Temperature-Responsive Competitive Inhibition of CRISPR-Cas9

Pamoic acid and carbenoxolone specifically inhibit CRISPR/Cas9 in bacteria, mammalian cells, and mice in a DNA topology-specific manner

BACKGROUND

Regulation of the target DNA cleavage activity of CRISPR/Cas has naturally evolved in a few bacteria or bacteriophages but is lacking in higher species. Thus, identification of bioactive agents and mechanisms that can suppress the activity of Cas9 is urgently needed to rebalance this new genetic pressure.

RESULTS

Here, we identify four specific inhibitors of Cas9 by screening 4607 compounds that could inhibit the endonuclease activity of Cas9 via three distinct mechanisms: substrate-competitive and protospacer adjacent motif (PAM)-binding site-occupation; substrate-targeting; and sgRNA-targeting mechanisms. These inhibitors inhibit, in a dose-dependent manner, the activity of Streptococcus pyogenes Cas9 (SpyCas9), Staphylococcus aureus Cas9 (SauCas9), and SpyCas9 nickase-based BE4 base editors in in vitro purified enzyme assays, bacteria, mammalian cells, and mice. Importantly, pamoic acid and carbenoxolone show DNA-topology selectivity and preferentially inhibit the cleavage of linear DNA compared with a supercoiled plasmid.

CONCLUSIONS

These pharmacologically selective inhibitors and new mechanisms offer new tools for controlling the DNA-topology selective activity of Cas9.

Conformational plasticity of SpyCas9 induced by AcrIIA4 and AcrIIA2: Insights from molecular dynamics simulation

CRISPR-Cas9 systems constitute bacterial adaptive immune systems that protect against phage infections. Bacteriophages encode anti-CRISPR proteins (Acrs) that mitigate the bacterial immune response. However, the structural basis for their inhibitory actions from a molecular perspective remains elusive. In this study, through microsecond atomistic molecular dynamics simulations, we demonstrated the remarkable flexibility of Streptococcus pyogenes Cas9 (SpyCas9) and its conformational adaptability during interactions with AcrIIA4 and AcrIIA2. Specifically, we demonstrated that the binding of AcrIIA4 and AcrIIA2 to SpyCas9 induces a conformational rearrangement that causes spatial separation between the nuclease and cleavage sites, thus making the endonuclease inactive. This separation disrupts the transmission of signals between the protospacer adjacent motif recognition and nuclease domains, thereby impeding the efficient processing of double-stranded DNA. The simulation also reveals that AcrIIA4 and AcrIIA2 cause different structural variations of SpyCas9. Our research illuminates the precise mechanisms underlying the suppression of SpyCas9 by AcrIIA4 and AcrIIA2, thus presenting new possibilities for controlling genome editing with higher accuracy.

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.

Inhibition mechanisms of CRISPR-Cas9 by AcrIIA25.1 and AcrIIA32

In the ongoing arms race between bacteria and bacteriophages, bacteriophages have evolved anti-CRISPR proteins to counteract bacterial CRISPR-Cas systems. Recently, AcrIIA25.1 and AcrIIA32 have been found to effectively inhibit the activity of SpyCas9 both in bacterial and human cells. However, their molecular mechanisms remain elusive. Here, we report the cryo-electron microscopy structures of ternary complexes formed by AcrIIA25.1 and AcrIIA32 bound to SpyCas9-sgRNA. Using structural analysis and biochemical experiments, we revealed that AcrIIA25.1 and AcrIIA32 recognize a novel, previously-unidentified anti-CRISPR binding site on SpyCas9. We found that both AcrIIA25.1 and AcrIIA32 directly interact with the WED domain, where they spatially obstruct conformational changes of the WED and PI domains, thereby inhibiting SpyCas9 from recognizing protospacer adjacent motif (PAM) and unwinding double-stranded DNA. In addition, they may inhibit nuclease activity by blocking the dynamic conformational changes of the SpyCas9 surveillance complex. In summary, our data elucidate the inhibition mechanisms of two new anti-CRISPR proteins, provide new strategies for the modulation of SpyCas9 activity, and expand our understanding of the diversity of anti-CRISPR protein inhibition mechanisms.

CRISPR Tools for Engineering Prokaryotic Systems: Recent Advances and New Applications

In the past decades, the broad selection of CRISPR-Cas systems has revolutionized biotechnology by enabling multimodal genetic manipulation in diverse organisms. Rooted in a molecular engineering perspective, we recapitulate the different CRISPR components and how they can be designed for specific genetic engineering applications. We first introduce the repertoire of Cas proteins and tethered effectors used to program new biological functions through gene editing and gene regulation. We review current guide RNA (gRNA) design strategies and computational tools and how CRISPR-based genetic circuits can be constructed through regulated gRNA expression. Then, we present recent advances in CRISPR-based biosensing, bioproduction, and biotherapeutics across in vitro and in vivo prokaryotic systems. Finally, we discuss forthcoming applications in prokaryotic CRISPR technology that will transform synthetic biology principles in the near future.

An anti-CRISPR that represses its own transcription while blocking Cas9-target DNA binding

AcrIIA15 is an anti-CRISPR (Acr) protein that inhibits Staphylococcus aureus Cas9 (SaCas9). Although previous studies suggested it has dual functions, the structural and biochemical basis for its two activities remains unclear. Here, we determined the cryo-EM structure of AcrIIA15 in complex with SaCas9-sgRNA to reveal the inhibitory mechanism of the Acr's C-terminal domain (CTD) in mimicking dsDNA to block protospacer adjacent motif (PAM) recognition. For the N-terminal domain (NTD), our crystal structures of the AcrIIA15-promoter DNA show that AcrIIA15 dimerizes through its NTD to recognize double-stranded (ds) DNA. Further, AcrIIA15 can simultaneously bind to both SaCas9-sgRNA and promoter DNA, creating a supercomplex of two Cas9s bound to two CTDs converging on a dimer of the NTD bound to a dsDNA. These findings shed light on AcrIIA15's inhibitory mechanisms and its autoregulation of transcription, enhancing our understanding of phage-host interactions and CRISPR defense.

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.

Leveraging QM/MM and Molecular Dynamics Simulations to Decipher the Reaction Mechanism of the Cas9 HNH Domain to Investigate Off-Target Effects

The clustered regularly interspaced short palindromic repeats (CRISPR) technology is an RNA-guided targeted genome-editing tool using Cas family proteins. Two magnesium-dependent nuclease domains of the Cas9 enzyme, termed HNH and RuvC, are responsible for cleaving the target DNA (t-DNA) and nontarget DNA strands, respectively. The HNH domain is believed to determine the DNA cleavage activity of both endonuclease domains and is sensitive to complementary RNA-DNA base pairing. However, the underlying molecular mechanisms of CRISPR-Cas9, by which it rebukes or accepts mismatches, are poorly understood. Thus, investigation of the structure and dynamics of the catalytic state of Cas9 with either matched or mismatched t-DNA can provide insights into improving its specificity by reducing off-target cleavages. Here, we focus on a recently discovered catalytic-active form of the Streptococcus pyogenes Cas9 (SpCas9) and employ classical molecular dynamics and coupled quantum mechanics/molecular mechanics simulations to study two possible mechanisms of t-DNA cleavage reaction catalyzed by the HNH domain. Moreover, by designing a mismatched t-DNA structure called MM5 (C to G at the fifth position from the protospacer adjacent motif region), the impact of single-guide RNA (sgRNA) and t-DNA complementarity on the catalysis process was investigated. Based on these simulations, our calculated binding affinities, minimum energy paths, and analysis of catalytically important residues provide atomic-level details of the differences between matched and mismatched cleavage reactions. In addition, several residues exhibit significant differences in their catalytic roles for the two studied systems, including K253, K263, R820, K896, and K913.

Highlights of bioinformatic tools and methods for validating bioinformatics derived hypotheses for microbial natural products research

Historically, bacterial natural products have served as an excellent source of drug leads, however, in recent decades the rate of discovery has slowed due to multiple challenges. Rapid advances in genome sequencing science in recent years have revealed the vast untapped encoded potential of bacteria to make natural products. To access these molecules, researchers can employ the ever-growing array of bioinformatic tools at their disposal and leverage newly developed experimental approaches to validate these bioinformatic-driven hypotheses. When used together effectively, bioinformatic and experimental tools enable researchers to deeply examine the full diversity of bacterial natural products. This review briefly outlines recent bioinformatic tools that can facilitate natural product research in bacteria including the use of CRISPR, co-occurrence network analysis, and combinatorial generation of microbial natural products to test bioinformatic hypotheses in the lab.

Optimization of Cas9 activity through the addition of cytosine extensions to single-guide RNAs

The precise regulation of the activity of Cas9 is crucial for safe and efficient editing. Here we show that the genome-editing activity of Cas9 can be constrained by the addition of cytosine stretches to the 5'-end of conventional single-guide RNAs (sgRNAs). Such a 'safeguard sgRNA' strategy, which is compatible with Cas12a and with systems for gene activation and interference via CRISPR (clustered regularly interspaced short palindromic repeats), leads to the length-dependent inhibition of the formation of functional Cas9 complexes. Short cytosine extensions reduced p53 activation and cytotoxicity in human pluripotent stem cells, and enhanced homology-directed repair while maintaining bi-allelic editing. Longer extensions further decreased on-target activity yet improved the specificity and precision of mono-allelic editing. By monitoring indels through a fluorescence-based allele-specific system and computational simulations, we identified optimal windows of Cas9 activity for a number of genome-editing applications, including bi-allelic and mono-allelic editing, and the generation and correction of disease-associated single-nucleotide substitutions via homology-directed repair. The safeguard-sgRNA strategy may improve the safety and applicability of genome editing.

Anti-CRISPR AcrIIC5 is a dsDNA mimic that inhibits type II-C Cas9 effectors by blocking PAM recognition

Anti-CRISPR proteins are encoded by phages to inhibit the CRISPR-Cas systems of the hosts. AcrIIC5 inhibits several naturally high-fidelity type II-C Cas9 enzymes, including orthologs from Neisseria meningitidis (Nme1Cas9) and Simonsiella muelleri (SmuCas9). Here, we solve the structure of AcrIIC5 in complex with Nme1Cas9 and sgRNA. We show that AcrIIC5 adopts a novel fold to mimic the size and charge distribution of double-stranded DNA, and uses its negatively charged grooves to bind and occlude the protospacer adjacent motif (PAM) binding site in the target DNA cleft of Cas9. AcrIIC5 is positioned into the crevice between the WED and PI domains of Cas9, and one end of the anti-CRISPR interacts with the phosphate lock loop and a linker between the RuvC and BH domains. We employ biochemical and mutational analyses to build a model for AcrIIC5's mechanism of action, and identify residues on both the anti-CRISPR and Cas9 that are important for their interaction and inhibition. Together, the structure and mechanism of AcrIIC5 reveal convergent evolution among disparate anti-CRISPR proteins that use a DNA-mimic strategy to inhibit diverse CRISPR-Cas surveillance complexes, and provide new insights into a tool for potent inhibition of type II-C Cas9 orthologs.

The Many (Inter)faces of Anti-CRISPRs: Modulation of CRISPR-Cas Structure and Dynamics by Mechanistically Diverse Inhibitors

The discovery of protein inhibitors of CRISPR-Cas systems, called anti-CRISPRs (Acrs), has enabled the development of highly controllable and precise CRISPR-Cas tools. Anti-CRISPRs share very little structural or sequential resemblance to each other or to other proteins, which raises intriguing questions regarding their modes of action. Many structure-function studies have shed light on the mechanism(s) of Acrs, which can act as orthosteric or allosteric inhibitors of CRISPR-Cas machinery, as well as enzymes that irreversibly modify CRISPR-Cas components. Only recently has the breadth of diversity of Acr structures and functions come to light, and this remains a rapidly evolving field. Here, we draw attention to a plethora of Acr mechanisms, with particular focus on how their action toward Cas proteins modulates conformation, dynamic (allosteric) signaling, nucleic acid binding, and cleavage ability.

Temperature dependent in vitro binding and release of target DNA by Cas9 enzyme

The CRISPR-associated protein 9 (Cas9) system has proven to be a powerful technology for genome editing in a wide variety of in vivo and in vitro applications. CRISPR-Cas9, when loaded with the guide RNA, cleaves the DNA at the target position as recognized by the guide RNA sequence. For successful application of this technology, it is important to study the biophysical parameters affecting its function. Temperature dependence of the Cas9 binding as well as energetics of product release after cleavage has not been well reported in the literature. In this work, we study the binding properties of Cas9 enzyme to the sequence specific target DNA at a range of temperatures and, surprisingly, find that the Cas9 enzyme, in our study, can find and bind its target DNA with 90 ± 20% efficiency at temperatures as low as 4 °C. Further, we show that the cleaved DNA products remain bound to the Cas9 enzyme strongly and is released from the enzyme only at higher temperatures. Using the gel shift assays, we quantify the rate of Cas9 binding to target DNA to be 0.8 ± 0.2 min-1 at 37 °C. We also tested denaturant (SDS) dependent release of cleaved product which showed a similar release pattern with a dissociation constant of 0.23 ± 0.04 mM. Our results of heat and denaturant dependence on Cas9-DNA binding and release mechanics will provide valuable insights for developing temperature dependent applications of the CRISPR-Cas9 technology.

Tips, Tricks, and Potential Pitfalls of CRISPR Genome Editing in Saccharomyces cerevisiae

The versatility of clustered regularly interspaced short palindromic repeat (CRISPR)-associated (Cas) genome editing makes it a popular tool for many research and biotechnology applications. Recent advancements in genome editing in eukaryotic organisms, like fungi, allow for precise manipulation of genetic information and fine-tuned control of gene expression. Here, we provide an overview of CRISPR genome editing technologies in yeast, with a particular focus on Saccharomyces cerevisiae. We describe the tools and methods that have been previously developed for genome editing in Saccharomyces cerevisiae and discuss tips and experimental tricks for promoting efficient, marker-free genome editing in this model organism. These include sgRNA design and expression, multiplexing genome editing, optimizing Cas9 expression, allele-specific editing in diploid cells, and understanding the impact of chromatin on genome editing. Finally, we summarize recent studies describing the potential pitfalls of using CRISPR genome targeting in yeast, including the induction of background mutations.

KPT330 improves Cas9 precision genome- and base-editing by selectively regulating mRNA nuclear export

CRISPR-based genome engineering tools are associated with off-target effects that constitutively active Cas9 protein may instigate. Previous studies have revealed the feasibility of modulating Cas9-based genome- and base-editing tools using protein or small-molecule CRISPR inhibitors. Here we screened a set of small molecule compounds with irreversible warhead, aiming to identifying small-molecule modulators of CRISPR-Cas9. It was found that selective inhibitors of nuclear export (SINEs) could efficiently inhibit the cellular activity of Cas9 in the form of genome-, base- and prime-editing tools. Interestingly, SINEs did not function as direct inhibitors to Cas9, but modulated Cas9 activities by interfering with the nuclear export process of Cas9 mRNA. Thus, to the best of our knowledge, SINEs represent the first reported indirect, irreversible inhibitors of CRISPR-Cas9. Most importantly, an FDA-approved anticancer drug KPT330, along with other examined SINEs, could improve the specificities of CRISPR-Cas9-based genome- and base editing tools in human cells. Our study expands the toolbox of CRISPR modulating elements and provides a feasible approach to improving the specificity of CRISPR-Cas9-based genome engineering tools.

The FDA-approved anti-cancer drug, KPT330, can indirectly inhibit Cas9 by interfering with Cas9 mRNA nuclear export and help reduce off-target editing in cells.

Smart Strategies for Precise Delivery of CRISPR/Cas9 in Genome Editing

The emergence of CRISPR/Cas technology has enabled scientists to precisely edit genomic DNA sequences. This approach can be used to modulate gene expression for the treatment of genetic disorders and incurable diseases such as cancer. This potent genome-editing tool is based on a single guide RNA (sgRNA) strand that recognizes the targeted DNA, plus a Cas nuclease protein for binding and processing the target. CRISPR/Cas has great potential for editing many genes in different types of cells and organisms both in vitro and in vivo. Despite these remarkable advances, the risk of off-target effects has hindered the translation of CRISPR/Cas technology into clinical applications. To overcome this hurdle, researchers have devised gene regulatory systems that can be controlled in a spatiotemporal manner, by designing special sgRNA, Cas, and CRISPR/Cas delivery vehicles that are responsive to different stimuli, such as temperature, light, magnetic fields, ultrasound (US), pH, redox, and enzymatic activity. These systems can even respond to dual or multiple stimuli simultaneously, thereby providing superior spatial and temporal control over CRISPR/Cas gene editing. Herein, we summarize the latest advances on smart sgRNA, Cas, and CRISPR/Cas nanocarriers, categorized according to their stimulus type (physical, chemical, or biological).

Discovery of potent and versatile CRISPR–Cas9 inhibitors engineered for chemically controllable genome editing

Anti-CRISPR (Acr) proteins are encoded by many mobile genetic elements (MGEs) such as phages and plasmids to combat CRISPR–Cas adaptive immune systems employed by prokaryotes, which provide powerful tools for CRISPR–Cas-based applications. Here, we discovered nine distinct type II-A anti-CRISPR (AcrIIA24–32) families from Streptococcus MGEs and found that most Acrs can potently inhibit type II-A Cas9 orthologs from Streptococcus (SpyCas9, St1Cas9 or St3Cas9) in bacterial and human cells. Among these Acrs, AcrIIA26, AcrIIA27, AcrIIA30 and AcrIIA31 are able to block Cas9 binding to DNA, while AcrIIA24 abrogates DNA cleavage by Cas9. Notably, AcrIIA25.1 and AcrIIA32.1 can inhibit both DNA binding and DNA cleavage activities of SpyCas9, exhibiting unique anti-CRISPR characteristics. Importantly, we developed several chemically inducible anti-CRISPR variants based on AcrIIA25.1 and AcrIIA32.1 by comprising hybrids of Acr protein and the 4-hydroxytamoxifen-responsive intein, which enabled post-translational control of CRISPR–Cas9-mediated genome editing in human cells. Taken together, our work expands the diversity of type II-A anti-CRISPR families and the toolbox of Acr proteins for the chemically inducible control of Cas9-based applications.

Interaction of Bare dSpCas9, Scaffold gRNA, and Type II Anti-CRISPR Proteins Highly Favors the Control of Gene Expression in the Yeast S. cerevisiae

Type II CRISPR-(d)SpCas9 and anti-CRISPR proteins (AcrIIs) show evidence of coevolution and competition for survival between bacteria and phages. In biotechnology, CRISPR-(d)SpCas9 is utilized for gene editing and transcriptional regulation. Moreover, its activity is controlled by AcrIIs. However, studies of dSpCas9/AcrII-based transcription regulation in Saccharomyces cerevisiae are rare. In this work, we used dSpCas9 as a template to engineer new transcription activators. We found that the most performant activation system requires the use of bare dSpCas9 in conjunction with scaffold gRNA (scRNA). This means that activation domains shall not be fused to dSpCas9 but rather interact with scRNA. We showed that a low amount of sgRNA is not a limiting factor in dSpCas9-driven transcription regulation. Moreover, a high quantity of sgRNA does not improve, generally, activation (and repression) efficiency. Importantly, we analyzed the performance of AcrIIA2, AcrIIA4, and AcrIIA5 in S. cerevisiae in depth. AcrIIA4 is the strongest of the three AcrIIs and also the only one able to induce high inhibition at low concentrations. However, the activation domains fused to dSpCas9 hindered interactions with the AcrIIs as well and limited their control of gene transcription regulation, confirming that bare dSpCas9 is the best solution for building synthetic genetic networks in yeast.

Inhibition mechanisms of CRISPR-Cas9 by AcrIIA17 and AcrIIA18

Mobile genetic elements such as phages and plasmids have evolved anti-CRISPR proteins (Acrs) to suppress CRISPR-Cas adaptive immune systems. Recently, several phage and non-phage derived Acrs including AcrIIA17 and AcrIIA18 have been reported to inhibit Cas9 through modulation of sgRNA. Here, we show that AcrIIA17 and AcrIIA18 inactivate Cas9 through distinct mechanisms. AcrIIA17 inhibits Cas9 activity through interference with Cas9-sgRNA binary complex formation. In contrast, AcrIIA18 induces the truncation of sgRNA in a Cas9-dependent manner, generating a shortened sgRNA incapable of triggering Cas9 activity. The crystal structure of AcrIIA18, combined with mutagenesis studies, reveals a crucial role of the N-terminal β-hairpin in AcrIIA18 for sgRNA cleavage. The enzymatic inhibition mechanism of AcrIIA18 is different from those of the other reported type II Acrs. Our results add new insights into the mechanistic understanding of CRISPR-Cas9 inhibition by Acrs, and also provide valuable information in the designs of tools for conditional manipulation of CRISPR-Cas9.

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.

Atomic-scale insights into allosteric inhibition and evolutional rescue mechanism of Streptococcus thermophilus Cas9 by the anti-CRISPR protein AcrIIA6

CRISPR-Cas systems are prokaryotic adaptive immunity against invading phages and plasmids. Phages have evolved diverse protein inhibitors of CRISPR-Cas systems, called anti-CRISPR (Acr) proteins, to neutralize this CRISPR machinery. In response, bacteria have co-evolved Cas variants to escape phage's anti-CRISPR strategies, called anti-anti-CRISPR systems. Here we explore the anti-CRISPR allosteric inhibition and anti-anti-CRISPR rescue mechanisms between Streptococcus thermophilus Cas9 (St1Cas9) and the anti-CRISPR protein AcrIIA6 at the atomic level, by generating mutants of key residues in St1Cas9. Extensive unbiased molecular dynamics simulations show that the functional motions of St1Cas9 in the presence of AcrIIA6 differ substantially from those of St1Cas9 alone. AcrIIA6 binding triggers a shift of St1Cas9 conformational ensemble towards a less catalytically competent state; this state significantly compromises protospacer adjacent motif (PAM) recognition and nuclease activity by altering interdependently conformational dynamics and allosteric signals among nuclease domains, PAM-interacting (PI) regions, and AcrIIA6 binding motifs. Via in vitro DNA cleavage assays, we further elucidate the rescue mechanism of efficiently escaping AcrIIA6 inhibition harboring St1Cas9 triple mutations (G993K/K1008M/K1010E) in the PI domain and identify the evolutionary landscape of such mutational escape within species. Our results provide mechanistic insights into Acr proteins as natural brakes for the CRISPR-Cas systems and a promising potential for the design of allosteric Acr peptidomimetics.

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.

CRISPR/Cas System: A Potential Technology for the Prevention and Control of COVID-19 and Emerging Infectious Diseases

The continued global pandemic of coronavirus disease 2019 (COVID-19) poses a serious threat to global public health and social stability and it has become a serious global public health problem. Unfortunately, existing diagnostic and therapeutic approaches for the prevention and control of COVID-19 have many shortcomings. In recent years, the emerging CRISPR/Cas technology can complement the problems of traditional methods. Biological tools based on CRISPR/Cas systems have been widely used in biomedicine. In particular, they are advantageous in pathogen detection, clinical antiviral therapy, drug, and vaccine development. Therefore, CRISPR/Cas technology may have great potential for application in the prevention and control of COVID-19 and emerging infectious diseases in the future. This article summarizes the existing applications of CRISPR/Cas technology in infectious diseases with the aim of providing effective strategies for the prevention and control of COVID-19 and other emerging infectious diseases in the future.

Efficient ssODN-Mediated Targeting by Avoiding Cellular Inhibitory RNAs through Precomplexed CRISPR-Cas9/sgRNA Ribonucleoprotein

Combined with CRISPR-Cas9 technology and single-stranded oligodeoxynucleotides (ssODNs), specific single-nucleotide alterations can be introduced into a targeted genomic locus in induced pluripotent stem cells (iPSCs); however, ssODN knockin frequency is low compared with deletion induction. Although several Cas9 transduction methods have been reported, the biochemical behavior of CRISPR-Cas9 nuclease in mammalian cells is yet to be explored. Here, we investigated intrinsic cellular factors that affect Cas9 cleavage activity in vitro. We found that intracellular RNA, but not DNA or protein fractions, inhibits Cas9 from binding to single guide RNA (sgRNA) and reduces the enzymatic activity. To prevent this, precomplexing Cas9 and sgRNA before delivery into cells can lead to higher genome editing activity compared with Cas9 overexpression approaches. By optimizing electroporation parameters of precomplexed ribonucleoprotein and ssODN, we achieved efficiencies of single-nucleotide correction as high as 70% and loxP insertion up to 40%. Finally, we could replace the HLA-C1 allele with the C2 allele to generate histocompatibility leukocyte antigen custom-edited iPSCs.

Optogenetic control of Neisseria meningitidis Cas9 genome editing using an engineered, light-switchable anti-CRISPR protein

Optogenetic control of CRISPR–Cas9 systems has significantly improved our ability to perform genome perturbations in living cells with high precision in time and space. As new Cas orthologues with advantageous properties are rapidly being discovered and engineered, the need for straightforward strategies to control their activity via exogenous stimuli persists. The Cas9 from Neisseria meningitidis (Nme) is a particularly small and target-specific Cas9 orthologue, and thus of high interest for in vivo genome editing applications. Here, we report the first optogenetic tool to control NmeCas9 activity in mammalian cells via an engineered, light-dependent anti-CRISPR (Acr) protein. Building on our previous Acr engineering work, we created hybrids between the NmeCas9 inhibitor AcrIIC3 and the LOV2 blue light sensory domain from Avena sativa. Two AcrIIC3-LOV2 hybrids from our collection potently blocked NmeCas9 activity in the dark, while permitting robust genome editing at various endogenous loci upon blue light irradiation. Structural analysis revealed that, within these hybrids, the LOV2 domain is located in striking proximity to the Cas9 binding surface. Together, our work demonstrates optogenetic regulation of a type II-C CRISPR effector and might suggest a new route for the design of optogenetic Acrs.

Gene and epigenetic editing in the treatment of primary ciliopathies

Primary ciliopathies are inherited human disorders that arise from mutations in ciliary genes. They represent a spectrum of severe, incurable phenotypes, differentially involving several organs, including the kidney and the eye. The development of gene-based therapies is opening up new avenues for the treatment of ciliopathies. Particularly attractive is the possibility of correcting in situ the causative genetic mutation, or pathological epigenetic changes, through the use of gene editing tools. Due to their versatility and efficacy, CRISPR/Cas-based systems represent the most promising gene editing toolkit for clinical applications. However, delivery and specificity issues have so far held back the translatability of CRISPR/Cas-based therapies into clinical practice, especially where systemic administration is required. The eye, with its characteristics of high accessibility and compartmentalization, represents an ideal target for in situ gene correction. Indeed, studies for the evaluation of a CRISPR/Cas-based therapy for in vivo gene correction to treat a retinal ciliopathy have reached the clinical stage. Further technological advances may be required for the development of in vivo CRISPR-based treatments for the kidney. We discuss here the possibilities and the challenges associated to the implementation of CRISPR/Cas-based therapies for the treatment of primary ciliopathies with renal and retinal phenotypes.

Diversity of molecular mechanisms used by anti-CRISPR proteins: the tip of an iceberg?

Bacteriophages (phages) and their preys are engaged in an evolutionary arms race driving the co-adaptation of their attack and defense mechanisms. In this context, phages have evolved diverse anti-CRISPR proteins to evade the bacterial CRISPR-Cas immune system, and propagate. Anti-CRISPR proteins do not share much resemblance with each other and with proteins of known function, which raises intriguing questions particularly relating to their modes of action. In recent years, there have been many structure-function studies shedding light on different CRISPR-Cas inhibition strategies. As the anti-CRISPR field of research is rapidly growing, it is opportune to review the current knowledge on these proteins, with particular emphasis on the molecular strategies deployed to inactivate distinct steps of CRISPR-Cas immunity. Anti-CRISPR proteins can be orthosteric or allosteric inhibitors of CRISPR-Cas machineries, as well as enzymes that irreversibly modify CRISPR-Cas components. This repertoire of CRISPR-Cas inhibition mechanisms will likely expand in the future, providing fundamental knowledge on phage-bacteria interactions and offering great perspectives for the development of biotechnological tools to fine-tune CRISPR-Cas-based gene edition.

Epigenome engineering: new technologies for precision medicine

Chromatin adopts different configurations that are regulated by reversible covalent modifications, referred to as epigenetic marks. Epigenetic inhibitors have been approved for clinical use to restore epigenetic aberrations that result in silencing of tumor-suppressor genes, oncogene addictions, and enhancement of immune responses. However, these drugs suffer from major limitations, such as a lack of locus selectivity and potential toxicities. Technological advances have opened a new era of precision molecular medicine to reprogram cellular physiology. The locus-specificity of CRISPR/dCas9/12a to manipulate the epigenome is rapidly becoming a highly promising strategy for personalized medicine. This review focuses on new state-of-the-art epigenome editing approaches to modify the epigenome of neoplasms and other disease models towards a more 'normal-like state', having characteristics of normal tissue counterparts. We highlight biomolecular engineering methodologies to assemble, regulate, and deliver multiple epigenetic effectors that maximize the longevity of the therapeutic effect, and we discuss limitations of the platforms such as targeting efficiency and intracellular delivery for future clinical applications.

Base editing: advances and therapeutic opportunities

Base editing - the introduction of single-nucleotide variants (SNVs) into DNA or RNA in living cells - is one of the most recent advances in the field of genome editing. As around half of known pathogenic genetic variants are due to SNVs, base editing holds great potential for the treatment of numerous genetic diseases, through either temporary RNA or permanent DNA base alterations. Recent advances in the specificity, efficiency, precision and delivery of DNA and RNA base editors are revealing exciting therapeutic opportunities for these technologies. We expect the correction of single point mutations will be a major focus of future precision medicine.

Discovery of multiple anti-CRISPRs highlights anti-defense gene clustering in mobile genetic elements

Many prokaryotes employ CRISPR-Cas systems to combat invading mobile genetic elements (MGEs). In response, some MGEs have developed strategies to bypass immunity, including anti-CRISPR (Acr) proteins; yet the diversity, distribution and spectrum of activity of this immune evasion strategy remain largely unknown. Here, we report the discovery of new Acrs by assaying candidate genes adjacent to a conserved Acr-associated (Aca) gene, aca5, against a panel of six type I systems: I-F (Pseudomonas, Pectobacterium, and Serratia), I-E (Pseudomonas and Serratia), and I-C (Pseudomonas). We uncover 11 type I-F and/or I-E anti-CRISPR genes encoded on chromosomal and extrachromosomal MGEs within Enterobacteriaceae and Pseudomonas, and an additional Aca (aca9). The acr genes not only associate with other acr genes, but also with genes encoding inhibitors of distinct bacterial defense systems. Thus, our findings highlight the potential exploitation of acr loci neighborhoods for the identification of previously undescribed anti-defense systems.

Intrinsic disorder is essential for Cas9 inhibition of anti-CRISPR AcrIIA5

Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins provide adaptive immunity to prokaryotes against invading phages and plasmids. As a countermeasure, phages have evolved anti-CRISPR (Acr) proteins that neutralize the CRISPR immunity. AcrIIA5, isolated from a virulent phage of Streptococcus thermophilus, strongly inhibits diverse Cas9 homologs, but the molecular mechanism underlying the Cas9 inhibition remains unknown. Here, we report the solution structure of AcrIIA5, which features a novel α/β fold connected to an N-terminal intrinsically disordered region (IDR). Remarkably, truncation of the N-terminal IDR abrogates the inhibitory activity against Cas9, revealing that the IDR is essential for Cas9 inhibition by AcrIIA5. Progressive truncations and mutations of the IDR illustrate that the disordered region not only modulates the association between AcrIIA5 and Cas9-sgRNA, but also alters the catalytic efficiency of the inhibitory complex. The length of IDR is critical for the Cas9-sgRNA recognition by AcrIIA5, whereas the charge content of IDR dictates the inhibitory activity. Conformational plasticity of IDR may be linked to the broad-spectrum inhibition of Cas9 homologs by AcrIIA5. Identification of the IDR as the main determinant for Cas9 inhibition expands the inventory of phage anti-CRISPR mechanisms.

Machine learning predicts new anti-CRISPR proteins

The increasing use of CRISPR–Cas9 in medicine, agriculture, and synthetic biology has accelerated the drive to discover new CRISPR–Cas inhibitors as potential mechanisms of control for gene editing applications. Many anti-CRISPRs have been found that inhibit the CRISPR–Cas adaptive immune system. However, comparing all currently known anti-CRISPRs does not reveal a shared set of properties for facile bioinformatic identification of new anti-CRISPR families. Here, we describe AcRanker, a machine learning based method to aid direct identification of new potential anti-CRISPRs using only protein sequence information. Using a training set of known anti-CRISPRs, we built a model based on XGBoost ranking. We then applied AcRanker to predict candidate anti-CRISPRs from predicted prophage regions within self-targeting bacterial genomes and discovered two previously unknown anti-CRISPRs: AcrllA20 (ML1) and AcrIIA21 (ML8). We show that AcrIIA20 strongly inhibits Streptococcus iniae Cas9 (SinCas9) and weakly inhibits Streptococcus pyogenes Cas9 (SpyCas9). We also show that AcrIIA21 inhibits SpyCas9, Streptococcus aureus Cas9 (SauCas9) and SinCas9 with low potency. The addition of AcRanker to the anti-CRISPR discovery toolkit allows researchers to directly rank potential anti-CRISPR candidate genes for increased speed in testing and validation of new anti-CRISPRs. A web server implementation for AcRanker is available online at http://acranker.pythonanywhere.com/.

The Impact of CRISPR-Cas9 on Age-related Disorders: From Pathology to Therapy

With advances in medical technology, the number of people over the age of 60 is on the rise, and thus, increasing the prevalence of age-related pathologies within the aging population. Neurodegenerative disorders, cancers, metabolic and inflammatory diseases are some of the most prevalent age-related pathologies affecting the growing population. It is imperative that a new treatment to combat these pathologies be developed. Although, still in its infancy, the CRISPR-Cas9 system has become a potent gene-editing tool capable of correcting gene-mediated age-related pathology, and therefore ameliorating or eliminating disease symptoms. Deleting target genes using the CRISPR-Cas9 system or correcting for gene mutations may ameliorate many different neurodegenerative disorders detected in the aging population. Cancer cells targeted by the CRISPR-Cas9 system may result in an increased sensitivity to chemotherapeutics, lower proliferation, and higher cancer cell death. Finally, reducing gene targeting inflammatory molecules production through microRNA knockout holds promise as a therapeutic strategy for both arthritis and inflammation. Here we present a review based on how the expanding world of genome editing can be applied to disorders and diseases affecting the aging population.

The Synergy between CRISPR and Chemical Engineering

Gene therapy and transgenic research have advanced quickly in recent years due to the development of CRISPR technology. The rapid development of CRISPR technology has been largely benefited by chemical engineering. Firstly, chemical or synthetic substance enables spatiotemporal and conditional control of Cas9 or dCas9 activities. It prevents the leaky expression of CRISPR components, as well as minimizes toxicity and off-target effects. Multi-input logic operations and complex genetic circuits can also be implemented via multiplexed and orthogonal regulation of target genes. Secondly, rational chemical modifications to the sgRNA enhance gene editing efficiency and specificity by improving sgRNA stability and binding affinity to on-target genomic loci, and hence reducing off-target mismatches and systemic immunogenicity. Chemically-modified Cas9 mRNA is also more active and less immunogenic than the native mRNA. Thirdly, nonviral vehicles can circumvent the challenges associated with viral packaging and production through the delivery of Cas9-sgRNA ribonucleoprotein complex or large Cas9 expression plasmids. Multi-functional nanovectors enhance genome editing in vivo by overcoming multiple physiological barriers, enabling ligand-targeted cellular uptake, and blood-brain barrier crossing. Chemical engineering can also facilitate viral-based delivery by improving vector internalization, allowing tissue-specific transgene expression, and preventing inactivation of the viral vectors in vivo. This review aims to discuss how chemical engineering has helped improve existing CRISPR applications and enable new technologies for biomedical research. The usefulness, advantages, and molecular action for each chemical engineering approach are also highlighted.

Keeping crispr in check: diverse mechanisms of phage-encoded anti-crisprs

CRISPR-Cas represents the only adaptive immune system of prokaryotes known to date. These immune systems are widespread among bacteria and archaea, and provide protection against invasion of mobile genetic elements, such as bacteriophages and plasmids. As a result of the arms-race between phages and their prokaryotic hosts, phages have evolved inhibitors known as anti-CRISPR (Acr) proteins to evade CRISPR immunity. In the recent years, several Acr proteins have been described in both temperate and virulent phages targeting diverse CRISPR-Cas systems. Here, we describe the strategies of Acr discovery and the multiple molecular mechanisms by which these proteins operate to inhibit CRISPR immunity. We discuss the biological relevance of Acr proteins and speculate on the implications of their activity for the development of improved CRISPR-based research and biotechnological tools.

Anti-CRISPRs: Protein Inhibitors of CRISPR-Cas Systems

Clustered regularly interspaced short palindromic repeats (CRISPR) together with their accompanying cas (CRISPR-associated) genes are found frequently in bacteria and archaea, serving to defend against invading foreign DNA, such as viral genomes. CRISPR-Cas systems provide a uniquely powerful defense because they can adapt to newly encountered genomes. The adaptive ability of these systems has been exploited, leading to their development as highly effective tools for genome editing. The widespread use of CRISPR-Cas systems has driven a need for methods to control their activity. This review focuses on anti-CRISPRs (Acrs), proteins produced by viruses and other mobile genetic elements that can potently inhibit CRISPR-Cas systems. Discovered in 2013, there are now 54 distinct families of these proteins described, and the functional mechanisms of more than a dozen have been characterized in molecular detail. The investigation of Acrs is leading to a variety of practical applications and is providing exciting new insight into the biology of CRISPR-Cas systems.

Structures and Strategies of Anti-CRISPR-Mediated Immune Suppression

More than 50 protein families have been identified that inhibit CRISPR (clustered regularly interspaced short palindromic repeats)-Cas-mediated adaptive immune systems. Here, we analyze the available anti-CRISPR (Acr) structures and describe common themes and unique mechanisms of stoichiometric and enzymatic suppressors of CRISPR-Cas. Stoichiometric inhibitors often function as molecular decoys of protein-binding partners or nucleic acid targets, while enzymatic suppressors covalently modify Cas ribonucleoprotein complexes or degrade immune signaling molecules. We review mechanistic insights that have been revealed by structures of Acrs, discuss some of the trade-offs associated with each of these strategies, and highlight how Acrs are regulated and deployed in the race to overcome adaptive immunity.

Efficient Genome Editing in Bacillus licheniformis Mediated by a Conditional CRISPR/Cas9 System

Bacillus licheniformis is widely used to produce multiple enzymes and chemicals in industrial fermentation. It is also an organism that is hard to genetically manipulate, which is mainly attributed to its extremely low transformation efficiency. The lack of genetic modification technology severely limits its further application. In this study, an all-in-one conditional clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 plasmid was developed for B. licheniformis with the cas9 gene under the control of a xylose-inducible promoter. By means of this design, the expression of the cas9 gene could be repressed without xylose, which significantly improved the transformation ratio from less than 0.1 cfu/μg to 2.42 cfu/μg DNA. Compared with this conditional system, a constitutive overexpression system led to significant growth retardation in bacterial cells. Both the biomass and specific growth rate decreased greatly. After transformation, successful genome editing could be triggered by 0.5% xylose. When the α-amylase gene amyL was used as a genomic target, the efficiencies of its disruption using three different protospacer-adjacent motif (PAM) sequences were 64.3%, 70.9%, and 47.1%, respectively. Moreover, temperature plays a pivotal role in the function of the constructed CRISPR system. The maximum success rate reached 97% at 20 °C, while higher temperatures negatively impacted the function of the system. These results suggested that the design with a cas9 gene under the strict control of a xylose-inducible promoter significantly improved the success rate of genome editing in this host. This work contributes to the development of genetic manipulation and furthers the use of B. licheniformis as an efficient industrial workhorse.

Broad-spectrum anti-CRISPR proteins facilitate horizontal gene transfer

CRISPR-Cas adaptive immune systems protect bacteria and archaea against their invading genetic parasites, including bacteriophages/viruses and plasmids. In response to this immunity, many phages have anti-CRISPR (Acr) proteins that inhibit CRISPR-Cas targeting. To date, anti-CRISPR genes have primarily been discovered in phage or prophage genomes. Here, we uncovered acr loci on plasmids and other conjugative elements present in Firmicutes using the Listeria acrIIA1 gene as a marker. The four identified genes, found in Listeria, Enterococcus, Streptococcus and Staphylococcus genomes, can inhibit type II-A SpyCas9 or SauCas9, and are thus named acrIIA16-19. In Enterococcus faecalis, conjugation of a Cas9-targeted plasmid was enhanced by anti-CRISPRs derived from Enterococcus conjugative elements, highlighting a role for Acrs in the dissemination of plasmids. Reciprocal co-immunoprecipitation showed that each Acr protein interacts with Cas9, and Cas9-Acr complexes were unable to cleave DNA. Northern blotting suggests that these anti-CRISPRs manipulate single guide RNA length, loading or stability. Mirroring their activity in bacteria, AcrIIA16 and AcrIIA17 provide robust and highly potent broad-spectrum inhibition of distinct Cas9 proteins in human cells (for example, SpyCas9, SauCas9, SthCas9, NmeCas9 and CjeCas9). This work presents a focused analysis of non-phage Acr proteins, demonstrating a role in horizontal gene transfer bolstered by broad-spectrum CRISPR-Cas9 inhibition.

Allosteric inhibition of CRISPR-Cas9 by bacteriophage-derived peptides

BACKGROUND

CRISPR-Cas9 has been developed as a therapeutic agent for various infectious and genetic diseases. In many clinically relevant applications, constitutively active CRISPR-Cas9 is delivered into human cells without a temporal control system. Excessive and prolonged expression of CRISPR-Cas9 can lead to elevated off-target cleavage. The need for modulating CRISPR-Cas9 activity over time and dose has created the demand of developing CRISPR-Cas off switches. Protein and small molecule-based CRISPR-Cas inhibitors have been reported in previous studies.

RESULTS

We report the discovery of Cas9-inhibiting peptides from inoviridae bacteriophages. These peptides, derived from the periplasmic domain of phage major coat protein G8P (G8PPD), can inhibit the in vitro activity of Streptococcus pyogenes Cas9 (SpCas9) proteins in an allosteric manner. Importantly, the inhibitory activity of G8PPD on SpCas9 is dependent on the order of guide RNA addition. Ectopic expression of full-length G8P (G8PFL) or G8PPD in human cells can inactivate the genome-editing activity of SpyCas9 with minimum alterations of the mutation patterns. Furthermore, unlike the anti-CRISPR protein AcrII4A that completely abolishes the cellular activity of CRISPR-Cas9, G8P co-transfection can reduce the off-target activity of co-transfected SpCas9 while retaining its on-target activity.

CONCLUSION

G8Ps discovered in the current study represent the first anti-CRISPR peptides that can allosterically inactivate CRISPR-Cas9. This finding may provide insights into developing next-generation CRISPR-Cas inhibitors for precision genome engineering.

Anti-CRISPR proteins targeting the CRISPR-Cas system enrich the toolkit for genetic engineering

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas adaptive immune defense systems, which are widely distributed in bacteria and Archaea, can provide sequence-specific protection against foreign DNA or RNA in some cases. However, the evolution of defense systems in bacterial hosts did not lead to the elimination of phages, and some phages carry anti-CRISPR genes that encode products that bind to the components mediating the defense mechanism and thus antagonize CRISPR-Cas immune systems of bacteria. Given the extensive application of CRISPR-Cas9 technologies in gene editing, in this review, we focus on the anti-CRISPR proteins (Acrs) that inhibit CRISPR-Cas systems for gene editing. We describe the discovery of Acrs in immune systems involving type I, II, and V CRISPR-Cas immunity, discuss the potential function of Acrs in inactivating type II and V CRISPR-Cas systems for gene editing and gene modulation, and provide an outlook on the development of important biotechnology tools for genetic engineering using Acrs.

Sharpening the Molecular Scissors: Advances in Gene-Editing Technology

The ability to precisely modify human genes has been made possible by the development of tools such as meganucleases, zinc finger nucleases, TALENs, and CRISPR/Cas. These now make it possible to generate targeted deletions, insertions, gene knock outs, and point variants; to modulate gene expression by targeting transcription factors or epigenetic machineries to DNA; or to target and modify RNA. Endogenous repair mechanisms are used to make the modifications required in DNA; they include non-homologous end joining, homology-directed repair, homology-independent targeted integration, microhomology-mediated end joining, base-excision repair, and mismatch repair. Off-target effects can be monitored using in silico prediction and sequencing and minimized using Cas proteins with higher accuracy, such as high-fidelity Cas9, enhanced-specificity Cas9, and hyperaccurate Cas9. Alternatives to Cas9 have been identified, including Cpf1, Cas12a, Cas12b, and smaller Cas9 orthologs such as CjCas9. Delivery of gene-editing components is performed ex vivo using standard techniques or in vivo using AAV, lipid nanoparticles, or cell-penetrating peptides. Clinical development of gene-editing technology is progressing in several fields, including immunotherapy in cancer treatment, antiviral therapy for HIV infection, and treatment of genetic disorders such as β-thalassemia, sickle cell disease, lysosomal storage disorders, and retinal dystrophy. Here we review these technological advances and the challenges to their clinical implementation.

Cas9 Allosteric Inhibition by the Anti-CRISPR Protein AcrIIA6

In the arms race against bacteria, bacteriophages have evolved diverse anti-CRISPR proteins (Acrs) that block CRISPR-Cas immunity. Acrs play key roles in the molecular coevolution of bacteria with their predators, use a variety of mechanisms of action, and provide tools to regulate Cas-based genome manipulation. Here, we present structural and functional analyses of AcrIIA6, an Acr from virulent phages, exploring its unique anti-CRISPR action. Our cryo-EM structures and functional data of AcrIIA6 binding to Streptococcus thermophilus Cas9 (St1Cas9) show that AcrIIA6 acts as an allosteric inhibitor and induces St1Cas9 dimerization. AcrIIA6 reduces St1Cas9 binding affinity for DNA and prevents DNA binding within cells. The PAM and AcrIIA6 recognition sites are structurally close and allosterically linked. Mechanistically, AcrIIA6 affects the St1Cas9 conformational dynamics associated with PAM binding. Finally, we identify a natural St1Cas9 variant resistant to AcrIIA6 illustrating Acr-driven mutational escape and molecular diversification of Cas9 proteins.

Structural insight into multistage inhibition of CRISPR-Cas12a by AcrVA4

Prokaryotes possess CRISPR-Cas systems to exclude parasitic predators, such as phages and mobile genetic elements (MGEs). These predators, in turn, encode anti-CRISPR (Acr) proteins to evade the CRISPR-Cas immunity. Recently, AcrVA4, an Acr protein inhibiting the CRISPR-Cas12a system, was shown to diminish Lachnospiraceae bacterium Cas12a (LbCas12a)-mediated genome editing in human cells, but the underlying mechanisms remain elusive. Here we report the cryo-EM structures of AcrVA4 bound to CRISPR RNA (crRNA)-loaded LbCas12a and found AcrVA4 could inhibit LbCas12a at several stages of the CRISPR-Cas working pathway, different from other characterized type I/II Acr inhibitors which target only 1 stage. First, it locks the conformation of the LbCas12a-crRNA complex to prevent target DNA-crRNA hybridization. Second, it interacts with the LbCas12a-crRNA-dsDNA complex to release the bound DNA before cleavage. Third, AcrVA4 binds the postcleavage LbCas12a complex to possibly block enzyme recycling. These findings highlight the multifunctionality of AcrVA4 and provide clues for developing regulatory genome-editing tools.

CRISPR-Cas, DNA Supercoiling, and Nucleoid-Associated Proteins

In this opinion article we highlight links between the H-NS nucleoid-associated protein, variable DNA topology, the regulation of CRISPR-cas locus expression, CRISPR-Cas activity, and the recruitment of novel genetic information by the CRISPR array. We propose that the requirement that the invading mobile genetic element be negatively supercoiled limits effective CRISPR action to a window in the bacterial growth cycle when DNA topology is optimal, and that this same window is used for the efficient integration of new spacer sequences at the CRISPR array. H-NS silences CRISPR promoters, and we propose that antagonists of H-NS, such as the LeuO transcription factor, provide a basis for a stochastic genetic switch that acts at random in each cell in the bacterial population. In addition, we wish to propose a mechanism by which mobile genetic elements can suppress CRISPR-cas transcription using H-NS homologues. Although the individual components of this network are known, we propose a new model in which they are integrated and linked to the physiological state of the bacterium. The model provides a basis for cell-to-cell variation in the expression and performance of CRISPR systems in bacterial populations.

Functional metagenomics-guided discovery of potent Cas9 inhibitors in the human microbiome

CRISPR-Cas systems protect bacteria and archaea from phages and other mobile genetic elements, which use small anti-CRISPR (Acr) proteins to overcome CRISPR-Cas immunity. Because Acrs are challenging to identify, their natural diversity and impact on microbial ecosystems are underappreciated. To overcome this discovery bottleneck, we developed a high-throughput functional selection to isolate ten DNA fragments from human oral and fecal metagenomes that inhibit Streptococcus pyogenes Cas9 (SpyCas9) in Escherichia coli. The most potent Acr from this set, AcrIIA11, was recovered from a Lachnospiraceae phage. We found that AcrIIA11 inhibits SpyCas9 in bacteria and in human cells. AcrIIA11 homologs are distributed across diverse bacteria; many distantly-related homologs inhibit both SpyCas9 and a divergent Cas9 from Treponema denticola. We find that AcrIIA11 antagonizes SpyCas9 using a different mechanism than other previously characterized Type II-A Acrs. Our study highlights the power of functional selection to uncover widespread Cas9 inhibitors within diverse microbiomes.