A recombination-based transgenic mouse system for genotoxicity testing.

It is well established that mutagens induce recombination in cultured cells and experimental organisms. Presumably, this is a consequence of the DNA-damage-triggering cellular-repair mechanisms. The relationship between recombination and mutagenicity has been exploited in submammalian organisms, such as yeast, to assay the ability of chemical agents and radiation to induce a form of recombination called gene conversion--the non-reciprocal transfer of genetic information. This work has demonstrated the efficacy of predicting mutagenicity on the basis of recombination induction. Here, we describe the utilization of a transgenic mouse system for efficient detection of germ-line gene-conversion events as a mutagen-screening tool. These mice contain two mutually defective reporter (lacZ) genes under the regulatory control of a spermatogenesis-specific promoter. A particular intrachromosomal gene conversion event must occur for the generation of functional lacZ activity. Conversion events are visualized by histochemical staining or flow cytometric analysis of transgenic spermatids. The highly mutagenic compound chlorambucil induced a several fold percentage-wise increase of lacZ-positive spermatids, whereas acrylamide, a weak genotoxin, produced no marked increase in converted spermatids. The results indicate that recombination-based transgenic mouse models for genotoxin screening present a viable option for inexpensive and rapid whole-animal mutagen testing. The particular mice we describe may ultimately prove to be a useful tool for identifying agents which can cause heritable genetic mutations in humans.