The thyroglobulin gene, like many other tissue-specific genes, appears to be specifically less methylated in the differentiated cell type where it is transcribed. The thyroglobulin gene promoter elements themselves are highly CG-deficient and do not contain any HpaII/MspI sites. In this study, using DNA constructs that were methylated in vitro with HpaII or MspI methylases, we show that DNA methylation of vector sequences is sufficient to repress the activity of the thyroglobulin gene promoter in transient transfection experiments. Reporter-gene expression from a plasmid containing only the proximal thyroglobulin gene promoter is sensitive to DNA methylation even in fully differentiated thyrocytes. Transcription from methylated plasmids containing the thyroglobulin gene enhancer and proximal promoter is also clearly reduced when the transfected cells are maintained under less-differentiated conditions. These results indicate that DNA methylation can influence, from a distance, the activity of an unmodified promoter. Our results also agree with the view that loss of DNA methylation does not constitute a prerequisite for thyroglobulin gene expression in differentiated thyrocytes, where the thyroglobulin gene enhancer and promoter are activated. However, the production of thyroglobulin transcripts could be severely impaired when this activation is not maximal, as is the case in less-differentiated cells or when the enhancer element is lacking. We suggest that DNA methylation helps to maintain the thyroglobulin gene in an inactive state unless all of the conditions required for its expression are fulfilled, and that the thyroid-specific demethylation events are a consequence of the activation state of the gene.

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