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Spi− selection: An efficient method to detect γ‐ray‐induced deletions in transgenic mice

Despite the importance of genome rearrangement in the etiology of cancer and human genetic disease, deletion mutations are poorly detectable by transgenic rodent mutagenicity tests. To facilitate the detection and molecular analysis of deletion mutations in vivo, we established a transgenic mouse model harboring a λEG10 shuttle vector that includes the red and gam genes for Spi− (sensitive to P2 interference) selection [Nohmi et al. (1996] Environ. Mol. Mutagen. 28:465–470]. This selection has a great advantage over other genetic systems, because phage deletion mutants can be preferentially selected as Spi− plaques, which can then be subjected to molecular analysis. Here, we show nucleotide sequences of 41 junctions of deletion mutations induced by γ‐irradiation. Unlike spontaneous deletion mutants, more than half of the large deletions occurred between short homologous sequences from one to eight bp. The remaining junctions had no such homologous sequences. Intriguingly, two Spi− mutants had P (palindrome)‐like nucleotide additions at the breakpoints, which are frequently observed in the coding junctions of V(D)J recombination, suggesting that broken DNA molecules with hairpin structures can be intermediates in the repair of radiation‐induced double‐strand breaks. We conclude that Spi− selection is useful for the efficient detection of deletion mutations in vivo and that most rearrangements induced by γ‐rays in mice are mediated by illegitimate recombination through DNA end‐joining. Environ. Mol. Mutagen. 34:9–15, 1999 © 1999 Wiley‐Liss, Inc.