A cell cycle-dependent CRISPR-Cas9 activation system based on an anti-CRISPR protein shows improved genome editing accuracy

Improving CRISPR-Cas9 directed faithful transgene integration outcomes by reducing unwanted random DNA integration

BACKGROUND

The field of genome editing has been revolutionized by the development of an easily programmable editing tool, the CRISPR-Cas9. Despite its promise, off-target activity of Cas9 posed a great disadvantage for genome editing purposes by causing DNA double strand breaks at off-target locations and causing unwanted editing outcomes. Furthermore, for gene integration applications, which introduce transgene sequences, integration of transgenes to off-target sites could be harmful, hard to detect, and reduce faithful genome editing efficiency.

METHOD

Here we report the development of a multicolour fluorescence assay for studying CRISPR-Cas9-directed gene integration at an endogenous locus in human cell lines. We examine genetic integration of reporter genes in transiently transfected cells as well as puromycin-selected stable cell lines to determine the fidelity of multiple CRISPR-Cas9 strategies.

RESULT

We found that there is a high occurrence of unwanted DNA integration which tarnished faithful knock-in efficiency. Integration outcomes are influenced by the type of DNA DSBs, donor design, the use of enhanced specificity Cas9 variants, with S-phase regulated Cas9 activity. Moreover, restricting Cas9 expression with a self-cleaving system greatly improves knock-in outcomes by substantially reducing the percentage of cells with unwanted DNA integration.

CONCLUSION

Our results highlight the need for a more stringent assessment of CRISPR-Cas9-mediated knock-in outcomes, and the importance of careful strategy design to maximise efficient and faithful transgene integration.

SpCas9-HF1 enhances accuracy of cell cycle-dependent genome editing by increasing HDR efficiency, and by reducing off-target effects and indel rates

In genome editing, it is important to avoid off-target mutations so as to reduce unexpected side effects, especially for therapeutic applications. Recently, several high-fidelity versions of SpCas9 have been developed to reduce off-target mutations. In addition to reducing off-target effects, highly efficient intended target gene correction is also essential to rescue protein functions that have been disrupted by single nucleotide polymorphisms. Homology-directed repair (HDR) corrects genes precisely using a DNA template. Our recent development of cell cycle-dependent genome editing has shown that regulation of Cas9 activation with an anti-CRISPR-Cdt1 fusion protein increases HDR efficiency and reduces off-target effects. In this study, to apply high-fidelity SpCas9 variants to cell cycle-dependent genome editing, we evaluated anti-CRISPR inhibition of high-fidelity SpCas9s. In addition, HDR efficiency of high-fidelity SpCas9s was addressed, identifying eSpCas9, SpCas9-HF1, and LZ3 Cas9 as promising candidates. Although eSpCas9 and LZ3 Cas9 showed decreased HDR efficiency in cell cycle-dependent genome editing, SpCas9-HF1 successfully achieved increased HDR efficiency and few off-target effects when co-expressed with an AcrIIA4-Cdt1 fusion.

Current Strategies for Increasing Knock-In Efficiency in CRISPR/Cas9-Based Approaches

Since its discovery in 2012, the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) system has supposed a promising panorama for developing novel and highly precise genome editing-based gene therapy (GT) alternatives, leading to overcoming the challenges associated with classical GT. Classical GT aims to deliver transgenes to the cells via their random integration in the genome or episomal persistence into the nucleus through lentivirus (LV) or adeno-associated virus (AAV), respectively. Although high transgene expression efficiency is achieved by using either LV or AAV, their nature can result in severe side effects in humans. For instance, an LV (NCT03852498)- and AAV9 (NCT05514249)-based GT clinical trials for treating X-linked adrenoleukodystrophy and Duchenne Muscular Dystrophy showed the development of myelodysplastic syndrome and patient's death, respectively. In contrast with classical GT, the CRISPR/Cas9-based genome editing requires the homologous direct repair (HDR) machinery of the cells for inserting the transgene in specific regions of the genome. This sophisticated and well-regulated process is limited in the cell cycle of mammalian cells, and in turn, the nonhomologous end-joining (NHEJ) predominates. Consequently, seeking approaches to increase HDR efficiency over NHEJ is crucial. This manuscript comprehensively reviews the current alternatives for improving the HDR for CRISPR/Cas9-based GTs.

Anti-CRISPR Proteins and Their Application to Control CRISPR Effectors in Mammalian Systems

CRISPR-Cas effectors are powerful tools for genome and transcriptome targeting and editing. Naturally, these protein-RNA complexes are part of the microbial innate immune system, which emerged from the evolutionary arms race between microbes and phages. This coevolution has also given rise to so-called anti-CRISPR (Acr) proteins that counteract the CRISPR-Cas adaptive immunity. Acrs constitutively block cognate CRISPR-Cas effectors, e.g., by interfering with guide RNA binding, target DNA/RNA recognition, or target cleavage. In addition to their important role in microbiology and evolution, Acrs have recently gained particular attention for being useful tools and switches to regulate or fine-tune the activity of CRISPR-Cas effectors. Due to their commonly small size, high inhibition potency, and structural and mechanistic versatility, Acrs offer a wide range of potential applications for controlling CRISPR effectors in heterologous systems, including mammalian cells.Here, we review the diverse applications of Acrs in mammalian cells and organisms and discuss the underlying engineering strategies. These applications include (i) persistent blockage of CRISPR-Cas function to create write-protected cells, (ii) reduction of CRISPR-Cas off-target editing, (iii) focusing CRISPR-Cas activity to specific cell types and tissues, (iv) spatiotemporal control of CRISPR effectors based on engineered, opto-, or chemogenetic Acrs, and (v) the use of Acrs for selective binding and detection of CRISPR-Cas effectors in complex samples. We will also highlight potential future applications of Acrs in a biomedical context and point out present challenges that need to be overcome on the way.

Engineering traits through CRISPR/cas genome editing in woody species to improve forest diversity and yield

Dangers confronting forest ecosystems are many and the strength of these biological systems is deteriorating, thus substantially affecting tree physiology, phenology, and growth. The establishment of genetically engineered trees into degraded woodlands, which would be adaptive to changing climate, could help in subsiding ecological threats and bring new prospects. This should not be resisted due to the apprehension of transgene dispersal in forests. Consequently, it is important to have a deep insight into the genetic structure and phenotypic limits of the reproductive capability of tree stands/population(s) to endure tolerance and survival. Importantly, for a better understanding of genes and their functional mechanisms, gene editing (GeEd) technology is an excellent molecular tool to unravel adaptation progressions. Therefore, GeEd could be harnessed for resolving the allelic interactions for the creation of gene diversity, and transgene dispersal may be alleviated among the population or species in different bioclimatic zones around the globe. This review highlights the potential of the CRISPR/Cas tools in genomic, transcriptomic, and epigenomic-based assorted and programmable alterations of genes in trees that might be able to fix the trait-specific gene function. Also, we have discussed the application of diverse forms of GeEd to genetically improve several traits, such as wood density, phytochemical constituents, biotic and abiotic stress tolerance, and photosynthetic efficiency in trees. We believe that the technology encourages fundamental research in the forestry sector besides addressing key aspects, which might fasten tree breeding and germplasm improvement programs worldwide.

Anti-CRISPR Protein AcrIIC5 Inhibits CRISPR-Cas9 by Occupying the Target DNA Binding Pocket

Anti-CRISPR proteins inhibit CRISPR-Cas immune systems through diverse mechanisms. Previously, the anti-CRISPR protein AcrIIC5_Smu was shown to potently inhibit a type II-C Cas9 from Neisseria meningitidis (Nme1Cas9). In this work, we explore the mechanism of activity of the AcrIIC5 homologue from Neisseria chenwenguii (AcrIIC5_Nch) and show that it prevents Cas9 binding to target DNA. We show that AcrIIC5_Nch targets the PAM-interacting domain (PID) of Nme1Cas9 for inhibition, agreeing with previous findings for AcrIIC5_Smu, and newly establish that strong binding of the anti-CRISPR requires guide RNA be pre-loaded on Cas9. We determined the crystal structure of AcrIIC5_Nch using X-ray crystallography and identified amino acid residues that are critical for its function. Using a protein docking algorithm we show that AcrIIC5_Nch likely occupies the Cas9 DNA binding pocket, thereby inhibiting target DNA binding through a mechanism similar to that previously described for AcrIIA2 and AcrIIA4.

A novel dual-targeting delivery system for specific delivery of CRISPR/Cas9 using hyaluronic acid, chitosan and AS1411

A facile method was designed that can specifically deliver CRISPR/Cas9 into target cells nuclei and reduce the off-target effects. A multifunctional delivery vector for FOXM1 knockout was composed by integration of cell targeting polymer (hyaluronic acid) and cell and nuclear targeting group (AS1411 aptamer) on the surface of nanoparticles formed by genome editing plasmid and chitosan (CS) as the core (Apt-HA-CS-CRISPR/Cas9). The data of cytotoxicity experiment and western blot confirmed this issue. The results of flow cytometry analysis and fluorescence imaging demonstrated that Apt-HA-CS-CRISPR/Cas9 was significantly internalized into target cells (MCF-7, SK-MES-1, HeLa) but not into nontarget cells (HEK293). Furthermore, the in vivo studies displayed that the Apt-HA-CS-CRISPR/Cas9 was strongly rendered tumor inhibitory effect and delivered efficiently CRISPR/Cas9 into the tumor with no detectable distribution in other organs compared with naked plasmid. This approach provides an avenue for specific in vivo gene editing therapeutics with the lowest side effect.

CRISPRthripsis: The Risk of CRISPR/Cas9-induced Chromothripsis in Gene Therapy

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 nuclease system has allowed the generation of disease models and the development of therapeutic approaches for many genetic and non-genetic disorders. However, the generation of large genomic rearrangements has raised safety concerns for the clinical application of CRISPR/Cas9 nuclease approaches. Among these events, the formation of micronuclei and chromosome bridges due to chromosomal truncations can lead to massive genomic rearrangements localized to one or few chromosomes. This phenomenon, known as chromothripsis, was originally described in cancer cells, where it is believed to be caused by defective chromosome segregation during mitosis or DNA double-strand breaks. Here, we will discuss the factors influencing CRISPR/Cas9-induced chromothripsis, hereafter termed CRISPRthripsis, and its outcomes, the tools to characterize these events and strategies to minimize them.

Advance trends in targeting homology-directed repair for accurate gene editing: An inclusive review of small molecules and modified CRISPR-Cas9 systems

Introduction: Clustered regularly interspaced short palindromic repeat and its associated protein (CRISPR-Cas)-based technologies generate targeted modifications in host genome by inducing site-specific double-strand breaks (DSBs) that can serve as a substrate for homology-directed repair (HDR) in both in vitro and in vivo models. HDR pathway could enhance incorporation of exogenous DNA templates into the CRISPR-Cas9-mediated DSB site. Owing to low rate of HDR pathway, the efficiency of accurate genome editing is diminished. Enhancing the efficiency of HDR can provide fast, easy, and accurate technologies based on CRISPR-Cas9 technologies.

Methods: The current study presents an overview of attempts conducted on the precise genome editing strategies based on small molecules and modified CRISPR-Cas9 systems.

Results: In order to increase HDR rate in targeted cells, several logical strategies have been introduced such as generating CRISPR effector chimeric proteins, anti-CRISPR proteins, modified Cas9 with donor template, and using validated synthetic or natural small molecules for either inhibiting non-homologous end joining (NHEJ), stimulating HDR, or synchronizing cell cycle. Recently, high-throughput screening methods have been applied for identification of small molecules which along with the CRISPR system can regulate precise genome editing through HDR.

Conclusion: The stimulation of HDR components or inhibiting NHEJ can increase the accuracy of CRISPR-Cas-mediated engineering systems. Generating chimeric programmable endonucleases provide this opportunity to direct DNA template close proximity of CRISPR-Cas-mediated DSB. Small molecules and their derivatives can also proficiently block or activate certain DNA repair pathways and bring up novel perspectives for increasing HDR efficiency, especially in human cells. Further, high throughput screening of small molecule libraries could result in more discoveries of promising chemicals that improve HDR efficiency and CRISPR-Cas9 systems.

Improving Homology-Directed Repair in Genome Editing Experiments by Influencing the Cell Cycle

Genome editing is currently widely used in biomedical research; however, the use of this method in the clinic is still limited because of its low efficiency and possible side effects. Moreover, the correction of mutations that cause diseases in humans seems to be extremely important and promising. Numerous attempts to improve the efficiency of homology-directed repair-mediated correction of mutations in mammalian cells have focused on influencing the cell cycle. Homology-directed repair is known to occur only in the late S and G2 phases of the cell cycle, so researchers are looking for safe ways to enrich the cell culture with cells in these phases of the cell cycle. This review surveys the main approaches to influencing the cell cycle in genome editing experiments (predominantly using Cas9), for example, the use of cell cycle synchronizers, mitogens, substances that affect cyclin-dependent kinases, hypothermia, inhibition of p53, etc. Despite the fact that all these approaches have a reversible effect on the cell cycle, it is necessary to use them with caution, since cells during the arrest of the cell cycle can accumulate mutations, which can potentially lead to their malignant transformation.

From DNA break repair pathways to CRISPR/Cas-mediated gene knock-in methods

Various DNA breaks created via programmable CRISPR/Cas9 nuclease activity results in different intracellular DNA break repair pathways. Based on the cellular repair pathways, CRISPR-based gene knock-in methods can be categorized into two major strategies: 1) Homology-independent strategies which are targeted insertion events based on non-homologous end joining, and 2) Homology-dependent strategies which are targeted insertion events based on the homology-directed repair. This review elaborates on various gene knock-in methods in mammalian cells using the CRISPR/Cas9 system and in sync with DNA-break repair pathways. Gene knock-in methods are applied in functional genomics and gene therapy. To compensate or correct genetic defects, different CRISPR-based gene knock-in strategies can be used. Thus, researchers need to make a conscious decision about the most suitable knock-in method. For a successful gene-targeted insertion, some determinant factors should be considered like cell cycle, dominant DNA repair pathway, size of insertions, and donor properties. In this review, different aspects of each gene knock-in strategy are discussed to provide a framework for choosing the most appropriate gene knock-in method in different applications.

Strategies for Enhancing the Homology-directed Repair Efficiency of CRISPR-Cas Systems

The CRISPR-Cas nuclease has emerged as a powerful genome-editing tool in recent years. The CRISPR-Cas system induces double-strand breaks that can be repaired via the non-homologous end joining or homology-directed repair (HDR) pathway. Compared to non-homologous end joining, HDR can be used for the treatment of incurable monogenetic diseases. Therefore, remarkable efforts have been dedicated to enhancing the efficacy of HDR. In this review, we summarize the currently used strategies for enhancing the HDR efficiency of CRISPR-Cas systems based on three factors: (1) regulation of the key factors in the DNA repair pathways, (2) modulation of the components in the CRISPR machinery, and (3) alteration of the intracellular environment around double-strand breaks. Representative cases and potential solutions for further improving HDR efficiency are also discussed, facilitating the development of new CRISPR technologies to achieve highly precise genetic manipulation in the future.

CRISPR/Cas9-mediated base-editing enables a chain reaction through sequential repair of sgRNA scaffold mutations

Cell behavior is controlled by complex gene regulatory networks. Although studies have uncovered diverse roles of individual genes, it has been challenging to record or control sequential genetic events in living cells. In this study, we designed two cellular chain reaction systems that enable sequential sgRNA activation in mammalian cells using a nickase Cas9 tethering of a cytosine nucleotide deaminase (nCas9-CDA). In these systems, thymidine (T)-to-cytosine (C) substitutions in the scaffold region of the sgRNA or the TATA box-containing loxP sequence (TATAloxP) are corrected by the nCas9-CDA, leading to activation of the next sgRNA. These reactions can occur multiple times, resulting in cellular chain reactions. As a proof of concept, we established a chain reaction by repairing sgRNA scaffold mutations in 293 T cells. Importantly, the results obtained in yeast or in vitro did not match those obtained in mammalian cells, suggesting that in vivo chain reactions need to be optimized in appropriate cellular contexts. Our system may lay the foundation for building cellular chain reaction systems that have a broad utility in the future biomedical research.

Fine-Tuning Cas9 Activity with a Cognate Inhibitor AcrIIA4 to Improve Genome Editing in Streptomyces

Efficient enabling technology is required for synthetic biology in Streptomyces due to its natural product reservoir. Though the CRISPR-Cas9 system is powerful for genome editing in this genus, the proposed Cas9 toxicity has limited its application. Here on the basis of previous inducible Cas9 expression at the transcriptional and translational levels coupled with atpD overexpression, a Cas9 cognate inhibitor AcrIIA4 was further introduced to fine-tune the Cas9 activity. In both laboratory and industrial Streptomyces species, we showed that, compared to the constitutively expressed Cas9, incorporating AcrIIA4 increased the conjugation efficiency from 700- to 7000-fold before induction, while a comparable 65%-90% editing efficiency was obtained even on multiple loci for simultaneous deletion after Cas9 expression was induced, along with no significant off-targets. Thus, AcrIIA4 could be a modulator to control Cas9 activity to significantly improve genome editing, and this new toolkit would be widely adaptable and fasten genetic engineering in Streptomyces.

The Off-Targets of Clustered Regularly Interspaced Short Palindromic Repeats Gene Editing

The repurposing of the CRISPR/Cas bacterial defense system against bacteriophages as simple and flexible molecular tools has revolutionized the field of gene editing. These tools are now widely used in basic research and clinical trials involving human somatic cells. However, a global moratorium on all clinical uses of human germline editing has been proposed because the technology still lacks the required efficacy and safety. Here we focus on the approaches developed since 2013 to decrease the frequency of unwanted mutations (the off-targets) during CRISPR-based gene editing.

Tissue specificity of DNA repair: the CRISPR compass

CRISPR-Cas9-mediated genome editing holds great promise for the correction of pathogenic variants in humans. However, its therapeutic implementation is hampered due to unwanted editing outcomes. A better understanding of cell type- and tissue-specific DNA repair processes will ultimately enable precise control of editing outcomes for safer and effective therapies.

Small nucleic acids and the path to the clinic for anti-CRISPR

CRISPR-based therapeutics have entered clinical trials but no methods to inhibit Cas enzymes have been demonstrated in a clinical setting. The ability to inhibit CRISPR-based gene editing or gene targeting drugs should be considered a critical step in establishing safety standards for many CRISPR-Cas therapeutics. Inhibitors can act as a failsafe or as an adjuvant to reduce off-target effects in patients. In this review we discuss the need for clinical inhibition of CRISPR-Cas systems and three existing inhibitor technologies: anti-CRISPR (Acr) proteins, small molecule Cas inhibitors, and small nucleic acid-based CRISPR inhibitors, CRISPR SNuBs. Due to their unique properties and the recent successes of other nucleic acid-based therapeutics, CRISPR SNuBs appear poised for clinical application in the near-term.

Resistance and Adaptation of Bacteria to Non-Antibiotic Antibacterial Agents: Physical Stressors, Nanoparticles, and Bacteriophages

Antimicrobial resistance is a significant threat to human health worldwide, forcing scientists to explore non-traditional antibacterial agents to support rapid interventions and combat the emergence and spread of drug resistant bacteria. Many new antibiotic-free approaches are being developed while the old ones are being revised, resulting in creating unique solutions that arise at the interface of physics, nanotechnology, and microbiology. Specifically, physical factors (e.g., pressure, temperature, UV light) are increasingly used for industrial sterilization. Nanoparticles (unmodified or in combination with toxic compounds) are also applied to circumvent in vivo drug resistance mechanisms in bacteria. Recently, bacteriophage-based treatments are also gaining momentum due to their high bactericidal activity and specificity. Although the number of novel approaches for tackling the antimicrobial resistance crisis is snowballing, it is still unclear if any proposed solutions would provide a long-term remedy. This review aims to provide a detailed overview of how bacteria acquire resistance against these non-antibiotic factors. We also discuss innate bacterial defense systems and how bacteriophages have evolved to tackle them.

Molecular Switch Engineering for Precise Genome Editing

Genome editing technology commenced in 1996 with the discovery of the first zinc-finger nuclease. Application of Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) associated protein 9 (Cas9) technology to genome editing of mammalian cells allowed researchers to use genome editing more easily and cost-effectively. However, one of the technological problems that remains to be solved is "off-target effects", which are unexpected mutations in nontarget DNA. One significant improvement in genome editing technology has been achieved with molecular/protein engineering. The key to this engineering is a "switch" to control function. In this review, we discuss recent efforts to design novel "switching" systems for precise editing using genome editing tools.