The OECD Test Guideline 476 describes in vitro mammalian cell gene mutation tests used to detect gene mutations that are induced by test chemicals. The guideline covers mutation assays targeting the endogenous hypoxanthine-guanine phosphoribosyl transferase gene (Hprt/HPRT) and the xanthine-guanine phosphoribosyl transferase transgene (gpt), referred to as the HPRT and XPRT tests, respectively.

The HPRT and XPRT tests detect a different set of genetic events. The HPRT test detecs mutations such as basepair substitutions, frameshifts, small deletions and insertions. Whereas, for XPRT test, the gpt transgene used is located on an autosome, and therefore, may additionally detect mutations arising from large deletions and possibly mitotic recombination.

The principle of the assay is based on resistance to the purine analogue 6-thioguanine (TG). Mutant cells deficient in Hprt or xprt enzyme activity are able to proliferate in the presence of TG, whereas cells with functional Hprt or gpt activity are sensitive to its cytostatic effects. Cells are exposed to the test chemical with and without an exogenous source of metabolic activation, followed by a phenotypic expression period and mutant selection. Mutant frequency is then determined from the number of mutant colonies, corrected for cloning efficiency.

Vipragen Biosciences offers in vitro mammalian cell gene mutation testing in accordance with OECD TG 476 as part of its genetic toxicology testing services.