R. Julian Preston

Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know

High doses of ionizing radiation clearly produce deleterious consequences in humans, including, but not exclusively, cancer induction. At very low radiation doses the situation is much less clear, but the risks of low-dose radiation are of societal importance in relation to issues as varied as screening tests for cancer, the future of nuclear power, occupational radiation exposure, frequent-flyer risks, manned space exploration, and radiological terrorism. We review the difficulties involved in quantifying the risks of low-dose radiation and address two specific questions. First, what is the lowest dose of x- or γ-radiation for which good evidence exists of increased cancer risks in humans? The epidemiological data suggest that it is ≈10–50 mSv for an acute exposure and ≈50–100 mSv for a protracted exposure. Second, what is the most appropriate way to extrapolate such cancer risk estimates to still lower doses? Given that it is supported by experimentally grounded, quantifiable, biophysical arguments, a linear extrapolation of cancer risks from intermediate to very low doses currently appears to be the most appropriate methodology. This linearity assumption is not necessarily the most conservative approach, and it is likely that it will result in an underestimate of some radiation-induced cancer risks and an overestimate of others.

Current status of cytogenetic procedures to detect and quantify previous exposures to radiation

The estimation of the magnitude of a dose of ionizing radiation to which an individual has been exposed (or of the plausibility of an alleged exposure) from chromosomal aberration frequencies determined in peripheral blood lymphocyte cultures is a well-established methodology, having first been employed over 25 years ago. The cytogenetics working group has reviewed the accumulated data and the possible applicability of the technique to the determination of radiation doses to which American veterans might have been exposed as participants in nuclear weapons tests in the continental U.S.A. or the Pacific Atolls during the late 1940s and 1950s or as members of the Occupation Forces entering Hiroshima or Nagasaki shortly after the nuclear detonations there. The working group believes that with prompt peripheral blood sampling, external doses to individuals of the order of about 10 rad (or less if the exposure was to high-LET radiation) can accurately be detected and measured. It also believes that exposures of populations to doses of the order of maximum permissible occupational exposures can also be detected (but only in populations; not in an individual). Large exposures of populations can also be detected even several decades after their exposure, but only in the case of populations, and of large doses (of the order of 100 to several hundred rad). The working group does not believe that cytogenetic measurements can detect internal doses from fallout radionuclides in individuals unless these are very large. The working group has approached the problem of detection of small doses (less than or equal to 10 or so rad) sampled decades after the exposure of individuals by using a Bayesian statistical approach. Only a preliminary evaluation of this approach was possible, but it is clear that it could provide a formal statement of the likelihood that any given observation of a particular number of chromosomal aberrations in a sample of any particular number of lymphocytes actually indicates an exposure to any given dose of radiation. It is also clear that aberration frequencies (and consequently doses) would have to be quite high before much confidence could be given to either exposure or dose estimation by this method, given the approximately 3 decades of elapsed time between the exposures and any future blood sampling.(ABSTRACT TRUNCATED AT 400 WORDS)

Chromosome aberrations, micronuclei, aneuploidy, sister chromatid exchanges, and cancer risk assessment

This paper describes the four cytogenetic endpoints most frequently used in hazard identification assays as the first step in the risk assessment process. These are structural chromosome aberrations, micronuclei, aneuploidy, and sister chromatid exchanges. The biological mechanisms involved in the formation of the alterations observed in each assay are briefly discussed. Variations in and recent improvements to each assay are described, with an emphasis on the use of molecular techniques to improve the sensitivity of the assay, and to allow for detection of specific alterations that are, or could be, associated with cancer induction. This, in turn, will make the data obtained in the cytogenetic assays more useful in cancer and genetic risk assessment. Thus, the aim of this paper is to encourage cytogeneticists to design their experiments in such a way that the data obtained will be of maximum possible benefit for characterizing and quantifying adverse human health effects, particularly cancer.

DNA-Reactive Carcinogens: Mode of Action and Human Cancer Hazard

It has been known for decades that mutagenicity plays an important role in the activity of most carcinogens. This mutagenicity can result from direct damage to DNA through a chemical being DNA reactive or from indirect effects, such as through the production of oxygen radicals that then react with DNA. This article presents a set of key events whereby DNA reactivity initiates the process of carcinogenicity that leads to the subsequent mutation induction and enhanced cell proliferation that ultimately results in tumor development. This set of key events for DNA-reactive chemicals was applied to two case studies (aflatoxin B1 and dichloromethane) with the aim of assessing the utility of the Human Relevance Framework (HRF) for this class of chemicals. The conclusions were that the HRF was a viable approach for the use of mechanistic data for DNA-reactive chemicals obtained from both laboratory animals and human cells in vivo and in vitro for predicting human carcinogenicity. In the case of aflatoxin B1, the HRF could be used to predict that carcinogenicity in humans was a likely outcome. In contrast, the HRF predicted that the human carcinogenic potential of dichloromethane was at best less likely than in rodents; this conclusion was supported by the available epidemiological data.

Systems Biology and Mode of Action Based Risk Assessment

The applications of systems biology approaches have greatly increased in the past decade largely as a consequence of the human genome project and technological advances in genomics and proteomics. Systems approaches have been used in the medical and pharmaceutical realm for diagnostic purposes and target identification. During this same period, the use of mode of action (MOA) for risk assessment has been increasing and there is a need for quantitative risk assessments on an ever-growing number of environmental chemicals. Genome-wide (i.e., global) measurements provide both a discovery engine for identifying MOA and an information base for subsequent evaluation of MOA when conducting a risk assessment. These genome-wide measurements are not chosen based on the hypothesized MOA and therefore represent an unbiased check of the comprehensiveness of an MOA. In addition, optimal design for MOA studies is critical to provide the time and dose dependent data required for quantitative model building. Finally, identification of biomarkers and bioindicators of disease in humans provides a viable way to extrapolate from disease outcomes measured at high exposure levels to those at low exposure levels and thus provide the opportunity to reduce or perhaps eliminate in vivo animal testing. To realize the full potential of these approaches, larger integrated projects which include all these individual components are necessary.

Application of Key Events Analysis to Chemical Carcinogens and Noncarcinogens

The existence of thresholds for toxicants is a matter of debate in chemical risk assessment and regulation. Current risk assessment methods are based on the assumption that, in the absence of sufficient data, carcinogenesis does not have a threshold, while noncarcinogenic endpoints are assumed to be thresholded. Advances in our fundamental understanding of the events that underlie toxicity are providing opportunities to address these assumptions about thresholds. A key events dose-response analytic framework was used to evaluate three aspects of toxicity. The first section illustrates how a fundamental understanding of the mode of action for the hepatic toxicity and the hepatocarcinogenicity of chloroform in rodents can replace the assumption of low-dose linearity. The second section describes how advances in our understanding of the molecular aspects of carcinogenesis allow us to consider the critical steps in genotoxic carcinogenesis in a key events framework. The third section deals with the case of endocrine disrupters, where the most significant question regarding thresholds is the possible additivity to an endogenous background of hormonal activity. Each of the examples suggests that current assumptions about thresholds can be refined. Understanding inter-individual variability in the events involved in toxicological effects may enable a true population threshold(s) to be identified.

Review of literature on chemical-induced aneuploidy in mammalian male germ cells

80 papers published between 1970 and 1984 were evaluated for results pertaining to chemical-induced aneuploidy in mammalian male germ cells. Diverse assays and end points were represented. The assays considered to involve direct measures of aneuploidy were based upon chromosome counts in premeiotic, meiotic, and embryonic cells, and the male pronucleus, or upon phenotypic expression of X-linked genetic markers. Assays in which indirect measures were interpreted as evidence for aneuploidy included those primarily assessing chiasma frequencies, univalent frequencies, and spermatid/sperm sex chromosome body counts. An initial screening to reject studies with insufficient data and those which did not involve a single chemical test agent led to the elimination of 39 papers from further review. The remaining 41 papers reported effects from 46 different chemicals. These papers were rigorously assessed for adequacy of experimental protocols, relevance of end points as direct measures of aneuploidy, and completeness of data presentation and statistical analysis. Criteria specific to each assay were also considered. 4 chemical tests were considered to provide reliable positive or negative aneuploidy data. Cyclophosphamide and chloral hydrate each caused metaphase II hyperploidy when injected into mice. Very limited analyses of trenimon and isoniazid provided negative results. Test findings for 44 chemicals were viewed as inconclusive. It was concluded that standardization of tests to evaluate chemical-induced aneuploidy in male germ cells and the application of these tests towards increasing the data base are badly needed.

Creating context for the use of DNA adduct data in cancer risk assessment: I. Data organization.

The assessment of human cancer risk from chemical exposure requires the integration of diverse types of data. Such data involve effects at the cell and tissue levels. This report focuses on the specific utility of one type of data, namely DNA adducts. Emphasis is placed on the appreciation that such DNA adduct data cannot be used in isolation in the risk assessment process but must be used in an integrated fashion with other information. As emerging technologies provide even more sensitive quantitative measurements of DNA adducts, integration that establishes links between DNA adducts and accepted outcome measures becomes critical for risk assessment. The present report proposes an organizational approach for the assessment of DNA adduct data (e.g., type of adduct, frequency, persistence, type of repair process) in concert with other relevant data, such as dosimetry, toxicity, mutagenicity, genotoxicity, and tumor incidence, to inform characterization of the mode of action. DNA adducts are considered biomarkers of exposure, whereas gene mutations and chromosomal alterations are often biomarkers of early biological effects and also can be bioindicators of the carcinogenic process.

Dominant lethal effects of subchronic acrylamide administration in the male Long-Evans rat

Acrylamide, a widely used vinyl monomer, is well known as a neurotoxin but inactive as a mutagen in bacterial test systems. The experiments reported demonstrate that after subchronic oral dosing in the male rat, acrylamide induced significant elevations in both pre- and post-implantation loss following dominant lethal testing. These effects were seen at doses which failed to produce clinical or pathological evidence of neurotoxicity. In an accompanying cytogenetic study, no increase in chromosome aberrations was observed in spermatogonia or spermatocytes of treated animals. When spermatocytes from treated spermatogonial stem cells were analyzed, reciprocal translocations (4) were observed in the treated animals and not in the control, but the significance of this result still needs to be established.

The induction of chromosome aberrations by restriction endonucleases that produce blunt-end or cohesive-end double-strand breaks

Restriction endonucleases have been used to study the involvement of specific types of DNA damages in the production of chromosome aberrations. In this study restriction endonucleases were introduced into viable CHO cells using osmolytic shock of pinocytic vesicles. We compared two cohesive-end cutters, Msp I (CCGG-2-base overlap) and Sau3A I (GATC-4-base overlap) with two blunt-end cutters, Alu I (AGCT) and Rsa I (GTAC). All 4 enzymes were effective at inducing aberrations. The 4-base overlap cohesive-end cutter Sau3A I was approximately as effective as the blunt-end cutter Alu I. We present evidence that cutting frequency rather than cut end-structure is important in determining efficiency of aberration induction. There is over-dispersion of the distribution of dicentrics and rings among cells, and the data could be fitted to a Neyman Type A distribution, a modified Poisson, that indicates that there is a probability distribution both for the entry of the enzyme into a cell nucleus and for the induction of aberrations once the enzyme has entered a cell nucleus. In addition, we used Alu I to determine the sensitivity of cells to aberration induction in the different stages of the cell cycle. Alu I induced aberrations in all stages of the cycle, chromatid-type in S/G2 and chromosome-type in G1. In agreement with data of others, there were variations in sensitivity with cycle stage, and changes in the proportions of the different aberration classes for chromatid-type aberrations.