Derivation of no significant risk levels for three lower acrylates: Conclusions and recommendations from an expert panel.

A panel of toxicology, mode of action (MOA), and cancer risk assessment experts was engaged to derive no-significant-risk-levels (NSRLs) for three lower acrylates: methyl acrylate (MA), ethyl acrylate (EA), and 2-ethylhexyl acrylate (2EHA) using the best available science, data, and methods. The review was structured as a five-round, modified Delphi format, a systematic process for collecting independent and deliberative input from panel members, and it included several procedural elements to reduce potential sources of bias and groupthink. Input from the panel for key decisions in the dose-response assessments resulted in NSRL values of 530 µg/day (330-800 µg/day), 640 µg/day (280-670 µg/day), and 1700 µg/day (1300-2700 µg/day) for MA, EA, and 2EHA, respectively. Novel to this approach were the use of nonneoplastic lesions reported at point of contact where tumors have been reported in laboratory rodents, along with nonlinear extrapolation to low doses (uncertainty factor approach) based upon panel recommendations. Confidence in these values is considered medium to high for exposures applied to the routes of exposure tested (inhalation for MA and EA, dermal for 2EHA), but confidence is considered lower when applied to other routes of exposure.

Cancer weight of evidence for three lower acrylates: Conclusions and recommendations from an expert panel.

An international panel of experts was engaged to assess the cancer weight of evidence (WOE) for three lower acrylates: methyl acrylate, ethyl acrylate, and 2-ethylhexyl acrylate. The review was structured as a three-round, modified Delphi format, a systematic process for collecting independent and deliberative input from panel members, and it included procedural elements to reduce bias and groupthink. Based upon the available science, the panel concluded: (1) The MOA for point of contact tumors observed in rodent cancer bioassays that is best supported by available data involves increased cell replication by cytotoxicity and regenerative proliferation; (2) The WOE supports a cancer classification of "Not likely to be carcinogenic to humans" a conclusion that is more in line with an IARC classification of Group 3 rather than Group 2 B; (3) Quantitative cancer potency values based on rodent tumor data are not required for these chemicals; and (4) Human health risk assessment for these chemicals should instead rely on non-cancer, precursor endpoints observed at the point of contact (e.g., hyperplasia). The degree of consensus (consensus scores of 0.84-0.91 out of a maximum score of 1) and degree of confidence (7.7-8.7 out of a maximum score of 10) in the WOE conclusions is considered high.

Genetic anomalies in mammalian germ cells and their significance for human reproductive and developmental risk.

The induction of heritable mutations in germ cells represents a potential health concern. This paper highlights data from mouse germ-cell mutagenesis studies that have implications in the assessment of reproductive and developmental risks. The paper discusses the developmental and reproductive consequences of induced chromosomal damage (structural rearrangements and numerical anomalies) and describes environmental agents that have been shown to produce such anomalies. Additionally, factors that influence the yield of genetic damage are addressed. Studies showing that the various germ-cell stages vary in their susceptibility to the induction of genetic damage are summarized. Of the chemicals evaluated in the male mouse, most appear to have their predominant or strongest effect on post-stem-cell stages. The differences between males and females in the susceptibility to mutagens is examined. Recent studies have shown that the female may be uniquely sensitive to certain mutagens. Finally, an important aspect of mutagenic risk is not only effects induced in developing germ cells but also the effects of environmental agents during the period from fertilization through the zygote and the two-cell embryo. Recent work in the mouse has demonstrated that exposure during these early developmental stages leads to high frequencies of external and visceral fetal malformations, as well as mid-to-late gestational death.

Mutagenicity of nitro compounds in Salmonella typhimurium in the presence of flavin mononucleotide in a preincubation assay.

A series of nitro compounds (18 aromatic and one aliphatic) was evaluated using a modification of the standard Salmonella typhimurium mutagenicity assay. A preincubation protocol was used with flavin mononucleotide (FMN) incorporated into the assay mixture to facilitate nitro reduction. Several aromatic nitro compounds (m-nitroaniline, p-nitroaniline, 2,6-dinitrotoluene, 2,4-dinitrotoluene,2,3-dinitrotoluene,1,8-dinitronaphthalene), which were negative or only weakly mutagenic when tested in the standard plate incorporation assay, showed FMN-dependent mutagenic responses with this procedure. For some nitro compounds, the addition of FMN was not needed for the detection of mutagenicity in the modified protocol. Not all nitro compounds were positive using the preincubation procedure with FMN. The lack of mutagenicity, however, does not appear to be the result of the inability of the modified method to reduce nitro compounds, since it was found that reduction does occur under the assay conditions for the two nonmutagens evaluated for nitro reduction (nitrobenzene and p-nitrophenol). It is suggested that the modified protocol may be useful for evaluating the mutagenicity of many nitro compounds.

Evolution of social concerns and environmental policies for chemical mutagens.

The founding of the Environmental Mutagen Society 20 years ago coincided with the beginning of general social concern about exposure to chemical mutagens. Initially, this concern focused on the potential of chemicals to induce heritable genetic damage in humans. Within a few years, however, mutagenicity tests came to be regarded primarily as short-term tests for carcinogenicity. Serious doubts have recently been cast upon the relationship between mutagenicity and carcinogenicity, and, as a result the real utility of mutagenicity tests is being questioned. Justification for the continued use of these tests will require 1) more detailed mechanistic knowledge concerning the role of genetic changes in the development of cancer and 2) an improved ability to relate the results of mutagenicity tests to the potential for inducing heritable genetic effects in humans.

Quantitative estimation of the genetic risk associated with the induction of heritable translocations at low-dose exposure: ethylene oxide as an example.

This paper explores how quantitative risk assessment methods might be extended to analysis of risks to the human germ line. High inhalation exposures to ethylene oxide are reported to cause heritable translocations in male mice with a steep and nonlinear dose-response-curve. We explore quantitative estimation of risk to humans from low exposures based on these animal data, addressing questions of tissue dosimetry for this alkylating agent, expected equivalency of doses across species, germ-cell sensitivity, and extrapolation of dose-response relationship to low exposure levels. Various dose-response models are discussed in terms of their applicability to genetic end points and their ability to reflect the underlying basis of induced heritable translocations.

Review of the mutagenicity of ethylene oxide.

Ethylene oxide has been shown to be an effective mutagen in a variety of organisms ranging from bacteria to mammalian cells. There is also an association between ethylene oxide exposure and human somatic cell cytogenetic damage. Furthermore, ethylene oxide has been shown to alkylate protein and DNA at exposure levels that have been encountered occupationally. Ethylene oxide is not only effective at producing somatic cell mutations but also at inducing genetic damage in germ cells. While it is clear that ethylene oxide is a germ cell mutagen in whole mammals, the mechanism(s) by which it produces genetic lesions in germ cells is uncertain.

Mutagenesis and human genetic disease: an introduction.

This special issue attempts to provide a fresh perspective on the importance of germ-cell mutagenesis studies and restate the questions and challenges inherent in efforts to minimize the incidence of human genetic diseases. We are working in a time when rapidly advancing molecular technologies provide the tools that permit a more detailed understanding of germ-cell mutagenesis and genetic disease. Meanwhile, discoveries of new genetic disease phenomena challenge our abilities to conceive and develop research models for their study. It is hoped that the collection of articles in this issue will serve to stimulate interest in scientists of varied disciplines and help focus those interests on the issues surrounding the relationship between environmental mutagens and human genetic disease.

Aneuploidy in germ cells: etiologies and risk factors.

A 2 1/2-day workshop on germ cell aneuploidy was convened September 11-13, 1995 at the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina to discuss current understandings of the etiology and origin of human aneuploidy, especially in regard to potential environmental causes, and to identify gaps in our research knowledge. The workshop was designed to facilitate interactions among research experts conducting studies on the fundamental biology of chromosomal movement and segregation, on aneuploidy as a human clinical problem, and on toxicological aspects of aneuploidy induction. Overview presentations provided perspectives on aneuploidy as a human clinical problem, the genetics of aneuploidy, and the issues of concern in toxicological testing and regulatory risk assessment. The four chairs introduced the topics for each of their workgroups, setting the stage for subsequent, in-depth discussions on (1) chromosome mover components, (2) altered recombination, (3) parental age effects, and (4) differential chromosome susceptibility. From these discussions, gaps in our research knowledge related to the role of the environment in the etiology of aneuploidy and associated molecular, cellular, and genetic processes involved were identified, and will be used to establish a research agenda for filling those gaps.

A mutagenicity assessment of acetaldehyde.

Acetaldehyde has been shown in studies by several different laboratories to be a clastogen (chromosome-breaking) and inducer of sister-chromatid exchanges in cultured mammalian cells (Chinese hamster cells and human lymphocytes). Although there have been very few studies in intact mammals, the available evidence suggests that acetaldehyde produces similar cytogenetic effects in vivo. The production of cytogenetic abnormalities may be related to the ability of acetaldehyde to form DNA-DNA and/or DNA-protein cross-links. Acetaldehyde apparently has not been evaluated for its ability to cause gene mutations in cultured mammalian cells, but it has been shown to produce sex-linked recessive lethals in Drosophila. In general, bacteria tests have been negative. Although acetaldehyde is a genotoxic cross-linking agent, it does not appear to cause DNA strand breaks. There were no studies available regarding the potential of acetaldehyde to produce genetic damage in mammalian germ cells in vivo. Most mutagenicity testing on acetaldehyde has been motivated by attempts to define the proximate mutagen in ethanol metabolism.

U.S. Environmental Protection Agency guidelines for carcinogen risk assessment: past and future.

The U.S. Environmental Protection Agency (USEPA) recently proposed new guidelines to update and replace the 1986 USEPA Guidelines for Carcinogen Risk Assessment. Today, there is a better understanding of the variety of modes by which carcinogens can operate that did not exist when the 1986 USEPA guidelines were published. Many laboratories are adding new test protocols in their programs directed at questions concerning the mechanisms of action of carcinogens. In response to the evolving science of carcinogenesis, the new guidelines provide an analytical framework for incorporating all relevant biological information and recognizing a variety of situations regarding cancer risk. In addition, the guidelines are flexible enough to allow consideration of future scientific advances.

U.S. Environmental Protection Agency's revised guidelines for carcinogen risk assessment: evaluating a postulated mode of carcinogenic action in guiding dose-response extrapolation.

There are new opportunities to using data from molecular and cellular studies in order to bring together a fuller biological understanding of how chemicals induce neoplasia. In 1996, the Environmental Protection Agency (EPA) published a proposal to replace its 1986 Guidelines for Carcinogen Risk Assessment to take advantage of these new scientific advances in cancer biology. The analytical framework within the new guidelines focuses on an understanding of the mode of carcinogenic action. Mode of action data come into play in a couple of ways in these new guidelines. For example, such information can inform the dose-response relationship below the experimental observable range of tumours. Thus, mode of action data can be useful in establishing more appropriate guidance levels for environmental contaminants. It is the understanding of the biological processes that lead to tumour development along with the response data derived from experimental studies that can help discern the shape of the dose-response at low doses (linear vs. nonlinear). Because it is experimentally difficult to establish "true thresholds" from others with a nonlinear dose-response relationship, the proposed guidelines take a practical approach to depart from low-dose linear extrapolation procedures when there is sufficient experimental support for a mode of action consistent with nonlinear biological processes (e.g., tumours resulting from the disruption of normal physiological processes).

An introduction to a series of U.S. Environmental Protection Agency special committee reports on testing approaches for the detection of chemically induced aneuploidy.

A committee of scientists was established by the U.S. Environmental Protection Agency to appraise the current state of aneuploidy test methodology, to compile and analyze published data on the chemical induction of aneuploidy, and to provide guidance for additional test development and validation. The reports that follow in this special issue of Mutation Research, document the urgent need for test method development and validation in this important area of environmental mutagenesis, and provide directions for further research.

Aneuploidy and health risk assessment: current status and future directions.

The U.S. Environmental Protection Agency (EPA) recently sponsored a workshop to discuss the contribution of aneuploidy to human disease and disability, the development of tests for detecting chemicals that induce aneuploidy and the relevance of these tests to human risk, and the current understanding of mechanisms by which aneuploidy arises. This summary is based on the presentations given at the workshop. It is hoped that this summary will stimulate thinking in this vitally important area of risk assessment and contribute to the establishment of priorities for basic research, development of new test methods, and validation of existing test approaches. Such research is needed to enhance the scientific basis of risk assessment for aneuploidy-producing chemicals.