Efficient Generation of Myostatin Mutations in Pigs Using the CRISPR/Cas9 System

Precise Insertion of AttB Sequences in Goat Genome Using Enhanced Prime Editor

Prime editor, an editing tool based on the CRISPR/Cas9 system, allows for all 12 types of nucleotide exchanges and arbitrary indels in genomic sequences without the need for inducing DNA double-strand breaks. Despite its flexibility and precision, prime editing efficiency is still low and hindered by various factors such as target sites, editing types, and the length of the primer binding site. In this study, we developed a prime editing system by incorporating an RNA motif at the 3' terminal of the pegRNA and integrating all twin prime editor factors into a single plasmid. These two strategies enhanced prime editing efficiency at target sites by up to 3.58-fold and 2.19-fold, respectively. Subsequently, enhanced prime editor was employed in goat cells and embryos to efficiently insert a 38 bp attB sequence into the Gt(ROSA)26Sor (Rosa26) and C-C motif chemokine receptor 5 (CCR5) loci. The enhanced prime editor can mediate 11.9% and 6.8% editing efficiency in parthenogenetic activation of embryos through embryo microinjection. In summary, our study introduces a modified prime editing system with improved editing and transfection efficiency, making it more suitable for inserting foreign sequences into primary cells and embryos. These results broaden the potential applications of prime editing technologies in the production of transgenic animals.

The key role of myostatin b in somatic growth in fishes derived from distant hybridization

The basic mechanism of heterosis has not been systematically and completely characterized. In previous studies, we obtained three economically important fishes that exhibit rapid growth, WR (WCC ♀ × RCC ♂), WR-II (WR ♀ × WCC ♂), and WR-III (WR-II ♀ × 4nAU ♂), through distant hybridization. However, the mechanism underlying this rapid growth remains unclear. In this study, we found that WR, WR-II, and WR-III showed muscle hypertrophy and higher muscle protein and fat contents compared with their parent species (RCC and WCC). Candidate genes responsible for this rapid growth were then obtained through an analysis of 12 muscle transcriptomes. Notably, the mRNA level of mstnb (myostatin b), which is a negative regulator of myogenesis, was significantly reduced in WR, WR-II, and WR-III compared with the parent species. To verify the function of mstnb, a mstnb-deficient mutant RCC line was generated using the CRISPR-Cas9 technique. The average body weight of mstnb-deficient RCC at 12 months of age was significantly increased by 29.57% compared with that in wild-type siblings. Moreover, the area and number of muscle fibers were significantly increased in mstnb-deficient RCC, indicating hypertrophy and hyperplasia. Furthermore, the muscle protein and fat contents were significantly increased in mstnb-deficient RCC. The molecular regulatory mechanism of mstnb was then revealed by transcription profiling, which showed that genes related to myogenesis (myod, myog, and myf5), protein synthesis (PI3K-AKT-mTOR), and lipogenesis (pparγ and fabp3) were highly activated in hybrid fishes and mstnb-deficient RCC. This study revealed that low expression or deficiency of mstnb regulates somatic growth by promoting myogenesis, protein synthesis, and lipogenesis in hybrid fishes and mstnb-deficient RCC, which provides evidence for the molecular mechanism of heterosis via distant hybridization.

CRISPR-mediated editing of β-lactoglobulin (BLG) gene in buffalo

Milk is a good source of nutrition but is also a source of allergenic proteins such as α-lactalbumin, β-lactoglobulin (BLG), casein, and immunoglobulins. The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas technology has the potential to edit any gene, including milk allergens. Previously, CRISPR/Cas has been successfully employed in dairy cows and goats, but buffaloes remain unexplored for any milk trait. In this study, we utilized the CRISPR/Cas9 system to edit the major milk allergen BLG gene in buffaloes. First, the editing efficiency of designed sgRNAs was tested in fibroblast cells using the T7E assay and Sanger sequencing. The most effective sgRNA was selected to generate clonal lines of BLG-edited cells. Analysis of 15 single-cell clones, through TA cloning and Sanger sequencing, revealed that 7 clones exhibited bi-allelic (-/-) heterozygous, bi-allelic (-/-) homozygous, and mono-allelic (-/+) disruptions in BLG. Bioinformatics prediction analysis confirmed that non-multiple-of-3 edited nucleotide cell clones have frame shifts and early truncation of BLG protein, while multiple-of-3 edited nucleotides resulted in slightly disoriented protein structures. Somatic cell nuclear transfer (SCNT) method was used to produce blastocyst-stage embryos that have similar developmental rates and quality with wild-type embryos. This study demonstrated the successful bi-allelic editing (-/-) of BLG in buffalo cells through CRISPR/Cas, followed by the production of BLG-edited blastocyst stage embryos using SCNT. With CRISPR and SCNT methods described herein, our long-term goal is to generate gene-edited buffaloes with BLG-free milk.

Advancement of animal and poultry nutrition: Harnessing the power of CRISPR-Cas genome editing technology

CRISPR-associated proteins and clustered regularly interspaced short palindromic repeats (CRISPR-Cas) technology has emerged as a groundbreaking advancement in animal and poultry nutrition to improve feed conversion efficiency, enhance disease resistance, and improve the nutritional quality of animal products. Despite significant advancements, there is a research gap in the systematic understanding and comprehensive use of the CRISPR-Cas method in animal and poultry nutrition. The purpose of this study is to elucidate the latest advancements in animal and poultry nutrition through CRISPR-Cas genome editing technology, focusing on gene manipulation in metabolism, immunity, and growth. Following preferred reporting items in meta-analysis and systematic reviews guidelines, we conducted a systematic search using several databases, including Scopus, PubMed, and Web of Science, until May 2024, and finally, we included a total of 108 articles in this study. This article explores the use of the CRISPR-Cas system in the advancement of feed additives like probiotics and enzymes, which could reduce the use of antibiotics in animal production. Furthermore, the article discusses ethical and regulatory issues related to gene editing in animal and poultry nutrition, including concerns about animal welfare, food safety, and environmental impacts. Overall, the CRISPR-Cas system holds substantial promise to overcome the challenges in modern animal agriculture. By enriching the nutritional quality of animal products, increasing disease resistance, and improving feed efficiency, it offers sustainable and cost-effective solutions that can revolutionize animal and poultry nutrition.

Genetically Engineered Goats as Efficient Mammary Gland Bioreactors for Production of Recombinant Human Neutrophil Peptide 1 Using CRISPR/Cas9

Mammary gland bioreactors are promising methods for recombinant protein production. Human neutrophil peptide 1 (HNP1) exhibits antibacterial and immune-modulating properties. This study aims to establish a method to generate goats secreting HNP1 using the mammary gland as bioreactors. HNP1 transgenic goats were generated by using CRISPR/Cas9 technology to knock-in (KI) the HNP1 sequence into exon 7 of the goat β-casein (CSN2) gene under the control of the CSN2 promoter. One-cell stage embryos were cytoplasmically injected with a mixture of Cas9 mRNA, sgRNA, and a homologous plasmid including the T2A-HNP1 sequences, followed by transfer to recipient goats. A total of 22 live offspring goats were delivered, and 21 of these goats (95.45%) exhibited targeted edits at the CSN2 locus, and 2 female goats (9.09%) demonstrated successful HNP1 integration. Western blot and ELISA analyses confirmed the presence of HNP1 protein at high levels in the milk of these HNP1-positive goats, with mean concentrations of 22.10 µg/mL and 0.0092 µg/mL during the initial 60 days of lactation. Furthermore, milk from these transgenic goats exhibited notable antibacterial activity against Escherichia coli and Staphylococcus aureus, demonstrating the functionality of the expressed HNP1 protein. In conclusion, we established an efficient method for developing new transgenic goat lines as a mammary gland bioreactor, and the bioactive HNP1 protein secreted by the transgenic goat has the potential to combat microbial resistance.

Genome editing: An insight into disease resistance, production efficiency, and biomedical applications in livestock

One of the primary concerns for the survival of the human species is the growing demand for food brought on by an increasing global population. New developments in genome-editing technology present promising opportunities for the growth of wholesome and prolific farm animals. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. Genome editing entails modifying genetic material by removing, adding, or manipulating particular DNA sequences from a particular locus in a way that does not happen naturally. The three primary genome editors are CRISPR/Cas 9, TALENs, and ZFNs. Each of these enzymes is capable of precisely severing nuclear DNA at a predetermined location. One of the most effective inventions is base editing, which enables single base conversions without the requirement for a DNA double-strand break (DSB). As reliable methods for precise genome editing in studies involving animals, cytosine and adenine base editing are now well-established. Effective zygote editing with both cytosine and adenine base editors (ABE) has resulted in the production of animal models. Both base editors produced comparable outcomes for the precise editing of point mutations in somatic cells, advancing the field of gene therapy. This review focused on the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of ZFNs, TALENs, and CRISPR/Cas9 base editors, and prime editing in diverse lab and farm animals. Additionally, we address the methodologies that can be used for gene regulation, base editing, and epigenetic alterations, as well as the significance of genome editing in animal models to better reflect real disease. We also look at methods designed to increase the effectiveness and precision of gene editing tools. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. This review is an overview of the existing knowledge of the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of zinc finger nucleases (ZFNs), transcription-activator-like endonucleases (TALENs), and clustered regularly interspaced short palindromic repeats associated protein 9 (CRISPR/Cas 9), base editors and prime editing in diverse lab and farm animals, which will offer better and healthier products for the entire human race.

Generation of Fel d 1 chain 2 genome-edited cats by CRISPR-Cas9 system

Allergens from domestic cats (Felis catus) cause allergy-related health problems worldwide. Fel d 1 is a major allergen that causes severe allergic reactions in humans, including rhinitis, conjunctivitis, and life-threatening asthma. Therefore, patients with cat allergies anticipate hypoallergenic cats. We successfully generated Fel d 1 chain 2 (CH2) genome-edited cats using the CRISPR-Cas9 system in this study. T7 endonuclease 1 assay and Sanger sequencing were used to confirm the mutation in CH2 genome-edited cats. Fel d 1 level in CH2 genome-edited cats were assessed by enzyme-linked immunosorbent assay (ELISA). Remarkably, ELISA showed that the level of Fel d 1 in the CH2 homozygous genome-edited cat (Name: Alsik) was extremely low compared with that in wild type domestic cats and could be hypoallergenic cats. Additionally, we successfully cloned the CH2 homozygous genome-edited cat using cytoplasm injection clone technology. The cloned CH2 homozygous genome-edited cat was verified using microsatellite analysis. Creating hypoallergenic cats using the CRISPR-Cas9 system is a significant step forward because these cats can safely approach allergic patients.

CRISPR Ribonucleoprotein-Mediated Precise Editing of Multiple Genes in Porcine Fibroblasts

The multi-gene editing porcine cell model can analyze the genetic mechanisms of multiple genes, which is beneficial for accelerating genetic breeding. However, there has been a lack of an effective strategy to simultaneously perform precise multi-gene editing in porcine cells. In this study, we aimed to improve the efficiency of CRISPR RNP-mediated precise gene editing in porcine cells. CRISPR RNP, including Cas9 protein, sgRNA, and ssODN, was used to generate precise nucleotide substitutions by homology-directed repair (HDR) in porcine fetal fibroblasts (PFFs). These components were introduced into PFFs via electroporation, followed by PCR for each target site. To enhance HDR efficacy, small-molecule M3814 and phosphorothioate-modified ssODN were employed. All target DNA samples were sequenced and analyzed, and the efficiencies of different combinations of the CRISPR RNP system in target sites were compared. The results showed that when 2 μM M3814, a small molecule which inhibits NHEJ-mediated repair by blocking DNA-PKs activity, was used, there was no toxicity to PFFs. The CRISPR RNP-mediated HDR efficiency increased 3.62-fold. The combination of CRISPR RNP with 2 μM M3814 and PS-ssODNs achieved an HDR-mediated precision gene modification efficiency of approximately 42.81% in mutated cells, a 6.38-fold increase compared to the control group. Then, we used the optimized CRISPR RNP system to perform simultaneous editing of two and three loci at the INS and RLN3 genes. The results showed that the CRISPR RNP system could simultaneously edit two and three loci. The efficiency of simultaneous editing of two loci was not significantly different from that of single-gene editing compared to the efficiency of single-locus editing. The efficiency of simultaneous precise editing of INS, RLN3 exon 1, and RLN3 exon 2 was 0.29%, 0.24%, and 1.05%, respectively. This study demonstrated that a 2 μM M3814 combination with PS-ssODNs improves the efficacy of CRISPR RNP-mediated precise gene editing and allows for precise editing of up to three genes simultaneously in porcine cells.

Current status and future of gene engineering in livestock

The application of gene engineering in livestock is necessary for various reasons, such as increasing productivity and producing disease resistance and biomedicine models. Overall, gene engineering provides benefits to the agricultural and research aspects, and humans. In particular, productivity can be increased by producing livestock with enhanced growth and improved feed conversion efficiency. In addition, the application of the disease resistance models prevents the spread of infectious diseases, which reduces the need for treatment, such as the use of antibiotics; consequently, it promotes the overall health of the herd and reduces unexpected economic losses. The application of biomedicine could be a valuable tool for understanding specific livestock diseases and improving human welfare through the development and testing of new vaccines, research on human physiology, such as human metabolism or immune response, and research and development of xenotransplantation models. Gene engineering technology has been evolving, from random, time-consuming, and laborious methods to specific, time-saving, convenient, and stable methods. This paper reviews the overall trend of genetic engineering technologies development and their application for efficient production of genetically engineered livestock, and provides examples of technologies approved by the United States (US) Food and Drug Administration (FDA) for application in humans. [BMB Reports 2024; 57(1): 50-59].

Efficient delivery of a large-size Cas9-EGFP vector in porcine fetal fibroblasts using a Lonza 4D-Nucleofector system

BACKGROUND

Porcine fetal fibroblasts (PFFs) are important donor cells for generating genetically modified pigs, but the transfection efficiencies of PFFs are often unsatisfactory especially when large-size vectors are to be delivered. In this study, we aimed to optimize the transfection conditions for delivery of a large-size vector in PFFs using Lonza 4D-Nucleofector™ vessels and strips.

METHODS

We firstly delivered a 13 kb Cas9-EGFP and a 3.5 kb pMAX-GFP vector into PFFs via 7 programs recommended by the Lonza basic protocol. We then tested 6 customized dual-electroporation programs for delivering the 13 kb plasmid into PFFs. In addition, we screened potential alternative electroporation buffers to the Nucleofector™ P3 solution. Finally, three CRISPR/Cas9-sgRNAs targeting Rosa26, H11, and Cep112 loci were delivered into PFFs with different single and dual-electroporation programs.

RESULTS

Notably lower transfection efficiencies were observed when delivering the 13 kb vector than delivering the 3.5 kb vector in PFFs via the single-electroporation programs. The customized dual-electroporation program FF-113 + CA-137 exhibited higher transfection efficiencies than any of the single-electroporation programs using vessels (98.1%) or strips (89.1%) with acceptable survival rates for the 13 kb vector. Entranster-E buffer generated similar transfection efficiencies and 24-hour survival rates to those from the P3 solution, thus can be used as an alternative electroporation buffer. In the genome-editing experiments, the FF-113 + CA-137 and CA-137 + CA-137 programs showed significantly superior (P < 0.01) efficiencies to ones from the single-electroporation programs in vessels and strips. Entranster-E buffer produced higher indel efficiencies than the P3 buffer.

CONCLUSIONS

We markedly increased the delivery efficiencies for a large vector via customized dual-electroporation programs using Lonza 4D-Nucleofector™ system, and Entranster-E buffer can be used as an alternative electroporation buffer to Nucleofector™ P3 buffer.

Generation of double knockout cattle via CRISPR-Cas9 ribonucleoprotein (RNP) electroporation

BACKGROUND

Genome editing has been considered as powerful tool in agricultural fields. However, genome editing progress in cattle has not been fast as in other mammal species, for some disadvantages including long gestational periods, single pregnancy, and high raising cost. Furthermore, technically demanding methods such as microinjection and somatic cell nuclear transfer (SCNT) are needed for gene editing in cattle. In this point of view, electroporation in embryos has been risen as an alternative.

RESULTS

First, editing efficiency of our electroporation methods were tested for embryos. Presence of mutation on embryo was confirmed by T7E1 assay. With first combination, mutation rates for MSTN and PRNP were 57.6% ± 13.7% and 54.6% ± 13.5%, respectively. In case of MSTN/BLG, mutation rates were 83.9% ± 23.6% for MSTN, 84.5% ± 18.0% for BLG. Afterwards, the double-KO embryos were transferred to surrogates and mutation rate was identified in resultant calves by targeted deep sequencing. Thirteen recipients were transferred for MSTN/PRNP, 4 calves were delivered, and one calf underwent an induction for double KO. Ten surrogates were given double-KO embryos for MSTN/BLG, and four of the six calves that were born had mutations in both genes.

CONCLUSIONS

These data demonstrated that production of genome edited cattle via electroporation of RNP could be effectively applied. Finally, MSTN and PRNP from beef cattle and MSTN and BLG from dairy cattle have been born and they will be valuable resources for future precision breeding.

Post-genomic era in agriculture and veterinary science: successful and proposed application of genetic targeting technologies

Gene editing tools have become an indispensable part of research into the fundamental aspects of cell biology. With a vast body of literature having been generated based on next generation sequencing technologies, keeping track of this ever-growing body of information remains challenging. This necessitates the translation of genomic data into tangible applications. In order to address this objective, the generated Next Generation Sequencing (NGS) data forms the basis for targeted genome editing strategies, employing known enzymes of various cellular machinery, in generating organisms with specifically selected phenotypes. This review focuses primarily on CRISPR/Cas9 technology in the context of its advantages over Zinc finger proteins (ZNF) and Transcription activator-like effector nucleases (TALEN) and meganucleases mutagenesis strategies, for use in agricultural and veterinary applications. This review will describe the application of CRISPR/Cas9 in creating modified organisms with custom-made properties, without the undesired non-targeted effects associated with virus vector vaccines and bioactive molecules produced in bacterial systems. Examples of the successful and unsuccessful applications of this technology to plants, animals and microorganisms are provided, as well as an in-depth look into possible future trends and applications in vaccine development, disease resistance and enhanced phenotypic traits will be discussed.

Efficient and Specific Generation of MSTN-Edited Hu Sheep Using C-CRISPR

Hu sheep, an indigenous breed in China known for its high fecundity, are being studied to improve their growth and carcass traits. MSTN is a negative regulator of muscle development, and its inactivation results in muscularity. The C-CRISPR system, utilizing multiple neighboring sgRNAs targeting a key exon, has been successfully used to generate genes for complete knockout (KO) monkeys and mice in one step. In this study, the C-CRISPR system was used to generate MSTN-edited Hu sheep; 70 embryos injected with Cas9 mRNA and four sgRNAs targeting exon 3 of sheep MSTN were transferred to 13 recipients. Out of 10 lambs born from five recipients after full-term pregnancies, nine had complete MSTN KO with various mutations. No off-target effects were found. These MSTN-KO Hu sheep showed a double-muscled (DM) phenotype, characterized by a higher body weight at 3 and 4 months old, prominent muscular protrusion, clearly visible intermuscular groves, and muscle hypertrophy. The molecular analysis indicated enhanced AKT and suppressed ERK1/2 signaling in the gluteus muscle of the edited Hu sheep. In conclusion, MSTN complete KO Hu sheep with a DM phenotype were efficiently and specifically generated using C-CRISPR, and the C-CRISPR method is a promising tool for farm animal breeding.

Combined Transcriptome and Metabolome Analysis of Smooth Muscle of Myostatin Knockout Cattle

Myostatin (MSTN), a growth and differentiation factor, plays an important role in regulating skeletal muscle growth and development. MSTN knockout (MSTN-KO) leads to skeletal muscle hypertrophy and regulates metabolic homeostasis. Moreover, MSTN is also detected in smooth muscle. However, the effect of MSTN-KO on smooth muscle has not yet been reported. In this study, combined metabolome and transcriptome analyses were performed to investigate the metabolic and transcriptional profiling in esophageal smooth muscles of MSTN-KO Chinese Luxi Yellow cattle (n = 5, 24 months, average body weight 608.5 ± 17.62 kg) and wild-type (WT) Chinese Luxi Yellow cattle (n = 5, 24 months, average body weight 528.25 ± 11.03 kg). The transcriptome was sequenced using the Illumina Novaseq™ 6000 sequence platform. In total, 337 significantly up- and 129 significantly down-regulated genes were detected in the MSTN-KO cattle compared with the WT Chinese Luxi Yellow cattle. Functional enrichment analysis indicated that the DEGs were mainly enriched in 67 signaling pathways, including cell adhesion molecules, tight junction, and the cGMP-PKG signaling pathway. Metabolomics analysis by liquid chromatography-tandem mass spectrometry (LC-MS/MS) identified 130 differential metabolites between the groups, with 56 up-regulated and 74 down-regulated in MSTN knockout cattle compared with WT cattle. Differential metabolites were significantly enriched in 31 pathways, including glycerophospholipid metabolism, histidine metabolism, glutathione metabolism, and purine metabolism. Transcriptome and metabolome were combined to analyze the significant enrichment pathways, and there were three metabolically related pathways, including histidine metabolism, purine metabolism, and arginine and proline metabolism. These results provide important references for in-depth research on the effect of MSTN knockout on smooth muscle.

Reproduction and viscera organ characteristics of MSTN and FGF5 dual-gene knockout sheep

Introduction

Myostatin (MSTN) negatively regulates skeletal muscle development. However, its function in reproductive performance and visceral organs has not been thoroughly investigated. Previously, we prepared a MSTN and fibroblast growth factor 5 (FGF5) double-knockout sheep, which was a MSTN and FGF5 dual-gene biallelic homozygous (MF−/−) mutant.

Methods

To understand the role of MSTN and FGF5 in reproductive performance and visceral organs, this study evaluated the ejaculation amount, semen pH, sperm motility, sperm density, acrosome integrity, rate of teratosperm, and seminal plasma biochemical indicators in adult MF−/− rams. We also compared the overall morphology, head, head-neck junction, middle segment and the transection of middle segment of spermatozoa between wildtype (WT) and MF−/− rams.

Results

Our results showed that the seminal plasma biochemical indicators, sperm structure and all sperm indicators were normal, and the fertilization rate also has no significant difference between WT and MF−/− rams, indicating that the MF−/− mutation did not affect the reproductive performance of sheep. Additional analysis evaluated the histomorphology of the visceral organs, digestive system and reproductive system of MF+/− sheep, the F1 generation of MF−/−, at the age of 12 months. There was an increased spleen index, but no significant differences in the organ indexes of heart, liver, lung, kidney and stomach, and no obvious differences in the histomorphology of visceral organs, digestive system and reproductive system in MF+/− compared with WT sheep. No MF+/− sheep were observed to have any pathological features.

Discussion

In summary, the MSTN and FGF5 double-knockout did not affect reproductive performance, visceral organs and digestive system in sheep except for differences previously observed in muscle and fat. The current data provide a reference for further elucidating the application of MSTN and FGF5 double-knockout sheep.

When Less Is More: Targeting the Myostatin Gene in Livestock for Augmenting Meat Production

How to increase meat production is one of the main questions in animal breeding. Selection for improved body weight has been made and, due to recent genomic advances, naturally occurring variants that are responsible for controlling economically relevant phenotypes have been revealed. The myostatin (MSTN) gene, a superstar gene in animal breeding, was discovered as a negative controller of muscle mass. In some livestock species, natural mutations in the MSTN gene could generate the agriculturally desirable double-muscling phenotype. However, some other livestock species or breeds lack these desirable variants. Genetic modification, particularly gene editing, offers an unprecedented opportunity to induce or mimic naturally occurring mutations in livestock genomes. To date, various MSTN-edited livestock species have been generated using different gene modification tools. These MSTN gene-edited models have higher growth rates and increased muscle mass, suggesting the high potential of utilizing MSTN gene editing in animal breeding. Additionally, post-editing investigations in most livestock species support the favorable influence of targeting the MSTN gene on meat quantity and quality. In this Review, we provide a collective discussion on targeting the MSTN gene in livestock to further encourage its utilization opportunities. It is expected that, shortly, MSTN gene-edited livestock will be commercialized, and MSTN-edited meat will be on the tables of ordinary customers.

Future of Lung Transplantation: Xenotransplantation and Bioengineering Lungs

Xenotransplantation promises to alleviate the issue of donor organ shortages and to decrease waiting times for transplantation. Recent advances in genetic engineering have allowed for the creation of pigs with up to 16 genetic modifications. Several combinations of genetic modifications have been associated with extended graft survival and life-supporting function in experimental heart and kidney xenotransplants. Lung xenotransplantation carries specific challenges related to the large surface area of the lung vascular bed, its innate immune system's intrinsic hyperreactivity to perceived 'danger', and its anatomic vulnerability to airway flooding after even localized loss of alveolocapillary barrier function. This article discusses the current status of lung xenotransplantation, and challenges related to immunology, physiology, anatomy, and infection. Tissue engineering as a feasible alternative to develop a viable lung replacement solution is discussed.

Myostatin increases the expression of matrix metalloproteinase genes to promote preadipocytes differentiation in pigs

ABSTACTMyostatin (MSTN) resulted in reduced backfat thickness in MSTN-knockout (MSTN-KO) pigs, whereas the underlying mechanism remains elusive. In this study, RNA sequencing (RNA-seq) was used to screen differentially expressed genes (DEGs) in porcine fat tissues. We identified 285 DEGs, including 4 adipocyte differentiation-related genes (ADRGs). Matrix Metalloproteinase-2/7 (MMP-2/7), fibronectin (FN), and laminin (LN) were differentially expressed in MSTN-KO pigs compared with wild-type (WT) pigs. To investigate the molecular mechanism, we treated the preadipocytes with siRNA and recombinant MSTN protein. The results indicated that MSTN increased the expression of MMP-2/7/9 and promoted the preadipocyte differentiation. To further validate the effect of MSTN on MMP-2/7/9 expression, we treated MSTN-KO PK15 cells with recombinant MSTN protein and detected the expression of MMP-2/7/9. The data showed that MSTN increases the expression of MMP-2/7/9 in PK15. This study revealed that MSTN promoted preadipocyte differentiation and provided the basis for the mechanism of fatty deposition in pigs.

Myostatin Knockout Affects Mitochondrial Function by Inhibiting the AMPK/SIRT1/PGC1α Pathway in Skeletal Muscle

Myostatin (Mstn) is a major negative regulator of skeletal muscle mass and initiates multiple metabolic changes. The deletion of the Mstn gene in mice leads to reduced mitochondrial functions. However, the underlying regulatory mechanisms remain unclear. In this study, we used CRISPR/Cas9 to generate myostatin-knockout (Mstn-KO) mice via pronuclear microinjection. Mstn-KO mice exhibited significantly larger skeletal muscles. Meanwhile, Mstn knockout regulated the organ weights of mice. Moreover, we found that Mstn knockout reduced the basal metabolic rate, muscle adenosine triphosphate (ATP) synthesis, activities of mitochondrial respiration chain complexes, tricarboxylic acid cycle (TCA) cycle, and thermogenesis. Mechanistically, expressions of silent information regulator 1 (SIRT1) and phosphorylated adenosine monophosphate-activated protein kinase (pAMPK) were down-regulated, while peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) acetylation modification increased in the Mstn-KO mice. Skeletal muscle cells from Mstn-KO and WT were treated with AMPK activator 5-aminoimidazole-4-carboxamide riboside (AICAR), and the AMPK inhibitor Compound C, respectively. Compared with the wild-type (WT) group, Compound C treatment further down-regulated the expression or activity of pAMPK, SIRT1, citrate synthase (CS), isocitrate dehydrogenase (ICDHm), and α-ketoglutarate acid dehydrogenase (α-KGDH) in Mstn-KO mice, while Mstn knockout inhibited the AICAR activation effect. Therefore, Mstn knockout affects mitochondrial function by inhibiting the AMPK/SIRT1/PGC1α signaling pathway. The present study reveals a new mechanism for Mstn knockout in regulating energy homeostasis.

CRISPR/Cas-edited pigs for personalized medicine: more than preclinical test-system

Novel CRISPR-Cas-based genome editing tools made it feasible to introduce a variety of precise genomic modifications in the pig genome, including introducing multiple edits simultaneously, inserting long DNA sequences into specifically targeted loci, and performing nucleotide transitions and transversions. Pigs serve as a vital agricultural resource and animal model in biomedical studies, given their advantages over the other models. Pigs share high similarities to humans regarding body/organ size, anatomy, physiology, and a metabolic profile. The pig genome can be modified to carry the same genetic mutations found in humans to replicate inherited diseases to provide preclinical trials of drugs. Moreover, CRISPR-based modification of pigs antigen profile makes it possible to offer porcine organs for xenotransplantation with minimal transplant rejection responses. This review summarizes recent advances in endonuclease-mediated genome editing tools and research progress of genome-edited pigs as personalized test-systems for preclinical trials and as donors of organs with human-fit antigen profile.

Graphical abstract:

Detection of Indiscriminate Genetic Manipulation in Thoroughbred Racehorses by Targeted Resequencing for Gene-Doping Control

The creation of genetically modified horses is prohibited in horse racing as it falls under the banner of gene doping. In this study, we developed a test to detect gene editing based on amplicon sequencing using next-generation sequencing (NGS). We designed 1012 amplicons to target 52 genes (481 exons) and 147 single-nucleotide variants (SNVs). NGS analyses showed that 97.7% of the targeted exons were sequenced to sufficient coverage (depth > 50) for calling variants. The targets of artificial editing were defined as homozygous alternative (HomoALT) and compound heterozygous alternative (ALT1/ALT2) insertion/deletion (INDEL) mutations in this study. Four models of gene editing (three homoALT with 1-bp insertions, one REF/ALT with 77-bp deletion) were constructed by editing the myostatin gene in horse fibroblasts using CRISPR/Cas9. The edited cells and 101 samples from thoroughbred horses were screened using the developed test, which was capable of identifying the three homoALT cells containing 1-bp insertions. Furthermore, 147 SNVs were investigated for their utility in confirming biological parentage. Of these, 120 SNVs were amenable to consistent and accurate genotyping. Surrogate (nonbiological) dams were excluded by 9.8 SNVs on average, indicating that the 120 SNV could be used to detect foals that have been produced by somatic cloning or embryo transfer, two practices that are prohibited in thoroughbred racing and breeding. These results indicate that gene-editing tests that include variant calling and SNV genotyping are useful to identify genetically modified racehorses.

Application of Gene Editing Technology in Resistance Breeding of Livestock

As a new genetic engineering technology, gene editing can precisely modify the specific gene sequence of the organism’s genome. In the last 10 years, with the rapid development of gene editing technology, zinc-finger nucleases (ZFNs), transcription activator-like endonucleases (TALENs), and CRISPR/Cas9 systems have been applied to modify endogenous genes in organisms accurately. Now, gene editing technology has been used in mice, zebrafish, pigs, cattle, goats, sheep, rabbits, monkeys, and other species. Breeding for disease-resistance in agricultural animals tends to be a difficult task for traditional breeding, but gene editing technology has made this easier. In this work, we overview the development and application of gene editing technology in the resistance breeding of livestock. Also, we further discuss the prospects and outlooks of gene editing technology in disease-resistance breeding.

Enhancing Animal Disease Resistance, Production Efficiency, and Welfare through Precise Genome Editing

The major goal of animal breeding is the genetic enhancement of economic traits. The CRISPR/Cas system, which includes nuclease-mediated and base editor mediated genome editing tools, provides an unprecedented approach to modify the mammalian genome. Thus, farm animal genetic engineering and genetic manipulation have been fundamentally revolutionized. Agricultural animals with traits of interest can be obtained in just one generation (and without long time selection). Here, we reviewed the advancements of the CRISPR (Clustered regularly interspaced short palindromic repeats)/Cas (CRISPR associated proteins) genome editing tools and their applications in animal breeding, especially in improving disease resistance, production performance, and animal welfare. Additionally, we covered the regulations on genome-edited animals (GEAs) and ways to accelerate their use. Recommendations for how to produce GEAs were also discussed. Despite the current challenges, we believe that genome editing breeding and GEAs will be available in the near future.

Application of Genetic, Genomic and Biological Pathways in Improvement of Swine Feed Efficiency

Despite the significant improvement of feed efficiency (FE) in pigs over the past decades, feed costs remain a major challenge for producers profitability. Improving FE is a top priority for the global swine industry. A deeper understanding of the biology underlying FE is crucial for making progress in genetic improvement of FE traits. This review comprehensively discusses the topics related to the FE in pigs including: measurements, genetics, genomics, biological pathways and the advanced technologies and methods involved in FE improvement. We first provide an update of heritability for different FE indicators and then characterize the correlations of FE traits with other economically important traits. Moreover, we present the quantitative trait loci (QTL) and possible candidate genes associated with FE in pigs and outline the most important biological pathways related to the FE traits in pigs. Finally, we present possible ways to improve FE in swine including the implementation of genomic selection, new technologies for measuring the FE traits, and the potential use of genome editing and omics technologies.

Enhanced Muscle Fibers of Epinephelus coioides by Myostatin Autologous Nucleic Acid Vaccine

Epinephelus coioides is a fish species with high economic value due to its delicious meat, high protein content, and rich fatty acid nutrition. It has become a high-economic fish in southern parts of China and some other Southeast Asian countries. In this study, the myostatin nucleic acid vaccine was constructed and used to immunize E. coioides. The results from body length and weight measurements indicated the myostatin nucleic acid vaccine promoted E. coioides growth performance by increasing muscle fiber size. The results from RT-qPCR analysis showed that myostatin nucleic acid vaccine upregulated the expression of myod, myog and p21 mRNA, downregulated the expression of smad3 and mrf4 mRNA. This preliminary study is the first report that explored the role of myostatin in E. coioides and showed positive effects of autologous nucleic acid vaccine on the muscle growth of E. coioides. Further experiments with increased numbers of animals and different doses are needed for its application to E. coiodes aquaculture production.

Identification and functional characterization of Pomstna in Japanese flounder (Paralichthys olivaceus)

Myostatin (MSTN) as a negative regulator of muscle growth has been identified in Japanese flounder. Yet, most fish experienced the teleost specific genome duplication and possess at least two mstn genes. In current study, the second mstn gene named Pomstna is identified in Japanese flounder. Pomstna is clustered with other mstn2 of teleosts and owned highly conserved TGF-beta domain. In addition to muscle, Pomstna also highly expressed in brain and spleen. Using the primarily cultured muscle cells of Japanese flounder, we found that Pomstna could inhibit the proliferation and differentiation of muscle cells in vitro. As a ligand of TGF-beta signaling pathway, Pomstnb could regulate the expression of p21 and myod by activating the TGF-beta signaling pathway. Different from the function of Pomstnb, Pomstna could not activate the TGF-beta signaling pathway in vitro. During the differentiation of PoM cells, the expression of Pomstnb decreased significantly but the expression of Pomstna showed no change. Our study suggests that Pomstna could negatively regulate the growth and differentiation of muscle like Pomstnb yet through a different regulatory mechanism than Pomstnb. The present study suggests that muscle proliferation and differentiation were regulated by mstn not only through the TGF-beta signaling pathway but also other unknown mechanisms.

Research progress and application prospects of stable porcine pluripotent stem cells

Pluripotent stem cells (PSCs) harbor the capacity of unlimited self-renewal and multi-lineage differentiation potential which are crucial for basic research and biomedical science. Establishment of PSCs with defined features were previously reported from mice and humans, while generation of stable large animal PSCs has experienced a relatively long trial stage and only recently has made breakthroughs. Pigs are regarded as ideal animal models for their similarities in physiology and anatomy to humans. Generation of porcine PSCs would provide cell resources for basic research, genetic engineering, animal breeding and cultured meat. In this review, we summarize the progress on the derivation of porcine PSCs and reprogrammed cells and elucidate the mechanisms of pluripotency changes during pig embryo development. This will be beneficial for understanding the divergence and conservation between different species involved in embryo development and the pluripotent regulated signaling pathways. Finally, we also discuss the promising future applications of stable porcine PSCs.

Generation of Heritable Prominent Double Muscle Buttock Rabbits via Novel Site Editing of Myostatin Gene Using CRISPR/Cas9 System

Rabbits have been domesticated for meat, wool, and fur production, and have also been cherished as a companion, artistic inspiration, and an experimental model to study many human diseases. In the present study, the muscle mass negative regulator gene myostatin (MSTN) was knocked out in rabbits at two novel sites in exon3, and the function of these mutations was determined in subsequent generations. The prominent double muscle phenotype with hyperplasia or hypertrophy of muscle fiber was observed in the MSTN-KO rabbits, and a similar phenotype was confirmed in the F1 generation. Moreover, the average weight of 80-day-old MSTN-KO rabbits (2,452 ± 63 g) was higher than that of wild-type rabbits (2,393.2 ± 106.88 g), and also the bodyweight of MSTN-KO rabbits (3,708 ± 43.06g) was significantly higher (P < 0.001) at the age of 180 days than wild-type (WT) rabbits (3,224 ± 48.64g). In MSTN-KO rabbits, fourteen rabbit pups from the F1 generation and thirteen from the F2 generation stably inherited the induced MSTN gene mutations. Totally, 194 pups were produced in the F1 generation of which 49 were MSTN-KO rabbits, while 47 pups were produced in the F2 generation of which 20 were edited rabbits, and the ratio of edited to wild-type rabbits in the F2 generation was approximately 1:1. Thus, we successfully generated a heritable double muscle buttocks rabbits via myostatin mutation with CRISPR/Cas9 system, which could be valuable in rabbit's meat production and also a useful animal model to study the development of muscles among livestock species and improve their important economic traits as well as the human muscle development-related diseases.

Application of CRISPR/Cas9 System in Establishing Large Animal Models

The foundation for investigating the mechanisms of human diseases is the establishment of animal models, which are also widely used in agricultural industry, pharmaceutical applications, and clinical research. However, small animals such as rodents, which have been extensively used to create disease models, do not often fully mimic the key pathological changes and/or important symptoms of human disease. As a result, there is an emerging need to establish suitable large animal models that can recapitulate important phenotypes of human diseases for investigating pathogenesis and developing effective therapeutics. However, traditional genetic modification technologies used in establishing small animal models are difficultly applied for generating large animal models of human diseases. This difficulty has been overcome to a great extent by the recent development of gene editing technology, especially the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). In this review, we focus on the applications of CRISPR/Cas9 system to establishment of large animal models, including nonhuman primates, pigs, sheep, goats and dogs, for investigating disease pathogenesis and treatment. We also discuss the limitations of large animal models and possible solutions according to our current knowledge. Finally, we sum up the applications of the novel genome editing tool Base Editors (BEs) and its great potential for gene editing in large animals.

Myostatin Alteration in Pigs Enhances the Deposition of Long-Chain Unsaturated Fatty Acids in Subcutaneous Fat

In animals, myostatin (MSTN) is a negative regulator that inhibits muscle growth and repair. The decreased level of functional MSTN gene expression can change the amount and proportions of fats in pigs. In this study we determined the lipidomics of subcutaneous fat in MSTN single copy mutant pigs and evaluated the variations in lipid contents of the subcutaneous fat from MSTN+/− and wild type Large White (LW) pigs via ultra-performance liquid chromatography–quadrupole/Orbitrap-mass spectrometry (MS). The results showed that the quantities of glycerolipids, sphingolipids, fatty acyls and glycerophospholipids were significantly changed, particularly, the molecular diacylglycerol in glycerolipids, long-chain unsaturated fatty acids, and ceramide non-hydroxy fatty acid-sphingosine in sphingolipids were remarkably increased in the MSTN+/− group. Due to their positive bioavailability demonstrated by previous researches, these three lipids might be beneficial for human health. Further, the results of our study confirm that MSTN participates in the regulation of fat metabolism, and reduced expression of MSTN can ultimately influence the accumulation of lipid contents in the subcutaneous fat of pigs.

Comparison of Microbial Community and Metabolites in Four Stomach Compartments of Myostatin-Gene-Edited and Non-edited Cattle

Myostatin (MSTN), a major negative regulator of skeletal muscle mass and an endocrine factor, can regulate the metabolism of various organisms. Inhibition of the MSTN gene can improve meat production from livestock. Rumen microorganisms are associated with production and health traits of cattle, but changes in the microbial composition and metabolome in the four stomach compartments of MSTN gene-edited cattle have not previously been studied. Our results indicated that microbial diversity and dominant bacteria in the four stomach compartments were very similar between MSTN gene-edited and wild-type (WT) cattle. The microbiota composition was significantly different between MSTN gene-edited and WT cattle. Our results show that the relative abundance of the phylum Proteobacteria in the reticulum of MSTN gene-edited cattle was lower than that of WT cattle, whereas the relative abundance of the genus Prevotella in the omasum of MSTN gene-edited cattle was significantly higher than that of WT cattle. Metabolomics analysis revealed that the intensity of L-proline and acetic acid was significantly different in the rumen, reticulum, and abomasum between the two types of cattle. Meanwhile, pathway topology analysis indicated that the differential metabolites were predominantly involved in arginine biosynthesis and glutamate metabolism in the rumen, reticulum, and omasum but were mainly involved in pyruvate metabolism and glycolysis/gluconeogenesis in the abomasum. Spearman correlation network analysis further demonstrated that there was a significant correlation between microflora composition and metabolic pathways. These findings provide clues for studying nutrient digestion and absorption ability of MSTN gene-edited cattle.

Viscera Characteristics of MSTN-Edited Heterozygous Pigs

Myostatin (MSTN) is a protein that negatively regulates growth of skeletal muscle, and inactivation of MSTN improves the mass of skeletal muscle. Our previous work found that MSTN^+/− pigs have higher muscle depth and lower fat depth compared to wild type without any developmental problems. Therefore, MSTN-edited pigs are most likely to appear as heterozygotes in the potential future market, but the characteristics of organs in digestive and reproductive system of pigs with MSTN gene editing remains unclear. Here, we investigated the histological of the organs in the digestive system and reproductive system in MSTN gene heterozygotes at adult stages. The length of intestine was further compared between adult heterozygous and wild type pigs. We found no significant differences in histomorphology of organs, including heart, duodenum, jejunum, ileum, cecum, colon, testis, epididymis, ovaries, oviducts and uterus, between individuals from two genotypes. Moreover, there was no significant difference in the average length of intestine in adult pigs. Our data provide a reference for further clarifying the applications of MSTN gene edited pigs.

Myostatin Knockout Regulates Bile Acid Metabolism by Promoting Bile Acid Synthesis in Cattle

Myostatin (MSTN) is a major negative regulator of skeletal muscle mass and causes a variety of metabolic changes. However, the effect of MSTN knockout on bile acid metabolism has rarely been reported. In this study, the physiological and biochemical alterations of serum in MSTN^+/- and wild type (WT) cattle were investigated. There were no significant changes in liver and kidney biochemical indexes. However, compared with the WT cattle, lactate dehydrogenase, total bile acid (TBA), cholesterol, and high-density lipoprotein (HDL) in the MSTN^+/- cattle were significantly increased, and glucose, low-density lipoprotein (LDL), and triglycerides (TG) were significantly decreased, indicating that MSTN knockout affected glucose and lipid metabolism and total bile acids content. Targeted metabolomic analysis of the bile acids and their derivatives was performed on serum samples and found that bile acids were significantly increased in the MSTN^+/- cattle compared with the WT cattle. As the only bile acid synthesis organ in the body, we performed metabolomic analysis on the liver to study the effect of MSTN knockout on hepatic metabolism. Metabolic pathway enrichment analysis of differential metabolites showed significant enrichment of the primary bile acid biosynthesis and bile secretion pathway in the MSTN^+/- cattle. Targeted metabolomics data further showed that MSTN knockout significantly increased bile acid content in the liver, which may have resulted from enhanced bile acid synthesis due to the expression of bile acid synthesis genes, cholesterol 7 alpha-hydroxylase (CYP7A1) and sterol 27-hydroxylase (CYP27A1), and upregulation in the liver of the MSTN^+/- cattle. These results indicate that MSTN knockout does not adversely affect bovine fitness but regulates bile acid metabolism via enhanced bile acid synthesis. This further suggests a role of MSTN in regulating metabolism.

The application of new breeding technology based on gene editing in pig industry

Genome/gene-editing (GE) techniques, characterized by a low technological barrier, high efficiency, and broad application among organisms, are now being employed not only in medical science but also in agriculture/veterinary science. Different engineered CRISPR/Cas9s have been identified to expand the application of this technology. In pig production, GE is a precise new breeding technology (NBT), and promising outcomes in improving economic traits, such as growth, lean or healthy meat production, animal welfare, and disease resistance, have already been documented and reviewed. These promising achievements in porcine gene editing, including the Myostatin gene knockout (KO) in indigenous breeds to improve lean meat production, the uncoupling protein 1 (UCP1) gene knock-in to enhance piglet thermogenesis and survival under cold stress, the generation of GGTA1 and CMP-N-glycolylneuraminic acid hydroxylase (CMAH) gene double KO (dKO) pigs to produce healthy red meat, and the KO or deletion of exon 7 of the CD163 gene to confer resistance to porcine reproductive and respiratory syndrome virus infection, are described in the present article. Other related approaches for such purposes are also discussed. The current trend of global regulations or legislation for GE organisms is that they are exempted from classification as genetically modified organisms (GMOs) if no exogenes are integrated into the genome, according to product-based and not process-based methods. Moreover, an updated case study in the EU showed that current GMO legislation is not fit for purpose in term of NBTs, which contribute to the objectives of the EU’s Green Deal and biodiversity strategies and even meet the United Nations’ sustainable development goals for a more resilient and sustainable agri-food system. The GE pigs generated via NBT will be exempted from classification as GMOs, and their global valorization and commercialization can be foreseen.

Rapid visual genotyping method for germline mutants with small genomic fragment deletion by allele-specific PCR and lateral flow nucleic acid biosensor

BACKGROUND

Genome-editing techniques incorporating artificial nucleases develop rapidly and enable efficient and precise modification of genomic DNA of numerous organisms. The present research aimed to establish a rapid, sensitive and visual method for genotyping of germline genome-edited mutants with small genomic fragment deletion.

METHODS AND RESULTS

The genome-edited pigs with 2-bp deletion and 11-bp deletion of Myostatin (MSTN) gene generated by TALENs system were used as test materials to check the proposed allele-specific PCR (AS-PCR) and lateral flow nucleic acid biosensor (LFNAB) cascade method. AS-PCR can produce products with different tags to distinguish genome-edited alleles and wild-type alleles. A LFNAB was applied to do visual detection of AS-PCR products without using additional instruments. Furthermore, we demonstrated that AS-PCR and LFNAB cascade could accurately and visually distinguish genome-edited pigs with small genomic fragment deletion of Myostatin (MSTN) gene and wild-type pigs with limit of detection (LOD) of 0.1 ng.

CONCLUSION

The proposed AS-PCR and LFNAB cascade can do rapid and visual genotyping of genome-edited mutants with small genomic fragment deletion, serving as a platform for genome-edited animal genotyping.

Transcriptomic Analysis of Short/Branched-Chain Acyl-Coenzyme a Dehydrogenase Knocked Out bMECs Revealed Its Regulatory Effect on Lipid Metabolism

The acyl-CoA dehydrogenase family of enzymes includes short/branched-chain acyl-CoA dehydrogenase (ACADSB), which catalyzes the dehydrogenation of acyl-CoA derivatives in fatty acid metabolism. Our previous findings suggested that ACADSB was a critical candidate gene affecting milk fat synthesis by comparing the transcriptome in bovine mammary epithelial cells (bMECs) from Chinese Holstein dairy cows producing high-fat and low-fat milk as well as gene functional validation studies on the cellular level. In the present study, ACADSB in bMECs was knocked out (KO) using a CRISPR/Cas9 system, and mRNA transcriptome was further sequenced to verify the function of the ACADSB gene and analyze its correlation with lipid metabolism. The findings revealed that 15,693 genes were expressed, 1,548 genes were differentially expressed genes (DEGs), and 6,098 GO terms were enriched, of which 637 GO terms were greatly enhanced, such as phospholipid-translocation ATPase activity (GO:0004012), lipoprotein lipase activity (GO:0004465), acyl-CoA desaturase activity (GO:0016215), and so on. The analysis by KEGG showed that DEGs were distributed over 247 pathogens, of which 49 were significantly enriched, including the metabolism of fatty acids (PATH: 01212), metabolism of glycerolipid (PATH: 00561), and signaling of adipocytokines (PATH: 04920). The CHOL, TGs and FFA contents in bMECs were reduced when the ACADSB gene was knocked out. The RT2 Profiler PCR array also revealed that the loss of the ACADSB gene changed the expression levels of functional genes involved in lipid metabolism, including ACADL, ACOX2, ACAT2, and FABP3. In conclusion, the current findings show that ACADSB is a key regulator of lipid metabolism in bMECs. The ACADSB-/- bMECs could also be useful genetic material and tools for future research into gene functions related to lipid and fatty acid metabolism. It will be valuable for revealing the gene regulatory roles and molecular mechanisms in milk fat synthesis.

Application of Gene Editing for Climate Change in Agriculture

Climate change imposes a severe threat to agricultural systems, food security, and human nutrition. Meanwhile, efforts in crop and livestock gene editing have been undertaken to improve performance across a range of traits. Many of the targeted phenotypes include attributes that could be beneficial for climate change adaptation. Here, we present examples of emerging gene editing applications and research initiatives that are aimed at the improvement of crops and livestock in response to climate change, and discuss technical limitations and opportunities therein. While only few applications of gene editing have been translated to agricultural production thus far, numerous studies in research settings have demonstrated the potential for potent applications to address climate change in the near future.

Evidence for and localization of proposed causative variants in cattle and pig genomes

BACKGROUND

This paper reviews the localization of published potential causative variants in contemporary pig and cattle reference genomes, and the evidence for their causality. In spite of the difficulties inherent to the identification of causative variants from genetic mapping and genome-wide association studies, researchers in animal genetics have proposed putative causative variants for several traits relevant to livestock breeding.

RESULTS

For this review, we read the literature that supports potential causative variants in 13 genes (ABCG2, DGAT1, GHR, IGF2, MC4R, MSTN, NR6A1, PHGK1, PRKAG3, PLRL, RYR1, SYNGR2 and VRTN) in cattle and pigs, and localized them in contemporary reference genomes. We review the evidence for their causality, by aiming to separate the evidence for the locus, the proposed causative gene and the proposed causative variant, and report the bioinformatic searches and tactics needed to localize the sequence variants in the cattle or pig genome.

CONCLUSIONS

Taken together, there is usually good evidence for the association at the locus level, some evidence for a specific causative gene at eight of the loci, and some experimental evidence for a specific causative variant at six of the loci. We recommend that researchers who report new potential causative variants use referenced coordinate systems, show local sequence context, and submit variants to repositories.

CRISPR/Cas9-Mediated Specific Integration of Fat-1 and IGF-1 at the pRosa26 Locus

Many researchers have focused on knock-in pigs for site-specific integration, but little attention has been given to genetically modified pigs with the targeted integration of multiple recombinant genes. To establish a multigene targeted knock-in editing system, we used the internal ribosome entry site (IRES) and self-cleaving 2A peptide technology to construct a plasmid coexpressing the fatty acid desaturase (Fat-1) and porcine insulin-like growth factor-1 (IGF-1) genes at equal levels. In this study, pigs were genetically modified with multiple genes that were precisely inserted into the pRosa26 locus by using the clustered regularly spaced short palindrome repeat sequence (CRISPR)/CRISPR-related 9 (Cas9) system and somatic cell nuclear transfer technology (SCNT) in combination. Single copies of the Fat-1 and IGF-1 genes were expressed satisfactorily in various tissues of F0-generation pigs. Importantly, gas chromatography analysis revealed a significantly increased n-3 polyunsaturated fatty acid (PUFA) level in these genetically modified pigs, which led to a significant decrease of the n-6 PUFA/n-3 PUFA ratio from 6.982 to 3.122 (*** p < 0.001). In conclusion, the establishment of an editing system for targeted double-gene knock-in in this study provides a reference for the precise integration of multiple foreign genes and lays a foundation for the development of new transgenic pig breeds with multiple excellent phenotypes.

Impact of CRISPR-Cas9-Based Genome Engineering in Farm Animals

Humans are sorely over-dependent on livestock for their daily basic need of food in the form of meat, milk, and eggs. Therefore, genetic engineering and transgenesis provide the opportunity for more significant gains and production in a short span of time. One of the best strategies is the genetic alteration of livestock to enhance the efficiency of food production (e.g., meat and milk), animal health, and welfare (animal population and disease). Moreover, genome engineering in the bovine is majorly focused on subjects such as disease resistance (e.g., tuberculosis), eradicate allergens (e.g., beta-lactoglobulin knock-out), products generation (e.g., meat from male and milk from female), male or female birth specifically (animal sexing), the introduction of valuable traits (e.g., stress tolerance and disease resistance) and their wellbeing (e.g., hornlessness). This review addressed the impressive genome engineering method CRISPR, its fundamental principle for generating highly efficient target-specific guide RNA, and the accompanying web-based tools. However, we have covered the remarkable roadmap of the CRISPR method from its conception to its use in cattle. Additionally, we have updated the comprehensive information on CRISPR-based gene editing in cattle.

HMEJ-mediated site-specific integration of a myostatin inhibitor increases skeletal muscle mass in porcine

As a robust antagonist of myostatin (MSTN), follistatin (FST) is an important regulator of skeletal muscle development, and the delivery of FST to muscle tissue represents a potential therapeutic strategy for muscular dystrophies. The N terminus and FSI domain of FST are the functional domains for MSTN binding. Here, we aimed to achieve site-specific integration of FSI-I-I, including the signal peptide, N terminus, and three FSI domains, into the last codon of the porcine MSTN gene using a homology-mediated end joining (HMEJ)-based strategy mediated by CRISPR-Cas9. Based on somatic cell nuclear transfer (SCNT) technology, we successfully obtained FSI-I-I knockin pigs. H&E staining of longissimus dorsi and gastrocnemius cross-sections showed larger myofiber sizes in FSI-I-I knockin pigs than in controls. Moreover, the Smad and Erk pathways were inhibited, whereas the PI3k/Akt pathway was activated in FSI-I-I knockin pigs. In addition, the levels of MyoD, Myf5, and MyoG transcription were upregulated while that of MRF4 was downregulated in FSI-I-I knockin pigs. These results indicate that the FSI-I-I gene mediates skeletal muscle hypertrophy through an MSTN-related signaling pathway and the expression of myogenic regulatory factors. Overall, FSI-I-I knockin pigs with hypertrophic muscle tissue hold great promise as a therapeutic model for human muscular dystrophies.

25th ANNIVERSARY OF CLONING BY SOMATIC-CELL NUCLEAR TRANSFER: Nuclear transfer and the development of genetically modified/gene edited livestock

The birth and adult development of 'Dolly' the sheep, the first mammal produced by the transfer of a terminally differentiated cell nucleus into an egg, provided unequivocal evidence of nuclear equivalence among somatic cells. This ground-breaking experiment challenged a long-standing dogma of irreversible cellular differentiation that prevailed for over a century and enabled the development of methodologies for reversal of differentiation of somatic cells, also known as nuclear reprogramming. Thanks to this new paradigm, novel alternatives for regenerative medicine in humans, improved animal breeding in domestic animals and approaches to species conservation through reproductive methodologies have emerged. Combined with the incorporation of new tools for genetic modification, these novel techniques promise to (i) transform and accelerate our understanding of genetic diseases and the development of targeted therapies through creation of tailored animal models, (ii) provide safe animal cells, tissues and organs for xenotransplantation, (iii) contribute to the preservation of endangered species, and (iv) improve global food security whilst reducing the environmental impact of animal production. This review discusses recent advances that build on the conceptual legacy of nuclear transfer and – when combined with gene editing – will have transformative potential for medicine, biodiversity and sustainable agriculture. We conclude that the potential of these technologies depends on further fundamental and translational research directed at improving the efficiency and safety of these methods.

Long-term, multidomain analyses to identify the breed and allelic effects in MSTN-edited pigs to overcome lameness and sustainably improve nutritional meat production

Beef and mutton production has been aided by breeding to integrate allelic diversity for myostatin (MSTN), but a lack of diversity in the MSTN germplasm has limited similar advances in pig farming. Moreover, insurmountable challenges with congenital lameness and a dearth of data about the impacts of feed conversion, reproduction, and meat quality in MSTN-edited pigs have also currently blocked progress. Here, in a largest-to-date evaluation of multiple MSTN-edited pig populations, we demonstrated a practical alternative edit-site-based solution that overcomes the major production obstacle of hindlimb weakness. We also provide long-term and multidomain datasets for multiple breeds that illustrate how MSTN-editing can sustainably increase the yields of breed-specific lean meat and the levels of desirable lipids without deleteriously affecting feed-conversion rates or litter size. Apart from establishing a new benchmark for the data scale and quality of genome-edited animal production, our study specifically illustrates how gene-editing site selection profoundly impacts the phenotypic outcomes in diverse genetic backgrounds.

The advancements, challenges, and future implications of the CRISPR/Cas9 system in swine research

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) genome editing technology has dramatically influenced swine research by enabling the production of high-quality disease-resistant pig breeds, thus improving yields. In addition, CRISPR/Cas9 has been used extensively in pigs as one of the tools in biomedical research. In this review, we present the advancements of the CRISPR/Cas9 system in swine research, such as animal breeding, vaccine development, xenotransplantation, and disease modeling. We also highlight the current challenges and some potential applications of the CRISPR/Cas9 technologies.

Porcine germline genome engineering

Germline editing, the process by which the genome of an individual is edited in such a way that the change is heritable, has been applied to a wide variety of animals [D. A. Sorrell, A. F. Kolb, Biotechnol. Adv. 23, 431-469 (2005); D. Baltimore et al., Science 348, 36-38 (2015)]. Because of its relevancy in agricultural and biomedical research, the pig genome has been extensively modified using a multitude of technologies [K. Lee, K. Farrell, K. Uh, Reprod. Fertil. Dev. 32, 40-49 (2019); C. Proudfoot, S. Lillico, C. Tait-Burkard, Anim. Front. 9, 6-12 (2019)]. In this perspective, we will focus on using pigs as the model system to review the current methodologies, applications, and challenges of mammalian germline genome editing. We will also discuss the broad implications of animal germline editing and its clinical potential.

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.

Practical Approaches for Knock-Out Gene Editing in Pigs

Pigs are an important resource for meat production and serve as a model for human diseases. Due to their physiological and anatomical similarities to humans, these animals can recapitulate symptoms of human diseases, becoming an effective model for biomedical research. Although, in the past pig have not been widely used partially because of the difficulty in genetic modification; nowadays, with the new revolutionary technology of programmable nucleases, and fundamentally of the CRISPR-Cas9 systems, it is possible for the first time to precisely modify the porcine genome as never before. To this purpose, it is necessary to introduce the system into early stage zygotes or to edit cells followed by somatic cell nuclear transfer. In this review, several strategies for pig knock-out gene editing, using the CRISPR-Cas9 system, will be summarized, as well as genotyping methods and different delivery techniques to introduce these tools into the embryos. Finally, the best approaches to produce homogeneous, biallelic edited animals will be discussed.