Structural Basis for Guide RNA Processing and Seed-Dependent DNA Targeting by CRISPR-Cas12a

A rapid on-site visualization platform based on RPA coupled with CRISPR-Cas12a for the detection of genetically modified papaya 'Huanong No.1'

The Papaya ringspot virus (PRSV)-resistant genetically modified (GM) papaya 'Huanong No.1' has been certified as safe for consumption and widely planted in China for about 18 years. To protect consumers' rights and facilitate government supervision and monitoring, it is necessary to establish a simple, rapid, and specific detection method for 'Huanong No.1'. Herein, we developed a platform based on recombinase polymerase amplification (RPA) coupled with CRISPR-Cas12a for the detection of 'Huanong No.1'. The RPA-CRISPR-Cas12a platform was found to have high specificity, with amplification signals only present in 'Huanong No.1'. Additionally, the platform was highly sensitive, with a limit of detection (LOD) of approximately 20 copies. The detection process was fast and could be completed in less than 1 h. This novel platform enables the rapid on-site visualization detection of 'Huanong No.1', eliminating dependence on laboratory conditions and specialized instruments, and can serve as a technical reference for the rapid detection of other GM plants.

EcCas6e-based antisense crRNA for gene repression and RNA editing in microorganisms

Precise gene regulation and programmable RNA editing are vital RNA-level regulatory mechanisms. Gene repression tools grounded in small non-coding RNAs, microRNAs, and CRISPR-dCas proteins, along with RNA editing tools anchored in Adenosine Deaminases acting on RNA (ADARs), have found extensive application in molecular biology and cellular engineering. Here, we introduced a novel approach wherein we developed an EcCas6e mediated crRNA-mRNA annealing system for gene repression in Escherichia coli and RNA editing in Saccharomyces cerevisiae. We found that EcCas6e possesses inherent RNA annealing ability attributed to a secondary positively charged cleft, enhancing crRNA-mRNA hybridization and stability. Based on this, we demonstrated that EcCas6e, along with its cognate crRNA repeat containing a complementary region to the ribosome binding site of a target mRNA, effectively represses gene expression up to 25-fold. Furthermore, we demonstrated that multiple crRNAs can be easily assembled and can simultaneously target up to 13 genes. Lastly, the EcCas6e-crRNA system was developed as an RNA editing tool by fusing it with the ADAR2 deaminase domain. The EcCas6e-crRNA mediated gene repression and RNA editing tools hold broad applications for research and biotechnology.

CRISPR-Cpf1 system and its applications in animal genome editing

The clustered regularly interspaced short palindromic repeats (CRISPR) and their associated protein (Cas) system is a gene editing technology guided by RNA endonuclease. The CRISPR-Cas12a (also known as CRISPR-Cpf1) system is extensively utilized in genome editing research due to its accuracy and high efficiency. In this paper, we primarily focus on the application of CRISPR-Cpf1 technology in the construction of disease models and gene therapy. Firstly, the structure and mechanism of the CRISPR-Cas system are introduced. Secondly, the similarities and differences between CRISPR-Cpf1 and CRISPR-Cas9 technologies are compared. Thirdly, the main focus is on the application of the CRISPR-Cpf1 system in cell and animal genome editing. Finally, the challenges faced by CRISPR-Cpf1 technology and corresponding strategies are analyzed. Although CRISPR-Cpf1 technology has certain off-target effects, it can effectively and accurately edit cell and animal genomes, and has significant advantages in the preclinical research.

A CRISPR/Cas12a-based fluorescence method for the amplified detection of total antioxidant capacity

The CRISPR/Cas12a system is a powerful signal amplification tool that has been widely used in nucleic acid detection. It has also been applied to the assay of non-nucleic acid targets, mainly relying on strategies for converting target determination into nucleic acid detection. Herein, we describe a CRISPR/Cas12a-based fluorescence method for sensitive detection of the total antioxidant capacity (TAC) by utilizing a strategy of converting TAC determination into Mn2+ detection. Specifically, the reduction of MnO2 nanosheets by antioxidants produces plenty of Mn2+, which accelerates the trans-cleavage activity of CRISPR/Cas12a. Thus, a fluorescence enhanced detection method for TAC was established, with a detection limit as low as 0.04 mg L-1 for a typical antioxidant, ascorbic acid. More importantly, this method has been proven to successfully analyze TAC in beverages. The excellent analytical performance of this method demonstrates the great potential of the CRISPR/Cas12a system in simple and sensitive TAC analysis.

Cas12a domain flexibility guides R-loop formation and forces RuvC resetting

The specific nature of CRISPR-Cas12a makes it a desirable RNA-guided endonuclease for biotechnology and therapeutic applications. To understand how R-loop formation within the compact Cas12a enables target recognition and nuclease activation, we used cryo-electron microscopy to capture wild-type Acidaminococcus sp. Cas12a R-loop intermediates and DNA delivery into the RuvC active site. Stages of Cas12a R-loop formation-starting from a 5-bp seed-are marked by distinct REC domain arrangements. Dramatic domain flexibility limits contacts until nearly complete R-loop formation, when the non-target strand is pulled across the RuvC nuclease and coordinated domain docking promotes efficient cleavage. Next, substantial domain movements enable target strand repositioning into the RuvC active site. Between cleavage events, the RuvC lid conformationally resets to occlude the active site, requiring re-activation. These snapshots build a structural model depicting Cas12a DNA targeting that rationalizes observed specificity and highlights mechanistic comparisons to other class 2 effectors.

A CRISPR/Cas12a-powered gold/nickel foam surface-enhanced Raman spectroscopy biosensor for nucleic acid specific detection in foods

Food is a necessary source of energy, but it also serves as a pathway for transmitting infectious pathogens, making food safety a matter of great concern. Rapid, accurate, and specific detection methods for foodborne viruses are crucial. Surface-Enhanced Raman Scattering (SERS), due to its superior sensitivity and characteristic fingerprint spectra, holds enormous potential. However, due to the limitations of SERS, it requires specific conditions to achieve specificity. In order to enhance the specificity and accuracy of nucleic acid detection based on SERS, we have developed a CRISPR-Cas12a-mediated SERS technique to identify target DNA, harnessing the targeting recognition capability of CRISPR-Cas12a and ultra-sensitive SERS tags and successfully addressing SERS' lack of specific detection capability. This system includes a gold/nickel foam substrate (Au-NFs) and a reporter (ssDNA-ROX). The phenomenon of colloidal gold/silver nano-aggregation due to magnesium ions, which is commonly encountered in CRISPR-SERS, was simultaneously solved using AuNFs. The qualitative and quantitative analysis of target DNA in drinking water was performed by monitoring the intensity change of ROX Raman reporter molecules. The results showed that the sensor detected DNA within 30 min and the limit of detection (LOD) was 8.23 fM. This is expected to become one of the alternative methods for nucleic acid detection for its rapid detection and high specificity.

CRISPR-Cas12a exhibits metal-dependent specificity switching

Cas12a is the immune effector of type V-A CRISPR-Cas systems and has been co-opted for genome editing and other biotechnology tools. The specificity of Cas12a has been the subject of extensive investigation both in vitro and in genome editing experiments. However, in vitro studies have often been performed at high magnesium ion concentrations that are inconsistent with the free Mg2+ concentrations that would be present in cells. By profiling the specificity of Cas12a orthologs at a range of Mg2+ concentrations, we find that Cas12a switches its specificity depending on metal ion concentration. Lowering Mg2+ concentration decreases cleavage defects caused by seed mismatches, while increasing the defects caused by PAM-distal mismatches. We show that Cas12a can bind seed mutant targets more rapidly at low Mg2+ concentrations, resulting in faster cleavage. In contrast, PAM-distal mismatches cause substantial defects in cleavage following formation of the Cas12a-target complex at low Mg2+ concentrations. We observe differences in Cas12a specificity switching between three orthologs that results in variations in the routes of phage escape from Cas12a-mediated immunity. Overall, our results reveal the importance of physiological metal ion conditions on the specificity of Cas effectors that are used in different cellular environments.

HLTF disrupts Cas9-DNA post-cleavage complexes to allow DNA break processing

The outcome of CRISPR-Cas-mediated genome modifications is dependent on DNA double-strand break (DSB) processing and repair pathway choice. Homology-directed repair (HDR) of protein-blocked DSBs requires DNA end resection that is initiated by the endonuclease activity of the MRE11 complex. Using reconstituted reactions, we show that Cas9 breaks are unexpectedly not directly resectable by the MRE11 complex. In contrast, breaks catalyzed by Cas12a are readily processed. Cas9, unlike Cas12a, bridges the broken ends, preventing DSB detection and processing by MRE11. We demonstrate that Cas9 must be dislocated after DNA cleavage to allow DNA end resection and repair. Using single molecule and bulk biochemical assays, we next find that the HLTF translocase directly removes Cas9 from broken ends, which allows DSB processing by DNA end resection or non-homologous end-joining machineries. Mechanistically, the activity of HLTF requires its HIRAN domain and the release of the 3'-end generated by the cleavage of the non-target DNA strand by the Cas9 RuvC domain. Consequently, HLTF removes the H840A but not the D10A Cas9 nickase. The removal of Cas9 H840A by HLTF explains the different cellular impact of the two Cas9 nickase variants in human cells, with potential implications for gene editing.

Gene therapy and gene editing strategies in inherited blood disorders

Gene therapy has shown significant potential in treating various diseases, particularly inherited blood disorders such as hemophilia, sickle cell disease, and thalassemia. Advances in understanding the regulatory network of disease-associated genes have led to the identification of additional therapeutic targets for treatment, especially for β-hemoglobinopathies. Erythroid regulatory factor BCL11A offers the most promising therapeutic target for β-hemoglobinopathies and reduction of its expression using the commercialized gene therapy product Casgevy was approved for use in the UK and USA in 2023. Notably, the emergence of innovative gene editing technologies has further broadened the gene therapy landscape, presenting new possibilities for treatment. Intensive studies indicate that base editing and prime editing, built upon CRISPR technology, enable precise single-base modification in hematopoietic stem cells for addressing inherited blood disorders ex vivo and in vivo. In this review, we present an overview of the current landscape of gene therapies, focusing on clinical research and gene therapy products for inherited blood disorders, evaluation of potential gene targets, and the gene editing tools employed in current gene therapy practices, which provides an insight for the establishment of safer and more effective gene therapy methods for a wider range of diseases in the future.

TnpB homologues exapted from transposons are RNA-guided transcription factors

Transposon-encoded tnpB and iscB genes encode RNA-guided DNA nucleases that promote their own selfish spread through targeted DNA cleavage and homologous recombination1-4. These widespread gene families were repeatedly domesticated over evolutionary timescales, leading to the emergence of diverse CRISPR-associated nucleases including Cas9 and Cas12 (refs. 5,6). We set out to test the hypothesis that TnpB nucleases may have also been repurposed for novel, unexpected functions other than CRISPR-Cas adaptive immunity. Here, using phylogenetics, structural predictions, comparative genomics and functional assays, we uncover multiple independent genesis events of programmable transcription factors, which we name TnpB-like nuclease-dead repressors (TldRs). These proteins use naturally occurring guide RNAs to specifically target conserved promoter regions of the genome, leading to potent gene repression in a mechanism akin to CRISPR interference technologies invented by humans7. Focusing on a TldR clade found broadly in Enterobacteriaceae, we discover that bacteriophages exploit the combined action of TldR and an adjacently encoded phage gene to alter the expression and composition of the host flagellar assembly, a transformation with the potential to impact motility8, phage susceptibility9, and host immunity10. Collectively, this work showcases the diverse molecular innovations that were enabled through repeated exaptation of transposon-encoded genes, and reveals the evolutionary trajectory of diverse RNA-guided transcription factors.

A One-Step RPA-CRISPR Assay Using crRNA Based on Suboptimal Protospacer Adjacent Motif for Vibrio vulnificus Detection

Vibrio vulnificus is a hazardous foodborne pathogen responsible for approximately 95% of seafood-related deaths. This highlights the urgent requirement for specialized detection tools to be developed and used by food enterprises and food safety authorities. The DETECTR (DNA endonuclease targeted CRISPR trans reporter) system that combines CRISPR/Cas and recombinase polymerase amplification (RPA) has been utilized to develop a molecular detection assay for V. vulnificus. However, because the incompatibility between RPA and Cas12a cleavage has not been addressed, it is a two-step assay that lacks convenience and presents contamination risk. Here, we developed a one-step RPA-CRISPR assay for V. vulnificus using a special crRNA targeting a sequence with a suboptimal protospacer adjacent motif (PAM). The entire assay, conducted at 37°C, takes only 40-60 min, yields results visualized under blue light, and exhibits exceptional specificity and sensitivity (detecting 4 pathogen genome copies per reaction). This study offers a valuable tool for detecting V. vulnificus, aiding in foodborne infection prevention, and exemplifies one-step RPA-CRISPR assays managing Cas-cleavage activity through PAM adjustments.

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.

CRISPR/Cas Technology: The Unique Synthetic Biology Genome-Editing Tool Shifting the Paradigm in Viral Diagnostics, Defense, and Therapeutics

The emergence of the COVID-19 pandemic has starkly exposed our significantly limited ability to promptly identify and respond to emergent biological threats. Consequently, there is an urgent need to advance biotechnological methods for addressing both known and unforeseen biological hazards. Recently, the CRISPR/Cas system has revolutionized genetic engineering, enabling precise and efficient synthetic biology applications. Therefore, this review aims to provide a comprehensive introduction to the fundamental principles underlying the CRISPR/Cas system and assess the advantages and limitations of various CRISPR/Cas-based techniques applicable to the detection of, defense against, and treatment of viral infections. These techniques include viral diagnostics, the development of antiviral vaccines, B cell engineering for antibody production, viral activation/interference, and epigenetic modifications. Furthermore, this review delves into the challenges and bioethical considerations associated with use of the CRISPR/Cas system. With the continuous evolution of technology, the CRISPR/Cas system holds considerable promise for addressing both existing and unforeseen biological threats.

Hybrid Cas12a Variants with Relaxed PAM Requirements Expand Genome Editing Compatibility

Cas12a is a widely used programmable nuclease for genome editing across a variety of organisms, but its application is limited by its PAM recognition restriction. To alleviate these PAM constraints, protein engineering efforts have been applied to expand the PAM recognition range. In this study, we designed and constructed 990 synthetic hybrid Cas12a chimeras through domain shuffling and screened an efficient hybrid Cas12a (ehCas12a) that could recognize a broad range PAM of 5'-TYYN-3' (Y is T or C and N is A, T, C, or G). Furthermore, we constructed an ehCas12a variant, ehCas12a RRVR (T167R/N572R/K578V/N582R), with expanded PAM preference to 5'-TNYN, TWRV-3' (W is A or T, R is A or G, and V is A, C, or G), which can efficiently recognize -2* A/G PAMs that are barely recognized by Cas12a-type proteins and their mutants. Finally, we demonstrated that the DNase-inactivated ehCas12a RRVR base editor (dehCas12a RRVR-BE) was capable of targeting noncanonical PAMs in vivo and disease-related loci for potential therapeutic applications. Overall, our findings highlight the modular design and reconfiguration of Cas proteins for enhanced functionality.

Circumvention of Topoisomerase IIα Intron 19 Intronic Polyadenylation in Acquired Etoposide-Resistant Human Leukemia K562 Cells

DNA topoisomerase IIα (TOP2α; 170 kDa, TOP2α/170) is an essential enzyme for proper chromosome dysjunction by producing transient DNA double-stranded breaks and is an important target for DNA damage-stabilizing anticancer agents, such as etoposide. Therapeutic effects of TOP2α poisons can be limited due to acquired drug resistance. We previously demonstrated decreased TOP2α/170 levels in an etoposide-resistant human leukemia K562 subline, designated K/VP.5, accompanied by increased expression of a C-terminal truncated TOP2α isoform (90 kDa; TOP2α/90), which heterodimerized with TOP2α/170 and was a determinant of resistance by exhibiting dominant-negative effects against etoposide activity. Based on 3'-rapid amplification of cDNA ends, we confirmed TOP2α/90 as the translation product of a TOP2α mRNA in which a cryptic polyadenylation site (PAS) harbored in intron 19 (I19) was used. In this report, we investigated whether the resultant intronic polyadenylation (IPA) would be attenuated by blocking or mutating the I19 PAS, thereby circumventing acquired drug resistance. An antisense morpholino oligonucleotide was used to hybridize/block the PAS in TOP2α pre-mRNA in K/VP.5 cells, resulting in decreased TOP2α/90 mRNA/protein levels in K/VP.5 cells and partially circumventing drug resistance. Subsequently, CRISPR/CRISPR-associated protein 9 with homology-directed repair was used to mutate the cryptic I19 PAS (AATAAA→ACCCAA) to prevent IPA. Gene-edited clones exhibited increased TOP2α/170 and decreased TOP2α/90 mRNA/protein and demonstrated restored sensitivity to etoposide and other TOP2α-targeted drugs. Together, results indicated that blocking/mutating a cryptic I19 PAS in K/VP.5 cells reduced IPA and restored sensitivity to TOP2α-targeting drugs. SIGNIFICANCE STATEMENT: The results presented in this study indicate that CRISPR/CRISPR-associated protein 9 gene editing of a cryptic polyadenylation site (PAS) within I19 of the TOP2α gene results in the reversal of acquired resistance to etoposide and other TOP2-targeted drugs. An antisense morpholino oligonucleotide targeting the PAS also partially circumvented resistance.

Progress and pitfalls of gene editing technology in CAR-T cell therapy: a state-of-the-art review

CAR-T cell therapy has shown remarkable promise in treating B-cell malignancies, which has sparked optimism about its potential to treat other types of cancer as well. Nevertheless, the Expectations of CAR-T cell therapy in solid tumors and non-B cell hematologic malignancies have not been met. Furthermore, safety concerns regarding the use of viral vectors and the current personalized production process are other bottlenecks that limit its widespread use. In recent years the use of gene editing technology in CAR-T cell therapy has opened a new way to unleash the latent potentials of CAR-T cell therapy and lessen its associated challenges. Moreover, gene editing tools have paved the way to manufacturing CAR-T cells in a fully non-viral approach as well as providing a universal, off-the-shelf product. Despite all the advantages of gene editing strategies, the off-target activity of classical gene editing tools (ZFNs, TALENs, and CRISPR/Cas9) remains a major concern. Accordingly, several efforts have been made in recent years to reduce their off-target activity and genotoxicity, leading to the introduction of advanced gene editing tools with an improved safety profile. In this review, we begin by examining advanced gene editing tools, providing an overview of how these technologies are currently being applied in clinical trials of CAR-T cell therapies. Following this, we explore various gene editing strategies aimed at enhancing the safety and efficacy of CAR-T cell therapy.

Unity among the diverse RNA-guided CRISPR-Cas interference mechanisms

CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated) systems are adaptive immune systems that protect bacteria and archaea from invading mobile genetic elements (MGEs). The Cas protein-CRISPR RNA (crRNA) complex uses complementarity of the crRNA "guide" region to specifically recognize the invader genome. CRISPR effectors that perform targeted destruction of the foreign genome have emerged independently as multi-subunit protein complexes (Class 1 systems) and as single multi-domain proteins (Class 2). These different CRISPR-Cas systems can cleave RNA, DNA, and protein in an RNA-guided manner to eliminate the invader, and in some cases, they initiate programmed cell death/dormancy. The versatile mechanisms of the different CRISPR-Cas systems to target and destroy nucleic acids have been adapted to develop various programmable-RNA-guided tools and have revolutionized the development of fast, accurate, and accessible genomic applications. In this review, we present the structure and interference mechanisms of different CRISPR-Cas systems and an analysis of their unified features. The three types of Class 1 systems (I, III, and IV) have a conserved right-handed helical filamentous structure that provides a backbone for sequence-specific targeting while using unique proteins with distinct mechanisms to destroy the invader. Similarly, all three Class 2 types (II, V, and VI) have a bilobed architecture that binds the RNA-DNA/RNA hybrid and uses different nuclease domains to cleave invading MGEs. Additionally, we highlight the mechanistic similarities of CRISPR-Cas enzymes with other RNA-cleaving enzymes and briefly present the evolutionary routes of the different CRISPR-Cas systems.

Structural mechanism of bridge RNA-guided recombination

Insertion sequence (IS) elements are the simplest autonomous transposable elements found in prokaryotic genomes1. We recently discovered that IS110 family elements encode a recombinase and a non-coding bridge RNA (bRNA) that confers modular specificity for target DNA and donor DNA through two programmable loops2. Here we report the cryo-electron microscopy structures of the IS110 recombinase in complex with its bRNA, target DNA and donor DNA in three different stages of the recombination reaction cycle. The IS110 synaptic complex comprises two recombinase dimers, one of which houses the target-binding loop of the bRNA and binds to target DNA, whereas the other coordinates the bRNA donor-binding loop and donor DNA. We uncovered the formation of a composite RuvC-Tnp active site that spans the two dimers, positioning the catalytic serine residues adjacent to the recombination sites in both target and donor DNA. A comparison of the three structures revealed that (1) the top strands of target and donor DNA are cleaved at the composite active sites to form covalent 5'-phosphoserine intermediates, (2) the cleaved DNA strands are exchanged and religated to create a Holliday junction intermediate, and (3) this intermediate is subsequently resolved by cleavage of the bottom strands. Overall, this study reveals the mechanism by which a bispecific RNA confers target and donor DNA specificity to IS110 recombinases for programmable DNA recombination.

A colorimetric tandem combination of CRISPR/Cas12a with dual functional hybridization chain reaction for ultra-sensitive detection of Mycobacterium bovis

Tuberculosis caused by Mycobacterium bovis poses a global infectious threat to humans and animals. Therefore, there is an urgent need to develop a sensitive, precise, and easy-to-readout strategy. Here, a novel tandem combination of a CRISPR/Cas12a system with dual HCR (denoted as CRISPR/Cas12a-D-HCR) was constructed for detecting Mycobacterium bovis. Based on the efficient trans-cleavage activity of the active CRISPR/Cas12a system, tandem-dsDNA with PAM sites was established using two flexible hairpins, providing multiple binding sites with CRISPR/Cas12a for further amplification. Furthermore, the activation of Cas12a initiated the second hybridization chain reaction (HCR), which integrated complete G-quadruplex sequences to assemble the hemin/G-quadruplex DNAzyme. With the addition of H2O2 and ABTS, a colorimetric signal readout strategy was achieved. Consequently, CRISPR/Cas12a-D-HCR achieved a satisfactory detection linear range from 20 aM to 50 fM, and the limit of detection was as low as 2.75 aM with single mismatched recognition capability, demonstrating good discrimination of different bacterial species. Notably, the practical application performance was verified via the standard addition method, with the recovery ranging from 96.0% to 105.2% and the relative standard deviations (RSD) ranging from 0.95% to 6.45%. The proposed CRISPR/Cas12a-D-HCR sensing system served as a promising application for accurate detection in food safety and agricultural fields.

Live-cell imaging reveals the trade-off between target search flexibility and efficiency for Cas9 and Cas12a

CRISPR-Cas systems have widely been adopted as genome editing tools, with two frequently employed Cas nucleases being SpyCas9 and LbCas12a. Although both nucleases use RNA guides to find and cleave target DNA sites, the two enzymes differ in terms of protospacer-adjacent motif (PAM) requirements, guide architecture and cleavage mechanism. In the last years, rational engineering led to the creation of PAM-relaxed variants SpRYCas9 and impLbCas12a to broaden the targetable DNA space. By employing their catalytically inactive variants (dCas9/dCas12a), we quantified how the protein-specific characteristics impact the target search process. To allow quantification, we fused these nucleases to the photoactivatable fluorescent protein PAmCherry2.1 and performed single-particle tracking in cells of Escherichia coli. From our tracking analysis, we derived kinetic parameters for each nuclease with a non-targeting RNA guide, strongly suggesting that interrogation of DNA by LbdCas12a variants proceeds faster than that of SpydCas9. In the presence of a targeting RNA guide, both simulations and imaging of cells confirmed that LbdCas12a variants are faster and more efficient in finding a specific target site. Our work demonstrates the trade-off of relaxing PAM requirements in SpydCas9 and LbdCas12a using a powerful framework, which can be applied to other nucleases to quantify their DNA target search.

Deciphering the Substrate Specificity Reveals that CRISPR-Cas12a Is a Bifunctional Enzyme with Both Endo- and Exonuclease Activities

The class 2 CRISPR-Cas9 and CRISPR-Cas12a systems, originally described as adaptive immune systems of bacteria and archaea, have emerged as versatile tools for genome-editing, with applications in biotechnology and medicine. However, significantly less is known about their substrate specificity, but such knowledge may provide instructive insights into their off-target cleavage and previously unrecognized mechanism of action. Here, we document that the Acidaminococcus sp. Cas12a (AsCas12a) binds preferentially, and independently of crRNA, to a suite of branched DNA structures, such as the Holliday junction (HJ), replication fork and D-loops, compared with single- or double-stranded DNA, and promotes their degradation. Further, our study revealed that AsCas12a binds to the HJ, specifically at the crossover region, protects it from DNase I cleavage and renders a pair of thymine residues in the HJ homologous core hypersensitive to KMnO4 oxidation, suggesting DNA melting and/or distortion. Notably, these structural changes enabled AsCas12a to resolve HJ into nonligatable intermediates, and subsequently their complete degradation. We further demonstrate that crRNA impedes HJ cleavage by AsCas12a, and that of Lachnospiraceae bacterium Cas12a, without affecting their DNA-binding ability. We identified a separation-of-function variant, which uncouples DNA-binding and DNA cleavage activities of AsCas12a. Importantly, we found robust evidence that AsCas12a endonuclease also has 3'-to-5' and 5'-to-3' exonuclease activity, and that these two activities synergistically promote degradation of DNA, yielding di- and mononucleotides. Collectively, this study significantly advances knowledge about the substrate specificity of AsCas12a and provides important insights into the degradation of different types of DNA substrates.

Intrinsic RNA Targeting Triggers Indiscriminate DNase Activity of CRISPR-Cas12a

The CRISPR-Cas12a system has emerged as a powerful tool for next-generation nucleic acid-based molecular diagnostics. However, it has long been believed to be effective only on DNA targets. Here, we investigate the intrinsic RNA-enabled trans-cleavage activity of AsCas12a and LbCas12a and discover that they can be directly activated by full-size RNA targets, although LbCas12a exhibits weaker trans-cleavage activity than AsCas12a on both single-stranded DNA and RNA substrates. Remarkably, we find that the RNA-activated Cas12a possesses higher specificity in recognizing mutated target sequences compared to DNA activation. Based on these findings, we develop the "Universal Nuclease for Identification of Virus Empowered by RNA-Sensing" (UNIVERSE) assay for nucleic acid testing. We incorporate a T7 transcription step into this assay, thereby eliminating the requirement for a protospacer adjacent motif (PAM) sequence in the target. Additionally, we successfully detect multiple PAM-less targets in HIV clinical samples that are undetectable by the conventional Cas12a assay based on double-stranded DNA activation, demonstrating unrestricted target selection with the UNIVERSE assay. We further validate the clinical utility of the UNIVERSE assay by testing both HIV RNA and HPV 16 DNA in clinical samples. We envision that the intrinsic RNA targeting capability may bring a paradigm shift in Cas12a-based nucleic acid detection and further enhance the understanding of CRISPR-Cas biochemistry.

Determinants of CRISPR Cas12a nuclease activation by DNA and RNA targets

The RNA-guided CRISPR-associated (Cas) enzyme Cas12a cleaves specific double-stranded (ds-) or single-stranded (ss-) DNA targets (in cis), unleashing non-specific ssDNA cleavage (in trans). Though this trans-activity is widely coopted for diagnostics, little is known about target determinants promoting optimal enzyme performance. Using quantitative kinetics, we show formation of activated nuclease proceeds via two steps whereby rapid binding of Cas12a ribonucleoprotein to target is followed by a slower allosteric transition. Activation does not require a canonical protospacer-adjacent motif (PAM), nor is utilization of such PAMs predictive of high trans-activity. We identify several target determinants that can profoundly impact activation times, including bases within the PAM (for ds- but not ssDNA targets) and sequences within and outside those complementary to the spacer, DNA topology, target length, presence of non-specific DNA, and ribose backbone itself, uncovering previously uncharacterized cleavage of and activation by RNA targets. The results provide insight into the mechanism of Cas12a activation, with direct implications on the role of Cas12a in bacterial immunity and for Cas-based diagnostics.

A high-fidelity DNAzyme-assisted CRISPR/Cas13a system with single-nucleotide resolved specificity

A CRISPR/Cas system represents an innovative tool for developing a new-generation biosensing and diagnostic strategy. However, the off-target issue (i.e., mistaken cleavage of nucleic acid targets and reporters) remains a great challenge for its practical applications. We hypothesize that this issue can be overcome by taking advantage of the site-specific cleavage ability of RNA-cleaving DNAzymes. To test this idea, we propose a DNAzyme Operation Enhances the Specificity of CRISPR/Cas13a strategy (termed DOES-CRISPR) to overcome the problem of relatively poor specificity that is typical of the traditional CRISPR/Cas13a system. The key to the design is that the partial hybridization of the CRISPR RNA (crRNA) with the cleavage fragment of off-target RNA was not able to activate the collateral cleavage activity of Cas13a. We showed that DOES-CRISPR can significantly improve the specificity of traditional CRISPR/Cas13a-based molecular detection by up to ∼43-fold. The broad utility of the strategy is illustrated through engineering three different systems for the detection of microRNAs (miR-17 and let-7e), CYP2C19*17 gene, SARS-Cov-2 variants (Gamma, Delta, and Omicron) and Omicron subtypes (BQ.1 and XBB.1) with single-nucleotide resolved specificity. Finally, clinical evaluation of this assay using 10 patient blood samples demonstrated a clinical sensitivity of 100% and specificity of 100% for genotyping CYP2C19*17, and analyzing 20 throat swab samples provided a diagnostic sensitivity of 95% and specificity of 100% for Omicron detection, and a clinical sensitivity of 92% and specificity of 100% for XBB.1 detection.

Discovery and structural mechanism of DNA endonucleases guided by RAGATH-18-derived RNAs

CRISPR-Cas systems and IS200/IS605 transposon-associated TnpBs have been utilized for the development of genome editing technologies. Using bioinformatics analysis and biochemical experiments, here we present a new family of RNA-guided DNA endonucleases. Our bioinformatics analysis initially identifies the stable co-occurrence of conserved RAGATH-18-derived RNAs (reRNAs) and their upstream IS607 TnpBs with an average length of 390 amino acids. IS607 TnpBs form programmable DNases through interaction with reRNAs. We discover the robust dsDNA interference activity of IS607 TnpB systems in bacteria and human cells. Further characterization of the Firmicutes bacteria IS607 TnpB system (ISFba1 TnpB) reveals that its dsDNA cleavage activity is remarkably sensitive to single mismatches between the guide and target sequences in human cells. Our findings demonstrate that a length of 20 nt in the guide sequence of reRNA achieves the highest DNA cleavage activity for ISFba1 TnpB. A cryo-EM structure of the ISFba1 TnpB effector protein bound by its cognate RAGATH-18 motif-containing reRNA and a dsDNA target reveals the mechanisms underlying reRNA recognition by ISFba1 TnpB, reRNA-guided dsDNA targeting, and the sensitivity of the ISFba1 TnpB system to base mismatches between the guide and target DNA. Collectively, this study identifies the IS607 TnpB family of compact and specific RNA-guided DNases with great potential for application in gene editing.

Flexible TAM requirement of TnpB enables efficient single-nucleotide editing with expanded targeting scope

TnpBs encoded by the IS200/IS605 family transposon are among the most abundant prokaryotic proteins from which type V CRISPR-Cas nucleases may have evolved. Since bacterial TnpBs can be programmed for RNA-guided dsDNA cleavage in the presence of a transposon-adjacent motif (TAM), these nucleases hold immense promise for genome editing. However, the activity and targeting specificity of TnpB in homology-directed gene editing remain unknown. Here we report that a thermophilic archaeal TnpB enables efficient gene editing in the natural host. Interestingly, the TnpB has different TAM requirements for eliciting cell death and for facilitating gene editing. By systematically characterizing TAM variants, we reveal that the TnpB recognizes a broad range of TAM sequences for gene editing including those that do not elicit apparent cell death. Importantly, TnpB shows a very high targeting specificity on targets flanked by a weak TAM. Taking advantage of this feature, we successfully leverage TnpB for efficient single-nucleotide editing with templated repair. The use of different weak TAM sequences not only facilitates more flexible gene editing with increased cell survival, but also greatly expands targeting scopes, and this strategy is probably applicable to diverse CRISPR-Cas systems.

An autocatalytic CRISPR-Cas amplification effect propelled by the LNA-modified split activators for DNA sensing

CRISPR-Cas systems with dual functions offer precise sequence-based recognition and efficient catalytic cleavage of nucleic acids, making them highly promising in biosensing and diagnostic technologies. However, current methods encounter challenges of complexity, low turnover efficiency, and the necessity for sophisticated probe design. To better integrate the dual functions of Cas proteins, we proposed a novel approach called CRISPR-Cas Autocatalysis Amplification driven by LNA-modified Split Activators (CALSA) for the highly efficient detection of single-stranded DNA (ssDNA) and genomic DNA. By introducing split ssDNA activators and the site-directed trans-cleavage mediated by LNA modifications, an autocatalysis-driven positive feedback loop of nucleic acids based on the LbCas12a system was constructed. Consequently, CALSA enabled one-pot and real-time detection of genomic DNA and cell-free DNA (cfDNA) from different tumor cell lines. Notably, CALSA achieved high sensitivity, single-base specificity, and remarkably short reaction times. Due to the high programmability of nucleic acid circuits, these results highlighted the immense potential of CALSA as a powerful tool for cascade signal amplification. Moreover, the sensitivity and specificity further emphasized the value of CALSA in biosensing and diagnostics, opening avenues for future clinical applications.

An Orthogonal CRISPR/dCas12a System for RNA Imaging in Live Cells

CRISPR/Cas technology has made great progress in the field of live-cell imaging beyond genome editing. However, effective and easy-to-use CRISPR systems for labeling multiple RNAs of interest are still needed. Here, we engineered a CRISPR/dCas12a system that enables the specific recognition of the target RNA under the guidance of a PAM-presenting oligonucleotide (PAMmer) to mimic the PAM recognition mechanism for DNA substrates. We demonstrated the feasibility and specificity of this system for specifically visualizing endogenous mRNA. By leveraging dCas12a-mediated precursor CRISPR RNA (pre-crRNA) processing and the orthogonality of dCas12a from different bacteria, we further demonstrated the proposed system as a simple and versatile molecular toolkit for multiplexed imaging of different types of RNA transcripts in live cells with high specificity. This programmable dCas12a system not only broadens the RNA imaging toolbox but also facilitates diverse applications for RNA manipulation.

MicroRNA Sensors Based on CRISPR/Cas12a Technologies: Evolution From Indirect to Direct Detection

MicroRNA (miRNA) has emerged as a promising biomarker for disease diagnosis and a potential therapeutic targets for drug development. The detection of miRNA can serve as a noninvasive tool in diseases diagnosis and predicting diseases prognosis. CRISPR/Cas12a system has great potential in nucleic acid detection due to its high sensitivity and specificity, which has been developed to be a versatile tool for nucleic acid-based detection of targets in various fields. However, conversion from RNA to DNA with or without amplification operation is necessary for miRNA detection based on CRISPR/Cas12a system, because dsDNA containing PAM sequence or ssDNA is traditionally considered as the activator of Cas12a. Until recently, direct detection of miRNA by CRISPR/Cas12a system has been reported. In this review, we provide an overview of the evolution of biosensors based on CRISPR/Cas12a for miRNA detection from indirect to direct, which would be beneficial to the development of CRISPR/Cas12a-based sensors with better performance for direct detection of miRNA.

Gene point mutation information translation and detection: Leveraging single base extension and CRISPR/Cas12a

Gene point mutations play a significant role in the development of cancer. Therefore, developing a sensitive, specific, and universally applicable method for detecting gene point mutation is crucial for clinical diagnosis, prognosis, and cancer treatment. Recently, gene point mutation detection methods based on CRISPR/Cas12a detection have emerged. However, existing methods generally lack universality and specificity. In this study, we have developed a CRISPR/Cas12a-based method that combines improved allele-specific polymerase chain reaction and single base extension to translate the point mutation information in the target dsDNA into length information in ssDNA activators to overcome the limitations associated with PAM sequences in the CRISPR/Cas12a system. Our method achieved a detection limit of 0.002% for clinically significant EGFR T790M mutation. The CRISPR/Cas12a system we constructed demonstrates high sensitivity, specificity, and universality in detecting gene point mutations, making it a promising tool for clinical cancer screening.

Past, present, and future of CRISPR genome editing technologies

Genome editing has been a transformative force in the life sciences and human medicine, offering unprecedented opportunities to dissect complex biological processes and treat the underlying causes of many genetic diseases. CRISPR-based technologies, with their remarkable efficiency and easy programmability, stand at the forefront of this revolution. In this Review, we discuss the current state of CRISPR gene editing technologies in both research and therapy, highlighting limitations that constrain them and the technological innovations that have been developed in recent years to address them. Additionally, we examine and summarize the current landscape of gene editing applications in the context of human health and therapeutics. Finally, we outline potential future developments that could shape gene editing technologies and their applications in the coming years.

Harnessing noncanonical crRNAs to improve functionality of Cas12a orthologs

There is a broad diversity among Cas12a endonucleases that possess nucleic acid detection and gene-editing capabilities, but few are studied extensively. Here, we present an exhaustive investigation of 23 Cas12a orthologs, with a focus on their cis- and trans-cleavage activities in combination with noncanonical crRNAs. Through biochemical assays, we observe that some noncanonical crRNA:Cas12a effector complexes outperform their corresponding wild-type crRNA:Cas12a. Cas12a can recruit crRNA with modifications such as loop extensions and split scaffolds. Moreover, the tolerance of Cas12a to noncanonical crRNA is also observed in mammalian cells through the formation of indels. We apply the adaptability of Cas12a:crRNA complexes to detect SARS-CoV-2 in clinical nasopharyngeal swabs, saliva samples, and tracheal aspirates. Our findings further expand the toolbox for next-generation CRISPR-based diagnostics and gene editing.

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.

Controllable Assembly of a Quantum Dot-Based Aptasensor Guided by CRISPR/Cas12a for Direct Measurement of Circulating Tumor Cells in Human Blood

Accurate and sensitive analysis of circulating tumor cells (CTCs) in human blood provides a non-invasive approach for the evaluation of cancer metastasis and early cancer diagnosis. Herein, we demonstrate the controllable assembly of a quantum dot (QD)-based aptasensor guided by CRISPR/Cas12a for direct measurement of CTCs in human blood. We introduce a magnetic bead@activator/recognizer duplex core-shell structure to construct a multifunctional platform for the capture and direct detection of CTCs in human blood, without the need for additional CTC release and re-identification steps. Notably, the introduction of magnetic separation ensures that only a target-induced free activator can initiate the downstream catalysis, efficiently avoiding the undesired catalysis triggered by inappropriate recognition of the activator/recognizer duplex structure by crRNAs. This aptasensor achieves high CTC-capture efficiency (82.72%) and sensitive detection of CTCs with a limit of detection of 2 cells mL-1 in human blood, holding great promise for the liquid biopsy of cancers.

An alpha-helical lid guides the target DNA toward catalysis in CRISPR-Cas12a

CRISPR-Cas12a is a powerful RNA-guided genome-editing system that generates double-strand DNA breaks using its single RuvC nuclease domain by a sequential mechanism in which initial cleavage of the non-target strand is followed by target strand cleavage. How the spatially distant DNA target strand traverses toward the RuvC catalytic core is presently not understood. Here, continuous tens of microsecond-long molecular dynamics and free-energy simulations reveal that an α-helical lid, located within the RuvC domain, plays a pivotal role in the traversal of the DNA target strand by anchoring the crRNA:target strand duplex and guiding the target strand toward the RuvC core, as also corroborated by DNA cleavage experiments. In this mechanism, the REC2 domain pushes the crRNA:target strand duplex toward the core of the enzyme, while the Nuc domain aids the bending and accommodation of the target strand within the RuvC core by bending inward. Understanding of this critical process underlying Cas12a activity will enrich fundamental knowledge and facilitate further engineering strategies for genome editing.

Engineered CRISPRa System for Precise and Multiplex Gene Regulation Using a Hybrid dCas12a Variant and Hairpin-Spacer crRNAs

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas12a nucleases have emerged as a promising alternative to CRISPR-Cas9 in gene editing and expression regulation. However, the adoption of Cas12a has been hindered due to general off-target activities and limited efficiency. Here, we utilized a hybrid engineered Cas12a variant and hairpin-spacer crRNAs (h-CAP) to enhance the specificity and efficiency of the CRISPR-Cas12a system. Leveraging the h-CAP strategy, we demonstrate both single-base-specific and multiplex gene expression regulation in human cells.

Utilization of nicking properties of CRISPR-Cas12a effector for genome editing

The CRISPR-Cas nickase system for genome editing has attracted considerable attention owing to its safety, efficiency, and versatility. Although alternative effectors to Cas9 have the potential to expand the scope of genome editing, their application has not been optimized. Herein, we used an enhanced CRISPR-Cas12a nickase system to induce mutations by targeting genes in a human-derived cell line. The optimized CRISPR-Cas12a nickase system effectively introduced mutations into target genes under a specific directionality and distance between nickases. In particular, the single-mode Cas12a nickase system can induce the target-specific mutations with less DNA double-strand breaks. By inducing mutations in the Thymine-rich target genes in single- or dual-mode, Cas12a nickase compensates the limitations of Cas9 nickase and is expected to contribute to the development of future genome editing technologies.

Next-generation forward genetic screens: uniting high-throughput perturbations with single-cell analysis

Programmable genome-engineering technologies, such as CRISPR (clustered regularly interspaced short palindromic repeats) nucleases and massively parallel CRISPR screens that capitalize on this programmability, have transformed biomedical science. These screens connect genes and noncoding genome elements to disease-relevant phenotypes, but until recently have been limited to individual phenotypes such as growth or fluorescent reporters of gene expression. By pairing massively parallel screens with high-dimensional profiling of single-cell types/states, we can now measure how individual genetic perturbations or combinations of perturbations impact the cellular transcriptome, proteome, and epigenome. We review technologies that pair CRISPR screens with single-cell multiomics and the unique opportunities afforded by extending pooled screens using deep multimodal phenotyping.

A minimally invasive, field-applicable CRISPR/Cas biosensor to aid in the detection of Pseudogymnoascus destructans, the causative fungal agent of white-nose syndrome in bats

The accessibility to CRISPR/Cas (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein) genetic tools has given rise to applications beyond site-directed genome editing for the detection of DNA and RNA. These tools include precise diagnostic detection of human disease pathogens, such as SARS-CoV-2 and Zika virus. Despite the technology being rapid and cost-effective, the use of CRISPR/Cas tools in the surveillance of the causative agents of wildlife diseases has not been prominent. This study presents the development of a minimally invasive, field-applicable and user-friendly CRISPR/Cas-based biosensor for the detection of Pseudogymnoascus destructans (Pd), the causative fungal agent of white-nose syndrome (WNS), an infectious disease that has killed more than five million bats in North America since its discovery in 2006. The biosensor assay combines a recombinase polymerase amplification (RPA) step followed by CRISPR/Cas12a nuclease cleavage to detect Pd DNA from bat dermal swab and guano samples. The biosensor had similar detection results when compared to quantitative PCR in distinguishing Pd-positive versus negative field samples. Although bat dermal swabs could be analysed with the biosensor without nucleic acid extraction, DNA extraction was needed when screening guano samples to overcome inhibitors. This assay can be applied to help with more rapid delineation of Pd-positive sites in the field to inform management decisions. With further optimization, this technology has broad translation potential to wildlife disease-associated pathogen detection and monitoring applications.

An Update on the Application of CRISPR Technology in Clinical Practice

The CRISPR/Cas system, an innovative gene-editing tool, is emerging as a promising technique for genome modifications. This straightforward technique was created based on the prokaryotic adaptive immune defense mechanism and employed in the studies on human diseases that proved enormous therapeutic potential. A genetically unique patient mutation in the process of gene therapy can be corrected by the CRISPR method to treat diseases that traditional methods were unable to cure. However, introduction of CRISPR/Cas9 into the clinic will be challenging because we still need to improve the technology's effectiveness, precision, and applications. In this review, we first describe the function and applications of the CRISPR-Cas9 system. We next delineate how this technology could be utilized for gene therapy of various human disorders, including cancer and infectious diseases and highlight the promising examples in the field. Finally, we document current challenges and the potential solutions to overcome these obstacles for the effective use of CRISPR-Cas9 in clinical practice.

CRISPR-Cas12a exhibits metal-dependent specificity switching

Cas12a is the immune effector of type V-A CRISPR-Cas systems and has been co-opted for genome editing and other biotechnology tools. The specificity of Cas12a has been the subject of extensive investigation both in vitro and in genome editing experiments. However, in vitro studies have often been performed at high magnesium ion concentrations that are inconsistent with the free Mg2+ concentrations that would be present in cells. By profiling the specificity of Cas12a orthologs at a range of Mg2+ concentrations, we find that Cas12a switches its specificity depending on metal ion concentration. Lowering Mg2+ concentration decreases cleavage defects caused by seed mismatches, while increasing the defects caused by PAM-distal mismatches. We show that Cas12a can bind seed mutant targets more rapidly at low Mg2+ concentrations, resulting in faster cleavage. In contrast, PAM-distal mismatches cause substantial defects in cleavage following formation of the Cas12a-target complex at low Mg2+ concentrations. We observe differences in Cas12a specificity switching between three orthologs that results in variations in the routes of phage escape from Cas12a-mediated immunity. Overall, our results reveal the importance of physiological metal ion conditions on the specificity of Cas effectors that are used in different cellular environments.

A CRISPR-Cas12a-based platform facilitates the detection and serotyping of Streptococcus suis serotype 2

Streptococcus suis serotype 2 is an economically important zoonotic pathogen that causes septicemia, arthritis, and meningitis in pigs and humans. S. suis serotype 2 is responsible for substantial economic losses to the swine industry and poses a serious threat to public health, and accurate and rapid detection is important for the prevention and control of epidemic disease. In this study, we developed a high-fidelity detection and serotyping platform for S. suis serotype 2 based on recombinase polymerase amplification (RPA) and a clustered regularly interspaced short palindromic repeat (CRISPR)-Cas12a system called Cards-SSJ/K. Cards-SSJ had a detection limit of 10 CFU, takes <60 min, and no cross-reaction was found with other S. suis serotypes, closely related Streptococcus spp., or common pig pathogens, and Cards-SSK could differentiate serotype 2 from serotype 1/2. Results from Cards-SSJ and qPCR were equivalent in detecting S. suis serotype 2 in tissue samples. Analysis indicated that despite a relatively high reagent cost compared to PCR and qPCR, Cards-SSJ was less time-consuming and had low requirements for equipment and personnel. Thus, it is an excellent method for point-of-care detection for S. suis serotype 2.

Cas12a-based direct visualization of nanoparticle-stabilized fluorescence signal for multiplex detection of DNA methylation biomarkers

The CRISPR-Cas12a RNA-guided complexes hold immense promise for nucleic acid detection. However, limitations arise from their specificity in detecting off-targets and the stability of the signal molecules. Here, we have developed a platform that integrates multiplex amplification and nanomolecular-reporting signals, allowing us to detect various clinically relevant nucleic acid targets with enhanced stability, sensitivity, and visual interpretation. Through the electrostatic co-assembly of the Oligo reporter with oppositely charged nanoparticles, we observed a significant enhancement in its stability in low-pollution environments, reaching up to a threefold increase compared to the original version. Additionally, the fluorescence efficiency was expanded by three orders of magnitude, broadening the detection range considerably. Utilizing a multiplex strategy, this assay can accomplish simultaneous detection of multiple targets and single-point indication detection of nine specific targets. This significant advancement heightened the sensitivity of disease screening and improved the accuracy of diagnosing disease-related changes. We tested this assay in a colorectal cancer model, demonstrating that it can identify DNA methylation features at the aM-level within 40-60 min. Validation using clinical samples yielded consistent results with qPCR and bisulfite sequencing, affirming the assay's reliability and potential for clinical applications.

The Development of a CRISPR-FnCpf1 System for Large-Fragment Deletion and Multiplex Gene Editing in Acinetobacter baumannii

Acinetobacter baumannii is a low-GC-content Gram-negative opportunistic pathogen that poses a serious global public health threat. Convenient and rapid genetic manipulation is beneficial for elucidating its pathogenic mechanisms and developing novel therapeutic methods. In this study, we report a new CRISPR-FnCpf1-based two-plasmid system for versatile and precise genome editing in A. baumannii. After identification, this new system prefers to recognize the 5'-TTN-3' (N = A, T, C or G) and the 5'-CTV-3' (V = A, C or G) protospacer-adjacent motif (PAM) sequence and utilize the spacer with lengths ranging from 19 to 25 nt. In direct comparison with the existing CRISPR-Cas9 system, it exhibits approximately four times the targetable range in A. baumannii. Moreover, by employing a tandem dual crRNA expression cassette, the new system can perform large-fragment deletion and simultaneous multiple gene editing, which is difficult to achieve via CRISPR-Cas9. Therefore, the new system is valuable and can greatly expand the genome editing toolbox of A. baumannii.

Recent application of CRISPR-Cas12 and OMEGA system for genome editing

In 2012, it was discovered that precise gene editing could be induced in target DNA using the reprogrammable characteristics of the CRISPR system. Since then, several studies have investigated the potential of the CRISPR system to edit various biological organisms. For the typical CRISPR system obtained from bacteria and archaea, many application studies have been conducted and have spread to various fields. To date, orthologs with various characteristics other than CRISPR-Cas9 have been discovered and are being intensively studied in the field of gene editing. CRISPR-Cas12 and its varied orthologs are representative examples of genome editing tools and have superior properties in terms of in vivo target gene editing compared with Cas9. Recently, TnpB and Fanzor of the OMEGA (obligate mobile element guided activity) system were identified to be the ancestor of CRISPR-Cas12 on the basis of phylogenetic analysis. Notably, the compact sizes of Cas12 and OMEGA endonucleases allow adeno-associated virus (AAV) delivery; hence, they are set to challenge Cas9 for in vivo gene therapy. This review is focused on these RNA-guided reprogrammable endonucleases: their structure, biochemistry, off-target effects, and applications in therapeutic gene editing.

Recent advances in genome engineering by CRISPR technology

Due to the development of CRISPR technology, the era of effective editing of target genes has arrived. However, the offtarget problem that occurs when recognizing target DNA due to the inherent nature of CRISPR components remains the biggest task to be overcome in the future. In this review, the principle of inducing such unintended off-target editing is analyzed from the structural aspect of CRISPR, and the methodology that has been developed to reduce off-target editing until now is summarized. [BMB Reports 2024; 57(1): 12-18].

Discovery of Diverse CRISPR-Cas Systems and Expansion of the Genome Engineering Toolbox

CRISPR systems mediate adaptive immunity in bacteria and archaea through diverse effector mechanisms and have been repurposed for versatile applications in therapeutics and diagnostics thanks to their facile reprogramming with RNA guides. RNA-guided CRISPR-Cas targeting and interference are mediated by effectors that are either components of multisubunit complexes in class 1 systems or multidomain single-effector proteins in class 2. The compact class 2 CRISPR systems have been broadly adopted for multiple applications, especially genome editing, leading to a transformation of the molecular biology and biotechnology toolkit. The diversity of class 2 effector enzymes, initially limited to the Cas9 nuclease, was substantially expanded via computational genome and metagenome mining to include numerous variants of Cas12 and Cas13, providing substrates for the development of versatile, orthogonal molecular tools. Characterization of these diverse CRISPR effectors uncovered many new features, including distinct protospacer adjacent motifs (PAMs) that expand the targeting space, improved editing specificity, RNA rather than DNA targeting, smaller crRNAs, staggered and blunt end cuts, miniature enzymes, promiscuous RNA and DNA cleavage, etc. These unique properties enabled multiple applications, such as harnessing the promiscuous RNase activity of the type VI effector, Cas13, for supersensitive nucleic acid detection. class 1 CRISPR systems have been adopted for genome editing, as well, despite the challenge of expressing and delivering the multiprotein class 1 effectors. The rich diversity of CRISPR enzymes led to rapid maturation of the genome editing toolbox, with capabilities such as gene knockout, base editing, prime editing, gene insertion, DNA imaging, epigenetic modulation, transcriptional modulation, and RNA editing. Combined with rational design and engineering of the effector proteins and associated RNAs, the natural diversity of CRISPR and related bacterial RNA-guided systems provides a vast resource for expanding the repertoire of tools for molecular biology and biotechnology.

Rapid in situ RNA imaging based on Cas12a thrusting strand displacement reaction

RNA In situ imaging through DNA self-assembly is advantaged in illustrating its structures and functions with high-resolution, while the limited reaction efficiency and time-consuming operation hinder its clinical application. Here, we first proposed a new strand displacement reaction (SDR) model (Cas12a thrusting SDR, CtSDR), in which Cas12a could overcome the inherent reaction limitation and dramatically enhance efficiency through energy replenishment and by-product consumption. The target-initiated CtSDR amplification was established for RNA analysis, with order of magnitude lower limit of detection (LOD) than the Cas13a system. The CtSDR-based RNA in situ imaging strategy was developed to monitor intra-cellular microRNA expression change and delineate the landscape of oncogenic RNA in 66 clinic tissue samples, possessing a clear advantage over classic in situ hybridization (ISH) in terms of operation time (1 h versus 14 h) while showing comparable sensitivity and specificity. This work presents a promising approach to developing advanced molecular diagnostic tools.

Emergence of RNA-guided transcription factors via domestication of transposon-encoded TnpB nucleases

Transposon-encoded tnpB genes encode RNA-guided DNA nucleases that promote their own selfish spread through targeted DNA cleavage and homologous recombination1-4. This widespread gene family was repeatedly domesticated over evolutionary timescales, leading to the emergence of diverse CRISPR-associated nucleases including Cas9 and Cas125,6. We set out to test the hypothesis that TnpB nucleases may have also been repurposed for novel, unexpected functions other than CRISPR-Cas. Here, using phylogenetics, structural predictions, comparative genomics, and functional assays, we uncover multiple instances of programmable transcription factors that we name TnpB-like nuclease-dead repressors (TldR). These proteins employ naturally occurring guide RNAs to specifically target conserved promoter regions of the genome, leading to potent gene repression in a mechanism akin to CRISPRi technologies invented by humans7. Focusing on a TldR clade found broadly in Enterobacteriaceae, we discover that bacteriophages exploit the combined action of TldR and an adjacently encoded phage gene to alter the expression and composition of the host flagellar assembly, a transformation with the potential to impact motility8, phage susceptibility9, and host immunity10. Collectively, this work showcases the diverse molecular innovations that were enabled through repeated exaptation of genes encoded by transposable elements, and reveals that RNA-guided transcription factors emerged long before the development of dCas9-based editors.