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

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 cumber 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 MTIG and CRABPI 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 MTIG gene in the cell line K2. Treatment with 5-Aza-dC alone restored CRABPI 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. MTIG and CRABPI are novel genes that are likely involved in the pathogenesis of sporadic PTC. (C) 2003 Wiley-Liss, Inc.