Improving the generation of genomic-type transgenic mice by ICSI

Preclinical tumor mouse models for studying esophageal cancer

Preclinical models are extensively employed in cancer research because they can be manipulated in terms of their environment, genome, molecular biology, organ systems, and physical activity to mimic human behavior and conditions. The progress made in in vivo cancer research has resulted in significant advancements, enabling the creation of spontaneous, metastatic, and humanized mouse models. Most recently, the remarkable and extensive developments in genetic engineering, particularly the utilization of CRISPR/Cas9, transposable elements, epigenome modifications, and liquid biopsies, have further facilitated the design and development of numerous mouse models for studying cancer. In this review, we have elucidated the production and usage of current mouse models, such as xenografts, chemical-induced models, and genetically engineered mouse models (GEMMs), for studying esophageal cancer. Additionally, we have briefly discussed various gene-editing tools that could potentially be employed in the future to create mouse models specifically for esophageal cancer research.

Animal models of SARS-CoV-2 and COVID-19 for the development of prophylactic and therapeutic interventions

Infections of the Coronavirus SARS-CoV-2 continue to spread around the globe, causing Coronavirus Disease (COVID)-19. Infected people are at risk of developing acute interstitial pneumonia, which can result in lethal complications, particularly in patients with pre-existing co-morbidities. Novel prophylactic and therapeutic interventions are urgently needed to limit the infection-associated health risk for the population and to contain the pandemic. Animal models are indispensable to assessing the efficacy and safety of potential new antivirals, vaccines, and other innovative therapies, such as nucleic acid agonists of innate immune sensing receptors. In this review, we provide an overview of the commonly used animal models to study SARS-CoV-2 and COVID-19, including a summary of their susceptibility to infection, the spectrum of symptoms elicited, and the potential for drug development in each model. We hope that this review will help researchers to decide on the right model organism to quickly address their specific scientific questions.

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.

Transgene Recombineering in Bacterial Artificial Chromosomes

Bacterial Artificial Chromosome (BAC) libraries are a valuable research resource. Any one of the clones in these libraries can carry hundreds of thousands of base pairs of genetic information. Often the entire coding sequence and significant upstream and downstream regions, including regulatory elements, can be found in a single BAC clone. BACs can be put to many uses, such as to study the function of human genes in knockout mice, to drive reporter gene expression in transgenic animals, and for gene discovery. In order to use BACs for experimental purposes it is often desirable to genetically modify them by introducing reporter elements or heterologous cDNA sequences. It is not feasible to use conventional DNA cloning approaches to modify BACs due to their size and complexity, thus a specialized field "recombineering" has developed to modify BAC clones through the use of homologous recombination in bacteria with short homology regions. Genetically engineered BACs can then be used in cell culture, mouse, or rat models to study cancer, neurology, and genetics.

Establishment of a transgenic mouse to model ETV7 expressing human tumors

The ETS transcription factor ETV7 has been characterized as a hematopoietic oncoprotein, which requires cooperating mutations for its leukemogenic activity. Although the ETV7 gene is highly conserved among vertebrates, part of the rodents, including Mus musculus, deleted the Etv7 gene locus. Many human hematopoietic malignancies upregulate ETV7 expression but contrary to ETV7’s role in oncogenesis, its physiological role in normal tissues is unknown. To determine the physiological function of ETV7 in vivo and determine its role in tumorigenesis in a mouse model, we have generated an ETV7 transgenic mouse that carries a single copy of human BAC DNA containing the ETV7 gene locus and its regulatory sequences. ETV7 heterozygous (ETV7Tg^+/WT) mice were fertile, normal in size and born at a normal Mendelian frequency. They had a normal blood count, did not display any gross physical or behavioral abnormalities, and were not tumor-prone. The ETV7 expression pattern in hematopoietic cells of ETV7Tg^+/WT mice is very similar to that in human hematopoietic cells. To examine the oncogenic potential of ETV7 in vivo, we crossed ETV7Tg^+/WT mice with tumor-prone mouse models. ETV7 greatly accelerated loss of Pten (phosphatase and tensin homolog)-evoked leukemogenesis in Pten^Δ/ΔETV7Tg^+/WT mice after deletion of the conditional Pten allele. Consistent with this observation, ETV7 expression enhanced the colony-forming and self-renewal activities of primary myeloid Pten^−/− cells. In this study we established a transgenic mouse in which we can more accurately model ETV7-associated human tumorigenesis in vivo.

Purification of Yeast Artificial Chromosome DNA for Microinjection Using a Two-Gel Electrophoresis Procedure

This protocol describes a method to purify yeast artificial chromosome (YAC) DNA for microinjection. YAC DNA solutions cannot be concentrated by standard DNA precipitation methods and resuspended into smaller volumes. Attempts to precipitate YAC DNA solutions will, unavoidably, break and destroy the DNA molecules. Therefore YAC DNA must always be kept in a solution that maintains the integrity of the YAC DNA molecules. In the method described here, YAC DNA is extracted from pulsed-field gel electrophoresis (PFGE) agarose plugs by using two gel electrophoresis steps. The first gel electrophoresis step is the PFGE itself. The region of agarose containing the YAC DNA of interest is excised and run on a standard electrophoresis gel setup at a 90° angle to the PFGE run. The YAC DNA migrates out of the agarose slice and enters into a thicker low-melting agarose gel, thereby promoting compaction of YAC DNA molecules. The aim of this method is to convert a slice of agarose into a cube of agarose of smaller volume. The volume of this cube will determine the final concentration of the YAC DNA precipitation.

Efficient generation of sFat-1 transgenic rabbits rich in n-3 polyunsaturated fatty acids by intracytoplasmic sperm injection

N-3 polyunsaturated fatty acids (n-3 PUFAs) have their first double bond at the third carbon from the methyl end of the fatty-acid chain and had been proven to be beneficial to human health. However, mammals cannot produce n-3 PUFAs by themselves because they lack the n-3 fatty-acid desaturase (Fat-1) gene. Thus, the possibility of producing sFat-1 transgenic rabbits was explored in this study. The transgenic cassette of pPGK1-sFat-1-CMV-EGFP was constructed and transgenic rabbit embryos were produced by intracytoplasmic sperm injection (ICSI). When 123 EGFP-positive embryos at the 2-8-cell stage were transplanted into the oviduct of four oestrous-synchronised recipients, two of them became pregnant and gave birth to seven pups. However, transfer of embryos into the uterus of oestrous-synchronised recipients and oviduct or uterus of oocyte donor rabbits did not result in pregnancy. The integration of the sFat-1 gene was confirmed in six of the seven live pups by PCR and Southern blot. The expression of the sFat-1 gene in the six transgenic pups was also detected by reverse transcription polymerase chain reaction (RT-PCR). Gas chromatography-mass spectrometry analysis revealed that transgenic rabbits exhibited an ~15-fold decrease in the ratio of n-6:n-3 PUFAs in muscle compared with wild-type rabbits and non-transgenic rabbits. These results demonstrate that sFat-1 transgenic rabbits can be produced by ICSI and display a low ratio of n-6:n-3 PUFAs.

RNA interference-based technology: what role in animal agriculture?

Animal agriculture faces a broad array of challenges, ranging from disease threats to adverse environmental conditions, while attempting to increase productivity using fewer resources. RNA interference (RNAi) is a biological phenomenon with the potential to provide novel solutions to some of these challenges. Discovered just 20 years ago, the mechanisms underlying RNAi are now well described in plants and animals. Intracellular double-stranded RNA triggers a conserved response that leads to cleavage and degradation of complementary mRNA strands, thereby preventing production of the corresponding protein product. RNAi can be naturally induced by expression of endogenous microRNA, which are critical in the regulation of protein synthesis, providing a mechanism for rapid adaptation of physiological function. This endogenous pathway can be co-opted for targeted RNAi either through delivery of exogenous small interfering RNA (siRNA) into target cells or by transgenic expression of short hairpin RNA (shRNA). Potentially valuable RNAi targets for livestock include endogenous genes such as developmental regulators, transcripts involved in adaptations to new physiological states, immune response mediators, and also exogenous genes such as those encoded by viruses. RNAi approaches have shown promise in cell culture and rodent models as well as some livestock studies, but technical and market barriers still need to be addressed before commercial applications of RNAi in animal agriculture can be realised. Key challenges for exogenous delivery of siRNA include appropriate formulation for physical delivery, internal transport and eventual cellular uptake of the siRNA; additionally, rigorous safety and residue studies in target species will be necessary for siRNA delivery nanoparticles currently under evaluation. However, genomic incorporation of shRNA can overcome these issues, but optimal promoters to drive shRNA expression are needed, and genetic engineering may attract more resistance from consumers than the use of exogenous siRNA. Despite these hurdles, the convergence of greater understanding of RNAi mechanisms, detailed descriptions of regulatory processes in animal development and disease, and breakthroughs in synthetic chemistry and genome engineering has created exciting possibilities for using RNAi to enhance the sustainability of animal agriculture.

Production of transgenic mice expressing tumor virus A under ovarian‑specific promoter 1 control using testis‑mediated gene transfer

The aim of the present study was to produce transgenic mice expressing tumor virus A (TVA) in the ovary under ovarian specific promoter 1 (OSP1) control. A transgenic mouse model was established in which TVA, an avian retroviral receptor gene driven by OSP1, was selectively expressed in the ovary. A recombinant plasmid containing TVA cDNA and an OSP1 promoter was constructed. The DNA fragment was repeatedly injected into male mouse testes at multiple sites. At 4‑7, 7‑10 and 10‑13 weeks following the final injection, two DNA‑injected male mice were mated with four wild‑type female mice to produce transgenic mice. The transgenic positive rate in mouse F1 offspring was 39.69%. When the positive F1 individuals were mated with wild‑type Imprinting Control Region mice (PxW) or with positive F1 individuals (PxP), the F2 individuals had a transgenic rate of 12.44%. The transgenic rates in the F1 offspring, produced following mating at the three time intervals, were 55.71 (39/70), 30.77 (4/13) and 18.75% (9/48), respectively. The transgenic rates of the F2 offspring decreased with the age of the F1 offspring, from 26.67% when PxP were mated at 6‑8 weeks of age to 6.52% when PxW were mated at 5‑6 months of age. The results indicate a high efficiency of gene transfer to F1 offspring using testis‑mediated gene transfer (TMGT). The transgenic rate in the F2 offspring was lower than that in the F1 offspring. The results reveal that TMGT is suitable for creating transgenic animals among F1 offspring. Semi‑quantitative reverse transcription-polymerase chain reaction results showed that TVA was expressed in the mice ovaries. The results demonstrate the importance of using the replication‑competent avian sarcoma‑leukosis virus long terminal repeat with a splice acceptor‑TVA system in ovarian tumorigenesis research.

Generation of transgenic mice with megabase-sized human yeast artificial chromosomes by yeast spheroplast-embryonic stem cell fusion

Introducing human genes into mice offers the opportunity to analyze their in vivo function or to obtain therapeutic molecules. For proper gene regulation, or in case of multigene families, megabase (Mb)-sized DNA fragments often have to be used. Yeast artificial chromosome (YAC)-mediated transgenesis is irreplaceable for this purpose, because alternative methods such as the use of bacterial artificial chromosomes (BACs) cannot introduce DNA fragments larger than 500 kb into the mouse germ line. However, YAC libraries often contain only partial gene loci. Time-consuming reconstruction of YACs, genetic instability and the difficulty in obtaining intact YAC DNA above a certain size impede the generation of humanized mice. Here we describe how to reconstruct YACs containing Mb-sized human DNA, such as the T cell receptor-α (TRA) gene locus, thus facilitating the introduction of large DNA fragments into the mouse germ line. Fusion of YAC-containing yeast and embryonic stem (ES) cells avoids the need for YAC DNA purification. These ES cells are then used to stably introduce the functional TRA gene locus into the mouse germ line. The protocol takes ∼1 year to complete, from reconstruction of the entire TRA gene locus from YACs containing partial but overlapping TRA regions to germline transmission of the YAC.

Modeling human neurodegenerative diseases in transgenic systems

Transgenic systems are widely used to study the cellular and molecular basis of human neurodegenerative diseases. A wide variety of model organisms have been utilized, including bacteria (Escherichia coli), plants (Arabidopsis thaliana), nematodes (Caenorhabditis elegans), arthropods (Drosophila melanogaster), fish (zebrafish, Danio rerio), rodents (mouse, Mus musculus and rat, Rattus norvegicus) as well as non-human primates (rhesus monkey, Macaca mulatta). These transgenic systems have enormous value for understanding the pathophysiological basis of these disorders and have, in some cases, been instrumental in the development of therapeutic approaches to treat these conditions. In this review, we discuss the most commonly used model organisms and the methodologies available for the preparation of transgenic organisms. Moreover, we provide selected examples of the use of these technologies for the preparation of transgenic animal models of neurodegenerative diseases, including Alzheimer's disease (AD), frontotemporal lobar degeneration (FTLD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and Parkinson's disease (PD) and discuss the application of these technologies to AD as an example of how transgenic modeling has affected the study of human neurodegenerative diseases.

Mendel: a simple excel workbook to compare the observed and expected distributions of genotypes/phenotypes in transgenic and knockout mouse crosses involving up to three unlinked loci by means of a χ2 test

The analysis of transgenic and knockout mice always involves the establishment of matings with individuals carrying different loci, segregating independently, whose presence is expected among the progeny, according to a Mendelian distribution. The appearance of distorted inheritance ratios suggests the existence of unexpected lethal or sub-lethal phenotypes associated with some genotypes. These situations are common in a number of cases, including: testing transgenic founder mice for germ-line transmission of their transgenes; setting up heterozygous crosses to obtain homozygous individuals, both for transgenic and knockout mice; establishing matings between floxed mouse lines and suitable cre transgenic mouse lines, etc. The Pearson's χ(2) test can be used to assess the significance of the observed frequencies of genotypes/phenotypes in relation to the expected values, in order to determine whether the observed cases fit the expected distribution. Here, I describe a simple Excel workbook to compare the observed and expected distributions of genotypes/phenotypes in transgenic and knockout mouse crosses involving up to three unlinked loci by means of a χ(2) test. The file is freely available for download from my laboratory's web page at: http://www.cnb.csic.es/~montoliu/Mendel.xls .

Prospects for the use of artificial chromosomes and minichromosome-like episomes in gene therapy

Artificial chromosomes and minichromosome-like episomes are large DNA molecules capable of containing whole genomic loci, and be maintained as nonintegrating, replicating molecules in proliferating human somatic cells. Authentic human artificial chromosomes are very difficult to engineer because of the difficulties associated with centromere structure, so they are not widely used for gene-therapy applications. However, OriP/EBNA1-based episomes, which they lack true centromeres, can be maintained stably in dividing cells as they bind to mitotic chromosomes and segregate into daughter cells. These episomes are more easily engineered than true human artificial chromosomes and can carry entire genes along with all their regulatory sequences. Thus, these constructs may facilitate the long-term persistence and physiological regulation of the expression of therapeutic genes, which is crucial for some gene therapy applications. In particular, they are promising vectors for gene therapy in inherited diseases that are caused by recessive mutations, for example haemophilia A and Friedreich's ataxia. Interestingly, the episome carrying the frataxin gene (deficient in Friedreich's ataxia) has been demonstrated to rescue the susceptibility to oxidative stress which is typical of fibroblasts from Friedreich's ataxia patients. This provides evidence of their potential to treat genetic diseases linked to recessive mutations through gene therapy.

Design of expression cassettes for the generation of transgenic animals (including insulators)

The use of transgenesis is relatively rare in rats, and this is because of the relative difficulty in adding foreign genes by the conventional methods. Gene knock out and knock in by the conventional techniques of homologous recombination remain difficult in rats. This situation would be less crucial if the gene constructs were more reliable for the expression of foreign genes. The present chapter describes the state of the art in vector design for various genetic modifications in rats.

[Applications of genetically modified animals]

The first transgenic animals, mice, were obtained in 1980. The techniques of gene transfer had to be adapted to obtain transgenic animals with an acceptable yield in about fifteen species. When the yield is low (low rate of random integration and targeted integration via homologous recombination), genetic modifications must be achieved in intermediate cells able to participate to the development of chimeric transgenic animals (ES cells, EG cells, iPS obtained by the dedifferentiation of somatic cells) or in somatic cells used as nuclear donor to generate transgenic clones. Various tools make possible a marked increase of homologous recombination efficiency (meganucleases and ZFN), or a gene inactivation at the genome level (direct or conditional knock out) or at the mRNA level (interfering RNAs). Vectors allow a more reliable transgene expression. Genetically modified animals are used mainly to obtain information on biological functions and human diseases. Transgenic animals produce recombinant pharmaceutical proteins in milk and soon in egg white. Pig organs adapted to be tolerated by patients might be tested in humans in five years. The projects based on the use of transgenesis to improve animal production are presently few. Transgenic salmon with accelerated growth might be on the market when their possible escape in oceans will be controlled.

Animal transgenesis: an overview

Transgenic animals are extensively used to study in vivo gene function as well as to model human diseases. The technology for producing transgenic animals exists for a variety of vertebrate and invertebrate species. The mouse is the most utilized organism for research in neurodegenerative diseases. The most commonly used techniques for producing transgenic mice involves either the pronuclear injection of transgenes into fertilized oocytes or embryonic stem cell-mediated gene targeting. Embryonic stem cell technology has been most often used to produce null mutants (gene knockouts) but may also be used to introduce subtle genetic modifications down to the level of making single nucleotide changes in endogenous mouse genes. Methods are also available for inducing conditional gene knockouts as well as inducible control of transgene expression. Here, we review the main strategies for introducing genetic modifications into the mouse, as well as in other vertebrate and invertebrate species. We also review a number of recent methodologies for the production of transgenic animals including retrovirus-mediated gene transfer, RNAi-mediated gene knockdown and somatic cell mutagenesis combined with nuclear transfer, methods that may be more broadly applicable to species where both pronuclear injection and ES cell technology have proven less practical.

A survey to establish performance standards for the production of transgenic mice

The generation of transgenic mice by microinjection of DNA into the pronuclei of fertilized oocytes was described in the early 1980s. A number of parameters affecting the efficiency of the technique were soon identified, including the type of DNA construct, the concentration of DNA being injected, and, most importantly, the strain of mice used for oocyte donors. Since then, hundreds of laboratories and transgenic core facilities across the world have successfully used this technique, essentially as originally described, to create thousands of new transgenic mouse lines. However, the overall procedure continues to be relatively inefficient, in terms of the number of fertilized oocytes required to produce a transgenic mouse, and variations in yields from day to day and construct to construct can be large. Consequently, core facilities often struggle to explain to their customers why a sufficient number of transgenic founders were not produced from a given construct. We believe the field (and individual facilities) would benefit from a rigorous assessment of average yields and expected variations in yields. To this end, we have initiated a survey from the International Society for Transgenic Technologies (ISTT) web site ( www.transtechsociety.org ), to obtain raw microinjection data from as many facilities as possible. We intend to use this data to establish performance standards for the field. Existing facilities will be able to refer to these standards in dealing with dissatisfied clients, and new facilities will be able to aim for an achievable goal. We may even be able to discover an optimum combination of factors that will allow every facility to achieve higher yields.

Use of intracytoplasmic sperm injection (ICSI) to generate transgenic animals

Even though intracytoplasmic sperm injection (ICSI) has been widely used for the production of offspring in human infertility clinics and in reproductive research laboratories using mice, many researchers engaged in animal transgenesis still consider it somewhat cumbersome. The greatest advantage of ICSI-mediated transgenesis is that it allows introduction of very large DNA transgenes (e.g., yeast artificial chromosomes), with relatively high efficiency into the genomes of hosts, as compared to pronuclear injection. Recently, we have developed an active form of intracytoplasmic sperm injection-mediated transgenesis (ICSI-Tr) with fresh sperm utilizing transposons. The transgenic efficiencies rival all transgenic techniques except that of lentiviral methods.

Transgene design and delivery into the mouse genome: keys to success

Mouse transgenesis by pronuclear injection generates random integration events resulting in variable overexpression efficiency. This chapter will outline current strategies for improved transgene design (using plasmids or bacterial artificial chromosomes) and delivery to the mouse genome. The choice of genetic background of embryos for microinjection will also be discussed.

Intracytoplasmic sperm injection in livestock species: an update

Intracytoplasmic sperm injection (ICSI) is a powerful technique in the field of assisted reproduction (ART) and provides exciting opportunities for studying the basic mechanisms of fertilization and early embryo development. Nevertheless, its application in agriculture and conservation biology has been greatly hampered by the low success rate reported for this method in respect of economically important species. Specifically, the rates of blastocyst formation and live newborn are greatly reduced when zygotes are generated by ICSI. Except for humans, ICSI remains a low efficiency technology in comparison with alternatives such as in vitro fertilization (IVF) and its application is less widespread. In this paper, we discuss the present status, applications and factors affecting ICSI in pigs and other species.

Production of transgenic farm animals by viral vector-mediated gene transfer

Transgenic technology holds considerable promise to advance understanding in biomedical and agricultural systems with some believing that one day transgenic animals may directly contribute to farming and breeding practice. Nevertheless, applications in livestock have been restricted in part by the inefficiency of the technology. The recent development of lentivirus vectors for transgene delivery may overcome some of this limitation. This presentation describes these vectors, their advantages and limitations.