Empower multiplex cell and tissue-specific CRISPR-mediated gene manipulation with self-cleaving ribozymes and tRNA

An Efficient Vector-Based CRISPR/Cas9 System in Zebrafish Cell Line

The clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system has been widely applied in animals as an efficient genome editing tool. However, the technique is difficult to implement in fish cell lines partially due to the lack of efficient promoters to drive the expression of both sgRNA and the Cas9 protein within a single vector. In this study, it was indicated that the zebrafish U6 RNA polymerase III (ZFU6) promoter could efficiently induce tyrosinase (tyr) gene editing and lead to loss of retinal pigments when co-injection with Cas9 mRNA in zebrafish embryo. Furthermore, an optimized all-in-one vector for expression of the CRISPR/Cas9 system in the zebrafish fibroblast cell line (PAC2) was constructed by replacing the human U6 promoter with ZFU6 promoter, basing on the lentiCRISPRV2 system that widely applied in mammal cells. This new vector could successfully target the cellular communication network factor 2a (ctgfa) gene and demonstrated its function in the PAC2 cell. Notably, the vector could also be used to edit the endogenous EMX1 gene in the mammal 293 T cell line, implying its wide application potential. In conclusion, we established a new gene editing tool for zebrafish cell line, which could be a useful in vitro platform for high-throughput analyzing gene function in fish.

Transforming the CRISPR/dCas9-based gene regulation technique into a forward screening tool in Plasmodium falciparum

It is a significant challenge to assess the functions of many uncharacterized genes in human malaria parasites. Here, we present a genetic screening tool to assess the contribution of essential genes from Plasmodium falciparum by the conditional CRISPR-/deadCas9-based interference and activation (i/a) systems. We screened both CRISPRi and CRISPRa sets, consisting of nine parasite lines per set targeting nine genes via their respective gRNAs. By conducting amplicon sequencing of gRNA loci, we identified the contribution of each targeted gene to parasite fitness upon drug (artemisinin, chloroquine) and stress (starvation, heat shock) treatment. The screening was highly reproducible, and the screening libraries were easily generated by transfection of mixed plasmids expressing different gRNAs. We demonstrated that this screening is straightforward, robust, and can provide a fast and efficient tool to study essential genes that have long presented a bottleneck in assessing their functions using existing genetic tools.

CROPseq-multi: a versatile solution for multiplexed perturbation and decoding in pooled CRISPR screens

Forward genetic screens seek to dissect complex biological systems by systematically perturbing genetic elements and observing the resulting phenotypes. While standard screening methodologies introduce individual perturbations, multiplexing perturbations improves the performance of single-target screens and enables combinatorial screens for the study of genetic interactions. Current tools for multiplexing perturbations are incompatible with pooled screening methodologies that require mRNA-embedded barcodes, including some microscopy and single cell sequencing approaches. Here, we report the development of CROPseq-multi, a CROPseq1-inspired lentiviral system to multiplex Streptococcus pyogenes (Sp) Cas9-based perturbations with mRNA-embedded barcodes. CROPseq-multi has equivalent per-guide activity to CROPseq and low lentiviral recombination frequencies. CROPseq-multi is compatible with enrichment screening methodologies and optical pooled screens, and is extensible to screens with single-cell sequencing readouts. For optical pooled screens, an optimized and multiplexed in situ detection protocol improves barcode detection efficiency 10-fold, enables detection of recombination events, and increases decoding efficiency 3-fold relative to CROPseq. CROPseq-multi is a widely applicable multiplexing solution for diverse SpCas9-based genetic screening approaches.

Paving the way for future gene therapies: A case study of scientific spillover from delandistrogene moxeparvovec

Gene therapies have potential to improve outcomes of severe diseases after only a single administration. Novel therapies are continually being developed using knowledge gained from prior successes, a concept known as scientific spillover. Gene therapy advancement requires extensive development at each stage: preclinical work to create and evaluate vehicles for delivery of the therapy, design of clinical development programs, and establishment of a large-scale manufacturing process. Pioneering gene therapies are generating spillover as investigators confront myriad issues specific to this treatment modality. These include frameworks for construct engineering, dose evaluation, patient selection, outcome assessment, and safety monitoring. Consequently, the benefits of these therapies extend beyond offering knowledge for treating any one disease to establishing new platforms and paradigms that will accelerate advancement of future gene therapies. This impact is even more profound in rare diseases, where developing therapies in isolation may not be possible. This review describes some instances of scientific spillover in healthcare, and specifically gene therapy, using delandistrogene moxeparvovec (SRP-9001), a gene therapy recently approved by the US Food and Drug Administration for the treatment of ambulatory pediatric patients aged 4-5 years with Duchenne muscular dystrophy with a confirmed mutation in the DMD gene, as a case study.

Cas13d-mediated multiplex RNA targeting confers a broad-spectrum resistance against RNA viruses in potato

CRISPR-Cas systems endow the bacterial and archaeal species with adaptive immune mechanisms to fend off invading phages and foreign plasmids. The class 2 type VI CRISPR/Cas effector Cas13d has been harnessed to confer the protection against RNA viruses in diverse eukaryotic species. However a vast number of different viruses can potentially infect the same host plant resulting in mixed infection, thus necessitating the generation of crops with broad-spectrum resistance to multiple viruses. Here we report the repurposing of CRISPR/Cas13d coupled with an endogenous tRNA-processing system (polycistronic tRNA-gRNA, PTG) to target the multiple potato RNA viruses. Expression of Cas13d and four different gRNAs were observed in transgenic potato lines expressing the Cas13d/PTG construct. We show that the Cas13d/PTG transgenic plants exhibit resistance to either PVY, PVS, PVX or PLRV alone or two/three viruses simultaneously by reducing viral accumulation in plant cells. In sum, our findings provide an efficient strategy for engineering crops that can simultaneously resist infection by multiple RNA viruses.

Intracellular RNA and DNA tracking by uridine-rich internal loop tagging with fluorogenic bPNA

The most widely used method for intracellular RNA fluorescence labeling is MS2 labeling, which generally relies on the use of multiple protein labels targeted to multiple RNA (MS2) hairpin structures installed on the RNA of interest (ROI). While effective and conveniently applied in cell biology labs, the protein labels add significant mass to the bound RNA, which potentially impacts steric accessibility and native RNA biology. We have previously demonstrated that internal, genetically encoded, uridine-rich internal loops (URILs) comprised of four contiguous UU pairs (8 nt) in RNA may be targeted with minimal structural perturbation by triplex hybridization with 1 kD bifacial peptide nucleic acids (bPNAs). A URIL-targeting strategy for RNA and DNA tracking would avoid the use of cumbersome protein fusion labels and minimize structural alterations to the RNA of interest. Here we show that URIL-targeting fluorogenic bPNA probes in cell media can penetrate cell membranes and effectively label RNAs and RNPs in fixed and live cells. This method, which we call fluorogenic U-rich internal loop (FLURIL) tagging, was internally validated through the use of RNAs bearing both URIL and MS2 labeling sites. Notably, a direct comparison of CRISPR-dCas labeled genomic loci in live U2OS cells revealed that FLURIL-tagged gRNA yielded loci with signal to background up to 7X greater than loci targeted by guide RNA modified with an array of eight MS2 hairpins. Together, these data show that FLURIL tagging provides a versatile scope of intracellular RNA and DNA tracking while maintaining a light molecular footprint and compatibility with existing methods.

Self-cleaving guide RNAs enable pharmacological selection of precise gene editing events in vivo

Expression of guide RNAs in the CRISPR/Cas9 system typically requires the use of RNA polymerase III promoters, which are not cell-type specific. Flanking the gRNA with self-cleaving ribozyme motifs to create a self-cleaving gRNA overcomes this limitation. Here, we use self-cleaving gRNAs to create drug-selectable gene editing events in specific hepatocyte loci. A recombinant Adeno Associated Virus vector targeting the Albumin locus with a promoterless self-cleaving gRNA to create drug resistance is linked in cis with the therapeutic transgene. Gene expression of both are dependent on homologous recombination into the target locus. In vivo drug selection for the precisely edited hepatocytes allows >30-fold expansion of gene-edited cells and results in therapeutic levels of a human Factor 9 transgene. Importantly, self-cleaving gRNA expression is also achieved after targeting weak hepatocyte genes. We conclude that self-cleaving gRNAs are a powerful system to enable cell-type specific in vivo drug resistance for therapeutic gene editing applications.

Modular, Synthetic Boolean Logic Gates Enabled in Saccharomyces cerevisiae through T7 Polymerases/CRISPR dCas9 Designs

Synthetic control of gene expression, whether simply promoter selection or higher-order Boolean-style logic, is an important tool for metabolic engineering and synthetic biology. This work develops a suite of orthogonal T7 RNA polymerase systems capable of exerting AND/OR switchlike control over transcription in the yeastSaccharomyces cerevisiae. When linked with CRISPR dCas9-based regulation systems, more complex circuitry is possible including AND/OR/NAND/NOR style control in response to combinations of extracellular copper and galactose. Additionally, we demonstrate that these T7 system designs are modular and can accommodate alternative stimuli sensing as demonstrated through blue light induction. These designs should greatly reduce the time and labor necessary for developing Boolean gene circuits in yeast with novel applications including metabolic pathway control in the future.

PAM-flexible dual base editor-mediated random mutagenesis and self-activation strategies to improve CRISPRa potency

VP64 is the smallest transactivation domain that can be packaged together with the sgRNA into a single adeno-associated virus (AAV) vector. However, VP64-based CRISPRa often exerts modest activation to the target gene when only one sgRNA is used. Herein, we used PAM-flexible dual base editor-mediated mutagenesis and self-activation strategies to derive VP64 variants with gain-of-function mutations. First, we generated an HEK293FT transgenic clone to stably expressing pTK-CRISPRa-GFP. The sgRNA of CRISPRa was designed to target the TK promoter, thereby allowing self-activation of CRISPRa-GFP. Base editors were then used to randomly mutagenesis VP64 in this transgenic cell. VP64 with enhanced potency would translate into increment of GFP fluorescence intensity, thereby allowing positive selection of the desired VP64 mutants. This strategy has enabled us to identify several VP64 variants that are more potent than the wild-type VP64. ΔCRISPRa derived from these VP64 variants also efficiently activated the endogenous promoter of anti-aging and longevity genes (KLOTHO, SIRT6, and NFE2L2) in human cells. Since the overall size of these ΔCRISPRa transgenes is not increased, it remains feasible for all-in-one AAV applications. The strategies described here can facilitate high-throughput screening of the desired protein variants and adapted to evolve any other effector domains.

Establishment of CRISPR/Cas9 Genome-Editing System Based on Dual sgRNAs in Flammulina filiformis

Flammulina filiformis, previously known as Asian Flammulina velutipes, is one of the most commercially important edible fungi, with nutritional value and medicinal properties worldwide. However, precision genome editing using CRISPR/Cas9, which is a revolutionary technology and provides a powerful tool for molecular breeding, has not been established in F. filiformis. Here, plasmids harboring expression cassettes of Basidiomycete codon-optimized Cas9 and dual sgRNAs targeting pyrG under the control of the gpd promoter and FfU6 promoter, respectively, were delivered into protoplasts of F. filiformis Dan3 strain through PEG-mediated transformation. The results showed that an efficient native U6 promoter of F. filiformis was identified, and ultimately several pyrG mutants exhibiting 5-fluorooric acid (5-FOA) resistance were obtained. Additionally, diagnostic PCR followed by Sanger sequencing revealed that fragment deletion between the two sgRNA target sites or small insertions and deletions (indels) were introduced in these pyrG mutants through the nonhomologous end joining (NHEJ) pathway, resulting in heritable changes in genomic information. Taken together, this is the first report in which a successful CRISPR/Cas9 genome-editing system based on dual sgRNAs was established in F. filiformis, which broadens the application of this advanced tool in Basidiomycetes.

Ribozyme-mediated CRISPR/Cas9 gene editing in pyrethrum (Tanacetum cinerariifolium) hairy roots using a RNA polymerase II-dependent promoter

BACKGROUND

Traditional CRISPR/Cas9 systems that rely on U6 or U3 snRNA promoters (RNA polymerase III-dependent promoters) can only achieve constitutive gene editing in plants, hampering the functional analysis of specifically expressed genes. Ribozyme-mediated CRISPR/Cas9 systems increase the types of promoters which can be used to transcribe sgRNA. Therefore, such systems allow specific gene editing; for example, transcription of the artificial gene Ribozyme-sgRNA-Ribozyme (RGR) is initiated by an RNA polymerase II-dependent promoter. Genetic transformation is indispensable for editing plant genes. In certain plant species, including pyrethrum, genetic transformation remains challenging to do, limiting the functional verification of novel CRISPR/Cas9 systems. Thus, this study's aim was to develop a simple Agrobacterium rhizogenes-mediated hairy root transformation system to analyze the function of a ribozyme-mediated CRISPR/Cas9 system in pyrethrum.

RESULTS

A hairy root transformation system for pyrethrum is described, with a mean transformation frequency of 7%. Transgenic hairy roots transformed with the pBI121 vector exhibited significantly increased beta-glucuronidase staining as a visual marker of transgene expression. Further, a ribozyme-based CRISPR/Cas9 vector was constructed to edit the TcEbFS gene, which catalyzes synthesis of the defense-related compound (E)-β-farnesene in pyrethrum. The vector was transferred into the hairy roots of pyrethrum and two stably transformed hairy root transgenic lines obtained. Editing of the TcEbFS gene in the hairy roots was evaluated by gene sequencing, demonstrating that both hairy root transgenic lines had DNA base loss at the editing target site. Gas chromatography-mass spectrometry showed that the (E)-β-farnesene content was significantly decreased in both hairy root transgenic lines compared with the empty vector control group. Altogether, these results show that RGR can be driven by the CaMV35S promoter to realize TcEbFS gene editing in pyrethrum hairy roots.

CONCLUSION

An A. rhizogenes-mediated hairy root transformation and ribozyme-mediated CRISPR/Cas9 gene editing system in pyrethrum was established, thereby facilitating gene editing in specific organs or at a particular developmental stage in future pyrethrum research.

Deciphering the Design Rules of Toehold-Gated sgRNA for Conditional Activation of Gene Expression and Protein Degradation in Mammalian Cells

A new class of toehold-gated gRNAs (thgRNAs) has been created to provide conditional gene regulation via RNA-mediated activation. However, the detailed design principles remain elusive. Here, we presented an investigation into the design rules for conditional gRNAs by systematically varying the toehold, stem, and flexible loop regions of thgRNA for optimal gene activation in HeLa cells. We determined that nonspecific interactions between the toehold region and the flexible loop are the main driver for the background leak observed in the OFF state. By trimming the toehold length from 15 to 5 nt, the improved thgNT-F design led to a 38-fold increase in the activated ON state with no observable background leak. The same design rule was successfully adapted to target two different regions on the mCherry mRNA with the same impressive fold change. Using the thgRNA to direct conditional protein degradation, we showed up to 8-fold knockdown of a reporter protein through activating expression of a bifunctional ubiquibody GS2-IpaH9.8. This new strategy may find many new applications for cell culture control or cell therapy by removing unwanted proteins in an RNA-responsive manner.

Efficient gene editing in a medaka (Oryzias latipes) cell line and embryos by SpCas9/tRNA-gRNA

Generation of mutants with clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) is commonly carried out in fish species by co-injecting a mixture of Cas9 messenger RNA (mRNA) or protein and transcribed guide RNA (gRNA). However, the appropriate expression system to produce functional gRNAs in fish embryos and cells is rarely present. In this study, we employed a poly-transfer RNA (tRNA)-gRNA (PTG) system driven by cytomegalovirus (CMV) promoter to target the medaka (Oryzias latipes) endogenous gene tyrosinase(tyr) or paired box 6.1 (pax6.1) and illustrated its function in a medaka cell line and embryos. The PTG system was combined with the CRISPR/Cas9 system under high levels of promoter to successfully induce gene editing in medaka. This is a valuable step forward in potential application of the CRISPR/Cas9 system in medaka and other teleosts.

Expansion of the Yeast Modular Cloning Toolkit for CRISPR-Based Applications, Genomic Integrations and Combinatorial Libraries

Standardisation of genetic parts has become a topic of increasing interest over the last decades. The promise of simplifying molecular cloning procedures, while at the same time making them more predictable and reproducible has led to the design of several biological standards, one of which is modular cloning (MoClo). The Yeast MoClo toolkit provides a large library of characterised genetic parts combined with a comprehensive and flexible assembly strategy. Here we aimed to (1) simplify the adoption of the standard by providing a simple design tool for including new parts in the MoClo library, (2) characterise the toolkit further by demonstrating the impact of a BglII site in promoter parts on protein expression, and (3) expand the toolkit to enable efficient construction of gRNA arrays, marker-less integration cassettes and combinatorial libraries. These additions make the toolkit more applicable for common engineering tasks and will further promote its adoption in the yeast biological engineering community.

CRISPR-Based Synthetic Transcription Factors In Vivo: The Future of Therapeutic Cellular Programming

Pinpoint control over endogenous gene expression in vivo has long been a fevered dream for clinicians and researchers alike. With the recent repurposing of programmable, RNA-guided DNA endonucleases from the CRISPR bacterial immune system, this dream is becoming a powerful reality. Engineered CRISPR/Cas9-based transcriptional regulators and epigenome editors have enabled researchers to perturb endogenous gene expression in vivo, allowing for the therapeutic reprogramming of cell and tissue behavior. For this technology to be of maximal use, a variety of technological hurdles still need to be addressed. Better understanding of the design principle controlling gene expression together with technologies that enable spatiotemporal control of transcriptional engineering are fundamental for rational design, improved efficacy, and ultimately safe translation to humans. In this review, we will discuss recent advances and integrative strategies that can help pave the path toward a new class of transcriptional therapeutics.

Efficient Cas9 multiplex editing using unspaced sgRNA arrays engineering in a Potato virus X vector

Systems based on the clustered, regularly interspaced, short palindromic repeat (CRISPR) and CRISPR-associated proteins (Cas) have revolutionized genome editing in many organisms, including plants. Most CRISPR-Cas strategies in plants rely on genetic transformation using Agrobacterium tumefaciens to supply the gene editing reagents, such as Cas nucleases or the synthetic guide RNA (sgRNA). While Cas nucleases are constant elements in editing approaches, sgRNAs are target-specific and a screening process is usually required to identify those most effective. Plant virus-derived vectors are an alternative for the fast and efficient delivery of sgRNAs into adult plants, due to the virus capacity for genome amplification and systemic movement, a strategy known as virus-induced genome editing. We engineered Potato virus X (PVX) to build a vector that easily expresses multiple sgRNAs in adult solanaceous plants. Using the PVX-based vector, Nicotiana benthamiana genes were efficiently targeted, producing nearly 80% indels in a transformed line that constitutively expresses Streptococcus pyogenes Cas9. Interestingly, results showed that the PVX vector allows expression of arrays of unspaced sgRNAs, achieving highly efficient multiplex editing in a few days in adult plant tissues. Moreover, virus-free edited progeny can be obtained from plants regenerated from infected tissues or infected plant seeds, which exhibit a high rate of heritable biallelic mutations. In conclusion, this new PVX vector allows easy, fast and efficient expression of sgRNA arrays for multiplex CRISPR-Cas genome editing and will be a useful tool for functional gene analysis and precision breeding across diverse plant species, particularly in Solanaceae crops.

Noncanonical Roles of tRNAs: tRNA Fragments and Beyond

As one of the most abundant and conserved RNA species, transfer RNAs (tRNAs) are well known for their role in reading the codons on messenger RNAs and translating them into proteins. In this review, we discuss the noncanonical functions of tRNAs. These include tRNAs as precursors to novel small RNA molecules derived from tRNAs, also called tRNA-derived fragments, that are abundant across species and have diverse functions in different biological processes, including regulating protein translation, Argonaute-dependent gene silencing, and more. Furthermore, the role of tRNAs in biosynthesis and other regulatory pathways, including nutrient sensing, splicing, transcription, retroelement regulation, immune response, and apoptosis, is reviewed. Genome organization and sequence variation of tRNA genes are also discussed in light of their noncanonical functions. Lastly, we discuss the recent applications of tRNAs in genome editing and microbiome sequencing.

Applications of CRISPR in a Microbial Cell Factory: From Genome Reconstruction to Metabolic Network Reprogramming

The well-designed microbial cell factory finds wide applications in chemical, pharmaceutical, and food industries due to its sustainable and environmentally friendly features. Recently, the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (CRISPR-Cas) systems have been developed into powerful tools to perform genome editing and transcriptional regulation in prokaryotic and eukaryotic cells. Accordingly, these tools are useful to build microbial cell factories not only by reconstructing the genome but also by reprogramming the metabolic network. In this review, we summarize the recent significant headway and potential uses of the CRISPR technology in the construction of efficient microbial cell factories. Moreover, the future perspectives on the improvement and upgradation of CRISPR-based tools are also discussed.

Synthetic immunomodulation with a CRISPR super-repressor in vivo

Transient modulation of the genes involved in immunity, without exerting a permanent change in the DNA code, can be an effective strategy to modulate the course of many inflammatory conditions. CRISPR-Cas9 technology represents a promising platform for achieving this goal. Truncation of guide RNA (gRNA) from the 5' end enables the application of a nuclease competent Cas9 protein for transcriptional modulation of genes, allowing multifunctionality of CRISPR. Here, we introduce an enhanced CRISPR-based transcriptional repressor to reprogram immune homeostasis in vivo. In this repressor system, two transcriptional repressors-heterochromatin protein 1 (HP1a) and Krüppel-associated box (KRAB)-are fused to the MS2 coat protein and subsequently recruited by gRNA aptamer binding to a nuclease competent CRISPR complex containing truncated gRNAs. With the enhanced repressor, we demonstrate transcriptional repression of the Myeloid differentiation primary response 88 (Myd88) gene in vitro and in vivo. We demonstrate that this strategy can efficiently downregulate Myd88 expression in lung, blood and bone marrow of Cas9 transgenic mice that receive systemic injection of adeno-associated virus (AAV)2/1-carrying truncated gRNAs targeting Myd88 and the MS2-HP1a-KRAB cassette. This downregulation is accompanied by changes in downstream signalling elements such as TNF-α and ICAM-1. Myd88 repression leads to a decrease in immunoglobulin G (IgG) production against AAV2/1 and AAV2/9 and this strategy modulates the IgG response against AAV cargos. It improves the efficiency of a subsequent AAV9/CRISPR treatment for repression of proprotein convertase subtilisin/kexin type 9 (PCSK9), a gene that, when repressed, can lower blood cholesterol levels. We also demonstrate that CRISPR-mediated Myd88 repression can act as a prophylactic measure against septicaemia in both Cas9 transgenic and C57BL/6J mice. When delivered by nanoparticles, this repressor can serve as a therapeutic modality to influence the course of septicaemia. Collectively, we report that CRISPR-mediated repression of endogenous Myd88 can effectively modulate the host immune response against AAV-mediated gene therapy and influence the course of septicaemia. The ability to control Myd88 transcript levels using a CRISPR-based synthetic repressor can be an effective strategy for AAV-based CRISPR therapies, as this pathway serves as a key node in the induction of humoral immunity against AAV serotypes.

BEON: A Functional Fluorescence Reporter for Quantification and Enrichment of Adenine Base-Editing Activity

Adenine base editor (ABE) is a new generation of genome-editing technology through fusion of Cas9 nickase with an evolved E. coli TadA (TadA∗) and holds great promise as novel genome-editing therapeutics for treating genetic disorders. ABEs can directly convert A-T to G-C in specific genomic DNA targets without introducing double-strand breaks (DSBs). We recently showed that computer program-assisted analysis of Sanger sequencing traces can be used as a low-cost and rapid alternative of deep sequencing to assess base-editing outcomes. Here we developed a rapid fluorescence-based reporter assay (Base Editing ON [BEON]) to quantify ABE efficiency. The assay relies on the restoration of the downstream green fluorescent protein (GFP) in ABE-mediated editing of a stop codon located within the guide RNA (gRNA). We showed that this assay can be used to screen for effective ABE variants, characterize the protospacer adjacent motif (PAM) requirement of a novel NNG-targeting ABE based on ScCas9, and enrich for edited cells. Finally, we demonstrated that the reporter assay allowed us to assess the feasibility of ABE editing to correct point mutations associated with dysferlinopathy. Taken together, the BEON assay would facilitate and simplify the studies with ABEs.

Recombinant Adeno-Associated Viral Vectors (rAAV)-Vector Elements in Ocular Gene Therapy Clinical Trials and Transgene Expression and Bioactivity Assays

Inherited retinal dystrophies and optic neuropathies cause chronic disabling loss of visual function. The development of recombinant adeno-associated viral vectors (rAAV) gene therapies in all disease fields have been promising, but the translation to the clinic has been slow. The safety and efficacy profiles of rAAV are linked to the dose of applied vectors. DNA changes in the rAAV gene cassette affect potency, the expression pattern (cell-specificity), and the production yield. Here, we present a library of rAAV vectors and elements that provide a workflow to design novel vectors. We first performed a meta-analysis on recombinant rAAV elements in clinical trials (2007-2020) for ocular gene therapies. We analyzed 33 unique rAAV gene cassettes used in 57 ocular clinical trials. The rAAV gene therapy vectors used six unique capsid variants, 16 different promoters, and six unique polyadenylation sequences. Further, we compiled a list of promoters, enhancers, and other sequences used in current rAAV gene cassettes in preclinical studies. Then, we give an update on pro-viral plasmid backbones used to produce the gene therapy vectors, inverted terminal repeats, production yield, and rAAV safety considerations. Finally, we assess rAAV transgene and bioactivity assays applied to cells or organoids in vitro, explants ex vivo, and clinical studies.

CRISPR Tools for Physiology and Cell State Changes: Potential of Transcriptional Engineering and Epigenome Editing

Given the large amount of genome-wide data that have been collected during the last decades, a good understanding of how and why cells change during development, homeostasis, and disease might be expected. Unfortunately, the opposite is true; triggers that cause cellular state changes remain elusive, and the underlying molecular mechanisms are poorly understood. Although genes with the potential to influence cell states are known, the historic dependency on methods that manipulate gene expression outside the endogenous chromatin context has prevented us from understanding how cells organize, interpret, and protect cellular programs. Fortunately, recent methodological innovations are now providing options to answer these outstanding questions, by allowing to target and manipulate individual genomic and epigenomic loci. In particular, three experimental approaches are now feasible due to DNA targeting tools, namely, activation and/or repression of master transcription factors in their endogenous chromatin context; targeting transcription factors to endogenous, alternative, or inaccessible sites; and finally, functional manipulation of the chromatin context. In this article, we discuss the molecular basis of DNA targeting tools and review the potential of these new technologies before we summarize how these have already been used for the manipulation of cellular states and hypothesize about future applications.

Direct-seq: programmed gRNA scaffold for streamlined scRNA-seq in CRISPR screen

CRISPR-based genome perturbation provides a new avenue to conveniently change DNA sequences, transcription, and epigenetic modifications in genetic screens. However, it remains challenging to assay the complex molecular readouts after perturbation at high resolution and at scale. By introducing an A/G mixed capture sequence into the gRNA scaffold, we demonstrate that gRNA transcripts could be directly reverse transcribed by poly (dT) primer together with the endogenous mRNA, followed by high-content molecular phenotyping in scRNA-seq (Direct-seq). With this method, the CRISPR perturbation and its transcriptional readouts can be profiled together in a streamlined workflow.

Efficient expression of multiple guide RNAs for CRISPR/Cas genome editing

The Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein system (CRISPR/Cas) has recently become the most powerful tool available for genome engineering in various organisms. With efficient and proper expression of multiple guide RNAs (gRNAs), the CRISPR/Cas system is particularly suitable for multiplex genome editing. During the past several years, different CRISPR/Cas expression strategies, such as two-component transcriptional unit, single transcriptional unit, and bidirectional promoter systems, have been developed to efficiently express gRNAs as well as Cas nucleases. Significant progress has been made to optimize gRNA production using different types of promoters and RNA processing strategies such as ribozymes, endogenous RNases, and exogenous endoribonuclease (Csy4). Besides being constitutively and ubiquitously expressed, inducible and spatiotemporal regulations of gRNA expression have been demonstrated using inducible, tissue-specific, and/or synthetic promoters for specific research purposes. Most recently, the emergence of CRISPR/Cas ribonucleoprotein delivery methods, such as engineered nanoparticles, further revolutionized transgene-free and multiplex genome editing. In this review, we discuss current strategies and future perspectives for efficient expression and engineering of gRNAs with a goal to facilitate CRISPR/Cas-based multiplex genome editing.

A multiplex guide RNA expression system and its efficacy for plant genome engineering

BACKGROUND

The Streptococcus pyogenes CRISPR system is composed of a Cas9 endonuclease (SpCas9) and a single-stranded guide RNA (gRNA) harboring a target-specific sequence. Theoretically, SpCas9 proteins could cleave as many targeted loci as gRNAs bind in a genome.

RESULTS

We introduce a PCR-free multiple gRNA cloning system for editing plant genomes. This method consists of two steps: (1) cloning the annealed products of two single-stranded oligonucleotide fragments harboring a complimentary target-binding sequence on each strand between tRNA and gRNA scaffold sequences in a pGRNA vector; and (2) assembling tRNA-gRNA units from several pGRNA vectors with a plant binary vector containing a SpCas9 expression cassette using the Golden Gate assembly method. We validated the editing efficiency and patterns of the multiplex gRNA expression system in wild tobacco (Nicotiana attenuata) protoplasts and in transformed plants by performing targeted deep sequencing. Two proximal cleavages by SpCas9-gRNA largely increased the editing efficiency and induced large deletions between two cleavage sites.

CONCLUSIONS

This multiplex gRNA expression system enables high-throughput production of a single binary vector and increases the efficiency of plant genome editing.

Does co-transcriptional regulation of alternative splicing mediate plant stress responses?

Plants display exquisite control over gene expression to elicit appropriate responses under normal and stress conditions. Alternative splicing (AS) of pre-mRNAs, a process that generates two or more transcripts from multi-exon genes, adds another layer of regulation to fine-tune condition-specific gene expression in animals and plants. However, exactly how plants control splice isoform ratios and the timing of this regulation in response to environmental signals remains elusive. In mammals, recent evidence indicate that epigenetic and epitranscriptome changes, such as DNA methylation, chromatin modifications and RNA methylation, regulate RNA polymerase II processivity, co-transcriptional splicing, and stability and translation efficiency of splice isoforms. In plants, the role of epigenetic modifications in regulating transcription rate and mRNA abundance under stress is beginning to emerge. However, the mechanisms by which epigenetic and epitranscriptomic modifications regulate AS and translation efficiency require further research. Dynamic changes in the chromatin landscape in response to stress may provide a scaffold around which gene expression, AS and translation are orchestrated. Finally, we discuss CRISPR/Cas-based strategies for engineering chromatin architecture to manipulate AS patterns (or splice isoforms levels) to obtain insight into the epigenetic regulation of AS.

Pooled library screening with multiplexed Cpf1 library

Capitalizing on the inherent multiplexing capability of AsCpf1, we developed a multiplexed, high-throughput screening strategy that minimizes library size without sacrificing gene targeting efficiency. We demonstrated that AsCpf1 can be used for functional genomics screenings and that an AsCpf1-based multiplexed library performs similarly as compared to currently available monocistronic CRISPR/Cas9 libraries, with only one vector required for each gene. We construct the smallest whole-genome CRISPR knock-out library, Mini-human, for the human genome (n = 17,032 constructs targeting 16,977 protein-coding genes), which performs favorably compared to conventional Cas9 libraries.

BCL6 corepressor contributes to Th17 cell formation by inhibiting Th17 fate suppressors

Th17 cells provide a protective immunity against extracellular bacterial and fungal pathogens. Kotov et al. identify and characterize a mechanism by which BCOR promotes Th17 formation after Streptococcus pyogenes infection by repressing genes that inhibit the Th17 lineage.

CD4⁺ T helper 17 (Th17) cells protect vertebrate hosts from extracellular pathogens at mucosal surfaces. Th17 cells form from naive precursors when signals from the T cell antigen receptor (TCR) and certain cytokine receptors induce the expression of the RORγt transcription factor, which activates a set of Th17-specific genes. Using T cell–specific loss-of-function experiments, we find that two components of the Polycomb repressive complex 1.1 (PRC1.1), BCL6 corepressor (BCOR) and KDM2B, which helps target the complex to unmethylated CpG DNA islands, are required for optimal Th17 cell formation in mice after Streptococcus pyogenes infection. Genome-wide expression and BCOR chromatin immunoprecipitation studies revealed that BCOR directly represses Lef1, Runx2, and Dusp4, whose products inhibit Th17 differentiation. Together, the results suggest that the PRC1.1 components BCOR and KDM2B work together to enhance Th17 cell formation by repressing Th17 fate suppressors.

Decoupling tRNA promoter and processing activities enables specific Pol-II Cas9 guide RNA expression

Spatial/temporal control of Cas9 guide RNA expression could considerably expand the utility of CRISPR-based technologies. Current approaches based on tRNA processing offer a promising strategy but suffer from high background. Here, to address this limitation, we present a screening platform which allows simultaneous measurements of the promoter strength, 5', and 3' processing efficiencies across a library of tRNA variants. This analysis reveals that the sequence determinants underlying these activities, while overlapping, are dissociable. Rational design based on the ensuing principles allowed us to engineer an improved tRNA scaffold that enables highly specific guide RNA production from a Pol-II promoter. When benchmarked against other reported systems this tRNA scaffold is superior to most alternatives, and is equivalent in function to an optimized version of the Csy4-based guide RNA release system. The results and methods described in this manuscript enable avenues of research both in genome engineering and basic tRNA biology.

TCR Affinity Biases Th Cell Differentiation by Regulating CD25, Eef1e1, and Gbp2

Naive CD4⁺ T lymphocytes differentiate into various Th cell subsets following TCR binding to microbial peptide:MHC class II (p:MHCII) complexes on dendritic cells (DCs). The affinity of the TCR interaction with p:MHCII plays a role in Th differentiation by mechanisms that are not completely understood. We found that low-affinity TCRs biased mouse naive T cells to become T follicular helper (Tfh) cells, whereas higher-affinity TCRs promoted the formation of Th1 or Th17 cells. We explored the basis for this phenomenon by focusing on IL-2R signaling, which is known to promote Th1 and suppress Tfh cell differentiation. SIRP⍺⁺ DCs produce abundant p:MHCII complexes and consume IL-2, whereas XCR1⁺ DCs weakly produce p:MHCII but do not consume IL-2. We found no evidence, however, of preferential interactions between Th1 cell-prone, high-affinity T cells and XCR1⁺ DCs or Tfh cell-prone, low-affinity T cells and SIRP⍺⁺ DCs postinfection with bacteria expressing the peptide of interest. Rather, high-affinity T cells sustained IL-2R expression longer and expressed two novel Th cell differentiation regulators, Eef1e1 and Gbp2, to a higher level than low-affinity T cells. These results suggest that TCR affinity does not influence Th cell differentiation by biasing T cell interactions with IL-2-consuming DCs, but instead, directly regulates genes in naive T cells that control the differentiation process.

A Multiplexed CRISPR/Cas9 Editing System Based on the Endogenous tRNA Processing

The CRISPR-Cas9 system has become a powerful and popular tool for genome editing due to its efficiency and simplicity. Multiplex genome editing is an important feature of the CRISPR-Cas9 system and requires simultaneous expression of multiple guide RNAs (gRNAs). Here we describe a general method to efficiently produce many gRNAs from a single gene transcript based on the endogenous tRNA-processing system. A step-by-step protocol is provided for the design and construction of the polycistronic tRNA-gRNA (PTG) gene. The PTG method has been demonstrated to be highly efficient for multiplex genome editing in various plant, animal, and microbial species.

In vivo epigenome editing and transcriptional modulation using CRISPR technology

The rapid advancement of CRISPR technology has enabled targeted epigenome editing and transcriptional modulation in the native chromatin context. However, only a few studies have reported the successful editing of the epigenome in adult animals in contrast to the rapidly growing number of in vivo genome editing over the past few years. In this review, we discuss the challenges facing in vivo epigenome editing and new strategies to overcome the huddles. The biggest challenge has been the difficulty in packaging dCas9 fusion proteins required for manipulation of epigenome into the adeno-associated virus (AAV) delivery vehicle. We review the strategies to address the AAV packaging issue, including small dCas9 orthologues, truncated dCas9 mutants, a split-dCas9 system, and potent truncated effector domains. We discuss the dCas9 conjugation strategies to recruit endogenous chromatin modifiers and remodelers to specific genomic loci, and recently developed methods to recruit multiple copies of the dCas9 fusion protein, or to simultaneous express multiple gRNAs for robust epigenome editing or synergistic transcriptional modulation. The use of Cre-inducible dCas9-expressing mice or a genetic cross between dCas9- and sgRNA-expressing flies has also helped overcome the transgene delivery issue. We provide perspective on how a combination use of these strategies can facilitate in vivo epigenome editing and transcriptional modulation.

A Multiplex Genome Editing Method for Escherichia coli Based on CRISPR-Cas12a

Various methods for editing specific sites in the Escherichia coli chromosome are available, and gene-size (∼1 kb) integration into a single site or to introduce deletions, short insertions or point mutations into multiple sites can be conducted in a short period of time. However, a method for rapidly integrating multiple gene-size sequences into different sites has not been developed yet. Here, we describe a method and plasmid system that makes it possible to simultaneously insert genes into multiple specific loci of the E. coli genome without the need for chromosomal markers. The method uses a CRISPR-Cas12a system to eliminate unmodified cells by double-stranded DNA cleavage in conjunction with the phage-derived λ-Red recombinases to facilitate recombination between the chromosome and the donor DNA. We achieved the insertion of up to 3 heterologous genes in one round of recombination and selection. To demonstrate the practical application of this gene-insertion method, we constructed a recombinant E. coli producing an industrially useful chemical, 5-aminolevulinic acid (ALA), with high-yield. Moreover, a similar two-plasmid system was built to edit the genome of the extremophile Halomonas bluephagenesis.

A tRNA-based multiplex sgRNA expression system in zebrafish and its application to generation of transgenic albino fish

The CRISPR/Cas9 system can be introduced into zebrafish as transgenes. Namely, expression of single-guide RNA (sgRNA) and controlled expression of Cas9 in transgenic zebrafish enables the study of gene functions in specific cell types. This transgenic CRISPR/Cas9 approach would be more useful if multiple sgRNAs could be expressed simultaneously since we could knock-out a gene more efficiently or disrupt multiple genes simultaneously. Here we describe a novel system to express multiple sgRNAs efficiently in zebrafish, that relies on the endogenous tRNA processing machinery. We cloned nine endogenous zebrafish tRNA genes, fused them to sgRNAs, and demonstrated that an active sgRNA can be produced from a precursor transcript containing either of these tRNAs. To show a proof of principle, we constructed transgenic fish expressing Cas9 under the control of the ubiquitin promoter and a single transcript containing three distinct sgRNAs, that targeted the slc45a2 (albino) gene, fused to tRNAs under the control of the U6 promoter. We found that the Tg(ubb:SpCas9,u6c:3xslc45a2-sgRNA) harbored mutations in all of the target sites in the albino gene and showed nearly complete albino phenotypes, which were amenable to imaging experiments. Thus, the tRNA-based multiplex sgRNA expression system should facilitate gene knock-out studies in transgenic zebrafish.

A Single Multiplex crRNA Array for FnCpf1-Mediated Human Genome Editing

Cpf1 has been harnessed as a tool for genome manipulation in various species because of its simplicity and high efficiency. Our recent study demonstrated that FnCpf1 could be utilized for human genome editing with notable advantages for target sequence selection due to the flexibility of the protospacer adjacent motif (PAM) sequence. Multiplex genome editing provides a powerful tool for targeting members of multigene families, dissecting gene networks, modeling multigenic disorders in vivo, and applying gene therapy. However, there are no reports at present that show FnCpf1-mediated multiplex genome editing via a single customized CRISPR RNA (crRNA) array. In the present study, we utilize a single customized crRNA array to simultaneously target multiple genes in human cells. In addition, we also demonstrate that a single customized crRNA array to target multiple sites in one gene could be achieved. Collectively, FnCpf1, a powerful genome-editing tool for multiple genomic targets, can be harnessed for effective manipulation of the human genome.

A novel rabbit model of Duchenne muscular dystrophy generated by CRISPR/Cas9

Duchenne muscular dystrophy (DMD) is an X-linked muscle-wasting disorder caused by mutations in the dystrophin gene, with an incidence of 1 in 3500 in new male births. Mdx mice are widely used as an animal model for DMD. However, these mice do not faithfully recapitulate DMD patients in many aspects, rendering the preclinical findings in this model questionable. Although larger animal models of DMD, such as dogs and pigs, have been generated, usage of these animals is expensive and only limited to several facilities in the world. Here, we report the generation of a rabbit model of DMD by co-injection of Cas9 mRNA and sgRNA targeting exon 51 into rabbit zygotes. The DMD knockout (KO) rabbits exhibit the typical phenotypes of DMD, including severely impaired physical activity, elevated serum creatine kinase levels, and progressive muscle necrosis and fibrosis. Moreover, clear pathology was also observed in the diaphragm and heart at 5 months of age, similar to DMD patients. Echocardiography recording showed that the DMD KO rabbits had chamber dilation with decreased ejection fraction and fraction shortening. In conclusion, this novel rabbit DMD model generated with the CRISPR/Cas9 system mimics the histopathological and functional defects in DMD patients, and could be valuable for preclinical studies.

This article has an associated First Person interview with the first author of the paper.

Summary: The DMD KO rabbit engineered by CRISPR genome editing faithfully recapitulates the DMD pathologies, and could be a valuable tool for basic and translational studies to combat this disease.

New and emerging uses of CRISPR/Cas9 to genetically manipulate apicomplexan parasites

Although the application of CRISPR/Cas9 genome engineering approaches was first reported in apicomplexan parasites only 3 years ago, this technology has rapidly become an essential component of research on apicomplexan parasites. This review briefly describes the history of CRISPR/Cas9 and the principles behind its use along with documenting its implementation in apicomplexan parasites, especially Plasmodium spp. and Toxoplasma gondii. We also discuss the recent use of CRISPR/Cas9 for whole genome screening of gene knockout mutants in T. gondii and highlight its use for seminal genetic manipulations of Cryptosporidium spp. Finally, we consider new variations of CRISPR/Cas9 that have yet to be implemented in apicomplexans. Whereas CRISPR/Cas9 has already accelerated rapid interrogation of gene function in apicomplexans, the full potential of this technology is yet to be realized as new variations and innovations are integrated into the field.

The multiplexed CRISPR targeting platforms

The discovery and engineering of the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) in the past several years have revolutionized biomedical research. The CRISPR technology showed great potential to advance detection, prevention, and treatment of human diseases in the near future. Compared to previous developed genome editing approaches, such as zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), the CRISPR-based systems have numerous advantages. One example is that the CRISPR systems can be easily adopted to efficiently target multiple genes simultaneously. Several strategies and toolboxes have been developed to achieve multiplexed targeting using the CRISPR systems. In this short review, we will discuss the principle, approach, and application of these strategies.

Delineation of the Exact Transcription Termination Signal for Type 3 Polymerase III

Type 3 Pol III promoters such as U6 are widely used for expression of small RNAs, including short hairpin RNA for RNAi applications and guide RNA in CRISPR genome-editing platforms. RNA polymerase III uses a T-stretch as termination signal, but the exact properties have not been thoroughly investigated. Here, we systematically measured the in vivo termination efficiency and the actual site of termination for different T-stretch signals in three commonly used human Pol III promoters (U6, 7SK, and H1). Both the termination efficiency and the actual termination site depend on the T-stretch signal. The T4 signal acts as minimal terminator, but full termination efficiency is reached only with a T-stretch of ≥6. The termination site within the T-stretch is quite heterogeneous, and consequently small RNAs have a variable U-tail of 1–6 nucleotides. We further report that such variable U-tails can have a significant negative effect on the functionality of the crRNA effector of the CRISPR-AsCpf1 system. We next improved these crRNAs by insertion of the HDV ribozyme to avoid U-tails. This study provides detailed design guidelines for small RNA expression cassettes based on Pol III.

Enabling Graded and Large-Scale Multiplex of Desired Genes Using a Dual-Mode dCas9 Activator in Saccharomyces cerevisiae

Standard approaches for dCas9-based modification of gene expression are limited in the ability to multiplex targets, establish streamlined cassettes, and utilize commonly studied Pol II promoters. In this work, we repurpose the dCas9-VPR activator to act as a dual-mode activator/repressor that can be programmed solely on the basis of target position at gene loci. Furthermore, we implement this approach using a streamlined Pol II-ribozyme system that allows expression of many sgRNAs from a single transcript. By "stepping" dCas9-VPR within the promoter region and ORF we create graded activation and repression (respectively) of target genes, allowing precise control over multiplexed gene modulation. Expression from the Pol II system increased the net amount of sgRNA production in cells by 3.88-fold relative to the Pol III SNR52 promoter, leading to a significant improvement in dCas9-VPR repression strength. Finally, we utilize our Pol II system to create galactose-inducible switching of gene expression states and multiplex constructs capable of modulating up to 4 native genes from a single vector. Our approach represents a significant step toward minimizing DNA required to assemble CRISPR systems in eukaryotes while enhancing the efficacy (greater repression strength), scale (more sgRNAs), and scope (inducibility) of dCas9-mediated gene regulation.

Engineered CRISPR Systems for Next Generation Gene Therapies

An ideal in vivo gene therapy platform provides safe, reprogrammable, and precise strategies which modulate cell and tissue gene regulatory networks with a high temporal and spatial resolution. Clustered regularly interspaced short palindromic repeats (CRISPR), a bacterial adoptive immune system, and its CRISPR-associated protein 9 (Cas9), have gained attention for the ability to target and modify DNA sequences on demand with unprecedented flexibility and precision. The precision and programmability of Cas9 is derived from its complexation with a guide-RNA (gRNA) that is complementary to a desired genomic sequence. CRISPR systems open-up widespread applications including genetic disease modeling, functional screens, and synthetic gene regulation. The plausibility of in vivo genetic engineering using CRISPR has garnered significant traction as a next generation in vivo therapeutic. However, there are hurdles that need to be addressed before CRISPR-based strategies are fully implemented. Some key issues center on the controllability of the CRISPR platform, including minimizing genomic-off target effects and maximizing in vivo gene editing efficiency, in vivo cellular delivery, and spatial-temporal regulation. The modifiable components of CRISPR systems: Cas9 protein, gRNA, delivery platform, and the form of CRISPR system delivered (DNA, RNA, or ribonucleoprotein) have recently been engineered independently to design a better genome engineering toolbox. This review focuses on evaluating CRISPR potential as a next generation in vivo gene therapy platform and discusses bioengineering advancements that can address challenges associated with clinical translation of this emerging technology.

Targeted genome editing in Caenorhabditis elegans using CRISPR/Cas9

Utilization of programmable nucleases to generate DNA lesions at precise endogenous sequences has transformed the ability to edit genomes from microbes to plants and animals. This is especially true in organisms that previously lacked the means to engineer precise genomic changes, like Caenorhabditis elegans. C. elegans is a 1 mm long free-living, nonparasitic, nematode worm, which is easily cultivated in a laboratory. Its detailed genetic map and relatively compact genome (~100 megabases) helped make it the first metazoan to have its entire genome sequenced. With detailed sequence information came development of numerous molecular tools to dissect gene function. Initially absent from this toolbox, however, were methods to make precise edits at chosen endogenous loci. Adapting site-specific nucleases for use in C. elegans, revolutionized studies of C. elegans biology. Zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and then CRISPR-associated protein 9 (Cas9) were used to target specific endogenous DNA sequences to make double-strand DNA breaks (DSBs). Precise changes could be engineered by providing repair templates targeting the DSB in trans. The ease of programming Cas9 to bind and cleave DNA sequences with few limitations has led to its widespread use in C. elegans research and sped the development of strategies to facilitate mutant recovery. Numerous innovative CRISPR/Cas9 methodologies are now primed for use in C. elegans. WIREs Dev Biol 2017, 6:e287. doi: 10.1002/wdev.287 For further resources related to this article, please visit the WIREs website.

SgRNA Expression of CRIPSR-Cas9 System Based on MiRNA Polycistrons as a Versatile Tool to Manipulate Multiple and Tissue-Specific Genome Editing

CRISPR/Cas9-mediated genome editing is a next-generation strategy for genetic modifications. Typically, sgRNA is constitutively expressed relying on RNA polymerase III promoters. Polymerase II promoters initiate transcription in a flexible manner, but sgRNAs generated by RNA polymerase II promoter lost their nuclease activity. To express sgRNAs in a tissue-specific fashion and endow CRISPR with more versatile function, a novel system was established in a polycistron, where miRNAs (or shRNAs) and sgRNAs alternately emerged and co-expressed under the control of a single polymerase II promoter. Effective expression and further processing of functional miRNAs and sgRNAs were achieved. The redundant nucleotides adjacent to sgRNA were degraded, and 5'- cap structure was responsible for the compromised nuclease capacity of sgRNA: Cas9 complex. Furthermore, this strategy fulfilled conducting multiplex genome editing, as well as executing neural- specific genome editing and enhancing the proportion of homologous recombination via inhibiting NHEJ pathway by shRNA. In summary, we designed a new construction for efficient expression of sgRNAs with miRNAs (shRNAs) by virtue of RNA polymerase II promoters, which will spur the development of safer, more controllable/regulable and powerful CRISPR/Cas9 system-mediated genome editing in a wide variety of further biomedical applications.

CRISPR/Cas9-Enabled Multiplex Genome Editing and Its Application

The CRISPR/Cas9 system is a prevalent and versatile genome-editing tool of choice for basic and applied biological research. An exchange of a 20-bp spacer sequence in the gRNA can easily reprogram Cas9 to target a different DNA site. By expressing or providing multiple gRNAs, the system also enables multiplex genome editing at high efficiencies. Current approaches for providing multiple gRNAs in vivo include the use of multigene cassettes to express several gRNAs, Csy4-based excision, arrays of crRNAs, ribozyme-flanked gRNAs, tRNA-dependent cleavage of gRNAs, and direct introduction of Cas9 proteins preloaded with different gRNAs. By simultaneously targeting multiple DNA sequences, multiplex genome editing can be used to knockout multiple genes or delete chromosomal fragments. Off-target risk can also be reduced by Cas9-dimers that require the simultaneous expression of two gRNAs. With multiple gRNAs, specific gene expression or methylation status can be efficiently controlled by dCas9 fused to activators, repressors, methyltransferase, demethylase, or other functional domains. As a result, multiplex genome editing is expected to accelerate functional discovery of plant genes as well as genetic improvement of agricultural crops.

How to Train a Cell-Cutting-Edge Molecular Tools

In biological systems, the formation of molecular complexes is the currency for all cellular processes. Traditionally, functional experimentation was targeted to single molecular players in order to understand its effects in a cell or animal phenotype. In the last few years, we have been experiencing rapid progress in the development of ground-breaking molecular biology tools that affect the metabolic, structural, morphological, and (epi)genetic instructions of cells by chemical, optical (optogenetic) and mechanical inputs. Such precise dissection of cellular processes is not only essential for a better understanding of biological systems, but will also allow us to better diagnose and fix common dysfunctions. Here, we present several of these emerging and innovative techniques by providing the reader with elegant examples on how these tools have been implemented in cells, and, in some cases, organisms, to unravel molecular processes in minute detail. We also discuss their advantages and disadvantages with particular focus on their translation to multicellular organisms for in vivo spatiotemporal regulation. We envision that further developments of these tools will not only help solve the processes of life, but will give rise to novel clinical and industrial applications.