Effects of ethylating agents on DNA synthesis in vitro: implications for the mechanism of carcinogenesis

A highly carcinogenic ethylating agent, ethylnitrosourea (ENU), and a weakly carcinogenic one, diethylsolfate (DES) react with DNA to roughly the same extent but DES produces about 6 times as many unstable ethylated bases, which are gradually lost spontaneously under physiological conditions. The different rates of loss for the different DNA bases have been studied using polydeoxyribonucleotides. Spontaneous strand breakage following base loss is slow, lagging more than a week behind base loss at 37°C; ultimately, DES results in much more spontaneous strand breakage than ENU. In DNA synthesis in vitro, using avian myeloblastosis virus (AMV) polymerase, no nucleotides are incorporated opposite missing bases in the template; when the template contains ethylated bases that are impaired in their ability to form specific hydrogen bonds, purine-pyrimidine mispairing can occur and mismatched nucleotides are incorporated into the daughter strand. ENU ethylates somewhat more sites leading to mispairing potential than DES. ENU also produces approximately 7.5 times more ethyl phosphotriesters than DES.