Lack of mutations in the thyroid hormone receptor (TR) α and β genes but frequent hypermethylation of the TRβ gene in differentiated thyroid tumors

Context: It remains inconclusive whether mutations in thyroid hormone receptor (TR) genes naturally occur in thyroid cancer and whether these genes could be suppressors of this cancer. Objectives: Our objectives were to examine further mutations of TRα and TRβ genes in thyroid cancer and also to examine their methylation as an epigenetic silencing mechanism in thyroid cancer. Experimental Design: Instead of using a cDNA sequencing approach used in previous studies, we used genomic DNA to sequence directly the coding regions of the TRα and TRβ genes to search mutations in various differentiated thyroid tumors and used methylation-specific PCR to analyze promoter methylation of these genes. Allelic zygosity status at TRβ was also analyzed. Results: We found no TRα gene mutation in 17 papillary thyroid cancers (PTCs) and 11 follicular thyroid cancers (FTCs), and no TRβ gene mutation in 16 PTCs and 12 FTCs. We also found no methylation of the TRα gene in 33 PTCs, 31 FTCs, 20 follicular thyroid adenomas (FTAs), and 10 thyroid tumor cell lines. In contrast, we found hypermethylation of the TRβ gene in 10 of 29 (34%) PTCs, 22 of 27 (81%) FTCs, five of 20 (25%) follicular thyroid adenomas, and three of 10 (30%) thyroid tumor cell lines, with the highest prevalence in FTC. We additionally examined loss of heterozygosity at TRβ and found it in three of nine (33%) PTCs and three of nine (33%) FTCs. Conclusions: Mutation is not common in TR genes, whereas hypermethylation of the TRβ gene as an alternative gene silencing mechanism is highly prevalent in thyroid cancer, particularly FTC, consistent with a possible tumor suppressor role of this gene for FTC.