Complex relationships between 5-aza-dC induced DNA demethylation and chromosome compaction at mitosis

A variety of treatments with 5-azadeoxycytidine (5-aza-dC) were applied to cultured human lymphocytes during one to four cell cycles. The effect of 5-aza-dC on DNA methylation was studied by using an antibody against 5-methylcytosine on mitotic chromosomes. 5-Azadeoxycytidine is known to induce strong and permanent demethylation of DNA. Unexpectedly complex relationships were observed between DNA methylation status and chromatid/chromosome compaction. The most dramatic alteration of compaction at mitosis was observed when pre-replicative chromosomes had unifilarly demethylated DNA. The compaction of chromosomes was found to depend only partially on the methylation of their DNA at the time of mitosis. Our results suggest that alteration of DNA methylation prevents the synchronization of chromatin compaction, inducing premature (or delayed) chromosome condensation, and that a crucial step is the DNA methylation status of the pre-replicative chromosome.