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Naturally Occurring Off-Switches for CRISPR-Cas9.
CRISPR-Cas9 technology would be enhanced by the ability to inhibit Cas9 function spatially, temporally, or conditionally. Previously, we discovered small proteins encoded by bacteriophages that inhibit the CRISPR-Cas systems of their host bacteria. These "anti-CRISPRs" were specific to type I CRISPR-Cas systems that do not employ the Cas9 protein. We posited that nature would also yield Cas9 inhibitors in response to the evolutionary arms race between bacteriophages and their hosts. Here, we report the discovery of three distinct families of anti-CRISPRs that specifically inhibit the CRISPR-Cas9 system of Neisseria meningitidis. We show that these proteins bind directly to N. meningitidis Cas9 (NmeCas9) and can be used as potent inhibitors of genome editing by this system in human cells. These anti-CRISPR proteins now enable "off-switches" for CRISPR-Cas9 activity and provide a genetically encodable means to inhibit CRISPR-Cas9 genome editing in eukaryotes. VIDEO ABSTRACT.
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Improved selection of zebrafish CRISPR editing by early next-generation sequencing based genotyping.
Despite numerous prior attempts to improve knock-in (KI) efficiency, the introduction of precise base pair substitutions by the CRISPR-Cas9 technique in zebrafish remains challenging. In our efforts to generate KI zebrafish models of human CACNA1C mutations, we have tested the effect of several CRISPR determinants on KI efficiency across two sites in a single gene and developed a novel method for early selection to ameliorate KI efficiency. We identified optimal KI conditions for Cas9 protein and non-target asymmetric PAM-distal single stranded deoxynucleotide repair templates at both cacna1c sites. An effect of distance to the cut site on the KI efficiency was only observed for a single repair template conformation at one of the two sites. By combining minimally invasive early genotyping with the zebrafish embryo genotyper (ZEG) device and next-generation sequencing, we were able to obtain an almost 17-fold increase in somatic editing efficiency. The added benefit of the early selection procedure was particularly evident for alleles with lower somatic editing efficiencies. We further explored the potential of the ZEG selection procedure for the improvement of germline transmission by demonstrating germline transmission events in three groups of pre-selected embryos.
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Engineered exosomes for CRISPR/Cas delivery to overcome oncogene-driven drug resistance.
Engineered exosomes are emerging as biocompatible nanocarriers for delivering CRISPR/Cas components to resistant tumor cells, enabling targeted disruption of oncogenic drivers and resistance-associated pathways. Engineered exosomes offer several delivery-platform advantages, including biocompatibility, membrane-mediated cargo protection, programmable tumor targeting, and potential tissue penetration. Selection of the CRISPR modality, Cas9 ribonucleoprotein, mRNA, base editor, or prime editor, depends on payload size, stability, editing duration, endosomal escape, and nuclear delivery requirements. Therapeutically, these systems may disrupt oncogenic drivers, inhibit resistance pathways, restore tumor-suppressor activity, and re-sensitize tumors to targeted therapy, chemotherapy, or immunotherapy. Clinical translation will require scalable manufacturing, reproducible cargo loading, standardized characterization, validated potency assays, off-target control, and clearly defined regulatory pathways. The goal of this review is to outline a realistic pathway featuring proof-of-concept research, through discoveries to the creation of manufacturable, safe, and effective exosome/CRISPR therapeutics that can trigger durable therapeutic responses in resistant malignancies.
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Enhancement of CRISPR-Cas12a system through universal circular RNA design.
Precise control of Cas12a activity is crucial to address incompatibility in isothermal amplification-CRISPR-Cas12a one-pot nucleic acid detection. We developed a light-triggerable circular RNA system for dynamic LbCas12a regulation. By employing circular CRISPR guide RNA (crRNA) or a split circular universal direct repeat region with a replaceable spacer, we resolved the incompatibility between isothermal amplification and CRISPR detection. This system demonstrated robust performance in detecting trace nucleic acids in clinical samples. Furthermore, DNA modifications on circular crRNA enabled CRISPR-Cas12a regulation via base excision repair (BER) enzymes, offering potential for BER enzyme detection and modulation of LbCas12a cleavage activity by BER enzymes. This programmable strategy holds promise for selective gene editing in cells with elevated BER enzyme expression, such as uracil DNA glycosylase (UDG) in colon cancer cells. The circular RNA-assisted approach represents a resource-efficient method with significant potential for medical diagnostics and future clinical gene therapy applications.
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CRISPR/Cas genome editing in soybean: challenges and new insights to overcome existing bottlenecks.
BACKGROUND: Soybean is a worldwide-cultivated crop due to its applications in the food, feed, and biodiesel industries. Genome editing in soybean began with ZFN and TALEN technologies; however, CRISPR/Cas has emerged and shortly became the preferable approach for soybean genome manipulation since it is more precise, easy to handle, and cost-effective. Recent reports have focused on the conventional Cas9 nuclease, Cas9 nickase (nCas9) derived base editors, and Cas12a (formally Cpf1) as the most commonly used genome editors in soybean. Nonetheless, several challenges in the complex plant genetic engineering pipeline need to be overcome to effectively edit the genome of an elite soybean cultivar. These challenges include (1) optimizing CRISPR cassette design (i.e., gRNA and Cas promoters, gRNA design and testing, number of gRNAs, and binary vector), (2) improving transformation frequency, (3) increasing the editing efficiency ratio of targeted plant cells, and (4) improving soybean crop production. AIM OF REVIEW: This review provides an overview of soybean genome editing using CRISPR/Cas technology, discusses current challenges, and highlights theoretical (insights) and practical suggestions to overcome the existing bottlenecks. KEY SCIENTIFIC CONCEPTS OF REVIEW: The CRISPR/Cas system was discovered as part of the bacterial innate immune system. It has been used as a biotechnological tool for genome editing and efficiently applied in soybean to unveil gene function, improve agronomic traits such as yield and nutritional grain quality, and enhance biotic and abiotic stress tolerance. To date, the efficiency of gRNAs has been validated using protoplasts and hairy root assays, while stable plant transformation relies on Agrobacterium-mediated and particle bombardment methods. Nevertheless, most steps of the CRISPR/Cas workflow require optimizations to achieve a more effective genome editing in soybean plants.
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Targeting transthyretin by one Cas9 variant with superfidelity and broad compatibility.
Amyloid transthyretin (ATTR) amyloidosis is a fatal disease caused by the accumulation of misfolded transthyretin proteins. Although knocking down the TTR gene by CRISPR-Cas9 represents a promising strategy for treating ATTR amyloidosis, its efficiency and safety remain to be further investigated. Here, we report a systematic investigation of SpCas9-based TTR editing. Besides the target site, wild-type SpCas9 and the reported variants induced extensive off-target edits. To improve the fidelity, we performed structural analysis and designed a series of SpCas9 variants. Studies demonstrated that SpCas9-Mut5 is an ultrahigh-fidelity variant, which induces extremely low levels of off-target edits and translocations without substantial impairment of on-target editing activity. SpCas9-Mut5 is compatible with the adenine base editor (ABE) system, markedly reducing off-target edits and narrowing the editing window. In conclusion, our study suggests that SpCas9-Mut5 is an excellent candidate for TTR gene editing. Besides ATTR amyloidosis, SpCas9-Mut5 and its derivative ABE could be widely used in the treatment of other diseases.
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Editing the Sickle Cell Disease Mutation in Human Hematopoietic Stem Cells: Comparison of Endonucleases and Homologous Donor Templates.
Site-specific correction of a point mutation causing a monogenic disease in autologous hematopoietic stem and progenitor cells (HSPCs) can be used as a treatment of inherited disorders of the blood cells. Sickle cell disease (SCD) is an ideal model to investigate the potential use of gene editing to transvert a single point mutation at the β-globin locus (HBB). We compared the activity of zinc-finger nucleases (ZFNs) and CRISPR/Cas9 for editing, and homologous donor templates delivered as single-stranded oligodeoxynucleotides (ssODNs), adeno-associated virus serotype 6 (AAV6), integrase-deficient lentiviral vectors (IDLVs), and adenovirus 5/35 serotype (Ad5/35) to transvert the base pair responsible for SCD in HBB in primary human CD34+ HSPCs. We found that the ZFNs and Cas9 directed similar frequencies of nuclease activity. In vitro, AAV6 led to the highest frequencies of homology-directed repair (HDR), but levels of base pair transversions were significantly reduced when analyzing cells in vivo in immunodeficient mouse xenografts, with similar frequencies achieved with either AAV6 or ssODNs. AAV6 also caused significant impairment of colony-forming progenitors and human cell engraftment. Gene correction in engrafting hematopoietic stem cells may be limited by the capacity of the cells to mediate HDR, suggesting additional manipulations may be needed for high-efficiency gene correction in HSPCs.
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CRISPR-Cas9 recognition of enzymatically synthesized base-modified nucleic acids.
An enzymatic method has been successfully established enabling the generation of partially base-modified RNA (previously named RZA) constructs, in which all G residues were replaced by isomorphic fluorescent thienoguanosine (thG) analogs, as well as fully modified RZA featuring thG, 5-bromocytosine, 7-deazaadenine and 5-chlorouracil. The transcriptional efficiency of emissive fully modified RZA was found to benefit from the use of various T7 RNA polymerase variants. Moreover, dthG could be incorporated into PCR products by Taq DNA polymerase together with the other three base-modified nucleotides. Notably, the obtained RNA products containing thG as well as thG together with 5-bromocytosine could function as effectively as natural sgRNAs in an in vitro CRISPR-Cas9 cleavage assay. N1-Methylpseudouridine was also demonstrated to be a faithful non-canonical substitute of uridine to direct Cas9 nuclease cleavage when incorporated in sgRNA. The Cas9 inactivation by 7-deazapurines indicated the importance of the 7-nitrogen atom of purines in both sgRNA and PAM site for achieving efficient Cas9 cleavage. Additional aspects of this study are discussed in relation to the significance of sgRNA-protein and PAM--protein interactions that were not highlighted by the Cas9-sgRNA-DNA complex crystal structure. These findings could expand the impact and therapeutic value of CRISPR-Cas9 and other RNA-based technologies.
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Efficient in vivo labeling of endogenous proteins with SMART delineates retina cellular and synaptic organization.
A key application of CRISPR/Cas9-based genomic editing is modification of genes to introduce engineered sequences. However, the editing flexibility is severely constrained by the requirement for targeting sites in proximity to the desired modification site, which makes many modifications intractable. Here, we develop a strategy that overcomes this key limitation to allow CRISPR-based editing at any position with high efficiency. It relies on reconstructing the targeted gene using Silently Mutate And Repair Template (SMART) where we mutate the gap sequence in the repair template to prevent its base pairing with the target DNA while maintaining the same amino acid coding. Using vertebrate retina as a neuronal model system we document the application of SMART editing for labeling endogenous proteins in vivo with high efficiency. We show that SMART editing allows us to access numerous cell types in the retina and address fundamental cell biological questions pertaining to its organization. We propose that this approach will facilitate functional genomic studies in a wide range of systems and increase the precision of corrective gene therapies.
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Application of the CRISPR/Cas9 system for modification of flower color in Torenia fournieri.
BACKGROUND: CRISPR/Cas9 technology is one of the most powerful and useful tools for genome editing in various living organisms. In higher plants, the system has been widely exploited not only for basic research, such as gene functional analysis, but also for applied research such as crop breeding. Although the CRISPR/Cas9 system has been used to induce mutations in genes involved in various plant developmental processes, few studies have been performed to modify the color of ornamental flowers. We therefore attempted to use this system to modify flower color in the model plant torenia (Torenia fournieri L.). RESULTS: We attempted to induce mutations in the torenia flavanone 3-hydroxylase (F3H) gene, which encodes a key enzyme involved in flavonoid biosynthesis. Application of the CRISPR/Cas9 system successfully generated pale blue (almost white) flowers at a high frequency (ca. 80% of regenerated lines) in transgenic torenia T0 plants. Sequence analysis of PCR amplicons by Sanger and next-generation sequencing revealed the occurrence of mutations such as base substitutions and insertions/deletions in the F3H target sequence, thus indicating that the obtained phenotype was induced by the targeted mutagenesis of the endogenous F3H gene. CONCLUSIONS: These results clearly demonstrate that flower color modification by genome editing with the CRISPR/Cas9 system is easily and efficiently achievable. Our findings further indicate that this system may be useful for future research on flower pigmentation and/or functional analyses of additional genes in torenia.
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