Target Nuclease Research Articles

Cas12a domain flexibility guides R-loop formation and forces RuvC resetting

The specific nature of CRISPR-Cas12a makes it a desirable RNA-guided endonuclease for biotechnology and therapeutic applications. To understand how R-loop formation within the compact Cas12a enables target recognition and nuclease activation, we used cryo-electron microscopy to capture wild-type Acidaminococcus sp. Cas12a R-loop intermediates and DNA delivery into the RuvC active site. Stages of Cas12a R-loop formation-starting from a 5-bp seed-are marked by distinct REC domain arrangements. Dramatic domain flexibility limits contacts until nearly complete R-loop formation, when the non-target strand is pulled across the RuvC nuclease and coordinated domain docking promotes efficient cleavage. Next, substantial domain movements enable target strand repositioning into the RuvC active site. Between cleavage events, the RuvC lid conformationally resets to occlude the active site, requiring re-activation. These snapshots build a structural model depicting Cas12a DNA targeting that rationalizes observed specificity and highlights mechanistic comparisons to other class 2 effectors.

SPLICER: A Highly Efficient Base Editing Toolbox That Enables In Vivo Therapeutic Exon Skipping.

Exon skipping technologies enable exclusion of targeted exons from mature mRNA transcripts, which has broad applications in molecular biology, medicine, and biotechnology. Existing exon skipping techniques include antisense oligonucleotides, targetable nucleases, and base editors, which, while effective for specific applications at some target exons, remain hindered by shortcomings, including transient effects for oligonucleotides, genotoxicity for nucleases and inconsistent exon skipping for base editors. To overcome these limitations, we created SPLICER, a toolbox of next-generation base editors consisting of near-PAMless Cas9 nickase variants fused to adenosine or cytosine deaminases for the simultaneous editing of splice acceptor (SA) and splice donor (SD) sequences. Synchronized SA and SD editing with SPLICER improves exon skipping, reduces aberrant outcomes, including cryptic splicing and intron retention, and enables skipping of exons refractory to single splice-site editing. To demonstrate the therapeutic potential of SPLICER, we targeted APP exon 17, which encodes the amino acid residues that are cleaved to form the Aβ plaques in Alzheimer's disease. SPLICER reduced the formation of Aβ42 peptides in vitro and enabled efficient exon skipping in a mouse model of Alzheimer's disease. Overall, SPLICER is a widely applicable and efficient toolbox for exon skipping with broad therapeutic applications.

Recent Updates of the CRISPR/Cas9 Genome Editing System: Novel Approaches to Regulate Its Spatiotemporal Control by Genetic and Physicochemical Strategies.

The genome editing approach by clustered regularly interspaced short palindromic repeats (CRISPR)/associated protein 9 (CRISPR/Cas9) is a revolutionary advancement in genetic engineering. Owing to its simple design and powerful genome-editing capability, it offers a promising strategy for the treatment of different infectious, metabolic, and genetic diseases. The crystal structure of Streptococcus pyogenes Cas9 (SpCas9) in complex with sgRNA and its target DNA at 2.5 Å resolution reveals a groove accommodating sgRNA:DNA heteroduplex within a bilobate architecture with target recognition (REC) and nuclease (NUC) domains. The presence of a PAM is significantly required for target recognition, R-loop formation, and strand scission. Recently, the spatiotemporal control of CRISPR/Cas9 genome editing has been considerably improved by genetic, chemical, and physical regulatory strategies. The use of genetic modifiers anti-CRISPR proteins, cell-specific promoters, and histone acetyl transferases has uplifted the application of CRISPR/Cas9 as a future-generation genome editing tool. In addition, interventions by chemical control, small-molecule activators, oligonucleotide conjugates and bioresponsive delivery carriers have improved its application in other areas of biological fields. Furthermore, the intermediation of physical control by using heat-, light-, magnetism-, and ultrasound-responsive elements attached to this molecular tool has revolutionized genome editing further. These strategies significantly reduce CRISPR/Cas9's undesirable off-target effects. However, other undesirable effects still offer some challenges for comprehensive clinical translation using this genome-editing approach. In this review, we summarize recent advances in CRISPR/Cas9 structure, mechanistic action, and the role of small-molecule activators, inhibitors, promoters, and physical approaches. Finally, off-target measurement approaches, challenges, future prospects, and clinical applications are discussed.

Current updates of CRISPR/Cas9-mediated genome editing and targeting within tumor cells: an innovative strategy of cancer management.

Clustered regularly interspaced short palindromic repeats-associated protein (CRISPR/Cas9), an adaptive microbial immune system, has been exploited as a robust, accurate, efficient and programmable method for genome targeting and editing. This innovative and revolutionary technique can play a significant role in animal modeling, in vivo genome therapy, engineered cell therapy, cancer diagnosis and treatment. The CRISPR/Cas9 endonuclease system targets a specific genomic locus by single guide RNA (sgRNA), forming a heteroduplex with target DNA. The Streptococcus pyogenes Cas9/sgRNA:DNA complex reveals a bilobed architecture with target recognition and nuclease lobes. CRISPR/Cas9 assembly can be hijacked, and its nanoformulation can be engineered as a delivery system for different clinical utilizations. However, the efficient and safe delivery of the CRISPR/Cas9 system to target tissues and cancer cells is very challenging, limiting its clinical utilization. Viral delivery strategies of this system may have many advantages, but disadvantages such as immune system stimulation, tumor promotion risk and small insertion size outweigh these advantages. Thus, there is a desperate need to develop an efficient non-viral physical delivery system based on simple nanoformulations. The delivery strategies of CRISPR/Cas9 by a nanoparticle-based system have shown tremendous potential, such as easy and large-scale production, combination therapy, large insertion size and efficient in vivo applications. This review aims to provide in-depth updates on Streptococcus pyogenic CRISPR/Cas9 structure and its mechanistic understanding. In addition, the advances in its nanoformulation-based delivery systems, including lipid-based, polymeric structures and rigid NPs coupled to special ligands such as aptamers, TAT peptides and cell-penetrating peptides, are discussed. Furthermore, the clinical applications in different cancers, clinical trials and future prospects of CRISPR/Cas9 delivery and genome targeting are also discussed.

Unique structural and dynamic properties of the HNH nuclease in mesophilic and thermophilic Cas9 revealed by NMR

CRISPR‐Cas9 is a widely utilized biochemical tool with applications in biotechnology and in medicine. Target DNA recognition and nuclease sites of Cas9 are spatially separated yet functionally coupled, indicative of an allosteric crosstalk throughout the enzyme. There are structural and dynamic changes occuring throughout Cas9 that are critical for this signal propagation. However, an understanding of the molecular motions guiding Cas9 and how this influences its mechanism are unknown. In this study we report on the structural and dynamic properties of the HNH domain between the extensively studied SpCas9 from Streptococcus pyogenes, and a recently discovered thermostable Cas9 homolog from a thermophilic bacterium Geobacillus stearothermophilus(GeoCas9). Here we show that despite sequential and structural conservation, SpHNH and GeoHNH have different dynamic profiles. We found the intrinsic dynamics of SpHNH to be on the slow (ms) timescale, while GeoHNH is highly influenced by fast (ps‐ns) timescale dynamics. Furthermore, we found that when the residue with the highest flexibility on the ps‐ns timescale is mutated in GeoHNH, its protein solubility, thermal stability, and dynamic profile are all drastically distorted. Our results demonstrate how there are mechanistic differences between the HNH domain of mesophilic and thermophilic Cas9 species. Our results also highlight how these fast timescale motions that regulate GeoHNH play a role in giving GeoCas9 its thermophilic characteristic.

Genomic Reporter Constructs to Monitor Pathway-Specific Repair of DNA Double-Strand Breaks.

Repair of DNA Double-Strand Breaks (DSBs) can be error-free or highly mutagenic, depending on which of multiple mechanistically distinct pathways repairs the break. Hence, DSB-repair pathway choice directly affects genome integrity, and it is therefore of interest to understand the parameters that direct repair towards a specific pathway. This has been intensively studied using genomic reporter constructs, in which repair of a site-specific DSB by the pathway of interest generates a quantifiable phenotype, generally the expression of a fluorescent protein. The current developments in genome editing with targetable nucleases like Cas9 have increased reporter usage and accelerated the generation of novel reporter constructs. Considering these recent advances, this review will discuss and compare the available DSB-repair pathway reporters, provide essential considerations to guide reporter choice, and give an outlook on potential future developments.

Characterization of WU-CART-007, an Allogeneic CD7-Targeted CAR-T Cell Therapy for T-Cell Malignancies

BackgroundT-cell Acute Lymphoblastic Leukemia (T-ALL) / Lymphoblastic Lymphoma (LBL) represent a class of devastating hematologic cancers with high rates of relapse and mortality in both children and adults. Development of CAR-T cell therapies for T-cell cancers has been complicated by induction of fratricide and the high risk of malignant cell contamination of the drug product in the autologous setting. Previously, Cooper et. al., demonstrated that CRISPR/Cas9 gene-editing to delete CD7 prevented self-killing, and deletion of the T-cell receptor alpha constant (TRAC) enabled the use of healthy donor-derived T-cells to manufacture CD7-targeted CAR-T cells without risk of malignancy and mitigating the risk of GvHD. Here we present preclinical data supporting the safety and efficacy of WU-CART-007, an IND ready, off-the-shelf, and fratricide resistant CD7-targeted CAR-T cell for the treatment of CD7+ T-cell malignancies.MethodsWU-CART-007 was manufactured using T cells isolated from healthy donors by deletion of CD7 and TRAC, followed by CAR transduction, cell expansion, depletion of residual TCRa/b+ cells and cryopreservation. Donors were confirmed negative for a panel of adventitious viruses. CD7/TRAC deletion and CAR transduction were confirmed by flow cytometry. Off-target editing profile was assessed by GUIDE-Seq. The binding kinetics to human CD7 were conducted by bio-layer interferometry and CD7 selectivity was confirmed by cell microarray with a library of HEK-293 cells expressing approximately 6000 human proteins. The in vitro activity of WU-CART-007 was interrogated by co-culture with human CD7+ CCRF-CEM T-ALL cells and the potential on-target, off-tumor activity was assessed by co-culture with a panel of immune and non-immune primary human cells. In vivo anti-tumor functionality was confirmed in immunocompromised NSG mice after the establishment of low or high tumor burden CCRF-CEM xenografts engineered to express green fluorescent protein (GFP) and click beetle red (CBR) luciferase. The impact of WU-CART-007 on normal hematopoiesis was assessed using CD34+ humanized NCG mice.ResultsSeveral successful full-scale manufacturing runs were completed with consistently high dual CD7/TRAC deletion, transduction efficiency, and cell viability. Drug product was primarily composed of a T cell memory phenotype. Off- target nuclease analysis by GUIDE-seq and targeted NGS confirmed no evidence of off-target editing events. The WU-CART-007 scFv exhibited high affinity and exquisite specificity for human CD7. In vitro co-incubation experiments confirmed strong cytotoxicity against CD7-expressing cells including CCRF-CEM T-ALL cells, primary T and NK cells, but not CD7- cells such as myeloid cells, B cells, hepatocytes, astrocytes, cardiomyocytes, epithelial cells, and endothelial cells. Importantly, no cytotoxicity was observed against hematopoietic progenitor cells in human bone marrow or cord blood following co-incubation with WU-CART-007. Similarly, WU-CART-007 treatment of a non-tumor bearing humanized mouse model resulted in transient reductions in CD7+ cells (T-cells and NK cells) but not CD7- cells (myeloid and B cells), and the impacted cells recovered after circulating WU-CART-007 cells were no longer detectable. Assessment of in vivo anti-tumor efficacy revealed that WU-CART-007 effectively inhibited tumor progression (>99% TGI) in both low and high burden CCRF-CEM tumor models and improved survival in a dose-dependent manner, while CAR- cells were inactive, confirming CD7-dependent activity.ConclusionsThese preclinical studies support the use of WU-CART-007 in clinical trials and highlight the potential of WU-CART-007 to be a well-tolerated and active therapy for patients with CD7+ T-cell malignancies. A first in human Phase 1/2 trial in patients with R/R T-ALL/LBL is currently open for enrollment (NCT# 04984356). DisclosuresNo relevant conflicts of interest to declare.

Mechanisms of Typhoid Toxin Neutralization by Antibodies Targeting Receptor-Binding and Nuclease Subunits

Nearly all clinical isolates of Salmonella Typhi, the cause of typhoid fever, are antibiotic-resistant. All S. Typhi isolates secrete an A2B5 exotoxin called typhoid toxin to benefit the pathogen during infection. Here, we demonstrate that antibiotic-resistant S. Typhi secretes typhoid toxin continuously during infection regardless of antibiotic treatment. Typhoid toxin neutralizing antibodies (nAbs) generated in this study target receptor-binding PltB or nuclease CdtB. All nAbs neutralize typhoid toxin in vitro and in vivo, as demonstrated by using typhoid toxin secreted by antibiotic-resistant S. Typhi during human cell infection and lethal-dose typhoid toxin challenge to mice. Cryo-EM analyses explain the mechanisms of how two nAbs, TyTx4 and TyTx11, are more productive; TyTx4 binds to all PltB subunits available per holotoxin, while TyTx11 makes CdtB inactive through CdtB catalytic-site conformational change. The identified toxin neutralizing epitopes are conserved across all S. Typhi clinical isolates, offering critical insights into typhoid toxin neutralizing strategies.

The era of editing plant genomes using CRISPR/Cas: A critical appraisal

The versatility of Clustered Regularly Interspaced Short Palindromic Repeats/Cas (CRISPR/Cas) genome editing tool ushered biologists into an exciting era of editing genomes with great efficiency and at a pace that was never imagined before. Though the CRISPR/Cas genome editing was developed after Zinc Finger Nucleases (ZFNs) and Transcription activator-like effector nucleases (TALENs), it is more popular and successful than these genome editing systems. The advent of targetable nucleases such as Cas9 has enabled manipulation of genomes in an accurate and precise manner. The CRISPR/Cas system of editing plant genomes has technical and economical advantages over conventional breeding methods. It has led to the development of traits within plant genomes that fulfill the needs of mankind. Advent of innovative procedures have paved the way for effective and efficient genome editing that has revolutionized genetic aspects and meets the safety regulations toward development of crops. The present review highlights the critical aspects of employing CRISPR/Cas for editing plant genomes in comparison with previously known editing approaches, such as ZFNs and TALENs. The study includes descriptive information on the approaches, procedural programs and applications in editing plant genomes for improving traits such as crop yield, resistance against emerging pathogens, abiotic stresses and herbicide tolerance thereof in the present-day world.

Editing of Endogenous Genes in Cellular Immunotherapies.

T cell-based cellular and antibody immunotherapies have dramatically altered the landscape of cancer treatment over the past decade. Over the same time span, gene editing technologies have enabled unprecedented degrees of genetic control. Knock-outs of endogenous genes, especially based on electroporation of targetable nucleases such as CRISPR/Cas9, have rapidly proliferated. Simultaneous introduction of large DNA sequences can integrate new synthetic genetic instructions with specific endogenous loci to alter T cell function and specificity. Recently developed discovery technologies to perform genome-wide knock-out and large-scale knock-in screens in T cells can rapidly identify endogenous gene targets and therapeutic knock-in programs. Endogenous gene knock-outs and targeted knock-ins may offer the chance to expand beyond the current limitations of randomly integrating viral vector-based T cell therapies, and extend immunotherapies' therapeutic advances to wider hematologic and solid tumor indications.

Degradation of Toxic RNA in Myotonic Dystrophy Using Gapmer Antisense Oligonucleotides.

Myotonic dystrophy (DM) types 1 (DM1) and 2 (DM2) are caused by autosomal dominant gain-of-function RNA which are, in turn, created by the expansion of repeat sequences in the DMPK and ZNF9 genes, respectively. The expansions are highly unstable and biased for further expansion in somatic cells and across generations. Despite the different genes involved, DM1 and DM2 share several clinical features due to having the similar underlying mechanism of repetitive RNA-mediated toxicity. Both disorders manifest as multisystemic conditions with features including myotonia, cataract development, and abnormalities in cardiac conduction. At present, there is no cure for DM and treatments mostly aim at symptom management. Among the therapeutics being developed, antisense therapy using gapmers is one of the most promising. Compared to other antisense oligonucleotides, gapmers maintain the ability to induce RNase H cleavage while having enhanced target binding affinity and nuclease resistance. This chapter will consolidate the different strategies studied thus far to develop a treatment for DM1 through the targeting of toxic repetitive RNA using gapmers.

Genome Editing in Mouse Embryos with CRISPR/Cas9.

Transgenic mouse models can be subdivided into two main categories based on genomic location: (1) targeted genomic manipulation and (2) random integration into the genome. Despite the potential confounding insertional mutagenesis and host locus-dependent expression, random integration transgenics allowed for rapid in vivo assessment of gene/protein function. Since precise genomic manipulation required the time-consuming prerequisite of first generating genetically modified embryonic stem cells, the rapid nature of generating random integration transgenes remained a strong benefit outweighing various disadvantages. The advent of targetable nucleases, such as CRISPR/Cas9, has eliminated the prerequisite of first generating genetically modified embryonic stem cells for some types of targeted genomic mutations. This chapter outlines the generation of mouse models with targeted genomic manipulation using the CRISPR/Cas9 system directly into single cell mouse embryos.

Abstract 1374: Modeling the CRISPR/Cas9 structural complex with sgRNA and DNA

Abstract The ability of genome editing tool to correct genetic disease, or edit the tumor causing gene, will not only benefit human race, but also plants and animal kingdom. The CRISPR/Cas9 system can be used to target specific genomic loci by single guide RNAs (sgRNAs). Using publicly available literature, we have modeled the structure of Streptococcus pyogenes Cas9 molecule in complex with sgRNA and its target DNA at 2.5 A° resolution. The crystal structure determination revealed a two-lobed architecture made of: (a) target recognition and (b) nuclease lobes, positioning the sgRNA:DNA hetero-duplex in a positively charged groove at their interface. While the recognition lobe interacts with the sgRNA and DNA, the nuclease lobe contains the HNH and RuvC nuclease domains that are responsible for cleavage of the complementary and non-complementary target DNA strands, respectively. The nuclease lobe also harbors a carboxyl-terminal domain to provide additional specificity for interaction with the protospacer adjacent motif (PAM) DNA sequence. This structural analyses show the molecular mechanism of RNA-guided DNA targeting by Cas9, thus providing the CRIPSR/Cas9 genetic machinery its ability to edit specific DNA sequence. (Key Ref: Nishimasu H et al., Cell. 2014;156:935-49) Citation Format: Anita Wary. Modeling the CRISPR/Cas9 structural complex with sgRNA and DNA [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1374.

Enhanced Proofreading Governs CRISPR-Cas9 Targeting Accuracy

The RNA-guided CRISPR-Cas9 nuclease from Streptococcus pyogenes (SpCas9) has been widely repurposed for genome editing. DNA cleavage of Cas9 is controlled by the conformational state of the HNH nuclease domain, but the mechanism that governs HNH activation at on-target DNA while reducing cleavage activity at off-target sites remains poorly understood. Using single-molecule Förster resonance energy transfer (smFRET), we identified an intermediate state of Streptococcus pyogenes Cas9, representing a conformational checkpoint between DNA binding and cleavage. Upon DNA binding, the HNH domain transitions between multiple conformations before docking into its active state. We showed that the high-fidelity (SpCas9-HF1) and enhanced specificity (eSpCas9(1.1)) Cas9 variants are trapped in an inactive state when bound to mismatched targets. We find that a non-catalytic domain within Cas9, REC3, recognizes target complementarity and governs the HNH nuclease to regulate overall catalytic competence. Exploiting this observation, we designed a new hyper-accurate Cas9 variant (HypaCas9) that demonstrates high genome-wide specificity without compromising on-target activity in human cells. These results offer a more comprehensive model to rationalize and modify the balance between target recognition and nuclease activation for precision genome editing.

2'-O-(2-Methoxyethyl) Nucleosides Are Not Phosphorylated or Incorporated Into the Genome of Human Lymphoblastoid TK6 Cells.

Nucleoside analogs with 2'-modified sugar moieties are often used to improve the RNA target affinity and nuclease resistance of therapeutic oligonucleotides in preclinical and clinical development. Despite their enhanced nuclease resistance, oligonucleotides could slowly degrade releasing nucleoside analogs that have the potential to become phosphorylated and incorporated into cellular DNA and RNA. For the first time, the phosphorylation and DNA/RNA incorporation of 2'-O-(2-methoxyethyl) (2'-O-MOE) nucleoside analogs have been investigated. Using liquid chromatography/tandem mass spectrometry, we showed that enzymes in the nucleotide salvage pathway including deoxycytidine kinase (dCK) and thymidine kinase (TK1) displayed poor reactivity toward 2'-O-MOE nucleoside analogs. On the other hand, 2'-fluoro (F) nucleosides, regardless of the nucleobase, were efficiently phosphorylated to their monophosphate forms by dCK and TK1. Consistent with their efficient phosphorylation by dCK and TK1, 2'-F nucleoside analogs were incorporated into cellular DNA and RNA while no incorporation was detected with 2'-O-MOE nucleoside analogs. In conclusion, these data suggest that the inability of dCK and TK1 to create the monophosphates of 2'-O-MOE nucleoside analogs reduces the risk of their incorporation into cellular DNA and RNA.

Non-transgenic Approach to Deliver ZFNs in Seeds for Targeted Genome Engineering.

In the post-genomic era, the efficient exploitation of the available information for plant breeding is a pressing problem. The discoveries that DNA double-stranded breaks (DSBs) are both recombinagenic and mutagenic have fuelled the development of targetable zinc-finger nucleases (ZFNs), which act as molecular scissors for the induction of controlled DSBs. These powerful tools are used by researchers to accelerate mutagenesis of the normal gene loci toward the development of useful traits in plants. Seeds contain the embryo, which is a multicellular system representing a micrography of a plant. Therefore, they can serve as a foundation for applying targeted genome engineering techniques. The following single-step method describes how to deliver and express transiently ZFNs in tomato (Solanum lycopersicum) seeds using electroporation. Unlike methods that rely on tissue culture and plant regeneration after transformation, the direct delivery of ZFNs to seeds provides a high-throughput breeding technology for safe and site-specific mutagenesis. Tomato is a leading crop in the world and biotechnological advances in this species have great impact.

Enhanced proofreading governs CRISPR-Cas9 targeting accuracy.

The RNA-guided CRISPR-Cas9 nuclease from Streptococcus pyogenes (SpCas9) has been widely repurposed for genome editing1–4. High-fidelity (SpCas9-HF1) and enhanced specificity (eSpCas9(1.1)) variants exhibit substantially reduced off-target cleavage in human cells, but the mechanism of target discrimination and the potential to further improve fidelity were unknown5–9. Using single-molecule Förster resonance energy transfer (smFRET) experiments, we show that both SpCas9-HF1 and eSpCas9(1.1) are trapped in an inactive state10 when bound to mismatched targets. We find that a non-catalytic domain within Cas9, REC3, recognizes target complementarity and governs the HNH nuclease to regulate overall catalytic competence. Exploiting this observation, we designed a new hyper-accurate Cas9 variant (HypaCas9) that demonstrates high genome-wide specificity without compromising on-target activity in human cells. These results offer a more comprehensive model to rationalize and modify the balance between target recognition and nuclease activation for precision genome editing.

Genome-scale measurement of off-target activity using Cas9 toxicity in high-throughput screens

CRISPR-Cas9 screens are powerful tools for high-throughput interrogation of genome function, but can be confounded by nuclease-induced toxicity at both on- and off-target sites, likely due to DNA damage. Here, to test potential solutions to this issue, we design and analyse a CRISPR-Cas9 library with 10 variable-length guides per gene and thousands of negative controls targeting non-functional, non-genic regions (termed safe-targeting guides), in addition to non-targeting controls. We find this library has excellent performance in identifying genes affecting growth and sensitivity to the ricin toxin. The safe-targeting guides allow for proper control of toxicity from on-target DNA damage. Using this toxicity as a proxy to measure off-target cutting, we demonstrate with tens of thousands of guides both the nucleotide position-dependent sensitivity to single mismatches and the reduction of off-target cutting using truncated guides. Our results demonstrate a simple strategy for high-throughput evaluation of target specificity and nuclease toxicity in Cas9 screens.

Enzymatic polymerization-based formation of fluorescent copper nanoparticles for the nuclease assay

In this work, inspired by the idea that the DNA fragments of nuclease cleavage exactly can serve as the primers for terminal deoxynucleotidyl transferase (TdT) mediated polymerization, a versatile and sensitive method for nuclease assay was developed based on the enzymatic polymerization and poly(thymine)-templated copper nanoparticles (CuNPs). Specifically, in the presence of target nuclease, the DNA substrate was firstly degraded as mononucleotides and oligonucleotide fragments. Employing the fragments as primers, the TdT-assisted polymerization was initiated to generate poly(thymine) templates on which fluorescent CuNPs could be synthesized. The fluorescence intensity of CuNPs was detected for the nuclease assay. So different nucleases assay could be realized just by changing the designs of DNA substrates of the corresponding nucleases. For the versatility demonstration, two nucleases, Exo III and EcoR V as the model of exonuclease and endonuclease, respectively, were detected with this method. It was proved that the proposed method was of high selectivity and sensitivity, and the limits of detections of Exo III and EcoR V were found to be 0.0138 and 0.0115U/mL, respectively. What’s more, the satisfactory result of EcoR V detection in human serum indicates that this method is capable to detect nuclease in real sample.

Homology-Independent Integration of Plasmid DNA into the Zebrafish Genome.

Targeting nucleases like zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly interspaced short palindromic repeats/CRISPR-associated (CRISPR/Cas) system have revolutionized genome-editing possibilities in many model organisms. They allow the generation of loss-of-function alleles by the introduction of double-strand breaks at defined sites within genes, but also more sophisticated genome-editing approaches have become possible. These include the integration of donor plasmid DNA into the genome by homology-independent repair mechanisms after CRISPR/Cas9-mediated cleavage. Here we present a protocol outlining the most important steps to target a genomic site and to integrate a donor plasmid at this defined locus.