CRISPR-Cas13 Inhibitors Block RNA Editing in Bacteria and Mammalian Cells

Comprehensive strategies for constructing efficient CRISPR/Cas based cancer therapy: Target gene selection, sgRNA optimization, delivery methods and evaluation

Cancer is a complicated disease that results from the interplay between specific changes in cellular genetics and diverse microenvironments. The application of high-performance and customizable clustered regularly interspaced palindromic repeats/associated protein (CRISPR/Cas) nuclease systems has significantly enhanced genome editing for accurate cancer modeling and facilitated simultaneous genetic modification for cancer therapy and mutation identification. Achieving an effective CRISPR/Cas platform for cancer treatment depends on the identification, selection, and optimization of specific mutated genes in targeted cancer tissues. However, overcoming the off-target effects, specificity, and immunogenicity are additional challenges that must be addressed while developing a gene editing system for cancer therapy. From this perspective, we briefly covered the pipeline of CRISPR/Cas cancer therapy, identified target genes to optimize gRNAs and sgRNAs, and explored alternative delivery modalities, including viral, non-viral, and extracellular vesicles. In addition, the list of patents and current clinical trials related to this unique cancer therapy method is discussed. In summary, we have discussed comprehensive start-to-end pipeline strategies for CRISPR/Cas development to advance the precision, effectiveness, and safety of clinical applications for cancer therapy.

Current updates on the structural and functional aspects of the CRISPR/Cas13 system for RNA targeting and editing: A next‑generation tool for cancer management (Review)

For centuries, a competitive evolutionary race between prokaryotes and related phages or other mobile genetic elements has led to the diversification of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR‑associated sequence (Cas) genome‑editing systems. Among the different CRISPR/Cas systems, the CRISPR/Cas9 system has been widely studied for its precise DNA manipulation; however, due to certain limitations of direct DNA targeting, off‑target effects and delivery challenges, researchers are looking to perform transient knockdown of gene expression by targeting RNA. In this context, the more recently discovered type VI CRISPR/Cas13 system, a programmable single‑subunit RNA‑guided endonuclease system that has the capacity to target and edit any RNA sequence of interest, has emerged as a powerful platform to modulate gene expression outcomes. All the Cas13 effectors known so far possess two distinct ribonuclease activities. Pre‑CRISPR RNA processing is performed by one RNase activity, whereas the two higher eukaryotes and prokaryotes nucleotide‑binding domains provide the other RNase activity required for target RNA degradation. Recent innovative applications of the type VI CRISPR/Cas13 system in nucleic acid detection, viral interference, transcriptome engineering and RNA imaging hold great promise for disease management. This genome editing system can also be employed by the Specific High Sensitivity Enzymatic Reporter Unlocking platform to identify any tumor DNA. The discovery of this system has added a new dimension to targeting, tracking and editing circulating microRNA/RNA/DNA/cancer proteins for the management of cancer. However, there is still a lack of thorough understanding of the mechanisms underlying some of their functions. The present review summarizes the recent updates on the type VI CRISPR/Cas system in terms of its structural and mechanistic properties and some novel applications of this genome‑editing tool in cancer management. However, some issues, such as collateral degradation of bystander RNA, impose major limitations on its in vivo application. Furthermore, additional challenges and future prospects for this genome editing system are described in the present review.

The growing repertoire of phage anti-defence systems

The biological interplay between phages and bacteria has driven the evolution of phage anti-defence systems (ADSs), which evade bacterial defence mechanisms. These ADSs bind and inhibit host defence proteins, add covalent modifications and deactivate defence proteins, degrade or sequester signalling molecules utilised by host defence systems, synthesise and restore essential molecules depleted by bacterial defences, or add covalent modifications to phage molecules to avoid recognition. Overall, 145 phage ADSs have been characterised to date. These ADSs counteract 27 of the 152 different bacterial defence families, and we hypothesise that many more ADSs are yet to be discovered. We discuss high-throughput approaches (computational and experimental) which are indispensable for discovering new ADSs and the limitations of these approaches. A comprehensive characterisation of phage ADSs is critical for understanding phage-host interplay and developing clinical applications, such as treatment for multidrug-resistant bacterial infections.

Characterizing the collateral activity of CRISPR/Cas13 in mammalian cells: Implications for RNA editing and therapeutic applications

The CRISPR/Cas13 system has garnered attention as a potential tool for RNA editing. However, the degree of collateral activity among various Cas13 orthologs and their cytotoxic effects in mammalian cells remain contentious, potentially impacting their applications. In this study, we observed differential collateral activities for LwaCas13a and RfxCas13d in 293T and U87 cells by applying both sensitive dual-fluorescence (mRuby/GFP) reporter and quantifiable dual-luciferase (Fluc/Rluc) reporter, with LwaCas13a displaying notable activity contrary to previous reports. However, significant collateral RNA cleavage exerted only a modest impact on cell viability. Furthermore, collateral activity of LwaCas13a mildly impeded, but did not arrest, porcine embryo development. Our findings reveal that distinct collateral RNA cleavage by Cas13 slightly suppresses mammalian cell proliferation and embryo development. This could account for the lack of reported collateral effects in numerous prior studies and offers new insights into the implications of the collateral activity of Cas13 for clinical application.

Current Updates of CRISPR/Cas System and Anti-CRISPR Proteins: Innovative Applications to Improve the Genome Editing Strategies

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated sequence (CRISPR/Cas) system is a cutting-edge genome-editing tool employed to explore the functions of normal and disease-related genes. The CRISPR/Cas system has a remarkable diversity in the composition and architecture of genomic loci and Cas protein sequences. Owing to its excellent efficiency and specificity, this system adds an outstanding dimension to biomedical research on genetic manipulation of eukaryotic cells. However, safe, efficient, and specific delivery of this system to target cells and tissues and their off-target effects are considered critical bottlenecks for the therapeutic applications. Recently discovered anti-CRISPR proteins (Acr) play a significant role in limiting the effects of this system. Acrs are relatively small proteins that are highly specific to CRISPR variants and exhibit remarkable structural diversity. The in silico approaches, crystallography, and cryo-electron microscopy play significant roles in elucidating the mechanisms of action of Acrs. Acrs block the CRISPR/Cas system mainly by employing four mechanisms: CRISPR/Cas complex assembly interruption, target-binding interference, target cleavage prevention, and degradation of cyclic oligonucleotide signaling molecules. Engineered CRISPR/Cas systems are frequently used in gene therapy, diagnostics, and functional genomics. Understanding the molecular mechanisms underlying Acr action may help in the safe and effective use of CRISPR/Cas tools for genetic modification, particularly in the context of medicine. Thus, attempts to regulate prokaryotic CRISPR/Cas surveillance complexes will advance the development of antimicrobial drugs and treatment of human diseases. In this review, recent updates on CRISPR/Cas systems, especially CRISPR/Cas9 and Acrs, and their novel mechanistic insights are elaborated. In addition, the role of Acrs in the novel applications of CRISPP/Cas biotechnology for precise genome editing and other applications is discussed.

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.

Structures, mechanisms and applications of RNA-centric CRISPR-Cas13

Prokaryotes are equipped with a variety of resistance strategies to survive frequent viral attacks or invading mobile genetic elements. Among these, CRISPR-Cas surveillance systems are abundant and have been studied extensively. This Review focuses on CRISPR-Cas type VI Cas13 systems that use single-subunit RNA-guided Cas endonucleases for targeting and subsequent degradation of foreign RNA, thereby providing adaptive immunity. Notably, distinct from single-subunit DNA-cleaving Cas9 and Cas12 systems, Cas13 exhibits target RNA-activated substrate RNase activity. This Review outlines structural, biochemical and cell biological studies toward elucidation of the unique structural and mechanistic principles underlying surveillance effector complex formation, precursor CRISPR RNA (pre-crRNA) processing, self-discrimination and RNA degradation in Cas13 systems as well as insights into suppression by bacteriophage-encoded anti-CRISPR proteins and regulation by endogenous accessory proteins. Owing to its programmable ability for RNA recognition and cleavage, Cas13 provides powerful RNA targeting, editing, detection and imaging platforms with emerging biotechnological and therapeutic applications.

CRISPR-Cas and CRISPR-based screening system for precise gene editing and targeted cancer therapy

Target cancer therapy has been developed for clinical cancer treatment based on the discovery of CRISPR (clustered regularly interspaced short palindromic repeat) -Cas system. This forefront and cutting-edge scientific technique improves the cancer research into molecular level and is currently widely utilized in genetic investigation and clinical precision cancer therapy. In this review, we summarized the genetic modification by CRISPR/Cas and CRISPR screening system, discussed key components for successful CRISPR screening, including Cas enzymes, guide RNA (gRNA) libraries, target cells or organs. Furthermore, we focused on the application for CAR-T cell therapy, drug target, drug screening, or drug selection in both ex vivo and in vivo with CRISPR screening system. In addition, we elucidated the advantages and potential obstacles of CRISPR system in precision clinical medicine and described the prospects for future genetic therapy.In summary, we provide a comprehensive and practical perspective on the development of CRISPR/Cas and CRISPR screening system for the treatment of cancer defects, aiming to further improve the precision and accuracy for clinical treatment and individualized gene therapy.

Structural basis of negative regulation of CRISPR-Cas7-11 by TPR-CHAT

CRISPR‒Cas7-11 is a Type III-E CRISPR-associated nuclease that functions as a potent RNA editing tool. Tetratrico-peptide repeat fused with Cas/HEF1-associated signal transducer (TPR-CHAT) acts as a regulatory protein that interacts with CRISPR RNA (crRNA)-bound Cas7-11 to form a CRISPR-guided caspase complex (Craspase). However, the precise modulation of Cas7-11's nuclease activity by TPR-CHAT to enhance its utility requires further study. Here, we report cryo-electron microscopy (cryo-EM) structures of Desulfonema ishimotonii (Di) Cas7-11-crRNA, complexed with or without the full length or the N-terminus of TPR-CHAT. These structures unveil the molecular features of the Craspase complex. Structural analysis, combined with in vitro nuclease assay and electrophoretic mobility shift assay, reveals that DiTPR-CHAT negatively regulates the activity of DiCas7-11 by preventing target RNA from binding through the N-terminal 65 amino acids of DiTPR-CHAT (DiTPR-CHATNTD). Our work demonstrates that DiTPR-CHATNTD can function as a small unit of DiCas7-11 regulator, potentially enabling safe applications to prevent overcutting and off-target effects of the CRISPR‒Cas7-11 system.

Inhibitors of bacterial immune systems: discovery, mechanisms and applications

To contend with the diversity and ubiquity of bacteriophages and other mobile genetic elements, bacteria have developed an arsenal of immune defence mechanisms. Bacterial defences include CRISPR-Cas, restriction-modification and a growing list of mechanistically diverse systems, which constitute the bacterial 'immune system'. As a response, bacteriophages and mobile genetic elements have evolved direct and indirect mechanisms to circumvent or block bacterial defence pathways and ensure successful infection. Recent advances in methodological and computational approaches, as well as the increasing availability of genome sequences, have boosted the discovery of direct inhibitors of bacterial defence systems. In this Review, we discuss methods for the discovery of direct inhibitors, their diverse mechanisms of action and perspectives on their emerging applications in biotechnology and beyond.

CRISPR-Enabled Graphene-Based Bio-Cyber Interface Model for In Vivo Monitoring of Non-Invasive Therapeutic Processes

In this paper, we present a model of the bio-cyber interface for the Internet of Bio-Nano Things application. The proposed model is inspired by the gains of integrating the Clustered Regularly Interspace Short Palindromic Repeats (CRISPR) technology with the Graphene-Field effect transistor (GFET). The capabilities of the integrated system are harnessed to detect nucleic acids transcribed by another component of the bio-cyber interface, a bioreporter, on being exposed to the signalling molecule of interest. The proposed model offers a label-free real-time signal transduction with multi-symbol signalling capability. We model the entire operation of the interface as a set of simultaneous differential equations representing the process's kinetics. The solution to the model is obtained using a numerical method. Numerical results show that the performance of the interface is influenced by parameters such as the concentrations of the input signalling molecules, the surface receptor on the bioreporter, and the CRISPR complex. The interface's performance also depends considerably on the elimination rate of the signalling molecules from the body. For multi-symbol molecular signalling, the rate of degradation of the transcribed RNAs influences the system's susceptibility to inter-symbol interference.

Exploring the oncogenic and tumor-suppressive roles of Circ-ADAM9 in cancer

Circular RNAs (circRNAs) constitute a recently identified category of closed continuous loop RNA transcripts, serving as a subset of competing endogenous RNAs (ceRNAs) with the capacity to modulate genes by acting as microRNA sponges. In the context of cancer growth, numerous investigations have explored the potential functions of circRNAs, revealing their diverse functions either as oncogenes, promoting cancer progression, or as tumor suppressors, mitigating disease development. Among these, circRNA ADAM9 (Circ-ADAM9) is now recognized as an important player in a variety of mechanisms, both physiological and pathological, especially in cancer. The aberrant expression of Circ-ADAM9 has been observed across multiple human malignancies, implying a significant involvement in tumorigenesis. This comprehensive review aims to synthesize recent findings elucidating the function of Circ-ADAM9 in many malignancies. Additionally, the review explores the possibility of Circ-ADAM9 as a valuable biomarker, offering insights into its prognostic, diagnostic, and therapeutic implications. By summarizing the latest discoveries in this field, the review contributes to our understanding of the multifaceted contribution of Circ-ADAM9 in tumor biology and its potential applications in clinical settings.

Cas-based bacterial detection: recent advances and perspectives

Persistent bacterial infections pose a formidable threat to global health, contributing to widespread challenges in areas such as food safety, medical hygiene, and animal husbandry. Addressing this peril demands the urgent implementation of swift and highly sensitive detection methodologies suitable for point-of-care testing and large-scale screening. These methodologies play a pivotal role in the identification of pathogenic bacteria, discerning drug-resistant strains, and managing and treating diseases. Fortunately, new technology, the CRISPR/Cas system, has emerged. The clustered regularly interspaced short joint repeats (CRISPR) system, which is part of bacterial adaptive immunity, has already played a huge role in the field of gene editing. It has been employed as a diagnostic tool for virus detection, featuring high sensitivity, specificity, and single-nucleotide resolution. When applied to bacterial detection, it also surpasses expectations. In this review, we summarise recent advances in the detection of bacteria such as Mycobacterium tuberculosis (MTB), methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli (E. coli), Salmonella and Acinetobacter baumannii (A. baumannii) using the CRISPR/Cas system. We emphasize the significance and benefits of this methodology, showcasing the capability of diverse effector proteins to swiftly and precisely recognize bacterial pathogens. Furthermore, the CRISPR/Cas system exhibits promise in the identification of antibiotic-resistant strains. Nevertheless, this technology is not without challenges that need to be resolved. For example, CRISPR/Cas systems must overcome natural off-target effects and require high-quality nucleic acid samples to improve sensitivity and specificity. In addition, limited applicability due to the protospacer adjacent motif (PAM) needs to be addressed to increase its versatility. Despite the challenges, we are optimistic about the future of bacterial detection using CRISPR/Cas. We have already highlighted its potential in medical microbiology. As research progresses, this technology will revolutionize the detection of bacterial infections.

Beyond the promise: evaluating and mitigating off-target effects in CRISPR gene editing for safer therapeutics

Over the last decade, CRISPR has revolutionized drug development due to its potential to cure genetic diseases that currently do not have any treatment. CRISPR was adapted from bacteria for gene editing in human cells in 2012 and, remarkably, only 11 years later has seen it's very first approval as a medicine for the treatment of sickle cell disease and transfusion-dependent beta-thalassemia. However, the application of CRISPR systems is associated with unintended off-target and on-target alterations (including small indels, and structural variations such as translocations, inversions and large deletions), which are a source of risk for patients and a vital concern for the development of safe therapies. In recent years, a wide range of methods has been developed to detect unwanted effects of CRISPR-Cas nuclease activity. In this review, we summarize the different methods for off-target assessment, discuss their strengths and limitations, and highlight strategies to improve the safety of CRISPR systems. Finally, we discuss their relevance and application for the pre-clinical risk assessment of CRISPR therapeutics within the current regulatory context.

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.

CRISPR genetic toolkits of classical food microorganisms: Current state and future prospects

Production of food-related products using microorganisms in an environmentally friendly manner is a crucial solution to global food safety and environmental pollution issues. Traditional microbial modification methods rely on artificial selection or natural mutations, which require time for repeated screening and reproduction, leading to unstable results. Therefore, it is imperative to develop rapid, efficient, and precise microbial modification technologies. This review summarizes recent advances in the construction of gene editing and metabolic regulation toolkits based on the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (CRISPR-Cas) systems and their applications in reconstructing food microorganism metabolic networks. The development and application of gene editing toolkits from single-site gene editing to multi-site and genome-scale gene editing was also introduced. Moreover, it presented a detailed introduction to CRISPR interference, CRISPR activation, and logic circuit toolkits for metabolic network regulation. Moreover, the current challenges and future prospects for developing CRISPR genetic toolkits were also discussed.

Progress of CRISPR-based programmable RNA manipulation and detection

Prokaryotic clustered regularly interspaced short palindromic repeats and CRISPR associated (CRISPR-Cas) systems provide adaptive immunity by using RNA-guided endonucleases to recognize and eliminate invading foreign nucleic acids. Type II Cas9, type V Cas12, type VI Cas13, and type III Csm/Cmr complexes have been well characterized and developed as programmable platforms for selectively targeting and manipulating RNA molecules of interest in prokaryotic and eukaryotic cells. These Cas effectors exhibit remarkable diversity of ribonucleoprotein (RNP) composition, target recognition and cleavage mechanisms, and self discrimination mechanisms, which are leveraged for various RNA targeting applications. Here, we summarize the current understanding of mechanistic and functional characteristics of these Cas effectors, give an overview on RNA detection and manipulation toolbox established so far including knockdown, editing, imaging, modification, and mapping RNA-protein interactions, and discuss the future directions for CRISPR-based RNA targeting tools. This article is categorized under: RNA Methods > RNA Analyses in Cells RNA Processing > RNA Editing and Modification RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

A comprehensive appraisal of mechanism of anti-CRISPR proteins: an advanced genome editor to amend the CRISPR gene editing

The development of precise and controlled CRISPR-Cas tools has been made possible by the discovery of protein inhibitors of CRISPR-Cas systems, called anti-CRISPRs (Acrs). The Acr protein has the ability to control off-targeted mutations and impede Cas protein-editing operations. Acr can help with selective breeding, which could help plants and animals improve their valuable features. In this review, the Acr protein-based inhibitory mechanisms that have been adopted by several Acrs, such as (a) the interruption of CRISPR-Cas complex assembly, (b) interference with target DNA binding, (c) blocking of target DNA/RNA cleavage, and (d) enzymatic modification or degradation of signalling molecules, were discussed. In addition, this review emphasizes the applications of Acr proteins in the plant research.

Bacteriophage strategies for overcoming host antiviral immunity

Phages and their bacterial hosts together constitute a vast and diverse ecosystem. Facing the infection of phages, prokaryotes have evolved a wide range of antiviral mechanisms, and phages in turn have adopted multiple tactics to circumvent or subvert these mechanisms to survive. An in-depth investigation into the interaction between phages and bacteria not only provides new insight into the ancient coevolutionary conflict between them but also produces precision biotechnological tools based on anti-phage systems. Moreover, a more complete understanding of their interaction is also critical for the phage-based antibacterial measures. Compared to the bacterial antiviral mechanisms, studies into counter-defense strategies adopted by phages have been a little slow, but have also achieved important advances in recent years. In this review, we highlight the numerous intracellular immune systems of bacteria as well as the countermeasures employed by phages, with an emphasis on the bacteriophage strategies in response to host antiviral immunity.

The applications of CRISPR/Cas-mediated genome editing in genetic hearing loss

Hearing loss (HL) can be caused by a number of different genetic factors. Non-syndromic HL refers that HL occurs as an isolated symptom in an individual, whereas syndromic HL refers that HL is associated with other symptoms or abnormalities. To date, more than 140 genes have been identified as being associated with non-syndromic HL, and approximately 400 genetic syndromes can include HL as one of the clinical symptoms. However, no gene therapeutic approaches are currently available to restore or improve hearing. Therefore, there is an urgent necessity to elucidate the possible pathogenesis of specific mutations in HL-associated genes and to investigate the promising therapeutic strategies for genetic HL. The development of the CRISPR/Cas system has revolutionized the field of genome engineering, which has become an efficacious and cost-effective tool to foster genetic HL research. Moreover, several in vivo studies have demonstrated the therapeutic efficacy of the CRISPR/Cas-mediated treatments for specific genetic HL. In this review, we briefly introduce the progress in CRISPR/Cas technique as well as the understanding of genetic HL, and then we detail the recent achievements of CRISPR/Cas technique in disease modeling and therapeutic strategies for genetic HL. Furthermore, we discuss the challenges for the application of CRISPR/Cas technique in future clinical treatments.

Improving recombinant protein production in CHO cells using the CRISPR-Cas system

Chinese hamster ovary (CHO) cells are among the most widely used mammalian cell lines in the biopharmaceutical industry. Therefore, it is not surprising that significant efforts have been made around the engineering of CHO cells using genetic engineering methods such as the CRISPR-Cas system. In this review, we summarize key recent studies that have used different CRISPR-Cas systems such as Cas9, Cas13 or dCas9 fused with effector domains to improve recombinant protein (r-protein) production in CHO cells. Here, every relevant stage of production was considered, underscoring the advantages and limitations of these systems, as well as discussing their bottlenecks and probable solutions. A special emphasis was given on how these systems could disrupt and/or regulate genes related to glycan composition, which has relevant effects over r-protein properties and in vivo activity. Furthermore, the related promising future applications of CRISPR to achieve a tunable, reversible, or highly stable editing of CHO cells are discussed. Overall, the studies covered in this review show that despite the complexity of mammalian cells, the synthetic biology community has developed many mature strategies to improve r-protein production using CHO cells. In this regard, CRISPR-Cas technology clearly provides efficient and flexible genetic manipulation and allows for the generation of more productive CHO cell lines, leading to more cost-efficient production of biopharmaceuticals, however, there is still a need for many emerging techniques in CRISPR to be reported in CHO cells; therefore, more research in these cells is needed to realize the full potential of this technology.

Anti-CRISPR Discovery: Using Magnets to Find Needles in Haystacks

CRISPR-Cas immune systems in bacteria and archaea protect against viral infection, which has spurred viruses to develop dedicated inhibitors of these systems called anti-CRISPRs (Acrs). Like most host-virus arms races, many diverse examples of these immune and counter-immune proteins are encoded by the genomes of bacteria, archaea, and their viruses. For the case of Acrs, it is almost certain that just a small minority of nature's true diversity has been described. In this review, I discuss the various approaches used to identify these Acrs and speculate on the future for Acr discovery. Because Acrs can determine infection outcomes in nature and regulate CRISPR-Cas activities in applied settings, they have a dual importance to both host-virus conflicts and emerging biotechnologies. Thus, revealing the largely hidden world of Acrs should provide important lessons in microbiology that have the potential to ripple far beyond the field.

Optimization of CRISPR-Cas system for clinical cancer therapy

Cancer is a genetic disease caused by alterations in genome and epigenome and is one of the leading causes for death worldwide. The exploration of disease development and therapeutic strategies at the genetic level have become the key to the treatment of cancer and other genetic diseases. The functional analysis of genes and mutations has been slow and laborious. Therefore, there is an urgent need for alternative approaches to improve the current status of cancer research. Gene editing technologies provide technical support for efficient gene disruption and modification in vivo and in vitro, in particular the use of clustered regularly interspaced short palindromic repeats (CRISPR)-Cas systems. Currently, the applications of CRISPR-Cas systems in cancer rely on different Cas effector proteins and the design of guide RNAs. Furthermore, effective vector delivery must be met for the CRISPR-Cas systems to enter human clinical trials. In this review article, we describe the mechanism of the CRISPR-Cas systems and highlight the applications of class II Cas effector proteins. We also propose a synthetic biology approach to modify the CRISPR-Cas systems, and summarize various delivery approaches facilitating the clinical application of the CRISPR-Cas systems. By modifying the CRISPR-Cas system and optimizing its in vivo delivery, promising and effective treatments for cancers using the CRISPR-Cas system are emerging.

Unveil the Secret of the Bacteria and Phage Arms Race

Bacteria have developed different mechanisms to defend against phages, such as preventing phages from being adsorbed on the surface of host bacteria; through the superinfection exclusion (Sie) block of phage's nucleic acid injection; by restricting modification (R-M) systems, CRISPR-Cas, aborting infection (Abi) and other defense systems to interfere with the replication of phage genes in the host; through the quorum sensing (QS) enhancement of phage's resistant effect. At the same time, phages have also evolved a variety of counter-defense strategies, such as degrading extracellular polymeric substances (EPS) that mask receptors or recognize new receptors, thereby regaining the ability to adsorb host cells; modifying its own genes to prevent the R-M systems from recognizing phage genes or evolving proteins that can inhibit the R-M complex; through the gene mutation itself, building nucleus-like compartments or evolving anti-CRISPR (Acr) proteins to resist CRISPR-Cas systems; and by producing antirepressors or blocking the combination of autoinducers (AIs) and its receptors to suppress the QS. The arms race between bacteria and phages is conducive to the coevolution between bacteria and phages. This review details bacterial anti-phage strategies and anti-defense strategies of phages and will provide basic theoretical support for phage therapy while deeply understanding the interaction mechanism between bacteria and phages.

CRISPR-Cas13 in malaria parasite: Diagnosis and prospective gene function identification

Malaria caused by Plasmodium is still a serious public health problem. Genomic editing is essential to understand parasite biology, elucidate mechanical pathways, uncover gene functions, identify novel therapeutic targets, and develop clinical diagnostic tools. Recent advances have seen the development of genomic diagnostic technologies and the emergence of genetic manipulation toolbox comprising a host of several systems for editing the genome of Plasmodium at the DNA, RNA, and protein level. Genomic manipulation at the RNA level is critical as it allows for the functional characterization of several transcripts. Of notice, some developed artificial RNA genome editing tools hinge on the endogenous RNA interference system of Plasmodium. However, Plasmodium lacks a robust RNAi machinery, hampering the progress of these editing tools. CRISPR-Cas13, which belongs to the VI type of the CRISPR system, can specifically bind and cut RNA under the guidance of crRNA, with no or minimal permanent genetic scar on genes. This review summarizes CRISPR-Cas13 system from its discovery, classification, principle of action, and diagnostic platforms. Further, it discusses the application prospects of Cas13-based systems in Plasmodium and highlights its advantages and drawbacks.

Enhanced RNA knockdown efficiency with engineered fusion guide RNAs that function with both CRISPR-CasRx and hammerhead ribozyme

BACKGROUND

CRISPR-Cas13 is a newly emerging RNA knockdown technology that is comparable to RNAi. Among all members of Cas13, CasRx degrades RNA in human cells with high precision and effectiveness. However, it remains unclear whether the efficiency of this technology can be further improved and applied to gene therapy.

RESULTS

In this study, we fuse CasRx crRNA with an antisense ribozyme to construct a synthetic fusion guide RNA that can interact with both CasRx protein and ribozyme and tested the ability of this approach in RNA knockdown and cancer gene therapy. We show that the CasRx-crRNA-ribozyme system (CCRS) is more efficient for RNA knockdown of mRNAs and non-coding RNAs than conventional methods, including CasRx, shRNA, and ribozyme. In particular, CCRS is more effective than wild-type CasRx when targeting multiple transcripts simultaneously. We next use bladder cancer as a model to evaluate the anticancer effects of CCRS targeting multiple genes in vitro and in vivo. CCRS shows a higher anticancer effect than conventional methods, consistent with the gene knockdown results.

CONCLUSIONS

Thus, our study demonstrates that CCRS expands the design ideas and RNA knockdown capabilities of Cas13 technology and has the potential to be used in disease treatment.

A CRISPR way for accelerating cereal crop improvement: Progress and challenges

Humans rely heavily on cereal grains as a key source of nutrients, hence regular improvement of cereal crops is essential for ensuring food security. The current food crisis at the global level is due to the rising population and harsh climatic conditions which prompts scientists to develop smart resilient cereal crops to attain food security. Cereal crop improvement in the past generally depended on imprecise methods like random mutagenesis and conventional genetic recombination which results in high off targeting risks. In this context, we have witnessed the application of targeted mutagenesis using versatile CRISPR-Cas systems for cereal crop improvement in sustainable agriculture. Accelerated crop improvement using molecular breeding methods based on CRISPR-Cas genome editing (GE) is an unprecedented tool for plant biotechnology and agriculture. The last decade has shown the fidelity, accuracy, low levels of off-target effects, and the high efficacy of CRISPR technology to induce targeted mutagenesis for the improvement of cereal crops such as wheat, rice, maize, barley, and millets. Since the genomic databases of these cereal crops are available, several modifications using GE technologies have been performed to attain desirable results. This review provides a brief overview of GE technologies and includes an elaborate account of the mechanisms and applications of CRISPR-Cas editing systems to induce targeted mutagenesis in cereal crops for improving the desired traits. Further, we describe recent developments in CRISPR-Cas-based targeted mutagenesis through base editing and prime editing to develop resilient cereal crop plants, possibly providing new dimensions in the field of cereal crop genome editing.

AcaFinder: Genome Mining for Anti-CRISPR-Associated Genes

Anti-CRISPR (Acr) proteins are encoded by (pro)viruses to inhibit their host's CRISPR-Cas systems. Genes encoding Acr and Aca (Acr associated) proteins often colocalize to form acr-aca operons. Here, we present AcaFinder as the first Aca genome mining tool. AcaFinder can (i) predict Acas and their associated acr-aca operons using guilt-by-association (GBA); (ii) identify homologs of known Acas using an HMM (Hidden Markov model) database; (iii) take input genomes for potential prophages, CRISPR-Cas systems, and self-targeting spacers (STSs); and (iv) provide a standalone program (https://github.com/boweny920/AcaFinder) and a web server (http://aca.unl.edu/Aca). AcaFinder was applied to mining over 16,000 prokaryotic and 142,000 gut phage genomes. After a multistep filtering, 36 high-confident new Aca families were identified, which is three times that of the 12 known Aca families. Seven new Aca families were from major human gut bacteria (Bacteroidota, Actinobacteria, and Fusobacteria) and their phages, while most known Aca families were from Proteobacteria and Firmicutes. A complex association network between Acrs and Acas was revealed by analyzing their operonic colocalizations. It appears very common in evolution that the same aca genes can recombine with different acr genes and vice versa to form diverse acr-aca operon combinations. IMPORTANCE At least four bioinformatics programs have been published for genome mining of Acrs since 2020. In contrast, no bioinformatics tools are available for automated Aca discovery. As the self-transcriptional repressor of acr-aca operons, Aca can be viewed as anti-anti-CRISPRs, with great potential in the improvement of CRISPR-Cas technology. Although all the 12 known Aca proteins contain a conserved helix-turn-helix (HTH) domain, not all HTH-containing proteins are Acas. However, HTH-containing proteins with adjacent Acr homologs encoded in the same genetic operon are likely Aca proteins. AcaFinder implements this guilt-by-association idea and the idea of using HMMs of known Acas for homologs into one software package. Applying AcaFinder in screening prokaryotic and gut phage genomes reveals a complex acr-aca operonic colocalization network between different families of Acrs and Acas.

The CRISPR-Cas system as a tool for diagnosing and treating infectious diseases

Emerging and relapsing infectious diseases pose a huge health threat to human health and a new challenge to global public health. Rapid, sensitive and simple diagnostic tools are keys to successful management of infectious patients and containment of disease transmission. In recent years, international research on Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-related proteins (Cas) has revolutionized our understanding of biology. The CRISPR-Cas system has the advantages of high specificity, high sensitivity, simple, rapid, low cost, and has begun to be used for molecular diagnosis and treatment of infectious diseases. In this paper, we described the biological principles, application fields and prospects of CRISPR-Cas system in the molecular diagnosis and treatment of infectious diseases, and compared it with existing molecular diagnosis methods, the advantages and disadvantages were summarized.

Engineered Bacteriophages Containing Anti-CRISPR Suppress Infection of Antibiotic-Resistant P. aeruginosa

The therapeutic use of bacteriophages (phages) provides great promise for treating multidrug-resistant (MDR) bacterial infections. However, an incomplete understanding of the interactions between phages and bacteria has negatively impacted the application of phage therapy. Here, we explored engineered anti-CRISPR (Acr) gene-containing phages (EATPs, eat Pseudomonas) by introducing Type I anti-CRISPR (AcrIF1, AcrIF2, and AcrIF3) genes into the P. aeruginosa bacteriophage DMS3/DMS3m to render the potential for blocking P. aeruginosa replication and infection. In order to achieve effective antibacterial activities along with high safety against clinically isolated MDR P. aeruginosa through an anti-CRISPR immunity mechanism in vitro and in vivo, the inhibitory concentration for EATPs was 1 × 108 PFU/mL with a multiplicity of infection value of 0.2. In addition, the EATPs significantly suppressed the antibiotic resistance caused by a highly antibiotic-resistant PA14 infection. Collectively, these findings provide evidence that engineered phages may be an alternative, viable approach by which to treat patients with an intractable bacterial infection, especially an infection by clinically MDR bacteria that are unresponsive to conventional antibiotic therapy. IMPORTANCE Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic Gram-negative bacterium that causes severe infection in immune-weakened individuals, especially patients with cystic fibrosis, burn wounds, cancer, or chronic obstructive pulmonary disease (COPD). Treating P. aeruginosa infection with conventional antibiotics is difficult due to its intrinsic multidrug resistance. Engineered bacteriophage therapeutics, acting as highly viable alternative treatments of multidrug-resistant (MDR) bacterial infections, have great potential to break through the evolutionary constraints of bacteriophages to create next-generation antimicrobials. Here, we found that engineered anti-CRISPR (Acr) gene-containing phages (EATPs, eat Pseudomonas) display effective antibacterial activities along with high safety against clinically isolated MDR P. aeruginosa through an anti-CRISPR immunity mechanism in vitro and in vivo. EATPs also significantly suppressed the antibiotic resistance caused by a highly antibiotic-resistant PA14 infection, which may provide novel insight toward developing bacteriophages to treat patients with intractable bacterial infections, especially infections by clinically MDR bacteria that are unresponsive to conventional antibiotic therapy.

Structure of AcrVIA2 and its binding mechanism to CRISPR-Cas13a

Phages and non-phage derived bacteria have evolved many anti-CRISPR proteins (Acrs) to escape the adaptive immune system of prokaryotes. Thus Acrs can be applied as a regulatory tool for gene edition by CRISPR system. Recently, a non-phage derived AcrVIA2 has been identified as an inhibitor that blocks the editing activity of Cas13a in vitro by binding to Cas13a. Here, we solved the crystal structure of AcrVIA2 at a resolution of 2.59 Å and confirmed that AcrVIA2 can bind to Helical-I domain in LshCas13a. Structural analysis show that the V-shaped acidic groove formed by β3-β3 hairpin of AcrVIA2 dimer is the key region that mediates the interaction between AcrVIA2 and Helical-I domain. In addition, we also reveal that Asp37 of AcrVIA2 plays an essential role in the functioning of the V-shaped acidic groove, and the functional dimer conformation of AcrVIA2 is stabilized by hydrogen bonds formed between Tyr41 of one monomer with Glu35 and Asp37 of the other monomer. These data expand the current understanding of the diverse interaction mechanisms between Acrs and Cas proteins, and also provide new ideas for the development of CRISPR-Cas13a regulatory tool.

Pseudomonas aeruginosa: pathogenesis, virulence factors, antibiotic resistance, interaction with host, technology advances and emerging therapeutics

Pseudomonas aeruginosa (P. aeruginosa) is a Gram-negative opportunistic pathogen that infects patients with cystic fibrosis, burn wounds, immunodeficiency, chronic obstructive pulmonary disorder (COPD), cancer, and severe infection requiring ventilation, such as COVID-19. P. aeruginosa is also a widely-used model bacterium for all biological areas. In addition to continued, intense efforts in understanding bacterial pathogenesis of P. aeruginosa including virulence factors (LPS, quorum sensing, two-component systems, 6 type secretion systems, outer membrane vesicles (OMVs), CRISPR-Cas and their regulation), rapid progress has been made in further studying host-pathogen interaction, particularly host immune networks involving autophagy, inflammasome, non-coding RNAs, cGAS, etc. Furthermore, numerous technologic advances, such as bioinformatics, metabolomics, scRNA-seq, nanoparticles, drug screening, and phage therapy, have been used to improve our understanding of P. aeruginosa pathogenesis and host defense. Nevertheless, much remains to be uncovered about interactions between P. aeruginosa and host immune responses, including mechanisms of drug resistance by known or unannotated bacterial virulence factors as well as mammalian cell signaling pathways. The widespread use of antibiotics and the slow development of effective antimicrobials present daunting challenges and necessitate new theoretical and practical platforms to screen and develop mechanism-tested novel drugs to treat intractable infections, especially those caused by multi-drug resistance strains. Benefited from has advancing in research tools and technology, dissecting this pathogen's feature has entered into molecular and mechanistic details as well as dynamic and holistic views. Herein, we comprehensively review the progress and discuss the current status of P. aeruginosa biophysical traits, behaviors, virulence factors, invasive regulators, and host defense patterns against its infection, which point out new directions for future investigation and add to the design of novel and/or alternative therapeutics to combat this clinically significant pathogen.

Lack of Cas13a inhibition by anti-CRISPR proteins from Leptotrichia prophages

CRISPR systems are prokaryotic adaptive immune systems that use RNA-guided Cas nucleases to recognize and destroy foreign genetic elements. To overcome CRISPR immunity, bacteriophages have evolved diverse families of anti-CRISPR proteins (Acrs). Recently, Lin et al. (2020) described the discovery and characterization of 7 Acr families (AcrVIA1-7) that inhibit type VI-A CRISPR systems. We detail several inconsistencies that question the results reported in the Lin et al. (2020) study. These include inaccurate bioinformatics analyses and bacterial strains that are impossible to construct. Published strains were provided by the authors, but MS2 bacteriophage plaque assays did not support the published results. We also independently tested the Acr sequences described in the original report, in E. coli and mammalian cells, but did not observe anti-Cas13a activity. Taken together, our data and analyses prompt us to question the claim that AcrVIA1-7 reported in Lin et al. are type VI anti-CRISPR proteins.

Anti-CRISPR prediction using deep learning reveals an inhibitor of Cas13b nucleases

As part of the ongoing bacterial-phage arms race, CRISPR-Cas systems in bacteria clear invading phages whereas anti-CRISPR proteins (Acrs) in phages inhibit CRISPR defenses. Known Acrs have proven extremely diverse, complicating their identification. Here, we report a deep learning algorithm for Acr identification that revealed an Acr against type VI-B CRISPR-Cas systems. The algorithm predicted numerous putative Acrs spanning almost all CRISPR-Cas types and subtypes, including over 7,000 putative type IV and VI Acrs not predicted by other algorithms. By performing a cell-free screen for Acr hits against type VI-B systems, we identified a potent inhibitor of Cas13b nucleases we named AcrVIB1. AcrVIB1 blocks Cas13b-mediated defense against a targeted plasmid and lytic phage, and its inhibitory function principally occurs upstream of ribonucleoprotein complex formation. Overall, our work helps expand the known Acr universe, aiding our understanding of the bacteria-phage arms race and the use of Acrs to control CRISPR technologies.

Hierarchical self-uncloaking CRISPR-Cas13a-customized RNA nanococoons for spatial-controlled genome editing and precise cancer therapy

CRISPR-Cas13a holds enormous potential for developing precise RNA editing. However, spatial manipulation of CRISPR-Cas13a activity remains a daunting challenge for elaborately regulating localized RNase function. Here, we designed hierarchical self-uncloaking CRISPR-Cas13a-customized RNA nanococoons (RNCOs-D), featuring tumor-specific recognition and spatial-controlled activation of Cas13a, for precise cancer synergistic therapy. RNCOs-D consists of programmable RNA nanosponges (RNSs) capable of targeted delivery and caging chemotherapeutic drug, and nanocapsules (NCs) anchored on RNSs for cloaking Cas13a/crRNA ribonucleoprotein (Cas13a RNP) activity. The acidic endo/lysosomal microenvironment stimulates the outer decomposition of NCs with concomitant Cas13a RNP activity revitalization, while the inner disassembly through trans-cleavage of RNSs initiated by cis-recognition and cleavage of EGFR variant III (EGFRvIII) mRNA. RNCOs-D demonstrates the effective EGFRvIII mRNA silencing for synergistic therapy of glioblastoma cancer cells in vitro and in vivo. The engineering of RNSs, together with efficient Cas13a activity regulation, holds immense prospect for multimodal and synergistic cancer therapy.

Mechanistic insights into the versatile class II CRISPR toolbox

The constantly expanding group of class II CRISPR-Cas (clustered regularly interspaced short palindromic repeats-associated) effectors and their engineered variants exhibit distinct editing modes and efficiency, fidelity, target range, and molecular size. Their enormous diversity of capabilities provides a formidable toolkit for a large array of technologies. We review the structural and biochemical mechanisms of versatile effector proteins from class II CRISPR-Cas systems to provide mechanistic insights into their target specificity, protospacer adjacent motif (PAM) restriction, and activity regulation, and discuss possible strategies to enhance genome-engineering tools in terms of accuracy, efficiency, applicability, and controllability.

Repurposable drugs for SARS-CoV-2 and influenza sepsis with scRNA-seq data targeting post-transcription modifications

Coronavirus disease 2019 (COVID-19) has impacted almost every part of human life worldwide, posing a massive threat to human health. The lack of time for new drug discovery and the urgent need for rapid disease control to reduce mortality have led to a search for quick and effective alternatives to novel therapeutics, for example drug repurposing. To identify potentially repurposable drugs, we employed a systematic approach to mine candidates from U.S. FDA-approved drugs and preclinical small-molecule compounds by integrating gene expression perturbation data for chemicals from the Library of Integrated Network-Based Cellular Signatures project with a publicly available single-cell RNA sequencing dataset from patients with mild and severe COVID-19 (GEO: GSE145926, public data available and accessed on 22 April 2020). We identified 281 FDA-approved drugs that have the potential to be effective against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, 16 of which are currently undergoing clinical trials to evaluate their efficacy against COVID-19. We experimentally tested and demonstrated the inhibitory effects of tyrphostin-AG-1478 and brefeldin-a, two chemical inhibitors of glycosylation (a post-translational modification) on the replication of the single-stranded ribonucleic acid (ssRNA) virus influenza A virus as well as on the transcription and translation of host cell cytokines and their regulators (IFNs and ISGs). In conclusion, we have identified and experimentally validated repurposable anti-SARS-CoV-2 and IAV drugs using a systems biology approach, which may have the potential for treating these viral infections and their complications (sepsis).

Digging into the lesser-known aspects of CRISPR biology

A long time has passed since regularly interspaced DNA repeats were discovered in prokaryotes. Today, those enigmatic repetitive elements termed clustered regularly interspaced short palindromic repeats (CRISPR) are acknowledged as an emblematic part of multicomponent CRISPR-Cas (CRISPR associated) systems. These systems are involved in a variety of roles in bacteria and archaea, notably, that of conferring protection against transmissible genetic elements through an adaptive immune-like response. This review summarises the present knowledge on the diversity, molecular mechanisms and biology of CRISPR-Cas. We pay special attention to the most recent findings related to the determinants and consequences of CRISPR-Cas activity. Research on the basic features of these systems illustrates how instrumental the study of prokaryotes is for understanding biology in general, ultimately providing valuable tools for diverse fields and fuelling research beyond the mainstream.

Virulence Comparison of Salmonella enterica Subsp. enterica Isolates from Chicken and Whole Genome Analysis of the High Virulent Strain S. Enteritidis 211

Background: Salmonellaenterica is one of the common pathogens in both humans and animals that causes salmonellosis and threatens public health all over the world. Methods and Results: Here we determined the virulence phenotypes of nine Salmonellaenterica subsp. enterica (S. enterica) isolates in vitro and in vivo, including pathogenicity to chicken, cell infection, biofilm formation and virulence gene expressions. S. Enteritidis 211 (SE211) was highly pathogenic with notable virulence features among the nine isolates. The combination of multiple virulence genes contributed to the conferring of the high virulence in SE211. Importantly, many mobile genetic elements (MGEs) were found in the genome sequence of SE211, including a virulence plasmid, genomic islands, and prophage regions. The MGEs and CRISPR-Cas system might function synergistically for gene transfer and immune defense. In addition, the neighbor joining tree and the minimum spanning tree were constructed in this study. Conclusions: This study provided both the virulence phenotypes and genomic features, which might contribute to the understanding of bacterial virulence mechanisms in Salmonella enterica subsp. enterica. The first completed genomic sequence for the high virulent S. Enteritidis isolate SE211 and the comparative genomics and phylogenetic analyses provided a preliminary understanding of S. enterica genetics and laid the foundation for further study.

RNA Editing: A New Therapeutic Target in Amyotrophic Lateral Sclerosis and Other Neurological Diseases

The conversion of adenosine to inosine in RNA editing (A-to-I RNA editing) is recognized as a critical post-transcriptional modification of RNA by adenosine deaminases acting on RNAs (ADARs). A-to-I RNA editing occurs predominantly in mammalian and human central nervous systems and can alter the function of translated proteins, including neurotransmitter receptors and ion channels; therefore, the role of dysregulated RNA editing in the pathogenesis of neurological diseases has been speculated. Specifically, the failure of A-to-I RNA editing at the glutamine/arginine (Q/R) site of the GluA2 subunit causes excessive permeability of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors to Ca2+, inducing fatal status epilepticus and the neurodegeneration of motor neurons in mice. Therefore, an RNA editing deficiency at the Q/R site in GluA2 due to the downregulation of ADAR2 in the motor neurons of sporadic amyotrophic lateral sclerosis (ALS) patients suggests that Ca2+-permeable AMPA receptors and the dysregulation of RNA editing are suitable therapeutic targets for ALS. Gene therapy has recently emerged as a new therapeutic opportunity for many heretofore incurable diseases, and RNA editing dysregulation can be a target for gene therapy; therefore, we reviewed neurological diseases associated with dysregulated RNA editing and a new therapeutic approach targeting dysregulated RNA editing, especially one that is effective in ALS.

Applications and challenges of CRISPR-Cas gene-editing to disease treatment in clinics

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated systems (Cas) are efficient tools for targeting specific genes for laboratory research, agricultural engineering, biotechnology, and human disease treatment. Cas9, by far the most extensively used gene-editing nuclease, has shown great promise for the treatment of hereditary diseases, viral infection, cancers, and so on. Recent reports have revealed that some other types of CRISPR-Cas systems may also have surprising potential to join the fray as gene-editing tools for various applications. Despite the rapid progress in basic research and clinical tests, some underlying problems present continuous, significant challenges, such as editing efficiency, relative difficulty in delivery, off-target effects, immunogenicity, etc. This article summarizes the applications of CRISPR-Cas from bench to bedside and highlights the current obstacles that may limit the usage of CRISPR-Cas systems as gene-editing toolkits in precision medicine and offer some viewpoints that may help to tackle these challenges and facilitate technical development. CRISPR-Cas systems, as a powerful gene-editing approach, will offer great hopes in clinical treatments for many individuals with currently incurable diseases.

Type II anti-CRISPR proteins as a new tool for synthetic biology

The clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated proteins) system represents, in prokaryotes, an adaptive and inheritable immune response against invading DNA. The discovery of anti-CRISPR proteins (Acrs), which are inhibitors of CRISPR-Cas, mainly encoded by phages and prophages, showed a co-evolution history between prokaryotes and phages. In the past decade, the CRISPR-Cas systems together with the corresponding Acrs have been turned into a genetic-engineering tool. Among the six types of CRISPR-Cas characterized so far, type II CRISPR-Cas system is the most popular in biotechnology. Here, we discuss about the discovery, the reported inhibitory mechanisms, and the applications in both gene editing and gene transcriptional regulation of type II Acrs. Moreover, we provide insights into future potential research and feasible applications.

Structure-based functional mechanisms and biotechnology applications of anti-CRISPR proteins

CRISPR loci and Cas proteins provide adaptive immunity in prokaryotes against invading bacteriophages and plasmids. In response, bacteriophages have evolved a broad spectrum of anti-CRISPR proteins (anti-CRISPRs) to counteract and overcome this immunity pathway. Numerous anti-CRISPRs have been identified to date, which suppress single-subunit Cas effectors (in CRISPR class 2, type II, V and VI systems) and multisubunit Cascade effectors (in CRISPR class 1, type I and III systems). Crystallography and cryo-electron microscopy structural studies of anti-CRISPRs bound to effector complexes, complemented by functional experiments in vitro and in vivo, have identified four major CRISPR-Cas suppression mechanisms: inhibition of CRISPR-Cas complex assembly, blocking of target binding, prevention of target cleavage, and degradation of cyclic oligonucleotide signalling molecules. In this Review, we discuss novel mechanistic insights into anti-CRISPR function that have emerged from X-ray crystallography and cryo-electron microscopy studies, and how these structures in combination with function studies provide valuable tools for the ever-growing CRISPR-Cas biotechnology toolbox, to be used for precise and robust genome editing and other applications.

Functional Features and Current Applications of the RNA-Targeting Type VI CRISPR-Cas Systems

CRISPR-Cas systems are a form of prokaryotic adaptive immunity that employs RNA-guided endonucleases (Cas effectors) to cleave foreign genetic elements. Due to their simplicity, targeting programmability, and efficiency, single-effector CRISPR-Cas systems have great potential for application in research, biotechnology, and therapeutics. While DNA-targeting Cas effectors such as Cas9 and Cas12a have become indispensable tools for genome editing in the past decade, the more recent discovery of RNA-targeting CRISPR-Cas systems has opened the door for implementation of CRISPR-Cas technology in RNA manipulation. With an increasing number of studies reporting their application in transcriptome engineering, viral interference, nucleic acid detection, and RNA imaging, type VI CRISPR-Cas systems and the associated Cas13 effectors particularly hold promise as RNA-targeting or RNA-binding tools. However, even though previous structural and biochemical characterization provided a firm basis for leveraging type VI CRISPR-Cas systems into such tools, the lack of comprehension of certain mechanisms underlying their functions hinders more sophisticated and conventional use. This review will summarize current knowledge on structural and mechanistic properties of type VI CRISPR-Cas systems, give an overview on the reported applications, and discuss functional features that need further investigation in order to improve performance of Cas13-based tools.

Structural basis of Staphylococcus aureus Cas9 inhibition by AcrIIA14

Bacteriophages have evolved a range of anti-CRISPR proteins (Acrs) to escape the adaptive immune system of prokaryotes, therefore Acrs can be used as switches to regulate gene editing. Herein, we report the crystal structure of a quaternary complex of AcrIIA14 bound SauCas9-sgRNA-dsDNA at 2.22 Å resolution, revealing the molecular basis for AcrIIA14 recognition and inhibition. Our structural and biochemical data analysis suggest that AcrIIA14 binds to a non-conserved region of SauCas9 HNH domain that is distinctly different from AcrIIC1 and AcrIIC3, with no significant effect on sgRNA or dsDNA binding. Further, our structural data shows that the allostery of the HNH domain close to the substrate DNA is sterically prevented by AcrIIA14 binding. In addition, the binding of AcrIIA14 triggers the conformational allostery of the HNH domain and the L1 linker within the SauCas9, driving them to make new interactions with the target-guide heteroduplex, enhancing the inhibitory ability of AcrIIA14. Our research both expands the current understanding of anti-CRISPRs and provides additional culues for the rational use of the CRISPR-Cas system in genome editing and gene regulation.

Spatiotemporal control of CRISPR/Cas9 gene editing

The clustered regularly interspaced short palindromic repeats (CRISPR)/associated protein 9 (CRISPR/Cas9) gene editing technology, as a revolutionary breakthrough in genetic engineering, offers a promising platform to improve the treatment of various genetic and infectious diseases because of its simple design and powerful ability to edit different loci simultaneously. However, failure to conduct precise gene editing in specific tissues or cells within a certain time may result in undesirable consequences, such as serious off-target effects, representing a critical challenge for the clinical translation of the technology. Recently, some emerging strategies using genetic regulation, chemical and physical strategies to regulate the activity of CRISPR/Cas9 have shown promising results in the improvement of spatiotemporal controllability. Herein, in this review, we first summarize the latest progress of these advanced strategies involving cell-specific promoters, small-molecule activation and inhibition, bioresponsive delivery carriers, and optical/thermal/ultrasonic/magnetic activation. Next, we highlight the advantages and disadvantages of various strategies and discuss their obstacles and limitations in clinical translation. Finally, we propose viewpoints on directions that can be explored to further improve the spatiotemporal operability of CRISPR/Cas9.

CRISPR-Cas13 System as a Promising and Versatile Tool for Cancer Diagnosis, Therapy, and Research

Over the past decades, significant progress has been made in targeted cancer therapy. In precision oncology, molecular profiling of cancer patients enables the use of targeted cancer therapeutics. However, current diagnostic methods for molecular analysis of cancer are costly and require sophisticated equipment. Moreover, targeted cancer therapeutics such as monoclonal antibodies and small-molecule drugs may cause off-target effects and they are available for only a minority of cancer driver proteins. Therefore, there is still a need for versatile, efficient, and precise tools for cancer diagnostics and targeted cancer treatment. In recent years, the CRISPR-based genome and transcriptome engineering toolbox has expanded rapidly. Particularly, the RNA-targeting CRISPR-Cas13 system has unique biochemical properties, making Cas13 a promising tool for cancer diagnosis, therapy, and research. Cas13-based diagnostic methods allow early detection and monitoring of cancer markers from liquid biopsy samples without the need for complex instrumentation. In addition, Cas13 can be used for targeted cancer therapy through degrading and manipulating cancer-associated transcripts with high efficiency and specificity. Moreover, Cas13-mediated programmable RNA manipulation tools offer invaluable opportunities for cancer research, identification of drug-resistance mechanisms, and discovery of novel therapeutic targets. Here, we review and discuss the current use and potential applications of the CRISPR-Cas13 system in cancer diagnosis, therapy, and research. Thus, researchers will gain a deep understanding of CRISPR-Cas13 technologies, which have the potential to be used as next-generation cancer diagnostics and therapeutics.