Diversity and evolution of class 2 CRISPR-Cas systems

Genetically Engineering the Nervous System with CRISPR-Cas

The multitude of neuronal subtypes and extensive interconnectivity of the mammalian brain presents a substantial challenge to those seeking to decipher its functions. While the molecular mechanisms of several neuronal functions remain poorly characterized, advances in next-generation sequencing (NGS) and gene-editing technology have begun to close this gap. The clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein (CRISPR-Cas) system has emerged as a powerful genetic tool capable of manipulating the genome of essentially any organism and cell type. This technology has advanced our understanding of complex neurologic diseases by enabling the rapid generation of novel, disease-relevant in vitro and transgenic animal models. In this review, we discuss recent developments in the rapidly accelerating field of CRISPR-mediated genome engineering. We begin with an overview of the canonical function of the CRISPR platform, followed by a functional review of its many adaptations, with an emphasis on its applications for genetic interrogation of the normal and diseased nervous system. Additionally, we discuss limitations of the CRISPR editing system and suggest how future modifications to existing platforms may advance our understanding of the brain.

Protein-Mutation-Induced Conformational Changes of the DNA and Nuclease Domain in CRISPR/Cas9 Systems by Molecular Dynamics Simulations

Class 2 CRISPR (clustered regularly interspaced short palindromic repeats) systems offer a unique protocol for genome editing in eukaryotic cells. The nuclease activity of Cas9 has been harnessed to perform precise genome editing by creating double-strand breaks. However, the nuclease activity of Cas9 can be triggered when there is imperfect complementarity between the RNA guide sequence and an off-target genomic site, which is a major limitation of the CRISPR technique for practical applications. Hence, understanding the binding mechanisms in CRISPR/Cas9 for predicting ways to increase cleavage specificity is a timely research target. One way to understand and tune the binding strength is to study wild-type and mutant Cas9, in complex with a guide RNA and a target DNA. We have performed classical all-atom MD simulations over a cumulative time scale of 13.5 μs of CRISPR/Cas9 ternary complexes with the wild-type Cas9 from Streptococcus pyogenes and three of its mutants: K855A, H982A, and the combination K855A+H982A, selected from the outcome of experimental work. Our results reveal significant structural impact of the mutations, with implications for specificity. We find that the "unwound" part of the nontarget DNA strand exhibits enhanced flexibility in complexes with Cas9 mutants and tries to move away from the HNH/RuvC interface, where it is otherwise stabilized by electrostatic couplings in the wild-type complex. Our findings refine an electrostatic model by which cleavage specificity can be optimized through protein mutations.

The rapidly advancing Class 2 CRISPR-Cas technologies: A customizable toolbox for molecular manipulations

The CRISPR-Cas technologies derived from bacterial and archaeal adaptive immune systems have emerged as a series of groundbreaking nucleic acid-guided gene editing tools, ultimately standing out among several engineered nucleases because of their high efficiency, sequence-specific targeting, ease of programming and versatility. Facilitated by the advancement across multiple disciplines such as bioinformatics, structural biology and high-throughput sequencing, the discoveries and engineering of various innovative CRISPR-Cas systems are rapidly expanding the CRISPR toolbox. This is revolutionizing not only genome editing but also various other types of nucleic acid-guided manipulations such as transcriptional control and genomic imaging. Meanwhile, the adaptation of various CRISPR strategies in multiple settings has realized numerous previously non-existing applications, ranging from the introduction of sophisticated approaches in basic research to impactful agricultural and therapeutic applications. Here, we summarize the recent advances of CRISPR technologies and strategies, as well as their impactful applications.

RNA-Targeting CRISPR-Cas Systems and Their Applications

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated (Cas) systems have revolutionized modern molecular biology. Numerous types of these systems have been discovered to date. Many CRISPR-Cas systems have been used as a backbone for the development of potent research tools, with Cas9 being the most widespread. While most of the utilized systems are DNA-targeting, recently more and more attention is being gained by those that target RNA. Their ability to specifically recognize a given RNA sequence in an easily programmable way makes them ideal candidates for developing new research tools. In this review we summarize current knowledge on CRISPR-Cas systems which have been shown to target RNA molecules, that is type III (Csm/Cmr), type VI (Cas13), and type II (Cas9). We also present a list of available technologies based on these systems.

Approaches to study CRISPR RNA biogenesis and the key players involved

Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins provide an inheritable and adaptive immune system against phages and foreign genetic elements in many bacteria and archaea. The three stages of CRISPR-Cas immunity comprise adaptation, CRISPR RNA (crRNA) biogenesis and interference. The maturation of the pre-crRNA into mature crRNAs, short guide RNAs that target invading nucleic acids, is crucial for the functionality of CRISPR-Cas defense systems. Mature crRNAs assemble with Cas proteins into the ribonucleoprotein (RNP) effector complex and guide the Cas nucleases to the cognate foreign DNA or RNA target. Experimental approaches to characterize these crRNAs, the specific steps toward their maturation and the involved factors, include RNA-seq analyses, enzyme assays, methods such as cryo-electron microscopy, the crystallization of proteins, or UV-induced protein-RNA crosslinking coupled to mass spectrometry analysis. Complex and multiple interactions exist between CRISPR-cas-encoded specific riboendonucleases such as Cas6, Cas5d and Csf5, endonucleases with dual functions in maturation and interference such as the enzymes of the Cas12 and Cas13 families, and nucleases belonging to the cell's degradosome such as RNase E, PNPase and RNase J, both in the maturation as well as in interference. The results of these studies have yielded a picture of unprecedented diversity of sequences, enzymes and biochemical mechanisms.

Clades of huge phages from across Earth's ecosystems

Bacteriophages typically have small genomes1 and depend on their bacterial hosts for replication2. Here we sequenced DNA from diverse ecosystems and found hundreds of phage genomes with lengths of more than 200 kilobases (kb), including a genome of 735 kb, which is-to our knowledge-the largest phage genome to be described to date. Thirty-five genomes were manually curated to completion (circular and no gaps). Expanded genetic repertoires include diverse and previously undescribed CRISPR-Cas systems, transfer RNAs (tRNAs), tRNA synthetases, tRNA-modification enzymes, translation-initiation and elongation factors, and ribosomal proteins. The CRISPR-Cas systems of phages have the capacity to silence host transcription factors and translational genes, potentially as part of a larger interaction network that intercepts translation to redirect biosynthesis to phage-encoded functions. In addition, some phages may repurpose bacterial CRISPR-Cas systems to eliminate competing phages. We phylogenetically define the major clades of huge phages from human and other animal microbiomes, as well as from oceans, lakes, sediments, soils and the built environment. We conclude that the large gene inventories of huge phages reflect a conserved biological strategy, and that the phages are distributed across a broad bacterial host range and across Earth's ecosystems.

CRISPR-Cas bioinformatics

Clustered regularly interspaced short palindromic repeats (CRISPR) and their associated proteins (Cas) are essential genetic elements in many archaeal and bacterial genomes, playing a key role in a prokaryote adaptive immune system against invasive foreign elements. In recent years, the CRISPR-Cas system has also been engineered to facilitate target gene editing in eukaryotic genomes. Bioinformatics played an essential role in the detection and analysis of CRISPR systems and here we review the bioinformatics-based efforts that pushed the field of CRISPR-Cas research further. We discuss the bioinformatics tools that have been published over the last few years and, finally, present the most popular tools for the design of CRISPR-Cas9 guides.

CRISPR-Cas12a: Functional overview and applications

Prokaryotes have developed an adaptive immune system called Clustered regularly interspaced short palindromic repeats (CRISPR) to combat attacks by foreign mobile genetic elements (MGEs) such as plasmids and phages. In the past decade, the widely characterized CRISPR-Cas9 enzyme has been redesigned to trigger a genome editing revolution. Class II type V CRISPR-Cas12a is a new RNA guided endonuclease that has been recently harnessed as an alternative genome editing tool, which is emerging as a powerful molecular scissor to consider in the genome editing application landscape. In this review, we aim to provide a mechanistic insight into the working mechanism of Cas12a, comparing it with Cas9, and eventually provide an overview of its current applications in genome editing and biotechnology applications.

Medium-throughput in vitro detection of DNA cleavage by CRISPR-Cas12a

Quantifying DNA cleavage by CRISPR-Cas nucleases is usually done by separating the cleaved products from the non-cleaved target by agarose gel electrophoresis. We devised a method that eliminates the quantification from band intensity on agarose gel, and uses a target with a fluorescent dye on the one end and a biotin on the other. Cleavage of the target will separate the dye from the biotin, and cause the dye to stay in solution when streptavidin beads are introduced. All non-cleaved target will be eliminated from solution and no longer contribute to detectable fluorescence. Cleavage will therefore increase the fluorescent signal. A control, which has no streptavidin treatment, is taken along to correct for any errors that might have been introduced by pipetting, inactivation of the fluorescent dye or release of the biotin during several steps of the procedure. With this method we were able to quantify the fraction of active Cas12a in a purification sample and assess the cleavage rate.

RNA-targeting CRISPR systems from metagenomic discovery to transcriptomic engineering

Deployment of RNA-guided DNA endonuclease CRISPR-Cas technology has led to radical advances in biology. As the functional diversity of CRISPR-Cas and parallel systems is further explored, RNA manipulation is emerging as a powerful mode of CRISPR-based engineering. In this Perspective, we chart progress in the RNA-targeting CRISPR-Cas (RCas) field and illustrate how continuing evolution in scientific discovery translates into applications for RNA biology and insights into mysteries, obstacles, and alternative technologies that lie ahead.

RNA editing as a therapeutic approach for retinal gene therapy requiring long coding sequences

RNA editing aims to treat genetic disease through altering gene expression at the transcript level. Pairing site-directed RNA-targeting mechanisms with engineered deaminase enzymes allows for the programmable correction of G>A and T>C mutations in RNA. This offers a promising therapeutic approach for a range of genetic diseases. For inherited retinal degenerations caused by point mutations in large genes not amenable to single-adeno-associated viral (AAV) gene therapy such as USH2A and ABCA4, correcting RNA offers an alternative to gene replacement. Genome editing of RNA rather than DNA may offer an improved safety profile, due to the transient and potentially reversible nature of edits made to RNA. This review considers the current site-directing RNA editing systems, and the potential to translate these to the clinic for the treatment of inherited retinal degeneration.

Atypical organizations and epistatic interactions of CRISPRs and cas clusters in genomes and their mobile genetic elements

Prokaryotes use CRISPR-Cas systems for adaptive immunity, but the reasons for the frequent existence of multiple CRISPRs and cas clusters remain poorly understood. Here, we analysed the joint distribution of CRISPR and cas genes in a large set of fully sequenced bacterial genomes and their mobile genetic elements. Our analysis suggests few negative and many positive epistatic interactions between Cas subtypes. The latter often result in complex genetic organizations, where a locus has a single adaptation module and diverse interference mechanisms that might provide more effective immunity. We typed CRISPRs that could not be unambiguously associated with a cas cluster and found that such complex loci tend to have unique type I repeats in multiple CRISPRs. Many chromosomal CRISPRs lack a neighboring Cas system and they often have repeats compatible with the Cas systems encoded in trans. Phages and 25 000 prophages were almost devoid of CRISPR-Cas systems, whereas 3% of plasmids had CRISPR-Cas systems or isolated CRISPRs. The latter were often compatible with the chromosomal cas clusters, suggesting that plasmids can co-opt the latter. These results highlight the importance of interactions between CRISPRs and cas present in multiple copies and in distinct genomic locations in the function and evolution of bacterial immunity.

The Complete Genome of Emcibacter congregatus ZYLT, a Marine Bacterium Encoding a CRISPR-Cas 9 Immune System

Emcibacter congregatus ZYL^T was isolated from a sediment sample cultured in situ in a coast located in the East China Sea. The genome of E. congregatus ZYL^T was sequenced and assembled into one single circular chromosome with the size of 4,189,011 bp and G+C content of 52.6%. Genomic annotation showed that E. congregatus ZYL^T had an intact Type II-C CRISPR-Cas system consists of three cas genes (cas 9, cas 1, and cas 2), 34 direct repeat sequences with the length of 36 bp, and 33 spacers. The predicted Cas 9 protein was smaller than most of existing genome editing tools. This structure might have potential in developing new gene editing system and uncovering the regulatory mechanisms of CRISPR-Cas system. Besides, the comparison between E. congregatus ZYL^T and its relative species living in neritic environments unraveled some common traits of the defective strategies of these bacteria to face inshore challenges including the motility, multidrug resistance, and universal efflux pumps.

CRISPRCasdb a successor of CRISPRdb containing CRISPR arrays and cas genes from complete genome sequences, and tools to download and query lists of repeats and spacers

In Archaea and Bacteria, the arrays called CRISPRs for 'clustered regularly interspaced short palindromic repeats' and the CRISPR associated genes or cas provide adaptive immunity against viruses, plasmids and transposable elements. Short sequences called spacers, corresponding to fragments of invading DNA, are stored in-between repeated sequences. The CRISPR-Cas systems target sequences homologous to spacers leading to their degradation. To facilitate investigations of CRISPRs, we developed 12 years ago a website holding the CRISPRdb. We now propose CRISPRCasdb, a completely new version giving access to both CRISPRs and cas genes. We used CRISPRCasFinder, a program that identifies CRISPR arrays and cas genes and determine the system's type and subtype, to process public whole genome assemblies. Strains are displayed either in an alphabetic list or in taxonomic order. The database is part of the CRISPR-Cas++ website which also offers the possibility to analyse submitted sequences and to download programs. A BLAST search against lists of repeats and spacers extracted from the database is proposed. To date, 16 990 complete prokaryote genomes (16 650 bacteria from 2973 species and 340 archaea from 300 species) are included. CRISPR-Cas systems were found in 36% of Bacteria and 75% of Archaea strains. CRISPRCasdb is freely accessible at https://crisprcas.i2bc.paris-saclay.fr/.

Harnessing Nature’s Diversity for Developing New Biotechnologies

Molecular biologists today make use of a number of technologies based on natural products or systems, essentially using the wonders of life to further discover wonderous things about life. This cycle is powered by the immense amount of natural diversity. Here I highlight how one natural system, CRISPR, has been engineered for a range of applications aimed at improving human health and discuss possible ways natural diversity may be leveraged in the future.

Emerging Genome Engineering Tools in Crop Research and Breeding

Recent advances in genome engineering are revolutionizing crop research and plant breeding. The ability to make specific modifications to a plant's genetic material creates opportunities for rapid development of elite cultivars with desired traits. The plant genome can be altered in several ways, including targeted introduction of nucleotide changes, deleting DNA segments, introducing exogenous DNA fragments and epigenetic modifications. Targeted changes are mediated by sequence specific nucleases (SSNs), such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR (clustered regularly interspersed short palindromic repeats)-Cas (CRISPR associated protein) systems. Recent advances in engineering chimeric Cas nucleases fused to base editing enzymes permit for even greater precision in base editing and control over gene expression. In addition to gene editing technologies, improvement in delivery systems of exogenous DNA into plant cells have increased the rate of successful gene editing events. Regeneration of fertile plants containing the desired edits remains challenging; however, manipulation of embryogenesis-related genes such as BABY BOOM (BBM) has been shown to facilitate regeneration through tissue culture, often a major hurdle in recalcitrant cultivars. Epigenome reprogramming for improved crop performance is another possibility for future breeders, with recent studies on MutS HOMOLOG 1 (MSH1) demonstrating epigenetic-dependent hybrid vigor in several crops. While these technologies offer plant breeders new tools in creating high yielding, better adapted crop varieties, constantly evolving government policy regarding the cultivation of plants containing transgenes may impede the widespread adoption of some of these techniques. This chapter summarizes advances in genome editing tools and discusses the future of these techniques for crop improvement.

Immunogenicity of Cas9 Protein

Clustered regularly interspaced short palindromic repeats (CRISPR) form the adaptive immune system in archaea and bacteria and have been modified for genome engineering in eukaryotic cells. CRISPR systems contain 2 components, a single-guide RNA, which is a short RNA composed of a 20 nucleotide sequence that targets specific sites in the genomic DNA and a scaffold necessary for its binding to the CRISPR-associated endonuclease (Cas9). Because of its high efficiency and accuracy, the CRISPR-Cas9 genome editing based therapies are poised to treat a multitude of human diseases with a promise to target previously "undruggable" proteins. As the first in-body clinical trial with CRISPR-Cas9 is embarked on, the risks associated with administering the genome editing machinery to patients become increasingly relevant. Recent studies have demonstrated an innate and adaptive cellular immune response to Cas9 in mouse models and the presence of anti-Cas9 antibodies and T-cells in human plasma. Pre-existing immunity against therapeutic Cas9 delivery could decrease its efficacy in vivo and may pose significant safety issues. This review focuses on the immunogenicity of the Cas9 protein and summarizes potential approaches to circumvent the problem of immune recognition.

Strategies for the CRISPR-Based Therapeutics

The CRISPR (clustered regularly interspaced short palindromic repeats)-based genome editing technology is an emerging RNA-guided nuclease system initially identified from the microbial adaptive immune systems. In recent years, the CRISPR system has been reprogrammed to target specific regions of the eukaryotic genome and has become a powerful tool for genetic engineering. Researchers have explored many approaches to improve the genome editing activity of the CRISPR-Cas system and deliver its components both ex vivo and in vivo. Moreover, these strategies have been applied to genome editing in preclinical research and clinical trials. In this review, we focus on representative strategies for regulation and delivery of the CRISPR-Cas system, and outline current therapeutic applications in their clinical translation.

Versatile and robust genome editing with Streptococcus thermophilus CRISPR1-Cas9

Targeting definite genomic locations using CRISPR-Cas systems requires a set of enzymes with unique protospacer adjacent motif (PAM) compatibilities. To expand this repertoire, we engineered nucleases, cytosine base editors, and adenine base editors from the archetypal Streptococcus thermophilus CRISPR1-Cas9 (St1Cas9) system. We found that St1Cas9 strain variants enable targeting to five distinct A-rich PAMs and provide a structural basis for their specificities. The small size of this ortholog enables expression of the holoenzyme from a single adeno-associated viral vector for in vivo editing applications. Delivery of St1Cas9 to the neonatal liver efficiently rewired metabolic pathways, leading to phenotypic rescue in a mouse model of hereditary tyrosinemia. These robust enzymes expand and complement current editing platforms available for tailoring mammalian genomes.

Precise gene replacement in plants through CRISPR/Cas genome editing technology: current status and future perspectives

CRISPR/Cas, as a simple, versatile, robust and cost-effective system for genome manipulation, has dominated the genome editing field over the past few years. The application of CRISPR/Cas in crop improvement is particularly important in the context of global climate change, as well as diverse agricultural, environmental and ecological challenges. Various CRISPR/Cas toolboxes have been developed and allow for targeted mutagenesis at specific genome loci, transcriptome regulation and epigenome editing, base editing, and precise targeted gene/allele replacement or tagging in plants. In particular, precise replacement of an existing allele with an elite allele in a commercial variety through homology-directed repair (HDR) is a holy grail in genome editing for crop improvement as it has been very difficult, laborious and time-consuming to introgress the elite alleles into commercial varieties without any linkage drag from parental lines within a few generations in crop breeding practice. However, it still remains very challenging in crop plants. This review intends to provide an informative summary of the latest development and breakthroughs in gene replacement using CRISPR/Cas technology, with a focus on achievements, potential mechanisms and future perspectives in plant biological science as well as crop improvement.

Methods for the Analysis and Characterization of Defense Mechanisms Against Horizontal Gene Transfer: CRISPR Systems

CRISPR-Cas systems provide RNA-guided adaptive immunity to the majority of archaea and many bacteria. They are able to capture pieces of invading genetic elements in the form of novel spacers in an array of repeats. These elements can then be used as a memory to destroy incoming DNA through the action of RNA-guided nucleases. This chapter describes general procedures to determine the ability of CRISPR-Cas systems to capture novel sequences and to use them to block phages and horizontal gene transfer. All protocols are performed in Staphylococcus aureus using Type II-A CRISPR-Cas systems. Nonetheless, the protocols provided can be adapted to work with other bacteria and other types of CRISPR-Cas systems.

Upgrading of efficient and scalable CRISPR-Cas-mediated technology for genetic engineering in thermophilic fungus Myceliophthora thermophila

BACKGROUND

Thermophilic filamentous fungus Myceliophthora thermophila has great capacity for biomass degradation and is an attractive system for direct production of enzymes and chemicals from plant biomass. Its industrial importance inspired us to develop genome editing tools to speed up the genetic engineering of this fungus. First-generation CRISPR-Cas9 technology was developed in 2017 and, since then, some progress has been made in thermophilic fungi genetic engineering, but a number of limitations remain. They include the need for complex independent expression cassettes for targeting multiplex genomic loci and the limited number of available selectable marker genes.

RESULTS

In this study, we developed an Acidaminococcus sp. Cas12a-based CRISPR system for efficient multiplex genome editing, using a single-array approach in M. thermophila. These CRISPR-Cas12a cassettes worked well for simultaneous multiple gene deletions/insertions. We also developed a new simple approach for marker recycling that relied on the novel cleavage activity of the CRISPR-Cas12a system to make DNA breaks in selected markers. We demonstrated its performance by targeting nine genes involved in the cellulase production pathway in M. thermophila via three transformation rounds, using two selectable markers neo and bar. We obtained the nonuple mutant M9 in which protein productivity and lignocellulase activity were 9.0- and 18.5-fold higher than in the wild type. We conducted a parallel investigation using our transient CRISPR-Cas9 system and found the two technologies were complementary. Together we called them CRISPR-Cas-assisted marker recycling technology (Camr technology).

CONCLUSIONS

Our study described new approaches (Camr technology) that allow easy and efficient marker recycling and iterative stacking of traits in the same thermophilic fungus strain either, using the newly established CRISPR-Cas12a system or the established CRISPR-Cas9 system. This Camr technology will be a versatile and efficient tool for engineering, theoretically, an unlimited number of genes in fungi. We expect this advance to accelerate biotechnology-oriented engineering processes in fungi.

Synthetic chimeric nucleases function for efficient genome editing

CRISPR-Cas systems have revolutionized genome editing across a broad range of biotechnological endeavors. Many CRISPR-Cas nucleases have been identified and engineered for improved capabilities. Given the modular structure of such enzymes, we hypothesized that engineering chimeric sequences would generate non-natural variants that span the kinetic parameter landscape, and thus provide for the rapid selection of nucleases fit for a particular editing system. Here, we design a chimeric Cas12a-type library with approximately 560 synthetic chimeras, and select several functional variants. We demonstrate that certain nuclease domains can be recombined across distantly related nuclease templates to produce variants that function in bacteria, yeast, and human cell lines. We further characterize selected chimeric nucleases and find that they have different protospacer adjacent motif (PAM) preferences and the M44 chimera has higher specificity relative to wild-type (WT) sequences. This demonstration opens up the possibility of generating nuclease sequences with implications across biotechnology.

CRISPR-Cas13d mediates robust RNA virus interference in plants

BACKGROUND

CRISPR-Cas systems endow bacterial and archaeal species with adaptive immunity mechanisms to fend off invading phages and foreign genetic elements. CRISPR-Cas9 has been harnessed to confer virus interference against DNA viruses in eukaryotes, including plants. In addition, CRISPR-Cas13 systems have been used to target RNA viruses and the transcriptome in mammalian and plant cells. Recently, CRISPR-Cas13a has been shown to confer modest interference against RNA viruses. Here, we characterized a set of different Cas13 variants to identify those with the most efficient, robust, and specific interference activities against RNA viruses in planta using Nicotiana benthamiana.

RESULTS

Our data show that LwaCas13a, PspCas13b, and CasRx variants mediate high interference activities against RNA viruses in transient assays. Moreover, CasRx mediated robust interference in both transient and stable overexpression assays when compared to the other variants tested. CasRx targets either one virus alone or two RNA viruses simultaneously, with robust interference efficiencies. In addition, CasRx exhibits strong specificity against the target virus and does not exhibit collateral activity in planta.

CONCLUSIONS

Our data establish CasRx as the most robust Cas13 variant for RNA virus interference applications in planta and demonstrate its suitability for studying key questions relating to virus biology.

Targeted transposition with Tn7 elements: safe sites, mobile plasmids, CRISPR/Cas and beyond

Transposon Tn7 is notable for the control it exercises over where transposition events are directed. One Tn7 integration pathways recognizes a highly conserved attachment (att) site in the chromosome, while a second pathway specifically recognizes mobile plasmids that facilitate transfer of the element to new hosts. In this review, I discuss newly discovered families of Tn7-like elements with different targeting pathways. Perhaps the most exciting examples are multiple instances where Tn7-like elements have repurposed CRISPR/Cas systems. In these cases, the CRISPR/Cas systems have lost their canonical defensive function to destroy incoming mobile elements; instead, the systems have been naturally adapted to use guide RNAs to specifically direct transposition into these mobile elements. The new families of Tn7-like elements also include a variety of novel att sites in bacterial chromosomes where genome islands can form. Interesting families have also been revealed where proteins described in the prototypic Tn7 element are fused or otherwise repurposed for the new dual activities. This expanded understanding of Tn7-like elements broadens our view of how genetic systems are repurposed and provides potentially exciting new tools for genome modification and genomics. Future opportunities and challenges to understanding the impact of the new families of Tn7-like elements are discussed.

CRISPR/Cas system: An emerging technology in stem cell research

The identification of new and even more precise technologies for modifying and manipulating the genome has been a challenge since the discovery of the DNA double helix. The ability to modify selectively specific genes provides a powerful tool for characterizing gene functions, performing gene therapy, correcting specific genetic mutations, eradicating diseases, engineering cells and organisms to achieve new and different functions and obtaining transgenic animals as models for studying specific diseases. Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology has recently revolutionized genome engineering. The application of this new technology to stem cell research allows disease models to be developed to explore new therapeutic tools. The possibility of translating new systems of molecular knowledge to clinical research is particularly appealing for addressing degenerative diseases. In this review, we describe several applications of CRISPR/Cas9 to stem cells related to degenerative diseases. In addition, we address the challenges and future perspectives regarding the use of CRISPR/Cas9 as an important technology in the medical sciences.

CRISPR/Cas9: Nature's gift to prokaryotes and an auspicious tool in genome editing

Clustered regularly interspaced short palindromic repeats (CRISPR) is a family of DNA direct repeats found in many prokaryotic genomes. It was discovered in bacteria as their (adaptive) immune system against invading viruses. Cas9 is an endonuclease enzyme linked with the CRISPR system in bacteria. Bacteria use the Cas9 enzyme to chop viral DNA sequences by unwinding it and then finding the complementary base pairs to the guide RNA. CRISPR/Cas9 is a modern and powerful molecular biology approach that is widely used in genome engineering (to activate/repress gene expression). It can be used in vivo to cause targeted genome modifications with better efficiency as compared to meganucleases, zinc-finger nucleases and transcription activator-like effector nucleases. CRISPR/Cas9 is a simple, reliable, and rapid method for causing gene alterations that open new horizons of gene editing in a variety of living organisms, including humans, for the treatment of several diseases. In this short review, we explored the basic mechanisms underlying its working principles along with some of its current applications in a number of diverse fields.

CRISPR-associated nucleases: the Dawn of a new age of efficient crop improvement

The world stands at a new threshold today. As a planet, we face various challenges, and the key one is how to continue to produce enough food, feed, fiber, and fuel to support the burgeoning population. In the past, plant breeding and the ability to genetically engineer crops contributed to increasing food production. However, both approaches rely on random mixing or integration of genes, and the process can be unpredictable and time-consuming. Given the challenge of limited availability of natural resources and changing environmental conditions, the need to rapidly and precisely improve crops has become urgent. The discovery of CRISPR-associated endonucleases offers a precise yet versatile platform for rapid crop improvement. This review summarizes a brief history of the discovery of CRISPR-associated nucleases and their application in genome editing of various plant species. Also provided is an overview of several new endonucleases reported recently, which can be utilized for editing of specific genes in plants through various forms of DNA sequence alteration. Genome editing, with its ever-expanding toolset, increased efficiency, and its potential integration with the emerging synthetic biology approaches hold promise for efficient crop improvement to meet the challenge of supporting the needs of future generations.

Making the cut(s): how Cas12a cleaves target and non-target DNA

CRISPR-Cas12a (previously named Cpf1) is a prokaryotic deoxyribonuclease that can be programmed with an RNA guide to target complementary DNA sequences. Upon binding of the target DNA, Cas12a induces a nick in each of the target DNA strands, yielding a double-stranded DNA break. In addition to inducing cis-cleavage of the targeted DNA, target DNA binding induces trans-cleavage of non-target DNA. As such, Cas12a-RNA guide complexes can provide sequence-specific immunity against invading nucleic acids such as bacteriophages and plasmids. Akin to CRISPR-Cas9, Cas12a has been repurposed as a genetic tool for programmable genome editing and transcriptional control in both prokaryotic and eukaryotic cells. In addition, its trans-cleavage activity has been applied for high-sensitivity nucleic acid detection. Despite the demonstrated value of Cas12a for these applications, the exact molecular mechanisms of both cis- and trans-cleavage of DNA were not completely understood. Recent studies have revealed mechanistic details of Cas12a-mediates DNA cleavage: base pairing of the RNA guide and the target DNA induces major conformational changes in Cas12a. These conformational changes render Cas12a in a catalytically activated state in which it acts as deoxyribonuclease. This deoxyribonuclease activity mediates cis-cleavage of the displaced target DNA strand first, and the RNA guide-bound target DNA strand second. As Cas12a remains in the catalytically activated state after cis-cleavage, it subsequently demonstrates trans-cleavage of non-target DNA. Here, I review the mechanistic details of Cas12a-mediated cis- and trans-cleavage of DNA. In addition, I discuss how bacteriophage-derived anti-CRISPR proteins can inhibit Cas12a activity.

CRISPR/Cas: from adaptive immune system in prokaryotes to therapeutic weapon against immune-related diseases

CRISPR/Cas evolved as an adaptive immune system in bacteria and archaea to inactivate foreign viral and plasmid DNA. However, the capacities of various CRISPR/Cas systems for precise genome editing based on sequence homology also allow their use as tools for genomic and epigenomic modification in eukaryotes. Indeed, these genetic characteristics have proven useful for disease modeling and testing the specific functions of target genes under pathological conditions. Moreover, recent studies provide compelling evidence that CRISPR/Cas systems could be useful therapeutic tools against human diseases, including cancer, monogenic disorders, and autoimmune disorders.HighlightsCRISPR/Cas evolved as an adaptive immune system in bacteria and archaea.CRISPR/Cas systems are nowadays used as tools for genomic modification.CRISPR/Cas systems could be useful therapeutic tools against human disease, including autoimmune conditions.

Evolutionary entanglement of mobile genetic elements and host defence systems: guns for hire

All cellular life forms are afflicted by diverse genetic parasites, including viruses and other types of mobile genetic elements (MGEs), and have evolved multiple, diverse defence systems that protect them from MGE assault via different mechanisms. Here, we provide our perspectives on how recent evidence points to tight evolutionary connections between MGEs and defence systems that reach far beyond the proverbial arms race. Defence systems incur a fitness cost for the hosts; therefore, at least in prokaryotes, horizontal mobility of defence systems, mediated primarily by MGEs, is essential for their persistence. Moreover, defence systems themselves possess certain features of selfish elements. Common components of MGEs, such as site-specific nucleases, are 'guns for hire' that can also function as parts of defence mechanisms and are often shuttled between MGEs and defence systems. Thus, evolutionary and molecular factors converge to mould the multifaceted, inextricable connection between MGEs and anti-MGE defence systems.

Establishment and application of a CRISPR-Cas12a assisted genome-editing system in Zymomonas mobilis

BACKGROUND

Efficient and convenient genome-editing toolkits can expedite genomic research and strain improvement for desirable phenotypes. Zymomonas mobilis is a highly efficient ethanol-producing bacterium with a small genome size and desirable industrial characteristics, which makes it a promising chassis for biorefinery and synthetic biology studies. While classical techniques for genetic manipulation are available for Z. mobilis, efficient genetic engineering toolkits enabling rapidly systematic and high-throughput genome editing in Z. mobilis are still lacking.

RESULTS

Using Cas12a (Cpf1) from Francisella novicida, a recombinant strain with inducible cas12a expression for genome editing was constructed in Z. mobilis ZM4, which can be used to mediate RNA-guided DNA cleavage at targeted genomic loci. gRNAs were then designed targeting the replicons of native plasmids of ZM4 with about 100% curing efficiency for three native plasmids. In addition, CRISPR-Cas12a recombineering was used to promote gene deletion and insertion in one step efficiently and precisely with efficiency up to 90%. Combined with single-stranded DNA (ssDNA), CRISPR-Cas12a system was also applied to introduce minor nucleotide modification precisely into the genome with high fidelity. Furthermore, the CRISPR-Cas12a system was employed to introduce a heterologous lactate dehydrogenase into Z. mobilis with a recombinant lactate-producing strain constructed.

CONCLUSIONS

This study applied CRISPR-Cas12a in Z. mobilis and established a genome editing tool for efficient and convenient genome engineering in Z. mobilis including plasmid curing, gene deletion and insertion, as well as nucleotide substitution, which can also be employed for metabolic engineering to help divert the carbon flux from ethanol production to other products such as lactate demonstrated in this work. The CRISPR-Cas12a system established in this study thus provides a versatile and powerful genome-editing tool in Z. mobilis for functional genomic research, strain improvement, as well as synthetic microbial chassis development for economic biochemical production.

The Expanding Class 2 CRISPR Toolbox: Diversity, Applicability, and Targeting Drawbacks

The class 2 clustered regularly interspaced short palindromic repeats (CRISPR)-Cas system, one of the prokaryotic adaptive immune systems, has sparked a lot of attention for its use as a gene editing tool. Currently, type II, V, and VI effector modules of this class have been characterized and extensively tested for nucleic acid editing, imaging, and disease diagnostics. Due to the unique composition of their nuclease catalytic center, the effector modules substantially vary in their function and possible biotechnology applications. In this review, we discuss the structural and functional diversity in class 2 CRISPR effectors, and debate their suitability for nucleic acid targeting and their shortcomings as gene editing tools.

The Role of Noncoding RNAs in Double-Strand Break Repair

Genome stability is constantly threatened by DNA lesions generated by different environmental factors as well as endogenous processes. If not properly and timely repaired, damaged DNA can lead to mutations or chromosomal rearrangements, well-known reasons for genetic diseases or cancer in mammals, or growth abnormalities and/or sterility in plants. To prevent deleterious consequences of DNA damage, a sophisticated system termed DNA damage response (DDR) detects DNA lesions and initiates DNA repair processes. In addition to many well-studied canonical proteins involved in this process, noncoding RNA (ncRNA) molecules have recently been discovered as important regulators of the DDR pathway, extending the broad functional repertoire of ncRNAs to the maintenance of genome stability. These ncRNAs are mainly connected with double-strand breaks (DSBs), the most dangerous type of DNA lesions. The possibility to intentionally generate site-specific DSBs in the genome with endonucleases constitutes a powerful tool to study, in vivo, how DSBs are processed and how ncRNAs participate in this crucial event. In this review, we will summarize studies reporting the different roles of ncRNAs in DSB repair and discuss how genome editing approaches, especially CRISPR/Cas systems, can assist DNA repair studies. We will summarize knowledge concerning the functional significance of ncRNAs in DNA repair and their contribution to genome stability and integrity, with a focus on plants.

Recruitment of Reverse Transcriptase-Cas1 Fusion Proteins by Type VI-A CRISPR-Cas Systems

Type VI CRISPR-Cas systems contain a single effector nuclease (Cas13) that exclusively targets single-stranded RNA. It remains unknown how these systems acquire spacers. It has been suggested that type VI systems with adaptation modules can acquire spacers from RNA bacteriophages, but sequence similarities suggest that spacers may provide immunity to DNA phages. We searched databases for Cas13 proteins with linked RTs. We identified two different type VI-A systems with adaptation modules including an RT-Cas1 fusion and Cas2 proteins. Phylogenetic reconstruction analyses revealed that these adaptation modules were recruited by different effector Cas13a proteins, possibly from RT-associated type III-D systems within the bacterial classes Alphaproteobacteria and Clostridia. These type VI-A systems are predicted to acquire spacers from RNA molecules, paving the way for future studies investigating their role in bacterial adaptive immunity and biotechnological applications.

Systematic prediction of functionally linked genes in bacterial and archaeal genomes

Functionally linked genes in bacterial and archaeal genomes are often organized into operons. However, the composition and architecture of operons are highly variable and frequently differ even among closely related genomes. Therefore, to efficiently extract reliable functional predictions for uncharacterized genes from comparative analyses of the rapidly growing genomic databases, dedicated computational approaches are required. We developed a protocol to systematically and automatically identify genes that are likely to be functionally associated with a 'bait' gene or locus by using relevance metrics. Given a set of bait loci and a genomic database defined by the user, this protocol compares the genomic neighborhoods of the baits to identify genes that are likely to be functionally linked to the baits by calculating the abundance of a given gene within and outside the bait neighborhoods and the distance to the bait. We exemplify the performance of the protocol with three test cases, namely, genes linked to CRISPR-Cas systems using the 'CRISPRicity' metric, genes associated with archaeal proviruses and genes linked to Argonaute genes in halobacteria. The protocol can be run by users with basic computational skills. The computational cost depends on the sizes of the genomic dataset and the list of reference loci and can vary from one CPU-hour to hundreds of hours on a supercomputer.

Generation of virus-resistant potato plants by RNA genome targeting

CRISPR/Cas systems provide bacteria and archaea with molecular immunity against invading phages and foreign plasmids. The class 2 type VI CRISPR/Cas effector Cas13a is an RNA-targeting CRISPR effector that provides protection against RNA phages. Here we report the repurposing of CRISPR/Cas13a to protect potato plants from a eukaryotic virus, Potato virus Y (PVY). Transgenic potato lines expressing Cas13a/sgRNA (small guide RNA) constructs showed suppressed PVY accumulation and disease symptoms. The levels of viral resistance correlated with the expression levels of the Cas13a/sgRNA construct in the plants. Our data further demonstrate that appropriately designed sgRNAs can specifically interfere with multiple PVY strains, while having no effect on unrelated viruses such as PVA or Potato virus S. Our findings provide a novel and highly efficient strategy for engineering crops with resistances to viral diseases.

Searching for a Match: Structure, Function and Application of Sequence-Specific RNA-Binding Proteins

Plants encode over 1800 RNA-binding proteins (RBPs) that modulate a myriad of steps in gene regulation from chromatin organization to translation, yet only a small number of these proteins and their target transcripts have been functionally characterized. Two classes of eukaryotic RBPs, pentatricopeptide repeat (PPR) and pumilio/fem-3 binding factors (PUF), recognize and bind to specific sequential RNA sequences through protein-RNA interactions. These modular proteins possess helical structural units containing key residues with high affinity for specific nucleotides, whose sequential order determines binding to a specific target RNA sequence. PPR proteins are nucleus-encoded, but largely regulate post-transcriptional gene regulation within plastids and mitochondria, including splicing, translation and RNA editing. Plant PUFs are involved in gene regulatory processes within the cell nucleus and cytoplasm. The modular structures of PPRs and PUFs that determine sequence specificity has facilitated identification of their RNA targets and biological functions. The protein-based RNA-targeting of PPRs and PUFs contrasts to the prokaryotic cluster regularly interspaced short palindromic repeats (CRISPR)-associated proteins (Cas) that target RNAs in prokaryotes. Together the PPR, PUF and CRISPR-Cas systems provide varied opportunities for RNA-targeted engineering applications.

CRISPR/Cas9 genome editing technology in filamentous fungi: progress and perspective

Filamentous fungi play an important role in human health and industrial/agricultural production. With the increasing number of full genomes available for fungal species, the study of filamentous fungi has brought about a wider range of genetic manipulation opportunities. However, the utilization of traditional methods to study fungi is time consuming and laborious. Recent rapid progress and wide application of a versatile genome editing technology, i.e., the CRISPR (clustered regularly interspaced short palindromic repeat)-Cas9 (CRISPR-related nuclease 9) system, has revolutionized biological research and has many innovative applications in a wide range of fields showing great promise in research and application of filamentous fungi. In this review, we introduce the CRISPR/Cas9 genome editing technology focusing on its application in research of filamentous fungi and we discuss the general considerations of genome editing using CRISPR/Cas9 system illustrating vector construction, multiple editing strategies, technical consideration of different sizes of homology arms on genome editing efficiency, off-target effects, and different transformation methodologies. In addition, we discuss the challenges encountered using CRISPR/Cas9 technology and give the perspectives of future applications of CRISPR/Cas9 technology for basic research and practical application of filamentous fungi.

BREX system of Escherichia coli distinguishes self from non-self by methylation of a specific DNA site

Prokaryotes evolved numerous systems that defend against predation by bacteriophages. In addition to well-known restriction-modification and CRISPR-Cas immunity systems, many poorly characterized systems exist. One class of such systems, named BREX, consists of a putative phosphatase, a methyltransferase and four other proteins. A Bacillus cereus BREX system provides resistance to several unrelated phages and leads to modification of specific motif in host DNA. Here, we study the action of BREX system from a natural Escherichia coli isolate. We show that while it makes cells resistant to phage λ infection, induction of λ prophage from cells carrying BREX leads to production of viruses that overcome the defense. The induced phage DNA contains a methylated adenine residue in a specific motif. The same modification is found in the genome of BREX-carrying cells. The results establish, for the first time, that immunity to BREX system defense is provided by an epigenetic modification.

Molecular Therapies for Inherited Retinal Diseases-Current Standing, Opportunities and Challenges

Inherited retinal diseases (IRDs) are both genetically and clinically highly heterogeneous and have long been considered incurable. Following the successful development of a gene augmentation therapy for biallelic RPE65-associated IRD, this view has changed. As a result, many different therapeutic approaches are currently being developed, in particular a large variety of molecular therapies. These are depending on the severity of the retinal degeneration, knowledge of the pathophysiological mechanism underlying each subtype of IRD, and the therapeutic target molecule. DNA therapies include approaches such as gene augmentation therapy, genome editing and optogenetics. For some genetic subtypes of IRD, RNA therapies and compound therapies have also shown considerable therapeutic potential. In this review, we summarize the current state-of-the-art of various therapeutic approaches, including the pros and cons of each strategy, and outline the future challenges that lie ahead in the combat against IRDs.

Cronobacter sakazakii, Cronobacter malonaticus, and Cronobacter dublinensis Genotyping Based on CRISPR Locus Diversity

Cronobacter strains harboring CRISPR-Cas systems are important foodborne pathogens that cause serious neonatal infections. CRISPR typing is a new molecular subtyping method to track the sources of pathogenic bacterial outbreaks and shows a promise in typing Cronobacter, however, this molecular typing procedure using routine PCR method has not been established. Therefore, the purpose of this study was to establish such methodology, 257 isolates of Cronobacter sakazakii, C. malonaticus, and C. dublinensis were used to verify the feasibility of the method. Results showed that 161 C. sakazakii strains could be divided into 129 CRISPR types (CTs), among which CT15 (n = 7) was the most prevalent CT followed by CT6 (n = 4). Further, 65 C. malonaticus strains were divided into 42 CTs and CT23 (n = 8) was the most prevalent followed by CT2, CT3, and CT13 (n = 4). Finally, 31 C. dublinensis strains belonged to 31 CTs. There was also a relationship among CT, sequence type (ST), food types, and serotype. Compared to multi-locus sequence typing (MLST), this new molecular method has greater power to distinguish similar strains and had better accordance with whole genome sequence typing (WGST). More importantly, some lineages were found to harbor conserved ancestral spacers ahead of their divergent specific spacer sequences; this can be exploited to infer the divergent evolution of Cronobacter and provide phylogenetic information reflecting common origins. Compared to WGST, CRISPR typing method is simpler and more affordable, it could be used to identify sources of Cronobacter food-borne outbreaks, from clinical cases to food sources and the production sites.

Extensive CRISPR RNA modification reveals chemical compatibility and structure-activity relationships for Cas9 biochemical activity

CRISPR (clustered regularly interspaced short palindromic repeat) endonucleases are at the forefront of biotechnology, synthetic biology and gene editing. Methods for controlling enzyme properties promise to improve existing applications and enable new technologies. CRISPR enzymes rely on RNA cofactors to guide catalysis. Therefore, chemical modification of the guide RNA can be used to characterize structure-activity relationships within CRISPR ribonucleoprotein (RNP) enzymes and identify compatible chemistries for controlling activity. Here, we introduce chemical modifications to the sugar–phosphate backbone of Streptococcus pyogenes Cas9 CRISPR RNA (crRNA) to probe chemical and structural requirements. Ribose sugars that promoted or accommodated A-form helical architecture in and around the crRNA ‘seed’ region were tolerated best. A wider range of modifications were acceptable outside of the seed, especially D-2′-deoxyribose, and we exploited this property to facilitate exploration of greater chemical diversity within the seed. 2′-fluoro was the most compatible modification whereas bulkier O-methyl sugar modifications were less tolerated. Activity trends could be rationalized for selected crRNAs using RNP stability and DNA target binding experiments. Cas9 activity in vitro tolerated most chemical modifications at predicted 2′-hydroxyl contact positions, whereas editing activity in cells was much less tolerant. The biochemical principles of chemical modification identified here will guide CRISPR-Cas9 engineering and enable new or improved applications.