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Methods for detecting off-target effects of CRISPR/Cas9.
The CRISPR/Cas9 system has emerged as a revolutionary tool for gene editing, widely used in the biomedical field due to its simplicity, efficiency, and cost-effectiveness. However, evidence suggests that CRISPR/Cas9 can induce off-target effects, leading to unintended mutations that may compromise the precision of gene modifications. Consequently, predicting，detecting and evaluating these off-target effects is crucial for optimizing the accuracy and reliability of CRISPR/Cas9 system. This paper provides an overview of the various methodologies and strategies, used or to be used for identifying off-target effects in CRISPR/Cas9-based genome editing, offering insights to improve the precision and safety of CRISPR applications in research and therapeutics.
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A benchmark of computational CRISPR-Cas9 guide design methods.
The popularity of CRISPR-based gene editing has resulted in an abundance of tools to design CRISPR-Cas9 guides. This is also driven by the fact that designing highly specific and efficient guides is a crucial, but not trivial, task in using CRISPR for gene editing. Here, we thoroughly analyse the performance of 18 design tools. They are evaluated based on runtime performance, compute requirements, and guides generated. To achieve this, we implemented a method for auditing system resources while a given tool executes, and tested each tool on datasets of increasing size, derived from the mouse genome. We found that only five tools had a computational performance that would allow them to analyse an entire genome in a reasonable time, and without exhausting computing resources. There was wide variation in the guides identified, with some tools reporting every possible guide while others filtered for predicted efficiency. Some tools also failed to exclude guides that would target multiple positions in the genome. We also considered two collections with over a thousand guides each, for which experimental data is available. There is a lot of variation in performance between the datasets, but the relative order of the tools is partially conserved. Importantly, the most striking result is a lack of consensus between the tools. Our results show that CRISPR-Cas9 guide design tools need further work in order to achieve rapid whole-genome analysis and that improvements in guide design will likely require combining multiple approaches.
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Editor's comment on "CRISPR/Cas9-mediated gene editing in human zygotes using Cas9 protein".
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Current and prospective strategies for advancing the targeted delivery of CRISPR/Cas system via extracellular vesicles.
Extracellular vesicles (EVs) have emerged as a promising platform for gene delivery owing to their natural properties and phenomenal functions, being able to circumvent the significant challenges associated with toxicity, problematic biocompatibility, and immunogenicity of the standard approaches. These features are of particularly interest for targeted delivery of the emerging clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) systems. However, the current efficiency of EV-meditated transport of CRISPR/Cas components remains insufficient due to numerous exogenous and endogenous barriers. Here, we comprehensively reviewed the current status of EV-based CRISPR/Cas delivery systems. In particular, we explored various strategies and methodologies available to potentially improve the loading capacity, safety, stability, targeting, and tracking for EV-based CRISPR/Cas system delivery. Additionally, we hypothesise the future avenues for the development of EV-based delivery systems that could pave the way for novel clinically valuable gene delivery approaches, and may potentially bridge the gap between gene editing technologies and the laboratory/clinical application of gene therapies.
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Kinetics of dCas9 target search in Escherichia coli.
How fast can a cell locate a specific chromosomal DNA sequence specified by a single-stranded oligonucleotide? To address this question, we investigate the intracellular search processes of the Cas9 protein, which can be programmed by a guide RNA to bind essentially any DNA sequence. This targeting flexibility requires Cas9 to unwind the DNA double helix to test for correct base pairing to the guide RNA. Here we study the search mechanisms of the catalytically inactive Cas9 (dCas9) in living Escherichia coli by combining single-molecule fluorescence microscopy and bulk restriction-protection assays. We find that it takes a single fluorescently labeled dCas9 6 hours to find the correct target sequence, which implies that each potential target is bound for less than 30 milliseconds. Once bound, dCas9 remains associated until replication. To achieve fast targeting, both Cas9 and its guide RNA have to be present at high concentrations.
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[CRISPR-Cas9, germinal cells and human embryo].
The performance of the molecular tool using CRISPR-Cas9, which makes it possible to induce targeted modifications of the DNA, has found numerous applications in research and open promising prospects in human clinic. CRISPR-Cas9 has been widely used to generate transgenic animals after targeted modification of the genome at the zygotic stage. It was also tested on human embryos on an experimental basis. Although there are potential medical indications that may justify a targeted modification of the embryo or germ cell genome, the uncertainties regarding the efficacy and safety of the method do not allow us to consider implementing such germline gene therapy in the short-term. However, it is necessary to weigh the scientific and ethical issues involved in this approach.
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Learning to utilize internal protein 3D nanoenvironment descriptors in predicting CRISPR-Cas9 off-target activity.
Despite advances in determining the factors influencing cleavage activity of a CRISPR-Cas9 single guide RNA (sgRNA) at an (off-)target DNA sequence, a comprehensive assessment of pertinent physico-chemical/structural descriptors is missing. In particular, studies have not yet directly exploited the information-rich internal protein 3D nanoenvironment of the sgRNA-(off-)target strand DNA pair, which we obtain by harvesting 634 980 residue-level features for CRISPR-Cas9 complexes. As a proof-of-concept study, we simulated the internal protein 3D nanoenvironment for all experimentally available single-base protospacer-adjacent motif-distal mutations for a given sgRNA-target strand pair. By determining the most relevant residue-level features for CRISPR-Cas9 off-target cleavage activity, we developed STING_CRISPR, a machine learning model delivering accurate predictive performance of off-target cleavage activity for the type of single-base mutations considered in this study. By interpreting STING_CRISPR, we identified four important Cas9 residue spatial hotspots and associated structural/physico-chemical descriptor classes influencing CRISPR-Cas9 (off-)target cleavage activity for the sgRNA-target strand pairs covered in this study.
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A New Class of Medicines through DNA Editing.
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CRISPRcon: What Does Rome Look Like?
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CRISPR/Cas9-mediated targeted T-DNA integration in rice.
Combining with a CRISPR/Cas9 system, Agrobacterium-mediated transformation can lead to precise targeted T-DNA integration in the rice genome. Agrobacterium-mediated T-DNA integration into the plant genomes is random, which often causes variable transgene expression and insertional mutagenesis. Because T-DNA preferentially integrates into double-strand DNA breaks, we adapted a CRISPR/Cas9 system to demonstrate that targeted T-DNA integration can be achieved in the rice genome. Using a standard Agrobacterium binary vector, we constructed a T-DNA that contains a CRISPR/Cas9 system using SpCas9 and a gRNA targeting the exon of the rice AP2 domain-containing protein gene Os01g04020. The T-DNA also carried a red fluorescent protein and a hygromycin resistance (hptII) gene. One version of the vector had hptII expression driven by an OsAct2 promoter. In an effort to detect targeted T-DNA insertion events, we built another T-DNA with a promoterless hptII gene adjacent to the T-DNA right border such that integration of T-DNA into the targeted exon sequence in-frame with the hptII gene would allow hptII expression. Our results showed that these constructs could produce targeted T-DNA insertions with frequencies ranging between 4 and 5.3% of transgenic callus events, in addition to generating a high frequency (50-80%) of targeted indel mutations. Sequencing analyses showed that four out of five sequenced T-DNA/gDNA junctions carry a single copy of full-length T-DNA at the target site. Our results indicate that Agrobacterium-mediated transformation combined with a CRISPR/Cas9 system can efficiently generate targeted T-DNA insertions.
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