Nuclear Methylation Levels in Normal and Cancerous Thyroid Cells

Background: Most cancers show abnormal DNA methylation and a positive correlation between hypomethylation and tumour progression. Patients and Methods: In our laboratory the extent of DNA methylation in individual nuclei in normal, cancer and non-cancer thyroid tissue samples was quantified according to a previously described method of computer-assisted semi-quantitative analysis. Cancer and non- cancer samples were obtained from nine patients with different thyroid pathologies (one multinodular goitre and eight carcinomas). Quantitative analysis was performed in two sets of samples, i.e. individual nuclei from touch preparations and from tissue sections. Results: In all cancer specimens a statistically significant decrease of heterochromatin methylation was consistently observed. In both sets of samples a direct correlation was consistently observed between the extent of chromatin demethylation and the degree of malignancy. Conclusion: Our preliminary results suggest that our method of cell-by-cell detection of intranuclear methylation abnormalities may be a useful tool in early identification of thyroid cancer lesions. Mammalian cells modify their DNA by means of a post- replicative process consisting of adding a methyl group to the 5' position of the cytosine ring in the two-base sequence 5'- CpG-3'. The location of 5-methylcytosine in the constitutive heterochromatin of mammalian cells was first described by Miller et al. (1). DNA methylation plays a role in the control of gene expression: expressed genes are hypomethylated while inactivated genes are generally highly methylated (2, 3). Aberrant DNA methylation has been detected in most human and mammalian cancers: the transformed cells often show increased methyltransferase levels and abnormal