Mutagenicity of nitric oxide is not caused by deamination of cytosine or 5-methylcytosine in double-stranded DNA.
Several human tumors of diverse histological origin have a high incidence of C:G to T:A transition mutations at methylated CpG sites in tumor suppressor genes. We used a sensitive genetic assay to examine the ability of nitric oxide (NO), a physiological intra- and intercellular messenger molecule, to promote these transitions by deaminating cytosine (C) or methylcytosine (5mC) in double-stranded DNA. Exposure of a test double-stranded plasmid containing C or 5mC at the target site to NO in phosphate-buffered solution at pH 7.4 followed by transformation into Escherichia coli ung- strain to avoid repair of U did not result in a significant increase in reversion frequency. In addition, exposure of E. coli transformed with the target plasmid to an NO-releasing spermine-NO complex during log-phase growth did not result in larger numbers of revertants, whereas Salmonella typhimurium strain TA1535 showed a dose-responsive increase in reversion frequency when treated in the same way. We conclude that genotoxicity of NO is not caused by deamination of C or 5mC to U or T, respectively, in double-stranded DNA. This is supported by the finding that extracts of TA1535 contained high uracil-DNA glycosylase activity, suggesting that the difference in mutagenesis between the strains is not due to a lack of uracil repair. Therefore, mutational hot-spots seen in human tumor tissues at CpG sites are probably not due to the action of NO at 5mC.
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Department of Biochemistry and Molecular Biology, Kenneth Norris, Jr, Comprehensive Cancer Center, University of Southern California, School of Medicine, Los Angeles 90033-0800.
Rideout
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Shen
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Jones
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D044503:5-Methylcytosine; D001483:Base Sequence; D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry*; D004247:DNA / Q000737:chemistry / Q000187:drug effects*; D004249:DNA Damage; D004260:DNA Repair; D004269:DNA, Bacterial / Q000187:drug effects; D004273:DNA, Neoplasm / Q000235:genetics; D003641:Deamination; D004926:Escherichia coli / Q000235:genetics; D006801:Humans; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009152:Mutagenicity Tests; D009569:Nitric Oxide / Q000633:toxicity*; D010957:Plasmids / Q000187:drug effects
D004269:DNA, Bacterial; D004273:DNA, Neoplasm; D009569:Nitric Oxide; D044503:5-Methylcytosine; D003596:Cytosine; D004247:DNA