Efficient rescue of integrated shuttle vectors from transgenic mice: A model for studying mutations in vivo

To study gene mutations in different organs and tissues of an experimental animal, we produced transgenic mice harboring bacteriophage λ shuttle vectors integrated in the genome in a head-to-tail arrangement. As a target for mutagenesis, the selectable bacterial lacZ gene was cloned in the vector. The integrated vectors were rescued from total genomic DNA with high efficiency by in vitro packaging and propagation of the phages in the LacZ^- strain of Escherichia coli C. The background mutation frequencies in brain and liver DNA appeared to be low, as was indicated by the absence of colorless plaques among 138,816 and 168,160 phage isolated from brain and liver DNA, respectively. Treatment of adult female transgenic mice with N-ethyl-N-nitrosourea resulted in a dose-dependent increase of the frequency of mutated vectors isolated from brain DNA, up to 7.4 x 10^-5 at 250 mg of the alkylating agent per kilogram of body weight. At this dose, in liver DNA of the same mice, mutation frequencies were ~3 x 10^-5. DNA sequence analysis of four mutant vectors isolated from brain DNA indicated predominantly G·C → A·T transitions. These results demonstrate the value of this transgenic mouse model in studying gene mutations in vivo. In addition to its use in fundamental research, the system could be used as a sensitive, organ-specific, short-term mutagenicity assay.