Mutagenicity of site-specifically located 1,N2-ethenoguanine in Chinese hamster ovary cell chromosomal DNA

The adduct 1,N-2-etheno(epsilon)-guanine (Gua) can be formed in DNA from exogenous or endogenous bifunctional electrophiles. Previous work with site-specifically modified oligonucleotides has shown all three possible base substitutions at the site of this residue in bacterial cells and in primer extension assays using purified polymerases (with the purified polymerases also showing deletions). A 10-mer was synthesized containing 1,N-2-epsilon-Gua at a specific position and ligated into a modified pCNheIA vector, which was used to insert the modified sequence into the chromosomes of AA8 (wild-type) and UV5 (nucleotide excision repair-deficient) Chinese hamster ovary cells. Transformants were selected by antibiotic resistance; DNA was amplified by polymerase chain reaction, and resistance to the restriction endonuclease NheI was used to estimate mutation frequency. In the AA8 cells, the apparent mutation frequency was elevated > 10-fold due to the presence of 1,N-2-epsilon-Gua (to 4.6%). In UV5 cells, the mutation frequency was even higher (7.8%), but the estimate of the frequency in the control system (vector and unmodified sequence only) was 4.5%. Sequence analysis of 21 clones derived from the mutant fraction yielded five that correspond to base pair mutations directly at the 1,N-2-epsilon-Gua site. The remainder of the mutants differed from those generated from the unmodified oligonucleotide and included deletions, rearrangements, double mutants, and base pair substitutions at sites nearby but not at the 1,N-2-epsilon-Gua site.