Autonomous zinc-finger nuclease pairs for targeted chromosomal deletion

Zinc-finger nucleases (ZFNs) have been successfully used for rational genome engineering in a variety of cell types and organisms. ZFNs consist of a non-specific FokI endonuclease domain and a specific zinc-finger DNA-binding domain. Because the catalytic domain must dimerize to become active, two ZFN subunits are typically assembled at the cleavage site. The generation of obligate heterodimeric ZFNs was shown to significantly reduce ZFN-associated cytotoxicity in single-site genome editing strategies. To further expand the application range of ZFNs, we employed a combination of in silico protein modeling, in vitro cleavage assays, and in vivo recombination assays to identify autonomous ZFN pairs that lack cross-reactivity between each other. In the context of ZFNs designed to recognize two adjacent sites in the human HOXB13 locus, we demonstrate that two autonomous ZFN pairs can be directed simultaneously to two different sites to induce a chromosomal deletion in ∼10% of alleles. Notably, the autonomous ZFN pair induced a targeted chromosomal deletion with the same efficacy as previously published obligate heterodimeric ZFNs but with significantly less toxicity. These results demonstrate that autonomous ZFNs will prove useful in targeted genome engineering approaches wherever an application requires the expression of two distinct ZFN pairs.     

应用SSA报告载体提高ZFN和CRISPR/Cas9对猪IGF2基因的打靶效率

IGF2(Insulin-like growth factor 2)基因作为最复杂多样的生长因子之一,对猪胎儿发育以及出生后生长发育和肌肉生成起着非常重要的作用。通过基因组编辑技术对我国本地猪种的IGF2基因作精确的遗传修饰,对于提高本地猪种的瘦肉率具有重要的育种意义。文章在蓝塘猪胎儿成纤维细胞(Porcine fetal fibroblasts, PEF)中检测了锌指核酸酶(Zinc finger nucleases, ZFN)和CRISPR/Cas9对IGF2基因的打靶效率,结果表明CRISPR/Cas9对IGF2基因的切割效率最高可达9.2%,显著高于ZFN的切割效率(<1%),但两者均未达到作为体细胞核移植(Somatic nuclear transfer, SCNT)供体细胞所需的打靶效率。应用SSA (Single-strand annealing)报告载体筛选技术来富集IGF2基因被ZFN和CRISPR/Cas9修饰过的PEF细胞,结果表明,该技术可使CRISPR/Cas9的打靶效率提高5倍左右,对ZFN的打靶效率具有更大的增强作用。     

Low frequency of zinc-finger nuclease-induced mutagenesis in <Emphasis Type="Italic">Populus</Emphasis>

Gene flow from recombinant-DNA-modified (GMO) trees is a major barrier to their public acceptance and regulatory approval. Because many intensively grown trees are vegetatively propagated, complete sexual sterility could be a powerful means to mitigate or prevent gene flow. We tested four pairs of zinc-finger nucleases (ZFNs) as mutagenic agents against the LEAFY and AGAMOUS orthologs in poplar that are expected to be required for sexual fertility. To reduce the potential for pleiotropic effects from mutagenesis, each of the pairs was functionally linked to a heat shock promoter to provide inducible ZFN expression. Using Agrobacterium tumefaciens, we transformed more than 21,000 total explants compromised of both male and female hybrid poplar. The rate of transformation for the ZFN constructs (2 %) was generally reduced compared to the transgenic control (8 %). We produced 391 ZFN transgenic shoots of which only two developed into plants with mutations in a target gene; both were 7-bp deletions in one allele of the PtAG2 locus. No mutations were observed in the PtAG1 or PtLFY loci. Our results indicate a mutation rate of zero to 0.3 % per explant per allele, among the lowest reported for ZFN mutagenesis in plants. The combined effects of low recovery of transgenic plants, a modest mutation frequency, and much higher reported rates of directed mutation for other gene editing methods suggest that the efficient use of ZFNs in poplar requires further technical improvements.     

Enhancing zinc-finger-nuclease activity with improved obligate heterodimeric architectures

Zinc-finger nucleases (ZFNs) drive efficient genome editing by introducing a double-strand break into the targeted gene. Cleavage is induced when two custom-designed ZFNs heterodimerize upon binding DNA to form a catalytically active nuclease complex. The importance of this dimerization event for subsequent cleavage activity has stimulated efforts to engineer the nuclease interface to prevent undesired homodimerization. Here we report the development and application of a yeast-based selection system designed to functionally interrogate the ZFN dimer interface. We identified critical residues involved in dimerization through the isolation of cold-sensitive nuclease domains. We used these residues to engineer ZFNs that have superior cleavage activity while suppressing homodimerization. The improvements were portable to orthogonal domains, allowing the concomitant and independent cleavage of two loci using two different ZFN pairs. These ZFN architectures provide a general means for obtaining highly efficient and specific genome modification.     

Application of ZFN for Site Directed Mutagenesis of Rice <Emphasis Type="Italic">SSIVa</Emphasis> Gene

Many successful studies on genome editing in plants have been reported and one of the popular genome editing technology used in plants is Zinc Finger Nucleases (ZFN), which are chimeric proteins composed of synthetic zinc finger-based DNA binding domain and a DNA cleavage domain. The objective of this research was to utilize ZFNs to induce a double-stranded break in SSIVa, a soluble starch synthase involved in starch biosynthesis pathway, leading to the regulation of the SSIVa expression. The isoform SSIVa is not yet well studied, thus, by modifying the endogenous loci in SSIVa, we can explore on the specific roles of this gene in starch biosynthesis and other possible functions it might play. In this study, we used ZFN-mediated targeted gene disruption in the coding sequence of the SSIVa rice gene in an effort to elucidate the functional role of the gene. Generation of transgenic plants carrying premature stop codons and substitution events, revealed no SSIVa mRNA expression, low starch contents and dwarf phenotypes. Remarkably, based on our analysis SSIVa gene disruption had no effect on other starch synthesis related genes as their expression remained at wild type levels. Therefore, the engineered ZFNs can efficiently cleave and stimulate mutations at SSIVa locus in rice to     

Genome editing with CompoZr custom zinc finger nucleases (ZFNs)

Genome editing is a powerful technique that can be used to elucidate gene function and the genetic basis of disease. Traditional gene editing methods such as chemical-based mutagenesis or random integration of DNA sequences confer indiscriminate genetic changes in an overall inefficient manner and require incorporation of undesirable synthetic sequences or use of aberrant culture conditions, potentially confusing biological study. By contrast, transient ZFN expression in a cell can facilitate precise, heritable gene editing in a highly efficient manner without the need for administration of chemicals or integration of synthetic transgenes. Zinc finger nucleases (ZFNs) are enzymes which bind and cut distinct sequences of double-stranded DNA (dsDNA). A functional CompoZr ZFN unit consists of two individual monomeric proteins that bind a DNA "half-site" of approximately 15-18 nucleotides (see Figure 1). When two ZFN monomers "home" to their adjacent target sites the DNA-cleavage domains dimerize and create a double-strand break (DSB) in the DNA. Introduction of ZFN-mediated DSBs in the genome lays a foundation for highly efficient genome editing. Imperfect repair of DSBs in a cell via the non-homologous end-joining (NHEJ) DNA repair pathway can result in small insertions and deletions (indels). Creation of indels within the gene coding sequence of a cell can result in frameshift and subsequent functional knockout of a gene locus at high efficiency. While this protocol describes the use of ZFNs to create a gene knockout, integration of transgenes may also be conducted via homology-directed repair at the ZFN cut site. The CompoZr Custom ZFN Service represents a systematic, comprehensive, and well-characterized approach to targeted gene editing for the scientific community with ZFN technology. Sigma scientists work closely with investigators to 1) perform due diligence analysis including analysis of relevant gene structure, biology, and model system pursuant to the project goals, 2) apply this knowledge to develop a sound targeting strategy, 3) then design, build, and functionally validate ZFNs for activity in a relevant cell line. The investigator receives positive control genomic DNA and primers, and ready-to-use ZFN reagents supplied in both plasmid DNA and in-vitro transcribed mRNA format. These reagents may then be delivered for transient expression in the investigator's cell line or cell type of choice. Samples are then tested for gene editing at the locus of interest by standard molecular biology techniques including PCR amplification, enzymatic digest, and electrophoresis. After positive signal for gene editing is detected in the initial population, cells are single-cell cloned and genotyped for identification of mutant clones/alleles.     

利用ZFN删除转基因猪细胞中的多拷贝EGFP基因

锌指核酸酶（zinc finger nuclease, ZFN）是由特异性识别DNA的锌指结构域和Fok I切割结构域组成,能够在基因组特定位点上切割DNA,引起DNA双链断裂（double-strand break, DSB）. 通过DSB修复机制,可以使基因修饰的效率比传统方法提高102～104倍．目前,利用ZFN对动物内源基因进行敲除的研究较多,但对转基因动物中外源多拷贝基因进行敲除的报道较少．本研究首先利用荧光定量PCR法对本实验室培育的两头转基因猪中增强型绿色荧光蛋白（enhanced green fluorescent protein, EGFP）基因的拷贝数进行鉴定,发现其拷贝数分别为11.95和17.36拷贝;然后将靶向EGFP的一对ZFN转染进拷贝数为1736的EGFP转基因猪的成纤维细胞中,并通过流式和CEL-1酶切方法检测敲除效率. 结果表明,转染400 ng、800 ng和1 200 ng ZFN的切割效率分别为0.97%、1.39%和1.76%,可见随着转染ZFN剂量的增加,ZFN的切割效率逐渐提高．但是,不发绿色荧光的细胞比例却没有明显提高,因此认为,ZFN敲除转基因动物中多拷贝基因的效率还是比较低．     

Trait stacking via targeted genome editing

Modern agriculture demands crops carrying multiple traits. The current paradigm of randomly integrating and sorting independently segregating transgenes creates severe downstream breeding challenges. A versatile, generally applicable solution is hereby provided: the combination of high‐efficiency targeted genome editing driven by engineered zinc finger nucleases (ZFNs) with modular ‘trait landing pads’ (TLPs) that allow ‘mix‐and‐match’, on‐demand transgene integration and trait stacking in crop plants. We illustrate the utility of nuclease‐driven TLP technology by applying it to the stacking of herbicide resistance traits. We first integrated into the maize genome an herbicide resistance gene, pat, flanked with a TLP (ZFN target sites and sequences homologous to incoming DNA) using WHISKERS?‐mediated transformation of embryogenic suspension cultures. We established a method for targeted transgene integration based on microparticle bombardment of immature embryos and used it to deliver a second trait precisely into the TLP via cotransformation with a donor DNA containing a second herbicide resistance gene, aad1, flanked by sequences homologous to the integrated TLP along with a corresponding ZFN expression construct. Remarkably, up to 5% of the embryo‐derived transgenic events integrated the aad1 transgene precisely at the TLP, that is, directly adjacent to the pat transgene. Importantly and consistent with the juxtaposition achieved via nuclease‐driven TLP technology, both herbicide resistance traits cosegregated in subsequent generations, thereby demonstrating linkage of the two independently transformed transgenes. Because ZFN‐mediated targeted transgene integration is becoming applicable across an increasing number of crop species, this work exemplifies a simple, facile and rapid approach to trait stacking.     

Evaluation of OPEN Zinc Finger Nucleases for Direct Gene Targeting of the ROSA26 Locus in Mouse Embryos

Zinc finger nucleases (ZFNs) enable precise genome modification in a variety of organisms and cell types. Commercial ZFNs were reported to enhance gene targeting directly in mouse zygotes, whereas similar approaches using publicly available resources have not yet been described. Here we report precise targeted mutagenesis of the mouse genome using Oligomerized Pool Engineering (OPEN) ZFNs. OPEN ZFN can be constructed using publicly available resources and therefore provide an attractive alternative for academic researchers. Two ZFN pairs specific to the mouse genomic locus gt(ROSA26)Sor were generated by OPEN selections and used for gene disruption and homology-mediated gene replacement in single cell mouse embryos. One specific ZFN pair facilitated non-homologous end joining (NHEJ)-mediated gene disruption when expressed in mouse zygotes. We also observed a single homologous recombination (HR)-driven gene replacement event when this ZFN pair was co-injected with a targeting vector. Our experiments demonstrate the feasibility of achieving both gene ablation through NHEJ and gene replacement by HR by using the OPEN ZFN technology directly in mouse zygotes.     

Sandwiched zinc-finger nucleases demonstrating higher homologous recombination rates than conventional zinc-finger nucleases in mammalian cells

We previously reported that our sandwiched zinc-finger nucleases (ZFNs), in which a DNA cleavage domain is inserted between two artificial zinc-finger proteins, cleave their target DNA much more efficiently than conventional ZFNs in vitro. In the present study, we compared DNA cleaving efficiencies of a sandwiched ZFN with those of its corresponding conventional ZFN in mammalian cells. Using a plasmid-based single-strand annealing reporter assay in HEK293 cells, we confirmed that the sandwiched ZFN induced homologous recombination more efficiently than the conventional ZFN; reporter activation by the sandwiched ZFN was more than eight times that of the conventional one. Western blot analysis showed that the sandwiched ZFN was expressed less frequently than the conventional ZFN, indicating that the greater DNA-cleaving activity of the sandwiched ZFN was not due to higher expression of the sandwiched ZFN. Furthermore, an MTT assay demonstrated that the sandwiched ZFN did not have any significant cytotoxicity under the DNA-cleavage conditions. Thus, because our sandwiched ZFN cleaved more efficiently than its corresponding conventional ZFN in HEK293 cells as well as in vitro, sandwiched ZFNs are expected to serve as an effective molecular tool for genome editing in living cells.     

Targeted disruption of the sheep MSTN gene by engineered zinc-finger nucleases

Prior to the development of zinc-finger nuclease technology, genetic manipulation by gene targeting achieved limited success in mammals, with the exception of mice and rat. Although ZFNs demonstrated highly effective gene targeted disruption in various model organisms, the activity of ZFNs in large domestic animals may be very low, and the probability of identifying ZFN-mediated positive targeted disruption events is small. In this paper, we used the context-dependent assembly method to synthesize two pairs of ZFNs targeted to the sheep MSTN gene. We verified the activity of these ZFNs using an mRFP-MBS-eGFP dual-fluorescence reporter system in HEK293T cells and, according to the expression level of eGFP, we obtained a pair of ZFNs that can recognize and cut the targeted MSTN site in the reporter vector. The activity of ZFN was increased by cold stimulation at 30 °C and by mutant the wildtype FokI in ZFN with its counterpart Sharkeys. Finally, the ZF-Sharkeys and reporter vector were cotransfected into sheep fetal fibroblasts and two MSTN mutant cell lines, identified by flow cytometry and sequencing, were obtained.     

Efficient disruption of endogenous Bombyx gene by TAL effector nucleases

Engineered nucleases are proteins that are able to cleave DNA at specified sites in the genome. These proteins have recently been used for gene targeting in a number of organisms. We showed earlier that zinc finger nucleases (ZFNs) can be used for generating gene-specific mutations in Bombyx mori by an error-prone DNA repair process of non-homologous end joining (NHEJ). Here we test the utility of another type of chimeric nuclease based on bacterial TAL effector proteins in order to induce targeted mutations in silkworm DNA. We designed three TAL effector nucleases (TALENs) against the genomic locus BmBLOS2, previously targeted by ZFNs. All three TALENs were able to induce mutations in silkworm germline cells suggesting a higher success rate of this type of chimeric enzyme. The efficiency of two of the tested TALENs was slightly higher than of the successful ZFN used previously. Simple design, high frequency of candidate targeting sites and comparable efficiency of induction of NHEJ mutations make TALENs an important alternative to ZFNs.     

Design, engineering, and characterization of zinc finger nucleases

Zinc finger nuclease (ZFN)-mediated gene targeting is rapidly becoming a powerful tool for "gene editing" and "directed mutagenesis" of plant and mammalian genomes including the human genome. ZFN-mediated gene targeting provides molecular biologists with the ability to site-specifically manipulate and permanently modify plant and mammalian genomes. Facile production of ZFNs and rapid characterization of their in vitro sequence-specific cleavage properties are a pre-requisite before ZFN-mediated gene targeting can become an efficient and effective practical tool for widespread use in biotechnology. Here, we report the design, engineering, and rapid in vitro characterization of ZFNs that target specific endogenous sequences within two mouse genes (mTYR and mCFTR), and two human genes (hCCR5 and hDMPK), respectively. These engineered ZFNs recognize their respective cognate DNA sites encoded in a plasmid substrate in a sequence-specific manner and, as expected, they induce a double-strand break at the chosen target site.     

Rapid "open-source" engineering of customized zinc-finger nucleases for highly efficient gene modification

Custom-made zinc-finger nucleases (ZFNs) can induce targeted genome modifications with high efficiency in cell types including Drosophila, C. elegans, plants, and humans. A bottleneck in the application of ZFN technology has been the generation of highly specific engineered zinc-finger arrays. Here we describe OPEN (Oligomerized Pool ENgineering), a rapid, publicly available strategy for constructing multifinger arrays, which we show is more effective than the previously published modular assembly method. We used OPEN to construct 37 highly active ZFN pairs which induced targeted alterations with high efficiencies (1%-50%) at 11 different target sites located within three endogenous human genes (VEGF-A, HoxB13, and CFTR), an endogenous plant gene (tobacco SuRA), and a chromosomally integrated EGFP reporter gene. In summary, OPEN provides an "open-source" method for rapidly engineering highly active zinc-finger arrays, thereby enabling broader practice, development, and application of ZFN technology for biological research and gene therapy.     

Gene editing in human stem cells using zinc finger nucleases and integrase-defective lentiviral vector delivery

Achieving the full potential of zinc-finger nucleases (ZFNs) for genome engineering in human cells requires their efficient delivery to the relevant cell types. Here we exploited the infectivity of integrase-defective lentiviral vectors (IDLV) to express ZFNs and provide the template DNA for gene correction in different cell types. IDLV-mediated delivery supported high rates (13-39%) of editing at the IL-2 receptor common gamma-chain gene (IL2RG) across different cell types. IDLVs also mediated site-specific gene addition by a process that required ZFN cleavage and homologous template DNA, thus establishing a platform that can target the insertion of transgenes into a predetermined genomic site. Using IDLV delivery and ZFNs targeting distinct loci, we observed high levels of gene addition (up to 50%) in a panel of human cell lines, as well as human embryonic stem cells (5%), allowing rapid, selection-free isolation of clonogenic cells with the desired genetic modification.