The asymmetric methylation of CG palindromic dinucleotides increases sister-chromatid exchanges

Treatment with the base analogue, 5azaC, increases SCEs in CHO but not in mosquito cells. On the other hand, both types of cells show equivalent increases in exchanges when treated with other compounds, such as mitomycin C. Vertebrate DNA is heavily methylated while diptera DNA is heavily demethylated. The sequence of events leading to an increase in SCEs in CHO cells is as follows: first of all, Cs are replaced by 5azaC; in the next cell cycle, CG palindromic dinucleotides exhibit an asymmetric configuration, the Cs in the parental DNA strand being methylated and the Cs in the daughter DNA strand demethylated; after one more cycle, half of the chromosomes show symmetric methylation and the other half symmetric demethylation of both Cs in CG palindromes. The increase of SCEs occurs in the second cell cycle when the hemimethylated DNA enters replication. DNA hemimethylation is believed to be an intermediate stage in the process of demethylation that accompanies gene expression. If so, gene demethylation would be a cause of SCE increase in normal vertebrate cells.