A gene drive biases the transmission of one of the two copies of a gene such that it is inherited more frequently than by random segregation. Highly efficient gene drive systems have recently been developed in insects, which leverage the sequence-targeted DNA cleavage activity of CRISPR–Cas9 and endogenous homology-directed repair mechanisms to convert heterozygous genotypes to homozygosity1,2,3,4. If implemented in laboratory rodents, similar systems would enable the rapid assembly of currently impractical genotypes that involve multiple homozygous genes (for example, to model multigenic human diseases). To our knowledge, however, such a system has not yet been demonstrated in mammals. Here we use an active genetic element that encodes a guide RNA, which is embedded in the mouse tyrosinase (Tyr) gene, to evaluate whether targeted gene conversion can occur when CRISPR–Cas9 is active in the early embryo or in the developing germline. Although Cas9 efficiently induces double-stranded DNA breaks in the early embryo and male germline, these breaks are not corrected by homology-directed repair. By contrast, Cas9 expression limited to the female germline induces double-stranded breaks that are corrected by homology-directed repair, which copies the active genetic element from the donor to the receiver chromosome and increases its rate of inheritance in the next generation. These results demonstrate the feasibility of CRISPR–Cas9-mediated systems that bias inheritance of desired alleles in mice and that have the potential to transform the use of rodent models in basic and biomedical research.
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All genotyping data for F3 offspring of constitutive crosses and F4 offspring of germline conditional crosses is available at Zenodo with the identifier https://doi.org/10.5281/zenodo.2003087. Annotated sequence data for the TyrCopyCat transgene is available in GenBank with the accession number MK160997.
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We thank K. Hanley for the DNA extraction protocol; A. Green and A.-C. Chen for genotyping assistance; M. Tran for laser-capture microdissection in an effort to genotype spermatogonia; P. Jain for assistance with fibroblast transfection; H. Cook-Andersen and M. Wilkinson for conversations about mouse germline development; L. Montoliu for discussion of the tyrosinase locus; M. Tuszynski for plasmids and for early support of the project. This work was funded by a Searle Scholar Award from the Kinship Foundation, a Pew Biomedical Scholar Award from the Pew Charitable Trusts, a Packard Fellowship in Science and Engineering from the David and Lucile Packard Foundation, and NIH grant R21GM129448 awarded to K.L.C. E.B. was supported by NIH grant R01GM117321, a Paul G. Allen Frontiers Group Distinguished Investigators Award and a gift from the Tata Trusts in India to TIGS-UCSD and TIGS-India. H.A.G. was supported by a Ruth Stern Graduate Fellowship and by the NIH Cell and Molecular Genetics training grant T32GM724039; V.M.G. was supported by NIH grant DP5OD023098.
Nature thanks B. Conklin, S. Qi and the other anonymous reviewer(s) for their contribution to the peer review of this work.