Hypermethylation, but not LOH, is associated with the low expression of MT1G and CRABP1 in papillary thyroid carcinoma

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Abstract
We previously obtained gene expression profiles of 8 matched papillary thyroid carcinoma (PTC) and normal tissues using DNA microarrays. To identify novel tumor suppressor genes involved in thyroid carcinogenesis, we here analyze genes showing lower expression in PTC tumors than in normal thyroid tissues. A search for loss of heterozygosity (LOH) in 49 regions that harbor consistently down‐regulated genes revealed LOH in only 4 regions and in just a very small number of tumors. To determine whether the underexpression might be due to promoter methylation, we used combined bisulfite restriction analysis and bisulfite sequencing to study 7 underexpressed genes. Loss of expression of MT1G and CRABP1 is accompanied by hypermethylation in the 5′ regions of these genes, but methylation was not seen in other genes tested. Combined treatment with the DNA methyltransferase inhibitor 5‐aza‐2′‐deoxycytidine (5‐Aza‐dC) and the histone deacetylase inhibitor trichostatin A (TSA) resulted in demethylation and re‐expression of the MT1G gene in the cell line K2. Treatment with 5‐Aza‐dC alone restored CRABP1 expression in a colorectal cancer cell line, SW48. In conclusion, LOH is a remarkably rare mechanism of loss of gene function in PTC. In contrast, hypermethylation of promoter CpG islands seems to occur at higher frequency. MT1G and CRABP1 are novel genes that are likely involved in the pathogenesis of sporadic PTC.