Human genome is continuously exposed to exogenous and endogenous genotoxic agents. The most hazardous and ubiquitous exogenous mutagen may be cigarette smoke, which contains more than 4,000 chemicals including about 60 known carcinogens. About 1% of oxygen metabolism leads to production of reactive oxygen species, a major source of endogenous mutagens. These genotoxic agents induce a variety of lesions in DNA, which results in mutations and chromosome aberrations upon replication. If such genetic alterations occurred in the genes involved in cell proliferations and/or maintenance of genome stability, the cells would proceed in multi-steps of carcinogenesis. The goal of environmental mutagenesis and genetic toxicology is to elucidate the mechanistic links between exposure to genotoxic agents and the health consequences, and to prevent the health hazard associated with DNA damage. To this end, we have investigated the mechanisms of mutagenesis induced by environmental chemicals and contributed to establish the paradigm that Y-family DNA polymerases play central roles in mutagenesis via translesion DNA synthesis across damaged bases in DNA. We also developed genotoxicity assays with bacteria and mice to evaluate the potential risk of environmental chemicals. Here, I review the roles of Y-family DNA polymerases in mutagenesis and introduce features of the novel bacterial and rodent genotoxicity assays. Future directions of environmental mutagenesis and carcinogenesis are also discussed.
Cisplatin is an active antitumor drug but its stereoisomer, i.e., transplatin, is clinically inactive. We characterized the gene mutations induced by both isomers using cell line GDL1 established from gpt delta transgenic mice. Because cisplatin exhibited about 100 times higher cytotoxicity than transplatin, the cells were treated with cisplatin at doses of 0.25, 0.5, and 1 μg/mL and with transplatin at doses of 12.5, 25, and 50 μg/mL for 24 h. After an additional 2- to 8-day culture, mutant frequencies (MFs) with both Spi- and 6-thioguanine (6-TG) selection were determined. In Spi- selection, MFs in cisplatin- or transplatin-treated cells showed an increase of up to twofold that of vehicle-treated cells. A midsize deletion less than 1 kilo base pair (kbp) in size and single base deletions in non-run sequences were significantly induced by treatment with both compounds. In 6-TG selection, MFs increased up to 3.7-fold in the cisplatin-treated cells and 2.6-fold in transplatin-treated cells compared to vehicle-treated cells. Hotspots of cisplatin- and transplatin-induced mutations were found in 5′-NGG-3′, 5′-GGN-3′, and 5′-GNG-3′ sequences (N is the mutated nucleotide) and 5′-GCG-3′, 5′-GCCG-3′, 5′-GCN-3′, and 5′-GGGN-3′ (G, C, or N is the mutated nucleotide), respectively. These findings are consistent with previous reports using cell-free systems that cisplatin induces intrastrand crosslinks between two purine bases in 5′-GG-3′, 5′-AG-3′, and 5′-GNG-3′ and that transplatin primarily forms mono adducts in the guanine bases and needs multiple guanine adducts to form crosslinks. We suggest that intrastrand crosslinks play key roles in the cytotoxicity and mutagenicity induced by these two platinum compounds and that the more efficient formation of intrastrand crosslinks of cisplatin compared to transplatin may account for the potent cytotoxicity and clinical activity. The spectral analysis of mutations using GDL1 cells would provide valuable information on the mechanisms underlying the mutagenesis induced by the platinum stereoisomers.
A new specimen preparation method named “cell suspension method” for in vivo micronucleus test is proposed. Generally, a specimen of the micronucleus test is prepared by smearing the sample cells on a slide glass before staining with Giemsa or acridine orange (A.O.). In this new method, the sample cells are suspended with formalin, and stained with A.O., then put on a slide glass and examined under a microscope without smearing. This method is more convenient than the conventional method in consideration of preparation and observation of specimens. Furthermore, preparation is repeatable after the observation in this method, the specimen is suitable for a long storage, and this method is applicable to specimen preparation using peripheral blood.
Oxidative stress is believed to increase the risk of lifestyle-related diseases, such as cancer and heart disease. For the measurement of oxidative stress in vivo, 8-hydroxydeoxyguanosine (8-OH-dG) in DNA or urine is the most popular biomarker. However, there are some difficulties in the reproducible and accurate analysis of 8-OH-dG in DNA. In this study, we found that artifactual 8-OH-dG was elevated significantly with time in an analytical sample kept at 10°C, but not for one kept at -80°C. Furthermore, we developed a method for urinary 8-OH-dG analysis with high accuracy by an HPLC-ECD system, using anion-exchange- and reverse-phase-columns. This method can also be used for urinary and serum 8-hydroxyguanine (8-OH-Gua, free base) analyses, with a slight modification. By applying these improved methods, we confirmed the induction of oxidative stress with low dose (2 Gy) whole body X-ray irradiation of mice. The 8-OH-dG levels in the mouse urine were increased about 4.2-fold by 2 Gy irradiation, in a dose-dependent manner. The 8-OH-Gua levels in the serum were also increased with 2 Gy of irradiation. These results suggest that our improved 8-OH-dG and 8-OH-Gua analysis methods are useful for measurements of oxidative stress in vivo.
It is known that genotoxic N-nitroso carcinogens induce DNA damage in mouse liver within a few hours and induce mutations within 28 days after their administration. However, related-gene expression changes at these time points in liver were not fully elucidated. Differential gene expression induced by two genotoxic N-nitroso carcinogens in mouse liver was examined 4 h and 28 days after their administration with in-house oligonucleotide microarray (268 genes) and quantitative real-time PCR, and compared to that of a non-genotoxic carcinogen and a non-carcinogenic toxin. Diethylnitrosamine (DEN, 80 mg/kg bw), dipropylnitrosamine (DPN, 250 mg/kg bw), phenobarbital sodium (30 mg/kg bw) and ethanol (1000 mg/kg bw) were injected intraperitoneally into groups of male 9-week-old B6C3F1 mice and liver was dissected after 4 h and 28 days. mRNA from pooled livers was reverse-transcribed to cDNA, and Cy3- and Cy5-labeled cDNA was competitively hybridized with in-house made microarray, scanned and analyzed; additionally, quantitative real-time PCR was performed for selected genes. Differential gene expression between two genotoxic N-nitroso carcinogens and phenobarbital and ethanol was observed in 11 genes 4 h after administration, including seven tumor suppressor p53 target genes, viz. c-Jun, Ccng1, Mdm2, p21, Bax, Hsp27 and Snk; the other genes were Mbd1, Hmox-1, Ccnf and Rad52. However, only some degree of differential gene expression of p21, Ccng1 and Snk was observed 28 days after administration; no other differentially-expressed genes were evident. The present results suggest that DEN and DPN induce differential gene expression in p53 target genes in liver within a few hours after administration and that these acute responses remained only partially in liver after 28 days.
To clarify the in vivo genotoxic potential of dietary style, the amounts of 8-hydroxydeoxyguanosine (8-OH-dG), a marker of oxidative DNA damage, were determined by a high-performance liquid chromatography system coupled to an electrochemical detector (HPLC-ECD) in the urine of female mice to which a vitamin-deficient diet (for two months) and a sweet beverage (for two weeks) were administered. The urinary 8-OH-dG levels were clearly increased in these studies. In the vitamin-deficient diet experiment, the urinary 8-OH-dG levels were increased to 1.2-fold and 1.4-fold after one month and two months, respectively. When mice were given a commercially available sweet beverage instead of water for two weeks, the urinary 8-OH-dG was increased to 1.4-fold. In the sweet beverage experiment, significant increases of the volume consumed per day were observed, as compared to the control group (water). Although the total caloric intake per day was not remarkably different between the sweet beverage- and control-group, the mice in the sweet beverage group obtained a higher ratio of calories from sugar components. These results indicated that the elevation of oxidative stress could be caused by the prolonged intake of an unbalanced diet, such as a vitamin-deficient diet or one including sweet beverages.