Evidence for AP site formation related to DNA-oxygen alkylation in CHO cells treated with ethylating agents

DNA single-strand breaks (ssb) induced by N-ethyl-N-nitrosourea (ENU) in CHO cells are quickly resealed within 10 min after treatment. This rapid repair kinetics is not explained by the rate of base excision repair which removes the main ethyl products with a half-life in the order of hours. We have explored the potential use of methoxyamine (MX), a chemical that reacts at neutral pH with AP sites in DNA in vitro, to clarify the origin of ENU-induced ssb. The presence of 50 mM MX during cell treatment with diethyl sulfate (DES) caused selective inhibition of the repair of AP sites generated during base excision repair and inhibited alkaline cleavage at these sites. The treatment of CHO cells with ENU in the presence of MX clearly showed that the burst of ssb observed immediately after treatment was due to AP site formation. Plasmid DNA treated in vitro with ENU did not present AP endonuclease-sensitive sites; therefore, the AP sites produced in CHO cells by ENU treatment are not due to the chemical hydrolysis of a very unstable ethyl adduct but rather are intermediates of an as yet undefined enzymatic pathway. This process occurs specifically after treatment with SN1-type ethylating agents (ENU and N-ethyl-N'-nitro-N-nitrosoguanidine) suggesting an association between this phenomenon and DNA-oxygen alkylation. We suggest that these breaks are generated by a mechanism of O6-ethylguanine processing without removal of the modified base.