Correlation between genetic alterations and microscopic features, clinical manifestations, and prognostic characteristics of thyroid papillary carcinomas

Incidentally simultaneous occurrence of RET/PTC, H4-PTEN and BRAF mutation in papillary thyroid carcinoma

Because interaction existed between PTEN and RET-RAS-RAF-MAPK pathway, H4-PTEN (a newly identified gene rearrangement), RET/PTC and BRAF mutation were scanned in 125 Chinese patients with papillary thyroid carcinoma (PTC). H4-PTEN were detected in 9.6% of PTC and the frequency of the occurrence of BRAF mutation and/or RET/PTC in H4-PTEN positive tumors was extremely high (75%). On the other hand, age has an important effect on the aberration formation and young age renders more prone to multi-genetic events. A combinational scanning of these involved changes will improve the predictive value of molecular aberrations in the treatment of PTC.

Molecular genetics of thyroid cancer: implications for diagnosis, treatment and prognosis

Thyroid cancer is the most common malignant tumor of the endocrine system and accounts for approximately 1% of all newly diagnosed cancer cases. The most frequent type of thyroid malignancy is papillary carcinoma, which constitutes approximately 80% of all cases. Papillary carcinomas frequently have genetic alterations leading to the activation of the MAPK signal pathway. Those include RET/PTC rearrangement and point mutations of the BRAF and RAS genes. Mutations in these genes are found in over 70% of papillary carcinomas and they rarely overlap in the same tumor. Frequent genetic alterations in follicular carcinomas, the second most common type of thyroid malignancy, include RAS mutations and PAX8-PPAR gamma rearrangement. RET point mutations are crucial for the development of medullary thyroid carcinomas. Many of these mutations, particularly those leading to the activation of the MAPK pathway, are being actively explored as therapeutic targets for thyroid cancer. Detection of these genetic alterations using molecular techniques is important for preoperative fine-needle aspiration diagnosis, prognosis and treatment of thyroid cancer.

Controversies in thyroid pathology: the diagnosis of follicular neoplasms

Thyroid nodules are common; almost 20% of the population has a palpable thyroid nodule and approximately 70% has a nodule detected by ultrasound. Thyroid cancer is the most frequent endocrine malignancy, and incidence rates have steadily increased over the last decades. Papillary carcinoma (PTC) is the most common malignant neoplasm of the thyroid; the diagnosis of this most frequent type (85-90%) has been increasing, possibly due to changing recognition of morphologic criteria. PTC is defined histologically as a malignant tumor showing evidence of follicular epithelial differentiation and characterized by distinctive nuclear features. However, there are borderline lesions that do not completely fulfill these criteria, making the diagnosis difficult. The use of immunohistochemical and molecular markers adds objective criteria to this confusing and controversial area of pathology. We review the differential diagnosis of well-differentiated follicular thyroid neoplasms and the ancillary techniques and molecular characteristics that have been proposed for application in the diagnosis of PTC.

BRAF mutation in papillary thyroid cancer: pathogenic role, molecular bases, and clinical implications

In recent years, the T1799A B-type Raf kinase (BRAF) mutation in thyroid cancer has received enthusiastic investigation, and significant progress has been made toward understanding its tumorigenic role and clinical significance. Among various thyroid tumors, this mutation occurs uniquely in papillary thyroid cancer (PTC), the most common endocrine malignancy, and some apparently PTC-derived anaplastic thyroid cancers. Many studies have found this mutation to be associated with those clinicopathological characteristics of PTC that are conventionally known to predict tumor progression and recurrence, including, for example, old patient age, extrathyroidal invasion, lymph node metastasis, and advanced tumor stages. Direct association of BRAF mutation with the clinical progression, recurrence, and treatment failure of PTC has also been demonstrated. The BRAF mutation has even been correlated with PTC recurrence in patients with conventionally low-risk clinicopathological factors. Some molecular mechanisms determining BRAF mutation-promoted progression and the aggressiveness of PTC have recently been uncovered. These include the down-regulation of major tumor suppressor genes and thyroid iodide-metabolizing genes and the up-regulation of cancer-promoting molecules, such as vascular endothelial growth factor, matrix metalloproteinases, nuclear transcription factor kappaB, and c-Met. Thus, BRAF mutation represents a novel indicator of the progression and aggressiveness of PTC. Significant advances have also occurred in the preclinical testing of new therapeutic strategies targeting the MAPK pathway aberrantly activated by BRAF mutation and other related mutations. New mitogen extracellular kinase (MEK) inhibitors developed recently are particularly promising therapeutic agents for thyroid cancer. With these advances, it has become clearer that BRAF mutation will likely have significant impact on the clinical management of PTC.

Selective growth inhibition in BRAF mutant thyroid cancer by the mitogen-activated protein kinase kinase 1/2 inhibitor AZD6244

CONTEXT

Activating mutations in the BRAF gene, primarily at V600E, are associated with poorer outcomes in patients with papillary thyroid cancer. MAPK kinase (MEK), immediately downstream of BRAF, is a promising target for ras-raf-MEK-ERK pathway inhibition.

OBJECTIVE

The objective of the investigation was to study the efficacy of a MEK1/2 inhibitor in thyroid cancer preclinical models with defined BRAF mutation status.

EXPERIMENTAL DESIGN

After treatment with the potent MEK 1/2 inhibitor AZD6244, MEK inhibition and cell growth were examined in four BRAF mutant (V600E) and two BRAF wild-type thyroid cancer cell lines and in xenografts from a BRAF mutant cell line.

RESULTS

AZD6244 potently inhibited MEK 1/2 activity in thyroid cancer cell lines regardless of BRAF mutation status, as evidenced by reduced ERK phosphorylation. Four BRAF mutant lines exhibited growth inhibition at low doses of the drug, with GI50 concentrations ranging from 14 to 50 nm, predominantly via a G0/G1 arrest, comparable with findings in a sensitive BRAF mutant melanoma cell line. In contrast, two BRAF wild-type lines were significantly less sensitive, with GI50 values greater than 200 nm. Nude mouse xenograft tumors derived from the BRAF mutant line ARO exhibited dose-dependent growth inhibition by AZD6244, with effective treatment at 10 mg/kg by oral gavage. This effect was primarily cytostatic and associated with marked inhibition of ERK phosphorylation.

CONCLUSION

AZD6244 inhibits the MEK-ERK pathway across a spectrum of thyroid cancer cells. MEK inhibition is cytostatic in papillary thyroid cancer and anaplastic thyroid cancer cells bearing a BRAF mutation and may have less impact on thyroid cancer cells lacking this mutation.

High frequency of level II-V lymph node involvement in RET/PTC positive papillary thyroid carcinoma

AIMS

To investigate the frequency and clinical significance of RET rearrangement in Chinese patients with papillary thyroid carcinoma (PTC) and discuss the role of RET rearrangement in therapeutic decision-making after the performance of level VI lymph node dissection and the 2002 AJCC staging system.

METHODS

RET/PTC-1 and RET/PTC-3 were detected in 126 PTCs using reverse transcription-polymerase chain reaction (RT-PCR) and direct sequencing.

RESULTS

RET rearrangement was detected in 18 cases of PTC. The patient group aged < 20 years had the highest frequency (3/6) of RET rearrangement among the age groups (< 20 years, 20-40 years and > or = 40 years; P=0.03). RET/PTC-1 positive patients were more likely to suffer from Hashimoto's thyroiditis simultaneously (P=0.02) while RET/PTC-3 positive patients had a higher frequency of extrathyroidal extension (P<0.01) and advanced T classification (P<0.01). RET rearrangement (OR=8.70, 95% CI 1.69-44.81), male (OR=3.88, 95% CI 1.41-10.69), age (OR=0.96, 95% CI 0.93-0.99), multifocality (OR=3.54, 95% CI 1.33-9.41) and advanced T classification (OR=7.32, 95% CI 2.91-18.40) were all identified as risk factors of level II-V lymph node involvement in the multivariate analysis.

CONCLUSIONS

The frequency of RET rearrangement in Chinese patients is low and age related. RET/PTC-1 and RET/PTC-3 are associated with different clinical pathological characteristics but not with lymph node involvement. The RET/PTC positive patients should receive more attention to lateral neck in the management of PTC.

Anaplastic carcinoma of the thyroid arising more often from follicular carcinoma than papillary carcinoma

BACKGROUND

Anaplastic thyroid carcinoma (ATC), a rare and highly malignant tumor, has long been thought to arise from well-differentiated carcinoma (WDC) such as follicular thyroid carcinoma (FTC) and papillary thyroid carcinoma (PTC). The purpose of this study was to test this notion by examining whether and, if so, how often ATC harbors the oncogenes that are commonly associated with WDC, such as RAS in FTC and BRAF in PTC.

METHODS

We analyzed the mutation hotspots of BRAF (codon 600) and N-, K-, and H-RAS (codons 12, 13, and 61) in 16 ATCs. We also examined two genes, PIK3CA (exons 9 and 20) and TP53 (exons 5-9), both of which have been reported in ATCs.

RESULTS

The results showed that approximately 31% (5 of 16) of ATCs harbored N-RAS mutation, 6% (1 of 16) had mutated BRAF, and approximately 56% (9 of 16) had mutated TP53. As to the three ATCs that had coexisted PTCs, mutated BRAF was detected in all PTC components but only in one ATC, while mutated PIK3CA was found in only one PTC component but not in the ATC.

CONCLUSION

A number of ATCs arise from WDCs, more often from RAS-mutant tumors than from BRAF-mutant tumors, implying that particular attention should be paid to the WDC harboring RAS mutation.

BRAF mutations in melanocytic lesions and papillary thyroid carcinoma samples identified using melting curve analysis of polymerase chain reaction products

CONTEXT

Mutations of the proto-oncogene B-raf (BRAF) have been detected in melanocytic lesions and papillary carcinomas of the thyroid, and identification of these mutations could be useful in resolving some diagnostic problems.

OBJECTIVE

To develop a method to evaluate mutations of BRAF that could provide results much more rapidly than conventional polymerase chain reaction and DNA sequencing assays.

DESIGN

An assay using a LightCycler was developed to evaluate DNA sequences encoding amino acids within the activation loop of BRAF.

RESULTS

Using this real-time polymerase chain reaction method, we analyzed 55 paraffin-embedded melanoma or nevus samples. The V600E mutation was found in 0 (0%) of 13 samples diagnosed histologically as Spitz nevi, 9 (24.3%) of 37 invasive melanomas, and 5 (100%) of 5 other melanocytic nevi. Two additional mutations, V600K and VK600-1E, also were identified in cases of invasive melanoma. We analyzed 14 paraffin-embedded papillary thyroid cancer (PTC) samples, 6 of which showed the V600E mutation. We found that our test worked efficiently with fine-needle aspirate specimens, and it identified 6 V600E mutations in 10 fine-needle aspirate specimens diagnosed as PTC. We also identified 4 V600E mutations in 6 specimens of PTC metastatic to lymph node. Unlike the melanocytic lesions, the PTC specimens yielded only V600E mutations. Comparison of our real-time polymerase chain reaction results with conventional polymerase chain reaction and DNA sequencing demonstrated 100% concordance. Surprisingly, we did not identify the previously reported VK600-1E or K601E mutations in our PTC specimens.

CONCLUSIONS

Our results show that the real-time polymerase chain reaction method is a rapid and accurate method for identifying BRAF mutations, such as V600E, in both paraffin-embedded tissue and fine-needle aspirate specimens.

The heterogeneous distribution of BRAF mutation supports the independent clonal origin of distinct tumor foci in multifocal papillary thyroid carcinoma

CONTEXT

Papillary thyroid carcinoma (PTC) is frequently multifocal. Independent PTC foci may occur either from intraglandular metastases from a single dominant tumor or as unrelated neoplastic clones. In rare cases, the simultaneous presence of PTC foci of different histopathological subtypes points to independent sites of tumor formation.

OBJECTIVES

We examined the pattern of BRAF mutations in noncontiguous tumor foci and node metastases from 69 patients affected by multicentric PTC. These included 19 cases characterized by the simultaneous presence of different PTC histopathological variants.

DESIGN

BRAF (exon 15) mutation was examined by PCR-single strand conformational polymorphism followed by DNA sequencing in laser-capture microdissected tissue samples.

RESULTS

Discordant patterns of BRAF mutation were found in about 40% of the multifocal PTCs. In node metastases, BRAF mutations were, in most but not all the cases, concordant with the dominant tumor. A discordant pattern of BRAF mutation was also found in about 50% of the cases in which multiple foci of different histopathological variants were present.

CONCLUSIONS

The heterogeneous distribution of BRAF mutations suggests that discrete tumor foci in multifocal PTC may occur as independent tumors. This information has to be considered in the design of targeted therapeutic approaches with BRAF pathway inhibitors.

BRAF in Papillary Thyroid Carcinoma of Ovary (Struma Ovarii)

BACKGROUND

Malignant struma ovarii (MSO) are rare tumors that arise from ectopic thyroid tissue in the ovary, benign struma ovarii (BSO). Most MSO are histologically classified as papillary thyroid carcinomas (PTC). Oncogenic activation of BRAF (35% to 69%), RAS (10%), or RET (5% to 30%) is common in PTC, and the mutations correlate with tumor subtype, patient age, and clinical behavior. In this study, we explored the possible role of these genes in the development of BSO and MSO.

DESIGN

Six paraffin-embedded cases of MSO with histopathologic features of PTC (4 follicular variants, 1 classic, and 1 metastasis of a classic) and 9 BSO were identified. BRAF, NRAS, and KRAS mutations were evaluated using a combination of polymerase chain reaction, denaturing high performance liquid chromatography, and automated sequencing. RET alterations were screened by fluorescence in situ hybridization with multicolor probes. Corresponding benign tissues were evaluated when available.

RESULTS

BRAF mutations were present in 4 of 6 MSO and none of 9 BSO. The BRAF mutations included V600E (2 cases), K601E, and a novel deletion/substitution TV599-600M. Neither MSO nor BSO contained alterations in NRAS, KRAS, or RET.

CONCLUSIONS

The development of MSOs with PTC features is associated with BRAF mutations of the type commonly observed in PTC, suggesting a common pathogenesis for all PTCs regardless of location. In contrast, mutations in the RET/RAS/RAF pathway are not found in BSO. The prognostic significance of BRAF mutation status in MSO remains to be determined.

Suppression of BRAF/MEK/MAP kinase pathway restores expression of iodide-metabolizing genes in thyroid cells expressing the V600E BRAF mutant

PURPOSE

The V600E BRAF mutant plays an important role in the pathogenesis of papillary thyroid cancer (PTC) and is associated with loss of expression of thyroid iodide-metabolizing genes. This study was done to investigate the restorability of expression of these genes by suppressing the BRAF/extracellular signal-regulated kinase kinase (MEK)/mitogen-activated protein (MAP) kinase pathway in V600E BRAF-harboring thyroid cells and to explore the mechanisms involved.

EXPERIMENTAL DESIGN

We used inducible expression of V600E BRAF, small interfering RNA transfection, and MEK-specific inhibitor to alter the MAP kinase pathway activities and subsequently examined the changes in expression, promoter activities, and methylation status of thyroid genes.

RESULTS

MEK inhibitor U0126 or cessation of V600E BRAF expression in PCCL3 cells restored expression of thyroid genes silenced by induced expression of V600E BRAF. U0126 also restored the expression of these genes in V600E BRAF-harboring PTC-derived NPA cells. Knockdown of BRAF by specific small interfering RNA restored expression of some of these genes in NPA cells. Luciferase reporter assay using thyroid-stimulating hormone receptor gene as a model showed that the promoter activity was modulated by the MAP kinase pathway. Promoter methylation in association with DNA methyltransferase expression played a role in gene silencing by MAP kinase pathway in NPA cells.

CONCLUSIONS

We showed the restorability of expression of thyroid iodide-metabolizing genes silenced by V600E BRAF, and linked this process to gene methylation in PTC cells. The results provide clinical implications that therapeutic targeting at the BRAF/MEK/MAP kinase pathway may be a good approach in restoring thyroid gene expression for effective radioiodine therapy for BRAF mutation-harboring PTC.

Diffuse sclerosing variant of papillary thyroid carcinoma: lack of BRAF mutation but occurrence of RET/PTC rearrangements

Diffuse sclerosing variant of papillary thyroid carcinoma (PTC) is a rare tumour with a characteristic morphology as well as a strong preponderance for younger female patients. The T1799A missense mutation in exon 15 of the BRAF gene and RET/PTC rearrangement have been identified as the dominant genetic tumour initiation events in the pathogenesis of PTC leading to a constitutive activation of the RAS-RAF-MAPK pathway. In order to elucidate the pathogenesis of diffuse sclerosing variant of PTC, the prevalence of BRAF mutation and RET/PTC were determined by RT-polymerase chain reaction and DNA-sequence analysis in tumour samples of seven patients with this variant (all female, age range 15-61 years, mean 33.3 years) without prior radiation exposure. None of these cases showed a BRAF mutation. RET/PTC1 (two out of seven) and RET/PTC3 (one out of seven), which have been shown in large PTC series to comprise together more than 90% of RET/PTC types, were found in <50% of the cases investigated. All seven samples expressed the RET tyrosine kinase domain but lacked its extracellular domain potentially suggesting the existence of rare types of RET/PTC rearrangement in the four remained cases of diffuse sclerosing variant of PTC. Regarding this subtype, our study confirmed the paradigm of a mutual exclusivity between RET/PTC and BRAF in PTC. Additionally, this rare variant of papillary thyroid carcinoma may represent a tumour type susceptible to RET-targeted therapies.

Clinical prognosis in BRAF-mutated PTC

BRAF mutation has recently emerged as a potential prognostic marker for papillary thyroid carcinoma (PTC) due to several studies suggesting that it may condition the development of tumors with aggressive behavior. A study of the phenotypes of thyroid follicular cell lines and transgenic mice characterized by targeted expression of BRAF mutation indicates that, at variance with RET/PTC rearrangement, it induces or facilitates genomic instability and higher invasiveness and eventually deeper tumor de-differentiation and more significant suppression of apoptosis. An analysis of differential gene expression of PTCs harboring BRAF mutation versus PTCs characterized by other genetic alterations shows an important impairment of the expression of genes related to intra-thyroidal iodine metabolism machinery, up-regulation of Glut-1 mRNA, methylation-induced gene silencing of tumor suppressor genes and up-regulation of pro-angiogenetic proteins such as VEGF. Correlation of BRAF mutation with PTC clinico-pathological features yields controversial results, with several studies showing the association with unfavourable clinico-pathological qualities, while others do not confirm the findings. This review will summarize the studies in favor of or in contrast with a role of BRAF mutation as a prognostic marker in PTC. We will also indicate what information we still need in order to routinely introduce this indicator in clinical practice.

Tall cell variant of papillary thyroid carcinoma without extrathyroid extension: biologic behavior and clinical implications

BACKGROUND

The tall cell variant (TCV) is a histologic subtype of papillary thyroid carcinoma (PTC) that is more aggressive than "classical" PTC. Most authors believe that TCV's worse prognosis is related to older age at presentation, larger tumor size, and high frequency of extrathyroid tumor extension (ETE). To assess the biologic and clinical behavior of TCV without ETE, we performed a detailed comparative clinicopathologic analysis of classical PTC and TCV without ETE.

METHODS

TCV was defined as a PTC harboring >50% tall cells, while classical PTC was restricted to those tumors containing >1% papillae and <30% tall cells. Microscopic analysis and chart review identified 62 cases of TCV and 83 classical PTC without ETE. These patients were analyzed for various pathologic, imaging, and clinical parameters including outcome.

RESULTS

There was no statistical difference between TCV and classical PTC in relation to age, gender, tumor size, risk stratification, type of therapy, and length of follow-up. TCV displayed more invasion of the tumor capsule and more often infiltrated into the thyroid capsule (p = 0.047 and 0.0004, respectively). Among patients with microscopically assessable regional lymph node (LN), 33 of 49 (67.3%) patients with TCV had LN metastasis at presentation, while only 24 of 60 (40%) classical PTC had positive nodes (p = 0.004). In multivariate analysis, histologic subtype (TCV vs. classical PTC) was the only independent factor associated with LN metastases (p = 0.007). In patients with adequate follow-up, 4 of 62 (6.5%) classical PTC and 7 of the 47 (14.9%) TCV had thyroid cancer recurrence (p = 0.202). TCV recurred at a distant site (3 of 47, 6.4%) while none of the 62 classical PTC developed distant metastases (p = 0.077).

CONCLUSION

TCV without ETE is biologically a more aggressive tumor than classical PTC without ETE independent of age, gender, and tumor size.

BRAF V600E maintains proliferation, transformation, and tumorigenicity of BRAF-mutant papillary thyroid cancer cells

CONTEXT

Although the BRAF V600E mutant can initiate the formation of papillary thyroid cancer (PTC), it is unclear whether it is required to maintain cell proliferation, transformation, and tumor growth of BRAF mutation-harboring PTC.

OBJECTIVE

The aim of the study was to investigate whether BRAF V600E is required for the proliferation, transformation, and tumorigenicity of BRAF mutation-harboring PTC cells.

DESIGN

We addressed this issue using BRAF small interference RNA (siRNA) to transfect stably several BRAF mutation-harboring PTC cell lines, isolated clones with stable suppression of BRAF, and assessed their ability to proliferate, transform, and grow xenograft tumors in nude mice.

RESULTS

PTC cell proliferation and transformation were suppressed in specific BRAF siRNA clones, but not in control scrambled siRNA clones. Specifically, taking the advantage of stable BRAF knockdown, we were able to show continued suppression of PTC cell proliferation and transformation, or anchorage-independent colony formation in soft agar, after long-term culture. Moreover, we also demonstrated that in vivo tumorigenicity and growth of tumors from the specific BRAF siRNA cell clones in nude mice were suppressed compared with control clones.

CONCLUSIONS

BRAF V600E is not only an initiator of PTC as demonstrated previously but is also a maintainer of proliferation, transformation, and tumorigenicity of PTC cells harboring BRAF mutation, and growth of tumors derived from such cells continues to depend on BRAF V600E. These results provide further support for potentially effective therapy targeted at BRAF for BRAF mutation-harboring PTC.

Perspectives and limitations of microarray-based gene expression profiling of thyroid tumors

Microarray technology has become a powerful tool to analyze the gene expression of tens of thousands of genes simultaneously. Microarray-based gene expression profiles are available for malignant thyroid tumors (i.e., follicular thyroid carcinoma, and papillary thyroid carcinoma), and for benign thyroid tumors (such as autonomously functioning thyroid nodules and cold thyroid nodules). In general, the two main foci of microarray investigations are improved understanding of the pathophysiology/molecular etiology of thyroid neoplasia and the detection of genetic markers that could improve the differential diagnosis of thyroid tumors. Their results revealed new features, not known from one-gene studies. Simultaneously, the increasing number of microarray analyses of different thyroid pathologies raises the demand to efficiently compare the data. However, the use of different microarray platforms complicates cross-analysis. In addition, there are other important differences between these studies: 1) some studies use intraindividual comparisons, whereas other studies perform interindividual comparisons; 2) the reference tissue is defined as strictly nonnodular healthy tissue or also contains benign lesions such as goiter, follicular adenoma, and hyperplastic nodules in some studies; and 3) the widely used Affymetrix GeneChip platform comprises several GeneChip generations that are only partially compatible. Moreover, the different studies are characterized by strong differences in data analysis methods, which vary from simple empiric filters to sophisticated statistic algorithms. Therefore, this review summarizes and compares the different published reports in the context of their study design. It also illustrates perspectives and solutions for data set integration and meta-analysis, as well as the possibilities to combine array analysis with other genetic approaches.

RET/PTC rearrangements and BRAF mutations in thyroid tumorigenesis

Thyroid papillary carcinoma is the most common type of endocrine cancer. It is frequently associated with genetic alterations leading to activation of the MAPK signaling pathway. The two most frequently affected genes, BRAF and RET, are activated by either point mutation or as a result of chromosomal rearrangement. These mutations are tumorigenic in thyroid follicular cells and correlate with specific phonotypical features and biological properties of papillary carcinomas, including tumor aggressiveness and response to radioiodine therapy. Molecular inhibitors that block RET/PTC or BRAF kinase activity have shown substantial therapeutic effects in the experimental systems and are currently being tested in clinical trials.

Controversies in the surveillance of patients with well differentiated thyroid cancer

PURPOSE OF REVIEW

Thyroid cancer incidence is rising in the USA, likely due to increased surveillance. Over the past several years, new approaches have developed for the long-term monitoring of differentiated thyroid cancer patients. The most recent developments in this area, as well as interesting data on molecular genetics of thyroid cancer will be discussed.

RECENT FINDINGS

The introduction of 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) imaging in the follow-up of patients with differentiated thyroid cancer is an important development in the last few years. Recent research has helped improve understanding of how 18F-FDG PET can best be applied. There is now improved understanding of the use of serum thyroglobulin levels to predict future risk of recurrence of differentiated thyroid cancer. Guidelines from the American Thyroid Association for management of thyroid neoplasia were updated in 2006 for the first time in a decade, helping physicians navigate the published data and provide evidence-based care. Ongoing advances in thyroid cancer genetics may help predict aggressiveness of individual thyroid cancers.

SUMMARY

Better tools are becoming available for physicians caring for thyroid cancer patients. Recent developments in thyroid cancer research will help physicians better anticipate a patient's future disease course and select appropriate surveillance testing.

Targeting BRAF in thyroid cancer

Activating mutations in the gene encoding BRAF are the most commonly identified oncogenic abnormalities in papillary thyroid cancer. In vitro and in vivo models have demonstrated that overexpression of activated BRAF induces malignant transformation and aggressive tumour behaviour. BRAF and other RAF kinases are frequently activated by other thyroid oncogenes and are important mediators of their biological effects including dedifferentiation and proliferation. Because current therapeutic options for patients with thyroid cancers that are aggressive and/or do not respond to standard therapies are limited, BRAF and its downstream effectors represent attractive therapeutic targets. In this review, data supporting a role for BRAF activation in thyroid cancer development and establishing the potential therapeutic efficacy of BRAF-targeted agents in patients with thyroid cancer will be reviewed.

Pathology and genetics of thyroid carcinoma

Carcinomas of the thyroid comprise a heterogeneous group of neoplasms with distinctive clinical and pathological characteristics. Over the past 15 years, the application of molecular technologies to the study of these neoplasms has elucidated critical genetic pathways associated with the development of specific thyroid tumor types. In papillary thyroid carcinoma (PTC), genetic events involve RET and TRK (rearrangements) and BRAF and RAS (mutations), although RAS mutations are uncommon except in the follicular variant of PTC. These genetic alterations, which rarely overlap in the same tumor, result in signaling abnormalities in the mitogen-activated protein kinase pathway. In contrast, genetic alterations in follicular carcinomas include PAX8-PPARgamma translocations and RAS mutations while mutations of CTNNB1 and p53 have been implicated in the development and progression of poorly differentiated and undifferentiated (anaplastic) thyroid carcinomas. Germline mutations of RET are responsible for the development of heritable forms of medullary thyroid carcinoma (MTC) while somatic mutations of this oncogene are found in a significant proportion of sporadic MTCs. The results of these studies not only have provided additional approaches to thyroid tumor classification, but also have stimulated the development of novel approaches to tumor diagnosis and additional parameters for prognostic assessment and potential biologic therapeutic strategies.

Genetic considerations in thyroid cancer

BACKGROUND

Recent molecular studies have described a number of abnormalities associated with the progression and dedifferentiation of thyroid carcinoma. These distinct molecular events are often associated with specific stages of tumor development. A better understanding of the mechanisms involved in thyroid cancer pathogenesis may help to translate these discoveries toward improvements in patient care.

METHODS

We reviewed the literature on the molecular pathogenesis of thyroid cancer and compared clinical, histopathologic, and genetic features important in defining the disease process.

RESULTS

The progression of thyroid cancer from well-differentiated to poorly differentiated and undifferentiated carcinoma represents a biological continuum. Specific genetic events serve as early initiating and late triggering events. Poorly differentiated thyroid carcinomas occupy an intermediate position in this progression model.

CONCLUSIONS

With sophisticated genetic tools generating a wealth of information, we have gained better insight into the mechanisms driving thyroid tumor progression. Recognition of these features is crucial to the management of patients with thyroid cancer. Novel treatments are being designed based on our enhanced understanding of this disease process.

No evidence of ARAF, CRAF and MET mutations in BRAFT1799A negative human papillary thyroid carcinoma

The MAPK signaling pathway plays a crucial role in tumorgenesis and cell proliferation in human papillary thyroid carcinoma (PTC). Ret/PTC rearrangements, RAS and BRAF mutations, the main non-overlapping genetic alterations all leading to MAPK cascade activation, are cumulatively identified in 60-80% of PTCs. In approximately one-fourth of the cases, oncogenic background potentially contributing to MAPK activation in PTC might be different. We therefore attempted to evaluate the mutational status of genes encoding other members of RAF family known to act upstream of MAPKs, ARAF and CRAF (RAF-1). In addition we also analyzed the MET gene that encodes hepatocyte growth factor/scatter factor receptor overexpressed in most of PTCs and a MAPK cascade contributor. In 129 Japanese patients with PTC, BRAF(T1799A) was detected in 65 cases (50.4%), and the remaining 64 tumor specimens were subjected to mutation analysis of kinase domains of ARAF, CRAF and MET genes, and hotspots of K- and N-RAS genes. No ARAF, CRAF, MET, K- and N-RAS mutations were revealed. Based on these observations, we concluded that despite the fact that ARAF, CRAF and MET are actively expressed, alterations of these genes are rare in PTC and unlikely to play a perceptible role in the molecular pathogenesis of this type of human malignancy.

Assessing the utility of a mutational assay for B-RAF as an adjunct to conventional fine needle aspiration of the thyroid gland

Thyroid carcinoma is the most common endocrine malignancy; it is typified by a number of classical genomic insults, which tend to cluster with the discrete histologic subtypes. The most common of these is a mutation in B-RAF, which is present in approximately 44% (29% to 83%) of cases. In this review we have assessed the potential utility of a molecular test for somatically acquired mutations in B-RAF using thyroid malignancy as a model system according to 3 fundamental questions: would a test enhance our ability to distinguish benign from malignant, would a test unveil a risk factor not otherwise known, and would detecting a mutation enable a therapeutic option specific to those patients who carry the mutation?

Different structural components of conventional papillary thyroid carcinoma display mostly identicalBRAF status

Activating BRAF(T1799A) mutation is closely associated with a papillary thyroid carcinoma (PTC) histotype. The transversion is frequently detected in the conventional type, Warthin-like and tall cell variants, but is rare in the follicular variant of PTC. Conventional PTC is often presented with tumors of mixed architecture, which besides the papillary structures also contain areas with follicular and solid morphology in which the details of BRAF mutational status are unknown. We set out to differentially investigate the presence of mutated BRAF in the individual structural components microdissected from 44 formalin-fixed, paraffin-embedded PTC tissues from 40 patients. The mutation was detected in at least 1 structural component in 23 tumors (52%). Different structural components of the same tumor had identical BRAF status in 41/44 tumors (93%). In 3 tumors the BRAF(T1799A) mutation was found only in the papillary, but not in the follicular component. Mutational patterns identical to those in the primary tumors were found in 11/12 lymph node metastases (92%, including both BRAF(T1799A)-positive and -negative cases). The high concordance of the BRAF mutational status in structurally distinct areas suggests a rather homogeneous distribution of neoplastic epithelial cells in a conventional PTC tumor in most cases. These results imply the reliability of preoperative molecular diagnosis of PTC regardless of the type of tumor component at the site of biopsy sampling and suggest that the majority of patients with BRAF mutation-positive PTC may benefit from the targeted pharmacotherapy.