The organ-specific induction of DNA adducts in 2-acetylaminofluorene-treated rats, studied by means of a sensitive immunochemical method

Influence of DNA Lesions on Polymerase-Mediated DNA Replication at Single-Molecule Resolution

Faithful replication of DNA is a critical aspect in maintaining genome integrity. DNA polymerases are responsible for replicating DNA, and high-fidelity polymerases do this rapidly and at low error rates. Upon exposure to exogenous or endogenous substances, DNA can become damaged and this can alter the speed and fidelity of a DNA polymerase. In this instance, DNA polymerases are confronted with an obstacle that can result in genomic instability during replication, for example, by nucleotide misinsertion or replication fork collapse. It is important to know how DNA polymerases respond to damaged DNA substrates to understand the mechanism of mutagenesis and chemical carcinogenesis. Single-molecule techniques have helped to improve our current understanding of DNA polymerase-mediated DNA replication, as they enable the dissection of mechanistic details that can otherwise be lost in ensemble-averaged experiments. These techniques have also been used to gain a deeper understanding of how single DNA polymerases behave at the site of the damage in a DNA substrate. In this review, we evaluate single-molecule studies that have examined the interaction between DNA polymerases and damaged sites on a DNA template.

On key lesions and all that: a tribute to Paul Lohman

This paper is a tribute to Paul Lohman at the occasion of his retirement from the position of Professor in the Medical Faculty at the Leiden University in The Netherlands and as Director of its Department of Radiation Genetics and Chemical Mutagenesis. Paul's contributions to the science of genetic toxicology are discussed in the context of more recent insights as to how mammalian cells process DNA damage, and how this may lead to cancer and, possibly, aging. Starting with his work on the characterization of UV-induced DNA repair in cultured cells from xeroderma pigmentosum patients and the development of methodology for monitoring the removal of UV-induced lesions in human cells, the concept of the key lesion is introduced. Among the myriad of DNA lesions that can be induced in DNA as a consequence of exposure to a range of natural or synthetic mutagens, key lesions are the ones responsible for subsequent adverse effects, for example, because they give rise to mutation. The development of methods using immunofluorescence microscopy to detect and identify such key lesions and quantitate them at the single cell level, is one of the highlights of Paul's career. Based on the perceived need to evaluate mutational end points in vivo in relation to specific lesions identified by his immunofluorescence methods, Paul subsequently made crucial contributions to the development of the first transgenic mouse model to measure mutations in chromosomally integrated reporter genes. In parallel to his experimental work, Paul greatly contributed to genetic toxicology at the theoretical level by his work on the development and evaluation of methods for assessment or prediction of risks of exposure to environmental mutagens. Finally, Paul has served the discipline of genetic toxicology in a more administrative role in various ways, both locally as one of the founders of the Medical Genetics Center South-West Netherlands and internationally by playing a prominent role in organizations such as ICPEMC. Here, his numerous contributions to the journal Mutation Research, both as author on many papers and as Executive Managing Editor should not go unmentioned.

Electrochemical detection and quantification of the acetylated and deacetylated C8-deoxyguanosine DNA adducts induced by 2-acetylaminofluorene

The genotoxic agent 2-acetylaminofluorene induces, upon metabolic activation, two main types of DNA adducts in animal tissue, i.e., (deoxyguanine-8-yl)-aminofluorene (dG-C8-AF) and N-(deoxyguanine-8-yl)-acetylaminofluorene (dG-C8-AAF). Quantification of the frequency of these adducts usually relies on the use of radioactively labeled 2-acetylaminofluorene. Here, we report the development of a sensitive, non-radioactive method for the quantification of dG-C8-AF and dG-C8-AAF. Essentially, the modified DNA bases are separated by high-performance liquid chromatography (HPLC) and quantified by electrochemical detection. We established that both modified bases guanine-C8-aminofluorene and guanine-C8-acetylaminofluorene are electrochemically active. Subsequently, a procedure was developed to quantify dG-C8-AF and dG-C8-AAF in genomic DNA. Following DNA hydrolysis the adducted bases were extracted by ethyl acetate, separated by HPLC, and detected electrochemically. This procedure has been applied in the analysis of dG-C8-AAF in N-acetoxy-2-acetylaminofluorene-modified calf thymus DNA and in the detection of dG-C8-AAF and dG-C8-AF in liver DNA of mice injected intraperitoneally with 150-450 mg N-hydroxy-2-acetylaminofluorene/kg. The quantification of relatively low dG-C8-AF and dG-C8-AAF adduct levels (i.e., 0.1-1 adduct/10(6) nucleotides) in mouse liver DNA demonstrates the sensitivity of this electrochemical detection procedure. The detection limit of the method is 1 adduct per 10(6) nucleotides for both adducts using 20 micrograms of DNA and 4 adducts per 10(8) nucleotides using 500 micrograms DNA.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Fifty years of research on N-acetyl-2-aminofluorene, one of the most versatile compounds in experimental cancer research

It is just about 50 years since the publication of the report on the toxicity and carcinogenicity of the potent carcinogen N-acetyl-2-aminofluorene (AAF). In 1940 very few reports on the carcinogenic activity of chemical compounds in experimental animals were available. The discovery of pure chemicals as carcinogens, such as AAF, azo dyes and benzo[a]pyrene, provided cancer researchers with a number of tools whereby the progressive changes involved in the induction of cancer could be studied in experimental systems. Contrary to the results with other carcinogens then known, AAF induced numerous types of tumors, but not at the site of application. This finding stimulated a great deal of interest in its use as an experimental carcinogen to study its metabolic fate and mechanism of action. During the following years an ever increasing number of reports appeared on the carcinogenicity of AAF in various species, on its metabolic fate, on the interaction of reactive metabolites with nucleic acids and proteins, and on its mutagenic activity. Particularly studies on the metabolism of AAF and the interaction with nucleic acids have contributed appreciably to our understanding of the mechanism of action of aromatic amines and also of other chemical carcinogens. It can be expected that AAF and its derivatives will continue to be used for specific applications in experimental cancer research. One of the most recent achievements is the preparation of site-specific AAF- and aminofluorene-modified DNA sequences for mutagenesis studies.

Immunocytochemical analysis of in vivo DNA modification

In the past decades a large number of DNA adducts induced in the intact animal by alkylating agents have been identified. The formation and repair of these adducts are important determinants, not only of mutagenesis, tumor initiation and DNA-mediated toxicity but probably also of tumor progression. Most studies on in vivo DNA modification have been performed on isolated bulk DNA. More recently, methods have been developed to study the distribution of DNA adducts at the level of either the individual gene or the individual cell. This paper reviews immunocytochemical methods to study the formation and repair of DNA adducts and other DNA modifications at the level of the individual cell. DNA modifications induced by alkylating agents and a variety of other agents including ultraviolet radiation, aromatic amines, polycyclic aromatic hydrocarbons and platinum anti-cancer drugs will be discussed. Up to now, immunocytochemical analysis of in vivo modified DNA has largely concentrated on experimental animals. These studies have revealed striking heterogeneities with regard to formation and/or repair of DNA adducts in tissues from rat, hamster and mouse. Immunocytochemical adduct analysis can be used to identify in a convenient, fast and detailed way cell types, cell stages and sites in which biological effects of the adducts might be expressed. More recently, immunocytochemical analysis of DNA adducts also proved to be feasible on in situ exposed human samples. A number of existing and potential applications in the field of chemical carcinogenesis, experimental chemotherapy and molecular epidemiology are discussed.

Application of new techniques for the detection of carcinogen adducts to human population monitoring

Several techniques have recently been developed for the detection and quantitation of carcinogen-DNA or -protein adducts without the requirement for radioactive carcinogens. These assays can be used to detect adducts in animals or cultured cells exposed to test compounds or in humans exposed to environmental carcinogens. Immunologic, 32P-postlabeling and fluorescence techniques, used on human samples for DNA adduct measurement, are reviewed here. Methods for the detection of carcinogen-protein adducts on human samples are also summarized.

Polyclonal antibodies to DNA modified with 4-nitroquinoline 1-oxide: application for the detection of 4-nitroquinoline 1-oxide-DNA adducts in vivo

Antibodies against 4-nitroquinoline 1-oxide (4NQO) adducts were elicited in rabbits immunized with 4NQO-modified DNA complexed with methylated bovine serum albumin. In enzyme-linked immunosorbent assay (ELISA), the antibodies could recognize either denatured or native 4NQO-modified DNA, but not unmodified DNA, DNA modified with other carcinogens or free 4NQO derivative. Modification levels as low as 5 mumol of adduct per one mole DNA nucleotide (5 adducts/10(6) nucleotides) can be easily detected by the competitive ELISA. Indirect immunofluorescence staining by anti 4NQO-DNA antibody indicated that the antibodies bound specifically to the nuclei of normal human skin fibroblast cells treated with 4NQO. The intensity of fluorescence was proportional to the dose of 4NQO used to treat the cells, and the fluorescence-positive cells could be detected after treatment with 0.25 microM 4NQO (which resulted in the formation of 10(4) adducts per cell). Applying the competitive ELISA to the quantitation of DNA-adducts in rats treated with 4NQO, it was confirmed that the sensitivity of immunochemical assays was equivalent to that of isotopic assays. These methods should be helpful in studies on the formation of adducts and their removal in cells and tissues.

A search for DNA alterations in the aging mammalian genome: an experimental strategy

In order to experimentally test the hypothesis that at the basal level senescence is caused by instabilities in DNA, techniques are required that allow the sensitive detection, quantification and characterization of DNA damage and DNA sequence changes in various organs and tissues of naturally aging mammals. In this article a strategy is presented that should allow the analysis of DNA damage metabolism in aging mammals from the original changes in the chemical structure of DNA (DNA damages) via their processing (DNA repair) to the molecular endpoints in terms of gene mutations and DNA rearrangements. With respect to the detection of DNA damage, a short overview is provided of recently emerged biochemical and immunochemical methods that can be applied immediately to study the spectrum of DNA damages in various organs and tissues of aging animals or humans. By contrast, before one is able to study low frequency changes in DNA sequence organization in somatic tissues, formidable problems have yet to be overcome. This is mainly due to the fact that the scale of highly advanced recombinant DNA techniques recently developed is almost totally devoted to the analysis of heritable genetic factors. In order to be able to study low frequency changes in DNA sequence organization in somatic tissues, we have initiated experimental approaches based on recently emerged shuttle vector technology in combination with transgenic mice, and on new electrophoretic separation principles. These approaches and their potentialities for testing somatic mutation theories of aging are discussed.

Monitoring the exposure of rats to 2-acetylaminofluorene by the estimation of mutagenic activity in excreta, sister-chromatid exchanges in peripheral blood cells and DNA adducts in peripheral blood, liver and spleen

The sensitivity of various methods suitable for biomonitoring the exposure to genotoxicants was compared in an animal model. The results were related to the presence of genotoxic effects in the target organ. Groups of male Wistar rats were given one oral dose of 0, 0.1, 10 or 200 mg 2-acetylaminofluorene (2-AAF)/5 ml dimethyl sulphoxide/kg body weight. Peripheral blood cells, excreta, liver and spleen were collected at different time intervals after dosing. Mutagenicity in urine and extracts of faeces was determined using the Ames test with Salmonella typhimurium TA98 with and without S9 and with and without beta-glucuronidase. Genotoxic effects were studied by measuring DNA-adduct formation in lymphocytes, liver and spleen, and sister-chromatid exchanges (SCEs) in lymphocytes. DNA adducts were measured with immunochemical techniques and postlabelling methods. Mutagenicity in urine and faeces, collected during the first 24 h after treatment, was detected at 2-AAF doses of 1 mg/kg b.w. and higher. At these doses DNA adducts also became apparent in the liver, the main target organ for tumour induction by 2-AAF. The adduct detected appeared to be the N-(deoxyguanosin-8-yl)-2-AAF adduct. There was no evidence of the presence of any other types of DNA adducts. At doses of 1 and 10 mg/kg b.w. no mutagenicity was detected in excreta collected during the second and third day after dosing. The DNA-adduct level in liver cells of the 1 mg/kg b.w. group was maximal 24 h after dosing. At 200 mg/kg b.w. a delay in excretion of mutagenicity with urine and faeces was seen and at 10 and 200 mg/kg b.w. the amount of DNA adducts continued to increase with time after dosing. At 24 and 48 h after treatment with 10 mg, the adduct levels were of the same order of magnitude as those found after the 20-fold higher dose. This points to overloading of the metabolizing system which in combination with the enterohepatic circulation, may lead to an increased retention of 2-AAF in the body. A slightly increased incidence of SCEs of doubtful significance was seen in lymphocytes, but only at the very high dose of 200 mg/kg b.w. No DNA adducts could be detected in blood lymphocytes or spleen cells at any of the dose levels applied, either with the immunochemical or with the postlabelling method.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunochemical detection of DNA in alkaline sucrose gradient fractions

A procedure for the detection of non-radioactive DNA in alkaline sucrose gradients is described. This method consists of the following steps: (1) fractionation of the DNA-containing gradients into microtiter plates, neutralization and overnight adsorption; (2) covalent labelling of the guanine bases of the adsorbed DNA by reaction with N-acetoxy-2-acetylaminofluorene; and (3) determination of the gradient profile by means of an enzyme-linked immunosorbent assay using antibodies with high affinity for dG-AAF. The method has been found suitable for the rapid and sensitive determination of gamma-ray-induced single-strand breaks and UV-induced pyrimidine dimers in mammalian cells in vitro. In addition, it has been shown that the method can be used for the determination of pyrimidine dimers induced in skin cells in vivo.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 2. Diet, mutation and cancer

Experimental research designed to determine the effects of variations in diet on the carcinogenic and mutagenic processes is difficult to conduct and even more difficult to interpret in terms of the likely response that such variations will have on the expression of human cancer and mutation. Although some of these difficulties may be due to a failure to persuade adequate numbers of highly trained nutritionists to enter into this type of research, a more germaine reason may be that the high level of complexity of both diet and the disease processes is such as to confound present efforts at interpretation. It is suggested that a stepwise analysis of the effects of dietary factors on each critical stage in carcinogenesis or mutagenesis may ultimately lead to results that are more easily interpreted in terms of human response. To this end it is proposed that studies of DNA-carcinogen or DNA-mutagen adduct formation, or other DNA damage, DNA replication and relevant DNA repair at the target site may be a useful guide to the effect of nutritional changes on mutation and/or cancer initiation. DNA replication at various stages of carcinogenesis, modification of hormonal levels, modification of immune response, or other factors as influenced by diet may provide markers for cancer development. The integration of this data to give an overall perception of the effects of nutrition is briefly discussed.