Engineered CRISPR-Cas12a variants with increased activities and improved targeting ranges for gene, epigenetic and base editing

A far-red light-inducible CRISPR-Cas12a platform for remote-controlled genome editing and gene activation

The CRISPR-Cas12a has been harnessed as a powerful tool for manipulating targeted gene expression. The possibility to manipulate the activity of CRISPR-Cas12a with a more precise spatiotemporal resolution and deep tissue permeability will enable targeted genome engineering and deepen our understanding of the gene functions underlying complex cellular behaviors. However, currently available inducible CRISPR-Cas12a systems are limited by diffusion, cytotoxicity, and poor tissue permeability. Here, we developed a far-red light (FRL)–inducible CRISPR-Cas12a (FICA) system that can robustly induce gene editing in mammalian cells, and an FRL-inducible CRISPR-dCas12a (FIdCA) system based on the protein-tagging system SUperNova (SunTag) that can be used for gene activation under light-emitting diode–based FRL. Moreover, we show that the FIdCA system can be deployed to activate target genes in mouse livers. These results demonstrate that these systems developed here provide robust and efficient platforms for programmable genome manipulation in a noninvasive and spatiotemporal fashion.

High-Throughput Profiling of Cas12a Orthologues and Engineered Variants for Enhanced Genome Editing Activity

CRISPR/Cas12a (formerly Cpf1), an RNA-guided endonuclease of the Class II Type V-A CRISPR system, provides a promising tool for genome engineering. Over 10 Cas12a orthologues have been identified and employed for gene editing in human cells. However, the functional diversity among emerging Cas12a orthologues remains poorly explored. Here, we report a high-throughput comparative profiling of editing activities across 16 Cas12a orthologues in human cells by constructing genome-integrated, self-cleaving, paired crRNA–target libraries containing >40,000 guide RNAs. Three Cas12a candidates exhibited promising potential owing to their compact structures and editing efficiency comparable with those of AsCas12a and LbCas12a, which are well characterized. We generated three arginine substitution variants (3Rv) via structure-guided protein engineering: BsCas12a-3Rv (K155R/N512R/K518R), PrCas12a-3Rv (E162R/N519R/K525R), and Mb3Cas12a-3Rv (D180R/N581R/K587R). All three Cas12a variants showed enhanced editing activities and expanded targeting ranges (NTTV, NTCV, and TRTV) compared with the wild-type Cas12a effectors. The base preference analysis among the three Cas12a variants revealed that PrCas12a-3Rv shows the highest activity at target sites with canonical PAM TTTV and non-canonical PAM TTCV, while Mb3Cas12a-3Rv exhibits recognition features distinct from the others by accommodating for more nucleotide A at position −3 for PAM TATV and at position −4 for PAM ATCV. Thus, the expanded Cas12a toolbox and an improved understanding of Cas12a activities should facilitate their use in genome engineering.

Simple-to-use CRISPR-SpCas9/SaCas9/AsCas12a vector series for genome editing in Saccharomyces cerevisiae

Genome editing using the CRISPR/Cas system has been implemented for various organisms and becomes increasingly popular even in the genetically tractable budding yeast Saccharomyces cerevisiae. Because each CRISPR/Cas system recognizes only the sequences flanked by its unique protospacer adjacent motif (PAM), a certain single system often fails to target a region of interest due to the lack of PAM, thus necessitating the use of another system with a different PAM. Three CRISPR/Cas systems with distinct PAMs, namely SpCas9, SaCas9, and AsCas12a, have been successfully used in yeast genome editing. Their combined use should expand the repertoire of editable targets. However, currently available plasmids for these systems were individually developed under different design principles, thus hampering their seamless use in the practice of genome editing. Here, we report a series of Golden Gate Assembly-compatible backbone vectors designed under a unified principle to exploit the three CRISPR/Cas systems in yeast genome editing. We also created a program to assist the design of genome-editing plasmids for individual target sequences using the backbone vectors. Genome editing with these plasmids demonstrated practically sufficient efficiency in the insertion of gene fragments to essential genes (median 52.1%), the complete deletion of an open reading frame (median 78.9%), and the introduction of single amino acid substitutions (median 79.2%). The backbone vectors with the program would provide a versatile toolbox to facilitate the seamless use of SpCas9, SaCas9, and AsCas12a in various types of genome manipulation, especially those that are difficult to perform with conventional techniques in yeast genetics.

Mg2+-dependent conformational rearrangements of CRISPR-Cas12a R-loop complex are mandatory for complete double-stranded DNA cleavage

CRISPR-Cas12a, an RNA-guided DNA targeting endonuclease, has been widely used for genome editing and nucleic acid detection. As part of the essential processes for both of these applications, the two strands of double-stranded DNA are sequentially cleaved by a single catalytic site of Cas12a, but the mechanistic details that govern the generation of complete breaks in double-stranded DNA remain to be elucidated. Here, using single-molecule fluorescence resonance energy transfer assay, we identified two conformational intermediates that form consecutively following the initial cleavage of the nontarget strand. Specifically, these two intermediates are the result of further unwinding of the target DNA in the protospacer-adjacent motif (PAM)-distal region and the subsequent binding of the target strand to the catalytic site. Notably, the PAM-distal DNA unwound conformation was stabilized by Mg2+ ions, thereby significantly promoting the binding and cleavage of the target strand. These findings enabled us to propose a Mg2+-dependent kinetic model for the mechanism whereby Cas12a achieves cleavage of the target DNA, highlighting the presence of conformational rearrangements for the complete cleavage of the double-stranded DNA target.

Making the cut with PAMless CRISPR-Cas enzymes

Genome editing technologies simplify our ability to rewrite genetic blueprints of life. However, CRISPR-Cas enzymes found in nature can only manipulate a fraction of the genome. To overcome this limitation, new Cas variants have been developed that unlock nearly the entire genome for editing.

Tumor microenvironment based stimuli-responsive CRISPR/Cas delivery systems: A viable platform for interventional approaches

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems have emerged as robust tools in cancer gene therapy due to their simplicity and versatility. Nevertheless, the genome editing efficiency in tumor sites and the clinical applications of CRISPR/Cas have been compromised by non-specific delivery and genotoxicity. Recently, intelligent delivery systems incorporating sensitive materials in response to endogenous stimuli of the tumor microenvironment (TME) have represented viable platforms for tumor-specific genome editing and reduced side effects of CRISPR/Cas. Spurred by this promising direction, this review first introduces the CRISPR/Cas systems widely employed in cancer therapeutic explorations. Various types of CRISPR/Cas delivery systems sensitive to the stimuli in TME and typical dual-/multiple-responsive CRISPR/Cas carriers are further discussed, emphasizing the correlations between sensitive components and spatiotemporal delivery mechanisms. The genome editing efficiencies of CRISPR/Cas-loaded stimuli-responsive carriers are also summarized both in vitro and in vivo. Collectively, stimuli-responsive CRISPR/Cas delivery systems hold great promise for potent cancer gene therapy.

In Silico Analysis of Pathogenic CRB1 Single Nucleotide Variants and Their Amenability to Base Editing as a Potential Lead for Therapeutic Intervention

Mutations in the Crumbs homolog 1 (CRB1) gene cause both autosomal recessive retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA). Since three separate CRB1 isoforms are expressed at meaningful levels in the human retina, base editing shows promise as a therapeutic approach. This retrospective analysis aims to summarise the reported pathogenic CRB1 variants and investigate their amenability to treatment with currently available DNA base editors. Pathogenic single nucleotide variants (SNVs) were extracted from the Leiden open-source variation database (LOVD) and ClinVar database and coded by mutational consequence. They were then analyzed for their amenability to currently available DNA base editors and available PAM sites from a selection of different Cas proteins. Of a total of 1115 unique CRB1 variants, 69% were classified as pathogenic SNVs. Of these, 62% were amenable to currently available DNA BEs. Adenine base editors (ABEs) alone have the potential of targeting 34% of pathogenic SNVs; 19% were amenable to a CBE while GBEs could target an additional 9%. Of the pathogenic SNVs targetable with a DNA BE, 87% had a PAM site for a Cas protein. Of the 33 most frequently reported pathogenic SNVs, 70% were targetable with a base editor. The most common pathogenic variant was c.2843G>A, p.Cys948Arg, which is targetable with an ABE. Since 62% of pathogenic CRB1 SNVs are amenable to correction with a base editor and 87% of these mutations had a suitable PAM site, gene editing represents a promising therapeutic avenue for CRB1-associated retinal degenerations.

Improving the efficiency of CRISPR-Cas12a-based genome editing with site-specific covalent Cas12a-crRNA conjugates

The CRISPR-Cas12a system shows unique features compared with widely used Cas9, making it an attractive and potentially more precise alternative. However, the adoption of this system has been hindered by its relatively low editing efficiency. Guided by physical chemical principles, we covalently conjugated 5' terminal modified CRISPR RNA (crRNA) to a site-specifically modified Cas12a through biorthogonal chemical reaction. The genome editing efficiency of the resulting conjugated Cas12a complex (cCas12a) was substantially higher than that of the wild-type complex. We also demonstrated that cCas12a could be used for precise gene knockin and multiplex gene editing in a chimeric antigen receptor T cell preparation with efficiency much higher than that of the wild-type system. Overall, our findings indicate that covalently linking Cas nuclease and crRNA is an effective approach to improve the Cas12a-based genome editing system and could potentially provide an insight into engineering other Cas family members with low efficiency as well.

Discovery of putative tumor suppressors from CRISPR screens reveals rewired lipid metabolism in acute myeloid leukemia cells

CRISPR knockout fitness screens in cancer cell lines reveal many genes whose loss of function causes cell death or loss of fitness or, more rarely, the opposite phenotype of faster proliferation. Here we demonstrate a systematic approach to identify these proliferation suppressors, which are highly enriched for tumor suppressor genes, and define a network of 145 such genes in 22 modules. One module contains several elements of the glycerolipid biosynthesis pathway and operates exclusively in a subset of acute myeloid leukemia cell lines. The proliferation suppressor activity of genes involved in the synthesis of saturated fatty acids, coupled with a more severe loss of fitness phenotype for genes in the desaturation pathway, suggests that these cells operate at the limit of their carrying capacity for saturated fatty acids, which we confirm biochemically. Overexpression of this module is associated with a survival advantage in juvenile leukemias, suggesting a clinically relevant subtype.

CRISPR-based knockout screens in cancer cells have suggested the existence of proliferation suppressor genes (PSG). Here, the authors develop an approach to systematically identify them, and reveal a PSG module involved in fatty acid synthesis and tumour suppression in acute myeloid leukemia cell lines.

Gene editing to enhance the efficacy of cancer cell therapies

Adoptive T cell therapies have shown impressive signals of activity, but their clinical impact could be enhanced by technologies to increase T cell potency and diminish the cost and labor involved in manufacturing these products. Gene editing platforms are under study in this arena to (1) enhance immune cell potency by knocking out molecules that inhibit immune responses; (2) deliver genetic payloads into precise genomic locations and thereby enhance safety and/or improve the gene expression profile by leveraging physiologic promoters, enhancers, and repressors; and (3) enable off-the-shelf therapies by preventing alloreactivity and immune rejection. This review discusses gene editing approaches that have been the best studied in the context of human T cells and adoptive T cell therapies, summarizing their current status and near-term potential for translation.

BEAR reveals that increased fidelity variants can successfully reduce the mismatch tolerance of adenine but not cytosine base editors

Adenine and cytosine base editors (ABE, CBE) allow for precision genome engineering. Here, Base Editor Activity Reporter (BEAR), a plasmid-based fluorescent tool is introduced, which can be applied to report on ABE and CBE editing in a virtually unrestricted sequence context or to label base edited cells for enrichment. Using BEAR-enrichment, we increase the yield of base editing performed by nuclease inactive base editors to the level of the nickase versions while maintaining significantly lower indel background. Furthermore, by exploiting the semi-high-throughput potential of BEAR, we examine whether increased fidelity SpCas9 variants can be used to decrease SpCas9-dependent off-target effects of ABE and CBE. Comparing them on the same target sets reveals that CBE remains active on sequences, where increased fidelity mutations and/or mismatches decrease the activity of ABE. Our results suggest that the deaminase domain of ABE is less effective to act on rather transiently separated target DNA strands, than that of CBE explaining its lower mismatch tolerance.

trans Single-Stranded DNA Cleavage via CRISPR/Cas14a1 Activated by Target RNA without Destruction

As a CRISPR-Cas system (clustered regularly interspaced short palindromic repeats and CRISPR associated proteins), Cas14a1 can cis/trans cleave single-stranded DNA (ssDNA). Here, we describe an unreported capacity of Cas14a1: RNA can trigger the trans ssDNA cleavage. This Cas14a1-based RNA-activated detection platform (Amplification, Transcription, Cas14a1-based RNA-activated trans ssDNA cleavage, ATCas-RNA) has an outstanding specificity for the detection of target RNAs with point mutation resolution, which is better than that of the Cas14a1-based ssDNA-activation. Using ATCas-RNA via a fluorophore quencher-labeled ssDNA reporter (FQ), we were able to detect 1 aM pathogenic nucleic acid within 1 h, and achieve 100 % accuracy with 25 milk samples. This platform can serve as a new tool for high-efficiency nucleic acid diagnostics. Importantly, this work can expand our understanding of Cas14a1 and inspire further mechanisms and applications of Class-2 Cas systems.

Get ready for the CRISPR/Cas system: A beginner's guide to the engineering and design of guide RNAs

The clustered regularly interspaced short palindromic repeats (CRISPR) system is a state-of-the-art tool for versatile genome editing that has advanced basic research dramatically, with great potential for clinic applications. The system consists of two key molecules: a CRISPR-associated (Cas) effector nuclease and a single guide RNA. The simplicity of the system has enabled the development of a wide spectrum of derivative methods. Almost any laboratory can utilize these methods, although new users may initially be confused when faced with the potentially overwhelming abundance of choices. Cas nucleases and their engineering have been systematically reviewed previously. In the present review, we discuss single guide RNA engineering and design strategies that facilitate more efficient, more specific and safer gene editing.

Controlling Gene Expression in Mammalian Cells Using Multiplexed Conditional Guide RNAs for Cas12a*

Spatiotemporal control of the activity of CRISPR-associated (Cas) proteins is of considerable interest for basic research and therapeutics. Here, we show that conditional guide RNAs (gRNAs) for Cas12a can be transcribed in mammalian cells by RNA polymerase II, followed by activation via input-dependent processing of the 3' tail of the gRNA transcript. We demonstrate processing using an RNA strand displacement mechanism, as well as microRNA-dependent processing, and cleavage by a guanine-responsive ribozyme. We further demonstrate that Cas12a along with several independently switchable gRNAs can be compactly integrated on a single transcript using stabilizing RNA triplexes, providing a route towards Cas12a-based gene regulation constructs with multi-input switching capabilities. The principle is shown to work in HEK and mouse fibroblast cells using luminescence, fluorescence, and is also demonstrated for the conditional upregulation of an endogenous gene.

Engineered miniature CRISPR-Cas system for mammalian genome regulation and editing

Compact and versatile CRISPR-Cas systems will enable genome engineering applications through high-efficiency delivery in a wide variety of contexts. Here, we create an efficient miniature Cas system (CasMINI) engineered from the type V-F Cas12f (Cas14) system by guide RNA and protein engineering, which is less than half the size of currently used CRISPR systems (Cas9 or Cas12a). We demonstrate that CasMINI can drive high levels of gene activation (up to thousands-fold increases), while the natural Cas12f system fails to function in mammalian cells. We show that the CasMINI system has comparable activities to Cas12a for gene activation, is highly specific, and allows robust base editing and gene editing. We expect that CasMINI can be broadly useful for cell engineering and gene therapy applications ex vivo and in vivo.

Engineered DNase-inactive Cpf1 variants to improve targeting scope for base editing in E. coli

The development of base editing (BE) technology has opened a new avenue for research studies in bacteriology, particularly for bacterial species in which the DNA double-strand breaks (DSBs) introduced by CRISPR/Cas system would lead to cell death. However, a major limitation of BE-mediated gene editing is the restricted editable sites in the target bacterial genome due to highly diverse genomic compositions, such as GC content. Herein, we developed a broad-spectrum DNase-inactive Cpf1 (dCpf1) variant from Francisella novicida (bsdFnCpf1) through directed evolution. The resulting optimized mutant showed a substantially expanded targeting range, including previously non-canonical protospacer-adjacent motifs (PAMs), especially the GC-rich PAMs. Cytidine deaminase APOBEC1 and uracil DNA glycosylase inhibitor (UGI) were fused with bsdFnCpf1 to achieve specific C to T mutations at multiple target sites with canonical or non-canonical PAMs in the E. coli genome without compromising cell growth. We anticipate that bsdFnCpf1 could be applied for multiplex gene regulation and BE in species that have been reported to be suitable for Cpf1.

Improving FnCas12a Genome Editing by Exonuclease Fusion

Among current reported Cas12a orthologs, Francisella novicida Cas12a (FnCas12a) is less restricted by protospacer adjacent motif (PAM). However, the activity of FnCas12a nuclease is relatively low or undetectable in human cells, limiting its application as desirable genome engineering tools. Here, we describe TEXT (Tethering EXonuclease T5 with FnCas12a)-a fusion strategy that significantly increased the knockout efficiency of FnCas12a in human cells at multiple genomic loci in three different cell lines. TEXT results in higher insertion and deletion efficiency than FnCas12a under different spacer lengths from 18 nt to 23 nt. Deep sequencing shows that TEXT substantially increased the deletion frequency and deletion size at the targeted locus. Compared to other Cas12a orthologs, including AsCas12a and LbCas12a, TEXT achieves the highest on-targeting efficiency and shows minimal off-targeting effects at all tested sites. TEXT enhances the activity of FnCas12a nuclease and expands its targeting scope and efficiency in human cell genome engineering.

Application of CHyMErA Cas9-Cas12a combinatorial genome-editing platform for genetic interaction mapping and gene fragment deletion screening

CRISPR-based forward genetic screening represents a powerful approach for the systematic characterization of gene function. Recent efforts have been directed toward establishing CRISPR-based tools for the programmable delivery of combinatorial genetic perturbations, most of which are mediated by a single nuclease and the expression of structurally identical guide backbones from two promoters. In contrast, we have developed CHyMErA (Cas hybrid for multiplexed editing and screening applications), which is based on the co-expression of Cas9 and Cas12a nucleases in conjunction with a hybrid guide RNA (hgRNA) engineered by the fusion of Cas9 and Cas12a guides and expressed from a single U6 promoter. CHyMErA is suitable for the high-throughput deletion of genetic segments including the excision of individual exons. Furthermore, CHyMErA enables the concomitant targeting of two or more genes and can thus be used for the systematic mapping of genetic interactions in mammalian cells. CHyMErA can also be applied for the perturbation of paralogous gene pairs, thereby allowing the capturing of phenotypic roles that would otherwise be masked because of genetic redundancy. Here, we provide instructions for the cloning of hgRNA screening libraries and individual hgRNA constructs and offer guidelines for designing and performing combinatorial pooled genetic screens using CHyMErA. Starting with the generation of Cas9- and Cas12a-expressing cell lines, CHyMErA screening can be implemented within 15-20 weeks.

Gene Targeting Facilitated by Engineered Sequence-Specific Nucleases: Potential Applications for Crop Improvement

Humans are currently facing the problem of how to ensure that there is enough food to feed all of the world's population. Ensuring that the food supply is sufficient will likely require the modification of crop genomes to improve their agronomic traits. The development of engineered sequence-specific nucleases (SSNs) paved the way for targeted gene editing in organisms, including plants. SSNs generate a double-strand break (DSB) at the target DNA site in a sequence-specific manner. These DSBs are predominantly repaired via error-prone non-homologous end joining and are only rarely repaired via error-free homology-directed repair if an appropriate donor template is provided. Gene targeting (GT), i.e. the integration or replacement of a particular sequence, can be achieved with combinations of SSNs and repair donor templates. Although its efficiency is extremely low, GT has been achieved in some higher plants. Here, we provide an overview of SSN-facilitated GT in higher plants and discuss the potential of GT as a powerful tool for generating crop plants with desirable features.

CRISPR-Cas Gene Perturbation and Editing in Human Induced Pluripotent Stem Cells

Directing the fates of human pluripotent stem cells (hPSC) to generate a multitude of differentiated cell types allows the study of the genetic regulation of human development and disease. The translational potential of hPSC is maximized by exploiting CRISPR to silence or activate genes with spatial and temporal precision permanently or reversibly. Here, we summarize the increasingly refined and diverse CRISPR toolkit for the latter forms of gene perturbation in hPSC and their downstream applications. We discuss newer methods to install edits efficiently with single nucleotide resolution and describe pooled CRISPR screens as a powerful means of unbiased discovery of genes associated with a phenotype of interest. Last, we discuss the potential of these combined technologies in the treatment of hitherto intractable human diseases and the challenges to their implementation in the clinic.

Advances in genomics and genome editing for breeding next generation of fruit and nut crops

Fruit tree crops are an essential part of the food production systems and are key to achieve food and nutrition security. Genetic improvement of fruit trees by conventional breeding has been slow due to the long juvenile phase. Advancements in genomics and molecular biology have paved the way for devising novel genetic improvement tools like genome editing, which can accelerate the breeding of these perennial crops to a great extent. In this article, advancements in genomics of fruit trees covering genome sequencing, transcriptome sequencing, genome editing technologies (GET), CRISPR-Cas system based genome editing, potential applications of CRISPR-Cas9 in fruit tree crops improvement, the factors influencing the CRISPR-Cas editing efficiency and the challenges for CRISPR-Cas9 applications in fruit tree crops improvement are reviewed. Besides, base editing, a recently emerging more precise editing system, and the future perspectives of genome editing in the improvement of fruit and nut crops are covered.

Enhanced Cas12a multi-gene regulation using a CRISPR array separator

The type V-A Cas12a protein can process its CRISPR array, a feature useful for multiplexed gene editing and regulation. However, CRISPR arrays often exhibit unpredictable performance due to interference between multiple guide RNA (gRNAs). Here, we report that Cas12a array performance is hypersensitive to the GC content of gRNA spacers, as high-GC spacers can impair activity of the downstream gRNA. We analyze naturally occurring CRISPR arrays and observe that natural repeats always contain an AT-rich fragment that separates gRNAs, which we term a CRISPR separator. Inspired by this observation, we design short, AT-rich synthetic separators (synSeparators) that successfully remove the disruptive effects between gRNAs. We further demonstrate enhanced simultaneous activation of seven endogenous genes in human cells using an array containing the synSeparator. These results elucidate a previously underexplored feature of natural CRISPR arrays and demonstrate how nature-inspired engineering solutions can improve multi-gene control in mammalian cells.

Forward and Reverse Genetics of B Cell Malignancies: From Insertional Mutagenesis to CRISPR-Cas

Cancer genome sequencing has identified dozens of mutations with a putative role in lymphomagenesis and leukemogenesis. Validation of driver mutations responsible for B cell neoplasms is complicated by the volume of mutations worthy of investigation and by the complex ways that multiple mutations arising from different stages of B cell development can cooperate. Forward and reverse genetic strategies in mice can provide complementary validation of human driver genes and in some cases comparative genomics of these models with human tumors has directed the identification of new drivers in human malignancies. We review a collection of forward genetic screens performed using insertional mutagenesis, chemical mutagenesis and exome sequencing and discuss how the high coverage of subclonal mutations in insertional mutagenesis screens can identify cooperating mutations at rates not possible using human tumor genomes. We also compare a set of independently conducted screens from Pax5 mutant mice that converge upon a common set of mutations observed in human acute lymphoblastic leukemia (ALL). We also discuss reverse genetic models and screens that use CRISPR-Cas, ORFs and shRNAs to provide high throughput in vivo proof of oncogenic function, with an emphasis on models using adoptive transfer of ex vivo cultured cells. Finally, we summarize mouse models that offer temporal regulation of candidate genes in an in vivo setting to demonstrate the potential of their encoded proteins as therapeutic targets.

Precise plant genome editing using base editors and prime editors

The development of CRISPR-Cas systems has sparked a genome editing revolution in plant genetics and breeding. These sequence-specific RNA-guided nucleases can induce DNA double-stranded breaks, resulting in mutations by imprecise non-homologous end joining (NHEJ) repair or precise DNA sequence replacement by homology-directed repair (HDR). However, HDR is highly inefficient in many plant species, which has greatly limited precise genome editing in plants. To fill the vital gap in precision editing, base editing and prime editing technologies have recently been developed and demonstrated in numerous plant species. These technologies, which are mainly based on Cas9 nickases, can introduce precise changes into the target genome at a single-base resolution. This Review provides a timely overview of the current status of base editors and prime editors in plants, covering both technological developments and biological applications.

Highly Multiplexed Analysis of CRISPR Genome Editing Outcomes in Mammalian Cells

CRISPR-Cas-based genome editing has enabled efficient genetic engineering of a range of organisms and sparked revolutions in many fields of biology. After Streptococcus pyogenes Cas9 was first demonstrated for mammalian genome editing, many CRISPR-associated (Cas) protein variants have been isolated from different species and adopted for genome editing. Furthermore, various effector domains have been fused to these Cas proteins to expand their genome-editing abilities. Although the number of genome-editing tools has been rapidly increasing, the throughput of cell-based characterization of new genome-editing tools remains limited. Here we describe a highly multiplexed genome editing and sequencing library preparation protocol that allows high-resolution analysis of mutation outcomes and frequencies induced by hundreds to thousands of different genome-editing reagents in mammalian cells. We have successful experiences of developing several key genome-editing tools using this protocol. The protocol also is designed to be compatible with robotic liquid handling systems for further scalability.

NNT mediates redox-dependent pigmentation via a UVB- and MITF-independent mechanism

Ultraviolet (UV) light and incompletely understood genetic and epigenetic variations determine skin color. Here we describe an UV- and microphthalmia-associated transcription factor (MITF)-independent mechanism of skin pigmentation. Targeting the mitochondrial redox-regulating enzyme nicotinamide nucleotide transhydrogenase (NNT) resulted in cellular redox changes that affect tyrosinase degradation. These changes regulate melanosome maturation and, consequently, eumelanin levels and pigmentation. Topical application of small-molecule inhibitors yielded skin darkening in human skin, and mice with decreased NNT function displayed increased pigmentation. Additionally, genetic modification of NNT in zebrafish alters melanocytic pigmentation. Analysis of four diverse human cohorts revealed significant associations of skin color, tanning, and sun protection use with various single-nucleotide polymorphisms within NNT. NNT levels were independent of UVB irradiation and redox modulation. Individuals with postinflammatory hyperpigmentation or lentigines displayed decreased skin NNT levels, suggesting an NNT-driven, redox-dependent pigmentation mechanism that can be targeted with NNT-modifying topical drugs for medical and cosmetic purposes.

DNA interference states of the hypercompact CRISPR-CasΦ effector

CRISPR-CasΦ, a small RNA-guided enzyme found uniquely in bacteriophages, achieves programmable DNA cutting as well as genome editing. To investigate how the hypercompact enzyme recognizes and cleaves double-stranded DNA, we determined cryo-EM structures of CasΦ (Cas12j) in pre- and post-DNA-binding states. The structures reveal a streamlined protein architecture that tightly encircles the CRISPR RNA and DNA target to capture, unwind and cleave DNA. Comparison of the pre- and post-DNA-binding states reveals how the protein rearranges for DNA cleavage upon target recognition. On the basis of these structures, we created and tested mutant forms of CasΦ that cut DNA up to 20-fold faster relative to wild type, showing how this system may be naturally attenuated to improve the fidelity of DNA interference. The structural and mechanistic insights into how CasΦ binds and cleaves DNA should allow for protein engineering for both in vitro diagnostics and genome editing.

Novel CRISPR/Cas applications in plants: from prime editing to chromosome engineering

In the last years, tremendous progress has been made in the development of CRISPR/Cas-mediated genome editing tools. A number of natural CRISPR/Cas nuclease variants have been characterized. Engineered Cas proteins have been developed to minimize PAM restrictions, off-side effects and temperature sensitivity. Both kinds of enzymes have, by now, been applied widely and efficiently in many plant species to generate either single or multiple mutations at the desired loci by multiplexing. In addition to DSB-induced mutagenesis, specifically designed CRISPR/Cas systems allow more precise gene editing, resulting not only in random mutations but also in predefined changes. Applications in plants include gene targeting by homologous recombination, base editing and, more recently, prime editing. We will evaluate these different technologies for their prospects and practical applicability in plants. In addition, we will discuss a novel application of the Cas9 nuclease in plants, enabling the induction of heritable chromosomal rearrangements, such as inversions and translocations. This technique will make it possible to change genetic linkages in a programmed way and add another level of genome engineering to the toolbox of plant breeding. Also, strategies for tissue culture free genome editing were developed, which might be helpful to overcome the transformation bottlenecks in many crops. All in all, the recent advances of CRISPR/Cas technology will help agriculture to address the challenges of the twenty-first century related to global warming, pollution and the resulting food shortage.

Small-molecule compounds boost genome-editing efficiency of cytosine base editor

Cytosine base editor (CBE) enables targeted C-to-T conversions at single base-pair resolution and thus has potential therapeutic applications in humans. However, the low efficiency of the system limits practical use of this approach. We reported a high-throughput human cells-based reporter system that can be harnessed for quickly measuring editing activity of CBE. Screening of 1813 small-molecule compounds resulted in the identification of Ricolinostat (an HDAC6 inhibitor) that can enhance the efficiency of BE3 in human cells (2.45- to 9.21-fold improvement). Nexturastat A, another HDAC6 inhibitor, could also increase BE3-mediated gene editing by 2.18- to 9.95-fold. Ricolinostat and Nexturastat A also boost base editing activity of the other CBE variants (BE4max, YE1-BE4max, evoAPOBEC1-BE4max and SpRY-CBE4max, up to 8.32-fold). Meanwhile, combined application of BE3 and Ricolinostat led to >3-fold higher efficiency of correcting a pathogenic mutation in ABCA4 gene related to Stargardt disease in human cells. Moreover, we demonstrated that our strategy could be applied for efficient generation of mouse models through direct zygote injection and base editing in primary human T cells. Our study provides a new strategy to improve the activity and specificity of CBE in human cells. Ricolinostat and Nexturastat A augment the effectiveness and applicability of CBE.

Next-Generation Breeding Strategies for Climate-Ready Crops

Climate change is a threat to global food security due to the reduction of crop productivity around the globe. Food security is a matter of concern for stakeholders and policymakers as the global population is predicted to bypass 10 billion in the coming years. Crop improvement via modern breeding techniques along with efficient agronomic practices innovations in microbiome applications, and exploiting the natural variations in underutilized crops is an excellent way forward to fulfill future food requirements. In this review, we describe the next-generation breeding tools that can be used to increase crop production by developing climate-resilient superior genotypes to cope with the future challenges of global food security. Recent innovations in genomic-assisted breeding (GAB) strategies allow the construction of highly annotated crop pan-genomes to give a snapshot of the full landscape of genetic diversity (GD) and recapture the lost gene repertoire of a species. Pan-genomes provide new platforms to exploit these unique genes or genetic variation for optimizing breeding programs. The advent of next-generation clustered regularly interspaced short palindromic repeat/CRISPR-associated (CRISPR/Cas) systems, such as prime editing, base editing, and de nova domestication, has institutionalized the idea that genome editing is revamped for crop improvement. Also, the availability of versatile Cas orthologs, including Cas9, Cas12, Cas13, and Cas14, improved the editing efficiency. Now, the CRISPR/Cas systems have numerous applications in crop research and successfully edit the major crop to develop resistance against abiotic and biotic stress. By adopting high-throughput phenotyping approaches and big data analytics tools like artificial intelligence (AI) and machine learning (ML), agriculture is heading toward automation or digitalization. The integration of speed breeding with genomic and phenomic tools can allow rapid gene identifications and ultimately accelerate crop improvement programs. In addition, the integration of next-generation multidisciplinary breeding platforms can open exciting avenues to develop climate-ready crops toward global food security.

A Cas12a-based CRISPR interference system for multigene regulation in mycobacteria

Mycobacteria are responsible for a heavy global disease burden, but their relative genetic intractability has long frustrated research efforts. The introduction of clustered regularly interspaced short palindromic repeats (CRISPR) interference (CRISPRi) has made gene repression in mycobacteria much more efficient, but limitations of the prototypical Cas9-based platform, for example, in multigene regulation, remain. Here, we introduce an alternative CRISPRi platform for mycobacteria that is based on the minimal type V Cas12a enzyme in combination with synthetic CRISPR arrays. This system is simple, tunable, reversible, can efficiently regulate essential genes and multiple genes simultaneously, and works as efficiently in infected macrophages as it does in vitro. Together, Cas12a-based CRISPRi provides a facile tool to probe higher-order genetic interactions in mycobacteria including Mycobacterium tuberculosis (Mtb), which will enable the development of synthetically lethal drug targets and the study of genes conditionally essential during infection.

Astrocytic interleukin-3 programs microglia and limits Alzheimer's disease

Communication within the glial cell ecosystem is essential for neuronal and brain health1-3. The influence of glial cells on the accumulation and clearance of β-amyloid (Aβ) and neurofibrillary tau in the brains of individuals with Alzheimer's disease (AD) is poorly understood, despite growing awareness that these are therapeutically important interactions4,5. Here we show, in humans and mice, that astrocyte-sourced interleukin-3 (IL-3) programs microglia to ameliorate the pathology of AD. Upon recognition of Aβ deposits, microglia increase their expression of IL-3Rα-the specific receptor for IL-3 (also known as CD123)-making them responsive to IL-3. Astrocytes constitutively produce IL-3, which elicits transcriptional, morphological, and functional programming of microglia to endow them with an acute immune response program, enhanced motility, and the capacity to cluster and clear aggregates of Aβ and tau. These changes restrict AD pathology and cognitive decline. Our findings identify IL-3 as a key mediator of astrocyte-microglia cross-talk and a node for therapeutic intervention in AD.

AsCas12a ultra nuclease facilitates the rapid generation of therapeutic cell medicines

Though AsCas12a fills a crucial gap in the current genome editing toolbox, it exhibits relatively poor editing efficiency, restricting its overall utility. Here we isolate an engineered variant, "AsCas12a Ultra", that increased editing efficiency to nearly 100% at all sites examined in HSPCs, iPSCs, T cells, and NK cells. We show that AsCas12a Ultra maintains high on-target specificity thereby mitigating the risk for off-target editing and making it ideal for complex therapeutic genome editing applications. We achieved simultaneous targeting of three clinically relevant genes in T cells at >90% efficiency and demonstrated transgene knock-in efficiencies of up to 60%. We demonstrate site-specific knock-in of a CAR in NK cells, which afforded enhanced anti-tumor NK cell recognition, potentially enabling the next generation of allogeneic cell-based therapies in oncology. AsCas12a Ultra is an advanced CRISPR nuclease with significant advantages in basic research and in the production of gene edited cell medicines.

Recent advances in CRISPR technologies for genome editing

The discovery of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) system, and its development into a set of powerful tools for manipulating the genome, has revolutionized genome editing. Precise, targeted CRISPR/Cas-based genome editing has become the most widely used platform in organisms ranging from plants to animals. The CRISPR/Cas system has been extensively modified to increase its efficiency and fidelity. In addition, the fusion of various protein motifs to Cas effector proteins has facilitated diverse set of genetic manipulations, such as base editing, transposition, recombination, and epigenetic regulation. The CRISPR/Cas system is undergoing continuous development to overcome current limitations, including off-target effects, narrow targeting scope, and issues associated with the delivery of CRISPR components for genome engineering and therapeutic approaches. Here, we review recent progress in a diverse array of CRISPR/Cas-based tools. We also describe limitations and concerns related to the use of CRISPR/Cas technologies.

CRISPAltRations: a validated cloud-based approach for interrogation of double-strand break repair mediated by CRISPR genome editing

CRISPR systems enable targeted genome editing in a wide variety of organisms by introducing single- or double-strand DNA breaks, which are repaired using endogenous molecular pathways. Characterization of on- and off-target editing events from CRISPR proteins can be evaluated using targeted genome resequencing. We characterized DNA repair fingerprints that result from non-homologous end joining (NHEJ) after double-stranded breaks (DSBs) were introduced by Cas9 or Cas12a for >500 paired treatment/control experiments. We found that building biological understanding of the repair into a novel analysis tool (CRISPAltRations) improved the quality of the results. We validated our software using simulated, targeted amplicon sequencing data (11 guide RNAs [gRNAs] and 603 on- and off-target locations) and demonstrated that CRISPAltRations outperforms other publicly available software tools in accurately annotating CRISPR-associated indels and homology-directed repair (HDR) events. We enable non-bioinformaticians to use CRISPAltRations by developing a web-accessible, cloud-hosted deployment, which allows rapid batch processing of samples in a graphical user interface (GUI) and complies with HIPAA security standards. By ensuring that our software is thoroughly tested, version controlled, and supported with a user interface (UI), we enable resequencing analysis of CRISPR genome editing experiments to researchers no matter their skill in bioinformatics.

Applications and Major Achievements of Genome Editing in Vegetable Crops: A Review

The emergence of genome-editing technology has allowed manipulation of DNA sequences in genomes to precisely remove or replace specific sequences in organisms resulting in targeted mutations. In plants, genome editing is an attractive method to alter gene functions to generate improved crop varieties. Genome editing is thought to be simple to use and has a lower risk of off-target effects compared to classical mutation breeding. Furthermore, genome-editing technology tools can also be applied directly to crops that contain complex genomes and/or are not easily bred using traditional methods. Currently, highly versatile genome-editing tools for precise and predictable editing of almost any locus in the plant genome make it possible to extend the range of application, including functional genomics research and molecular crop breeding. Vegetables are essential nutrient sources for humans and provide vitamins, minerals, and fiber to diets, thereby contributing to human health. In this review, we provide an overview of the brief history of genome-editing technologies and the components of genome-editing tool boxes, and illustrate basic modes of operation in representative systems. We describe the current and potential practical application of genome editing for the development of improved nutritious vegetables and present several case studies demonstrating the potential of the technology. Finally, we highlight future directions and challenges in applying genome-editing systems to vegetable crops for research and product development.

RNA transcripts stimulate homologous recombination by forming DR-loops

Homologous recombination (HR) repairs DNA double-strand breaks (DSBs) in the S and G2 phases of the cell cycle1-3. Several HR proteins are preferentially recruited to DSBs at transcriptionally active loci4-10, but how transcription promotes HR is poorly understood. Here we develop an assay to assess the effect of local transcription on HR. Using this assay, we find that transcription stimulates HR to a substantial extent. Tethering RNA transcripts to the vicinity of DSBs recapitulates the effects of local transcription, which suggests that transcription enhances HR through RNA transcripts. Tethered RNA transcripts stimulate HR in a sequence- and orientation-dependent manner, indicating that they function by forming DNA-RNA hybrids. In contrast to most HR proteins, RAD51-associated protein 1 (RAD51AP1) only promotes HR when local transcription is active. RAD51AP1 drives the formation of R-loops in vitro and is required for tethered RNAs to stimulate HR in cells. Notably, RAD51AP1 is necessary for the DSB-induced formation of DNA-RNA hybrids in donor DNA, linking R-loops to D-loops. In vitro, RAD51AP1-generated R-loops enhance the RAD51-mediated formation of D-loops locally and give rise to intermediates that we term 'DR-loops', which contain both DNA-DNA and DNA-RNA hybrids and favour RAD51 function. Thus, at DSBs in transcribed regions, RAD51AP1 promotes the invasion of RNA transcripts into donor DNA, and stimulates HR through the formation of DR-loops.

Detect and destroy: CRISPR-based technologies for the response against viruses

Despite numerous viral outbreaks in the last decade, including a devastating global pandemic, diagnostic and therapeutic technologies remain severely lacking. CRISPR-Cas systems have the potential to address these critical needs in the response against infectious disease. Initially discovered as the bacterial adaptive immune system, these systems provide a unique opportunity to create programmable, sequence-specific technologies for detection of viral nucleic acids and inhibition of viral replication. This review summarizes how CRISPR-Cas systems-in particular the recently discovered DNA-targeting Cas12 and RNA-targeting Cas13, both possessing a unique trans-cleavage activity-are being harnessed for viral diagnostics and therapies. We further highlight the numerous technologies whose development has accelerated in response to the COVID-19 pandemic.

CRISPR-Cas Tools and Their Application in Genetic Engineering of Human Stem Cells and Organoids

CRISPR-Cas technology has revolutionized biological research and holds great therapeutic potential. Here, we review CRISPR-Cas systems and their latest developments with an emphasis on application to human cells. We also discuss how different CRISPR-based strategies can be used to accomplish a particular genome engineering goal. We then review how different CRISPR tools have been used in genome engineering of human stem cells in vitro, covering both the pluripotent (iPSC/ESC) and somatic adult stem cell fields and, in particular, 3D organoid cultures. Finally, we discuss the progress and challenges associated with CRISPR-based genome editing of human stem cells for therapeutic use.

CRISPR/Cas-Based Epigenome Editing: Advances, Applications, and Clinical Utility

The epigenome dynamically regulates gene expression and guides cellular differentiation throughout the lifespan of eukaryotic organisms. Recent advances in clustered regularly interspaced palindromic repeats (CRISPR)/Cas-based epigenome editing technologies have enabled researchers to site-specifically program epigenetic modifications to endogenous DNA and histones and to manipulate the architecture of native chromatin. As a result, epigenome editing has helped to uncover the causal relationships between epigenetic marks and gene expression. As epigenome editing tools have continued to develop, researchers have applied them in new ways to explore the function of the epigenome in human health and disease. In this review, we discuss the recent technical improvements in CRISPR/Cas-based epigenome editing that have advanced clinical research and examine how these technologies could be improved for greater future utility.

Cas12a Base Editors Induce Efficient and Specific Editing with Low DNA Damage Response

The advent of base editors (BEs) holds great potential for correcting pathogenic-related point mutations to treat relevant diseases. However, Cas9 nickase (nCas9)-derived BEs lead to DNA double-strand breaks, which can trigger unwanted DNA damage response (DDR). Here, we show that the original version of catalytically dead Cas12a (dCas12a)-conjugated BEs induce a basal level of DNA breaks and minimally activate DDR proteins, including H2AX, ATM, ATR, and p53. By fusing dCas12a with engineered human apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3A (APOBEC3A), we further develop the BEACON (base editing induced by human APOBEC3A and Cas12a without DNA break) system to achieve enhanced deamination efficiency and editing specificity. Efficient C-to-T editing is achieved by BEACON in mammalian cells at levels comparable to AncBE4max, with only low levels of DDR and minimal RNA off-target mutations. Importantly, BEACON induces in vivo base editing in mouse embryos, and targeted C-to-T conversions are detected in F0 mice.

Saccharomyces cerevisiae Synthetic Transcriptional Networks Harnessing dCas12a and Type V-A anti-CRISPR Proteins

Type V-A anti-CRISPR proteins (AcrVAs) represent the response from phages to the CRISPR-Cas12a prokaryotic immune system. CRISPR-Cas12a was repurposed, in high eukaryotes, to carry out gene editing and transcription regulation, the latter via a nuclease-dead Cas12a (dCas12a). Consequently, AcrVAs were adopted to regulate (d)Cas12a activity. However, the usage of both dCas12a-based transcription factors and AcrVAs in the yeast Saccharomyces cerevisiae has not been explored. In this work, we show that, in the baker's yeast, two dCas12a proteins (denAsCas12a and dLbCas12a) work both as activators (upon fusion to a strong activation domain) and repressors, whereas dMbCa12a is nonfunctional. The activation efficiency of dCas12a-ADs manifests a dependence on the number of crRNA binding sites, whereas it is not directly correlated to the amount of crRNA in the cells. Moreover, AcrVA1, AcrVA4, and AcrVA5 are able to inhibit dLbCa12a in yeast, and denAsCas12a is only inhibited by AcrVA1. However, AcrVA1 performs well at high concentration only. Coexpression of two or three AcrVAs does not enhance inhibition of dCas12a(-AD), suggesting a competition between different AcrVAs. Further, AcrVA4 significantly limits gene editing by LbCas12a. Overall, our results indicate that dCas12a:crRNA and AcrVA proteins are highly performant components in S. cerevisiae synthetic transcriptional networks.

CRISPR-based detection of SARS-CoV-2: A review from sample to result

The current pandemic of the 2019 novel coronavirus (COVID-19) caused by SARS-CoV-2 (severe acute respiratory syndrome coronavirus-2) has raised significant public health concern. Rapid, affordable, and accurate diagnostics of SARS-CoV-2 is essential for early treatment and control of the disease spread. In the past few years, CRISPR technology has shown great potential for highly sensitive and specific molecular diagnostics. Amid the ongoing COVID-19 pandemic, there is an increasing interest in implementing CRISPR-based diagnostic principles to develop fast and precise methods for detecting SARS-CoV-2. In this work, we reviewed and summarized these CRISPR-based diagnostic systems as well as their characteristics and challenges. We also provided future perspectives of CRISPR-based sensing towards point-of-care molecular diagnosis applications.

COPII collar defines the boundary between ER and ER exit site and does not coat cargo containers

COPII and COPI mediate the formation of membrane vesicles translocating in opposite directions within the secretory pathway. Live-cell and electron microscopy revealed a novel mode of function for COPII during cargo export from the ER. COPII is recruited to membranes defining the boundary between the ER and ER exit sites, facilitating selective cargo concentration. Using direct observation of living cells, we monitored cargo selection processes, accumulation, and fission of COPII-free ERES membranes. CRISPR/Cas12a tagging, the RUSH system, and pharmaceutical and genetic perturbations of ER-Golgi transport demonstrated that the COPII coat remains bound to the ER–ERES boundary during protein export. Manipulation of the cargo-binding domain in COPII Sec24B prohibits cargo accumulation in ERES. These findings suggest a role for COPII in selecting and concentrating exported cargo rather than coating Golgi-bound carriers. These findings transform our understanding of coat proteins’ role in ER-to-Golgi transport.

A short overview of CRISPR-Cas technology and its application in viral disease control

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) together with CRISPR-associated (Cas) proteins have catalysed a revolution in genetic engineering. Native CRISPR-Cas systems exist in many bacteria and archaea where they provide an adaptive immune response through sequence-specific degradation of an invading pathogen's genome. This system has been reconfigured for use in genome editing, drug development, gene expression regulation, diagnostics, the prevention and treatment of cancers, and the treatment of genetic and infectious diseases. In recent years, CRISPR-Cas systems have been used in the diagnosis and control of viral diseases, for example, CRISPR-Cas12/13 coupled with new amplification techniques to improve the specificity of sequence-specific fluorescent probe detection. Importantly, CRISPR applications are both sensitive and specific and usually only require commonly available lab equipment. Unlike the canonical Cas9 which is guided to double-stranded DNA sites of interest, Cas13 systems target RNA sequences and thus can be employed in strategies directed against RNA viruses or for transcriptional silencing. Many challenges remain for these approach, including issues with specificity and the requirement for better mammalian delivery systems. In this review, we summarize the applications of CRISPR-Cas systems in controlling mammalian viral infections. Following necessary improvements, it is expected that CRISPR-Cas systems will be used effectively for such applications in the future.

CRISPR technologies for the treatment of Duchenne muscular dystrophy

The emerging clustered regularly interspaced short palindromic repeats (CRISPR)-mediated genome editing technologies have progressed remarkably in recent years, opening up the potential of precise genome editing as a therapeutic approach to treat various diseases. The CRISPR-CRISPR-associated (Cas) system is an attractive platform for the treatment of Duchenne muscular dystrophy (DMD), which is a neuromuscular disease caused by mutations in the DMD gene. CRISPR-Cas can be used to permanently repair the mutated DMD gene, leading to the expression of the encoded protein, dystrophin, in systems ranging from cells derived from DMD patients to animal models of DMD. However, the development of more efficient therapeutic approaches and delivery methods remains a great challenge for DMD. Here, we review various therapeutic strategies that use CRISPR-Cas to correct or bypass DMD mutations and discuss their therapeutic potential, as well as obstacles that lie ahead.

Recent advances of Cas12a applications in bacteria

Clustered regularly interspaced short palindromic repeats (CRISPR)-mediated genome engineering and related technologies have revolutionized biotechnology over the last decade by enhancing the efficiency of sophisticated biological systems. Cas12a (Cpf1) is an RNA-guided endonuclease associated to the CRISPR adaptive immune system found in many prokaryotes. Contrary to its more prominent counterpart Cas9, Cas12a recognizes A/T rich DNA sequences and is able to process its corresponding guide RNA directly, rendering it a versatile tool for multiplex genome editing efforts and other applications in biotechnology. While Cas12a has been extensively used in eukaryotic cell systems, microbial applications are still limited. In this review, we highlight the mechanistic and functional differences between Cas12a and Cas9 and focus on recent advances of applications using Cas12a in bacterial hosts. Furthermore, we discuss advantages as well as current challenges and give a future outlook for this promising alternative CRISPR-Cas system for bacterial genome editing and beyond. KEY POINTS: • Cas12a is a powerful tool for genome engineering and transcriptional perturbation • Cas12a causes less toxic side effects in bacteria than Cas9 • Self-processing of crRNA arrays facilitates multiplexing approaches.