Novel mosaic mice with diverse applications

Gene-deficient mouse models are indispensable for interrogating mammalian gene functions, but the conventional models allow the study of only one or few genes per mouse line, which has been a bottleneck in functional genomics. To confront the challenge, we have combined the CRISPR-Cas and Cre-Lox systems to develop a novel type of mosaic mice termed MARC ( M osaic A nimal based on g R NA and C re) for targeting many genes per mouse but only one gene per cell. This technology employs a transgene comprising a modified U6 promoter upstream of a series of floxed gRNA genes linked together in tandem, with one gRNA expressed per cell following Cre-mediated recombination. At least 61 gRNA genes can be stably maintained in the transgene, and importantly, enables robust proof-of-principle in vivo screens, demonstrating the potential for quickly evaluating the functions of many genes in diverse tissues in a single MARC line. In theory, MARC can also be analyzed by single-cell sequencing, and should enable cost-effective derivation of conventional single-gene-KO lines via simple breeding. Our study establishes MARC as an important addition to the mouse genetics toolbox.