Point mutations, genotoxicity testing

The test is commonly employed as an initial screen for genotoxic activity and, in particular, for point mutation-inducing activity. It detects point mutations, which involve substimtion, addition or deletion of one or a few DNA base pairs. The reverse mutation test in either Salmonella typhimurium or Escherichia coli detects mutation in an amino acid requiring strain (histidine or tryptophan, respectively) to produce a strain independent of an outside supply of amino acid. The principle of the test is that it detects mutations, which revert mutations present in the test strains and restore the functional capability of the bacteria to synthesize an essential amino acid. The revertant bacteria are detected by their ability to grow in the absence of the amino acid required by the parent test strain. 

It is tempting to classify all in vivo techniques as risk-assessment methods. For example, host-mediated-type studies have been incorrectly assumed to be more applicable to risk assessment than related in vitro studies. This is not necessarily true, since the genotoxic end points are still measured by in vitro test systems coupled with the animal host, and therefore the data are still valid only to detect potential. Attributing greater relevance to such tests may result in erroneous conclusions regarding safety or risk. Actually, there are very few practical risk-assessment assays available to evaluate genotoxicity, and our ability to extrapolate results from these to humans is uncertain, since evidence demonstrating mutation induction in humans is technically not possible. 

Genotoxicity studies are required to identify compounds that can induce genetic damage ranging from single point gene mutations to gross alterations of chromosomal structure. Such effects are taken as indicative of the potential to cause cancer or heritable defects in humans. A standard battery of three types of test is recommended ... 

Mutagenicity (genotoxicity). A bacterial mutagenicity test that demonstrates the induction of point (gene) mutations is always required. Some mutations result in the development of cancer. 

A number of short-term tests can be used to determine the genotoxic potential of chemicals. These tests use both prokaryotic and eukaryotic cells and measure such end points as gene mutations, chromosomal aberrations, and interactions with critical macromolecules It is widely recognized chat no test can detect all genotoxic compounds, and multiple end points are required to provide a reliable assessment of genotoxiclty. Information from several tests can be combined to reveal two important toxic effects carcinogenesis and mutagenesis. 

DIPE has been tested for genotoxic activity in bacterial mutation assays, a yeast assay for mitotic gene conversion, and in tests using rat liver and Chinese hamster ovary cells with structural chromosome damaging the end point. Negative responses were observed in the bacterial and yeast assays. 

The genotoxicity of PCBs has been tested in in vivo and in vitro studies with generally negative results. End points that have been examined in these studies include gene mutations in bacteria and Chinese hamster V79 cells, chromosomal aberrations in human lymphocytes and rat and mouse bone marrow cells and spermatogonia, micronuclei in mouse bone marrow cells, and dominant lethal mutations in rat sperm cells. 

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