Changes in the size of pulse-labelled DNA fragments induced in human cells by inhibitors of uracil-DNA glycosylase and DNA methylation.

An inhibitor of uracil-DNA glycosylase, uracil, induces an increase in the size of pulse-labelled DNA fragments in human cells in vivo suggesting that dUMP incorporation into DNA and uracil-DNA glycosylase contribute to the small size of pulse-labelled DNA. It is also shown that inhibition of DNA methylation in vivo by ethionine and 5-azacytidine induces a decrease in the size of pulse-labelled DNA, and the effect is partially suppressed by uracil. In vitro experiments with purified uracil-DNA glycosylase from human placenta show that DNA hypermethylation inhibits the enzyme. The data make it possible to explain the antimutagenic effect of ethionine in mammalian cells [1] by stimulation of the repair of DNA containing incorporated uracil on the basis of the hypothesis that DNA-uracil repair stimulates mismatch correction leading to preferential excision of misincorporated nucleotides from daughter DNA strands.