Evidence that a burst of DNA depurination in SENCAR mouse skin induces error-prone repair and forms mutations in the H-ras gene

Treatment of SENCAR mouse skin with dibenzo[a, l]pyrene results in abundant formation of abasic sites that undergo error-prone excision repair, forming oncogenic H-ras mutations in the early preneoplastic period. To examine whether the abundance of abasic sites causes repair infidelity, we treated SENCAR mouse skin with estradiol-3, 4-quinone (E₂-3, 4-Q) and determined adduct leveis 1 h after treatment, as well as mutation spectra in the H-ras gene between 6 h and 3 days after treatment. E₂-3, 4-Q formed predominantly (≥99%) the rapidly-depurinating 4-hydroxy estradiol (4-OHE₂)-1-N3Ade adduct and the slower-depurinating 4-OHE₂-1-N7Gua adduct. Between 6 h and 3 days, E₂-3, 4-Q induced abundant A to G mutations in H-ras DNA, frequently in the context of a 3′-G residue. Using a T.G-DNA glycosylase (TDG)-PCR assay, we determined that the early A to G mutations (6 and 12 h) were in the form of G.T heteroduplexes, suggesting misrepair at A-specific depurination sites. Since G-specific mutations were infrequent in the spectra, it appears that the slow rate of depurination of the N7Gua adducts during active repair may not generate a threshold level of G-specific abasic sites to affect repair fidelity. These results also suggest that E₂-3, 4-Q, a suspected endogenous carcinogen, is a genotoxic compound and could cause mutations.