Frequent spontaneous deletions at a shuttle vector locus in transgenic mice

Transgenic mice carrying multiple copies of a recoverable lambda phage shuttle vector (lambda supF) were constructed for the purpose of studying mutagenesis in a whole animal. Spontaneous mutations in rescued supF target genes from several different lines of transgenic mice were analyzed. One mouse line, 1139, was identified in which the frequency of spontaneous mutations was unusually high (3.15 x 10(-4)), 20-fold higher than in other transgenic mice carrying a similar number of copies of the lambda transgene (approximately 100). Over 75% of the spontaneous mutations from 1139 mice were found to be deletions, whereas mostly point mutations were recovered from the other mice. In 1139 no significant variation among adult tissues has been detected. However, embryonic tissue yielded a 3- to 4-fold lower frequency of mutations, most of which were point mutations rather than deletions. The frequency of mutations at another locus, the hypoxanthine phosphoribosyl transferase gene, was not elevated in fibroblast lines established in culture from the 1139 mice. Overall, these results suggest that the deletion mutagenesis affecting the transgene sequences in 1139 mice is a locus-specific effect occurring during growth and development. The increased mutagenesis could not be explained by the degree of methylation of the transgene sequences, since hypermethylation was seen in both 1139 mice and other mice with a low frequency of shuttle vector mutations. The integrated lambda vector DNA in 1139 mice was mapped to a single site on chromosome 7, but no mechanism for the mutagenesis was suggested by this localization. It is proposed that the lambda DNA may have either integrated into an unstable genomic site or created a newly unstable locus in the process of integration.