Use of phi X174 as a shuttle vector for the study of in vivo mammalian mutagenesis

The most promising new techniques for the study of in vivo mammalian mutagenesis make use of transgenic mice carrying a recoverable vector. Mutation systems in mammals can be based on the selection of altered phenotypes among cells sampled from the whole animal, but they are then limited to the very few cell types in which the marker gene is expressed. Such systems require both in vivo and in vitro cell proliferation for expression and verification of the mutations. To avoid these complications, the study of mutations in most tissues must be based on the detection of genetic alterations in a vector that is independent of the phenotype of the mammalian cell. The vector is only a small portion of the mammalian genome, and many of the procedures for recovering the vector are inhibited by the host DNA. For this reason, partial purification is necessary. The purification is made possible by using vectors which are not cut by restriction enzymes that cut the host DNA to pieces of an average size considerably smaller than the vector. The efficiency for measuring mutation frequencies depends on the number of vectors which can be recovered from a certain amount of DNA and is affected by the number of vectors per mammalian genome and the transfection efficiency of the partially-purified vector. In order to avoid selection against or for the spontaneous or induced mutations, the transfection efficiency of the vector from the transformed DNA and of the pure vector DNA should be of the same order of magnitude. Differences in the response to mutagens between the mammalian genome and the procaryotic vector may be expected due to the lack of unique mammalian topographical features in the vectors. Any mutation induction which depends preferentially on these unique features of the mammalian genome may not be detected in a shuttle vector system unless the vector has been engineered or specifically designed to include such topographical characters. The shortcoming of short-term tests that use mutagenicity for predicting human carcinogenicity is usually lack of correlation between mutagenesis in the short-term tests and the corresponding results in carcinogenesis bioassays in mammals. One factor which could contribute to the lack of correlation between the short-term test systems and the bioassays is that we are comparing mutations in totally different genes in different organisms. By using the phi X174 shuttle system, one of the variables may be eliminated.(ABSTRACT TRUNCATED AT 400 WORDS)