Implications for genetic toxicology of the chromosomal breakage syndromes

The activation of oncogenes and our knowledge of the chromosome breakage syndromes show that both intragenic mutations and chromosomal aberrations are important in carcinogenesis. Each suggests that an agent could produce genetic changes in a tissue without producing cancer there, if the types of genetic change do not match: chromosomal aberrations may be irrelevant in the mammary epithelium but be very significant in the bone marrow, and vice versa. This has vital implications for genetic toxicology: (1) both gene mutations and chromosomal aberrations should be measured, and (2) carcinogens may be mutagenic in tissues in which they are not carcinogenic. One might therefore expect in vivo assays for mutagenicity to correlate rather well with cancer bioassays; unfortunately, the bioassays themselves seem faulty. If cancer bioassays are valid, they would be reproducible. If bioassays are reproducible, they would be internally consistent. The information supplied by Tennant et al. (1987) for their validation of in vitro assays gives data from both sexes in rats and mice for 70 chemicals. When the data are analyzed site-by-site, positive results were not replicated in the other sex or in the other species much of the time: in half the cases the other sex does not give the same result; in two-thirds of the cases the other species does not give the same result. There are 3 potential explanations for these differing results: (1) genuine sex-specific carcinogens are common, (2) genuine species-specific carcinogens are common, or (3) the bioassay does not replicate well, i.e., is erratic. The third possibility best explains the data. The apparent inability of short-term in vitro tests to discriminate well between carcinogens and non-carcinogens may be more a reflection of the cancer bioassays that were used to determine which chemicals were carcinogenic than any defect in the assays. In this situation in vivo assays can scarcely be expected to do better even if they are better.