Detection of global hypermethylation in well-differentiated thyroid neoplasms by immunohistochemical (5-methylcytidine) analysis

Purpose: While global hypomethylation of DNA has been found in several malignancies, studies on thyroid tumours have shown controversial results using different techniques. To help resolve this issue, we assessed methylation status using two different techniques in papillary thyroid carcinomas (PTC) and follicular adenomas (FA) and carcinomas (FTC), comparing adjacent non-neoplastic thyroid tissue. Methods: A series of 15 FA, 18 FTC and 17 PTC were assessed by: (1) measurement of methylation levels of long interspersed nuclear elements (LINE-1) using a combined bisulfite restriction analysis polymerase chain reaction protocol and (2) immunostaining with an anti-5-methylcytidine antibody that detects methylated DNA regardless of the DNA sequence. Immunostaining was scored by image analysis. Results: Methylation levels of LINE-1 in FA, FTC and PTC were not significantly different from adjacent normal tissue. There was no significant difference in methylation levels of LINE-1 between FA, FTC and PTC (p = 0.44). By immunohistochemical staining for methylation, the 5-methylcytidine score was significantly higher in tumours than in normal tissue counterparts, for FA (p < 0.001), FTC (p = 0.04) and PTC (p = 0.02). PTC showed the highest 5-methylcytidine expression amongst all tumours which was significantly different from FTC (p = 0.015), but not FA (p = 0.09). There was no correlation in methylation level between LINE-1 and 5-methylcytidine scores for each group and overall. Conclusions: Well-differentiated thyroid neoplasms (FA, FTC and PTC) were not found by two independent methods to undergo global hypomethylation as part of an oncogenic sequence from normal tissue to carcinoma. Instead, hypermethylation was detected in all types of tumours, implying that this epigenetic event may contribute to oncogenic development of thyroid neoplasms (both benign and malignant).