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Within the context of current international initiatives on the control of persistent organic pollutants (POPs), an overview is given of the scientific knowledge relating to POP sources, emissions, transport, fate and effects. At the regional scale, improvements in mass balance models for well-characterised POPs are resulting in an ability to estimate their environmental concentrations with sufficient accuracy to be of help for some regulatory purposes. The relevance of the parameters used to define POPs within these international initiatives is considered with an emphasis on mechanisms for adding new substances to the initial lists. A tiered approach is proposed for screening the large number of untested chemical substances according to their long-range transport potential, persistence and bioaccumulative potential prior to more detailed risk assessments. The importance of testing candidate POPs for chronic toxicity (i.e. for immunotoxicity, endocrine disruption and carcinogenicity) is emphasised as is a need for the further development of relevant SAR (structure activity relationship) models and in vitro and in vivo tests for these effects. Where there is a high level of uncertainty at the risk assessment stage, decision-makers may have to rely on expert judgement and weight-of-evidence, taking into account the precautionary principle and the views of relevant stake-holders. Close co-operation between the various international initiatives on POPs will be required to ensure that assessment criteria and procedures are as compatible as possible.

Aneuploidy plays a significant role in adverse human health conditions including birth defects, pregnancy wastage and cancer. Although there is clear evidence of chemically induced aneuploidy in experimental systems, to date there are insufficient data to determine with certainty if chemically induced aneuploidy contributes to human disease. However, since there is no reason to assume that chemically induced aneuploidy will not occur in human beings, it is prudent to address the aneugenic potential of chemicals in the safety assessment process. A wide range of methods has been described for the detection of chemically induced aneuploidy including subcellular systems, tests with fungi, plants and Drosophila as well as in vitro mammalian systems and in vivo mammalian somatic and germ cell assays. However, none of these methods is sufficiently validated or widely used in routine screening. Underlying the efforts to develop aneuploidy-specific assays is the presumption that current genetic toxicology tests do not detected chemicals that have aneuploidy-inducing potential. To address this, we have critically evaluated data from standard genetic toxicology assays for 16 known or suspected aneugens. The conclusions from the review are listed below. 1. At present there are only nine chemicals that can be classified as definitive aneugens, as determined by positive results in in vivo rodent assays. 2. As expected, the majority of definitive and suspected aneugens are negative in the bacterial mutation assay. 3. The majority of definitive aneugens evaluated induce polyploidy in vitro. With few exception, they also induced structural chromosome aberrations in vitro. 4. All of the definitive aneugens that have been sufficiently tested induce micronuclei in rodent bone marrow cells in vivo. A number of these chemicals also induced structural chromosome aberrations in vivo. 5. There is no evidence for a unique germ cell aneugen, that is a chemical that induces aneuploidy in germ cells and not in somatic cells. Furthermore, an analysis of several databases indicates the proportion of chemicals which induce polyploidy and not chromosome aberrations in vitro is low. Based on these conclusions, the following recommendations are made: for screening purposes, a standard genotoxicity test battery (including an in vitro cytogenetic assay with an assessment of polyploidy and clastogenicity at the same harvest time) should be performed; in the absence of polyploidy induction in vitro no further evaluation of aneuploidy-inducing potential is needed; if polyploidy is observed, in vitro follow-up testing to investigate further the aneuploidy-inducing potential should be conducted; such follow-up testing will generally start with the conduct of a standard in vivo somatic cell micronucleus assay; if the in vivo somatic cell micronucleus assay is negative, with adequate evidence of exposure of the bone marrow to the test compound, no further testing of aneuploidy-inducing potential is needed; if the in vivo somatic cell micronucleus assay is positive, further information on mechanisms of micronucleus induction can be obtained by using kinetochore/centromeric staining in vitro and/or in vivo; an assessment of potential germ cell aneuploidy activity may then be considered; aneuploidy induction which does not involve the direct interaction of a chemical or its metabolite(s) with DNA is expected to have a threshold. This must be considered in the risk assessment of such chemicals; this is not addressed by current risk assessment guidelines.

Isoprene monomer is derived from petroleum cracking and is used in the manufacturing of polyisoprene, butyl rubber and a variety of copolymers. It is also the monomeric unit of naturally occurring terpenes (e.g. lycopenes, the pigmentation in tomatoes). Isoprene occurs in tobacco smoke and it is released by a wide range of plants and mammals (including man). Portion of the isoprene present in the atmosphere is therefore from natural origin. Isoprene is chemically closely related to butadiene. Both substances are being used in similar industrial processes and because of the structural analogy, it is often speculated that the toxicological properties of isoprene must resemble those of butadiene. In fact, the acute toxicity of isoprene is very similar and also the biotransformation to mono- and diepoxides is qualitatively alike. There is however a difference; isoprene is asymmetric and therefore more metabolic enantiomers are possible. Pharmacokinetic studies have demonstrated species differences (as with butadiene) in the maximum metabolic elimination rate: in mice this was determined to be at least three times higher than in rats which implies a species sensitivity in isoprene metabolism in the mouse. Both isoprene and its major metabolites were tested for mutagenic activity in Salmonella strains. Only the diepoxide was found to be positive in TA100. In in vivo cytogenetic tests in mice, no chromosomal aberrations were observed but an increase in SCEs and micronuclei was evident. As is the case with butadiene there are clear indications from long-term rat/mice inhalational studies with isoprene of a species difference in tumour response. Based on cytogenetic findings and tumorigenicity data it is clear that isoprene is less potent than butadiene. Such observations are important and useful to reduce uncertainties in risk-characterisation procedures (e.g. occupational exposure standard setting; EU existing chemicals regulations).

This paper discusses genotoxicity testing and data interpretation as applied in The Netherlands in the context of the regulation of chemicals. Guidelines were first formulated in 1981 and their use evolved in practice, on the basis of increasing experience at the national and international levels. The distinction between in vitro assays to detect intrinsic genotoxic properties and in vivo assays as a subsequent phase to show the realization of this potential in an intact organism has always been a cornerstone of the Dutch approach. Several critical aspects of the use of short-term genotoxicity tests in sequential schemes are discussed, such as their predictivity for carcinogenicity, the limited database concerning the performance of short-term in vivo assays, the relevance of devising separate strategies to test for possible carcinogenicity and germ cell mutagenicity, and the use of short-term tests to discriminate between genotoxic and non-genotoxic carcinogens. Examples are given of how short-term tests contributed to the toxicological evaluation of chemicals in The Netherlands.

We report on a microcephalic, growth-retarded newborn girl without major anomalies who has chromosome instability in lymphocytes and fibroblasts. Frequent involvement of bands 7p13, 7q34, 14q11, and 14q32 suggested the diagnosis of ataxia telangiectasia (AT) or a related disorder. Supportive evidence was radioresistant DNA synthesis in fibroblasts and radiation hypersensitivity of short-term lymphocyte cultures. Follow-up for nearly 4 years showed largely normal development, and no signs of telangiectasia, ataxia, or immunodeficiency. Serum AFP levels turned from elevated at age 5 months to normal at age 2 years. We propose that our patient belongs to the expanding category of "AT-related" genetic disorders, probably to the Nijmegen breakage syndrome.

Immunological and cytogenetic studies were performed in 6 patients with ataxia telangiectasia (AT). Immunological disturbances were found in these patients: immunoglobulin deficiencies (IgA, IgE, IgG2 and IgG4), decreased cellular immunity and a defect in the synthesis of specific antibodies. Cytogenetic studies revealed chromosome 7 and/or 14 abnormalities in all patients. X-irradiation of AT cells induced an excessive increase in chromosome and chromatid breaks. The DNA synthesis inhibition after X rays was less in AT patients compared to controls. The possibilities of early diagnosis and the eventual relationship between immunological and cytogenetic findings are discussed.

The induction of sister-chromatid exchanges (SCEs) by cyclophosphamide (CP) in human, rat and rabbit lymphocytes was studied in vitro without exogenous metabolic activation. In contrast with published reports, CP induced SCEs in lymphocytes from all three species, indicating that cultured lymphocytes possess the metabolic capacity to convert this indirect-acting mutagen.

Fibroblast cultures from six unrelated patients having a familial type of immunodeficiency combined with microcephaly, developmental delay, and chromosomal instability were studied with respect to their response to ionizing radiation. The cells from five of them resembled those from individuals with ataxia telangiectasia (AT) in that they were two to three times more radiosensitive on the basis of clonogenic cell survival. In addition, after exposure to either X-rays or bleomycin, they showed an inhibition of DNA replication that was less pronounced than that in normal cells and characteristic of AT fibroblasts. However, the patients are clinically very different from AT patients, not showing any signs of neurocutaneous symptoms. Genetic complementation studies in fused cells, with the radioresistant DNA synthesis used as a marker, showed that the patients' cells could complement representatives of all presently known AT complementation groups. Furthermore, they were shown to constitute a genetically heterogeneous group as well. It is concluded that these patients are similar to AT patients with respect to cytological parameters. The clinical differences between these patients and AT patients are a reflection of genetic heterogeneity. The data indicate that the patients suffer from a chromosome-instability syndrome that is distinct from AT.

A 10-year-old boy with microcephaly, stunted growth, mental retardation, café-au-lait spots and immunodeficiency is described. An older brother of the patient had the same clinical symptoms and a more severe immunodeficiency. Cytogenetic studies in the proband revealed a typical form of chromosome instability with multiple rearrangements of chromosomes 7 and 14. Such abnormalities were also present, though in very low frequencies, in the father and three of the phenotypically normal sibs. The similarity of the symptoms in the two sibs, the close consanguinity of their parents and the results of the cytogenetic studies in the family favour the hypothesis that the disorder is an inherited one. The clinical features and the chromosome aberrations as present in the proband are usually found in chromosomal breakage syndromes, but it was possible to exclude each of the classical chromosomal breakage syndromes on clinical and/or cytogenetic grounds.

We report on five independent families with a chromosome instability disorder that earlier had been called the Nijmegen breakage syndrome (NBS). These families, two from the Netherlands and three from Czechoslovakia, had a total of eight patients, five of whom are still alive. The main clinical manifestations were microcephaly, short stature, a "bird-like" face, immunological defects involving both the humoral and cellular system. In four of the five living patients it has been possible to study the chromosomes of cultured lymphocytes. The basic karyotype in these patients were normal, but in 17% to 35% of the metaphases rearrangements were found, preferentially involving chromosomes 7 and/or 14 at the sites 7p13, 7q34, and 14q11. The chromosomes of all five living patients were very sensitive to ionizing radiation. In addition, the DNA synthesis in their cultured lymphocytes and fibroblasts was more resistant to X-rays than in cells from controls. The NBS shares a number of important features with ataxia telangiectasia (AT). Both syndromes are characterized by the occurrence of typical rearrangements of chromosomes 7 and/or 14, cellular and chromosomal hypersensitivity to X-irradiation, radioresistance of DNA replication and immunodeficiency. However, there are also obvious differences: NBS patients have microcephaly but neither ataxia nor telangiectasia, and in contrast to the situation in AT the alpha-fetoprotein level in their serum is normal.

Chromosome analysis was performed in 17 children with IgA-deficiency. In two patients a constitutional structural chromosome abnormality was found. A ring chromosome 22 was seen in one, while in the other a mosaicism of ring chromosome 18/18p+ was observed. Both patients were mentally retarded and showed distinct congenital defects. From ten asymptomatic patients, spontaneous as well as X-ray-induced chromosome instability was investigated. There was no increased spontaneous instability, and also after irradiation the induced chromosome damage was within normal control levels. A relationship between IgA-deficiency and X-ray hypersensitivity, as might be suggested by the frequently occurring coincidence of radiosensitivity and IgA-deficiency in ataxia telangiectasia patients, is not established.

The Nijmegen Breakage Syndrome (NBS) is a new chromosomal instability disorder different from ataxia telangiectasia (AT) and other chromosome-breakage syndromes. Cells from an NBS patient appeared hypersensitive to X-irradiation. X-rays induced significantly more chromosomal damage in NBS lymphocytes and fibroblasts than in normal cells. The difference was most pronounced after irradiation in G2. Further, NBS fibroblasts were more readily killed by X-rays than normal fibroblasts. In addition, the DNA synthesis in NBS cells was more resistant to X-rays and bleomycin than that in normal cells. The reaction of NBS cells to X-rays and bleomycin was similar to that of cells from patients with ataxia telangiectasia. Our results indicate that NBS and AT, which also have similar chromosomal characteristics, must be closely related.