TnpB structure reveals minimal functional core of Cas12 nuclease family

Generation of Anti-Mastitis Gene-Edited Dairy Goats with Enhancing Lysozyme Expression by Inflammatory Regulatory Sequence using ISDra2-TnpB System

Gene-editing technology has become a transformative tool for the precise manipulation of biological genomes and holds great significance in the field of animal disease-resistant breeding. Mastitis, a prevalent disease in animal husbandry, imposes a substantial economic burden on the global dairy industry. In this study, a regulatory sequence gene editing breeding strategy for the successful creation of a gene-edited dairy (GED) goats with enhanced mastitis resistance using the ISDra2-TnpB system and dairy goats as the model animal is proposed. This included the targeted integration of an innate inflammatory regulatory sequence (IRS) into the promoter region of the lysozyme (LYZ) gene. Upon Escherichia Coli (E. coli) mammary gland infection, GED goats exhibited increased LYZ expression, showing robust anti-mastitis capabilities, mitigating PANoptosis activation, and alleviating blood-milk-barrier (BMB) damage. Notably, LYZ is highly expressed only in E. coli infection. This study marks the advent of anti-mastitis gene-edited animals with exogenous-free gene expression and demonstrates the feasibility of the gene-editing strategy proposed in this study. In addition, it provides a novel gene-editing blueprint for developing disease-resistant strains, focusing on disease specificity and biosafety while providing a research basis for the widespread application of the ISDra2-TnpB system.

RNA-guided genome engineering: paradigm shift towards transposons

CRISPR-Cas systems revolutionized the genome engineering field but need to induce double-strand breaks (DSBs) and may be difficult to deliver due to their large protein size. Tn7-like transposons such as CRISPR-associated transposons (CASTs) can be repurposed for RNA-guided DSB-free integration, and obligate mobile element guided activity (OMEGA) proteins of the IS200/IS605 transposon family have been developed as hypercompact RNA-guided genome editing tools. CASTs and OMEGA are exciting, innovative genome engineering tools that can improve the precision and efficiency of editing. This review explores the recent developments and uses of CASTs and OMEGA in genome editing across prokaryotic and eukaryotic cells. The pros and cons of these transposon-based systems are deliberated in comparison to other CRISPR systems.

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.

CRISPR beyond: harnessing compact RNA-guided endonucleases for enhanced genome editing

The CRISPR-Cas system, an adaptive immunity system in prokaryotes designed to combat phages and foreign nucleic acids, has evolved into a groundbreaking technology enabling gene knockout, large-scale gene insertion, base editing, and nucleic acid detection. Despite its transformative impact, the conventional CRISPR-Cas effectors face a significant hurdle-their size poses challenges in effective delivery into organisms and cells. Recognizing this limitation, the imperative arises for the development of compact and miniature gene editors to propel advancements in gene-editing-related therapies. Two strategies were accepted to develop compact genome editors: harnessing OMEGA (Obligate Mobile Element-guided Activity) systems, or engineering the existing CRISPR-Cas system. In this review, we focus on the advances in miniature genome editors based on both of these strategies. The objective is to unveil unprecedented opportunities in genome editing by embracing smaller, yet highly efficient genome editors, promising a future characterized by enhanced precision and adaptability in the genetic interventions.

Antagonistic conflict between transposon-encoded introns and guide RNAs

TnpB nucleases represent the evolutionary precursors to CRISPR-Cas12 and are widespread in all domains of life. IS605-family TnpB homologs function as programmable RNA-guided homing endonucleases in bacteria, driving transposon maintenance through DNA double-strand break-stimulated homologous recombination. In this work, we uncovered molecular mechanisms of the transposition life cycle of IS607-family elements that, notably, also encode group I introns. We identified specific features for a candidate "IStron" from Clostridium botulinum that allow the element to carefully control the relative levels of spliced products versus functional guide RNAs. Our results suggest that IStron transcripts evolved an ability to balance competing and mutually exclusive activities that promote selfish transposon spread while limiting adverse fitness costs on the host. Collectively, this work highlights molecular innovation in the multifunctional utility of transposon-encoded noncoding RNAs.

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.

Zero-shot prediction of mutation effects with multimodal deep representation learning guides protein engineering

Mutations in amino acid sequences can provoke changes in protein function. Accurate and unsupervised prediction of mutation effects is critical in biotechnology and biomedicine, but remains a fundamental challenge. To resolve this challenge, here we present Protein Mutational Effect Predictor (ProMEP), a general and multiple sequence alignment-free method that enables zero-shot prediction of mutation effects. A multimodal deep representation learning model embedded in ProMEP was developed to comprehensively learn both sequence and structure contexts from ~160 million proteins. ProMEP achieves state-of-the-art performance in mutational effect prediction and accomplishes a tremendous improvement in speed, enabling efficient and intelligent protein engineering. Specifically, ProMEP accurately forecasts mutational consequences on the gene-editing enzymes TnpB and TadA, and successfully guides the development of high-performance gene-editing tools with their engineered variants. The gene-editing efficiency of a 5-site mutant of TnpB reaches up to 74.04% (vs 24.66% for the wild type); and the base editing tool developed on the basis of a TadA 15-site mutant (in addition to the A106V/D108N double mutation that renders deoxyadenosine deaminase activity to TadA) exhibits an A-to-G conversion frequency of up to 77.27% (vs 69.80% for ABE8e, a previous TadA-based adenine base editor) with significantly reduced bystander and off-target effects compared to ABE8e. ProMEP not only showcases superior performance in predicting mutational effects on proteins but also demonstrates a great capability to guide protein engineering. Therefore, ProMEP enables efficient exploration of the gigantic protein space and facilitates practical design of proteins, thereby advancing studies in biomedicine and synthetic biology.

Methanotrophic Methanoperedens archaea host diverse and interacting extrachromosomal elements

Methane emissions are mitigated by anaerobic methane-oxidizing archaea, including Methanoperedens. Some Methanoperedens host huge extrachromosomal genetic elements (ECEs) called Borgs that may modulate their activity, yet the broader diversity of Methanoperedens ECEs is understudied. Here we report small enigmatic linear ECEs, circular viruses and unclassified ECEs that are predicted to replicate within Methanoperedens. Linear ECEs have inverted terminal repeats, tandem repeats and coding patterns that are strongly reminiscent of Borgs, but they are only 52-145 kb in length. As they share proteins with Borgs and Methanoperedens, we refer to them as mini-Borgs. Mini-Borgs are genetically diverse and can be assigned to at least five family-level groups. We identify eight families of Methanoperedens viruses, some of which encode multi-haem cytochromes, and circular ECEs encoding transposon-associated TnpB genes with proximal population-heterogeneous CRISPR arrays. These ECEs exchange genetic information with each other and with Methanoperedens, probably impacting their archaeal host activity and evolution.

Rapid DNA unwinding accelerates genome editing by engineered CRISPR-Cas9

Thermostable clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas9) enzymes could improve genome-editing efficiency and delivery due to extended protein lifetimes. However, initial experimentation demonstrated Geobacillus stearothermophilus Cas9 (GeoCas9) to be virtually inactive when used in cultured human cells. Laboratory-evolved variants of GeoCas9 overcome this natural limitation by acquiring mutations in the wedge (WED) domain that produce >100-fold-higher genome-editing levels. Cryoelectron microscopy (cryo-EM) structures of the wild-type and improved GeoCas9 (iGeoCas9) enzymes reveal extended contacts between the WED domain of iGeoCas9 and DNA substrates. Biochemical analysis shows that iGeoCas9 accelerates DNA unwinding to capture substrates under the magnesium-restricted conditions typical of mammalian but not bacterial cells. These findings enabled rational engineering of other Cas9 orthologs to enhance genome-editing levels, pointing to a general strategy for editing enzyme improvement. Together, these results uncover a new role for the Cas9 WED domain in DNA unwinding and demonstrate how accelerated target unwinding dramatically improves Cas9-induced genome-editing activity.

Conformational landscape of the type V-K CRISPR-associated transposon integration assembly

CRISPR-associated transposons (CASTs) are mobile genetic elements that co-opt CRISPR-Cas systems for RNA-guided DNA transposition. CASTs integrate large DNA cargos into the attachment (att) site independently of homology-directed repair and thus hold promise for eukaryotic genome engineering. However, the functional diversity and complexity of CASTs hinder an understanding of their mechanisms. Here, we present the high-resolution cryoelectron microscopy (cryo-EM) structure of the reconstituted ∼1 MDa post-transposition complex of the type V-K CAST, together with different assembly intermediates and diverse TnsC filament lengths, thus enabling the recapitulation of the integration complex formation. The results of mutagenesis experiments probing the roles of specific residues and TnsB-binding sites show that transposition activity can be enhanced and suggest that the distance between the PAM and att sites is determined by the lengths of the TnsB C terminus and the TnsC filament. This singular model of RNA-guided transposition provides a foundation for repurposing the system for genome-editing applications.

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.

Evolutionary mining and functional characterization of TnpB nucleases identify efficient miniature genome editors

As the evolutionary ancestor of Cas12 nuclease, the transposon (IS200/IS605)-encoded TnpB proteins act as compact RNA-guided DNA endonucleases. To explore their evolutionary diversity and potential as genome editors, we screened TnpBs from 64 annotated IS605 members and identified 25 active in Escherichia coli, of which three are active in human cells. Further characterization of these 25 TnpBs enables prediction of the transposon-associated motif (TAM) and the right-end element RNA (reRNA) directly from genomic sequences. We established a framework for annotating TnpB systems in prokaryotic genomes and applied it to identify 14 additional candidates. Among these, ISAam1 (369 amino acids (aa)) and ISYmu1 (382 aa) TnpBs demonstrated robust editing activity across dozens of genomic loci in human cells. Both RNA-guided genome editors demonstrated similar editing efficiency as SaCas9 (1,053 aa) while being substantially smaller. The enormous diversity of TnpBs holds potential for the discovery of additional valuable genome editors.

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.

Innate programmable DNA binding by CRISPR-Cas12m effectors enable efficient base editing

Cas9 and Cas12 nucleases of class 2 CRISPR-Cas systems provide immunity in prokaryotes through RNA-guided cleavage of foreign DNA. Here we characterize a set of compact CRISPR-Cas12m (subtype V-M) effector proteins and show that they provide protection against bacteriophages and plasmids through the targeted DNA binding rather than DNA cleavage. Biochemical assays suggest that Cas12m effectors can act as roadblocks inhibiting DNA transcription and/or replication, thereby triggering interference against invaders. Cryo-EM structure of Gordonia otitidis (Go) Cas12m ternary complex provided here reveals the structural mechanism of DNA binding ensuring interference. Harnessing GoCas12m innate ability to bind DNA target we fused it with adenine deaminase TadA-8e and showed an efficient A-to-G editing in Escherichia coli and human cells. Overall, this study expands our understanding of the functionally diverse Cas12 protein family, revealing DNA-binding dependent interference mechanism of Cas12m effectors that could be harnessed for engineering of compact base-editing tools.

CoCoNuTs: A diverse subclass of Type IV restriction systems predicted to target RNA

A comprehensive census of McrBC systems, among the most common forms of prokaryotic Type IV restriction systems, followed by phylogenetic analysis, reveals their enormous abundance in diverse prokaryotes and a plethora of genomic associations. We focus on a previously uncharacterized branch, which we denote CoCoNuTs (coiled-coil nuclease tandems) for their salient features: the presence of extensive coiled-coil structures and tandem nucleases. The CoCoNuTs alone show extraordinary variety, with 3 distinct types and multiple subtypes. All CoCoNuTs contain domains predicted to interact with translation system components, such as OB-folds resembling the SmpB protein that binds bacterial transfer-messenger RNA (tmRNA), YTH-like domains that might recognize methylated tmRNA, tRNA, or rRNA, and RNA-binding Hsp70 chaperone homologs, along with RNases, such as HEPN domains, all suggesting that the CoCoNuTs target RNA. Many CoCoNuTs might additionally target DNA, via McrC nuclease homologs. Additional restriction systems, such as Type I RM, BREX, and Druantia Type III, are frequently encoded in the same predicted superoperons. In many of these superoperons, CoCoNuTs are likely regulated by cyclic nucleotides, possibly, RNA fragments with cyclic termini, that bind associated CARF (CRISPR-Associated Rossmann Fold) domains. We hypothesize that the CoCoNuTs, together with the ancillary restriction factors, employ an echeloned defense strategy analogous to that of Type III CRISPR-Cas systems, in which an immune response eliminating virus DNA and/or RNA is launched first, but then, if it fails, an abortive infection response leading to PCD/dormancy via host RNA cleavage takes over.

Targeted mutagenesis in mice via an engineered AsCas12f1 system

SpCas9 and AsCas12a are widely utilized as genome editing tools in human cells, but their applications are largely limited by their bulky size. Recently, AsCas12f1 protein, with a small size (422 amino acids), has been demonstrated to be capable of cleaving double-stranded DNA protospacer adjacent motif (PAM). However, low editing efficiency and large differences in activity against different genomic loci have been a limitation in its application. Here, we show that engineered AsCas12f1 sgRNA has significantly improved the editing efficiency in human cells and mouse embryos. Moreover, we successfully generated three stable mouse mutant disease models using the engineered CRISPR-AsCas12f1 system in this study. Collectively, our work uncovers the engineered AsCas12f1 system expands mini CRISPR toolbox, providing a remarkable promise for therapeutic applications.

Engineering a transposon-associated TnpB-ωRNA system for efficient gene editing and phenotypic correction of a tyrosinaemia mouse model

Transposon-associated ribonucleoprotein TnpB is known to be the ancestry endonuclease of diverse Cas12 effector proteins from type-V CRISPR system. Given its small size (408 aa), it is of interest to examine whether engineered TnpB could be used for efficient mammalian genome editing. Here, we showed that the gene editing activity of native TnpB from Deinococcus radiodurans (ISDra2 TnpB) in mouse embryos was already higher than previously identified small-sized Cas12f1. Further stepwise engineering of noncoding RNA (ωRNA or reRNA) component of TnpB significantly elevated the nuclease activity of TnpB. Notably, an optimized TnpB-ωRNA system could be efficiently delivered in vivo with single adeno-associated virus (AAV) and corrected the disease phenotype in a tyrosinaemia mouse model. Thus, the engineered miniature TnpB system represents a new addition to the current genome editing toolbox, with the unique feature of the smallest effector size that facilitate efficient AAV delivery for editing of cells and tissues.

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.

Rapid DNA unwinding accelerates genome editing by engineered CRISPR-Cas9

Thermostable CRISPR-Cas9 enzymes could improve genome editing efficiency and delivery due to extended protein lifetimes. However, initial experimentation demonstrated Geobacillus stearothermophilus Cas9 (GeoCas9) to be virtually inactive when used in cultured human cells. Laboratory-evolved variants of GeoCas9 overcome this natural limitation by acquiring mutations in the wedge (WED) domain that produce >100-fold higher genome editing levels. Cryo-EM structures of the wildtype and improved GeoCas9 (iGeoCas9) enzymes reveal extended contacts between the WED domain of iGeoCas9 and DNA substrates. Biochemical analysis shows that iGeoCas9 accelerates DNA unwinding to capture substrates under the magnesium-restricted conditions typical of mammalian but not bacterial cells. These findings enabled rational engineering of other Cas9 orthologs to enhance genome editing levels, pointing to a general strategy for editing enzyme improvement. Together, these results uncover a new role for the Cas9 WED domain in DNA unwinding and demonstrate how accelerated target unwinding dramatically improves Cas9-induced genome editing activity.

Eukaryotic RNA-guided endonucleases evolved from a unique clade of bacterial enzymes

RNA-guided endonucleases form the crux of diverse biological processes and technologies, including adaptive immunity, transposition, and genome editing. Some of these enzymes are components of insertion sequences (IS) in the IS200/IS605 and IS607 transposon families. Both IS families encode a TnpA transposase and a TnpB nuclease, an RNA-guided enzyme ancestral to CRISPR-Cas12s. In eukaryotes, TnpB homologs occur as two distinct types, Fanzor1s and Fanzor2s. We analyzed the evolutionary relationships between prokaryotic TnpBs and eukaryotic Fanzors, which revealed that both Fanzor1s and Fanzor2s stem from a single lineage of IS607 TnpBs with unusual active site arrangement. The widespread nature of Fanzors implies that the properties of this particular lineage of IS607 TnpBs were particularly suited to adaptation in eukaryotes. Biochemical analysis of an IS607 TnpB and Fanzor1s revealed common strategies employed by TnpBs and Fanzors to co-evolve with their cognate transposases. Collectively, our results provide a new model of sequential evolution from IS607 TnpBs to Fanzor2s, and Fanzor2s to Fanzor1s that details how genes of prokaryotic origin evolve to give rise to new protein families in eukaryotes.

New faces of prokaryotic mobile genetic elements: guide RNAs link transposition with host defense mechanisms

Most life forms harbor multiple, diverse mobile genetic elements (MGE) that widely differ in their rates and mechanisms of mobility. Recent findings on two classes of MGE in prokaryotes revealed a novel mechanism, RNA-guided transposition, where a transposon-encoded guide RNA directs the transposase to a unique site in the host genome. Tn7-like transposons, on multiple occasions, recruited CRISPR systems that lost the capacity to cleave target DNA and instead mediate RNA-guided transposition via CRISPR RNA. Conversely, the abundant transposon-associated, RNA-guided nucleases IscB and TnpB that appear to promote proliferation of IS200/IS605 and IS607 transposons were the likely evolutionary ancestors of type II and type V CRISPR systems, respectively. Thus, RNA-guided target recognition is a major biological phenomenon that connects MGE with host defense mechanisms. More RNA-guided defensive and MGE-associated functionalities are likely to be discovered.

Antagonistic conflict between transposon-encoded introns and guide RNAs

TnpB nucleases represent the evolutionary precursors to CRISPR-Cas12 and are widespread in all domains of life, presumably due to the critical roles they play in transposon proliferation. IS605family TnpB homologs function in bacteria as programmable homing endonucleases by exploiting transposon-encoded guide RNAs to cleave vacant genomic sites, thereby driving transposon maintenance through DSB-stimulated homologous recombination. Whether this pathway is conserved in other genetic contexts, and in association with other transposases, is unknown. Here we uncover molecular mechanisms of transposition and RNA-guided DNA cleavage by IS607-family elements that, remarkably, also encode catalytic, self-splicing group I introns. After reconstituting and systematically investigating each of these biochemical activities for a candidate 'IStron' derived from Clostridium botulinum, we discovered sequence and structural features of the transposon-encoded RNA that satisfy molecular requirements of a group I intron and TnpB guide RNA, while still retaining the ability to be faithfully mobilized at the DNA level by the TnpA transposase. Strikingly, intron splicing was strongly repressed not only by TnpB, but also by the secondary structure of ωRNA alone, allowing the element to carefully control the relative levels of spliced products versus functional guide RNAs. Our results suggest that IStron transcripts have evolved a sensitive equilibrium to balance competing and mutually exclusive activities that promote transposon maintenance while limiting adverse fitness costs on the host. Collectively, this work explains how diverse enzymatic activities emerged during the selfish spread of IS607-family elements and highlights molecular innovation in the multi-functional utility of transposon-encoded noncoding RNAs.

Fanzor: a compact programmable RNA-guided endonuclease from eukaryotes

The IS200/605 transposons in prokaryotes are known to harbor programmable endonucleases. Despite carrying their own transposable elements, no such effector has been characterized in eukaryotes. Saito et al. recently reported compact and programmable RNA-guided eukaryotic endonucleases, called Fanzors, that can induce targeted genetic modifications, thus expanding the genome-editing toolbox.

Fanzor is a eukaryotic programmable RNA-guided endonuclease

RNA-guided systems, which use complementarity between a guide RNA and target nucleic acid sequences for recognition of genetic elements, have a central role in biological processes in both prokaryotes and eukaryotes. For example, the prokaryotic CRISPR-Cas systems provide adaptive immunity for bacteria and archaea against foreign genetic elements. Cas effectors such as Cas9 and Cas12 perform guide-RNA-dependent DNA cleavage1. Although a few eukaryotic RNA-guided systems have been studied, including RNA interference2 and ribosomal RNA modification3, it remains unclear whether eukaryotes have RNA-guided endonucleases. Recently, a new class of prokaryotic RNA-guided systems (termed OMEGA) was reported4,5. The OMEGA effector TnpB is the putative ancestor of Cas12 and has RNA-guided endonuclease activity4,6. TnpB may also be the ancestor of the eukaryotic transposon-encoded Fanzor (Fz) proteins4,7, raising the possibility that eukaryotes are also equipped with CRISPR-Cas or OMEGA-like programmable RNA-guided endonucleases. Here we report the biochemical characterization of Fz, showing that it is an RNA-guided DNA endonuclease. We also show that Fz can be reprogrammed for human genome engineering applications. Finally, we resolve the structure of Spizellomyces punctatus Fz at 2.7 Å using cryogenic electron microscopy, showing the conservation of core regions among Fz, TnpB and Cas12, despite diverse cognate RNA structures. Our results show that Fz is a eukaryotic OMEGA system, demonstrating that RNA-guided endonucleases are present in all three domains of life.

Progress and Prospects of Gene Editing in Pluripotent Stem Cells

Applying programmable nucleases in gene editing has greatly shaped current research in basic biology and clinical translation. Gene editing in human pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), is highly relevant to clinical cell therapy and thus should be examined with particular caution. First, since all mutations in PSCs will be carried to all their progenies, off-target edits of editors will be amplified. Second, due to the hypersensitivity of PSCs to DNA damage, double-strand breaks (DSBs) made by gene editing could lead to low editing efficiency and the enrichment of cell populations with defective genomic safeguards. In this regard, DSB-independent gene editing tools, such as base editors and prime editors, are favored due to their nature to avoid these consequences. With more understanding of the microbial world, new systems, such as Cas-related nucleases, transposons, and recombinases, are also expanding the toolbox for gene editing. In this review, we discuss current applications of programmable nucleases in PSCs for gene editing, the efforts researchers have made to optimize these systems, as well as new tools that can be potentially employed for differentiation modeling and therapeutic applications.

Mechanistic and evolutionary insights into a type V-M CRISPR-Cas effector enzyme

RNA-guided type V CRISPR-Cas12 effectors provide adaptive immunity against mobile genetic elements (MGEs) in bacteria and archaea. Among diverse Cas12 enzymes, the recently identified Cas12m2 (CRISPR-Cas type V-M) is highly compact and has a unique RuvC active site. Although the non-canonical RuvC triad does not permit dsDNA cleavage, Cas12m2 still protects against invading MGEs through transcriptional silencing by strong DNA binding. However, the molecular mechanism of RNA-guided genome inactivation by Cas12m2 remains unknown. Here we report cryo-electron microscopy structures of two states of Cas12m2-CRISPR RNA (crRNA)-target DNA ternary complexes and the Cas12m2-crRNA binary complex, revealing structural dynamics during crRNA-target DNA heteroduplex formation. The structures indicate that the non-target DNA strand is tightly bound to a unique arginine-rich cluster in the recognition (REC) domains and the non-canonical active site in the RuvC domain, ensuring strong DNA-binding affinity of Cas12m2. Furthermore, a structural comparison of Cas12m2 with TnpB, a putative ancestor of Cas12 enzymes, suggests that the interaction of the characteristic coiled-coil REC2 insertion with the protospacer-adjacent motif-distal region of the heteroduplex is crucial for Cas12m2 to engage in adaptive immunity. Collectively, our findings improve mechanistic understanding of diverse type V CRISPR-Cas effectors and provide insights into the evolution of TnpB to Cas12 enzymes.

Cas12n nucleases, early evolutionary intermediates of type V CRISPR, comprise a distinct family of miniature genome editors

Type V CRISPR-associated systems (Cas)12 family nucleases are considered to have evolved from transposon-associated TnpB, and several of these nucleases have been engineered as versatile genome editors. Despite the conserved RNA-guided DNA-cleaving functionality, these Cas12 nucleases differ markedly from the currently identified ancestor TnpB in aspects such as guide RNA origination, effector complex composition, and protospacer adjacent motif (PAM) specificity, suggesting the presence of earlier evolutionary intermediates that could be mined to develop advanced genome manipulation biotechnologies. Using evolutionary and biochemical analyses, we identify that the miniature type V-U4 nuclease (referred to as Cas12n, 400-700 amino acids) is likely the earliest evolutionary intermediate between TnpB and large type V CRISPR systems. We demonstrate that with the exception of CRISPR array emergence, CRISPR-Cas12n shares several similar characteristics with TnpB-ωRNA, including a miniature and likely monomeric nuclease for DNA targeting, origination of guide RNA from nuclease coding sequence, and generation of a small sticky end following DNA cleavage. Cas12n nucleases recognize a unique 5'-AAN PAM sequence, of which the A nucleotide at the -2 position is also required for TnpB. Moreover, we demonstrate the robust genome-editing capacity of Cas12n in bacteria and engineer a highly efficient CRISPR-Cas12n (termed Cas12Pro) with up to 80% indel efficiency in human cells. The engineered Cas12Pro enables base editing in human cells. Our results further expand the understanding regarding type V CRISPR evolutionary mechanisms and enrich the miniature CRISPR toolbox for therapeutic applications.

The Transposon-Encoded Protein TnpB Processes Its Own mRNA into ωRNA for Guided Nuclease Activity

TnpB is a member of the Obligate Mobile Element Guided Activity (OMEGA) RNA-guided nuclease family, is harbored in transposons, and likely functions to maintain the transposon in genomes. Previously, it was shown that TnpB cleaves double- and single-stranded DNA substrates in an RNA-guided manner, but the biogenesis of the TnpB ribonucleoprotein (RNP) complex is unknown. Using in vitro purified apo TnpB, we demonstrate the ability of TnpB to generate guide omegaRNA (ωRNA) from its own mRNA through 5' processing. We also uncover a potential cis-regulatory mechanism whereby a region of the TnpB mRNA inhibits DNA cleavage by the RNP complex. We further expand the characterization of TnpB by examining ωRNA processing and RNA-guided nuclease activity in 59 orthologs spanning the natural diversity of the TnpB family. This work reveals a new functionality, ωRNA biogenesis, of TnpB, and characterizes additional members of this biotechnologically useful family of programmable enzymes.