Molecular Alterations and Expression Dynamics in the Etiopathogenesis of Thyroid Cancer

Thyroid carcinoma is the most prevalent endocrine malignancy and accounts for 2% of all human cancers. In the past decade, knowledge of genetic alterations of thyroid cancer (TC) has rapidly expanded, which has provided new insights into thyroid cancer etiology and has offered novel diagnostic tools and prognostic markers that enable improved and personalized management of thyroid cancer patients. Alterations in key signaling effectors seem to be the hallmark of distinct forms of thyroid neoplasia. Mutations or rearrangements in genes that encode Mitogen activated protein kinase (MAPK) pathway effectors seem to be required for transformation. Mutations in BRAF were the most recently identified MAPK effector in thyroid cancer. BRAF V600E is the most common alteration in sporadic papillary carcinoma. Three RAS proto-oncogenes (NRAS, HRAS & KRAS) are implicated in human thyroid tumorigenesis. High incidence of thyroid cancer worldwide indicates the importance of studying genetic alterations that lead to its carcinogenesis. BRAF and RAS alterations represent a novel indicator of the progression and aggressiveness of thyroid carcinogenesis. The GSα-adenylyl cyclase-cyclic AMP (cAMP) cascade is effected in thyroid cancer. Promoter hyperme‐ thylation of multiple genes especially TSHR has been identified to play a role in thyroid cancers, in particular showing a close association with BRAF mutational status. So, the main aim of the study was to elucidate the involvement of BRAF and RAS gene mutations along with BRAF expression and thyroid-stimulating hormone receptor (TSHR) hypermethylation in North Indian patients and investigate their association with clinicopathological characteristics. We screened exon 15 of BRAF gene and exons 1 and 2 of RAS genes (HRAS, KRAS, and NRAS) in 60 consecutive thyroid tissue (tumor and adjacent normal) samples. Overall mutations in BRAF were found to be 25% (15 of 60) affecting codon 600 (valine to glutamine) and restricted only to papillary thyroid cancer and well-differentiated grade. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. BRAF mutations were significantly associated with well-differentiated disease and elevated thyroid-stimulating hormone (TSH) levels (P < 0.05). Overall, increased expression of BRAF was found in 90% (54 of 60) of thyroid cancer cases and signifi‐ cantly associated with nonsmokers. Totally, 86.7% (13 of 15) of BRAF mutationpositive patients were having BRAF protein overexpression compared to 91.2% (41 of 45) of patients with wild-type BRAF status (P > 0.05). We screened 60 consecutive thyroid tumor and adjacent normal tissues for mutations, if any, in the exons 1 and 2 of RAS genes (HRAS, KRAS, and NRAS) and 140 blood samples from thyroid cancer patients for HRAS T81C polymorphism in codon 27 in comparison with 170 cancer-free controls from a Kashmiri population. Thyroid tumor tissue samples were devoid of any mutation, but a frequent nucleotide change at position 81 (T > C) in exon 1 of HRAS gene was seen. In HRAS T81C SNP, frequencies of TT, TC, and CC genotypes among cases were 41.4, 38.6, and 20.0%, while in controls genotype frequencies were 84.1, 11.7, and 4.2%, respectively. A significant difference was observed in variant allele frequen‐ cies (TC + CC) between the cases and controls (58.6 vs. 16%) with odds ratio of 7.4 (CI = 04.3–12.7; P < 0.05). Interestingly, combined TC and CC genotype abundantly presented in follicular thyroid tumor (P < 0.05). Moreover, a significant association of the variant allele (TC + CC) was found with nonsmokers (P < 0.05). TSHR gene was found to be hypermethylated in 25% (15 of 60) of the cases with strong association with elevated TSH levels (OR = 4.0, P = 0.02). TSHR promoter was hypermethylated in 73.3% (11 of 15) of patients with BRAF V600E mutation compared to 26.7% (4 of 15) of patients having absence of TSHR promoter methylation and the association was significant (P < 0.05). We conclude that both mutational events and overexpression of BRAF gene are highly implicated in pathogenesis of thyroid cancer and the BRAF protein overexpression is independent of the BRAF mutational status of thyroid cancer patients. RAS gene mutation does not prevail in this population. Contrary to this, HRAS T81C polymor‐ phism moderately increases thyroid cancer risk with rare allele as a predictive marker for follicular tumors. Our study showed a high implication of TSHR gene methylation and its significant association with BRAF V600E mutation in thyroid tumors, depict‐ ing a positive connection between TSHR pathway and MAP kinase pathway.