Efficient generation of GGTA1-deficient pigs by electroporation of the CRISPR/Cas9 system into in vitro-fertilized zygotes

Programmed cell death-1-modified pig developed using electroporation-mediated gene editing for in vitro fertilized zygotes

Programmed cell death-1 (PD-1) is an immunoinhibitory receptor required to suppress inappropriate immune responses such as autoimmunity. Immune checkpoint antibodies that augment the PD-1 pathway lead to immune-related adverse events (irAEs), organ non-specific side effects due to autoimmune activation in humans. In this study, we generated a PD-1 mutant pig using electroporation-mediated introduction of the CRISPR/Cas9 system into porcine zygotes to evaluate the PD-1 gene deficiency phenotype. We optimized the efficient guide RNAs (gRNAs) targeting PD-1 in zygotes and transferred electroporated embryos with the optimized gRNAs and Cas9 into recipient gilts. One recipient gilt became pregnant and gave birth to two piglets. Sequencing analysis revealed that both piglets were biallelic mutants. At 18 mo of age, one pig showed non-purulent arthritis of the left elbow/knee joint and oligozoospermia, presumably related to PD-1 modification. Although this study has a limitation because of the small number of cases, our phenotypic analysis of PD-1 modification in pigs will provide significant insight into human medicine and PD-1-deficient pigs can be beneficial models for studying human irAEs.

Comparison of ICSI, IVF, and in vivo derived embryos to produce CRISPR-Cas9 gene-edited pigs for xenotransplantation

Genome editing in pigs for xenotransplantation has seen significant advances in recent years. This study compared three methodologies to generate gene-edited embryos, including co-injection of sperm together with the CRISPR-Cas9 system into oocytes, named ICSI-MGE (mediated gene editing); microinjection of CRISPR-Cas9 components into oocytes followed by in vitro fertilization (IVF), and microinjection of in vivo fertilized zygotes with the CRISPR-Cas9 system. Our goal was to knock-out (KO) porcine genes involved in the biosynthesis of xenoantigens responsible for the hyperacute rejection of interspecific xenografts, namely GGTA1, CMAH, and β4GalNT2. Additionally, we attempted to KO the growth hormone receptor (GHR) gene with the aim of limiting the growth of porcine organs to a size that is physiologically suitable for human transplantation. Embryo development, pregnancy, and gene editing rates were evaluated. We found an efficient mutation of the GGTA1 gene following ICSI-MGE, comparable to the results obtained through the microinjection of oocytes followed by IVF. ICSI-MGE also showed higher rates of biallelic mutations compared to the other techniques. Five healthy piglets were born from in vivo-derived embryos, all of them exhibiting biallelic mutations in the GGTA1 gene, with three displaying mutations in the GHR gene. No mutations were observed in the CMAH and β4GalNT2 genes. In conclusion, in vitro methodologies showed high rates of gene-edited embryos. Specifically, ICSI-MGE proved to be an efficient technique for obtaining homozygous biallelic mutated embryos. Lastly, only live births were obtained from in vivo-derived embryos showing efficient multiple gene editing for GGTA1 and GHR.

Mosaic TP53 Mutation on Tumour Development in Pigs: A Case Study

Pigs rarely develop cancer; however, tumour protein p53 (TP53)-modified pigs may have an increased incidence of cancer. In this study, two pigs with mosaic mutations induced by gene editing were compared to determine the role of the wild-type TP53 sequence in tumorigenesis and to speculate how amino acid changes in TP53 sequences are related to tumorigenesis. The pig without tumours had a wild-type TP53 sequence and a 1-bp deletion in the TP53 sequence that resulted in a premature stop codon. In contrast, the pig with nephroblastoma had 6- and 7-bp deletions in the TP53 sequence, resulting in the absence of two amino acids and a premature stop codon, respectively. Our results indicated that TP53 mutations with truncated amino acids may be related to tumour formation.

GHR-mutant pig derived from domestic pig and microminipig hybrid zygotes using CRISPR/Cas9 system

BACKGROUND

Pigs are excellent large animal models with several similarities to humans. They provide valuable insights into biomedical research that are otherwise difficult to obtain from rodent models. However, even if miniature pig strains are used, their large stature compared with other experimental animals requires a specific maintenance facility which greatly limits their usage as animal models. Deficiency of growth hormone receptor (GHR) function causes small stature phenotypes. The establishment of miniature pig strains via GHR modification will enhance their usage as animal models. Microminipig is an incredibly small miniature pig strain developed in Japan. In this study, we generated a GHR mutant pig using electroporation-mediated introduction of the CRISPR/Cas9 system into porcine zygotes derived from domestic porcine oocytes and microminipig spermatozoa.

METHODS AND RESULTS

First, we optimized the efficiency of five guide RNAs (gRNAs) designed to target GHR in zygotes. Embryos that had been electroporated with the optimized gRNAs and Cas9 were then transferred into recipient gilts. After embryo transfer, 10 piglets were delivered, and one carried a biallelic mutation in the GHR target region. The GHR biallelic mutant showed a remarkable growth-retardation phenotype. Furthermore, we obtained F1 pigs derived from the mating of GHR biallelic mutant with wild-type microminipig, and GHR biallelic mutant F2 pigs through sib-mating of F1 pigs.

CONCLUSIONS

We have successfully demonstrated the generation of biallelic GHR-mutant small-stature pigs. Backcrossing of GHR-deficient pig with microminipig will establish the smallest pig strain which can contribute significantly to the field of biomedical research.

- Invited Review - Reproductive technologies needed for the generation of precise gene-edited pigs in the pathways from laboratory to farm

Gene editing (GE) offers a new breeding technique (NBT) of sustainable value to animal agriculture. There are 3 GE working sites covering 5 feasible pathways to generate GE pigs along with the crucial intervals of GE/genotyping, microinjection/electroporation, induced pluripotent stem cells, somatic cell nuclear transfer, cryopreservation, and nonsurgical embryo transfer. The extension of NBT in the new era of pig breeding depends on the synergistic effect of GE and reproductive biotechnologies; the outcome relies not only on scientific due diligence and operational excellence but also on the feasibility of application on farms to improve sustainability.

Pigs with an INS point mutation derived from zygotes electroporated with CRISPR/Cas9 and ssODN

Just one amino acid at the carboxy-terminus of the B chain distinguishes human insulin from porcine insulin. By introducing a precise point mutation into the porcine insulin (INS) gene, we were able to generate genetically modified pigs that secreted human insulin; these pigs may be suitable donors for islet xenotransplantation. The electroporation of the CRISPR/Cas9 gene-editing system into zygotes is frequently used to establish genetically modified rodents, as it requires less time and no micromanipulation. However, electroporation has not been used to generate point-mutated pigs yet. In the present study, we introduced a point mutation into porcine zygotes via electroporation using the CRISPR/Cas9 system to generate INS point-mutated pigs as suitable islet donors. We first optimized the efficiency of introducing point mutations by evaluating the effect of Scr7 and the homology arm length of ssODN on improving homology-directed repair-mediated gene modification. Subsequently, we prepared electroporated zygotes under optimized conditions and transferred them to recipient gilts. Two recipients became pregnant and delivered five piglets. Three of the five piglets carried only the biallelic frame-shift mutation in the INS gene, whereas the other two successfully carried the desired point mutation. One of the two pigs mated with a WT boar, and this desired point mutation was successfully inherited in the next F1 generation. In conclusion, we successfully established genetically engineered pigs with the desired point mutation via electroporation-mediated introduction of the CRISPR/Cas9 system into zygotes, thereby avoiding the time-consuming and complicated micromanipulation method.

CRISPR/Cas9-editing of KISS1 to generate pigs with hypogonadotropic hypogonadism as a castration free trait

Introduction: Most male pigs are surgically castrated to avoid puberty-derived boar taint and aggressiveness. However, this surgical intervention represents a welfare concern in swine production. Disrupting porcine KISS1 is hypothesized to delay or abolish puberty by inducing variable hypogonadotropism and thus preventing the need for castration. Methods: To test this hypothesis, we generated the first KISS1-edited large animal using CRISPR/Cas9-ribonucleoproteins and single-stranded donor oligonucleotides. The targeted region preceded the sequence encoding a conserved core motif of kisspeptin. Genome editors were intracytoplasmically injected into 684 swine zygotes and transferred to 19 hormonally synchronized surrogate sows. In nine litters, 49 American Yorkshire and 20 Duroc liveborn piglets were naturally farrowed. Results: Thirty-five of these pigs bore KISS1-disruptive alleles ranging in frequency from 5% to 97% and did not phenotypically differ from their wild-type counterparts. In contrast, four KISS1-edited pigs (two boars and two gilts) with disruptive allele frequencies of 96% and 100% demonstrated full hypogonadotropism, infantile reproductive tracts, and failed to reach sexual maturity. Change in body weight during development was unaffected by editing KISS1. Founder pigs partially carrying KISS1-disruptive alleles were bred resulting in a total of 53 KISS1 ^+/+, 60 KISS1 ^+/-, and 34 KISS1 ^-/- F1 liveborn piglets, confirming germline transmission. Discussion: Results demonstrate that a high proportion of KISS1 alleles in pigs must be disrupted before variation in gonadotropin secretion is observed, suggesting that even a small amount of kisspeptin ligand is sufficient to confer proper sexual development and puberty in pigs. Follow-on studies will evaluate fertility restoration in KISS1 KO breeding stock to fully realize the potential of KISS1 gene edits to eliminate the need for surgical castration.

Electroporation-Mediated CRISPR/Cas9 Genome Editing in Rat Zygotes

Genetic engineering in the rat has been revolutionized by the development of CRISPR-based genome editing tools. Conventional methods for inserting genome editing elements such as CRISPR/Cas9 reagents into rat zygotes include cytoplasmic or pronuclear microinjections. These techniques are labor-intensive, require specialized micromanipulator equipment, and are technically challenging. Here, we describe a simple and effective method for zygote electroporation in which CRISPR/Cas9 reagents are introduced into rat zygotes via pores produced by precise electrical pulses applied to the cells. Zygote electroporation allows for high-throughput efficient genome editing in rat embryos.

Genome centric engineering using ZFNs, TALENs and CRISPR-Cas9 systems for trait improvement and disease control in Animals

Livestock is an essential life commodity in modern agriculture involving breeding and maintenance. The farming practices have evolved mainly over the last century for commercial outputs, animal welfare, environment friendliness, and public health. Modifying genetic makeup of livestock has been proposed as an effective tool to create farmed animals with characteristics meeting modern farming system goals. The first technique used to produce transgenic farmed animals resulted in random transgene insertion and a low gene transfection rate. Therefore, genome manipulation technologies have been developed to enable efficient gene targeting with a higher accuracy and gene stability. Genome editing (GE) with engineered nucleases-Zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) regulates the targeted genetic alterations to facilitate multiple genomic modifications through protein-DNA binding. The application of genome editors indicates usefulness in reproduction, animal models, transgenic animals, and cell lines. Recently, CRISPR/Cas system, an RNA-dependent genome editing tool (GET), is considered one of the most advanced and precise GE techniques for on-target modifications in the mammalian genome by mediating knock-in (KI) and knock-out (KO) of several genes. Lately, CRISPR/Cas9 tool has become the method of choice for genome alterations in livestock species due to its efficiency and specificity. The aim of this review is to discuss the evolution of engineered nucleases and GETs as a powerful tool for genome manipulation with special emphasis on its applications in improving economic traits and conferring resistance to infectious diseases of animals used for food production, by highlighting the recent trends for maintaining sustainable livestock production.

Multiple gene editing in porcine embryos using a combination of microinjection, electroporation, and transfection methods

Background and Aim:

Mosaicism – the presence of both wild-type and mutant alleles – is a serious problem for zygotic gene modification through gene editing using the Clustered regularly interspaced short palindromic repeats-Cas9 (CRISPR/Cas9) system. Different delivery methods, such as microinjection (MI), electroporation (EP), and transfection (TF), can be used to transfer CRISPR/Cas9 components into porcine zygotes. This study aimed to develop a method that combines MI, EP, and TF to improve mutation efficiency mediated through the CRISPR/Cas9 system for a triple-gene knockout in pigs.

Materials and Methods:

The study consisted of three groups: The MI group with three simultaneously microinjected guide RNAs (gRNAs) targeting α-1,3-galactosyltransferase (GGTA1), cytidine 32 monophosphate-N-acetylneuraminic acid hydroxylase (CMAH), and β-1,4-N-acetyl-galactosaminyltransferase 2 (B4GALNT2); the MI + EP group with two gRNAs targeting GGTA1 and B4GALNT2 genes delivered into zygotes through MI, followed by EP of gRNA targeting the CMAH 1 h later; and the MI + EP + TF group with MI of gRNA targeting GGTA1 gene into zygotes, followed by EP of gRNA targeting CMAH 1 h later, and then TF of gRNA targeting the B4GALNT2 gene into zona-free zygotes after another hour.

Results:

The rate of blastocysts carrying mutations in one or two gene(s) was significantly higher in the MI + EP + TF group than in the MI group. However, the blastocyst formation rate of zygotes in the MI + EP + TF group was lower than that of the zygotes in the other treatment groups.

Conclusion:

The combination of CRISPR/Cas9 delivery methods may improve the mutation efficiency of triple-gene edited porcine blastocysts.

Genetically modified immunomodulatory cell-based biomaterials in tissue regeneration and engineering

To date, the wide application of cell-based biomaterials in tissue engineering and regeneration is remarkably hampered by immune rejection. Reducing the immunogenicity of cell-based biomaterials has become the latest direction in biomaterial research. Recently, genetically modified cell-based biomaterials with immunomodulatory genes have become a feasible solution to the immunogenicity problem. In this review, recent advances and future challenges of genetically modified immunomodulatory cell-based biomaterials are elaborated, including fabrication approaches, mechanisms of common immunomodulatory genes, application and, more importantly, current preclinical and clinical advances. The fabrication approaches can be categorized into commonly used (e.g., virus transfection) and newly developed approaches. The immunomodulatory mechanisms of representative genes involve complicated cell signaling pathways and metabolic activities. Wide application in curing multiple end-term diseases and replacing lifelong immunosuppressive therapy in multiple cell and organ transplantation models is demonstrated. Most significantly, practices of genetically modified organ transplantation have been conducted on brain-dead human decedent and even on living patients after a series of experiments on nonhuman primates. Nevertheless, uncertain biosecurity, nonspecific effects and overlooked personalization of current genetically modified immunomodulatory cell-based biomaterials are shortcomings that remain to be overcome.

Advances in pig models of human diseases

Animal models of human diseases play a critical role in medical research. Pigs are anatomically and physiologically more like humans than are small rodents such as mice, making pigs an attractive option for modeling human diseases. Advances in recent years in genetic engineering have facilitated the rapid rise of pig models for use in studies of human disease. In the present review, we summarize the current status of pig models for human cardiovascular, metabolic, neurodegenerative, and various genetic diseases. We also discuss areas that need to be improved. Animal models of human diseases play a critical role in medical research. Advances in recent years in genetic engineering have facilitated the rapid rise of pig models for use in studies of human disease. In the present review, we summarize the current status of pig models for human cardiovascular, metabolic, neurodegenerative, various genetic diseases and xenotransplantation.

Generation of mutant pigs by lipofection-mediated genome editing in embryos

The specificity and efficiency of CRISPR/Cas9 gene-editing systems are determined by several factors, including the mode of delivery, when applied to mammalian embryos. Given the limited time window for delivery, faster and more reliable methods to introduce Cas9-gRNA ribonucleoprotein complexes (RNPs) into target embryos are needed. In pigs, somatic cell nuclear transfer using gene-modified somatic cells and the direct introduction of gene editors into the cytoplasm of zygotes/embryos by microinjection or electroporation have been used to generate gene-edited embryos; however, these strategies require expensive equipment and sophisticated techniques. In this study, we developed a novel lipofection-mediated RNP transfection technique that does not require specialized equipment for the generation of gene-edited pigs and produced no detectable off-target events. In particular, we determined the concentration of lipofection reagent for efficient RNP delivery into embryos and successfully generated MSTN gene-edited pigs (with mutations in 7 of 9 piglets) after blastocyst transfer to a recipient gilt. This newly established lipofection-based technique is still in its early stages and requires improvements, particularly in terms of editing efficiency. Nonetheless, this practical method for rapid and large-scale lipofection-mediated gene editing in pigs has important agricultural and biomedical applications.

Development and characterization of Gal KO porcine bone marrow-derived mesenchymal stem cells

BACKGROUND

We demonstrated that neonatal porcine bone marrow-derived mesenchymal stem cell (npBM-MSCs) could improve a critical ischemic limb disease in rat model more efficiently compared with human MSCs. However, since porcine MSC presents galactosyl-alpha 1,3-galactose antigen (Gal antigen), MSC could be eliminated by the xenogeneic rejection. Recently, we established Gal knockout (KO) pigs by a technique of the electroporation of the CRISPR/Cas9 system into vitro-fertilized zygotes. In this study, we hypothesized that MSC from the established Gal KO pigs could further improve the efficacy. Before examining the hypothesis, in this study, we have established and characterized bone marrow-derived MSC from the Gal KO adult pigs (apBM-MSCs).

METHODS

Mononuclear cells (MNCs) were isolated from bone marrow cells of both Gal KO adult pigs and wild-type (WT) adult pigs. MNCs were further manipulated to create Gal KO apBM-MSCs and WT apBM-MSCs. Both MSCs were assessed by their surface markers, the capability of differentiation into adipocytes, osteocytes and chondrocytes, grow speed and colony-forming assay. To assess the efficacy of Gal KO apBM-MSCs, angiogenesis-related genes and immunosuppression-related genes were assessed by cytokine stimulation.

RESULTS

Gal KO apBM-MSC showed no Gal antigen on their cell surfaces. Both Gal KO apBM-MSCs and WT apBM-MSCs, presented little or no negative surface markers of MSCs, while they presented positive surface markers of MSCs. Furthermore, Gal KO apBM-MSCs were able to differentiate into adipocytes, osteocytes, and chondrocytes as well as WT apBM-MSCs. There was no difference in doubling time between Gal KO apBM-MSCs and WT apBM-MSCs. Interestingly, the colony-forming efficiency of Gal KO apBM-MSCs was about half that of WT apBM-MSC. However, angiogenesis and immunosuppression-related genes were equally upregulated in both Gal KO apBM-MSCs and WT apBM-MSCs by cytokine stimulation.

CONCLUSION

We created and characterized Gal KO apBM-MSCs which showed similar characteristics and cytokine-induced gene upregulation to the WT apBM-MSCs.

Zona pellucida treatment before CRISPR/Cas9-mediated genome editing of porcine zygotes

Background

Increasing the permeability of the zona pellucida (ZP) of oocytes before CRISPR/Cas9 electroporation may improve the efficiency of gene editing; however, the effects of this approach on subsequent developmental processes are unclear. In this study, the effects of ZP treatment before electroporation on embryonic development and gene editing in porcine embryos were evaluated.

Methods

The ZP of zygotes was weakened or removed by exposure to 0.5% actinase E, followed by electroporation of the Cas9 protein with guide RNA targeting GGTA1.

Results

The blastocyst formation rate of ZP‐free zygotes after electroporation was significantly lower (p < 0.05) than that of ZP‐intact zygotes. The mutation rate in blastocysts from ZP‐weakened zygotes was similar to that in ZP‐intact zygotes, whereas ZP removal increased the mutation rate. The mutation efficiency in blastocysts from electroporated zygotes did not differ among ZP treatment groups.

Conclusions

Our results indicate that weakening the ZP does not affect the developmental competence, mutation rate, or mutation efficiency of electroporated zygotes, whereas ZP removal has a detrimental effect on embryonic development but may increase the mutation rate.

Timing and duration of lipofection-mediated CRISPR/Cas9 delivery into porcine zygotes affect gene-editing events

OBJECTIVE

Lipofection-mediated introduction of the CRISPR/Cas9 system in porcine zygotes provides a simple method for gene editing, without requiring micromanipulation. However, the gene editing efficiency is inadequate. The aim of this study was to improve the lipofection-mediated gene editing efficiency by optimizing the timing and duration of lipofection.

RESULTS

Zona pellucida (ZP)-free zygotes collected at 5, 10, and 15 h from the start of in vitro fertilization (IVF) were incubated with lipofection reagent, guide RNA (gRNA) targeting GGTA1, and Cas9 for 5 h. Lipofection of zygotes collected at 10 and 15 h from the start of IVF yielded mutant blastocysts. Next, ZP-free zygotes collected at 10 h from the start of IVF were incubated with lipofection reagent, gRNA, and Cas9 for 2.5, 5, 10, or 20 h. The blastocyst formation rate of zygotes treated for 20 h was significantly lower (p < 0.05) than those of the other groups, and no mutant blastocysts were obtained. Moreover, the mutation rates of the resulting blastocysts decreased as the incubation time increased. In conclusion, a lipofection-mediated gene editing system using the CRISPR/Cas9 system in ZP-zygotes is feasible; however, further improvements in the gene editing efficiency are required.

Exosome/Liposome-like Nanoparticles: New Carriers for CRISPR Genome Editing in Plants

Rapid developments in the field of plant genome editing using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems necessitate more detailed consideration of the delivery of the CRISPR system into plants. Successful and safe editing of plant genomes is partly based on efficient delivery of the CRISPR system. Along with the use of plasmids and viral vectors as cargo material for genome editing, non-viral vectors have also been considered for delivery purposes. These non-viral vectors can be made of a variety of materials, including inorganic nanoparticles, carbon nanotubes, liposomes, and protein- and peptide-based nanoparticles, as well as nanoscale polymeric materials. They have a decreased immune response, an advantage over viral vectors, and offer additional flexibility in their design, allowing them to be functionalized and targeted to specific sites in a biological system with low cytotoxicity. This review is dedicated to describing the delivery methods of CRISPR system into plants with emphasis on the use of non-viral vectors.

Electroporation-Mediated Genome Editing of Livestock Zygotes

The introduction of genome editing reagents into mammalian zygotes has traditionally been accomplished by cytoplasmic or pronuclear microinjection. This time-consuming procedure requires expensive equipment and a high level of skill. Electroporation of zygotes offers a simplified and more streamlined approach to transfect mammalian zygotes. There are a number of studies examining the parameters used in electroporation of mouse and rat zygotes. Here, we review the electroporation conditions, timing, and success rates that have been reported for mice and rats, in addition to the few reports about livestock zygotes, specifically pigs and cattle. The introduction of editing reagents at, or soon after, fertilization can help reduce the rate of mosaicism, the presence of two of more genotypes in the cells of an individual; as can the introduction of nuclease proteins rather than mRNA encoding nucleases. Mosaicism is particularly problematic in large livestock species with long generation intervals as it can take years to obtain non-mosaic, homozygous offspring through breeding. Gene knockouts accomplished via the non-homologous end joining pathway have been more widely reported and successfully accomplished using electroporation than have gene knock-ins. Delivering large DNA plasmids into the zygote is hindered by the zona pellucida (ZP), and the majority of gene knock-ins accomplished by electroporation have been using short single stranded DNA (ssDNA) repair templates, typically less than 1 kb. The most promising approach to deliver larger donor repair templates of up to 4.9 kb along with genome editing reagents into zygotes, without using cytoplasmic injection, is to use recombinant adeno-associated viruses (rAAVs) in combination with electroporation. However, similar to other methods used to deliver clustered regularly interspaced palindromic repeat (CRISPR) genome-editing reagents, this approach is also associated with high levels of mosaicism. Recent developments complementing germline ablated individuals with edited germline-competent cells offer an approach to avoid mosaicism in the germline of genome edited founder lines. Even with electroporation-mediated delivery of genome editing reagents to mammalian zygotes, there remain additional chokepoints in the genome editing pipeline that currently hinder the scalable production of non-mosaic genome edited livestock.

Current status of the application of gene editing in pigs

Genetically modified animals, especially rodents, are widely used in biomedical research. However, non-rodent models are required for efficient translational medicine and preclinical studies. Owing to the similarity in the physiological traits of pigs and humans, genetically modified pigs may be a valuable resource for biomedical research. Somatic cell nuclear transfer (SCNT) using genetically modified somatic cells has been the primary method for the generation of genetically modified pigs. However, site-specific gene modification in porcine cells is inefficient and requires laborious and time-consuming processes. Recent improvements in gene-editing systems, such as zinc finger nucleases, transcription activator-like effector nucleases, and the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (CRISPR/Cas) system, represent major advances. The efficient introduction of site-specific modifications into cells via gene editors dramatically reduces the effort and time required to generate genetically modified pigs. Furthermore, gene editors enable direct gene modification during embryogenesis, bypassing the SCNT procedure. The application of gene editors has progressively expanded, and a range of strategies is now available for porcine gene engineering. This review provides an overview of approaches for the generation of genetically modified pigs using gene editors, and highlights the current trends, as well as the limitations, of gene editing in pigs.

One-Step Generation of Multiple Gene-Edited Pigs by Electroporation of the CRISPR/Cas9 System into Zygotes to Reduce Xenoantigen Biosynthesis

Xenoantigens cause hyperacute rejection and limit the success of interspecific xenografts. Therefore, genes involved in xenoantigen biosynthesis, such as GGTA1, CMAH, and B4GALNT2, are key targets to improve the outcomes of xenotransplantation. In this study, we introduced a CRISPR/Cas9 system simultaneously targeting GGTA1, CMAH, and B4GALNT2 into in vitro-fertilized zygotes using electroporation for the one-step generation of multiple gene-edited pigs without xenoantigens. First, we optimized the combination of guide RNAs (gRNAs) targeting GGTA1 and CMAH with respect to gene editing efficiency in zygotes, and transferred electroporated embryos with the optimized gRNAs and Cas9 into recipient gilts. Next, we optimized the Cas9 protein concentration with respect to the gene editing efficiency when GGTA1, CMAH, and B4GALNT2 were targeted simultaneously, and generated gene-edited pigs using the optimized conditions. We achieved the one-step generation of GGTA1/CMAH double-edited pigs and GGTA1/CMAH/B4GALNT2 triple-edited pigs. Immunohistological analyses demonstrated the downregulation of xenoantigens; however, these multiple gene-edited pigs were genetic mosaics that failed to knock out some xenoantigens. Although mosaicism should be resolved, the electroporation technique could become a primary method for the one-step generation of multiple gene modifications in pigs aimed at improving pig-to-human xenotransplantation.

Lipofection-Mediated Introduction of CRISPR/Cas9 System into Porcine Oocytes and Embryos

Simple Summary

Liposome-mediated gene transfer has become an alternative method for establishing a gene targeting framework, and the production of mutant animals may be feasible even in laboratories without specialized equipment. However, whether blastocyst genome editing can be performed by treatment with lipofection reagent, guide RNA, and Cas9, without performing electroporation or microinjection, remains unclear. In this study, we demonstrated that lipofection treatment successfully induced mutation into zygotes during in vitro fertilization and in embryos at the 2- and 4-cell stages. Although liposome-mediated gene editing is a feasible system for use with zona-pellucida-free oocytes/embryos, several challenges must be overcome.

Abstract

Liposome-mediated gene transfer has become an alternative method for establishing a gene targeting framework, and the production of mutant animals may be feasible even in laboratories without specialized equipment. However, how this system functions in mammalian oocytes and embryos remains unclear. The present study was conducted to clarify whether blastocyst genome editing can be performed by treatment with lipofection reagent, guide RNA, and Cas9 for 5 h without using electroporation or microinjection. A mosaic mutation was observed in blastocysts derived from zona pellucida (ZP)-free oocytes following lipofection treatment, regardless of the target genes. When lipofection treatment was performed after in vitro fertilization (IVF), no significant differences in the mutation rates or mutation efficiency were found between blastocysts derived from embryos treated at 24 and 29 h from the start of IVF. Only blastocysts from embryos exposed to lipofection treatment at 29 h after IVF contained biallelic mutant. Furthermore, there were no significant differences in the mutation rates or mutation efficiency between blastocysts derived from embryos at the 2- and 4-cell stages. This suggests that lipofection-mediated gene editing can be performed in ZP-free oocytes and ZP-free embryos; however, other factors affecting the system efficiency should be further investigated.

Improvements in Gene Editing Technology Boost Its Applications in Livestock

Accelerated development of novel CRISPR/Cas9-based genome editing techniques provides a feasible approach to introduce a variety of precise modifications in the mammalian genome, including introduction of multiple edits simultaneously, efficient insertion of long DNA sequences into specific targeted loci as well as performing nucleotide transitions and transversions. Thus, the CRISPR/Cas9 tool has become the method of choice for introducing genome alterations in livestock species. The list of new CRISPR/Cas9-based genome editing tools is constantly expanding. Here, we discuss the methods developed to improve efficiency and specificity of gene editing tools as well as approaches that can be employed for gene regulation, base editing, and epigenetic modifications. Additionally, advantages and disadvantages of two primary methods used for the production of gene-edited farm animals: somatic cell nuclear transfer (SCNT or cloning) and zygote manipulations will be discussed. Furthermore, we will review agricultural and biomedical applications of gene editing technology.

Methodologies and Challenges for CRISPR/Cas9 Mediated Genome Editing of the Mammalian Brain

Neurons and glia are highly polarized cells with extensive subcellular structures extending over large distances from their cell bodies. Previous research has revealed elaborate protein signaling complexes localized within intracellular compartments. Thus, exploring the function and the localization of endogenous proteins is vital to understanding the precise molecular mechanisms underlying the synapse, cellular, and circuit function. Recent advances in CRISPR/Cas9-based genome editing techniques have allowed researchers to rapidly develop transgenic animal models and perform single-cell level genome editing in the mammalian brain. Here, we introduce and comprehensively review the latest techniques for genome-editing in whole animals using fertilized eggs and methods for gene editing in specific neuronal populations in the adult or developing mammalian brain. Finally, we describe the advantages and disadvantages of each technique, as well as the challenges that lie ahead to advance the generation of methodologies for genome editing in the brain using the current CRISPR/Cas9 system.