| Significant amounts of 5-methylcytosine have been shown to occur in all eucaryotic DNAs examined. This minor base is produced by the enzymatic transfer of methyl groups from S-adenosylmethionine to specific cytosine residues in the DNA polymer. The biological function of DNA methylation in eucaryotes is unknown. However, recent studies indicate that it may be involved in the regulation of gene expression.;The relationship between DNA methylation and changes in gene expression during differentiation was further examined by determining whether loss of methyl groups occurs at specific sites within and around globin genes during mouse erythroleukemia cell differentiation. Although it was possible to demonstrate differences in the patterns of methylation of C methylation-sensitive restriction endonuclease (Hpa II and Hha I) sites in the (alpha)-like globin gene regions of two separate mouse erythroleukemia cell clones, the methylation of Msp I, Hpa II and Hha I sites located within the globin gene regions was not found to be altered during erythroid differentiation.;Thus, while the results of this study suggest that hypomethylation of DNA is an integral part of the process of cell differentiation, it is clear that the degree of methylation of many sites within a gene remains unchanged even during a period when the expression of that gene is enhanced.;I have found that mouse erythroleukemia cells grown in the presence of either hypomethylating agents which cause their differentiation (L-ethionine and 5-azacytidine) or known inducing agents which are not general inhibitors of transmethylation reactions (dimethylsulfoxide, butyrate, hexamethylene-bisacetamide or pentamethylene-bisacetamide) synthesize undermethylated DNA. A direct correlation between the extent to which DNA becomes hypomethylated and the number of cells in the population which become committed to differentiation is not observed. Nevertheless, the two processes are connected. Hypomethylated DNA is synthesized only in cells chemically induced to differentiate. DNA isolated from a dimethylsulfoxide resistant clone grown in the presence of that agent is not undermethylated, while DNA prepared from these cells after exposure to agents which induce their differentiation is undermethylated. |
