Genotoxicity

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METHODPRINCIPLE OF THE TEST
OECD TG 471: Bacterial Reverse
Mutation Test (Ames test)
The Ames test uses at least five strains of amino acid–requiring Salmonella typhimurium and Escherichia coli to detect point mutations by base substitutions or frameshifts. It detects mutations that revert other mutations present in the test strains, restoring the functional capability of the bacteria to synthesise an essential amino acid. A very large database of results for a wide variety of structures is available for bacterial reverse mutation tests, and well-established methodologies have been developed for testing chemicals with different physicochemical properties, including volatile compounds.
OECD TG 476: In Vitro Mammalian
Cell Gene Mutation Tests Using the HPRT and XPRT Genes
This test measures mutation at hypoxanthine-guanine phosphoribosyl transferase (HPRT) and at a transgene of xanthine-guanine phosphoribosyl transferase (XPRT). The HPRT and XPRT mutation tests detect different spectra of genetic events. Mutant frequency is determined by seeding known numbers of cells in a medium containing the selective agent in order to detect mutant cells and in a medium without the selective agent in order to determine the cloning efficiency (viability). After a suitable incubation time, colonies are counted.
OECD TG 487: In Vitro Mammalian
Cell Micronucleus Test (MNvit)
The MNvit detects micronuclei in the cytoplasm of interphase cells. Micronuclei may originate in acentric chromosome fragments (i.e. those lacking a centromere) or whole chromosomes that are unable to migrate to the poles during the anaphase stage of cell division. The assay detects the activity of clastogenic and aneugenic test substances in cells that have undergone cell division during or after exposure to the test substance.
OECD TG 473: In Vitro Mammalian Chromosomal Aberration TestThis method identifies agents that cause structural chromosomal aberrations in cultured mammalian somatic cells. At predetermined intervals after the exposure of cell cultures to the test substance, the cells are treated with a metaphase-arresting substance, harvested, and stained. Metaphase cells are then analysed microscopically for the presence of chromosomal aberrations.
OECD TG 490: In Vitro Mammalian Cell Gene Mutation Tests Using the Thymidine Kinase GeneThis TG includes two distinct in vitro mammalian gene mutation assays requiring two specific thymidine kinase (TK) heterozygous cell lines: L5178Y tk+/− 3.7.2C cells for the mouse lymphoma assay and TK6 tk+/− cells for the TK6 assay. Genetic events detected using the tk locus include both gene mutations and chromosomal events. Mutant frequency is determined by seeding known numbers of cells in a medium containing the selective agent in order to detect mutant cells and in a medium without the selective agent in order to determine the cloning efficiency (viability). After a suitable incubation time, colonies are counted.
In Vitro Human Reconstructed
Skin Comet Assay
The Comet assay detects the migration of DNA in an agarose gel following electrophoresis, which results in a “head” of intact DNA and a “tail” of fragmented DNA. Single cells are isolated from a highly differentiated three-dimensional tissue model consisting of normal, human-derived epidermal keratinocytes cultured on specially prepared tissue culture inserts and evaluated by fluorescence microscopy after staining with a DNA-binding fluorescent agent. The percentage of DNA in the tail of the comet is used as a measure of DNA damage.
In Vitro Human Reconstructed
Skin Micronucleus (RSMN) Assay
Like the MNvit, the RSMN test detects micronuclei in the cytoplasm of interphase cells – in this case, a highly differentiated three-dimensional tissue model consisting of normal, human-derived epidermal keratinocytes cultured on specially prepared tissue culture inserts.

The proposed two-test battery consisting of OECD TG 471 and OECD TG 487 covers all three endpoints to be assessed in the standard testing battery (gene mutation, structural chromosomal aberration, and numerical chromosomal aberration). The set of tests used in the in vitro battery should be as small as scientifically justifiable in order to reduce the number of false positives.

  • Notes

    False positive results in OECD TG 471 can be reduced via the following methods: 1) using p53-competent human cells, 2) choosing measures of cytotoxicity based on cell proliferation, 3) carefully checking the source and the characterisation  of the cells, and 4) testing at reduced maximum concentration.

    The in vitro human reconstructed Comet assay and in vitro human reconstructed skin micronucleus assay are most applicable to testing dermally applied  substances. When assessing substances administered by other routes, test methods such as the liver or airway human three-dimensional reconstructed tissue assays may be more relevant.

    This may include information on toxicodynamics, toxicogenomics, toxicokinetics, mechanism of toxicity, involvement of reactive oxygen species and saturation of detoxification mechanisms, gene expression profiles, and cell transformation assays.