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[The Development of SpCas9 Variants with High Specificity and Efficiency Based on the HH Theory].
Streptococcus pyogenes Cas9 (SpCas9) is the most popular tool in gene editing; however, off-target mutagenesis is one of the biggest impediments in its application. In our previous study, we proposed the HH theory, which states that sgRNA/DNA hybrid (hybrid) extrusion-induced enhancement of hydrophobic interactions between the hybrid and REC3/HNH is a key factor in cleavage initiation. Based on the HH theory, we analyzed the interactions between the REC3 domain and hybrid and obtained 8 mutant sites. We designed 8 SpCas9 variants (V1-V8), used digital droplet PCR to assess SpCas9-induced DNA indels in human cells, and developed high-fidelity variants. Thus, the HH theory may be employed to further optimize SpCas9-mediated genome editing systems, and the resultant V3, V6, V7, and V8 SpCas9 variants may be valuable for applications requiring high-precision genome editing.
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Generation of 3'UTR knockout cell lines by CRISPR/Cas9-mediated genome editing.
In addition to the protein code, messenger RNAs (mRNAs) also contain untranslated regions (UTRs). 3'UTRs span the region between the translational stop codon and the poly(A) tail. Sequence elements located in 3'UTRs are essential for pre-mRNA processing. 3'UTRs also contain elements that can regulate protein abundance, localization, and function. At least half of all human genes use alternative cleavage and polyadenylation (APA) to further diversify the regulatory potential of protein functions. Traditional gene editing approaches are designed to disrupt the production of functional proteins. Here, we describe a method that allows investigators to manipulate 3'UTR sequences of endogenous genes for both single- 3'UTR and multi-3'UTR genes. As 3'UTRs can regulate individual functions of proteins, techniques to manipulate 3'UTRs at endogenous gene loci will help to disentangle multi-functionality of proteins. Furthermore, the ability to directly examine the impact of gene regulatory elements in 3'UTRs will provide further insights into their functional significance.
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A Novel System for Simple Rapid Adenoviral Vector Construction to Facilitate CRISPR/Cas9-Mediated Genome Editing.
Recombinant adenoviruses have broad applications for gene delivery and expression. Furthermore, the adenovirus packaging system facilitates the expression of RNA-guided CRISPR/Cas9 nuclease complexes. In this study, we developed a novel system, named AdBlue, for the construction of recombinant adenoviruses using an enzymatic assembly strategy. This system could significantly reduce the time and labor required to generate adenoviral vectors. When applied to CRISPR/Cas9 design, it simplifies the preparation of recombinant adenoviruses carrying nuclease complexes and can induce high levels of site-specific mutagenesis. Our system has outstanding advantages for adenovirus preparation and could be a useful molecular engineering tool for gene delivery and editing.
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Two Plasmid-Based Systems for CRISPR/Cas9 Mediated Knockout of Target Genes.
CRISPR/Cas9-based gene editing is a recent advance that allows for the knockout or alteration of target genes within mammalian cells. Many variations of the technique exist, but here we describe two systems of plasmid-based CRISPR gene knockout which together allow for the selective knockout of virtually any gene target. Compared with other CRISPR-based systems, these plasmids have the advantages of delivering all the necessary components in one plasmid, choice of multiple selectable markers, and choice of route of administration into target cells. In addition, potential off-target effects from one system (dependent upon selection of target gene) can be overcome through use of the second system. Strategies for optimizing the knockout process and selection of finished cell lines are also presented.
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T cell engineering by targeted small extracellular vesicles delivering CRISPR/Cas9 system for PD-1 knock-out.
Among the emerging cancer therapies, immunotherapy using T cell engineering stands out as one of the most promising approaches. While the clustered regularly interspaced palindromic repeats (CRISPR/Cas) system is widely recognized as one of the most common gene-editing system tools, the main challenge for in vivo delivery applications remains the lack of an efficient and safe intracellular carrier. Small extracellular vesicles (s-EVs), as natural nanoparticles with low immunogenicity, emerge as a promising candidate for delivering the CRISPR/Cas system. Two guide RNAs (gRNAs) targeting the programmed cell death (PD-1) gene were cloned into the PX-459 vector, and these CRISPR/Cas9 plasmids were loaded into targeted s-EVs by electroporation. Subsequently, T cells were exposed to these s-EVs, and the disruption to the PD-1 gene was examined using flow cytometry and sequencing. Flow cytometry results indicated that newly produced targeted s-EVs exhibited a higher uptake by T cells compared to non-targeted s-EVs. The successful loading of PX-459 plasmids into targeted s-EVs and their functional delivery to T cells were confirmed. Subsequent carboxyfluorescein succinimidyl ester (CFSE) staining and enzyme-linked immunosorbent assay (ELISA) analyses demonstrated that the knockout (KO) of the PD-1 gene in T cells significantly enhanced both their cytotoxicity and cytokine release compared to untreated T cells. Targeted s-EVs were used in the current study as a means to deliver the CRISPR/Cas9 system into T cells, enabling genetic modification of target cells. This approach holds the potential to improve and simplify immunotherapy. This study demonstrates a novel approach to genetic modification in T cells using targeted s-EVs containing the CRISPR/Cas9 gene-editing system. KEY POINTS: •CRISPR/Cas9 is a kind of powerful gene editing system in cancer therapeutics. •s-EVs can be applied as a biological carrier for CRISPR/Cas9 system. • T cell engineering-based immunotherapy is a promising form of anticancer therapy.
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A cyclical marker system enables indefinite series of oligonucleotide-directed gene editing in Chlamydomonas reinhardtii.
CRISPR/Cas9 gene editing in the model green alga Chlamydomonas reinhardtii relies on the use of selective marker genes to enrich for nonselectable target mutations. This becomes challenging when many sequential modifications are required in a single-cell line, as useful markers are limited. Here, we demonstrate a cyclical selection process which only requires a single marker gene to identify an almost infinite sequential series of CRISPR-based target gene modifications. We used the NIA1 (Nit1, NR; nitrate reductase) gene as the selectable marker in this study. In the forward stage of the cycle, a stop codon was engineered into the NIA1 gene at the CRISPR target location. Cells retaining the wild-type NIA1 gene were killed by chlorate, while NIA1 knockout mutants survived. In the reverse phase of the cycle, the stop codon engineered into the NIA1 gene during the forward phase was edited back to the wild-type sequence. Using nitrate as the sole nitrogen source, only the reverted wild-type cells survived. By using CRISPR to specifically deactivate and reactivate the NIA1 gene, a marker system was established that flipped back and forth between chlorate- and auxotrophic (nitrate)-based selection. This provided a scarless cyclical marker system that enabled an indefinite series of CRISPR edits in other, nonselectable genes. We demonstrate that this "Sequential CRISPR via Recycling Endogenous Auxotrophic Markers (SCREAM)" technology enables an essentially limitless series of genetic modifications to be introduced into a single-cell lineage of C. reinhardtii in a fast and efficient manner to complete complex genetic engineering.
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CRISPR/CAS9-Mediated Antiviral Activity: A Tool to Combat Viral Infection.
Viruses hijack host cellular receptors and functions for replication, thereby posing a complication in identifying therapeutic targets. The CRISPR/Cas 9 platform has revolutionized gene-editing modalities in a wide range of cells and organisms with high potential in therapeutics. Recently, it has been put to work targeting human pathogenic viruses that interrupt receptors and functions with viral replication. This review encompasses major discoveries in CRISPR/Cas as an antiviral strategy. Additionally, challenges that demand consideration prior to its use in the clinic as part of the antiviral armamentarium are briefly addressed.
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Fast Fission Yeast Genome Editing by CRISPR/Cas9 Using Gap Repair and Fluoride Selection.
We present a protocol to perform CRISPR/Cas9-mediated genome editing in the fission yeast Schizosaccharomyces pombe that does not require cloning and uses the fluoride exporter channel Fex1 as the selection marker. Transformation is typically carried out on the same day of PCR primer arrival and successfully edited strains are selected 5 days after transformation. We expect the adoption of this protocol to further accelerate the throughput of genome editing in S. pombe.
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High-Throughput Imaging of CRISPR- and Recombinant Adeno-Associated Virus-Induced DNA Damage Response in Human Hematopoietic Stem and Progenitor Cells.
CRISPR-Cas technology has revolutionized gene editing, but concerns remain due to its propensity for off-target interactions. This, combined with genotoxicity related to both CRISPR-Cas9-induced double-strand breaks and transgene delivery, poses a significant liability for clinical genome-editing applications. Current best practice is to optimize genome-editing parameters in preclinical studies. However, quantitative tools that measure off-target interactions and genotoxicity are costly and time-consuming, limiting the practicality of screening large numbers of potential genome-editing reagents and conditions. Here, we show that flow-based imaging facilitates DNA damage characterization of hundreds of human hematopoietic stem and progenitor cells per minute after treatment with CRISPR-Cas9 and recombinant adeno-associated virus serotype 6. With our web-based platform that leverages deep learning for image analysis, we find that greater DNA damage response is observed for guide RNAs with higher genome-editing activity, differentiating even single on-target guide RNAs with different levels of off-target interactions. This work simplifies the characterization and screening process of genome-editing parameters toward enabling safer and more effective gene-therapy applications.
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Harnessing CRISPR/Cas systems for programmable transcriptional and post-transcriptional regulation.
Genome editing has enabled broad advances and novel approaches in studies of gene function and structure; now, emerging methods aim to precisely engineer post-transcriptional processes. Developing precise, efficient molecular tools to alter the transcriptome holds great promise for biotechnology and synthetic biology applications. Different approaches have been employed for targeted degradation of RNA species in eukaryotes, but they lack programmability and versatility, thereby limiting their utility for diverse applications. The CRISPR/Cas9 system has been harnessed for genome editing in many eukaryotic species and, using a catalytically inactive Cas9 variant, the CRISPR/dCas9 system has been repurposed for transcriptional regulation. Recent studies have used other CRISPR/Cas systems for targeted RNA degradation and RNA-based manipulations. For example, Cas13a, a Type VI-A endonuclease, has been identified as an RNA-guided RNA ribonuclease and used for manipulation of RNA. Here, we discuss different modalities for targeted RNA interference with an emphasis on the potential applications of CRISPR/Cas systems as programmable transcriptional regulators for broad uses, including functional biology, biotechnology, and synthetic biology applications.
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