Efficient introgression of allelic variants by embryo-mediated editing of the bovine genome

The recent development of designer nucleases that can be custom-designed to target essentially any site within the genome now allows for the efficient and precise introduction of genetic change into livestock genomes. Most studies have so far focused on the introduction of random mutations in cultured cells and the use of nuclear transfer to generate animals with edited genotypes. To circumvent the intrinsic uncertainties of random mutations and the inefficiencies of nuclear transfer we directed our efforts to the introduction of specific genetic changes by homology-driven repair directly in in vitro produced embryos. Initially, we injected zinc finger nuclease (ZFN)-encoding mRNA or DNA into bovine zygotes to verify cleavage activity at their target site within the gene for beta lactoglobulin (LGB) and could detect ZFN-induced random mutations in 30 % to 80 % of embryos. Next, to precisely change the LGB sequence, we co-injected ZFNs or transcription activator-like effector nucleases (TALENs) with DNA oligonucleotides (ODNs) as homologous repair templates. Analysis of ZFN- and TALEN-injected blastocysts showed that the ODNs induced targeted changes in approximately 30 % and close to 46 % of blastocysts, respectively. Deep sequence analysis of selected embryos revealed contributions of the targeted LGB allele of up to 100 % which implies that genome editing by zygote injections can facilitate the one-step generation of non-mosaic livestock animals with pre-designed biallelic modifications.