A DNA molecule is vulnerable to many types of DNA-damaging agents of both endogenous and exogenous origins. To date, the majority of DNA repair and mutagenesis studies are based on the actions of DNA polymerases during the replication process. The presence of DNA lesions interferes not only with replication but also with transcription. Therefore, to provide an accurate estimate risk of damaged DNA in living cells, it is an essential factor to understand the behavior of transcription elongation complexes on the transcribed strand containing DNA lesions, which are induced by mutagens and carcinogens in the environment. Notably, the vast majority of cells living outside the artificial environment in growth factor-enriched media cannot continue to grow. Although such cells do not replicate their genome DNA, they only need to transcribe a large number of genes accurately to produce the necessary proteins for normal physiological processes. In this review, we describe the mechanism of RNA polymerases (RNAPs) stalled at DNA lesions, which is an implication of transcription-coupled nucleotide excision repair. The mutant transcripts derived from translesion RNA synthesis of RNAPs implicate the occurrence of transcriptional mutagenesis. The biological risks of DNA lesions induced by mutagens and carcinogens with regard to transcription elongation are discussed.
2-Acetylaminofluorene (AAF) is a procarcinogen and its activation mechanisms have been investigated in detail. AAF was metabolized to 2-acetylamino-9-fluorenone (AAF=O) and 2-acetylamino-9-fluorenol by S9 mix. The mutagenicity of AAF=O in the presence of S9 mix was equal in potency to that of AAF in Salmonella typhimurium TA1538, but the activation mechanism of AAF=O was poorly reported. In this study, we investigated possible ultimate species derived from AAF=O; N-hydroxy-2-acetylamino-9-fluorenone (N-OH-AAF=O), N-acetoxy-2-acetylamino-9-fluorenone (N-OAc-AAF=O), N-hydroxy-2-amino-9-fluorenone (N-OH-AF=O), and N-acetoxy-2-trifluoroacetylamino-9-fluorenone (N-OAc-TFAAF=O), a model compound for N-acetoxy-2-amino-9-fluorenone (N-OAc-AF=O), were synthesized and their mutagenicity was examined in S. typhimurium TA1538. The activation mechanism in S. typhimurium TA1538 was also investigated. The compounds in order of decreasing mutagenicity are N-OAc-TFAAF=O>N-OH-AF=O>N-OAc-AAF=O> N-OH-AAF=O. AAF=O is at least partially responsible for the mutagenicity of AAF, since a small amount of AAF is oxidized to AAF=O in the presence of S9 mix. Furthermore we suggest that an ultimate active species of AAF=O in S. typhimurium is N-OAc-AF=O, with the same activation manner as AAF.
In the acellular comet assay, slides with gels prepared from untreated cells are exposed after lysis to the test agents and then processed according to the standard comet assay protocol. The sensitivity of acellular comet assay was compared with that of standard assay using human lymphoblastoid WTK1 cells. Selected model mutagens were N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), methyl nitrosourea (MNU), ethyl nitrosourea (ENU), methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), bleomycin (BLM), and UVC. In the acellular assay, lysed slides were exposed to model chemical mutagens for 2 h or irradiated to UVC, and then electrophoresed immediately after chemical mutagen-treatment or 2 h after UVC-irradiation. The slides for standard assay were prepared immediately after 2 h-exposure to each chemical mutagen or 2 h after UVC-irradiation. In both assays, slides were electrophoresed at pH>13 or pH 12 for 20 min after 20 min unwinding. BLM was positive at pH>13 and pH 12 in both assays. UVC was positive in the standard assay but not in the acellular assay. In spite of positive responses of alkylating agents in the acellular assay at pH>13 and pH 12, they were positive at pH>13 but not at pH 12 in the standard assay. The positive responses in the acellular assay were greater than those in the standard assay. Our present results suggest that acellular comet assay can detect DNA single strand breaks (SSBs) as initial lesions but not alkali-labile sites generated from DNA lesion such as alkylated bases and that the sensitivity to detect SSBs as initial lesions is lower in the standard than in the acellular assay.
Base excision repair (BER) is the major pathway to repair oxidized bases in many organisms, but the BER mutants of Schizosaccharomyces pombe are substantially resistant to hydrogen peroxide. To reduce the reactive oxygen species (ROS) scavenging activity in cells, we disrupted a catalase gene, ctt1, in S. pombe. The ctt1 mutant became sensitive to hydrogen peroxide, but had no mutator phenotype. Deletion of the BER genes (nth1, apn1, apn2, or uve1) from ctt1 mutant further increased the hydrogen peroxide sensitivity, indicating that the catalase activity obscures the functions of BER enzymes in vivo. The nth1 and apn2 mutants exhibited a moderate mutator phenotype. Double mutants in both ctt1 and BER genes showed extremely high spontaneous mutation rates, especially in the ctt1/nth1 mutant. Vitamin C relieved the mutator phenotype of the ctt1/nth1 mutant. The ctt1/apn1 and ctt1/uve1 mutants also had high mutation rates, even though each single mutant showed no mutator phenotype. Our results provide evidences that BER enzymes as well as calatase and antioxidant contribute in vivo to avoidance of ROS-induced mutagenesis and cell death.
Phosphorylated histone H2AX (γH2AX) has been thought to be a marker of DNA double-strand breaks (DSBs). We examined whether polyaromatic hydrocarbons (PAH), benzo[a]pyrene, 1-nitropyrene, 1,8-dinitropyrene, 3-nitrobenzanthrone and N-hydroxy-3-aminobenzanthrone, can phosphorylate H2AX in human HeLa cells by using immunofluorescence microscopy. These substances do not cause DSB directly but produce purine adducts in cellular DNA. After exposure of the cells to these chemicals with a concentration giving 10% cell survival followed by incubation for one hour, γH2AX foci appeared in the cell nuclei. All cells expressing γH2AX corresponded to those that incorporated bromodeoxyuridine after PAH treatment, indicating that PAH-induced H2AX phosphorylation correlated to S-phase entry of the cells. Cells exposed to ultraviolet light and camptothecin, which produce pyrimidine dimers and single-strand breaks (SSBs) in DNA, respectively, showed the same response. On the other hand, cells exposed to X-ray and etoposide, which produce DSBs, expressed γH2AX not only in S-phase cells. These results suggest that H2AX phosphorylation signals are transmitted from directly produced DSBs in all cell phases as well as from DNA strand stalling in the S-phase. H2AX phosphorylation by all substances used in this experiment is inhibited by wortmannin, implying that H2AX phosphorylation is regulated by ATM and/or ATR pathways.