Mammalian Cell HPRT Gene Mutation Assay: Test Methods

Using the combination of bacterial gene mutation assay and chromosomal aberrations test in mammalian cells may not detect a small proportion of mammalian specific mutagenic agents. Therefore, at the current time a third assay should be used, except for compounds for which there is little or no exposure (DOH (2000) Department of Health Guidance for the testing of chemicals for Mutagenicity. Committee on Mutagenicity of Chemicals in Food, Consumer Products and the Environment). The hypoxanthine phosphorybosyl transferase (HPRT ) gene is on the X chromosome of mammalian cells, and it is used as a model gene to investigate gene mutations in mammalian cell lines. The assay can detect a wide range of chemicals capable of causing DNA damage that leads to gene mutation. The test follows a very similar methodology to the thymidine kinase (TK) mouse lymphoma assay (MLA), and both are included in the guidelines for mammalian gene mutation tests (OECD (1997) Organisation for Economic Co-operation and Development. Ninth addendum to the OECD Guidelines for the Testing of Chemicals. In Vitro Mammalian Cell Gene Mutation Test: 476). The HPRT methodology is such that mutations which destroy the functionality of the HPRT gene and or/protein are detected by positive selection using a toxic analogue, and HPRT ⁻ mutants are seen as viable colonies. Unlike bacterial reverse mutation assays, mammalian gene mutation assays respond to a broad spectrum of mutagens, since any mutation resulting in the ablation of gene expression/function produces a HPRT ⁻ mutant. Human cells are readily used, and mechanistic studies using the HPRT test methodology with modifications, such as knock-out cell lines for DNA repair, can provide details of the mode of action (MOA) of the test compound (24).