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The ability to alter single bases without homology directed repair (HDR) of double‐strand breaks provides a potential solution for editing livestock genomes for economic traits, which are often multigenic. Progress toward multiplex editing in large animals has been hampered by the costly inefficiencies of HDR via microinjection of in vitro manipulated embryos. Here, we designed sgRNAs to induce nonsense codons (C‐to‐T transitions) at four target sites in caprine FGF5, which is a crucial regulator of hair length in mammals. Initial transfections of the third generation Base Editor (BE3) plasmid and four different sgRNAs into caprine fibroblasts were ineffective in altering FGF5. In contrast, all five progenies produced from microinjected single‐cell embryos had alleles with a targeted nonsense mutation. The effectiveness of BE3 to make single base changes varied considerably based on sgRNA design. In addition, the rate of mosaicism differed between animals, target sites, and tissue type. The phenotypic effects on hair fiber were characterized by hematoxylin and eosin, immunofluorescence staining, and western blotting. Differences in morphology were detectable, even though mosaicism was probably affecting the levels of FGF5 expression. PCR amplicon and whole‐genome resequencing analyses for off‐target changes caused by BE3 were low at a genome‐wide scale. This study provided the first evidence of base editing in large mammals produced from microinjected single‐cell embryos. Our results support further optimization of BEs for introgressing complex human disease alleles into large animal models, to evaluate potential genetic improvement of complex health and production traits in a single generation. [ABSTRACT FROM AUTHOR]

The bacterial clustered regularly interspaced short palindromic repeats ( CRISPR)/ CRISPR-associated 9 (Cas9) system is a versatile RNA-guided mammalian genome modification system. One-step generation of mouse genome targeting has been achieved by co-microinjection of one-cell stage embryos with Cas9 m RNA and small/single guide (sg) RNA. Many studies have focused on enhancing the efficiency of this system. In the present study, we report that simultaneous use of dual sg RNAs to target an individual gene significantly improved the Cas9-mediated genome targeting with a bi-allelic modification efficiency of up to 78%. We further observed that the target gene modifications were characterized by efficient germline transmission and site-dependent off-target effects, and also that the apolipoprotein E gene knockout-mediated defects in blood biochemical parameters were recapitulated by CRISPR/Cas9-mediated heritable gene modification. Our results provide a dual sg RNAs strategy to facilitate CRISPR/Cas9-mediated mouse genome targeting. [ABSTRACT FROM AUTHOR]