Spi(-) selection: An efficient method to detect gamma-ray-induced deletions in transgenic mice

Despite the importance of genome rearrangement in the etiology of cancer an d human genetic disease, deletion mutations are poorly detectable by transg enic rodent mutagenicity tests. To facilitate the detection and molecular a nalysis of deletion mutations in vivo, we established a transgenic mouse mo del harboring a lambda EG10 shuttle vector that includes the red and gam ge nes for Spi(-) (sensitive to P2 interference) selection [Nohmi et al. [1996 ] Environ. Mel. Mutagen. 28:465-470]. This selection has a great advantage over other genetic systems, because phage deletion mutants can be preferent ially selected as Spi(-) plaques, which can then be subjected to molecular analysis. Here, we show nucleotide sequences of 41 junctions of deletion mu tations induced by gamma-irradiation. Unlike spontaneous deletion mutants, more than half of the large deletions occurred between short homologous seq uences from one to eight bp. The remaining junctions had no such homologous sequences. Intriguingly, two Spi(-) mutants had P (palindrome)-like nucleo tide additions at the breakpoints, which are frequently observed in the cod ing junctions of V(D)J recombination, suggesting that broken DNA molecules with hairpin structures can be intermediates in the repair of radiation-ind uced double-strand breaks. We conclude that Spi(-) selection is useful for the efficient detection of deletion mutations in vivo and that most rearran gements induced by gamma-rays in mice are mediated by illegitimate recombin ation through DNA end-joining. Environ. Mol. Mutagen. 34:9-15, 1999. (C) 19 99 Wiley-Liss, Inc.