Production of thyroid tumours in mice by demethylating agents.

Previous studies on thyroid tumorigenesis in rodents have explored the role of oncogene mutation in tumour development. However, many mutagens are also known to decrease DNA methylation, another factor known to be important in the regulation of gene expression. We report the results of a small study to examine whether the demethylating agents 5-azacytidine and 5-aza-2-deoxycytidine could promote radiation- or goitrogen-induced tumorigenesis in the mouse, and whether they could potentiate the effect of a combined radiation and goitrogen regime. Three single doses of either 5-azacytidine or 5-aza-2-deoxycytidine increased the frequency of lesions in the goitrogen-treated animals; one of these lesions was a metastasising carcinoma. Thyroid carcinomas are very rare in mice in the absence of mutagen treatment, and metastasis is particularly unusual. There was no significant difference, at the dosages used, between the number of tumours induced by demethylating agents and those induced by a mutagen (radiation) in goitrogen-treated animals. Unlike goitrogens, demethylating agents did not promote radiation-induced tumorigenesis, and they did not produce any significant potentiation of the conventional regime of radiation and goitrogen at the dosages used. This study suggests that the role of non-genotoxic factors, such as agents affecting patterns of DNA methylation, warrants consideration in thyroid tumorigenesis.