| Abstract
| - Guidelines have been proposed to assess the potential of chemicals to affect human health. Written into these guidelines is the requirement that information be submitted on mutagenic activity. Although regulatory agencies accept mutagenicity data from both the hprt and tk loci in mammalian cells, many studies suggest that the L5178Y mouse lymphoma assay at the thymidine kinase locus is likely to detect a greater spectrum of mutagenic lesions. Thus, there is increasing emphasis being placed on this assay in many proposed and published guidelines. The L5178Y mouse lymphoma suspension protocol produces both small and large colonies which are the products of mutants growing at different rates. There is a reduction in the proportion of slowly growing mutants with respect to the total population of cells when expression is carried out in suspension. This potentially leads to quantitatively inaccurate assessments of the mutagenic activity of chemicals. Therefore an in situ procedure was developed that more accurately assesses the mutagenic activity of chemicals by maximizing the detection of small colonies. Many guidelines recommend tests that assess the clastogenic activity of chemicals. Some regulatory agencies accept data from the mouse lymphoma mutation assay to detect clastogens if the protocol is optimized for the detection of small colonies or if colony sizing data are submitted. The conventional suspension assay protocol is not sufficiently validated for this purpose. The in situ protocol has greater potential to meet these requirements. However, although this strategy might be suitable for detecting compounds that induce gene mutations and chromosomal aberrations in cells, it should not be used to distinguish these two responses from each other because there are insufficient data showing that small and large colony populations always represent the induction of chromosome aberrations and gene mutations, respectively. It is, nevertheless, possible to infer not only mutation and clastogenesis but also aneugenesis in one culture using mouse lymphoma cells. To achieve this, we recommend an assessment of micronucleus formation with an analysis of micronuclei for the presence of whole chromosomes and chromosomal fragments in mouse lymphoma cells coupled with the measurement of TFT resistance using the in situ protocol with these same cells. This should result in eventual validtion of a simpler and more accurater method for asscertaining these endpoints.
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