Correspondence between effects of 5-azacytidine on SCE formation, cell cycling and DNA methylation in Chinese hamster cells.

The effects of 5-azacytidine (5-Aza-C), alone and in combination with mitomycin C, were measured on sister-chromatid exchange (SCE) formation and DNA methylation in different genomic regions of Chinese hamster ovary cells and in Chinese hamster cells containing amplified, dihydrofolate reductase sequences and resistant to methotrexate. 5-Aza-C, when present for the penultimate preharvest cell cycle, induced SCEs in a manner consistent with a directly measured reduction in deoxycytosine methylation in cellular DNA. At higher 5-Aza-C concentrations, cell cycling was inhibited and both SCE induction and DNA demethylation tended to level off. Under appropriate conditions, 5-Aza-C also potentiated the induction of SCEs by mitomycin C. 5-Aza-C-induced DNA demethylation could also be detected in the vicinity of different DNA sequences with the use of comparative HpaII/MspI digestion, DNA blotting, and molecular probes. The efficiency of an individual demethylation event in inducing SCE induction appeared to be very low, compared with alkylating agents such as 8-methoxypsoralen, suggesting that SCE induction by 5-Aza-C might be an indirect effect from long range changes induced in cellular DNA or chromatin conformation.