International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC Publication No. 11. The use of recombinant DNA technology to study gene alteration

Evolution of testing strategies for genetic toxicity

Shortly following the inception of genetic toxicology as a distinct discipline within toxicology, questions arose regarding the type and number of tests needed to classify a chemical as a mutagenic hazard or as a potential carcinogen. To some degree the discipline separated into two sub-specialties, (1) genetic risk assessment and (2) cancer prediction since data from experimental oncology also supports the existence of a genotoxic step in tumor initiation. The issue of which and how many tests continued to be debated, but is now focused more tightly around two independent phenomena. Tier or sequential testing was initially proposed as a logical and cost-effective method, but was discarded on the basis that the lower tier tests appeared to have too many false responses to force or exclude further testing of the test agent. Matrix (battery) testing was proposed for screening on the hypothesis that combinations of endpoints and multiple phylogenetic target organisms were needed to achieve satisfactory predictability. As the results from short-term test 'validation' studies for carcinogen prediction and evaluations of EPA's Gene-Tox data accumulated, it became obvious that qualitative differences remained between predictive and definitive tests and by assembling different combinations of short-term assays investigators did not appear to resolve the lack of concordance. Recent trends in genetic toxicology testing have focused on mathematical models for test selection, and standardized systems for multi-test data assessment.

Deployment of short-term assays for the detection of carcinogens; genetic and molecular considerations

The deployment of short-term assays for the detection of carcinogens inevitably has to be based on the genetic alterations actually involved in carcinogenesis. This paper gives an overview of oncogene activation and other mutagenic events connected with cancer induction. It is emphasized that there are indications of DNA alterations in carcinogenicity, which are not in accordance with "conventional" mutations and mutation frequencies, as measured by short-term assays of point mutations, chromosome aberrations and numerical chromosome changes. This discrepancy between DNA alterations in carcinogenicity and the endpoints of short-term assays in current use include transpositions, insertion mutations, polygene mutations, gene amplifications and DNA methylations. Furthermore, tumourigenicity may imply an induction of a genetic instability, followed by a cascade of genetic alterations. The evaluation of short-term assays for carcinogenesis mostly involves two correlations that is, between mutation and animal cancer data on the one hand and between animal cancer data and human carcinogenicity on the other. It should be stressed that animal bioassays for cancer in general imply tests specifically for the property of chemicals to function as complete carcinogens, which may be a rather poor reflection of the actual situation in human populations. The primary aim of short-term mutagenicity assays is to provide evidence as to whether a compound can be expected to cause mutations in humans, and such evidence has to be considered seriously even against a background of negative cancer data. For the evaluation of data from short-term assays the massive amount of empirical data from different assays should be used and new computer systems in that direction can be expected to provide improved predictions of carcinogenicity.