Analysis of spontaneous and induced mutations in transgenic mice using a lambda ZAP/lacl shuttle vector

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Abstract
A short term, in vivo mutagenesis assay has been developed utilizing a lacl target gene contained within a lambda ZAP shuttle vector which has been incorporated into transgenic mice. Following chemical exposure, the target gene was recovered from mouse genomic DNA by mixing the DNA with in vitro lambda phage packaging extract. Mutations within the lacl target were identified by infecting host E. coli with the packaged phage and plating on indicator plates containing Xgal. Phage plaques with mutations in the lacl appeared blue, while intact phage were colorless. The ratio of blue plaques to colorless plaques is a measure of the mutagenicity of the compound. This system was used to obtain significant induction (up to 74-fold) over background levels for a variety of compounds, including N-ethyl-N-nitrosourea, benzo(a)pyrene (BaP), cyclophosphamide, and methylnitrosourea. Sequence analysis of selected mutant clones derived from this system was accomplished through the use of partial filamentous phage origins which allow rapid transfer of the target gene from phage to plasmid. Sequence analysis of spontaneous mutants derived from the mice primarily found of base substitutions, differing markedly from the previous data for spontaneous mutations in lacl derived from E. coli, where the preponderance of mutations were found at a single site, a repeated tetramer sequence. Upon sequence analysis of BaP derived base substitutions, only transversions were obtained, consistent with the known mechanism of BaP mutagenesis. Use of the well-characterized lacl gene in transgenic mice should allow for extrapolation of the extensive pool of in vitro data to whole animals, as well as provide insight into the tissue specific effects of mutagenic compounds.