Neonatal islets from human PD-L1 transgenic pigs reduce immune cell activation and cellular rejection in humanized nonobese diabetic-scid IL2rγnull mice.

Strong xenorejection limits the clinical application of porcine islet transplantation in type 1 diabetes. Targeting T cell-mediated rejection is one of the main approaches to improve long-term graft survival. Here we study engraftment and survival of porcine islet cells expressing human programmed cell death ligand-1 (hPD-L1) in a humanized mouse model. Neonatal islet-like clusters (NPICCs) from transgenic hPD-L1 (hPD-L1-Tg) and wild-type (Wt) pigs were transplanted into nonobese diabetic-scid IL2rγnull mice stably reconstituted with human immune cells (hPD-L1 n = 10; Wt n = 6). Primary endpoint was development of normoglycemia during a 16-week observation period after transplantation. Secondary endpoints were porcine C-peptide levels and immune cell infiltration. Animals transplanted with hPD-L1-Tg neonatal islet-like clusters achieved a superior normoglycemic rate (50% versus 0%) and significantly higher plasma C-peptide levels as compared to the Wt group, indicating long-term beta cell function. Intracytoplasmic fluorescence-activated cell sorting analysis and immunohistochemistry revealed significantly decreased frequencies of interferonγ-expressing splenic hCD8-positive T cells and reduced intragraft-infiltrating immune cells. We here demonstrate that expression of hPD-L1 provides strong islet xenograft protection without administration of immunosuppressive drugs. These findings support the hypothesis that hPD-L1 has the capacity to control cellular rejection and therefore represents a very promising transgene candidate for clinical porcine islet xenotransplantation.

Generation of sheep with defined FecBB and TBXT mutations and porcine blastocysts with KCNJ5G151R/+ mutation using prime editing.

BACKGROUND: Rewriting the genomes of living organisms has been a long-standing aim in the biological sciences. The revelation of the CRISPR/Cas9 technology has revolutionized the entire biological field. Since its emergence, this technology has been widely applied to induce gene knockouts, insertions, deletions, and base substitutions. However, the classical version of this system was imperfect for inducing or correcting desired mutations. A subsequent development generated more advanced classes, including cytosine and adenine base editors, which can be used to achieve single nucleotide substitutions. Nevertheless, these advanced systems still suffer from several limitations, such as the inability to edit loci without a suitable PAM sequence and to induce base transversions. On the other hand, the recently emerged prime editors (PEs) can achieve all possible single nucleotide substitutions as well as targeted insertions and deletions, which show promising potential to alter and correct the genomes of various organisms. Of note, the application of PE to edit livestock genomes has not been reported yet. RESULTS: In this study, using PE, we successfully generated sheep with two agriculturally significant mutations, including the fecundity-related FecBB p.Q249R and the tail length-related TBXT p.G112W. Additionally, we applied PE to generate porcine blastocysts with a biomedically relevant point mutation (KCNJ5 p.G151R) as a porcine model of human primary aldosteronism. CONCLUSIONS: Our study demonstrates the potential of the PE system to edit the genomes of large animals for the induction of economically desired mutations and for modeling human diseases. Although prime-edited sheep and porcine blastocysts could be generated, the editing frequencies are still unsatisfactory, highlighting the need for optimizations in the PE system for efficient generation of large animals with customized traits.

Optimized Cas9:sgRNA delivery efficiently generates biallelic MSTN knockout sheep without affecting meat quality.

BACKGROUND: CRISPR/Cas9-based genome-editing systems have been used to efficiently engineer livestock species with precise genetic alterations intended for biomedical and agricultural applications. Previously, we have successfully generated gene-edited sheep and goats via one-cell-stage embryonic microinjection of a Cas9 mRNA and single-guide RNAs (sgRNAs) mixture. However, most gene-edited animals produced using this approach were heterozygotes. Additionally, non-homozygous gene-editing outcomes may not fully generate the desired phenotype in an efficient manner. RESULTS: We report the optimization of a Cas9 mRNA-sgRNA delivery system to efficiently generate homozygous myostatin (MSTN) knockout sheep for improved growth and meat production. Firstly, an sgRNA selection software (sgRNAcas9) was used to preliminarily screen for highly efficient sgRNAs. Ten sgRNAs targeting the MSTN gene were selected and validated in vitro using sheep fibroblast cells. Four out of ten sgRNAs (two in exon 1 and two in exon 2) showed a targeting efficiency > 50%. To determine the optimal CRISPR/Cas9 microinjection concentration, four levels of Cas9 mRNA and three levels of sgRNAs in mixtures were injected into sheep embryos. Microinjection of 100 ng/µL Cas9 mRNA and 200 ng/µL sgRNAs resulted in the most improved targeting efficiency. Additionally, using both the highly efficient sgRNAs and the optimal microinjection concentration, MSTN-knockout sheep were generated with approximately 50% targeting efficiency, reaching a homozygous knockout efficiency of 25%. Growth rate and meat quality of MSTN-edited lambs were also investigated. MSTN-knockout lambs exhibited increased body weight and average daily gain. Moreover, pH, drip loss, intramuscular fat, crude protein, and shear force of gluteal muscles of MSTN-knockout lambs did not show changes compared to the wild-type lambs. CONCLUSIONS: This study highlights the importance of in vitro evaluation for the optimization of sgRNAs and microinjection dosage of gene editing reagents. This approach enabled efficient engineering of homozygous knockout sheep. Additionally, this study confirms that MSTN-knockout lambs does not negatively impact meat quality, thus supporting the adoption of gene editing as tool to improve productivity of farm animals.

Highly efficient generation of sheep with a defined FecBB mutation via adenine base editing.

Base editing has the potential to improve important economic traits in agriculture and can precisely convert single nucleotides in DNA or RNA sequences into minimal double-strand DNA breaks (DSB). Adenine base editors (ABE) have recently emerged as a base editing tool for the conversion of targeted A:T to G:C, but have not yet been used in sheep. ABEmax is one of the latest versions of ABE, which consists of a catalytically-impaired nuclease and a laboratory-evolved DNA-adenosine deaminase. The Booroola fecundity (FecBB) mutation (g.A746G, p.Q249R) in the bone morphogenetic protein receptor 1B (BMPR1B) gene influences fecundity in many sheep breeds. In this study, by using ABEmax we successfully obtained lambs with defined point mutations that result in an amino acid substitution (p.Gln249Arg). The efficiency of the defined point mutations was 75% in newborn lambs, since six lambs were heterozygous at the FecBB mutation site (g.A746G, p.Q249R), and two lambs were wild-type. We did not detect off-target mutations in the eight edited lambs. Here, we report the validation of the first gene-edited sheep generated by ABE and highlight its potential to improve economically important traits in livestock.

Low incidence of SNVs and indels in trio genomes of Cas9-mediated multiplex edited sheep.

BACKGROUND: The simplicity of the CRISPR/Cas9 system has enabled its widespread applications in generating animal models, functional genomic screening and in treating genetic and infectious diseases. However, unintended mutations produced by off-target CRISPR/Cas9 nuclease activity may lead to negative consequences. Especially, a very recent study found that gene editing can introduce hundreds of unintended mutations into the genome, and have attracted wide attention. RESULTS: To address the off-target concerns, urgent characterization of the CRISPR/Cas9-mediated off-target mutagenesis is highly anticipated. Here we took advantage of our previously generated gene-edited sheep and performed family trio-based whole genome sequencing which is capable of discriminating variants in the edited progenies that are inherited, naturally generated, or induced by genetic modification. Three family trios were re-sequenced at a high average depth of genomic coverage (~ 25.8×). After developing a pipeline to comprehensively analyze the sequence data for de novo single nucleotide variants, indels and structural variations from the genome; we only found a single unintended event in the form of a 2.4 kb inversion induced by site-specific double-strand breaks between two sgRNA targeting sites at the MSTN locus with a low incidence. CONCLUSIONS: We provide the first report on the fidelity of CRISPR-based modification for sheep genomes targeted simultaneously for gene breaks at three coding sequence locations. The trio-based sequencing approach revealed almost negligible off-target modifications, providing timely evidences of the safe application of genome editing in vivo with CRISPR/Cas9.

Basics of genome editing technology and its application in livestock species.

In the last decade, the research community has witnessed a blooming of targeted genome editing tools and applications. Novel programmable DNA nucleases such as zinc finger nucleases (ZFNs), transcription activator-like endonucleases (TALENs) and the clustered regularly interspaced short palindromic repeats/Cas9 system (CRISPR/Cas9) possess long recognition sites and are capable of cutting DNA in a very specific manner. These DNA nucleases mediate targeted genetic alterations by enhancing the DNA mutation rate via induction of double-strand breaks at a predetermined genomic site. Compared to conventional homologous recombination-based gene targeting, DNA nucleases, also referred to as Genome Editors (GEs), can increase the targeting rate around 10,000- to 100,000-fold. The successful application of different GEs has been shown in a myriad of different organisms, including insects, amphibians, plants, nematodes and several mammalian species, including human cells and embryos. In contrast to all other DNA nucleases, that rely on protein-DNA binding, CRISPR/Cas9 uses RNA to establish a specific binding of its DNA nuclease. Besides its capability to facilitate multiplexed genomic modifications in one shot, the CRISPR/Cas is much easier to design compared to all other DNA nucleases. Current results indicate that any DNA nuclease can be successfully employed in a broad range of organisms which renders them useful for improving the understanding of complex physiological systems such as reproduction, producing transgenic animals, including creating large animal models for human diseases, creating specific cell lines, and plants, and even for treating human genetic diseases. This review provides an update on DNA nucleases, their underlying mechanism and focuses on their application to edit the genome of livestock species.

The production of multi-transgenic pigs: update and perspectives for xenotransplantation.

The domestic pig shares many genetic, anatomical and physiological similarities to humans and is thus considered to be a suitable organ donor for xenotransplantation. However, prior to clinical application of porcine xenografts, three major hurdles have to be overcome: (1) various immunological rejection responses, (2) physiological incompatibilities between the porcine organ and the human recipient and (3) the risk of transmitting zoonotic pathogens from pig to humans. With the introduction of genetically engineered pigs expressing high levels of human complement regulatory proteins or lacking expression of α-Gal epitopes, the HAR can be consistently overcome. However, none of the transgenic porcine organs available to date was fully protected against the binding of anti-non-Gal xenoreactive natural antibodies. The present view is that long-term survival of xenografts after transplantation into primates requires additional modifications of the porcine genome and a specifically tailored immunosuppression regimen compliant with current clinical standards. This requires the production and characterization of multi-transgenic pigs to control HAR, AVR and DXR. The recent emergence of new sophisticated molecular tools such as Zinc-Finger nucleases, Transcription-activator like endonucleases, and the CRISPR/Cas9 system has significantly increased efficiency and precision of the production of genetically modified pigs for xenotransplantation. Several candidate genes, incl. hTM, hHO-1, hA20, CTLA4Ig, have been explored in their ability to improve long-term survival of porcine xenografts after transplantation into non-human primates. This review provides an update on the current status in the production of multi-transgenic pigs for xenotransplantation which could bring porcine xenografts closer to clinical application.

Efficient production of biallelic GGTA1 knockout pigs by cytoplasmic microinjection of CRISPR/Cas9 into zygotes.

BACKGROUND: Xenotransplantation is considered to be a promising solution to the growing demand for suitable donor organs for transplantation. Despite tremendous progress in the generation of pigs with multiple genetic modifications thought to be necessary to overcoming the severe rejection responses after pig-to-non-human primate xenotransplantation, the production of knockout pigs by somatic cell nuclear transfer (SCNT) is still an inefficient process. Producing genetically modified pigs by intracytoplasmic microinjection of porcine zygotes is an alluring alternative. The porcine GGTA1 gene encodes for the α1,3-galactosyltransferase that synthesizes the Gal epitopes on porcine cells which constitute the major antigen in a xenotransplantation setting. GGTA1-KO pigs have successfully been produced by transfecting somatic cells with zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), or CRISPR/Cas targeting GGTA1, followed by SCNT. METHODS: Here, we microinjected a CRISPR/Cas9 vector coding for a single-guide RNA (sgRNA) targeting exon 8 of the GGTA1 gene into the cytoplasm of 97 in vivo-derived porcine zygotes and transferred 86 of the microinjected embryos into three hormonally synchronized recipients. Fetuses and piglets were analyzed by flow cytometry for remaining Gal epitopes. DNA was sequenced to detect mutations at the GGTA1 locus. RESULTS: Two of the recipients remained pregnant as determined by ultrasound scanning on day 25 of gestation. One pregnancy was terminated on day 26, and six healthy fetuses were recovered. The second pregnancy was allowed to go to term and resulted in the birth of six healthy piglets. Flow cytometry analysis revealed the absence of Gal epitopes in four of six fetuses (66%), indicating a biallelic KO of GGTA1. Additionally, three of the six live-born piglets (50%) did not express Gal epitopes on their cell surface. Two fetuses and two piglets showed a mosaicism with a mixed population of Gal-free and Gal-expressing cells. Only a single piglet did not have any genomic modifications. Genomic sequencing revealed indel formation at the GGTA1 locus ranging from +17 bp to -20 bp. CONCLUSIONS: These results demonstrate the efficacy of CRISPR/Cas to generate genetic modifications in pigs by simplified technology, such as intracytoplasmic microinjection into zygotes, which would significantly facilitate the production of genetically modified pigs suitable for xenotransplantation. Importantly, this simplified injection protocol avoids the penetration of the vulnerable pronuclear membrane, and is thus compatible with higher survival rates of microinjected embryos, which in turn facilitates production of genetically modified piglets.

Advances in genetic modification of farm animals using zinc-finger nucleases (ZFN).

Genome editing tools (GET), including zinc-finger nucleases (ZFN), transcription activator-like endonucleases (TALENS), and meganucleases possess long recognition sites and are thus capable of cutting DNA in a very specific manner. These genome editing tools mediate targeted genetic alterations by enhancing DNA mutation frequency via induction of double-strand breaks at a predetermined genomic site. Compared to conventional homologous recombination based gene targeting, GETs can increase gene targeting and gene disruption via mutagenic DNA repair more than 10,000-fold. Recently, a novel class of genome editing tools was described that uses RNAs to target a specific genomic site. The CRISPR/Cas9 system is capable of targeting even multiple genomic sites in one shot and thus could be superior to ZFNs or TALEN. Current results indicate that these tools can be successfully employed in a broad range of organisms which renders them useful for improving the understanding of complex physiological systems, producing transgenic animals, including creating large animal models for human diseases, creating specific cell lines, and plants, and even for treating human genetic diseases. This review provides an update on the use of ZFNs to modify the genome of farm animals, summarizes current knowledge on the underlying mechanism, and discusses new opportunities for generating genetically modified farm animals.

Molecular scissors and their application in genetically modified farm animals.

Molecular scissors (MS), incl. Zinc Finger Nucleases (ZFN), Transcription-activator like endoncleases (TALENS) and meganucleases possess long recognition sites and are thus capable of cutting DNA in a very specific manner. These molecular scissors mediate targeted genetic alterations by enhancing the DNA mutation rate via induction of double-strand breaks at a predetermined genomic site. Compared to conventional homologous recombination based gene targeting, MS can increase the targeting rate 10,000-fold, and gene disruption via mutagenic DNA repair is stimulated at a similar frequency. The successful application of different MS has been shown in different organisms, including insects, amphibians, plants, nematodes, and mammals, including humans. Recently, another novel class of molecular scissors was described that uses RNAs to target a specific genomic site. The CRISPR/Cas9 system is capable of targeting even multiple genomic sites in one shot and thus could be superior to ZFNs or TALEN, especially by its easy design. MS can be successfully employed for improving the understanding of complex physiological systems, producing transgenic animals, incl. creating large animal models for human diseases, creating specific cell lines, and plants, and even for treating human genetic diseases. This review provides an update on molecular scissors, their underlying mechanism and focuses on new opportunities for generating genetically modified farm animals.

Efficient generation of a biallelic knockout in pigs using zinc-finger nucleases.

Zinc-finger nucleases (ZFNs) are powerful tools for producing gene knockouts (KOs) with high efficiency. Whereas ZFN-mediated gene disruption has been demonstrated in laboratory animals such as mice, rats, and fruit flies, ZFNs have not been used to disrupt an endogenous gene in any large domestic species. Here we used ZFNs to induce a biallelic knockout of the porcine α1,3-galactosyltransferase (GGTA1) gene. Primary porcine fibroblasts were treated with ZFNs designed against the region coding for the catalytic core of GGTA1, resulting in biallelic knockout of ∼1% of ZFN-treated cells. A galactose (Gal) epitope counter-selected population of these cells was used in somatic cell nuclear transfer (SCNT). Of the resulting six fetuses, all completely lacked Gal epitopes and were phenotypically indistinguishable from the starting donor cell population, illustrating that ZFN-mediated genetic modification did not interfere with the cloning process. Neither off-target cleavage events nor integration of the ZFN-coding plasmid was detected. The GGTA1-KO phenotype was confirmed by a complement lysis assay that demonstrated protection of GGTA1-KO fibroblasts relative to wild-type cells. Cells from GGTA1-KO fetuses and pooled, transfected cells were used to produce live offspring via SCNT. This study reports the production of cloned pigs carrying a biallelic ZFN-induced knockout of an endogenous gene. These findings open a unique avenue toward the creation of gene KO pigs, which could benefit both agriculture and biomedicine.

Hyperbaric oxygen treatment reveals spatiotemporal OXPHOS plasticity in the porcine heart.

Cardiomyocytes meet their high ATP demand almost exclusively by oxidative phosphorylation (OXPHOS). Adequate oxygen supply is an essential prerequisite to keep OXPHOS operational. At least two spatially distinct mitochondrial subpopulations facilitate OXPHOS in cardiomyocytes, i.e. subsarcolemmal (SSM) and interfibrillar mitochondria (IFM). Their intracellular localization below the sarcolemma or buried deep between the sarcomeres suggests different oxygen availability. Here, we studied SSM and IFM isolated from piglet hearts and found significantly lower activities of electron transport chain enzymes and F1FO-ATP synthase in IFM, indicative for compromised energy metabolism. To test the contribution of oxygen availability to this outcome, we ventilated piglets under hyperbaric hyperoxic (HBO) conditions for 240 min. HBO treatment raised OXPHOS enzyme activities in IFM to the level of SSM. Complexome profiling analysis revealed that a high proportion of the F1FO-ATP synthase in the IFM was in a disassembled state prior to the HBO treatment. Upon increased oxygen availability, the enzyme was found to be largely assembled, which may account for the observed increase in OXPHOS complex activities. Although HBO also induced transcription of genes involved in mitochondrial biogenesis, a full proteome analysis revealed only minimal alterations, meaning that HBO-mediated tissue remodeling is an unlikely cause for the observed differences in OXPHOS. We conclude that a previously unrecognized oxygen-regulated mechanism endows cardiac OXPHOS with spatiotemporal plasticity that may underlie the enormous metabolic and contractile adaptability of the heart.

Suggestions for the diagnosis and elimination of hepatitis E virus in pigs used for xenotransplantation.

The hepatitis E virus (HEV) is considered a zoonotic pathogen. In xenotransplantation, given the high prevalence of HEV infection in pigs, the risk of zoonotic transmission from a porcine source is considered high. Currently no clear data are available on how to diagnose and eliminate HEV in herds used for medical purposes and the importance of viral infection at the stage of harvest. In this study, several groups of animals currently used for medical purposes were found RNA positive in both serum and faeces for HEV genotype 3. In addition, viraemia was found in animals up to 3.6 yr of age, which is much longer than originally expected. Herd transmission rates appeared to be significantly lower in animals kept under minimal barrier conditions, compared with those observed for commercial animals, and as expected, segregation of animals at an early age prevented spread of infection. This study makes suggestions to ensure appropriate detection and eradication of HEV from a donor herd to be used for xenotransplantation purposes.

Generation and first characterization of TRDC-knockout pigs lacking γδ T cells.

The TRDC-locus encodes the T cell receptor delta constant region, one component of the γδ T cell receptor which is essential for development of γδ T cells. In contrast to peptide recognition by αß T cells, antigens activating γδ T cells are mostly MHC independent and not well characterized. Therefore, the function of γδ T cells and their contribution to protection against infections is still unclear. Higher numbers of circulating γδ T cells compared to mice, render the pig a suitable animal model to study γδ T cells. Knocking-out the porcine TRDC-locus by intracytoplasmic microinjection and somatic cell nuclear transfer resulted in healthy living γδ T cell deficient offspring. Flow cytometric analysis revealed that TRDC-KO pigs lack γδ T cells in peripheral blood mononuclear cells (PBMC) and spleen cells. The composition of the remaining leucocyte subpopulations was not affected by the depletion of γδ T cells. Genome-wide transcriptome analyses in PBMC revealed a pattern of changes reflecting the impairment of known or expected γδ T cell dependent pathways. Histopathology did not reveal developmental abnormalities of secondary lymphoid tissues. However, in a vaccination experiment the KO pigs stayed healthy but had a significantly lower neutralizing antibody titer as the syngenic controls.

Sheep and Goat Genome Engineering: From Random Transgenesis to the CRISPR Era.

Sheep and goats are valuable livestock species that have been raised for their production of meat, milk, fiber, and other by-products. Due to their suitable size, short gestation period, and abundant secretion of milk, sheep and goats have become important model animals in agricultural, pharmaceutical, and biomedical research. Genome engineering has been widely applied to sheep and goat research. Pronuclear injection and somatic cell nuclear transfer represent the two primary procedures for the generation of genetically modified sheep and goats. Further assisted tools have emerged to enhance the efficiency of genetic modification and to simplify the generation of genetically modified founders. These tools include sperm-mediated gene transfer, viral vectors, RNA interference, recombinases, transposons, and endonucleases. Of these tools, the four classes of site-specific endonucleases (meganucleases, ZFNs, TALENs, and CRISPRs) have attracted wide attention due to their DNA double-strand break-inducing role, which enable desired DNA modifications based on the stimulation of native cellular DNA repair mechanisms. Currently, CRISPR systems dominate the field of genome editing. Gene-edited sheep and goats, generated using these tools, provide valuable models for investigations on gene functions, improving animal breeding, producing pharmaceuticals in milk, improving animal disease resistance, recapitulating human diseases, and providing hosts for the growth of human organs. In addition, more promising derivative tools of CRISPR systems have emerged such as base editors which enable the induction of single-base alterations without any requirements for homology-directed repair or DNA donor. These precise editors are helpful for revealing desirable phenotypes and correcting genetic diseases controlled by single bases. This review highlights the advances of genome engineering in sheep and goats over the past four decades with particular emphasis on the application of CRISPR/Cas9 systems.

Programmable Base Editing of the Sheep Genome Revealed No Genome-Wide Off-Target Mutations.

Since its emergence, CRISPR/Cas9-mediated base editors (BEs) with cytosine deaminase activity have been used to precisely and efficiently introduce single-base mutations in genomes, including those of human cells, mice, and crop species. Most production traits in livestock are induced by point mutations, and genome editing using BEs without homology-directed repair of double-strand breaks can directly alter single nucleotides. The p.96R > C variant of Suppressor cytokine signaling 2 (SOCS2) has profound effects on body weight, body size, and milk production in sheep. In the present study, we successfully obtained lambs with defined point mutations resulting in a p.96R > C substitution in SOCS2 by the co-injection of BE3 mRNA and a single guide RNA (sgRNA) into sheep zygotes. The observed efficiency of the single nucleotide exchange in newborn animals was as high as 25%. Observations of body size and body weight in the edited group showed that gene modification contributes to enhanced growth traits in sheep. Moreover, targeted deep sequencing and unbiased family trio-based whole genome sequencing revealed undetectable off-target mutations in the edited animals. This study demonstrates the potential for the application of BE-mediated point mutations in large animals for the improvement of production traits in livestock species.

Base pair editing in goat: nonsense codon introgression into FGF5 results in longer hair.

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.

Efficient inhibition of African swine fever virus replication by CRISPR/Cas9 targeting of the viral p30 gene (CP204L).

African swine fever is a devastating viral disease of domestic and wild pigs against which no vaccine or therapy is available. Therefore, we applied the CRISPR (clustered regularly interspaced short palindromic repeats) - Cas9 nuclease system to target the double-stranded DNA genome of African swine fever virus (ASFV). To this end, a permissive wild boar lung (WSL) cell line was modified by stable transfection with a plasmid encoding Cas9 and a guide RNA targeting codons 71 to 78 of the phosphoprotein p30 gene (CP204L) of ASFV. Due to targeted Cas9 cleavage of the virus genome, plaque formation of ASFV was completely abrogated and virus yields were reduced by four orders of magnitude. The specificity of these effects could be demonstrated by using a natural ASFV isolate and escape mutants possessing nucleotide exchanges within the target sequence, which were not inhibited in the Cas9-expressing cell line. Growth of the cell line was not affected by transgene expression which, as well as virus inhibition, proved to be stable over at least 50 passages. Thus, CRISPR-Cas9 mediated targeting of the ASFV p30 gene is a valid strategy to convey resistance against ASF infection, which may also be applied in its natural animal host.