Cytidine base editors (CBEs) and adenine base editors (ABEs), composed of a cytidine deaminase or an evolved adenine deaminase fused to Cas9 nickase, enable the conversion of C·G to T·A or A·T to G·C base pair in organisms, respectively. Here, we show that BE3 and ABE7.10 systems can achieve a targeted mutation efficiency of 53–88% and 44–100%, respectively, in both blastocysts and Founder (F0) rabbits. Meanwhile, this strategy can be used to precisely mimic human pathologies by efficiently inducing nonsense or missense mutations as well as RNA mis-splicing in rabbit. In addition, the reduced frequencies of indels with higher product purity are also determined in rabbit blastocysts by BE4-Gam, which is an updated version of the BE3 system. Collectively, this work provides a simple and efficient method for targeted point mutations and generation of disease models in rabbit.

CRISPR-Cas9 and base editors (BEs) systems are poised to become the gene-editing tool of choice in clinical contexts; however, large-fragment deletion was found in Cas9-mediated mutation cells and mice. In this study, by analyzing 16 gene-edited rabbit lines (including 112 rabbits) generated using SpCas9, BEs, xCas9, and xCas9-BEs with long-range PCR genotyping and long-read sequencing by the PacBio platform, we show the extension of thousands of base fragment deletions in single-guide RNA/Cas9 and xCas9 system mutation rabbits, but no deletions were found in BE-induced mutation rabbits. Thus, we first validated that no large-fragment deletion was induced by the BEs system, suggesting that BE systems can be beneficial tools for the further development of highly accurate and secure gene therapy for the clinical treatment of human genetic disorders.

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Background: Cytidine base editors (CBEs), composed of a cytidine deaminase fused to Cas9 nickase (nCas9), enable efficient C-toT conversion in various organisms. However, current base editors can induce unwanted bystander C-toT conversions when multiple Cs are present in the~5-nucleotide activity window of cytidine deaminase, which negatively affects their precision. Here, we develop a new base editor which significantly reduces unwanted bystander activities. Results: We used an engineered human APOBEC3G (eA3G) C-terminal catalytic domain with preferential cytidine-deaminase activity in motifs with a hierarchy CCC>CCC>CC (where the preferentially deaminated C is underlined), to develop an eA3G-BE with distinctive CC context-specificity and reduced generation of bystander mutations. Targeted editing efficiencies of 18.3-58.0% and 54.5-92.2% with excellent CC context-specificity were generated in human cells and rabbit embryos, respectively. In addition, a base editor that can further recognize relaxed NG PAMs is achieved by combining hA3G with an engineered SpCas9-NG variant. The A3G-BEs were used to induce accurate single-base substitutions which led to nonsense mutation with an efficiency of 83-100% and few bystander mutations in Founder (F0) rabbits at Tyr loci. Conclusions: These novel base editors with improved precision and CC context-specificity will expand the toolset for precise gene modification in organisms.

Cytidine base editors, composed of a cytidine deaminase fused to Cas9 nickase, enable efficient C-to-T conversion in various organisms. However, current base editors suffer from severe trade-off between editing efficiency and precision. Here, based on rationally mutated cytidine deaminase domain, we develop a new base editor, YFE-BE4max, effectively narrow the editing width to as little as approximately three nucleotides while maintaining high efficiency in rabbits. Moreover, YFE-BE4max successfully mediated the Tyr p. Q68Stop and Lmna p. G607G mutation in F0 rabbit with high efficiency and precision, which precisely recapitulates the pathological features of human OCA1 and HGPS, respectively. Collectively, YFE-BE4max system provide promising tools to perform efficient base editing with high precision in rabbits and enhances its capacity to precisely model human diseases.