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The generation and characterization of novel Col1a1FRT-Cre-ER-T2-FRT and Col1a1FRT-STOP-FRT-Cre-ER-T2 mice for sequential mutagenesis

ABSTRACT Novel genetically engineered mouse models using the Cre-loxP or the Flp-FRT systems have generated useful reagents to manipulate the mouse genome in a temporally-regulated and tissue-specific manner. By incorporating a constitutive Cre driver line into a mouse model in which FRT-regulated genes in other cell types are regulated by Flp-FRT recombinase, gene expression can be manipulated simultaneously in separate tissue compartments. This application of dual recombinase technology can be used to dissect the role of stromal cells in tumor development and cancer therapy. Generating mice in which Cre-ERT2 is expressed under Flp-FRT-mediated regulation would enable step-wise manipulation of the mouse genome using dual recombinase technology. Such next-generation mouse models would enable sequential mutagenesis to better model cancer and define genes required for tumor maintenance. Here, we generated novel genetically engineered mice that activate or delete Cre-ERT2 in response to Flp recombinase. To potentially utilize the large number of Cre-loxP-regulated transgenic alleles that have already been targeted into the Rosa26 locus, such as different reporters and mutant genes, we targeted the two novel Cre-ERT2 alleles into the endogenous Col1a1 locus for ubiquitous expression. In the Col1a1FRT-Cre-ER-T2-FRT mice, Flp deletes Cre-ERT2, so that Cre-ERT2 is only expressed in cells that have never expressed Flp. In contrast, in the Col1a1FRT-STOP-FRT-Cre-ER-T2 mice, Flp removes the STOP cassette to allow Cre-ERT2 expression so that Cre-ERT2 is only expressed in cells that previously expressed Flp. These two new novel mouse strains will be complementary to each other and will enable the exploration of complex biological questions in development, normal tissue homeostasis and cancer. Summary: We generated two mouse strains expressing Cre-ERT2 under Flp-FRT regulation. These tools enable sequential mutagenesis in the same or different cells to study development, tissue homeostasis and diseases such as cancer.