Biology of Ras in thyroid cells

The significance of gene mutations across eight major cancer types

Mutations occur spontaneously, which can be induced by either chemicals (e.g. benzene) or biological factors (e.g. virus). Not all mutations cause noticeable changes in cellular functions. However, mutation in key cellular genes leads to developmental disorders. It is one of the main ways in which proto-oncogenes can be changed into their oncogenic state. The progressive accumulation of multiple mutations throughout life leads to cancer. In the past few decades, extensive research on cancer biology has discovered many genes and pathways having role in cancer development. In this review, we tried to summarize the current knowledge of mutational effect on different cancer types and its consequences in brief for future reference and guidance of researchers in cancer biology.

The management of thyroid nodules and cancer in the molecular era

The incidence of thyroid cancer is increasing worldwide. Current standards in the diagnosis and management of thyroid cancer are limited by the uncertainty of fine-needle aspiration samples that are indeterminate in nature. Molecular markers have the potential to improve the accuracy of thyroid fine-needle aspiration and to aid the physician in giving a more accurate diagnosis and prognosis. This paper summarizes the various molecular markers currently available.

Treatment of advanced thyroid cancer: role of molecularly targeted therapies

Advanced thyroid cancer is not amenable to therapy with conventional cytotoxic chemotherapy. However, newer advances in the understanding of the molecular pathogenesis of different subtypes of thyroid cancer have provided new opportunities for the evaluation of molecularly targeted therapies. This has led to multiple clinical trials using various multi-kinase inhibitors and the subsequent US FDA approval of sorafenib for differentiated thyroid cancer and vandetanib and cabozantinib for medullary thyroid carcinoma. This review provides a summary of the current literature for the treatment of advanced thyroid carcinoma and future directions in this disease.

Targeting RAS-MAPK-ERK and PI3K-AKT-mTOR signal transduction pathways to chemosensitize anaplastic thyroid carcinoma

Anaplastic thyroid carcinoma (ATC) is a rare, but aggressive and chemoresistant tumor with dismal prognosis. Most ATCs harbor mutations that activate RAS/MAPK/ERK and PI3K/AKT/mTOR pathways. Therefore, we investigated and correlated the expression of phosphatase and tensin homolog, pERK, and pAKT proteins as well as mutations of BRAF, RAS, and p53 genes in samples of patients with ATC. Furthermore, we evaluated the potential of inhibition of these pathways on chemosensitization of ATC using 2 thyroid carcinoma cell lines (FRO and SW1736). Our results revealed a negative correlation between the activity of RAS-MAPK-ERK and PI3K-AKT-mTOR pathways in samples of patients. To be specific, the PI3K-AKT-mTOR pathway was suppressed in patients with activated NRAS or high pERK expression. In vitro results suggest that the inhibition of either RAS-MAPK-ERK or PI3K-AKT-mTOR components may confer sensitivity of thyroid cancer cells to classic chemotherapeutics. This may form a basis for the development of novel genetic-based therapeutic approach for this cancer type.

Genetic mutations, molecular markers and future directions in research

Recent molecular studies have described a number of abnormalities associated with the pathogenesis of thyroid carcinoma. These distinct molecular events are often associated with specific stages of tumor development and may serve as prognostic factors and therapeutic targets. A better understanding of the mechanisms involved in thyroid cancer pathogenesis, will hopefully help translate these discoveries to improved patient care.

What are the keys to successful thyroid FNA interpretation?

There is much concern expressed in the literature regarding the lack of predictive power of the thyroid fine needle aspiration/biopsy (TFNAB) approach to defining the nature of clinically detected thyroid nodules. This has been exacerbated in the past decade or more by the routine use of ultrasonography (US) in examining the thyroid as well as the introduction of molecular testing in the realm of thyroid pathology. Some have even gone so far as to suggest replacing the TFNAB with molecular signature testing to reduce the degree of uncertainty for a specific cytological diagnosis. This review addresses those concerns with a re-emphasis on understanding the basic keys to successfully evaluating a patient with a thyroid nodule by routine TFNAB examination. These keys include bringing to bear an experienced group of physicians in an integrated team approach, a sharpened focus on the TFNAB cytological categories and a grounded understanding of the predictive ability of molecular testing in a given patient when the cytological interpretation creates too much uncertainty in the minds of the patients and clinicians in attempting to reach a decision on how to manage a thyroid nodule. With this practical approach in mind, the false-negative and false-positive rates of "negative (benign)" and "positive (malignant)" thyroid aspirates should be no more than 1%; and the prevalence of an "indeterminate" aspirate - the area in TFNAB attracting the most attention for improvement with novel biomarkers - should be 10% or less. Thus, physicians should be capable of managing at least 90% of patients undergoing TFNAB in a confident manner without further testing beyond the routine, future re-examination of the patient's nodule to re-assess for any change in its nature or its impact on the patient's quality of life. The other 10% can then be considered for molecular testing in a manner tailored to those individuals truly in need of a more sophisticated - and expensive - approach to the characterization of their thyroid nodules. As the era of US has matured - just as with the era of serum PSA testing for prostate cancer in men - we have experienced a paradigm shift: Given the incidence of thyroid nodules in the U.S. adult population in comparison with the risk of dying of thyroid malignancy, the main role of TFNAB is to reduce the need for surgical intervention.

Recent advances in molecular diagnosis of thyroid cancer

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. In particular, remarkable advances have occurred in several major biological areas of thyroid cancer, including the molecular alterations for the loss of radioiodine avidity of thyroid cancer, the pathogenic role of the MAP kinase and PI3K/Akt pathways and their related genetic alterations, and the aberrant methylation of functionally important genes in thyroid tumorigenesis and pathogenesis. 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.

Downregulation of Rap1GAP contributes to Ras transformation

Although abundant in well-differentiated rat thyroid cells, Rap1GAP expression was extinguished in a subset of human thyroid tumor-derived cell lines. Intriguingly, Rap1GAP was downregulated selectively in tumor cell lines that had acquired a mesenchymal morphology. Restoring Rap1GAP expression to these cells inhibited cell migration and invasion, effects that were correlated with the inhibition of Rap1 and Rac1 activity. The reexpression of Rap1GAP also inhibited DNA synthesis and anchorage-independent proliferation. Conversely, eliminating Rap1GAP expression in rat thyroid cells induced a transient increase in cell number. Strikingly, Rap1GAP expression was abolished by Ras transformation. The downregulation of Rap1GAP by Ras required the activation of the Raf/MEK/extracellular signal-regulated kinase cascade and was correlated with the induction of mesenchymal morphology and migratory behavior. Remarkably, the acute expression of oncogenic Ras was sufficient to downregulate Rap1GAP expression in rat thyroid cells, identifying Rap1GAP as a novel target of oncogenic Ras. Collectively, these data implicate Rap1GAP as a putative tumor/invasion suppressor in the thyroid. In support of that notion, Rap1GAP was highly expressed in normal human thyroid cells and downregulated in primary thyroid tumors.

Mechanisms of Disease: molecular genetics of childhood thyroid cancers

Childhood thyroid cancers are uncommon and have a fairly good prognosis. Papillary adenocarcinoma is the most prevalent malignant tumor of the thyroid in children and adults with radiation-induced or sporadic cancer. The incidence of thyroid cancer in children increased dramatically in the territories affected by the Chernobyl nuclear accident; this increase is probably attributable to (131)I and other short-lived isotopes of iodine released into the environment. There was a broad range of latency periods in children who developed thyroid cancer; some periods were less than 5 years. The mutational spectrum of childhood thyroid cancers demonstrates that gene rearrangements that lead to the activation of mitogen-activated protein kinase signaling seem to have a pivotal role; point mutations are rare. So far none of the cancer genes or tumor suppressors, or a peculiar gene expression pattern, has been specifically implicated in radiation-induced thyroid carcinogenesis. The frequency of certain oncogenes does, however, vary in tumors that develop after different periods of latency. Such differences in the distribution of gene abnormalities in radiation-related cancers implies that they associate with patients' age at exposure and diagnosis, clinicopathological manifestations of disease and depend on an individual's genetic characteristics. Here we review results of pathological and molecular studies in childhood thyroid cancer.

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.

Ras triggers ataxia-telangiectasia-mutated and Rad-3-related activation and apoptosis through sustained mitogenic signaling

Genetic evidence indicates that Ras plays a critical role in the initiation and progression of human thyroid tumors. Paradoxically, acute expression of activated Ras in normal rat thyroid cells induced deregulated cell cycle progression and apoptosis. We investigated whether cell cycle progression was required for Ras-stimulated apoptosis. Ras increased CDK-2 activity following its introduction into quiescent cells. Apoptotic cells exhibited a sustained increase in CDK-2 activity, accompanied by the loss of CDK-2-associated p27. Blockade of Ras-induced CDK-2 activity and S phase entry via overexpression of p27 inhibited apoptosis. Inactivation of the retinoblastoma protein in quiescent cells through expression of HPV-E7 stimulated cell cycle progression and apoptosis, indicating that deregulated cell cycle progression is sufficient to induce apoptosis. Ras failed to induce G1 phase growth arrest in normal rat thyroid cells. Rather, Ras-expressing thyroid cells progressed into S and G2 phases and evoked a checkpoint response characterized by the activation of ATR. Ras-stimulated ATR activity, as evidenced by Chk1 and p53 phosphorylation, was blocked by p27, suggesting that cell cycle progression triggers checkpoint activation, likely as a consequence of replication stress. These data reveal that Ras is capable of inducing a DNA damage response with characteristics similar to those reported in precancerous lesions. Our findings also suggest that the frequent mutational activation of Ras in thyroid tumors reflects the ability of Ras-expressing cells to bypass checkpoints and evade apoptosis rather than to simply increase proliferative potential.