DNA hypomethylation and differentiation in Friend leukemia cell variants

The occurrence, upon differentiation, of a transient DNA hypomethylation has been observed in Friend erythroleukemia cells. Treatment with hexamethylenebisacetamide induces within 24 h a 20% hypomethylation of newly synthesized DNA, that is followed by re-methylation before completion of the differentiative process, as measured by the appearance of benzidine-positive cells. We examined a series of mutant clones which continue to grow in the presence of an inducer. Methylcytosine content of DNA was measured by HPLC, after cell labeling with [3H]uridine. We found that one of these continuously growing clones, which was still capable of hemoglobin synthesis, showed the same degree of hypomethylation as the parental one. The re-methylation process did not occur, however, unless erythroid differentiation was reverted by the removal of the inducer. In another clone which had lost the capacity to synthesize hemoglobin, no DNA hypomethylation was detectable. These experiments show that DNA hypomethylation is an early event strictly related to cell differentiation but not to cell growth arrest.