Formation of S-formyl-2′-deoxycytidine from 5-methyl-2′-deoxycytidine in duplex DNA by Fenton-type reactions and γ-irradiation

5-Methyl-2′-deoxycytidine (5-Me-dC) is formed by the enzymatic methylation of dC, primarily in CpG sequences in DNA, and is involved in the regulation of gene expression. In the present study, 5-Me-dC and double-stranded DNA fragments containing 5-Me-dC were either γirradiated or aerobically treated with Fenton-type reagents, Fe(ll)-EDTA, Fe(ll)-nitrilotri-acetic acid, Fe(lll)-EDTA-H2O2-catechol or ascorbic acid-H2O2 under neutral conditions. The formation of 5-formyl-2′-deoxycytidine (5-CHO-dC) was observed upon treatment of both 5-Me-dC and DNA fragments containing 5-Me-dC. The yields of 5-CHO-dC from 5-Me-dC and those of 5-formyl-2′-deoxyuridine from dT were comparable. These results suggest that 5-Me-dC in DNA is as susceptible to oxidation as dT in cells, and raise the possibility that 5-CHO-dC may contribute to the high mutagenic rate observed in CpG sequences in genomic DNA.