International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC publication no. 10. Review of the evidence for the presence or absence of thresholds in the induction of genetic effects by genotoxic chemicals

Hypersensitivity of mouse embryonic fibroblast cells defective for DNA polymerases η, ι and κ to various genotoxic compounds: Its potential for application in chemical genotoxic screening

Genotoxic agents cause modifications of genomic DNA, such as alkylation, oxidation, bulky adduct formation, and strand breaks, which potentially induce mutations and changes to the structure or number of genes. Majority of point mutations are generated during error-prone bypass of modified nucleotides (translesion DNA synthesis, TLS); however, when TLS fails, replication forks stalled at lesions eventually result in more lethal effects, formation of double-stranded breaks (DSBs). Here we compared sensitivities to various compounds among mouse embryonic fibroblasts derived from wild-type and knock-out mice lacking one of the three Y-family TLS DNA polymerases (Polη, Polι, and Polκ) or all of them (TKO). The compounds tested in this study include genotoxins such as methyl methanesulfonate (MMS) and nongenotoxins such as ammonium chloride. We found that TKO cells exhibited the highest sensitivities to most of the tested genotoxins, but not to the non-genotoxins. In order to quantitatively evaluate the hypersensitivity of TKO cells to different chemicals, we calculated ratios of half-maximal inhibitory concentration for WT and TKO cells. The ratios for 9 out of 10 genotoxins ranged from 2.29 to 5.73, while those for 5 nongenotoxins ranged from 0.81 to 1.63. Additionally, the two markers for DNA damage, ubiquitylated proliferating cell nuclear antigen and γ-H2AX after MMS treatment, were accumulated in TKO cells more greatly than in WT cells. Furthermore, following MMS treatment, TKO cells exhibited increased frequency of sister chromatid exchange compared with WT cells. These results indicated that the hypersensitivity of TKO cells to genotoxins resulted from replication fork stalling and subsequent DNA double-strand breaks, thus demonstrating that TKO cells should be useful for evaluating chemical genotoxicity.

Dose-response analysis of bromate-induced DNA damage and mutagenicity is consistent with low-dose linear, nonthreshold processes

Mutagenic agents have long been inferred to act through low-dose linear, nonthreshold processes. However, there is debate about this assumption, with various studies interpreting datasets as showing thresholds for DNA damage and mutation. We have applied rigorous statistical analyses to investigate the shape of dose-response relationships for a series of in vitro and in vivo genotoxicity studies using potassium bromate (KBrO(3) ), a water ozonation byproduct that is bioactivated to a reactive species causing oxidative damage to DNA. We analyzed studies of KBrO(3) genotoxicity where no-effect/threshold levels were reported as well as other representative datasets. In all cases, the data were consistent with low-dose linear models. In the majority of cases, the data were fit either by a linear (straight line) model or a model which was linear at low doses and showed a saturation-like downward curvature at high doses. Other datasets with apparent upward curvature were still adequately represented by models that were linear at low dose. Sensitivity analysis of datasets showing upward curvature revealed that both low-dose linear and nonlinear models provide adequate fits. Additionally, a simple biochemical model of selected key processes in bromate-induced DNA damage was developed and illustrated a situation where response for early primary events suggested an apparent threshold while downstream events were linear. Overall, the statistical analyses of DNA damage and mutations induced by KBrO(3) are consistent with a low-dose linear response and do not provide convincing evidence for the presence of a threshold.

Miniaturized flow cytometric in vitro micronucleus assay represents an efficient tool for comprehensively characterizing genotoxicity dose-response relationships

This laboratory has developed a flow cytometric approach for scoring in vitro micronuclei (In Vitro MicroFlow(®)) whose characteristics are expected to benefit studies designed to comprehensively investigate genotoxicity dose-response relationships. In particular, new experimental designs become possible when automated scoring is combined with treatment, processing and sampling that all occur in microtiter plates. To test this premise, experiments described herein investigated micronucleus (MN) formation in TK6 cells treated with genotoxic agents applied at 22 closely spaced concentrations in quadruplicate, with 10,000 cells analyzed per replicate. The genotoxicants colchicine, vinblastine sulfate, ethyl methanesulfonate, methyl methanesulfonate, ethyl nitrosourea, methyl nitrosourea, and bleomycin were applied continuously for 24-30 h. Following treatment, all cell processing, sampling and data acquisition steps were accomplished in the same 96-well plate. Data acquisition occurred in a walk-away mode via the use of a high throughput sampling device. The resulting flow cytometric MN values were evaluated with a statistical model that indicated non-linear relationships describe the data better than linear fits. The one exception was bleomycin, where MN induction was consistently best described by a linear dose-response relationship. Collectively, these results suggest that flow cytometry represents a practical and efficient approach for thoroughly examining the dose-response relationship, and clearly benefits studies that seek to characterize no observable genotoxic effect levels, lowest observable genotoxic effect levels, and/or benchmark doses.

Flow cytometry peripheral blood micronucleus test in vivo: determination of potential thresholds for aneuploidy induced by spindle poisons

Non-DNA binding genotoxins (e.g., aneugens), unlike DNA-binding genotoxins, are theoretically expected to show thresholded concentration-effect response curves. This is a major issue in genetic toxicology testing because the identification of thresholds in vivo can provide a safety margin for exposure to a particular compound. In the current study we measured micronucleus induction by flow cytometry to determine the dose-response curves for tubulin interacting agents, a specific class of aneugens. All experiments with aneugens, which include colchicine, vinblastine, vincristine, as well as the clastogen cyclophosphamide (CP) were performed in mice to avoid the splenic elimination of micronucleated reticulocytes, which has been described in rats. Flow cytometry analysis revealed a non-linear dose-dependent increase in micronuclei frequencies for all tested aneugens, and a linear dose response curve for the clastogen, CP. To determine whether micronucleus induction at higher doses was due to chromosome loss (aneuploidy) or chromosome breakage (clastogenicity), flow sorting of the micronucleated reticulocytes and fluorescent in situ hybridization (FISH) with a mouse pan centromeric probe were performed for vinblastine, vincristine, and colchicine. Statistical evaluation of the flow cytometry and FISH data was performed to determine the threshold levels for chromosome loss in vivo. The threshold concentrations for vinblastine, vincristine, and colchicine were found at 0.35, 0.017, and 0.49 mg kg(-1), respectively.

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.

A review of the toxicology of acrylamide

Acrylamide (ACR) is a chemical used in many industries around the world and more recently was found to form naturally in foods cooked at high temperatures. Acrylamide was shown to be a neurotoxicant, reproductive toxicant, and carcinogen in animal species. Only the neurotoxic effects were observed in humans and only at high levels of exposure in occupational settings. The mechanism underlying neurotoxic effects of ACR may be basic to the other toxic effects seen in animals. This mechanism involves interference with the kinesin-related motor proteins in nerve cells or with fusion proteins in the formation of vesicles at the nerve terminus and eventual cell death. Neurotoxicity and resulting behavioral changes can affect reproductive performance of ACR-exposed laboratory animals with resulting decreased reproductive performance. Further, the kinesin motor proteins are important in sperm motility, which could alter reproduction parameters. Effects on kinesin proteins could also explain some of the genotoxic effects on ACR. These proteins form the spindle fibers in the nucleus that function in the separation of chromosomes during cell division. This could explain the clastogenic effects of the chemical noted in a number of tests for genotoxicity and assays for germ cell damage. Other mechanisms underlying ACR-induced carcinogenesis or nerve toxicity are likely related to an affinity for sulfhydryl groups on proteins. Binding of the sulfhydryl groups could inactive proteins/enzymes involved in DNA repair and other critical cell functions. Direct interaction with DNA may or may not be a major mechanism for cancer induction in animals. The DNA adducts that form do not correlate with tumor sites and ACR is mostly negative in gene mutation assays except at high doses that may not be achievable in the diet. All epidemiologic studies fail to show any increased risk of cancer from either high-level occupational exposure or the low levels found in the diet. In fact, two of the epidemiologic studies show a decrease in cancer of the large bowel. A number of risk assessment studies were performed to estimate increased cancer risk. The results of these studies are highly variable depending on the model. There is universal consensus among international food safety groups in all countries that examined the issue of ACR in the diet that not enough information is available at this time to make informed decisions on which to base any regulatory action. Too little is known about levels of this chemical in different foods and the potential risk from dietary exposure. Avoidance of foods containing ACR would result in worse health issues from an unbalanced diet or pathogens from under cooked foods. There is some consensus that low levels of ACR in the diet are not a concern for neurotoxicity or reproductive toxicity in humans, although further research is need to study the long-term, low-level cumulative effects on the nervous system. Any relationship to cancer risk from dietary exposure is hypothetical at this point and awaits more definitive studies.

Evaluation of nonthreshold leukemogenic response to methyl nitrosourea in p53-deficient C3H/He mice

The classic controversy of whether genotoxic chemicals induce cancers with or without a certain low-dose limit, i.e., the threshold, is revisited because of a number of current publications available addressing the plausibility of "practical" thresholds even for genotoxic carcinogens, the mechanism of which may be hypothesized to be due, in part, to a repair system composed of ordinarily available various defense mechanisms under the steady-state DNA damage. The question of whether an absolute nonthreshold or a relative nonthreshold, i.e., a "practical" threshold specifically in the low-dose level, is present may not be answered even with the use of a prohibitively large number of wild-type mice. Could the excessive incidence of tumorigenesis in p53-deficient mice contribute to our understanding of the threshold vs nonthreshold issue in genotoxic carcinogenesis? This is considered because an exaggeration of tumorigenesis in p53-deficient mice is hypothesized to reduce or eliminate the range of threshold due to the p53-deficiency-mediated reduction of DNA repair and apoptosis. The present study of chemical leukemogenesis in p53-deficient mice by transplantation assay was designed to answer this question. Briefly, 218 C3H/He mice were lethally irradiated and repopulated with bone marrow cells from wild-type, heterozygous p53-deficient, and homozygous p53-deficient C3H/He mice. This was followed by treatment with a single and graded dose of methyl nitrosourea at 6.6, 14.8, 33.3, 50.0, and 75.0 mg/kg body wt, with the vehicle-treated control groups treated with zero dose for each genotype. Whereas mice repopulated with p53-deficient bone marrow cells showed a marked reduction of the threshold for leukemogenicity, mice repopulated with wild-type bone marrow cells did not exhibit leukemia at a dose of 33.3 mg/kg body wt and showed a curve with a high probability for the linear regression model with a positive dose intercept, predicting a threshold by the likelihood ratio test. Thus, the failure of wild-type mice to show an increase in incidence of leukemogenesis at low doses of genotoxic carcinogens may be due not to a statistical rarity, but to various p53-related pharmacophysiological functions, possibly including DNA repair and apoptosis that may account for a threshold.

Susceptibility differences in chemical carcinogenesis linearize the dose-response relationship: threshold doses can be defined only for individuals

The debate on thresholds in dose-response relationships for chemical carcinogenesis concentrates on the question of mechanisms of action that come into play only at dose levels that overwhelm compensatory control mechanism, such as DNA repair or regulation of cell proliferation and death. In this article, individual susceptibility differences are introduced. It is postulated that one single threshold dose cannot be defined for a heterogeneous population because both the background rate of carcinogenesis and specific exposure-related effects differ between individuals. A threshold dose can therefore be defined only on an individual basis and for a given organ. Expressed as a time-to-tumor, the threshold dose results in tumor manifestation at exactly the end of the specified observation period. For those individuals who do not have cancer by the end of this period, the dose was below their individual threshold dose, for those who do have cancer, the dose was above their threshold dose. Based on this concept, a distinction between genotoxic and non-genotoxic carcinogens is no longer required; both types modulate time-to-tumor. Although the present analysis does not allow to define a threshold dose for a population, the setting of a "limit value" for regulatory purposes can be considered if regulators are aware of the fact that this splits a population at some percentile into a group for which the chosen standard is protective and a group for which it might not be. Investigation of factors that confer particular susceptibility to individuals is the key to an understanding of the dose-response relationship at low dose.

Threshold effects in genetic toxicity: perspective of chemicals regulation in Germany

In the regulation of chemical substances, it is generally agreed that there are no thresholds for genotoxic effects of chemicals, i.e. , that there are no doses without genotoxic effects. When classifying and labelling chemicals, dangerous properties of chemicals are to be identified. In this context, in general, the mode of action (threshold or not) is not considered for genotoxic substances. In the process of quantitative risk assessment, however, determination of the type of dose-effect relationships is decisive for the outcome and the type of risk management. The presence of a threshold must be justified specifically in each individual case. Inter alia, the following aspects may be discussed in this respect: aneugenic activity, indirect modes of action, extremely steep dose-effect relationships in combination with strong toxicity, specific toxicokinetic conditions which may lead to 'metabolic protection' prior to an attack of DNA. In the practice of the regulation of chemical substances with respect to their genotoxic effects, the discussion of thresholds has played a minor role. For notified new substances, there are, in general, no data available that would allow a reasonable discussion. Concerning substances out of the European programme on existing substances, so far 29 have been assessed in our institute with respect to genetic toxicity. Eight out of these have shown considerable evidence for genotoxicity. For two of them, a possible threshold is discussed: one substance is an aneugen, the other one is metabolised to an endogenic compound with genotoxic potential. In the practice of risk assessment of genotoxic substances, the discussion of the mode of action for genotoxicity is frequently associated with the evaluation of potential carcinogenic effects. Here, tissue-specific genotoxic effects in target organs for carcinogenicity are to be discussed. Moreover, the contribution of genotoxicity to the multifactorial process of tumour development should be assessed.

Interpretation of the biological relevance of genotoxicity test results: the importance of thresholds

Despite recent improvements in genotoxicity protocols, we have observed an increase in the occurrence of positive results, particularly in chromosomal aberration tests in vitro, yet very few of these are accompanied by positive responses in vivo. Thus, the positive results may not be biologically relevant either for rodents or humans in vivo, but how should we determine "biological relevance"? Chemicals that produce thresholded dose-responses may well not pose a genotoxic risk at low (relevant to human) exposures, but thresholds should not just be "seen"; there must be an explanation and understanding of the underlying mechanism. In addition to extremes of pH, ionic strength and osmolality, as have been identified previously, such mechanisms include indirect genotoxicity resulting from interaction with non-DNA targets, chemicals/metabolites which are inherently genotoxic but which, at low concentrations, are effectively conjugated and unable to form adducts, and production of specific metabolites under in vitro conditions that are not formed in rodents or humans in vivo. If such thresholded mechanisms can be identified at exposures which are well in excess of expected human exposure, then there may be a strong argument that the positive results are not biologically relevant.

Concepts of threshold in mutagenesis and carcinogenesis

Although the existence of a threshold in the dose effect relationship is well documented for many, if not most, types of toxicological effects the existence of a threshold for the mutagenic effects of ionising radiation and of certain chemicals has been questioned since the middle of the century and only recently the question of thresholds for radiation and chemical carcinogenesis has been addressed. The essential facts for the interpretation of threshold dose-response curves are common to all type of effects and are: (i) the number and the identity of the target; (ii) the type and sensitivity of the endpoint used to quantify the effect. We therefore will first try to model the type of interactions which may be expected between a mutagen and its target and define from this whether a threshold dose-effect can be expected; in a second step the concept will be extended to heritable mutations and carcinogenesis.

Acrylamide: a review of its genotoxicity and an assessment of heritable genetic risk

An updated review of the genotoxicity studies with acrylamide is provided. Then, using data from the studies generating quantitative information concerning heritability of genetic effects, an assessment of the heritable genetic risk presented by acrylamide is presented. The review offers a discussion of the reactions and possible mechanisms of genotoxic action by acrylamide and its epoxide metabolite glycidamide. Several genetic risk approaches are discussed, including the parallelogram, direct (actually a modified direct), and doubling dose approaches. Using data from the specific-locus and heritable translocation assays, the modified direct and doubling dose approaches are utilized to quantitate genetic risk. Exposures of male parents to acrylamide via inhalation, ingestion, and dermal routes are also quantitated. With these approaches and measurements and their underlying assumptions concerning extrapolation factors (including germ cell stage specificity, DNA repair variability, locus specificity), number of human loci associated with dominant disease alleles, and spontaneous mutation rates, an assessment of heritable genetic risk for humans is calculated for the three exposure scenarios. The calculated estimates for offspring from fathers exposed to acrylamide via drinking water are up to three offspring potentially affected with induced genetic disease per 10(8) offspring. Estimates for inhalation or dermal exposures suggest higher risks for induced genetic disease in offspring from fathers exposed in occupational settings.

Cytogenetic toxicity and no-effect limit dose of pesticides

The no-effect limit dose (NELD) of three commonly used pesticides with respect to their cytogenetic toxicity was determined in a number of test systems using a sufficient number of lower doses to characterize the dose-effect relationship. For lindane, malathion and metacid, this dose was 3.2, 7.0 and 3.0 mg/litre, respectively, for mitosis inhibition and 9.0, 55 and 60 mg/litre, respectively, for chromosome clastogeny in onion root-tip cells. For chromosome clastogeny in mice bone marrow cells, the NELDs of the three pesticides were 1.6, 1.5 and 2.0 mg/kg body weight/day, respectively. These values for dominant lethals and X-chromosome-linked recessive lethals in Drosophila were 20 and 5 micrograms lindane/litre, 2 and 3.5 micrograms malathion/litre and 4 and 5.5 micrograms metacid/litre, respectively. Thus, the NELDs are not only pesticide specific but also organism specific, tissue specific and even damage specific. Furthermore, the NELD values determined are so small that the real human exposure to pesticides cannot be reduced below these levels without compromising the effectiveness of pesticides in use.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 6. Estimation of genetic risks of exposure to chemical mutagens: relevance of data on spontaneous mutations and of experience with ionizing radiation

This paper examines the impact of advances in knowledge on the molecular biology of human Mendelian diseases on the estimation of genetic risks of exposure to ionizing radiation and to chemical mutagens. More specifically, it addresses the question of whether and to what extent naturally occurring Mendelian diseases can be used as a baseline for efforts in this area. Data on the molecular nature and mechanisms of origin of spontaneous mutations underlying naturally occurring Mendelian diseases and on radiation-induced mutations in experimental systems suggest that for ionizing radiation, naturally occurring Mendelian diseases may not constitute an entirely adequate frame of reference and that current risk estimates for this class of diseases are conservative; these estimates however provide a margin of safety in formulating radiation protection guidelines. Currently available data on mechanisms and specificities of action of chemical mutagens, molecular dosimetry, repair of chemically induced adducts in the DNA, adduct-mutation relationships etc., permit the tentative conclusion that naturally occurring Mendelian diseases may provide a better baseline for genetic risk estimation for chemical mutagens than for ionizing radiation. With both ionizing radiation and chemical mutagens, the question of which Mendelian diseases are potentially inducible will become answerable in the near future when more molecular data on human genetic diseases become available. It is therefore essential that risk estimators keep abreast of advances in human genetics and integrate these into their conceptual framework. However, induced Mendelian diseases (especially the dominant ones which are of more immediate concern) are likely to represent a very small fraction of the adverse genetic effects of induced mutations. More attention therefore needs to be devoted to studies on the heterozygous effects of induced mutations.

The risks of handling cytotoxic drugs. I. Methods of testing exposure

Results of various biological and physical/chemical tests of the urines or blood of health-care personnel working with cytotoxic drugs are discussed. The outcomes of these tests are conflicting and inconclusive. The physical/chemical tests seem to be an alternative method. However, until now it has not been possible to establish the threshold concentration in urine or blood beneath which no effect has to be expected. Therefore, the interpretation of the concentration of cytotoxic drugs and/or the metabolites in the urine or blood is difficult. As long as one will not be able to provide conclusive data on the health hazards when working with cytotoxic drugs, protective measures have to be taken in order to lower the risk as much as possible.

A dose-response analysis of ethylnitrosourea-induced recessive specific-locus mutations in treated spermatogonia of the mouse

A dose-response analysis was carried out with 2 independent data sets available for ethylnitrosourea-induced specific-locus mutations in spermatogonia of the mouse. It was assumed that the occurrence of mutation is binomially distributed and maximum-likelihood procedures were employed to determine the appropriateness of 4 alternative models, Linear, Linear-Quadratic, Power, and Threshold, in describing the dependence of the binomial parameter on dose. For both data sets, the Threshold model yielded a far superior fit and the threshold dose was estimated to be between 34 and 39 mg/kg. These results are supported by the relatively inefficient response of ethylnitrosourea at lower doses in inducing DNA adducts. Relevant specific-locus mutation results in the mouse for low-dose fractionated treatment as well as the recovery of mutation mosaics indicate the threshold model to be an oversimplification. Rather than a threshold dose below which 100% of the induced DNA adducts are repaired, we propose that some DNA adducts which may eventually be fixed as a mutation persist through a number of repair-competent cell divisions and do not interfere with normal cell function nor do they induce a repair response before being eventually fixed as a mutation. We interpret the thresholded response for ethylnitrosourea-induced specific-locus mutations to be due to a saturable repair process which at lower doses results in ethylnitrosourea being less efficient in inducing mutation. Once this repair process is saturated, a clear dose-related increase in the mutation rate is observed.

Reducing uncertainty in risk assessment

This article presents a summary of the proceedings of the Symposium and Workshop on Reducing Uncertainty in Risk Assessment, held at Michigan State University, on May 18-19, 1987. Participants addressed four topic areas: safety factors in noncarcinogen risk assessment; relevance of biological data in risk assessment; upper and lower bounds in carcinogenic risk assessment; and exposure assessment. One additional issue, risk communication, was discussed as a result of participant interest. Consensus recommendations in these five areas, resulting from the deliberations of the workshop groups, are presented. In addition to the specific recommendations, some general conclusions could be drawn. One was that the increased understanding of underlying mechanisms of toxicity, gained in the last decade, should be incorporated as much as possible into the risk assessment process. A second conclusion was that more effort should be devoted to increasing this understanding and developing the best methods for applying this knowledge to risk assessment. Last, more effort should be made to improve the communication of these assessments to the public and to policy makers so that the best and most complete information is utilized in risk management decisions.

[Quantification of chemical genetic risk]

Associated with technical advances of our civilization is the possibility of an increase of radiation- and chemically induced germ-cell mutations in man. This would result in an increase in the frequency of genetic diseases and would be detrimental to future generations. It is the duty of our generation to keep this risk as low as possible. In order to achieve this goal it is necessary to identify, evaluate and quantify the genetic risk of chemical mutagens. The possibilities and the shortcomings of the quantification of radiation- and chemically induced mutations are discussed. The discrepancy between the high safety standards for radiation protection and the low level of knowledge for the toxicological evaluation of chemicals is emphasized.

Mechanisms of inhibitors of mutagenesis and carcinogenesis. Classification and overview

The mechanisms of action of inhibitors of mutagenesis and carcinogenesis are reviewed in the light of our present knowledge. The identified mechanisms are classified into several categories and sub-categories, depending on the stage of intervention in the mutagenesis and carcinogenesis processes, and on the patterns of modulation of the host defense devices. Although a number of the known mechanisms fit into the proposed scheme, the available information on these problems is still fragmentary, and often inhibitors act through multiple mechanisms or can interact with other inhibitors. Moreover, due to the double-edged nature of many protective factors of the organism, and to the wide array of biological properties displayed by several inhibitors, the beneficial effects are in many instances counter-balanced by adverse reactions. Nevertheless, the present data-base on mechanisms of inhibitors, which is expected to grow rapidly in the near future, provides an extremely useful scientific premise for the primary prevention of mutation-related diseases. In this prospect, the elucidation of the underlying mechanisms complements the results emerging from the monitoring of protective end-points in mutagenicity and carcinogenicity test systems.

Extrapolation from in vitro tests to human risk: experience with sodium fluoride clastogenicity

Genotoxic effects observed in vitro, only at high doses or high levels of cytotoxicity, will be false positives if such conditions are not achieved or cannot be tolerated in vivo. However, for such effects to be disregarded there must be a threshold dose or level of cytotoxicity below which genotoxicity is absent. Sodium fluoride (NaF) has previously been shown to be clastogenic in vitro in Syrian hamster cells and human fibroblasts. We have extended these studies in human fibroblasts and included a positive control (mitomycin C, MMC) which is clastogenic in vivo and carcinogenic, and a chemically related control (NaCl). Cytotoxicity was measured as mitotic inhibition and cell death (loss of clonogenicity). The results are used to illustrate the problems associated with quantitative extrapolation from in vitro tests to human risk, as follows. (1) There appears to be a threshold response (clastogenicity vs. dose) with NaF at around 10 micrograms/ml (48 h exposure) but a more definitive conclusion must await elucidation of the mechanisms of clastogenicity. (2) NaCl is weakly clastogenic at 1000 times the threshold dose for NaF. The mechanisms are unlikely to be similar. (3) No clastogenicity was detected with NaF below about 30% mitotic inhibition but the relationship between clastogenicity and mitotic inhibition was similar for NaF and MMC. (4) There was no obvious threshold in the relationship between clastogenicity and cell killing with NaF. MMC was less clastogenic than NaF at equitotoxic doses. Observations 3 and 4 preclude the possibility of regarding the clastogenicity of NaF as a false positive by virtue of associated cytotoxicity.

Measurement of low levels of x-ray mutagenesis in relation to human disease

We previously demonstrated that conventional methods for measurement of mutagenesis in mammalian cells are subject to serious error that causes underestimation of environmental contributions to cancer and genetic disease. This error has been corrected by use of somatic cell hybrids containing a single human chromosome on which the marker genes are carried and by using doses of mutagenic agents so low that little cell killing occurs. This method permits direct measurement of the effects of low doses of radiation and other mutagens without resort to the controversial extrapolation procedure customarily used to estimate effects of doses in the neighborhood of actual human exposures. The new data demonstrate that the true mutagenesis efficiency at the low doses of ionizing radiation that approximate human exposures is more than 200 times greater than those obtained with conventional methods. This methodology also permits evaluation of localized mutations, large and small chromosomal deletions, and nondisjunctional processes and can be used for mutagens that need metabolic activation as well as for cooperatively acting agents. The two opposing classical views that in mammalian cells extrapolation to low doses of x-radiation is linear, on the one hand, or involves a threshold, on the other, are both demonstrated to be incorrect at least for the conditions here considered. The actual curve exhibits a downward concavity so that the mutational efficiency is maximal at low doses. These data may have important implications for human health.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 3. The concept of negativity in experimental carcinogenesis

The problems that arise in the interpretation of experimental data on chemical carcinogenesis are addressed. In particular, the difficulties in demonstrating negative results are shown to present problems in delineating carcinogens from noncarcinogens. The use of the virtually safe dose estimated under the assumption of low dose linearity is shown to lead to potentially anomalous results if used indiscriminately in bioassays in which no statistically significant increase in tumor occurrence is induced. It is suggested that there is a need to establish an operational definition of negativity in carcinogenesis, with the realization that this definition may be revised in light of new information. The establishment of negativity in aligning data from positive and negative experiments and in considering possible thresholds is also discussed.

Immunologic quantification of carcinogen-DNA adducts

Sensitive immunological methods for the detection of carcinogen-DNA adducts have recently been developed. These techniques are particularly useful for screening human populations for exposure to environmental carcinogens. Measurement of the biologically effective dose in humans may be useful in detecting carcinogenic hazards and carrying out risk estimates. We have developed monoclonal antibodies to several carcinogen-DNA adducts. These have included DNA modified by a benzo[a]pyrene diol epoxide (BPDE-I), 1-aminopyrene (1-AP) and 8-methoxypsoralen (8-MOP). BALB/cCr mice were immunized with the modified DNAs complexed electrostatically to methylated bovine serum albumin. Several stable clones have been isolated for each of the modified DNAs and characterized by enzyme-linked immunosorbent assay (ELISA). All antibodies are highly specific for the appropriate modified DNA and do not cross-react with nonmodified DNA. The antibodies to BPDE-I-DNA have significant cross-reactivity with DNAs modified by similar antitrans diol epoxides of benz[a]anthracene and chrysene. These DNAs all contain N-2 of guanine adducts. The antibody probably recognizes a shared determinant encompassing the guanine base and the hydrocarbon ring containing the hydroxide groups. The antibodies cross-react with BPDE-I-dG, the monoadduct isolated from DNA, but with lower sensitivity than for the intact modified DNA. They do not react with acetylaminofluorene (AAF) or 1-AP modified DNA, both of which contain C-8 of guanosine adducts. The antibodies to 1-AP-modified DNA demonstrate cross-reactivity with 8-nitro-1-aminopyrene- and 6-nitro-1-aminopyrene-modified DNA, as well as some slight cross-reactivity with BPDE-I-DNA and AAF-DNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for cell-replacement repair of X-ray-induced teratogenic damage in male genital imaginal discs of Drosophila melanogaster

Male genital imaginal discs from old (late-third-instar) larvae of Drosophila that had been X-irradiated with appropriate doses developed into severely damaged adult genitalia when implanted into old larvae, but they developed into completely normal adult genitalia when transplanted into 2-day-younger larvae. The major cause of X-ray-initiated teratogenesis in the genital disc was DNA damage because development of the genital discs of strain mei-9a with defective DNA repair was twice as sensitive as that of the wild-type strain to impairment by X-irradiation, just as larvae of this strain were twice as sensitive to killing by X-irradiation as those of the wild-type strain. Complete repair of X-ray-induced teratogenic damage in the genital discs on transplantation into young host larvae was similar in the wild-type and mei-9a strains. These results are discussed in relation to the hypothesis that repair of X-ray-induced teratogenic damage depends not on DNA repair but on replacement of damage-bearing primordial cells by healthy ones after suicidal elimination of the former.

DNA modification by chemical carcinogens

The chemistry and molecular biology of DNA adducts is only one part of the carcinogenic process. Many other factors will determine whether a particular chemical will exert a carcinogenic effect. For example, the size of particles upon which a carcinogenic may be adsorbed will influence whether or not, and if so where, deposition within the lung will occur. The simultaneous exposure to several different agents may enhance or inhibit the metabolism of a chemical to its ultimate carcinogenic form (Rice et al., 1984; Smolarek and Baird, 1984). The ultimate carcinogenic metabolites may be influenced in their ability to react with DNA by a number of factors such as internal levels of detoxifying enzymes, the presence of other metabolic intermediates such as glutathione with which they could react either enzymatically or non-enzymatically, and the state of DNA which is probably most heavily influenced by whether or not the cell is undergoing replication or particular sequences being expressed. Replicating forks have been shown to be more extensively modified than other areas of DNA. Another critical factor which can influence the final outcome of the DNA damage is whether or not the modifications can be repaired. If this occurs with high fidelity and the cell has not previously undergone replication then the effect of the damage by the carcinogen is likely to be minimal. The major area in which progress is needed is an understanding of what this damage really does to the cell such that after an additional period of time, which may be as long as twenty or more years, these prior events are expressed and cell proliferation occurs. Clearly additional stimulatory factors, for example tumor promoting agents such as the phorbol esters or phenobarbital, are often needed. After such prolonged periods it seems likely that the DNA adducts would no longer be present. However, the way in which their earlier presence is remembered is not clear. Simple mutations do not explain all the characteristics of tumor progression and, when it occurs, regression. Even if a specific site mutation does occur then its expression must be under other types of control. Any explanation of the action of DNA modification at the molecular level also requires that account be taken of the diverse nature of the DNA adducts from simple modifications such as methylation to bulkier adducts such as benzo[a]pyrene, aflatoxin or aromatic amines.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of tumor promotion in affecting the multi-hit nature of carcinogenesis

The assessment of risk from low-level exposure to radiation and chemicals is hindered by the basic lack of scientific understanding of the complex nature of the multiple levels of protective or synergistic interactions. Radiation and chemicals have the potential of inducing mutations, cell death and altered gene expression. The biological consequences of each of these effects is again complex, since many factors can enhance or mask the effect of a single mutation, cytotoxic or gene modulatory change. Carcinogenesis, representing but one chronic disease state, is a multi-step process, involving the clonal expansion of a single altered cell (that is, the initiation event). Radiation and chemicals appear to contribute to the initiation process by their ability to induce viable mutations. Insufficient theoretical and empirical knowledge precludes a determination of whether mutagenesis and, hence, initiation exhibit a threshold phenomenon. Because of a variety of redundancy mechanisms on the genetic and cellular levels, the physiological impact of a single dysfunctional cell is felt only after it is amplified to a large number (the promotion phase of carcinogenesis). Radiation and chemical-induced cytotoxicity, as well as noncytotoxic chemical induction of mitogenesis, can induce surviving single dysfunctional stem cells to multiply. If during this multiplication of dysfunctional cells, the initiated cells are further exposed chronically to low levels of mutagens, there is an enhanced probability of additional genetic changes. The accumulation of foci of dysfunctional cells can occur in any stem cell population of any tissue. As with mutagens, the existence of threshold levels for promoting conditions and chemicals is still not yet scientifically validated. However, specific examples of the actions of a few promoters does seem to be consistent with that idea. The concepts of initiation and promotion imply the existence of anti-initiation and antipromotion conditions. Together with genetic factors, the complex and unpredictable interactions of radiation and chemicals as initiators, anti-initiators, promoters and antipromoters, defy prospects of an easy means to predict the consequences of exposure to chronic low levels of radiation and chemicals.

A quantitative analysis of the cytotoxic action of chemical mutagens

A quantitative hypothesis is developed to explain the cytotoxic action of chemical mutagens on eukaryotic cells. The hypothesis forms an extrapolation of previously developed concepts used to explain the effect of ionizing radiation and the cytotoxic action of UV light. The crucial potentially lethal lesion is assumed to be a DNA double-strand lesion which may be an interstrand cross-link or a pair of DNA single-strand alkylations, for example. The effect of repair processes is included in the analytical equation derived to describe cell survival. The analysis of several sets of cell survival data for different chemical mutagens is used to demonstrate the applicability of the hypothesis. The logical extension of the hypothesis permits a division of chemical mutagens into 4 separate classes on the basis of the mechanisms proposed for the cytotoxic activity, and the relative importance of the risk associated with low-level exposure to each class is discussed. The hypothesis is amenable to further experimental verification.

The evaluation of the carcinogenicity of environmental substances

A review of the current status of carcinogenicity assessment leads to a number of observations that raise specific questions about the conduct and techniques of these bioassays. An approach is suggested that avoids many of the scientific controversies associated with the use of excessively high doses that cause secondary toxicity, leading to a promoter-like effect. Recommendations are made to utilize toxicokinetic data that relate metabolic characteristics of a chemical to its potential carcinogenic effects.

The International Commission for Protection against Environmental Mutagens and Carcinogens (ICPEMC): the first seven years

Careful, independent scientific reviews of issues of importance in mutagenesis and carcinogenesis evaluations have been made over the past seven years by a group of leading members of several environmental mutagen societies. The International Commission for Protection against Environmental Mutagens and Carcinogens (ICPEMC) supports five main committees concerned with general principles and has published working papers and reports on an unbiased scientific basis. The work and goals of the Commission are described and future plans outlined.

The principle of a threshold dose in chemical carcinogenesis

The nature of at least three stages in carcinogenesis by chemical agents justifies the existence of a threshold dose: (1) a cytoplasmic stage during which most carcinogenic molecules are eliminated, (2) a nuclear stage during which certain DNA lesions are repaired and therefore cannot help to bring about mutations, and (3) an extracellular stage during which mutations are controlled for a long time by positive or negative epigenetic factors. All these findings are very difficult to collate. Since threshold doses cannot be detected by direct experimentation, the knowledge that a threshold exists is of little practical use and does not much alter the data permitting extrapolation of the results obtained with high doses of carcinogens. At the theoretical level only, this conclusion renders obsolete the opinion that all carcinogenic agents should be completely eliminated from our environment, and also the view that DNA needs only one hit from such an agent for a carcinogenic effect to be produced. The above conclusion also provides a more rational basis for the concept of a 'virtually safe dose' (VSD) or 'allowable daily intake' (ADI).

Implications of multiple mechanisms of carcinogenesis for short-term testing

The attempt has been made recently to categorize carcinogens into two mechanistic types based on their mechanism of action: genotoxic (capable of reacting with and damaging DNA) and epigenetic (unable to damage DNA to any detectable extent). By requiring that a given chemical fit into one or the other of these narrowly defined categories for regulatory purposes, we are probably oversimplifying potential biological effects. In fact, based on our limited understanding of carcinogenic mechanisms, this artificial distinction should probably be abandoned in favor of a more precise statement of each chemical's mechanism or relative potency of initiating and promoting effects. Since the standard short-term tests by which carcinogenicity of chemicals is screened were designed to detect certain chemical classes with active electrophilic intermediates, weak or specialized carcinogens may be missed and may be assumed erroneously to be nongenotoxic. The mechanisms of carcinogenicity for such carcinogens may include particulate deposition, active radical formation, liver toxicity, and hormonal interactions. Not all of these secondary mechanisms depend upon a detectable level of binding to DNA, damage to DNA, or modification of the DNA sequence, even though they may demonstrate other characteristics of a complete carcinogen (that is, irreversibility and lack of a threshold). Certain agents have been labeled as epigenetic. However, a consideration of the literature on sample agents (diethylstilbestrol, asbestos, and urethane) reveals that these are not epigenetic carcinogens despite their being labeled as such. Agents with irreversibility and no threshold have initiating potential and, as such, are genotoxic, whereas carcinogens that are classified as nongenotoxic are largely agents that promote the growth of liver tumors. Even promotion can be a mechanistically specialized phenomenon. For example, some promoters are cytotoxic to the liver, but not all liver toxins are liver tumor promoters or liver carcinogens. Further, the carcinogens commonly labeled as epigenetic might cause a unique specialized genotoxicity not detected by common screening tests routinely used for detecting genotoxicity. If we assume that this unrecognized but necessary initiating potential is mediated by some specialized genotoxicity, extra care must be taken to establish a genuine lack of genotoxicity before an agent can be classified (and regulated) as a promoter (lacking the ability to initiate tumor growth but still enhancing tumor development).(ABSTRACT TRUNCATED AT 400 WORDS)