Comparison of the mutagenic properties of 8-oxo-7,8-dihydro-2 '-deoxyadenosine and 8-oxo-7,8-dihydro-2 '-deoxyguanosine DNA lesions in mammalian cells

The comparative mutagenicity of 8-oxo-7,8-dihydro-2'-deoxyadenosine (8-oxod A) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) was explored using sim ian kidney (COS-7) cells, Oligodeoxynucleotides [5'-TCCTCCT-G(1)X(2)CCTCTC or 5'-TCCTCCTX(1)G(2)CCTCTC (X = dA, dG, 8-oxodA or 8-oxodG)] containing 8- oxodA or 8-oxodG positioned within codon 60 or 61 of the non-coding strand of human c-Ha-ras1 gene were inserted into a single-stranded phagemid shutt le vector. The vector was replicated in COS-7 cells and the progeny plasmid s were used to transform Escherichia coli DH10B, The transformants were ana lyzed by oligodeoxynucleotide hybridization and DNA sequence analysis to es tablish the mutation frequency and specificity. When 8-oxodA was positioned at X-1, targeted A(oxo)-->C transversions were detected; the mutation freq uency was 1.2%. When 8-oxodA was positioned at X-2, one targeted mutant amo ng 416 colonies screened (an A(oxo)-->G transition) was detected. Thus, the mutation frequency and spectrum of 8-oxodA depend on the sequence context of the lesion. The mutation frequency of 8-oxodG at X-1 and X-2 was 5.2 and 6.8%, respectively. G(oxo)-->T transversions dominated the spectrum, accom panied by small numbers of G(oxo)-->A transitions and G(oxo)-->C transversi ons. We conclude that 8-oxodA has mutagenic potential in mammalian cells, g enerating A-->C transversions. However, when tested under similar condition s, the mutation frequency of 8-oxodA is at least four times lower than that of 8-oxodG.