Base excision repair intermediates are mutagenic in mammalian cells

Base excision repair (BER) is the main pathway for repair of DNA damage in mammalian cells. This path-way leads to the formation of DNA repair intermedi-ates which, if still unsolved, cause cell lethality and mutagenesis. To characterize mutations induced by BER intermediates in mammalian cells, an SV-40 derived shuttle vector was constructed carrying a site-specific lesion within the recognition sequence of a restriction endonuclease. The mutation spectra of abasic(AP)sites,50-deoxyribose-5-phosphate(50dRp) and 30-[2,3-didehydro-2,3-dideoxy-ribose] (30ddR5p) single-strand breaks (ssb) in mammalian cells was analysed by RFLP/PCR and mutation frequency was estimated by quantitative PCR. Point mutations were the predominant events occurring at all BER interme-diates. The AP site-induced mutation spectrum sup-ports evidence for the ‘A-rule ’ and is also consistent with the use of the 50 neighbouring base to instruct nucleotide incorporation (50-rule). Preferential aden-ine insertion was also observed after in vivo replica-tion of 50dRp or 30ddR5p ssb. We provide original evidence that notonly theabasic sitebutalso its deriv-atives ‘faceless ’ BER intermediates are mutagenic, with a similar mutation frequency, in mammalian cells. Our findings support the hypothesis that unat-tended BER intermediates could be a constant threat for genome integrity as well as a spontaneous source of mutations