Transgenic sheep generated by lentiviral vectors: safety and integration analysis of surrogates and their offspring

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

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

Effects of AANAT overexpression on the inflammatory responses and autophagy activity in the cellular and transgenic animal levels

To explore the anti-inflammatory activity of endogenous produced melatonin, a melatonin-enriched animal model (goat) with AANAT transfer was successfully generated with somatic cell nuclear transfer (SCNT) technology. Basically, a pIRES2-EGFP-AANAT expression vector was constructed and was transferred into the female fetal fibroblast cells (FFCs) via electrotransfection and then the nuclear of the transgenic FFC was transferred to the eggs of the donor goats. The peripheral blood mononuclear cells (PBMCs) of the transgenic offspring expressed significantly higher levels of AANAT and melatonin synthetic function than those PBMCs from the wild-type (WT) animals. After challenge with lipopolysaccharide (LPS), the transgenic PBMCs had increased autophagosomes and LC3B expression while they exhibited suppressed production of the proinflammatory cytokines, IL1B and IL12 (IL12A-IL12B/p70), compared to their WT. The mechanistic analysis indicated that the anti-inflammatory activity of endogenous melatonin was mediated by MTNR1B (melatonin receptor 1B). MTNR1B stimulation activated the MAPK14 signaling pathway to promote cellular macroautophagy/autophagy, thus, suppressing the excessive inflammatory response of cellular. However, when the intact animals challenged with LPS, the serum proinflammatory cytokines were significantly higher in the transgenic goats than that in the WT. The results indicated that endogenous melatonin inhibited the MAPK1/3 signaling pathway and ROS production, subsequently downregulated gene expression of BECN1, ATG5 in PMBCs and then suppressed the autophagy activity of PBMCs and finally elevated levels of serum proinflammatory cytokines in transgenic animals, Herein we provided a novel melatonin-enriched animal model to study the potential effects of endogenously produced melatonin on inflammatory responses and autophagy activity.

Multiplex gene editing via CRISPR/Cas9 exhibits desirable muscle hypertrophy without detectable off-target effects in sheep

The CRISPR/Cas9 system provides a flexible approach for genome engineering of genetic loci. Here, we successfully achieved precise gene targeting in sheep by co-injecting one-cell-stage embryos with Cas9 mRNA and RNA guides targeting three genes (MSTN, ASIP, and BCO2). We carefully examined the sgRNAs:Cas9-mediated targeting effects in injected embryos, somatic tissues, as well as gonads via cloning and sequencing. The targeting efficiencies in these three genes were within the range of 27–33% in generated lambs, and that of simultaneously targeting the three genes was 5.6%, which demonstrated that micro-injection of zygotes is an efficient approach for generating gene-modified sheep. Interestingly, we observed that disruption of the MSTN gene resulted in the desired muscle hypertrophy that is characterized by enlarged myofibers, thereby providing the first detailed evidence supporting that gene modifications had occurred at both the genetic and morphological levels. In addition, prescreening for the off-target effect of sgRNAs was performed on fibroblasts before microinjection, to ensure that no detectable off-target mutations from founder animals existed. Our findings suggested that the CRISPR/Cas9 method can be exploited as a powerful tool for livestock improvement by simultaneously targeting multiple genes that are responsible for economically significant traits.

New insights and current tools for genetically engineered (GE) sheep and goats

Genetically engineered sheep and goats represent useful models applied to proof of concepts, large-scale production of novel products or processes, and improvement of animal traits, which is of interest in biomedicine, biopharma, and livestock. This disruptive biotechnology arose in the 80s by injecting DNA fragments into the pronucleus of zygote-staged embryos. Pronuclear microinjection set the transgenic concept into people's mind but was characterized by inefficient and often frustrating results mostly because of uncontrolled and/or random integration and unpredictable transgene expression. Somatic cell nuclear transfer launched the second wave in the late 90s, solving several weaknesses of the previous technique by making feasible the transfer of a genetically modified and fully characterized cell into an enucleated oocyte, capable of cell reprogramming to generate genetically engineered animals. Important advances were also achieved during the 2000s with the arrival of new techniques like the lentivirus system, transposons, RNA interference, site-specific recombinases, and sperm-mediated transgenesis. We are now living the irruption of the third technological wave in which genome edition is possible by using endonucleases, particularly the CRISPR/Cas system. Sheep and goats were recently produced by CRISPR/Cas9, and for sure, cattle will be reported soon. We will see new genetically engineered farm animals produced by homologous recombination, multiple gene editing in one-step generation and conditional modifications, among other advancements. In the following decade, genome edition will continue expanding our technical possibilities, which will contribute to the advancement of science, the development of clinical or commercial applications, and the improvement of people's life quality around the world.

Embryo development, fetal growth and postnatal phenotype of eGFP lambs generated by lentiviral transgenesis

Lentiviral technology has been recently proposed to generate transgenic farm animals more efficiently and easier than traditional techniques. The objective was to evaluate several parameters of lambs obtained by lentiviral transgenesis in comparison with non-transgenic counterparts. In vitro produced embryos were microinjected (TG group) at two-cell stage with a lentiviral construct containing enhanced green fluorescent protein (eGFP) gene, while embryos produced by in vitro fertilization (IVF group) or intrauterine insemination (IUI group) were not microinjected. Microinjection technique efficiently generated eight-cell transgenic embryos (97.4%; 114/117). Development rate on day 5 after fertilization was similar for TG (39.3%, 46/117) and IVF embryos (39.6%, 44/111). Pregnancy rate was detected in 50.0% (6/12) of recipient ewes with TG embryos, in 46.7% (7/15) with IVF embryos, and in 65.0% (13/20) of IUI ewes (P = NS). Nine lambs were born in TG group, six lambs in IVF group, and 16 lambs in IUI group. All TG lambs (9/9) were GFP positive to real-time PCR and eight (88.9%) showed a strong and evident GFP expression in mucosae, eyes and keratin tissues. Fetal growth monitored every 15 day by ultrasonography did not show significant differences. Transgenic lambs neither differ in morphometric variables in comparison with non transgenic IVF lambs within 3 months after birth. Transmission of the transgene to the progeny was observed in green fluorescent embryos produced by IVF using semen from the TG founder lambs. In conclusion, this study demonstrates the high efficiency of lentiviral technology to produce transgenic sheep, with no clinic differences in comparison with non transgenic lambs.