DNA methylation in differentiating mouse teratocarcinoma and erythroleukemia cells

We have measured the content of 5-methylcytosine (5MC) in the genomic DNA of differentiated and undifferentiated cultures of murine embryonal carcinoma (EC) and murine erythroleukemia (MEL) cells. A large proportion of deoxycytosine residues were methylated in EC cells (4.6%) and this proportion dropped significantly (4.1%) following differentiation. The alpha-1 globin genes were heavily methylated at HpaII sites in EC cells and in differentiated derivatives of these EC cells. The MEL cells had only 3.3% of their genomic deoxycytidine methylated and no significant change occurred following differentiation. The alpha-1 globin genes of MEL cells were much less methylated than the same genes in EC cells and no change in this methylation pattern accompanied the induction of hemoglobin synthesis. In EC X MEL cell hybrids which express the alpha-1 globin genes from both parents, the EC-derived genes had become demethylated. These results are consistent with the general model that DNA hypomethylation is correlated with expression of that DNA and that gene activation is accompanied by DNA demethylation. We have also measured the 5MC content of DNA isolated from nuclease-treated EC nuclei. Unexpectedly, the DNase I sensitive chromatin contained a large proportion of 5MC. This result, along with the work of others, suggests that nuclease sensitivity may often reflect the transcriptional activity of chromatin in somatic cells, but is not indicative of the active state in pluripotent EC cells.