208 Diana Anderson tumour and is a very emotive issue, the end-point of genetic damage in man is more obscure. With current techniques we are able to distinguish between molecular changes in the gene and small chromosome aberrations. Most human traits are inherited as Mendelian units which can be divided into autosomal dominant, autosomal recessive and X-linked recessive units. There are also X-linked dominants and there may be a few Y-linked traits but these are numerically insignificant (10). McKusick (11) lists such traits. Th, dominant expression of a mutant gene is sufficient to cause a recognisable abnormality or disease and there are also some well-established recessive traits. However, whilst a change in a dominant trait is immediately evident in the next generation, recessive traits may take very many generations before they are expressed. Therefore, the effect of chemi- cal mutagenesis at this level of the gene may not show immediate results and the data generated by mutagenic tests lose their impact. Chromosome abnormalities may arise by errors in the distribution of chromosomes leading to abnormalities of chromosome numbers such as non-disjunction, where the effect is seen in the next generation, or by the consequence of chromosome breakage. Consequences of chromosome damage are physical or mental abnormalities, sterility or embryonic death. With regard to the last condition, epidemiological evidence suggests that there is an increased incidence in foetal wastage in wives of workers exposed to vinyl chloride (12, 13, 14). There is however, a conflict concerning the statistical interpretation of the data (15), and as yet there is thus no unequivocal epidemiological evidence that vinyl chloride can cause germ cell mutations in man. Married women anaesthetists were claimed to have a higher incidence of spontaneous abortion when they worked than when they did not work and the incidence was also higher by comparison with non- anaesthetist married women doctors (16) but, it is difficult to determine if the abortions were due to genetic events or embryotoxicity because of systemic involvement after exposure of the women. Many constitutional and degenerative diseases such as epilepsy and schizophrenia may be caused by other irregularities of the gene expression or arise from multiple genes (10). The effect of chemical mutagenesis both on multiple gene effects as well as at the level of the cromosome may also not be immediately obvious. Nevertheless, since the world population is rapidly increasing the chance for natural selection in civilised communities is being sharply reduced, and whilst it is recognised that not all mutagens are harmful and some are necessary for evolution, any increase in the mutagenic factors in the environment could cause a potential risk to the population. It is estimated that more than 4•o of the population are affected by genetic anomalies and about 1 •o of children born each year have chromosomal mutations (17). Mutations that cause sterility or early embryonic loss are detrimental in the Darwinian sense but have little impact on society. Mutations that are more fit biologically may be a heavy burden to society if the affected persons require medical or institutional care (18). Thus it is necessary to monitor potential chemical mutagens which could increase the percentage of genetic aberrations in the population, and although a great number of relatively simple and practical methods are available the evaluation of mutagenic effects is a complex and extremely difficult task. FACTORS DETERMINING A MUTAGENIC RESPONSE The mutagenic activity of chemical substances has been studied in a number of organisms, using different methods for calculating both gene and chromosome mutations in somatic

The current state of mutagenicity testing 209 and germ cells. Systems from micro-organisms, plants, insects, mammalian and human cells in vivo and in vitro have been used, and whilst the mutagenic effect of a chemical may be detectable in one system it may not be in another or even in different organs of the same system. There may be many interactions between a chemical and an organism, all of which may determine whether genetic damage is expressed- unlike ionising radiation, which is immediately active in terms of its mutagenic potential. With radiation extremely short-lived free radicals are generated randomly in cells and tissues very close to or within the genetic target molecules. There are many factors that determine whether a chemical compound reaches and reacts with the critical genetic targets. Among these factors in mammals are chemical structure of the compound, duration of treatment, route of administration, absorption, distribution, excretion, drug-protein binding, metabolic transformation, pharmacogenetic make-up (species, strain, sex) membrane barriers, numbers of SH groups, etc (19). The following examples will illustrate such factors: Vinyl chloride, a known carcinogen (20), requires metabolic activation to exert its effect (21). Vinyl chloride exposure causes mutations in micro-organisms (22) and in some test systems involving higher organisms. It causes chromosome damage in human peripheral lymphocytes (23), but no effect is seen in the dominant lethal assay in mice (24). This suggests that various tissues metabolise the substance to differing extents and maybe the testes metabolise the substance least. This is certainly the case with dimethylnitrosamine, another indirect mutagen, where there is less alkylation in the testes than in other organs (25). This compound also gives a negative result in the dominant lethal assay (26), and in a host-mediated assay with yeast cells, mutagenic activity is high in the liver, moderate in the lungs, but only very weak in the testes (27, 28). Other compounds such as methylnitrosoguanidine, a direct mutagen, are metabolised rapidly in mammals to non-mutagenic activity and 4-nitroquinoline-l-oxide has high mutagenic activity in yeast cells recovered from the lungs and slight activity in the liver but is not active in the testes (28). Another possibility is that such compounds may be detoxified by organs where little mutagenic activity is detected. Other factors determine whether the damage in the target molecules is expressed as genetic damage. Such factors are the innate susceptibility of the cell (e.g. repair capacity), numbers of susceptible cells, type of genetic target of the genetic target locus, germinal and somatic selection processes involved, mode of inheritance etc (19). It has been found with ethyl methanesulphonate (EMS) for example, a compound which can alkylate all tissues directly (29), that at doses below 100 mg/kg a dominant lethal effect is not observed (30) but at higher doses an effect is clearly manifest in CD-1 mice at 150 mg/kg body weight (31), confirmed by Anderson et al. (32, 24, 33). Sperm proteins and sperm DNA are alkylated by EMS at doses as low as 3 mg/kg body weight (34). Such discrepancies may occur because the numbers of animals screened in a dominant lethal test may be statistically too small to detect a weak effect at low doses, or unknown factors may affect the expression of damage in germ-cell development preventing the production of muta- tions (19). The factors described above as influencing a mutagenic response complicate muta- genicity testing. Many compounds that would require testing will not behave in the same way as model mutagens, which bind covalently with biologically important molecules. Many will react reversibly with receptors by mechanisms such as hydrogen bonding and electrostatic binding. Such compounds may not react with the genetic material unless they become metabolised to highly reactive forms. Other factors such as temperature, ageing, pH, hypoxia and viral infections can also produce genetic damage (19). Chemicals