Reflections on the impact of advances in the assessment of genetic risks of exposure to ionizing radiation on international radiation protection recommendations between the mid-1950s and the present.

Efforts at protecting people against the harmful effects of radiation had their beginnings in the early 1900s with the intent of protecting individuals in medicine and associated professions. Such efforts remain vital for all of us more than 100 years later as part of our 'learning to live with ionizing radiation.' The field of radiation protection has evolved slowly over time with advances in knowledge on hereditary (i.e., genetic) and carcinogenic effects of radiation continually improving our ability to make informed judgments about how best to balance risks against benefits of radiation exposure. This paper examines just one aspect of these efforts, namely, how advances in knowledge of genetic effects of radiation have impacted on the recommendations of the International Commission on Radiological Protection (ICRP). The focus is on the period from the mid-1950s (when genetic risk estimates were first made) to 2007. This article offers a detailed historical analysis and personal perspective, and concludes with a synopsis of key developments in radiation protection.

A toxicity evaluation and predictive system based on neural networks and wavelets.

A computational approach has been developed for performing efficient and reasonably accurate toxicity evaluation and prediction. The approach is based on computational neural networks linked to modern computational chemistry and wavelet methods. In this paper, we present details of this approach and results demonstrating its accuracy and flexibility for predicting diverse biological endpoints including metabolic processes, mode of action, and hepato- and neurotoxicity. The approach also can be used for automatic processing of microarray data to predict modes of action.

Ionizing radiation and genetic risks XIV. Potential research directions in the post-genome era based on knowledge of repair of radiation-induced DNA double-strand breaks in mammalian somatic cells and the origin of deletions associated with human genomic disorders.

Recent estimates of genetic risks from exposure of human populations to ionizing radiation are those presented in the 2001 report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). These estimates incorporate two important concepts, namely, the following: (1) most radiation-induced mutations are DNA deletions, often encompassing multiple genes, but only a small proportion of the induced deletions is compatible with offspring viability; and (2) the viability-compatible deletions induced in germ cells are more likely to manifest themselves as multi-system developmental anomalies rather than as single gene disorders. This paper: (a) pursues these concepts further in the light of knowledge of mechanisms of origin of deletions and other rearrangements from two fields of contemporary research: repair of radiation-induced DNA double-strand breaks (DSBs) in mammalian somatic cells and human molecular genetics; and (b) extends them to deletions induced in the germ cell stages of importance for radiation risk estimation, namely, stem cell spermatogonia in males and oocytes in females. DSB repair studies in somatic cells have elucidated the roles of two mechanistically distinct pathways, namely, homologous recombination repair (HRR) that utilizes extensive sequence homology and non-homologous end-joining (NHEJ) that requires little or no homology at the junctions. A third process, single-strand annealing (SSA), which utilizes short direct repeat sequences, is considered a variant of HRR. HRR is most efficient in late S and G2 phases of the cell cycle and is a high fidelity mechanism. NHEJ operates in all cell cycle phases, but is especially important in G1. In the context of radiation-induced DSBs, NHEJ is error-prone. SSA is also an error-prone mechanism and its role is presumably similar to that of HRR. Studies in human molecular genetics have demonstrated that the occurrence of large deletions, duplications or other rearrangements in certain regions of the genome is related to the presence of large segments of repetitive DNA called segmental duplications (also called duplicons or low copy repeats, LCRs) in such regions. The mechanism that is envisaged for the origin of deletions and other rearrangements involves misalignment of region-specific LCRs of homologous chromosomes in meiosis followed by unequal crossing-over (i.e., non-allelic homologous recombination, NAHR). We hypothesize that: (a) in spermatogonial stem cells, NHEJ is probably the principal mechanism underlying the origin of radiation-induced deletions, although SSA and NAHR may also be involved to some extent, especially at low doses; and (b) in irradiated oocytes, NAHR is likely to be the main mechanism for generating deletions. We suggest future research possibilities, including the development of models for identifying regions of the genome that are susceptible to radiation-induced deletions. Such efforts may have particular significance in the context of the estimation of genetic risks of radiation exposure of human females, a problem that is still with us.

TBASE: a computerized database for transgenic animals and targeted mutations.

A computerized database, called TBASE, has been developed to organize and make available information on transgenic animals and targeted mutations by using resources at the Oak Ridge National Laboratory (ORNL) and Johns Hopkins University (JHU). The database is available through the JHU Computational Biology Gopher Server. To ensure that all interested users have access, several mechanisms will be installed to accommodate varying levels of telecommunication network connectivity.

Current status of the Gene-Tox Program.

The U.S. Environmental Protection Agency's Gene-Tox Program is a multiphased effort to review and evaluate the existing literature in assay systems available in the field of genetic toxicology. The first phase of the Gene-Tox Program selected assay systems for evaluation, generated expert panel reviews of the data from the scientific literature, and recommended testing protocols for the systems. Phase II established and evaluated the database of chemical genetic toxicity data for its relevance to identifying human health hazards. The ongoing phase III continues reviewing and updating chemical data in selected assay systems. Currently, data exist on over 4000 chemicals in 27 assay systems; two additional assay systems will be included in phase III. The review data are published in the scientific literature and are also publicly available through the National Library of Medicine TOXNET system. The review and analysis components of Gene-Tox comprise 45 published papers, and several others are in preparation. Differences that have been observed between Gene-Tox and National Toxicology Program databases relative to the sensitivity, specificity, accuracy, and predictivity of genetic toxicity data compared to carcinogenesis data are ascribable to differences between the two databases in chemical selection criteria, testing protocols, and chemical class distributions.

Origins of genetic toxicology and the Environmental Mutagen Society.

A brief history of events that contributed to the establishment of genetic toxicology as a distinct research area and influenced the formation of the Environmental Mutagen Society is presented.

A review and analysis of the Chinese hamster ovary/hypoxanthine guanine phosphoribosyl transferase assay to determine the mutagenicity of chemical agents. A report of phase III of the U.S. Environmental Protection Agency Gene-Tox Program.

Published literature on the Chinese hamster ovary cell/hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) assay from mid-1979 through June 1986 was reviewed and evaluated. Data from the papers considered acceptable include test results on 121 chemicals belonging to 25 chemical classes. A total of 87 chemicals were evaluated positive, 3 negative, and 31 inconclusive. Mutagenicity data on 49 of the 121 chemicals evaluated could also be compared with in vivo animal carcinogenicity data. 40 of the 43 reported animal carcinogens were considered mutagenic. Caprolactam, the only definitive noncarcinogen in the group of 49, was not mutagenic. The CHO/HGPRT assay was concluded to be an appropriate assay system for use in the screening of chemicals for genotoxicity.

An approach to identifying specialized batteries of bioassays for specific classes of chemicals: class analysis using mutagenicity and carcinogenicity relationships and phylogenetic concordance and discordance patterns. 1. Composition and analysis of the overall data base. A report of phase II of the U.S. Environmental Protection Agency Gene-Tox Program.

This report of the Gene-Tox Assessment Panel is a compilation of data that documents the chemical testing efforts in genetic toxicology through mid-1979. It thus provides an historical perspective of the major efforts in this field and the utility of test models. The total number of chemicals tested in assays reflects chemical availability, commercial interest in specific structural types, the ease or difficulty in assay performance, as well as methodological development resulting from testing experience. Other factors that have been important in assay selection and utility are the perceptions of relevance to hazard evaluation of chemicals and the role that genetic factors may have in other disease states as well as in heritable defects. The phylogenetic diversity of test systems attests to the tremendous effort that has been applied to the testing and evaluation of the effect chemicals can have on genetic structure. The data also illustrate the fact that certain chemicals have an intrinsic capability to alter the genetic structure of cells of diverse biological origin in an heritable manner, whereas others do not. Any attempt to summarize and analyze a data base of this magnitude is a formidable task that would be almost impossible without a computer capability. A computerized system of analysis has been developed at the Environmental Mutagen Information Center (EMIC) that makes it possible to examine the performance of any particular assay in any of 30 chemical classes and to make comparisons with all the other assays individually or in designated groupings. Components of this system include: A distribution of the 2622 chemicals into 30 chemical classes with results of testing in each class. A tabulation of assay results showing the total numbers of chemicals tested, with their definitive and nondefinitive results. A subdivision of assays and results of testing into four major groups: gene mutation, chromosomal aberrations, other genotoxic effects, and in vitro cell transformation assays. These major groups are further subdivided into phylogenetic categories and type of assay. A system of analysis of results utilizing mutagenicity and carcinogenicity comparisons and phylogenetic concordance and discordance. The major utility and/or benefit of this compilation will be derived from a chemical class by chemical class comparative analysis of individual assay performance. Obviously, the data base will serve as a resource for safety evaluation of chemicals through structural correlations and biological end point analyses.(ABSTRACT TRUNCATED AT 400 WORDS)

Toxicological review of busulfan (Myleran).

Busulfan is a bifunctional alkylating agent that appears to be cytotoxic to slowly proliferating or non-proliferating stem cell compartments, although its specific molecular and cellular mechanisms are unknown. It is the drug of preference in treatment of chronic myelogenous or granulocytic leukemia because its cytotoxic activity results in primary damage or destruction of hematopoietic cells. Additional effects resulting from the cytotoxicity of busulfan in hematological and other tissues, as documented by both human and animal model studies, include lethality, sterility, teratogenicity, and alteration of immune function. Busulfan has been shown to be mutagenic to microorganisms, mammalian cells in culture, Drosophila, and rodents. This agent is also considered potentially carcinogenic to humans. Various tissue hyperplasia and preneoplastic cells have been observed in animal model studies with busulfan, and case reports on human patients implicate busulfan as the causative agent in induction of secondary malignancies. Reports from human and animal studies of busulfan's cytotoxicity, teratogenicity, carcinogenicity, and mutagenicity have been reviewed. This information may be useful in a quantitative assessment of the effects of this agent and the identification of significant deficiencies in the data base. Demonstration that busulfan induces mutations in both somatic and germ cells suggests the need to assess its risk to humans.

Use of selected toxicology information resources in assessing relationships between chemical structure and biological activity.

This paper addresses the subject of the use of selected toxicology information resources in assessing relationships between chemical structure and specific biological end points. To assist the researcher in how to access the primary literature of genetic toxicology, teratogenesis, and carcinogenesis, three specific specialized information centers are discussed--Environmental Mutagen Information Center, Environmental Teratology Information Center, and Environmental Carcinogenesis Information Center. Also included are descriptions of information resources that contain evaluated (peer-reviewed) biological research results. The U.S. Environmental Protection Agency Genetic Toxicology Program, the International Agency for Research on Cancer Monographs, and the Toxicology Data Bank are the best sources currently available to obtain peer-reviewed results for compounds tested for genotoxicity, carcinogenicity, and other toxicological end points. The value of published information lies in its use. It has become evident that most information cannot be accepted at face value for interpretation and analysis when subjected to stringent quality evaluation criteria. This deficit can be corrected by rigid editorship and the cognizance of authors. Increased interest in alternative methods to in vivo animal testing will be exemplified by use of short-term bioassays and in structure-activity relationship studies. With respect to this latter area, it must be remembered that mechanically (computer generated) derived data cannot substitute, at least at this stage, for data obtained from actual animal testing. The future of structure-activity relationship studies will rest only in their use as a predictive tool.

Mammalian in vivo and in vitro cytogenetic assays: a report of the U.S. EPA's gene-tox program.

This report presents an assessment made by the U.S. Environmental Protection Agency Gene-Tox Program's Work Group on mammalian cytogenetics of the clastogenic effects of chemicals in in vivo and in vitro mammalian cell assays. This assessment is based on information provided by the Environmental Mutagen Information Center, Oak Ridge National Laboratory, with the proviso that the experimental protocol used in these papers was adjudged to be acceptable by standards outlined by the Work Group. Some data were accepted as "qualitative only" because the protocol used was fairly close to that proposed as suitable. Using these criteria, 177 papers were selected for review. 6 assays were reviewed: bone marrow (32 papers, 31 chemicals), spermatogonial (10 papers, 10 chemicals), spermatocyte (25 papers, 25 chemicals), oocyte or early embryo (18 papers, 19 chemicals), in vitro cell culture (30 papers, 66 chemicals), and leukocyte (66 papers, 53 chemicals). Each assay was considered separately, and comparisons were then made between them for their similarities or differences in producing a positive or negative clastogenic effect of a particular chemical or chemical class. A large proportion of the available cytogenetic data was not suitable for inclusion in the final data base because of poor experimental design or unsatisfactory reporting of the information. It was not possible to recommend any one assay for determining potential clastogenicity because each had its own particular advantages and limitations and provided unique information. For demonstrating in vivo effects, the bone-marrow assay is probably the simplest and most economical. If only in vitro exposures were considered, leukocytes or cultured mammalian cell lines would be suitable. However, there are advantages to using leukocytes because they are a synchronous population, at least through their cell division, and because of the ready availability of human cells. In general, there was good agreement between clastogenicity and carcinogenicity.

Literature survey of bacterial, fungal, and Drosophila assay systems used in the evaluation of selected chemical compounds for mutagenic activity.

Literature reports were surveyed, with results noted from experiments in seven nonmammalian assay systems used for the detection of mutagenicity or other related genetic effects. A comparison was made of the activities of 54 selected noncarcinogens, procarcinogens, and ultimate carcinogens as revealed by these test systems. Of the compounds tested, 49 (91%) were active in one or more of the assays, and 42 (78%) were positive in at least one system without having to be metabolically activated. In one or more test systems, 17/17 (100%) of the ultimate carcinogens, 27/28 (96%) of the procarcinogens, and 6/9 (67%) of the noncarcinogens were positive. The Ames Salmonella-microsome assay responded with increased mutation frequency to 37/44 (84%) of the carcinogenic compounds but to only 2/8 (25%) of the noncarcinogens tested. The Drosophila system responded to 19/21 (90%) of the carcinogens and to 3/6 (50%) of the noncarcinogens. Prophages were induced when lysogenic bacteria were exposed to 12/21 (57%) of the carcinogens, but not enough tests were done with the noncarcinogens (1/3, or 33%) for a comparison. The other systems reviewed, such as the killing of repair-deficient bacteria, mutations in Escherichia coli and Neurospora crassa, and the host-mediated assay, were not challenged by enough of the compounds for valid comparisons.

A review of the genetic toxicology of chlorinated dibenzo-p-dioxins.

Information from both published and unpublished sources considered relevant to the understanding of the genetic toxicology of chlorinated dibenzo-p-dioxins is summarized in this review. Interest in writing this paper was stimulated by the fact that this class of compounds, particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has gained notoriety as an extreme environmental and industrial hazard. The potential for human exposure occurs in the work place when dioxins are formed during the synthesis of a number of commercially important compounds such as 2,4,5-trichlorophenoxyacetic acid, hexachlorophene, and pentachlorophenol. Environmental contamination may result from manufacturing processes and from dioxin contaminants in marketed products. Research on dioxins as potential mutagens was initiated because of their structural similarity to acridines, a class of known intercalating agents. To date, only 4 dioxin compounds have been evaluated for mutagenicity: the di-, tetra-, and octa-chlorinated derivatives and the unsubstituted dibenzo-p-dioxin. Since only a few of the many possible structural forms of dioxins have been tested, no definite conclusions can be made about their potential mutagenicity. Furthermore, the positive mutagenicity and cytological effects reported thus far with the few dioxin isomers examined seems to depend on the position of chlorine substitution. The most active form of the molecule is the 2,3,7,8-derivative (TCDD). Data available for assessing the mutagenic potential of TCDD are conflicting and scarce. Differences in testing results reported in these studies could be attributed to solubility problems with the test chemical, treatment protocols, purity of test samples, or toxicity. Because there are conflicting data, additional experiments are needed before the mutagenic potential of TCDD and other dioxins can be determined. Studies exploring the promoting effect of dioxins on the mutagenicity of other compounds are also recommended because experiments have shown TCDD to be an extremely active liver enzyme inducing agent that enhances the mutagenicity of certain polycyclic hydrocarbons such as 3-methylcholanthrene in vitro. The importance of discerning the hazards to human health from dioxin compounds became apparent after an accidental release of TCDD from a chemical plant contaminated the Seveso, Italy area in July 1976. This accident revealed that insufficient data were available to properly evaluate the long-term health risks posed by dioxin compounds. Several research projects were therefore initiated after the Seveso incident; it is hoped that many of the questions concerning the mutagenicity of TCDD and possibly of other dioxin congeners will be answered as a result of this work.