Transgenic models for detection of mutations in tumors and normal tissues of rodents

Transgenic rodents that contain easily retrievable target genes allow the rapid quantitation of mutations in any tissue from which DNA can be isolated. We are using the Stratagene Big Blue transgenic mouse system that contains a lacI target and an alpha lacZ reporter gene to study the parameters that affect mutations. We have evaluated a number of chemicals to determine mutant frequency (MF) in specific target tissues of C57Bl/6 and B6C3F1 mice. The correlation between mutagenesis and carcinogenesis in this system is excellent. For example, the liver carcinogen dimethylnitrosamine produces significant increases in MF in mouse liver, whereas the nonhepatocarcinogenic mutagen methylmethane sulfonate does not. We have also evaluated the induction of mutations by radiation and demonstrated that this system is suitable for the study of agents that produce deletion mutations. This system is also useful for studying changes in MF in developing tumors. We have used an initiation-promotion protocol to induce hepatocellular carcinomas, and we then measured MF in normal liver, tumors, and metastases from these mice. Animals initiated with diethylnitrosamine maintain an elevated MF in normal liver, even 1 year after initiation. This MF increases exponentially in developing liver tumors, possibly owing to a breakdown in the fidelity of DNA replication and DNA repair in tumors. This system offers a unique tool for the study of mutations induced in specific target tissues of rodents and should become an important assay for evaluating the mutagenic risk of drugs and chemicals.