This review summarizes the available evidence on the biological and predictive role of RET gene fusions in thyroid carcinoma patients and the latest screening assays currently used to detect these genomic alterations in histological and cytological specimens.

EXPERT OPINION

Management of advanced thyroid carcinoma has significantly evolved over the last decade thanks to the approval of three multikinase inhibitors, i.e. sorafenib, lenvatinib, cabozantinib, and of two selective RET-tyrosine inhibitors, i.e. selpercatinib and pralsetinib. In this setting, the detection of RET-fusions in advanced thyroid cancer specimens through the use of next-generation sequencing has become a commonly used strategy in clinical practice to select the best treatment options.

Novel rearrangements involving the RET gene in papillary thyroid carcinoma

BACKGROUND

In the field of gene fusions driving tumorigenesis in papillary thyroid carcinoma (PTC), rearrangement of the proto-oncogene RET is the most frequent alteration. Apart from the most common rearrangement of RET to CCDC6, more than 15 partner genes are yet reported. The landscape of RET rearrangements in PTC ("RET-PTC") can notably be enlarged by modern targeted next-generation sequencing, indicating similarities between oncogenic pathways in other cancer types with identical genetic alterations.

METHODS

Targeted next-generation sequencing was performed for two cases of BRAF-wild type PTC with confirmation of the results by Sanger sequencing. A "UniProt" database research was performed to assess protein alterations resulting from RET rearrangements.

RESULTS

RUFY2-RET and KIAA1468-RET were detected. The fusion genes were not present in normal tissue of the index patients. The rearrangement RUFY2-RET lead to a fusion of the RET tyrosine kinase domain to a RUN domain and a coiled-coil domain. For KIAA1468-RET, a fusion to a LisH domain and two coiled-coil domains resulted.

CONCLUSIONS

RUFY2-RET and KIAA1468-RET are novel RET/PTC rearrangements. The fusions were previously described in non-small cell lung cancer. The rearrangement results in a fusion of the RET tyrosine kinase to regulatory domains of RUFY2 and KIAA1468.

Ret Oncogene Protein Expression in Papillary Thyroid Carcinoma and Related Lesions

Background

Activation of Ret oncogenes, particularly Ret/PTC, has been identified in papillary thyroid carcinoma (PTC). The purpose of this study was to investigate the immunostaining pattern of Ret oncogene protein in PTC and nodular non-PTC lesions with a fine chromatin pattern.

Materials and Methods

Ninety-three PTC and 139 nodular non-PTC lesions were microscopically reviewed to identify the nuclear changes of “limited nuclear features of PTC” (focal nuclear grooves, nuclear inclusions or optically clear nuclei) and areas of infiltrating carcinoma (IC) and were submitted for immunostaining with Ret oncogene protein antiserum.

Results

Immunoreactivity for Ret protein ranged from negative in follicular adenoma (FA) with a coarse chromatin pattern, to negative or weak reactivity in FA with a fine chromatin pattern, to strong reactivity in PTC with areas of infiltrating carcinoma (IC). In FA with fine chromatin, FA and follicular carcinoma (FC) containing an admixture of areas of coarse and fine chromatin, areas with nuclear changes with “limited nuclear features of PTC” displayed varying degrees of immunoreactivity. The intensity of immunostaining varied with the degree of nuclear change. The noninvasive component of PTC with IC usually showed more extensive and stronger reactivity than PTC without IC. PTCs with and without IC were associated with a rate of lymph node metastasis of 48% and 3%, respectively.

Conclusions

The expression of Ret oncogenes (Ret/PTC, other unknown variants or wild type) is focally or extensively present in all PTC with IC. The degree of immunoreactivity is likely to be proportional to the potential for lymph node metastasis of PTC. In the context of this study and due to the specificity of Ret oncogenes, it is likely that nodular non-PTC lesions with a fine chromatin pattern and focal positive reactivity for Ret oncogene represent PTC-related lesions.

Papillary Thyroid Carcinoma and Related Thyroid Neoplastic Lesions: A Light Microscopic Study with Emphasis on Nuclear Changes

A total of 187 thyroid lesions consisting of 2 cases of Grave's disease, 21 cases of multinodular goiter, 40 follicular adenomas and 124 low-grade papillary thyroid carcinomas were studied to identify intermediate neoplastic lesions in the spectrum of nuclear changes between benign reactive thyroid follicles and low-grade thyroid papillary carcinoma. The lesions were examined and classified on the basis of the following nuclear features: fine chromatin seen in the thyroid papillary carcinomas and coarse chromatin seen in follicular carcinomas. Cases with Hürthle cell changes were excluded from the study. Cases with nuclei containing coarse chromatin were classified in the group of follicular adenomas with a coarse chromatin pattern. The neoplastic thyroid lesions containing fine chromatin showed a spectrum of nuclear changes ranging between reactive follicular lesions and papillary thyroid carcinoma with lymph node metastasis. Such lesions were classified as follicular adenomas with a fine chromatin pattern. The nuclei of these lesions were graded into mild to marked "nuclear atypia with a fine chromatin pattern". The degree of atypia depended on the degree and extent of nuclear changes. Encapsulated follicular adenomas with a fine chromatin pattern and with mild atypia (11 cases), moderate atypia (13 cases), marked atypia (27 cases), and encapsulated or nonencapsulated papillary thyroid carcinoma were characterized by uniform nuclei; with mild, moderate and marked nuclear atypia in less than 2/3 of the cell population and marked nuclear atypia in more than 2/3 of the cell population; and measuring 5.4-6.3, 6.0-7.2, 6.3-9 and 7.2-10 microns in diameter, respectively. Follow-up of cases of papillary thyroid carcinoma fulfilling the above criteria showed lymph node metastasis in 33% of cases, whereas follicular adenomas with a fine chromatin pattern, including cases originally diagnosed as papillary carcinoma, showed no evidence of lymph node or distant metastasis in a follow-up period of 30 months to 15 years. In the thyroid tissue surrounding papillary thyroid carcinoma or encapsulated follicular adenoma with a fine chromatin pattern and marked atypia, adenomatous nodules with a fine chromatin pattern and with low-grade nuclear atypia were identified. The adenomatous nodules with a fine chromatin pattern and with mild, moderate and marked atypia showed architectural, cytoplasmic and nuclear features similar to those of follicular adenoma with a fine chromatin pattern of the same grade. Of interest, a large number of cases of follicular adenoma with a fine chromatin pattern had areas with features of follicular adenoma with a coarse chromatin pattern.

Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes

The gene encoding the receptor-tyrosine kinase RET was first discovered more than three decades ago, and activating RET rearrangements and mutations have since been identified as actionable drivers of oncogenesis. Several multikinase inhibitors with activity against RET have been explored in the clinic, and confirmed responses to targeted therapy with these agents have been observed in patients with RET-rearranged lung cancers or RET-mutant thyroid cancers. Nevertheless, response rates to RET-directed therapy are modest compared with those achieved using targeted therapies matched to other oncogenic drivers of solid tumours, such as sensitizing EGFR or BRAF^V600E mutations, or ALK or ROS1 rearrangements. To date, no RET-directed targeted therapeutic has received regulatory approval for the treatment of molecularly defined populations of patients with RET-mutant or RET-rearranged solid tumours. In this Review, we discuss how emerging data have informed the debate over whether the limited success of multikinase inhibitors with activity against RET can be attributed to the tractability of RET as a drug target or to the lack, until 2017, of highly specific inhibitors of this oncoprotein in the clinic. We emphasize that novel approaches to targeting RET-dependent tumours are necessary to improve the clinical efficacy of single-agent multikinase inhibition and, thus, hasten approvals of RET-directed targeted therapies.

Current Standards in Treatment of Radioiodine Refractory Thyroid Cancer

OPINION STATEMENT

Radioiodine refractory differentiated thyroid cancer (RAI-R DTC) is a challenging malignancy with limited prognosis and treatment options. Recently, clinical trials with targeted therapies have advanced the outlook of these patients, and inhibition of the vascular endothelial growth factor (VEGF) axis has led to the approval of small-molecule tyrosine kinase inhibitors (TKIs) for first-line treatment of radioiodine refractory disease. In addition to approved therapies (sorafenib and lenvatinib), other multi-targeted tyrosine kinase inhibitors that are commercially available have been recognized as viable treatment options for RAI-R DTC. Our preference is to initially use lenvatinib, given the dramatic progression-free survival (PFS) improvement versus placebo, with the caveat that 24 mg daily is not often tolerated and lower doses often used. In patients with BRAF V600E mutation, BRAF inhibitors are now considered for treatment, especially if patients are at high risk from antiangiogenic therapy. Research is continuing to evolve in identifying mechanisms related to radioiodine refractoriness, and trials are evaluating therapeutic molecules to overcome this resistance. Clinical care of patients with RAI-R DTC requires careful consideration of both patient and disease characteristics. Many patients with asymptomatic and indolent disease can be followed for years without treatment while others with high volume or rapidly progressive disease merit early intervention.

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.

Differentiated thyroid cancer: focus on emerging treatments for radioactive iodine-refractory patients

BACKGROUND

The treatment of differentiated thyroid cancer refractory to radioactive iodine (RAI) had been hampered by few effective therapies. Recently, tyrosine kinase inhibitors (TKIs) have shown activity in this disease. Clinical guidance on the use of these agents in RAI-refractory thyroid cancer is warranted.

MATERIALS AND METHODS

Molecular mutations found in RAI-refractory thyroid cancer are summarized. Recent phase II and III clinical trial data for TKIs axitinib, lenvatinib, motesanib, pazopanib, sorafenib, sunitinib, and vandetinib are reviewed including efficacy and side effect profiles. Molecular targets and potencies of these agents are compared. Inhibitors of BRAF, mammalian target of rapamycin, and MEK are considered.

RESULTS

Routine testing for molecular alterations prior to therapy is not yet recommended. TKIs produce progression-free survival of approximately 1 year (range: 7.7-19.6 months) and partial response rates of up to 50% by Response Evaluation Criteria in Solid Tumors. Pazopanib and lenvatinib are the most active agents. The majority of patients experienced tumor shrinkage with TKIs. Common adverse toxicities affect dermatologic, gastrointestinal, and cardiovascular systems.

CONCLUSION

Multiple TKIs have activity in RAI-refractory differentiated thyroid cancer. Selection of a targeted agent should depend on disease trajectory, side effect profile, and goals of therapy.

Molecular pathogenesis and mechanisms of thyroid cancer

Thyroid cancer is a common endocrine malignancy. There has been exciting progress in understanding its molecular pathogenesis in recent years, as best exemplified by the elucidation of the fundamental role of several major signalling pathways and related molecular derangements. Central to these mechanisms are the genetic and epigenetic alterations in these pathways, such as mutation, gene copy-number gain and aberrant gene methylation. Many of these molecular alterations represent novel diagnostic and prognostic molecular markers and therapeutic targets for thyroid cancer, which provide unprecedented opportunities for further research and clinical development of novel treatment strategies for this cancer.

Rearrangement Analysis in Archival Thyroid Tissues: Punching Microdissection and Artificial RET/PTC 1–12 Transcripts

BACKGROUND

In few papillary thyroid carcinomas (PTC) and oxyphilic thyroid carcinoma, the clinical impact of the 15 known RET hybrid oncogene variants (RET/PTC 1 to 12, 1L, 3r2, 3r3) is subject to controversial discussions. Large patient cohorts and exploitation of pathological thyroid tissue archives are essential to study the prognostic significance of RET/PTC chimeras.

MATERIALS AND METHODS

Formalin-fixed and paraffin-embedded thyroid neoplasms were subjected to manual punching macrodissection and subsequent extraction of total RNA. Following reverse transcriptase polymerase chain reaction (RT-PCR)-based screening for RET rearrangements, hybrid-specific expression analyses were carried out for samples indicative of chimeric transcripts. Due to lack of tissue specimen harboring the rare RET chimeras, artificially constructed hybrid sequences of all known RET/PTC variants served as PCR controls.

RESULTS

Manual punching dissection successfully diminished RET wild-type contamination originating from C-cells dispersed throughout normal thyroid tissues. The average amount of 27.4 mug RNA extracted allowed for repeated molecular analyses (>60 PCRs). Hybrid-specific expression analysis identified 10 of 15 RET rearrangements (8x RET/PTC 1, 2x RET/PTC 3, 5x RET/PTC x) to be found in 54 oxyphilic thyroid tumors examined. Successful amplification of each artificial hybrid sequence ensured the absence of rare chimeric transcripts. Therefore, RET/PTC x represent either common chimeras not amplifiable due to archival RNA degradation or truly novel hybrid oncoproducts.

CONCLUSIONS

The fast and simple techniques described here were used to examine oxyphilic carcinomas and adenomas. These microdissection and RT-PCR procedures can easily be put into practice in any molecular biology research laboratory to enable screening of large numbers of archival thyroid tumors for known as well as yet unknown RET rearrangements.

Radiation-induced thyroid cancer: what we have learned from chernobyl

An increased incidence of thyroid cancer in the exposed children remains the most well-documented long-term effect of radioactive contamination after the Chernobyl nuclear accident in April, 1986. Multiple studies on approx 4000 children and adolescents with thyroid cancer have provided important new information about the epidemiological, clinical, pathological, and molecular aspects of radiation-induced carcinogenesis in the thyroid gland. They revealed that environmental exposure to 131I during childhood carries an increased risk of thyroid cancer and the risk is radiation dose dependent. The youngest children are most sensitive to radiation-induced carcinogenesis, and the minimal latent period for thyroid cancer development after exposure is as short as 4 yr. The vast majority of these cancers are papillary carcinomas, many of which have characteristic solid or solid-follicular microscopic appearance. On the molecular level, post-Chernobyl tumors are characterized by frequent occurrence of chromosomal rearrangements, such as RET/PTC, whereas point mutations of BRAF and other genes are much less common in this population.

Multiple signaling pathways converge on beta-catenin in thyroid cancer

The beta-catenin pathway has been conclusively demonstrated to regulate differentiation and patterning in multiple model systems. In thyroid cancer, alterations are often seen in proteins that regulate beta-catenin, including those of the RAS, PI3K/AKT, and peroxisome proliferation activated receptor-gamma (PPARgamma) pathways, and evidence from the literature suggests that beta-catenin may play a direct role in the dedifferentiation commonly observed in late-stage disease. RET/PTC rearrangements are frequent in thyroid cancer and appear to be exclusive from mutational events in RAS and BRAF. Activation of AKT by phosphatidylinositide-3 kinase (PI3K), a RAS effector, results in GSK3beta phosphorylation and deactivation and subsequent beta-catenin upregulation in thyroid cancer. Activating mutations in beta-catenin, which have been demonstrated in late-stage thyroid tumors, correlate with beta-catenin nuclear localization and poor prognosis. We hypothesize that activation of the RAS, PI3K/AKT, and PPARgamma pathways ultimately impinges upon beta-catenin. We further propose that if mutations in BRAF, RAS, and RET/PTC rearrangements are mutually exclusive in certain thyroid tumors or tumor types, as has already been shown for papillary thyroid cancer, then these interconnected pathways may cooperate in the initiation and promotion of the disease. We believe that clinical benefit for thyroid cancer patients could be derived from disrupting the middle or distal pathway effectors of these pathways, such as AKT or beta-catenin.

BRAF mutations are uncommon in papillary thyroid cancer of young patients

Mortality is low for young patients (younger than 21 years) with papillary thyroid cancer (PTC), and different mutations might contribute to this. Previous studies detected ret/PTC rearrangements more frequently in PTC from children than adults, and recent reports describe a high incidence of BRAF T1796A transversion in adult PTC. However, BRAF mutations have not been adequately studied in PTC from young patients. We amplified and sequenced segments of the BRAF gene spanning the T1796A transversion site in 14 PTC from patients 10-21 years of age (mean, 17.5 +/- 3.5 years). The PTC (7 = class 1; 5 = class 2; 1 = class 3) ranged from 0.7-2.9 cm in diameter (mean, 1.4 +/- 0.75 cm). None of them (0/14) contained BRAF T1796A and none recurred (mean follow-up, 66 +/- 40 months). This incidence of BRAF T1796A is significantly less than that reported for adult PTC (270/699, 38.6%, p = 0.0015) in several series. None of our PTC (0/10) contained ras mutations, but 7/12 (58%) contained ret/PTC rearrangements. We conclude that BRAF mutations are less common in PTC from young patients, and ret/PTC rearrangements were the most common mutation found in these childhood PTC.

Immunohistochemical localisation of RET and p53 mutant protein of thyroid lesions in a North-Eastern Malaysian population and its prognostic implications

AIMS

To investigate RET and p53 expression in local thyroid lesions, in order to shed light on the pathogenesis of papillary carcinoma and explain the high prevalence of this condition among the nodular hyperplasia (multi-nodular goitre) cases.

METHODS

Archival thyroid tissue was retrieved from Hospital Universiti Sains Malaysia (HUSM) Pathology Department files and studied by immunohistochemistry for RET and p53 mutant protein. Normal tissues from 74 cases served as controls.

RESULTS

Fifty follicular adenoma, 66 nodular hyperplasia and 53 papillary carcinoma cases were studied. RET was expressed in 5.4% of normal thyroid tissue, 18% of follicular adenomas, 22.7% of nodular hyperplasia cases and 71.7% of papillary carcinomas. Its expression in papillary carcinoma was not associated with the coexistence of nodular hyperplasia lesions. p53 was expressed by 17% of papillary carcinomas. No association was found between p53 expression of nodular hyperplasia with or without co-existing papillary carcinoma. p53, rather than RET, was an excellent predictor of tumour lymph node metastasis and capsular invasion. p53 was also a significant prognosticator of survival outcome.

CONCLUSIONS

RET expression is highly prevalent in local papillary carcinoma, indicating a significant role in the pathogenesis of this tumour, with no apparent role in tumour behaviour and survival outcome. p53 on the other hand appears to be a significant factor in the latter events. The two genes appear to act in two different pathways: the former being an initiator, and the later a propagator of papillary carcinoma.

Novel tumorigenic rearrangement, Delta rfp/ret, in a papillary thyroid carcinoma from externally irradiated patient

Molecular analysis of cDNA derived from a papillary thyroid carcinoma (PTC) (follicular variant of papillary thyroid carcinoma on histology) which developed in an externally irradiated patient 4 years after exposure identified a portion of the 5' region, exons 1-3, of the rfp gene juxtaposed upstream of the fragment encoding the tyrosine kinase (TK) domain of the ret gene. The fusion gene, termed Delta rfp/ret, was the result of a balanced chromosomal translocation t(6;10) (p21.3;q11.2) confirmed by interphase FISH painting, with breakpoints occurring in introns 3 and 11 of the rfp and ret genes, respectively. Both Delta rfp/ret and reciprocal ret/rfp chimeric introns had small deletions around breakpoints consistent with presumed misrepair of a radiation-induced double-strand DNA break underlying the rearrangement. No extensive sequence homology was found between the fragments flanking the breakpoints. The fusion protein retained the propensity to form oligomers likely to be mediated by a coiled-coil of the RFP polypeptide as assessed by a yeast two-hybrid system. NIH 3T3 fibroblasts stably transfected with a mammalian expression vector encoding full-length Delta RFP/RET readily gave rise to the tumors in athymic mice suggestive of high transforming potential of the fusion protein. Thus, the Delta rfp/ret rearrangement may be involved in a causative manner in cancerogenesis and provides additional evidence of the role of activated ret oncogene in the development of a subset of papillary thyroid carcinoma.

Hybrid thyroid carcinoma with a coarse chromatin pattern and nuclear features of papillary thyroid carcinoma

Hybrid follicular carcinoma (FC) and papillary thyroid carcinoma (PTC) have not been previously well described. Consecutive cases of 29 FC, 12 Hurthle cell carcinomas (HC), 247 PTC and 13 Hurthle cell PTC (HPTC) were reviewed with special attention to the coarse (CC) and fine chromatin patterns (FIC), as well as to the presence of nuclear grooves, pseudoinclusions or optically clear appearance. Limited nuclear features of PTC (LNF-PTC) are defined as areas of tumor with FIC in addition to some other nuclear features, but insufficient for the diagnosis of PTC. Tumors with nuclei showing an admixture of CC and PTC or LNFPTC were submitted for immunostaining for cytokeratin 19, HBME and Ret/PTC. FC and HC contained areas of LNFPTC in 25 tumors and focal PTC in 3 tumors. None of these cases was associated with lymph node metastasis. Areas with CC were found in 54 PTC and 3 HPTC. The rates of vascular invasion and distant metastasis tended to be higher for PTC with areas of coarse chromatin pattern than for PTC without such areas; however, the difference was not statistically significant. Immunoreactivity for cytokeratin 19 and HBME was moderate to strong for PTC and focal areas of PTC or LNFPTC in FC without Hurthle cell changes. Ret/PTC immunostaining was positive in areas of LNFPTC or focal PTC in three FC. Focal PTC or areas of LNFPTC are frequently seen in FC. Likewise, areas of CC are often present in PTC. The presence of these focal areas does not appear to change the clinical behavior of the tumor and therefore does not warrant a change of nomenclature.

Differentiated thyroid carcinomas with vascular invasion: a comparative study of follicular, Hürthle cell and papillary thyroid carcinoma

AIM

Non-medullary thyroid carcinomas arise from follicular cells. The purpose of this study is to correlate clinical and pathological properties of these tumours with the rate of distant metastasis from a series of thyroid tumours excised at one institution.

METHODS

A total of 311 non-medullary thyroid tumours were identified and divided into: 29 follicular carcinoma (FC), 12 Hürthle cell carcinoma (HC), 13 Hürthle cell papillary thyroid carcinoma (HPTC) with vascular invasion (VI), 32 papillary thyroid carcinoma (PTC) with VI and 225 PTC without VI. The mean follow-up was 6.5 years with a range of 1-17 years. The tumours were histologically subdivided into minimal or wide invasion for FC and HC and focal or extensive invasion for PTC and HPTC, and stratified according to status of VI.

RESULTS

The rate of distant metastasis was similar for FC, malignant Hürthle cell tumours and PTC with VI, and increased with extent of invasion. VI was seen in 12% of all PTC and 0% of HPTC in this study. PTC without VI were associated with a much lower potential of distant metastasis, were smaller in size and occurred in patients of younger age than PTC with VI. In addition, there was a tendency for increased potential for distant metastases with increased tumour size and patient age for all groups of tumours in the study. Patient age and tumour size appeared to play a smaller role than that of VI in predicting distant metastasis.

CONCLUSIONS

Our study suggests that the rate of distant metastasis relates to VI, patient age and tumour size, regardless of Hürthle cell, FC or PTC differentiation. PTC of large size, and in patients older than 45 years, have a high propensity for vascular invasion.

Establishment of a non-tumorigenic papillary thyroid cell line (FB-2) carrying the RET/PTC1 rearrangement

A novel human thyroid papillary carcinoma cell line (FB-2) has been established and characterized. FB-2 cells harbor the RET/PTC1 chimeric oncogene in which the RET kinase domain is fused to the H4 gene. FB-2 cells neither formed colonies in semisolid media nor induced tumors after heterotransplant into severe combined immunodeficient mice. However, HMGI(Y), HMGI-C and c-myc genes, which are associated to thyroid cell transformation, were abundantly expressed in FB-2 cells but not in normal thyroid cells. FB-2 cells only partially retained the differentiated thyroid phenotype. In fact, the PAX-8 gene, which codes for a transcriptional factor required for thyroid cell differentiation, was expressed, while thyroglobulin, TSH-receptor and thyroperoxidase genes were not. Moreover, FB-2 cells produced high levels of interleukin (IL)-6 and IL-8.

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.

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.

Comparative analysis of the NF2, TP53, PTEN, KRAS, NRAS and HRAS genes in sporadic and radiation-induced human meningiomas

Irradiation to the head is associated with a significantly increased incidence of meningiomas. Radiation-induced meningiomas morphologically resemble their sporadically arising counterparts; however, they frequently exhibit a more malignant phenotype. Several genes have been shown to carry mutations in meningiomas, with the NF2 gene being most frequently affected. To examine whether the NF2 gene also plays a role in the development of radiation-induced meningiomas, we compiled a series of meningiomas from 25 patients with a history of previous cranial radiation. This series was compared with 21 atypical WHO grade II meningiomas and 15 anaplastic WHO grade III meningiomas, all from patients without a history of prior irradiation. NF2 mutations occurred significantly more often in sporadic atypical and anaplastic than in radiation-induced meningiomas (p < 0.02). In addition, all meningiomas were examined for mutations in the PTEN, TP53, HRAS, KRAS and NRAS genes. Two mutations in the TP53 gene in a sporadic and a radiation-induced tumor were detected. PTEN mutations were observed in 1 anaplastic and 1 radiation-induced meningioma. No structural alterations were seen in the RAS genes. Our data suggest that, while there is a certain overlap in the mutational spectrum, NF2 mutations may not play such a prominent role in the pathogenesis of radiation-induced compared to sporadic meningiomas.

A novel chromosomal translocation t(3;5)(q12;p15.3) and loss of heterozygosity on chromosome 22 in a multifocal follicular variant of papillary thyroid carcinoma presenting with skin metastases

Classic genetic rearrangements in papillary carcinoma of the thyroid involve the RET- or TRK proto-oncogenes. We report a novel chromosomal translocation t(3;5)(q12;p15.3), confirmed by fluorescence in situ hybridization, in a multifocal follicular variant of a papillary carcinoma of the thyroid in a 79-year-old woman, with skin metastases as a presenting symptom. Three years earlier, another cutaneous metastasis on her scalp was misdiagnosed as hidradenoma. Four tumour foci were recognized in the thyroid, two with a follicular variant of papillary carcinoma. To detect loss of heterozygosity, 14 chromosomes were investigated with 59 microsatellite markers. A clonal relationship was detected between the two foci of tumour in the thyroid gland containing follicular variant of papillary carcinoma and one of the skin lesions tested, all demonstrating loss of heterozygosity (LOH) in the same region of chromosome 22. Based on earlier reports, the low rate of LOH detected is in agreement with the diagnosis papillary carcinoma of the thyroid. Whole body scintigraphy performed after ablative therapy with radioiodine revealed multiple metastases in the lungs and skeleton. After repeated radioiodine therapy, thyroglobulin under thyroxine suppression became undetectable and post-therapeutic scintigraphy revealed important regression of metastases.

Immunostaining for ret oncogene proteins in papillary thyroid carcinoma: a correlation of ret immunoreactivity and potential of lymph node metastasis

Ret oncogenes, particularly Ret/PTC, have been associated with the potential of local invasion of papillary thyroid carcinoma (PTC). The purpose of this study was to investigate the correlation between the Ret oncogene expression and the potential of lymph node metastasis of PTC. A total of 107 PTC were microscopically reviewed to identify areas of infiltrating carcinoma (IC). IC was defined as tumor cells disposed in a haphazard pattern and in lobules, nests, follicles, or single cells within a desmoplastic or sclerotic stroma. All cases were submitted to immunostaining for Ret oncogene. There were 36 noninfiltrating PTC with lymph node metastasis in 1 case and 71 infiltrating PTC with lymph node metastasis in 40 cases. For non-PTC, the positive immunoreactivity was often weak to moderate and focal. For infiltrating PTC with IC, the IC displayed strong immunoreactivity. The noninfiltrating component of PTC with IC usually showed stronger reactivity than PTC without an infiltrating component. Furthermore, 36 of 40 metastatic PTC in lymph node were immunoreactive. Three follicular adenomas with areas of scar caused by fine-needle aspiration biopsy were not immunoreactive for Ret. In view of the high potential of infiltrating PTC for lymph node metastasis, distinction of this type of carcinoma from its noninfiltrating form is clinically important. Because immunoreactivity for Ret is usually positive in areas of infiltrating PTC and is often negative or focally positive in noninfiltrating PTC, immunostaining for Ret is helpful to identify infiltrating PTC and distinguish it from changes caused by fine-needle aspiration biopsy in benign thyroid lesions.

Prognostic factors in well-differentiated follicular-derived carcinoma and medullary thyroid carcinoma

This paper reviews both clinicopathologic and biological factors in differentiated thyroid carcinoma of follicular and C-cell derivation. The value of specific histological parameters including tumor subtypes, vascular invasion as predictors of prognosis in these tumors is discussed and the pertinent literature reviewed. The utility of ploidy analysis and immunohistochemical assessment of hormonal and tumor markers are described. Current information on biological and molecular markers that may be helpful in predicting behavior is discussed. The commonly used staging systems for thyroid cancer are briefly described.

Gene rearrangements in radiation-induced thyroid carcinogenesis

BACKGROUND

Radiation is an accepted risk factor for thyroid carcinogenesis in children. Recent observations in large cohorts of children and young adults who developed papillary thyroid carcinomas (PTC) related to accidental radiation exposure after the Chernobyl reactor accident revealed typical genetic aberrations shedding light on genetic determinants and mechanisms of radiation-induced carcinogenesis.

PROCEDURE

A molecular genetic analysis was performed on 191 post-Chernobyl PTC by RT-PCR, multiplex PCR, DNA sequencing, and in some cases 5'RACE. Determination of point mutations was by means of PCR and either allele-specific oligonucleotide hybridization or SSCP and DNA sequencing.

RESULTS

In various sporadic thyroid tumor types of adults structural genetic aberrations have been found involving mutations of RAS (codon 12, 13, 61), p53 (exons 5 to 8), Gsalpha (codon 201 and 227), and, at a low prevalence, the receptor tyrosine kinases RET or NTRK1. In contrast, in radiation-induced PTC of children RET rearrangements are by far the most prevalent genetic aberrations. In these RET rearrangements, the transmembrane and extracellular domains of RET are lost, and are replaced by parts of other genes at the 5' end. These genes always contain coiled-coil domains with dimerization potential and lead to constitutive, ligand-independent activation of the ret tyrosine kinase domain at the 3' end of the fusion product. The most frequent radiation-induced RET gene fusions involve the ELE1 (ARA70) gene, a transcription coactivator of the androgen receptor (PTC3), and H4, a gene of unknown function (PTC1). Both rearrangements originate from DNA double strand breaks with repair by intrachromosomal balanced paracentric inversion and recombination by illegitimate DNA endjoining at small stretches of homologous nucleotide sequences and direct or inverted repeats, without significant breakpoint clusters in the involved introns. In addition, five different RET-fused genes, RIalpha, GOLGA5, HTIF, RFG7 and RFG8, have been detected leading to the PTC2, 5, 6, 7 and 8 types of RET rearrangements, respectively. Each fusion leads, in principle, to the same effect: The ret tyrosine kinase is uncoupled from its stringent physiological regulation by replacement of its 5' end and is aberrantly activated by the 5' parts of fused genes in thyrocytes that do not normally express ret tyrosine kinase. Ectopic ret expression, clonal expansion and early invasion are peculiar to the affected cells. The RET-fused gene is obviously decisive for modulating tumor development: ELE1/RET rearrangements lead to most rapid tumor progression and are related to the solid variant of PTC, in contrast to H4/RET rearrangements connected with papillary or follicular variants of PTC.

CONCLUSIONS

Typical genetic aberrations are produced by radioiodine uptake in the juvenile thyroid gland. They act as determinants of phenotype, biology, and clinical course of radiation-induced papillary thyroid carcinomas.

RET rearrangements in radiation-induced papillary thyroid carcinomas: high prevalence of topoisomerase I sites at breakpoints and microhomology-mediated end joining in ELE1 and RET chimeric genes

Children exposed to radioactive iodine after the Chernobyl reactor accident frequently developed papillary thyroid carcinomas (PTC). The predominant molecular lesions in these tumors are rearrangements of the RET receptor tyrosine kinase gene. Various types of RET rearrangements have been described. More than 90% of PTC with RET rearrangement exhibit a PTC1 or PTC3 type of rearrangement with an inversion of the H4 or ELE1 gene, respectively, on chromosome 10. To obtain closer insight into the mechanisms underlying PTC3 inversions, we analyzed the genomic breakpoints of 22 reciprocal and 4 nonreciprocal ELE1 and RET rearrangements in 26 post-Chernobyl tumor samples. In contrast to previous assumptions, an accumulation of breakpoints at the two Alu elements in the ELE1 sequence was not observed. Instead, breakpoints are distributed in the affected introns of both genes without significant clustering. When compared to the corresponding wildtype sequences, the majority of breakpoints (92%) do not contain larger deletions or insertions. Most remarkably, at least one topoisomerase I site was found exactly at or in close vicinity to all breakpoints, indicating a potential role for this enzyme in the formation of DNA strand breaks and/or ELE1 and RET inversions. The presence of short regions of sequence homology (microhomologies) and short direct and inverted repeats at the majority of breakpoints furthermore indicates a nonhomologous DNA end-joining mechanism in the formation of chimeric ELE1/Ret and Ret/ELE1 genes.

The RFG oligomerization domain mediates kinase activation and re-localization of the RET/PTC3 oncoprotein to the plasma membrane

The RET/PTC3 oncogene arises from the fusion between the N-terminal encoding domain of the RFG gene and the tyrosine kinase encoding domain of RET receptor. RET/PTC3 is very frequent in papillary thyroid carcinomas, especially in children exposed to the Chernobyl accident. We have studied the functional consequences of the RFG-RET fusion. Here we show that the N-terminal coiled-coil domain of RGF mediates oligomerization and activation of the kinase and of the transforming capability of RET/PTC3. In addition, the RFG coiled-coil domain mediates a physical association between RET/PTC3 and RGF proteins, rendering RFG a bona fide substrate of RET/PTC3 kinase. Finally, we show that the coiled-coil domain of RGF is essential for the distribution of the RET/PTC3 protein at the membrane/particulate cell compartment level, where also most of the RFG protein is localized. We propose that fusion to the RFG coiled-coil domain provides RET kinase with a scaffold that mediates oligomerization and re-localization of the RET/PTC3 protein, a process that may be crucial for the signalling of this specific RET/PTC variant.

Poorly differentiated follicular thyroid carcinoma with rhabdoid phenotype: a clinicopathologic, immunohistochemical and electron microscopic study of two cases

Poorly differentiated thyroid carcinomas with follicular cell phenotype are not well defined. Different diagnostic criteria have been employed for these tumors, including solid growth, nodular, trabecular, and insular patterns. Cytologic features, such as a predominance of tall and columnar cells, have been considered to be diagnostic of poorly differentiated carcinoma. However, there is no agreement among surgical pathologists regarding morphologic criteria for poorly differentiated thyroid carcinoma. We report two unique thyroid neoplasms that we interpreted as poorly differentiated follicular carcinomas. Nodular, trabecular, and sheetlike patterns predominated in both tumors. They were composed of cells that were focally immunoreactive for thyroglobulin and had large vesicular nuclei with prominent nucleoli. A variable number of cells showed rhabdoid phenotype. The rhabdoid inclusions did not stain for thyroglobulin but contained whorls of intermediate filaments that were vimentin positive. There were foci of necrosis and numerous mitotic figures. Both patients were adults and died with multiple pulmonary metastases. The presence of rhabdoid cells in poorly differentiated follicular carcinomas broadens the spectrum of tumors with rhabdoid phenotype. More cases are needed to determine whether the rhabdoid phenotype is a marker for poorly differentiated follicular carcinoma as well as an independent adverse prognostic factor.

New breakpoints in both the H4 and RET genes create a variant of PTC-1 in a post-Chernobyl papillary thyroid carcinoma

Two main types of RET/PTC oncogene, named RET/PTC-1 and 3, occur in papillary thyroid carcinomas especially in those from Belarus children after the Chernobyl nuclear accident. Several variants of RET/PTC-3 have also been found, having different break points with respect to the classical RET/PTC-3. To our knowledge, no variant of RET/PTC-1 has been described up to now. We found a post-Chernobyl papillary thyroid carcinoma with an RET/PTC-1 rearrangement characterized by a transcript longer than expected. Sequence analysis of the PCR product obtained after RT-PCR revealed new fusion points between H4 and RET genes. The genomic sequence showed new breakpoints in both H4 intronic and in RET exonic regions. The RET gene breakpoint occurred within exon 11, at variance with the classical form of RET/PTC-1, in which it is in intron 11. As a consequence of this new fusion point, the transcript included 132 nucleotides of exon 11, coding for 44 amino acids of RET protein. Regarding the H4 gene, the classical breakpoint is in the first intron and the cDNA contains a fragment of 339 nucleotides. In our case the cDNA had a longer fragment of H4 involving a total of 1266 nucleotides. Sequencing of genomic DNA revealed a rearrangement breakpoint at position 886 of a new H4 intron located downstream of the 1266 coding region. Furthermore, as a consequence of the activation of a cryptic splicing site, 132 nucleotides of this intron were spliced between the H4 and RET genes. Sequence analysis of the new chimera showed that the original frames of H4 and RET were joint with the intronic sequence without disruption of the open reading frame (ORF). Moreover, the genomic DNA of this case showed transforming activity in the DNA-mediated transfection assay using NIH-3T3 cells. In conclusion, we describe here the first variant of RET/PTC-1 oncogene, which we have termed 'long'-PTC-1, characterized by new breakpoints of both genes involved in the rearrangement and having transforming activity. Similar to previously reported PTC-3 variants, long-PTC-1 has been found in a post-Chernobyl papillary thyroid carcinoma confirming that RET/PTC rearrangements other than the classical forms (RET/PTC-1 and -3) are specifically associated with radiation-induced papillary thyroid cancer.

High prevalence of RET/PTC rearrangements in Ukrainian and Belarussian post-Chernobyl thyroid papillary carcinomas: a strong correlation between RET/PTC3 and the solid-follicular variant

A sharp increase in the incidence of pediatric thyroid papillary cancer was documented after the Chernobyl power plant explosion. An increased prevalence of rearrangements of the RET protooncogene (RET/PTC rearrangements) has been reported in Belarussian post-Chernobyl papillary carcinomas arising between 1990 and 1995. We analyzed 67 post-Chernobyl pediatric papillary carcinomas arising in 1995-1997 for RET/PTC activation: 28 were from Ukraine and 39 were from Belarus. The study, conducted by a combined immunohistochemistry and RT-PCR approach, demonstrated a high frequency (60.7% of the Ukrainian and 51.3% of the Belarussian cases) of RET/PTC activation. A strong correlation was observed between the solid-follicular subtype of papillary carcinoma and the RET/PTC3 isoform: 19 of the 24 RET/PTC-positive solid-follicular carcinomas harbored a RET/PTC3 rearrangement, whereas only 5 had a RET/PTC1 rearrangement. Taken together these results support the concept that RET/PTC activation plays a central role in the pathogenesis of thyroid papillary carcinomas in both Ukraine and Belarus after the Chernobyl accident.

The transcription coactivator HTIF1 and a related protein are fused to the RET receptor tyrosine kinase in childhood papillary thyroid carcinomas

Children exposed to radioactive iodine as a consequence of the Chernobyl reactor accident have an increased risk of papillary thyroid carcinomas (PTC). The predominant molecular lesions in these tumors are rearrangements of the RET receptor tyrosine kinase (tk). Here we report on two novel types of RET rearrangement, PTC6 and 7, and describe the fusion products and the ret fused gene (rfg) proteins. Like the other rfg proteins identified so far they are ubiquitously expressed, not membrane-bound and contain coiled coil domains required for constitutive activation of the ret tk domain. In the PTC6 rearrangement the ret tk domain is fused to the aminoterminal part of the human transcription intermediary factor htif 1. In the PTC7 rearrangement the ret tk domain is fused to a novel protein that is strongly related to htif1. Like htif1 it contains a RBCC motif (ring finger, B boxes, coiled coil domain) located in the aminoterminal part and a phd finger and a bromodomain in the carboxyterminal part. Htif1 and related proteins are transcription coactivators for nuclear receptors, thus participating in controlling cellular development, differentiation and homeostasis. This is the first report on their involvement in human thyroid carcinogenesis.

Different mutations of the RET gene cause different human tumoral diseases

The RET gene encodes a tyrosine kinase receptor for neurotrophic molecules. RET is a conceptually valuable example of how different mutations of a single gene may cause different diseases. Gene rearrangements activate the oncogenic potential of RET in human thyroid papillary carcinomas. On the other side, different point mutations activate RET in familial multiple endocrine neoplasia syndromes. Finally, inactivating mutations of RET can be present in Hirschsprung's disease patients. The detailed knowledge of the specific RET mutations responsible for human tumors provides relevant tools for the clinical management of these diseases. Moreover, the recent discovery of the growth factors which in vivo stimulate its signaling may shed new light on the role played by RET in the development and differentiation of the central and peripheral nervous system.

NTRK1 re-arrangement in papillary thyroid carcinomas of children after the Chernobyl reactor accident

The prevalence of NTRK1 re-arrangement was determined in papillary thyroid carcinomas (PTCs) of children from Belarus who had been exposed to radioactive iodine after the Chernobyl reactor accident; 81 tumors were included, all of which were devoid of RET re-arrangement as analyzed in a current study on genomic alterations in PTC. Oncogenic fusion of the NTRK1 tyrosine kinase domain with the amino-terminal part of the tropomyosin gene (TPM3/NTRK1, trk) was observed in 5 tumors. A single tumor exhibited a TPR/NTRK1 fusion (TRK-T2). Reciprocal NTRK1/TPM3 transcripts were found in 4 of 5 tumors with TPM3/NTRK1 re-arrangement, indicating an intra-chromosomal balanced reciprocal inversion. No phenotypic differences from other post-Chernobyl childhood PTCs were detected. As compared with the high prevalence of RET re-arrangements reported for thyroid carcinomas of children after the Chernobyl reactor accident, NTRK1 re-arrangements appear rare. Our results confirm that activation of receptor tyrosine kinase genes plays the predominant role in post-Chernobyl childhood thyroid carcinogenesis.

Molecular genetics of childhood papillary thyroid carcinomas after irradiation: high prevalence of RET rearrangement

Epidemiological studies have revealed a connection between thyroid carcinogenesis and a history of radiation. The molecular mechanisms involved are not well understood. It has been claimed that RAS, p53 or GSP mutations and RET or TRK rearrangements might play a role in adult thyroid tumors. In childhood, the thyroid gland is particularly sensitive to ionizing radiation. The reactor accident in Chernobyl provided a unique chance to study molecular genetic aberrations in a cohort of children who developed papillary thyroid carcinomas after a short latency time after exposure to high doses of radioactive iodine isotopes. According to the concepts of molecular genetic epidemiology, exposure to a specific type of irradiation might result in a typical molecular lesion. Childhood papillary thyroid tumors after Chernobyl exhibit a high prevalence of RET rearrangement as almost the only molecular alteration. The majority showed RET/PTC3 (i.e., ELE/RET rearrangements), including several subtypes. Less frequently, RET/PTC1 (i.e., H4/RET rearrangements), and a novel type (RET/PTC5, i.e., RFG5/RET) were observed. Proof of reciprocal transcripts suggests that a balanced intrachromosomal inversion leads to this rearrangement. Breakpoint analyses revealed short homologous nucleotide stretches at the fusion points. In all types of rearrangement, the RET tyrosine kinase domain becomes controlled by 5' fused regulatory sequences of ubiquitously expressed genes that display coiled-coil regions with dimerization potential. Oncogenic activation of RET is apparently due to ligand-independent constitutive ectopic RET tyrosine kinase activity. The analysis of this cohort of children with radiation-induced thyroid tumors after Chernobyl provides insights into typical molecular aberrations in relation to a specific mode of environmental exposure and may serve as a paradigm for molecular genetic epidemiology.

Low prevalence of the ret/PTC3r1 rearrangement in a series of papillary thyroid carcinomas presenting in Belarus ten years post-Chernobyl

After the Chernobyl accident in 1986, there was a significant increase in the incidence of papillary thyroid cancer in fallout-exposed children from Belarus. Radiation-induced rearrangements of chromosome 10 involving the c-ret proto-oncogene have been implicated in the pathogenesis of these cancers. The ret/PTC3r1 rearrangement was the most prevalent molecular lesion identified in post-Chernobyl papillary thyroid cancers arising in 1991 and 1992. We identified the ret/PTC1 rearrangement in 29% of 31 papillary thyroid cancers presenting in Belarus in 1996. In the present report, we examined 14 cases from this series (plus 1 additional case) and found a ret/PTC3r1 rearrangement in only 1 (7%). The prevalence of ret/PTC3r1 in this series is significantly lower than previously reported (p = 0.0006, Fisher exact test). This result suggests a switch in the ratio of ret/PTC3 to ret/PTC1 rearrangements in late (1996) versus early (1991-1992) post-Chernobyl papillary thyroid cancers.