Illegitimate Cre-dependent chromosome rearrangements in transgenic mouse spermatids

The bacteriophage P1 Cre/loxP system has become a powerful tool for in vivo manipulation of the genomes of transgenic mice. Although in vitro studies have shown that Cre can catalyze recombination between cryptic "pseudo-loxP '' sites in mammalian genomes, to date there have been no reports of loxP-s ite infidelity in transgenic animals. We produced lines of transgenic mice that use the mouse Protamine 1 (Prm1) gene promoter to express Cre recombin ase in postmeiotic spermatids. All male founders and all Cre-bearing male d escendents of female founders were sterile; females were unaffected. Sperm counts, sperm motility, and sperm morphology were normal, as was the mating behavior of the transgenic males and the production of two-celled embryos after mating. Mice that expressed similar levels of a derivative transgene that carries an inactive Cre exhibited normal male fertility. Analyses of e mbryos from matings between sterile Cre-expressing males and wild-type fema les indicated that Cre-catalyzed chromosome rearrangements in the spermatid s that lead to abortive pregnancies with 100% penetrance. Similar Cre-media ted, hut loxP-independent, genomic alterations may also occur in somatic ti ssues that express Cre, but, because of the greater difficulty of assessing deleterious effects of somatic mutations, these may go undetected. This st udy indicates that, following the use of the Cre/loxP site-specific recombi nation systems in vivo, it is prudent to eliminate or inactivate the Cre re combinase gene as rapidly as possible.