Quantification of endogenous carcinogens. The ethylene oxide paradox

HPLC-ESI-HRMS2 Determination of N-(2-Hydroxyethyl)-l-valyl-l-leucine in Human Urine: Method Validation

Biomonitoring of human exposure to reactive electrophilic chemicals such as ethylene oxide (EO) has been commonly based on the determination of adducts with N-terminal valine in blood protein globin, but a systematic search has also been undertaken to find surrogate markers enabling non-invasive sampling. Recently, N-(2-hydroxyethyl)-L-valyl-L-leucine (HEVL) has been identified as an ultimate cleavage product of EO-adducted globin in the urine of occupationally exposed workers. Herein, full validation of the analytical procedure consisting of solid-phase extraction of HEVL from urine samples (2 mL) followed by high-performance liquid chromatography-electrospray ionization-high-resolution mass spectrometry determination using deuterium-labeled HEVL as an internal standard (IS) is described. Method limit of quantitation is 0.25 ng/mL, and its selectivity is excellent as demonstrated by the invariable ratio of the qualifier and quantifier ion intensities across diverse urine samples and synthetic standard. The linear calibration model was applicable over the whole concentration range tested (0.25-10 ng/mL). The method accuracy assessed as a recovery of HEVL using a spiking experiment was 98-100%. Within-day precision of the method ranged from 1.8% to 3.0%, while the results from consecutive analytical runs conducted within 1 week or within 10-150 weeks differed in the range of 2.2-9.7%. The stability study on urine samples (-20°C up to 3 years, freeze-and-thaw up to 10 cycles) as well as on aqueous solutions (5°C up to 4 months) indicated no relevant changes in HEVL concentration (≤4%) over the time tested. Analytical responses of both HEVL and IS correlated with urinary creatinine as an index of matrix composition, but this matrix effect was mostly eliminated using the HEVL/IS peak area ratio, attaining the IS-normalized relative matrix effect <3%. In conclusion, the method complied successfully with the bioanalytical method validation criteria, making it a reliable tool for HEVL determination in human biomonitoring.

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

The role of endogenous versus exogenous sources in the exposome of putative genotoxins and consequences for risk assessment

The "totality" of the human exposure is conceived to encompass life-associated endogenous and exogenous aggregate exposures. Process-related contaminants (PRCs) are not only formed in foods by heat processing, but also occur endogenously in the organism as physiological components of energy metabolism, potentially also generated by the human microbiome. To arrive at a comprehensive risk assessment, it is necessary to understand the contribution of in vivo background occurrence as compared to the ingestion from exogenous sources. Hence, this review provides an overview of the knowledge on the contribution of endogenous exposure to the overall exposure to putative genotoxic food contaminants, namely ethanol, acetaldehyde, formaldehyde, acrylamide, acrolein, α,β-unsaturated alkenals, glycation compounds, N-nitroso compounds, ethylene oxide, furans, 2- and 3-MCPD, and glycidyl esters. The evidence discussed herein allows to conclude that endogenous formation of some contaminants appears to contribute substantially to the exposome. This is of critical importance for risk assessment in the cases where endogenous exposure is suspected to outweigh the exogenous one (e.g. formaldehyde and acrolein).

A speculative discussion of four animal carcinogens endogenously produced in humans and a formula for cancer development

The major factors (macro) which cause human cancer have been elucidated and include tobacco use, diet, infection, reproductive and sexual behavior and, to a lesser extent, alcohol consumption and occupational factors. Several reports have been published about endogenous chemicals made in humans which produce DNA adducts; however, few have linked them to possible carcinogenic activity. This paper discussed four chemicals made in humans (formaldehyde, acetaldehyde, isoprene and ethylene oxide), pathways of their formation, their animal carcinogenicity and questions about these and other endogenous chemicals’ possible role in human cancer. In addition, the author posits a simplified formula for development of cancer and a formula for causing mutations by various agents.

Monitoring the effects of different conservation treatments on paper-infecting fungi

Fungi are among the most degradative organisms inducing biodeterioration of paper-based items of cultural heritage. Appropriate conservation measures and restoration treatments to deal with fungal infections include mechanical, chemical, and biological methods, which entail effects on the paper itself and health hazards for humans. Three different conservation treatments, namely freeze-drying, gamma rays, and ethylene oxide fumigation, were compared and monitored to assess their short- (one month, T1) and long-term (one year, T2) effectiveness to inhibit fungal growth. After the inoculation with fungi possessing cellulose hydrolysis ability - Chaetomium globosum, Trichoderma viride, and Cladosporium cladosporioides - as single strains or as a mixture, different quality paper samples were treated and screened for fungal viability by culture-dependent and -independent techniques. Results derived from both strategies were contradictory. Both gamma irradiation and EtO fumigation showed full efficacy as disinfecting agents when evaluated with cultivation techniques. However, when using molecular analyses, the application of gamma rays showed a short-term reduction in DNA recovery and DNA fragmentation; the latter phenomenon was also observed in a minor degree in samples treated with freeze-drying. When RNA was used as an indicator of long-term fungal viability, differences in the RNA recovery from samples treated with freeze-drying or gamma rays could be observed in samples inoculated with the mixed culture. Only the treatment with ethylene oxide proved negative for both DNA and RNA recovery. Therefore, DNA fragmentation after an ethylene oxide treatment can hamper future paleogenetic and archaeological molecular studies on the objects.

Susceptibility of lung epithelial cells to alkylating genotoxic insult

Alkylation is one of the most common types of DNA damage that can lead to mutations and cancer. Lung is the primary target organ of airborne alkylators such as ethylene oxide (EO). However, the ability of EO to cause lung cancer has not been clearly demonstrated yet. The aim of this study was to investigate the susceptibility of lung cells to alkylating DNA insult by detecting EO-mediated DNA damage with the alkaline comet assay in human lung epithelial cells, peripheral blood lymphocytes, and keratinocytes. The susceptibility of these cell types toward the alkylating insult induced by EO was compared against the oxidative DNA insult induced by hydrogen peroxide (H2 O2 ). Due to the volatility of EO, its active concentrations were monitored by gas chromatography during exposure and were found to decrease significantly in a time-dependent manner. EO induced a statistically significant genotoxic effect at the lowest concentration used (16.4 µM) in lung epithelial cells and in lymphocytes, while in keratinocytes, a genotoxic effect was not detected until 55.5 µM EO. However, lung epithelial cells demonstrated increased resistance to oxidative insult. In fact, oxidative DNA damage detectable by endonuclease treatment was minimal in lung cells compared with the other cell types. These results suggest an increased sensitivity of lung epithelial cells toward the alkylating effects of EO, which was not observed for oxidative DNA damage. Our findings point out the importance of DNA alkylation and the possible role of EO on the induction of lung cancer.

A case report - Volatile metabolomic signature of malignant melanoma using matching skin as a control

Melanoma is the most serious form of skin cancer. The quest for melanoma diagnostic biomarkers is paramount since early detection of melanoma and surgical excision represent the only effective treatment of this capricious disease. Our recent study tested the hypothesis that melanoma forms a unique volatile signature that is different than control, healthy tissue. Here, we are reporting a case study, the analysis of the volatile metabolic signature of a malignant melanoma using matched, non-neoplastic skin tissue from the same patient as a control. This is a significant improvement in the methodology, since it is well known that diet, skin type, genetic background, age, sex and environment all contribute to individual variation in the skin volatile signature. In the present study, we have identified 32 volatile compounds; 9 volatile compounds were increased in melanoma when compared to normal skin and 23 volatile compounds were detected only in melanoma and not in normal skin. Out of these 32 compounds, 10 have been reported previously by our group, thus confirming our results and adding additional confidence in our untargeted metabolomics approach for detection of melanoma biomarkers.

Rapid and sensitive on-line liquid chromatographic/tandem mass spectrometric determination of an ethylene oxide-DNA adduct, N7-(2-hydroxyethyl)guanine, in urine of nonsmokers

Ethylene oxide (EtO) is classified as a known human carcinogen. The formation of EtO-DNA adducts is considered as an important early event in the EtO carcinogenic process. An isotope-dilution on-line solid-phase extraction and liquid chromatography coupled with tandem mass spectrometry method was then developed to analyze one of the EtO-DNA adducts, N7-(2-hydroxyethyl)guanine (N7-HEG), in urine of 46 nonsmokers with excellent accuracy, sensitivity and specificity. The merits of this method include small sample volume (only 120 microL urine required), automated sample cleanup, and short total run time (12 minutes per sample). This method demonstrates its high-throughput capacity for future molecular epidemiology studies on the potential health effects resulting from the low-dose EtO exposure.

Biomarkers in toxicology and risk assessment: informing critical dose-response relationships

Tremendous advances have been made in the study of biomarkers related to carcinogenesis during the past 20 years. This perspective will briefly review improvements in methodology and instrumentation that have increased our abilities to measure the formation, repair, and consequences of DNA adducts. These biomarkers of exposure, along with surrogates such as protein adducts, have greatly improved our understanding of species differences in metabolism and effects of chemical stability and DNA repair on tissue differences in molecular dose. During this same time frame, improvements in assays for biomarkers of effect have provided better data and an improved understanding of the dose responses for both gene and chromosomal mutations. A framework analysis approach was used to examine the mode of action of genotoxic chemicals and the default assumption that cancer can be expected to be linear at very low doses. This analysis showed that biomarkers of exposure are usually linear at low doses, with the exception being when identical adducts are formed endogenously. Whereas biomarkers of exposure extrapolate down to zero, biomarkers of effect can only be interpolated back to the spontaneous or background number of mutations. The likely explanation for this major difference is that at high exposures, the biology that results in mutagenesis is driven by DNA damage resulting from the chemical exposure. In contrast, at very low exposures, the biology that results in mutagenesis is driven by endogenous DNA damage. The shapes of the dose-response curves for biomarkers of exposure and effect can be very different, with biomarkers of effect better informing quantitative estimates of risk for cancer, a disease that results from multiple mutations. It is also clear, however, that low dose data on mutagenesis are needed for many more chemicals.

De minimus non curat lex--virtual thresholds for cancer initiation by tobacco specific nitrosamines--prospects for harm reduction by smokeless tobacco

Whereas the impact of tobacco specific nitrosamines in smokers is obscured by the presence of numerous other carcinogens and promoters, for smokeless tobacco virtually all the carcinogenic potential is associated with 4-(nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN). In some countries exposure to smokeless tobacco with extremely high nitrosamine concentrations have been found to induce cancers in the head-neck region, whereas three recent large epidemiological studies failed to detect any such risk with respect to Swedish low-nitrosamine snuff. This review deals with quantitative aspects of DNA adduct formation from NNN and NNK in relation to the background levels ubiquitously found in healthy humans without known exposures to either tobacco or alkylating agents. The lack of significant increases of pro-mutagenic O6-methylations and DNA pyridyloxobutylations seen in smokers, as well as the negative outcome of the Swedish epidemiological studies, can be expected on basis of extrapolation of the dose response relationships found in rodents to actual exposures to NNK and NNN in Swedish snuff or from smoking. Sweden has the lowest prevalence of male smokers and smoking related diseases in the Western World, which has been ascribed to the fact that more than 20% of the grown up male population uses snuff. Smokeless tobacco represents an inexpensive and effective alternative to nicotine delivering products like nicotine patch, spray or gum. Considering that all other tobacco products are freely marketed, the ban on low-nitrosamine snuff in all countries in EU except Sweden is difficult to defend on either medical or ethical grounds.

Carcinogenicity categorization of chemicals-new aspects to be considered in a European perspective

Existing systems of classification of carcinogens are a matter of discussion, world-wide. There is agreement that it should be distinguished between genotoxic and non-genotoxic chemicals. The risk assessment approach used for non-genotoxic chemicals is similar among different regulatory bodies: insertion of an uncertainty (safety) factor permits the derivation of permissible exposure levels at which no relevant human cancer risks are anticipated. For genotoxic carcinogens, case studies of chemicals point to a whole array of possibilities. Positive data of chromosomal effects only, in the absence of mutagenicity, may support the characterization of a compound that produces carcinogenic effects only at high, toxic doses. Non-DNA-reactive genotoxins, such as topoisomerase inhibitors or inhibitors of the spindle apparatus are considered in this respect. In such cases, arguments are in favour of the existence of "practical" thresholds. Taking existing concepts together, it is proposed to basically distinguish between "perfect" and "practical" thresholds. There is a wide consensus that for non-DNA-reactive genotoxins such as aneugens (aneuploidy, chromosome loss, non-disjunction) thresholds should be defined. It is being discussed as to whether the identification of possible threshold effects should also include other mechanisms of genotoxicity, in addition to aneugenic effects. Specific mechanisms of clastogenicity have been repeatedly addressed as also having thresholds, such as topoisomerase II poisons or mechanisms based on reactive oxygen. Oxidative stress as an important mechanism is triggered by exposure to exogenous factors such as ultraviolet (UV) and ionizing radiation, anoxia and hyperoxia, and by chemicals producing reactive oxygen species. The idea is receiving increased support that reactive oxygen species (ROS)-mediated processes of carcinogenesis have practical thresholds. Since reactive oxygen species are genotoxic in principle, questions arise whether chemicals that increase ROS production will superimpose to an endogenously produced background level of DNA lesions, related to mechanisms that may result in non-linear dose-effect relationships. The existence of "endogenous" DNA adducts has been generally accepted, and possible regulatory implications of the presence of endogenous carcinogens have been discussed. It is now becoming evident that a diversity of methods of carcinogenic risk extrapolation to low doses must be considered, dependent on the mode of action. Although there is an increasing international awareness of these developments, the system of classification of carcinogens of the European Union still remains static. This should be changed, as the philosophy of separation of a strictly sequential "hazard assessment" and "risk assessment" appears out-of-date.

Markers of genetic susceptibility in human environmental hygiene and toxicology: the role of selected CYP, NAT and GST genes

Inherited genetic traits co-determine the susceptibility of an individual to a toxic chemical. Special emphasis has been put on individual responses to environmental and industrial carcinogens, but other chronic diseases are of increasing interest. Polymorphisms of relevant xenobiotic metabolising enzymes may be used as toxicological susceptibility markers. A growing number of genes encoding enzymes involved in biotransformation of toxicants and in cellular defence against toxicant-induced damage to the cells has been identified and cloned, leading to increased knowledge of allelic variants of genes and genetic defects that may result in a differential susceptibility toward environmental toxicants. "Low penetrating" polymorphisms in metabolism genes tend to be much more common in the population than allelic variants of "high penetrating" cancer genes, and are therefore of considerable importance from a public health point of view. Positive associations between cancer and CYP1A1 alleles, in particular the *2C I462V allele, were found for tissues following the aerodigestive tract. Again, in most cases, the effect of the variant CYP1A1 allele becomes apparent or clearer in connection with the GSTM1 null allele. The CYP1B1 codon 432 polymorphism (CYP1B1*3) has been identified as a susceptibility factor in smoking-related head-and-neck squameous cell cancer. The impact of this polymorphic variant of CYP1B1 on cancer risk was also reflected by an association with the frequency of somatic mutations of the p53 gene. Combined genotype analysis of CYP1B1 and the glutathione transferases GSTM1 or GSTT1 has also pointed to interactive effects. Of particular interest for the industrial and environmental field is the isozyme CYP2E1. Several genotypes of this isozyme have been characterised which seem to be associated with different levels of expression of enzyme activity. The acetylator status for NAT2 can be determined by genotyping or by phenotyping. In the pathogenesis of human bladder cancer due to occupational exposure to "classical" aromatic amines (benzidine, 4-aminodiphenyl, 1-naphthylamine) acetylation by NAT2 is regarded as a detoxication step. Interestingly, the underlying European findings of a higher susceptibility of slow acetylators towards aromatic amines are in contrast to findings in Chinese workers occupationally exposed to aromatic amines which points to different mechanisms of susceptibility between European and Chinese populations. Regarding human bladder cancer, the hypothesis has been put forward that genetic polymorphism of GSTM1 might be linked with the occurrence of this tumour type. This supports the hypothesis that exposure to PAH might causally be involved in urothelial cancers. The human polymorphic GST catalysing conjugation of halomethanes, dihalomethanes, ethylene oxide and a number of other industrial compounds could be characterised as a class theta enzyme (GSTT1) by means of molecular biology. "Conjugator" and "non-conjugator" phenotypes are coincident with the presence and absence of the GSTT1 gene. There are wide variations in the frequencies of GSTT1 deletion (GSTT1*0/0) among different ethnicities. Human phenotyping is facilitated by the GST activity towards methyl bromide or ethylene oxide in erythrocytes which is representative of the metabolic GSTT1 competence of the entire organism. Inter-individual variations in xenobiotic metabolism capacities may be due to polymorphisms of the genes coding for the enzymes themselves or of the genes coding for the receptors or transcription factors which regulate the expression of the enzymes. Also, polymorphisms in several regions of genes may cause altered ligand affinity, transactivation activity or expression levels of the receptor subsequently influencing the expression of the downstream target genes. Studies of individual susceptibility to toxicants and gene-environment interaction are now emerging as an important component of molecular epidemiology.

Carcinogenicity and genotoxicity of ethylene oxide: new aspects and recent advances

Long-term inhalation studies in rodents have presented unequivocal evidence of experimental carcinogenicity of ethylene oxide, based on the formation of malignant tumors at multiple sites. However, despite a considerable body of epidemiological data only limited evidence has been obtained of its carcinogenicity in humans. Ethylene oxide is not only an important exogenous toxicant, but it is also formed from ethylene as a biological precursor. Ethylene is a normal body constituent; its endogenous formation is evidenced by exhalation in rats and in humans. Consequently, ethylene oxide must also be regarded as a physiological compound. The most abundant DNA adduct of ethylene oxide is 7-(2-hydroxyethyl)guanine (HOEtG). Open questions are the nature and role of tissue-specific factors in ethylene oxide carcinogenesis and the physiological and quantitative role of DNA repair mechanisms. The detection of remarkable individual differences in the susceptibility of humans has promoted research into genetic factors that influence the metabolism of ethylene oxide. With this background it appears that current PBPK models for trans-species extrapolation of ethylene oxide toxicity need to be refined further. For a cancer risk assessment at low levels of DNA damage, exposure-related adducts must be discussed in relation to background DNA damage as well as to inter- and intraindividual variability. In rats, subacute ethylene oxide exposures on the order of 1 ppm (1.83 mg/m3) cause DNA adduct levels (HOEtG) of the same magnitude as produced by endogenous ethylene oxide. Based on very recent studies the endogenous background levels of HOEtG in DNA of humans are comparable to those that are produced in rodents by repetitive exogenous ethylene oxide exposures of about 10 ppm (18.3 mg/m3). Experimentally, ethylene oxide has revealed only weak mutagenic effects in vivo, which are confined to higher doses. It has been concluded that long-term human occupational exposure to low airborne concentrations to ethylene oxide, at or below current occupational exposure limits of 1 ppm (1.83 mg/m3), would not produce unacceptable increased genotoxic risks. However, critical questions remain that need further discussions relating to the coherence of animal and human data of experimental data in vitro vs. in vivo and to species-specific dynamics of DNA lesions.

Biomarkers of exposure and effect as indicators of potential carcinogenic risk arising from in vivo metabolism of ethylene to ethylene oxide

The purposes of the present study were: (i) to investigate the potential use of several biomarkers as quantitative indicators of the in vivo conversion of ethylene (ET) to ethylene oxide (EO); (ii) to produce molecular dosimetry data that might improve assessment of human risk from exogenous ET exposures. Groups (n = 7/group) of male F344 rats and B6C3F1 mice were exposed by inhalation to 0 and 3000 p. p.m. ET for 1, 2 or 4 weeks (6 h/day, 5 days/week) or to 0, 40, 1000 and 3000 p.p.m. ET for 4 weeks. N:-(2-hydroxyethyl)valine (HEV), N:7-(2-hydroxyethyl) guanine (N7-HEG) and HPRT: mutant frequencies were assessed as potential biomarkers for determining the molecular dose of EO resulting from exogenous ET exposures of rats and mice, compared with background biomarker values. N7-HEG was quantified by gas chromatography coupled with high resolution mass spectrometry (GC-HRMS), HEV was determined by Edman degradation and GC-HRMS and HPRT: mutant frequencies were measured by the T cell cloning assay. N7-HEG accumulated in DNA with repeated exposure of rodents to 3000 p.p.m. ET, reaching steady-state concentrations around 1 week of exposure in most tissues evaluated (brain, liver, lung and spleen). The dose-response curves for N7-HEG and HEV were supralinear in exposed rats and mice, indicating that metabolic activation of ET was saturated at exposures >/=1000 p.p.m. ET. Exposures of mice and rats to 200 p.p.m. EO for 4 weeks (as positive treatment controls) led to significant increases in HPRT: mutant frequencies over background in splenic T cells from exposed rats and mice, however, no significant mutagenic response was observed in the HPRT: gene of ET-exposed animals. Comparisons between the biomarker data for both unexposed and ET-exposed animals, the dose-response curves for the same biomarkers in EO-exposed rats and mice and the results of the rodent carcinogenicity studies of ET and EO suggest that too little EO arises from exogenous ET exposure to produce a significant mutagenic response or a carcinogenic response under standard bioassay conditions.

Formation of DNA adducts and induction of mutagenic effects in rats following 4 weeks inhalation exposure to ethylene oxide as a basis for cancer risk assessment

Ethylene oxide (EO) is mutagenic in various in vitro and in vivo test systems and carcinogenic in rodents. EO forms different adducts upon reaction with DNA, N7-(2-hydroxyethyl)guanine (N7-HEG) being the main adduct. The major objectives of this study were: (a) to determine the formation and persistence of N7-HEG adducts in liver DNA of adult male rats exposed to 0, 50, 100 and 200 ppm by inhalation (4 weeks, 5 days/week, 6 h/day) and (b) to assess dose-response relationships for Hprt gene mutations and various types of chromosomal changes in splenic lymphocytes.N7-HEG adducts were measured 5, 21, 35 and 49 days after cessation of exposure. By extrapolation, the mean concentrations of N7-HEG immediately after cessation of exposure ('day 0') to 50, 100 and 200 ppm were calculated as 310, 558 and 1202 adducts/10(8) nucleotides, respectively, while the mean concentration in control rats was 2.6 adducts/10(8) nucleotides. At 49 days, N7-HEG values had returned close to background levels. The mean levels of N-(2-hydroxyethylvaline) adducts in haemoglobin were also determined and amounted 61.7, 114 and 247 nmol/g globin, respectively. Statistically significant linear relationships were found between mean N7-HEG levels ('day 0') and Hprt mutant frequencies at expression times 21/22 and 49/50 days and between mean N7-HEG ('day 0') and sister-chromatid exchanges (SCEs) or high frequency cells (HFC) measured 5 days post-exposure. At day 21 post-exposure, SCEs and HFCs in-part persisted and were significantly correlated with persistent N7-HEG adducts. No statistically significant dose effect relationships were observed for induction of micronuclei, nor for chromosome breaks or translocations. In conclusion, this study indicates that following sub-chronic exposure, EO is only weakly mutagenic in adult rats. Using the data of this study to predict cancer risk in man resulting from low level EO exposures in conjunction with other published data, i.e., those on (a) genotoxic effects of EO in humans and rats, (b) DNA binding of other carcinogens, (c) natural background DNA binding and (d) genotoxic potency of low energy transfer (LET) radiation, it is not expected that long term occupational exposure to airborne concentrations of EO at or below 1 ppm EO produces an unacceptable increased risk in man.

Internal hazards: baseline DNA damage by endogenous products of normal metabolism

Recent improvements in the ability to detect chemically modified bases in DNA have revealed that not only does the genetic material incur damage by foreign chemicals, but that it also sustains injury by reactive products of normal physiological processes. This review summarises current understanding of the DNA-damaging potential of various substances of endogenous origin, including oxidants, lipid peroxidation products, alkylating agents, estrogens, chlorinating agents, reactive nitrogen species, and certain intermediates of various metabolic pathways. The strengths and weaknesses of the existing database for DNA damage by each class of substance are discussed, as are future strategies for resolving the difficult question of whether endogenous chemicals are significant contributors to spontaneous mutagenesis and cancer development in vivo.

The carcinogenic risk of ethene (ethylene)

On occasion of the 25th year of publication of Toxicologic Pathology, the Editor has asked for a report about recent progress in the area addressed by an article entitled “Olefinic Hydrocarbons: A First Risk Estimate,” one of the top 10 most frequently cited papers of the journal (3). Because general issues of risk assessment have very recently been addressed in this journal (6), I will focus on new specific aspects of ethene carcinogenicity.

Formation of novel C1-oxidised abasic sites in alkylperoxyl radical-damaged plasmid DNA

We have recently shown that peroxyl radicals react with DNA to form alkali-labile sites. To further characterise these lesions, we studied their susceptibility to digestion by repair endonucleases that recognise different types of abasic sites. We found that peroxyl radical-damaged pSP189 plasmids were resistant to cleavage by T4 endonuclease V, an enzyme that incises DNA at "regular" and C4-oxidised abasic residues. In contrast, the DNA was digested by exonuclease III, an enzyme that recognises "regular" and C1-oxidised abasic sites. The presence of Trolox during exposure to peroxyl radicals reduced subsequent DNA cleavage by exonuclease III, while prior incubation of damaged plasmids with methoxyamine potentiated digestion by this enzyme. These findings suggest that peroxyl radical-induced DNA damage involves the generation of novel C1-oxidised deoxyribose residues.