DNA modification, differentiation, and transformation

Substantial evidence has accumulated over the last 5 years that the methylation of cytosine residues in vertebrate DNA is implicated in the control of gene expression. We have used analogs of cytidine, modified in the 5 position, as specific inhibitors of DNA methylation to probe the relationship between this process and cellular differentiation. 5-Azacytidine effected marked changes in the differentiated state of cultured cells and induced the formation of biochemically differentiated muscle, fat, and chondrocytes from mouse fibroblast cell lines. Since the analog is a powerful inhibitor of DNA methylation, we suggest that this inhibition is causally related to the mechanism of phenotypic conversion. DNA extracted from cells treated with 5-azacytidine was hemimethylated and was used as an efficient acceptor of methyl groups in an in vitro reaction in the presence of eukaryotic methylases. In vitro methylation was inhibited if the substrate DNA was preincubated with a diverse range of chemical carcinogens including benzo(a)pyrene diolepoxide. Thus, chemical carcinogens may induce changes in gene expression by alteration of cellular methylation patterns. Recent experiments have also demonstrated that freshly explanted diploid fibroblasts from mice, hamsters, and humans lose substantial quantities of 5-methylcytosine during cell division and aging in culture. Taken together, these experiments suggest that the genomic distribution of 5-methylcytosine might have importance in normal differentiation and also in the aberrant gene expression found in cancer and senescence in culture.