Evaluation of OPEN zinc finger nucleases for direct gene targeting of the ROSA26 locus in mouse embryos

Glycogen storage disease type I: Genetic etiology, clinical manifestations, and conventional and gene therapies

Glycogen storage disease type I (GSDI) is an inherited metabolic disorder characterized by a deficiency of enzymes or proteins involved in glycogenolysis and gluconeogenesis, resulting in excessive intracellular glycogen accumulation. While GSDI is classified into four different subtypes based on molecular genetic variants, GSDIa accounts for approximately 80%. GSDIa and GSDIb are autosomal recessive disorders caused by deficiencies in glucose-6-phosphatase (G6Pase-α) and glucose-6-phosphate-transporter (G6PT), respectively. For the past 50 years, the care of patients with GSDI has been improved following elaborate dietary managements. GSDI patients currently receive dietary therapies that enable patients to improve hypoglycemia and alleviate early symptomatic signs of the disease. However, dietary therapies have many limitations with a risk of calcium, vitamin D, and iron deficiency and cannot prevent long-term complications, such as progressive liver and renal failure. With the deepening understanding of the pathogenesis of GSDI and the development of gene therapy technology, there is great progress in the treatment of GSDI. Here, we review the underlying molecular genetics and the current clinical management strategies of GSDI patients with an emphasis on promising experimental gene therapies.

Progress in and Prospects of Genome Editing Tools for Human Disease Model Development and Therapeutic Applications

Programmable nucleases, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas, are widely accepted because of their diversity and enormous potential for targeted genomic modifications in eukaryotes and other animals. Moreover, rapid advances in genome editing tools have accelerated the ability to produce various genetically modified animal models for studying human diseases. Given the advances in gene editing tools, these animal models are gradually evolving toward mimicking human diseases through the introduction of human pathogenic mutations in their genome rather than the conventional gene knockout. In the present review, we summarize the current progress in and discuss the prospects for developing mouse models of human diseases and their therapeutic applications based on advances in the study of programmable nucleases.

Targeted Integration of Transgenes at the Mouse Gt(ROSA)26Sor Locus

The targeting of transgenic constructs at single copy into neutral genomic loci avoids the unpredictable outcomes associated with conventional random integration approaches. The Gt(ROSA)26Sor locus on chromosome 6 has been used many times for the integration of transgenic constructs and is known to be permissive for transgene expression and disruption of the gene is not associated with a known phenotype. Furthermore, the transcript made from the Gt(ROSA)26Sor locus is ubiquitously expressed and subsequently the locus can be used to drive the ubiquitous expression of transgenes.Here we report a protocol for the generation of targeted transgenic alleles at Gt(ROSA)26Sor, taking as an example a conditional overexpression allele, by PhiC31 integrase/recombinase-mediated cassette exchange of an engineered Gt(ROSA)26Sor locus in mouse embryonic stem cells. The overexpression allele is initially silenced by the presence of a loxP flanked stop sequence but can be strongly activated through the action of Cre recombinase.

Production of Transgenic Handmade Cloned Goat (Capra hircus) Embryos by Targeted Integration into Rosa 26 Locus Using Transcription Activator-like Effector Nucleases

Transgenic goats are ideal bioreactors for the production of therapeutic proteins in their mammary glands. However, random integration of the transgene within-host genome often culminates in unstable expression and unpredictable phenotypes. Targeting desired genes to a safe locus in the goat genome using advanced targeted genome-editing tools, such as transcription activator-like effector nucleases (TALENs) might assist in overcoming these hurdles. We identified Rosa 26 locus, a safe harbor for transgene integration, on chromosome 22 in the goat genome for the first time. We further demonstrate that TALEN-mediated targeting of GFP gene cassette at Rosa 26 locus exhibited stable and ubiquitous expression of GFP gene in goat fetal fibroblasts (GFFs) and after that, transgenic cloned embryos generated by handmade cloning (HMC). The transfection of GFFs by the TALEN pair resulted in 13.30% indel frequency at the target site. Upon cotransfection with TALEN and donor vectors, four correctly targeted cell colonies were obtained and all of them showed monoallelic gene insertions. The blastocyst rate for transgenic cloned embryos (3.92% ± 1.12%) was significantly (p < 0.05) lower than cloned embryos (7.84% ± 0.68%) used as control. Concomitantly, 2 out of 15 embryos of morulae and blastocyst stage (13.30%) exhibited site-specific integration. In conclusion, the present study demonstrates TALEN-mediated transgene integration at Rosa 26 locus in caprine fetal fibroblasts and the generation of transgenic cloned embryos using HMC.

The evolution and history of gene editing technologies

Scientific enquiry must be the driving force of research. This sentiment is manifested as the profound impact gene editing technologies are having in our current world. There exist three main gene editing technologies today: Zinc Finger Nucleases, TALENs and the CRISPR-Cas system. When these systems were being uncovered, none of the scientists set out to design tools to engineer genomes. They were simply trying to understand the mechanisms existing in nature. If it was not for this simple sense of wonder, we probably would not have these breakthrough technologies. In this chapter, we will discuss the history, applications and ethical issues surrounding these technologies, focusing on the now predominant CRISPR-Cas technology. Gene editing technologies, as we know them now, are poised to have an overwhelming impact on our world. However, it is impossible to predict the route they will take in the future or to comprehend the full impact of its repercussions.

Recent advances in the CRISPR genome editing tool set

Genome editing took a dramatic turn with the development of the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated proteins (Cas) system. The CRISPR-Cas system is functionally divided into classes 1 and 2 according to the composition of the effector genes. Class 2 consists of a single effector nuclease, and routine practice of genome editing has been achieved by the development of the Class 2 CRISPR-Cas system, which includes the type II, V, and VI CRISPR-Cas systems. Types II and V can be used for DNA editing, while type VI is employed for RNA editing. CRISPR techniques induce both qualitative and quantitative alterations in gene expression via the double-stranded breakage (DSB) repair pathway, base editing, transposase-dependent DNA integration, and gene regulation using the CRISPR-dCas or type VI CRISPR system. Despite significant technical improvements, technical challenges should be further addressed, including insufficient indel and HDR efficiency, off-target activity, the large size of Cas, PAM restrictions, and immune responses. If sophisticatedly refined, CRISPR technology will harness the process of DNA rewriting, which has potential applications in therapeutics, diagnostics, and biotechnology.

A high-efficiency CRISPR/Cas9 system for targeted mutagenesis in Cotton (Gossypium hirsutum L.)

The complex allotetraploid genome is one of major challenges in cotton for repressing gene expression. Developing site-specific DNA mutation is the long-term dream for cotton breeding scientists. The clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system is emerging as a robust biotechnology for targeted-DNA mutation. In this study, two sgRNAs, GhMYB25-like-sgRNA1 and GhMYB25-like-sgRNA2, were designed in the identical genomic regions of GhMYB25-like A and GhMYB25-like D, which were encoded by cotton A subgenome and the D subgenome, respectively, was assembled to direct Cas9-mediated allotetraploid cotton genome editing. High proportion (14.2–21.4%) CRISPR/Cas9-induced specific truncation events, either from GhMYB25-like A DNA site or from GhMYB25-like D DNA site, were detected in 50% examined transgenic cotton through PCR amplification assay and sequencing analyses. Sequencing results also demonstrated that 100% and 98.8% mutation frequency were occurred on GhMYB25-like-sgRNA1 and GhMYB25-like-sgRNA2 target site respectively. The off-target effect was evaluated by sequencing two putative off-target sites, which have 3 and 1 mismatched nucleotides with GhMYB25-like-sgRNA1 and GhMYB25-like-sgRNA2, respectively; all the examined samples were not detected any off-target-caused mutation events. Thus, these results demonstrated that CRISPR/Cas9 is qualified for generating DNA level mutations on allotetraploid cotton genome with high-efficiency and high-specificity.

Concepts and tools for gene editing

Gene editing is a relatively recent concept in the molecular biology field. Traditional genetic modifications in animals relied on a classical toolbox that, aside from some technical improvements and additions, remained unchanged for many years. Classical methods involved direct delivery of DNA sequences into embryos or the use of embryonic stem cells for those few species (mice and rats) where it was possible to establish them. For livestock, the advent of somatic cell nuclear transfer platforms provided alternative, but technically challenging, approaches for the genetic alteration of loci at will. However, the entire landscape changed with the appearance of different classes of genome editors, from initial zinc finger nucleases, to transcription activator-like effector nucleases and, most recently, with the development of clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (Cas). Gene editing is currently achieved by CRISPR–Cas-mediated methods, and this technological advancement has boosted our capacity to generate almost any genetically altered animal that can be envisaged.

Gene editing and its application for hematological diseases

The use of precise, rationally designed gene-editing nucleases allows for targeted genome and transcriptome modification, and at present, four major classes of nucleases are being employed: zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), meganucleases (MNs), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9. Each reagent shares the ability to recognize and bind a target sequence of DNA. Depending on the properties of the reagent, the DNA can be cleaved on one or both strands, or epigenetic changes can be mediated. These novel properties can impact hematological disease by allowing for: (1) direct modification of hematopoietic stem/progenitor cells (HSPCs), (2) gene alteration of hematopoietic lineage committed terminal effectors, (3) genome engineering in non-hematopoietic cells with reprogramming to a hematopoietic phenotype, and (4) transcriptome modulation for gene regulation, modeling, and discovery.

T cell-specific inactivation of mouse CD2 by CRISPR/Cas9

The CRISPR/Cas9 system can be used to mutate target sequences by introduction of double-strand breaks followed by imprecise repair. To test its use for conditional gene editing we generated mice transgenic for CD4 promoter-driven Cas9 combined with guide RNA targeting CD2. We found that within CD4(+) and CD8(+) lymphocytes from lymph nodes and spleen 1% and 0.6% were not expressing CD2, respectively. T cells lacking CD2 carryied mutations, which confirmed that Cas9 driven by cell-type specific promoters can edit genes in the mouse and may thus allow targeted studies of gene function in vivo.

Efficient genome engineering by targeted homologous recombination in mouse embryos using transcription activator-like effector nucleases

Generation of mouse models by introducing transgenes using homologous recombination is critical for understanding fundamental biology and pathology of human diseases. Here we investigate whether artificial transcription activator-like effector nucleases (TALENs)-powerful tools that induce DNA double-strand breaks at specific genomic locations-can be combined with a targeting vector to induce homologous recombination for the introduction of a transgene in embryonic stem cells and fertilized murine oocytes. We describe the generation of a conditional mouse model using TALENs, which introduce double-strand breaks at the genomic locus of the special AT-rich sequence-binding protein-1 in combination with a large 14.4 kb targeting template vector. We report successful germline transmission of this allele and demonstrate its recombination in primary cells in the presence of Cre-recombinase. These results suggest that TALEN-assisted induction of DNA double-strand breaks can facilitate homologous recombination of complex targeting constructs directly in oocytes.

Finding of a highly efficient ZFN pair for Aqpep gene functioning in murine zygotes

The generation efficiencies of mutation-induced mice when using engineered zinc-finger nucleases (ZFNs) have been generally 10 to 20% of obtained pups in previous studies. The discovery of high-affinity DNA-binding modules can contribute to the generation of various kinds of novel artificial chromatin-targeting tools, such as zinc-finger acetyltransferases, zinc-finger histone kinases and so on, as well as improvement of reported zinc-finger recombinases and zinc-finger methyltransferases. Here, we report a novel ZFN pair that has a highly efficient mutation-induction ability in murine zygotes. The ZFN pair induced mutations in all obtained mice in the target locus, exon 17 of aminopeptidase Q gene, and almost all of the pups had biallelic mutations. This high efficiency was also shown in the plasmid DNA transfected in a cultured human cell line. The induced mutations were inherited normally in the next generation. The zinc-finger modules of this ZFN pair are expected to contribute to the development of novel ZF-attached chromatin-targeting tools.

Homology-directed repair in rodent zygotes using Cas9 and TALEN engineered proteins

The generation of genetically-modified organisms has been revolutionized by the development of new genome editing technologies based on the use of gene-specific nucleases, such as meganucleases, ZFNs, TALENs and CRISPRs-Cas9 systems. The most rapid and cost-effective way to generate genetically-modified animals is by microinjection of the nucleic acids encoding gene-specific nucleases into zygotes. However, the efficiency of the procedure can still be improved. In this work we aim to increase the efficiency of CRISPRs-Cas9 and TALENs homology-directed repair by using TALENs and Cas9 proteins, instead of mRNA, microinjected into rat and mouse zygotes along with long or short donor DNAs. We observed that Cas9 protein was more efficient at homology-directed repair than mRNA, while TALEN protein was less efficient than mRNA at inducing homology-directed repair. Our results indicate that the use of Cas9 protein could represent a simple and practical methodological alternative to Cas9 mRNA in the generation of genetically-modified rats and mice as well as probably some other mammals.

Efficient gene targeting of the Rosa26 locus in mouse zygotes using TALE nucleases

Gene targeting in mice mainly employs homologous recombination (HR) in embryonic stem (ES) cells. Although it is a standard way for production of genetically modified mice, the procedure is laborious and time-consuming. This study describes targeting of the mouse Rosa26 locus by transcription activator-like effector nucleases (TALENs). We employed TALEN-assisted HR in zygotes to introduce constructs encoding TurboRFP and TagBFP fluorescent proteins into the first intron of the Rosa26 gene, and in this way generated two transgenic mice. We also demonstrated that these Rosa26-specific TALENs exhibit high targeting efficiency superior to that of zinc-finger nucleases (ZFNs) specific for the same targeting sequence. Moreover, we devised a reporter assay to assess TALENs activity and specificity to improve the quality of TALEN-assisted targeting.

The new CRISPR-Cas system: RNA-guided genome engineering to efficiently produce any desired genetic alteration in animals

The CRISPR-Cas system is the newest targeted nuclease for genome engineering. In less than 1 year, the ease, robustness and efficiency of this method have facilitated an immense range of genetic modifications in most model organisms. Full and conditional gene knock-outs, knock-ins, large chromosomal deletions and subtle mutations can be obtained using combinations of clustered regularly interspaced short palindromic repeats (CRISPRs) and DNA donors. In addition, with CRISPR-Cas compounds, multiple genetic modifications can be introduced seamlessly in a single step. CRISPR-Cas not only brings genome engineering capacities to species such as rodents and livestock in which the existing toolbox was already large, but has also enabled precise genetic engineering of organisms with difficult-to-edit genomes such as zebrafish, and of technically challenging species such as non-human primates. The CRISPR-Cas system allows generation of targeted mutations in mice, even in laboratories with limited or no access to the complex, time-consuming standard technology using mouse embryonic stem cells. Here we summarize the distinct applications of CRISPR-Cas technology for obtaining a variety of genetic modifications in different model organisms, underlining their advantages and limitations relative to other genome editing nucleases. We will guide the reader through the many publications that have seen the light in the first year of CRISPR-Cas technology.

Highly efficient targeted mutagenesis in one-cell mouse embryos mediated by the TALEN and CRISPR/Cas systems

Since the establishment of embryonic stem (ES) cell lines, the combined use of gene targeting with homologous recombination has aided in elucidating the functions of various genes. However, the ES cell technique is inefficient and time-consuming. Recently, two new gene-targeting technologies have been developed: the transcription activator-like effector nuclease (TALEN) system, and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) system. In addition to aiding researchers in solving conventional problems, these technologies can be used to induce site-specific mutations in various species for which ES cells have not been established. Here, by targeting the Fgf10 gene through RNA microinjection in one-cell mouse embryos with the TALEN and CRISPR/Cas systems, we produced the known limb-defect phenotypes of Fgf10-deficient embryos at the F0 generation. Compared to the TALEN system, the CRISPR/Cas system induced the limb-defect phenotypes with a strikingly higher efficiency. Our results demonstrate that although both gene-targeting technologies are useful, the CRISPR/Cas system more effectively elicits single-step biallelic mutations in mice.

Efficient gene targeting by homology-directed repair in rat zygotes using TALE nucleases

The generation of genetically modified animals is important for both research and commercial purposes. The rat is an important model organism that until recently lacked efficient genetic engineering tools. Sequence-specific nucleases, such as ZFNs, TALE nucleases, and CRISPR/Cas9 have allowed the creation of rat knockout models. Genetic engineering by homology-directed repair (HDR) is utilized to create animals expressing transgenes in a controlled way and to introduce precise genetic modifications. We applied TALE nucleases and donor DNA microinjection into zygotes to generate HDR-modified rats with large new sequences introduced into three different loci with high efficiency (0.62%–5.13% of microinjected zygotes). Two of these loci (Rosa26 and Hprt1) are known to allow robust and reproducible transgene expression and were targeted for integration of a GFP expression cassette driven by the CAG promoter. GFP-expressing embryos and four Rosa26 GFP rat lines analyzed showed strong and widespread GFP expression in most cells of all analyzed tissues. The third targeted locus was Ighm, where we performed successful exon exchange of rat exon 2 for the human one. At all three loci we observed HDR only when using linear and not circular donor DNA. Mild hypothermic (30°C) culture of zygotes after microinjection increased HDR efficiency for some loci. Our study demonstrates that TALE nuclease and donor DNA microinjection into rat zygotes results in efficient and reproducible targeted donor integration by HDR. This allowed creation of genetically modified rats in a work-, cost-, and time-effective manner.

Emerging gene editing strategies for Duchenne muscular dystrophy targeting stem cells

The progressive loss of muscle mass characteristic of many muscular dystrophies impairs the efficacy of most of the gene and molecular therapies currently being pursued for the treatment of those disorders. It is becoming increasingly evident that a therapeutic application, to be effective, needs to target not only mature myofibers, but also muscle progenitors cells or muscle stem cells able to form new muscle tissue and to restore myofibers lost as the result of the diseases or during normal homeostasis so as to guarantee effective and lost lasting effects. Correction of the genetic defect using oligodeoxynucleotides (ODNs) or engineered nucleases holds great potential for the treatment of many of the musculoskeletal disorders. The encouraging results obtained by studying in vitro systems and model organisms have set the groundwork for what is likely to become an emerging field in the area of molecular and regenerative medicine. Furthermore, the ability to isolate and expand from patients various types of muscle progenitor cells capable of committing to the myogenic lineage provides the opportunity to establish cell lines that can be used for transplantation following ex vivo manipulation and expansion. The purpose of this article is to provide a perspective on approaches aimed at correcting the genetic defect using gene editing strategies and currently under development for the treatment of Duchenne muscular dystrophy (DMD), the most sever of the neuromuscular disorders. Emphasis will be placed on describing the potential of using the patient own stem cell as source of transplantation and the challenges that gene editing technologies face in the field of regenerative biology.

Mouse genome engineering using designer nucleases

Transgenic mice carrying site-specific genome modifications (knockout, knock-in) are of vital importance for dissecting complex biological systems as well as for modeling human diseases and testing therapeutic strategies. Recent advances in the use of designer nucleases such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) 9 system for site-specific genome engineering open the possibility to perform rapid targeted genome modification in virtually any laboratory species without the need to rely on embryonic stem (ES) cell technology. A genome editing experiment typically starts with identification of designer nuclease target sites within a gene of interest followed by construction of custom DNA-binding domains to direct nuclease activity to the investigator-defined genomic locus. Designer nuclease plasmids are in vitro transcribed to generate mRNA for microinjection of fertilized mouse oocytes. Here, we provide a protocol for achieving targeted genome modification by direct injection of TALEN mRNA into fertilized mouse oocytes.

Binary recombinase systems for high-resolution conditional mutagenesis

Conditional mutagenesis using Cre recombinase expressed from tissue specific promoters facilitates analyses of gene function and cell lineage tracing. Here, we describe two novel dual-promoter-driven conditional mutagenesis systems designed for greater accuracy and optimal efficiency of recombination. Co-Driver employs a recombinase cascade of Dre and Dre-respondent Cre, which processes loxP-flanked alleles only when both recombinases are expressed in a predetermined temporal sequence. This unique property makes Co-Driver ideal for sequential lineage tracing studies aimed at unraveling the relationships between cellular precursors and mature cell types. Co-InCre was designed for highly efficient intersectional conditional transgenesis. It relies on highly active trans-splicing inteins and promoters with simultaneous transcriptional activity to reconstitute Cre recombinase from two inactive precursor fragments. By generating native Cre, Co-InCre attains recombination rates that exceed all other binary SSR systems evaluated in this study. Both Co-Driver and Co-InCre significantly extend the utility of existing Cre-responsive alleles.

A modified TALEN-based strategy for rapidly and efficiently generating knockout mice for kidney development studies

The transcription activator-like effector nucleases (TALENs) strategy has been widely used to delete and mutate genes in vitro. This strategy has begun to be used for in vivo systemic gene manipulation, but not in an organ-specific manner. In this study, we developed a modified, highly efficient TALEN strategy using a dual-fluorescence reporter. We used this modified strategy and, within 5 weeks, we successfully generated kidney proximal tubule-specific gene Ttc36 homozygous knockout mice. Unilateral nephrectomy was performed on the 6-week-old founders (F0) to identify the knockout genotype prior to the birth of the offspring. This strategy was found to have little effect on reproduction in the knockout mice and inheritability of the knockout genotypes. The modified TALEN knockout strategy in combination with unilateral nephrectomy can be readily used for studies of gene function in kidney development and diseases.

Efficient generation of large-scale genome-modified mice using gRNA and CAS9 endonuclease

The generation of genome-modified animals is a powerful approach to analyze gene functions. The CAS9/guide RNA (gRNA) system is expected to become widely used for the efficient generation of genome-modified animals, but detailed studies on optimum conditions and availability are limited. In the present study, we attempted to generate large-scale genome-modified mice with an optimized CAS9/gRNA system, and confirmed the transmission of these mutations to the next generations. A comparison of different types of gRNA indicated that the target loci of almost all pups were modified successfully by the use of long-type gRNAs with CAS9. We showed that this system has much higher mutation efficiency and much lower off-target effect compared to zinc-finger nuclease. We propose that most of these off-target effects can be avoided by the careful control of CAS9 mRNA concentration and that the genome-modification efficiency depends rather on the gRNA concentration. Under optimized conditions, large-scale (~10 kb) genome-modified mice can be efficiently generated by modifying two loci on a single chromosome using two gRNAs at once in mouse zygotes. In addition, the normal transmission of these CAS9/gRNA-induced mutations to the next generation was confirmed. These results indicate that CAS9/gRNA system can become a highly effective tool for the generation of genome-modified animals.

Comparing zinc finger nucleases and transcription activator-like effector nucleases for gene targeting in Drosophila

Zinc-finger nucleases have proven to be successful as reagents for targeted genome manipulation in Drosophila melanogaster and many other organisms. Their utility has been limited, however, by the significant failure rate of new designs, reflecting the complexity of DNA recognition by zinc fingers. Transcription activator-like effector (TALE) DNA-binding domains depend on a simple, one-module-to-one-base-pair recognition code, and they have been very productively incorporated into nucleases (TALENs) for genome engineering. In this report we describe the design of TALENs for a number of different genes in Drosophila, and we explore several parameters of TALEN design. The rate of success with TALENs was substantially greater than for zinc-finger nucleases , and the frequency of mutagenesis was comparable. Knockout mutations were isolated in several genes in which such alleles were not previously available. TALENs are an effective tool for targeted genome manipulation in Drosophila.

Repeatable construction method for engineered zinc finger nuclease based on overlap extension PCR and TA-cloning

Zinc finger nuclease (ZFN) is a useful tool for endogenous site-directed genome modification. The development of an easier, less expensive and repeatedly usable construction method for various sequences of ZFNs should contribute to the further widespread use of this technology. Here, we establish a novel construction method for ZFNs. Zinc finger (ZF) fragments were synthesized by PCR using short primers coding DNA recognition helices of the ZF domain. DNA-binding domains composed of 4 to 6 ZFs were synthesized by overlap extension PCR of these PCR products, and the DNA-binding domains were joined with a nuclease vector by TA cloning. The short primers coding unique DNA recognition helices can be used repeatedly for other ZFN constructions. By using this novel OLTA (OverLap extension PCR and TA-cloning) method, arbitrary ZFN vectors were synthesized within 3 days, from the designing to the sequencing of the vector. Four different ZFN sets synthesized by OLTA showed nuclease activities at endogenous target loci. Genetically modified mice were successfully generated using ZFN vectors constructed by OLTA. This method, which enables the construction of intended ZFNs repeatedly and inexpensively in a short period of time, should contribute to the advancement of ZFN technology.

Is BAC transgenesis obsolete? State of the art in the era of designer nucleases

DNA constructs based on bacterial artificial chromosomes (BACs) are frequently used to generate transgenic animals as they reduce the influence of position effects and allow predictable expression patterns for genes whose regulatory sequences are not fully identified. Despite these advantages BAC transgenics suffer from drawbacks such as complicated vector construction, low efficiency of transgenesis, and some remaining expression variegation. The recent development of transcription activator-like effector nucleases (TALENs) and zinc finger nucleases (ZFNs) has resulted in new transgenic techniques which do not have the drawbacks associated with BAC transgenesis. Initial reports indicate that such designer nucleases (DNs) allow the targeted insertion of transgenes into endogenous loci by direct injection of the targeting vector and mRNA/DNA encoding the predesigned nucleases into oocytes. This results in the transgene being inserted at a specific locus in the mouse genome, thus circumventing the drawbacks associated with BAC transgenesis.