Expression of CRISPR/Cas single guide RNAs using small tRNA promoters

A gln-tRNA-based CRISPR/Cas9 knockout system enables the functional characterization of genes in the genetically recalcitrant brassica anthracnose fungus Colletotrichum higginsianum

Colletotrichum higginsianum causes anthracnose disease in brassicas. The availability of the C. higginsianum genome has paved the way for the genome-wide exploration of genes associated with virulence/pathogenicity. However, delimiting the biological functions of these genes remains an arduous task due to the recalcitrance of C. higginsianum to genetic manipulations. Here, we report a CRISPR/Cas9-based system that can knock out the genes in C. higginsianum with a staggering 100% homologous recombination frequency (HRF). The system comprises two vectors: pCas9-Ch_tRp-sgRNA, in which a C. higginsianum glutaminyl-tRNA drives the expression of sgRNA, and pCE-Zero-HPT carrying a donor DNA cassette containing the marker gene HPT flanked by homology arms. Upon co-transformation of the C. higginsianum protoplasts, pCas9-Ch_tRp-sgRNA causes a DNA double-strand break in the targeted gene, followed by homology-directed replacement of the gene with HPT by pCE-Zero-HPT, thereby generating loss-of-function mutants. Using the system, we generated the knockout mutants of two effector candidates (ChBas3 and OBR06881) with a 100% HRF. Interestingly, the ΔChBas3 and ΔOBR06881 mutants did not seem to affect the C. higginsianum infection of Arabidopsis thaliana. Altogether, the CRISPR/Cas9 system developed in the study enables the targeted deletion of genes, including effectors, in C. higginsianum, thus determining their biological functions.

Differential cell-cycle control by oscillatory versus sustained Hes1 expression via p21

Oscillatory Hes1 expression activates cell proliferation, while high and sustained Hes1 expression induces quiescence, but the mechanism by which Hes1 differentially controls cell proliferation depending on its expression dynamics is unclear. Here, we show that oscillatory Hes1 expression down-regulates the expression of the cyclin-dependent kinase inhibitor p21 (Cdkn1a), which delays cell-cycle progression, and thereby activates the proliferation of mouse neural stem cells (NSCs). By contrast, sustained Hes1 overexpression up-regulates p21 expression and inhibits NSC proliferation, although it initially down-regulates p21 expression. Compared with Hes1 oscillation, sustained Hes1 overexpression represses Dusp7, a phosphatase for phosphorylated Erk (p-Erk), and increases the levels of p-Erk, which can up-regulate p21 expression. These results indicate that p21 expression is directly repressed by oscillatory Hes1 expression, but indirectly up-regulated by sustained Hes1 overexpression, suggesting that depending on its expression dynamics, Hes1 differentially controls NSC proliferation via p21.

Evolution of giant pandoravirus revealed by CRISPR/Cas9

Giant viruses (GVs) are a hotspot of unresolved controversies since their discovery, including the definition of "Virus" and their origin. While increasing knowledge of genome diversity has accumulated, GV functional genomics was largely neglected. Here, we describe an experimental framework to genetically modify nuclear GVs and their host Acanthamoeba castellanii using CRISPR/Cas9, shedding light on the evolution from small icosahedral viruses to amphora-shaped GVs. Ablation of the icosahedral major capsid protein in the phylogenetically-related mollivirus highlights a transition in virion shape and size. We additionally demonstrate the existence of a reduced core essential genome in pandoravirus, reminiscent of their proposed smaller ancestors. This proposed genetic expansion led to increased genome robustness, indicating selective pressures for adaptation to uncertain environments. Overall, we introduce new tools for manipulation of the unexplored genome of nuclear GVs and provide experimental evidence suggesting that viral gigantism has aroused as an emerging trait.

Building Blocks of Artificial CRISPR-Based Systems beyond Nucleases

Tools developed in the fields of genome engineering, precise gene regulation, and synthetic gene networks have an increasing number of applications. When shared with the scientific community, these tools can be used to further unlock the potential of precision medicine and tissue engineering. A large number of different genetic elements, as well as modifications, have been used to create many different systems and to validate some technical concepts. New studies have tended to optimize or improve existing elements or approaches to create complex synthetic systems, especially those based on the relatively new CRISPR technology. In order to maximize the output of newly developed approaches and to move from proof-of-principle experiments to applications in regenerative medicine, it is important to navigate efficiently through the vast number of genetic elements to choose those most suitable for specific needs. In this review, we have collected information regarding the main genetic elements and their modifications, which can be useful in different synthetic systems with an emphasis of those based on CRISPR technology. We have indicated the most suitable elements and approaches to choose or combine in planning experiments, while providing their deeper understanding, and have also stated some pitfalls that should be avoided.

Combinatorial gene targeting in primary human hematopoietic stem and progenitor cells

The CRISPR/Cas9 system offers enormous versatility for functional genomics but many applications have proven to be challenging in primary human cells compared to cell lines or mouse cells. Here, to establish a paradigm for multiplexed gene editing in primary human cord blood-derived hematopoietic stem and progenitor cells (HSPCs), we used co-delivery of lentiviral sgRNA vectors expressing either Enhanced Green Fluorescent Protein (EGFP) or Kusabira Orange (KuO), together with Cas9 mRNA, to simultaneously edit two genetic loci. The fluorescent markers allow for tracking of either single- or double-edited cells, and we could achieve robust double knockout of the cell surface molecules CD45 and CD44 with an efficiency of ~ 70%. As a functional proof of concept, we demonstrate that this system can be used to model gene dependencies for cell survival, by simultaneously targeting the cohesin genes STAG1 and STAG2. Moreover, we show combinatorial effects with potential synergy for HSPC expansion by targeting the Aryl Hydrocarbon Receptor (AHR) in conjunction with members of the CoREST complex. Taken together, our traceable multiplexed CRISPR/Cas9 system enables studies of genetic dependencies and cooperation in primary HSPCs, and has important implications for modelling polygenic diseases, as well as investigation of the underlying mechanisms of gene interactions.

Efficient CRISPR/Cas9 system based on autonomously replicating plasmid with an AMA1 sequence and precisely targeted gene deletion in the edible fungus, Cordyceps militaris

Cordyceps militaris is a popular edible fungus with important economic value worldwide. In this study, an efficient CRISPR/Cas9 genome-editing system based on an autonomously replicating plasmid with an AMA1 sequence was constructed. Further, a precisely targeted gene deletion via homology-directed repair was effectively introduced in C. militaris. Gene editing was successful, with efficiencies of 55.1% and 89% for Cmwc-1 and Cmvvd, respectively. Precisely targeted gene deletion was achieved at an efficiency of 73.9% by a single guide RNA supplementation with donor DNAs. Double genes, Cmwc-1 and Cmvvd, were edited simultaneously with an efficiency of 10%. Plasmid loss was observed under non-selective culture conditions, which could permit recycling of the selectable marker and avoid the adverse effects of the CRISPR/Cas9 system on the fungus, which is beneficial for the generation of new cultivars. RNA Pol III promoters, endogenous tRNAPro of C. militaris, and chimeric AfU6-tRNAGly can be used to improve the efficiency. Polyethylene glycol-mediated protoplast transformation was markedly more efficient than Agrobacterium tumefaciens-mediated transformation of C. militaris. To our knowledge, this is the first description of genome editing and precisely targeted gene deletion in mushrooms based on AMA1 plasmids. Our findings will enable the modification of multiple genes in both functional genomics research and strain breeding.

Development of a gRNA Expression and Processing Platform for Efficient CRISPR-Cas9-Based Gene Editing and Gene Silencing in Candida tropicalis

Candida tropicalis, a nonmodel diploid microbe, has been applied in industry as a chassis cell. Metabolic engineering of C. tropicalis is challenging due to a lack of gene editing and regulation tools. Here, we report a tRNA:guide RNA (gRNA) platform for boosting gene editing and silencing efficiency in C. tropicalis. As the endogenous tRNA-processing system enables autocleavage for producing a large number of mature gRNAs, a tRNAGly sequence from the genome of C. tropicalis ATCC 20336 was selected for constructing the tRNA:gRNA platform. In the CRISPR-Cas9 system, the tRNA:gRNA platform proved to be efficient in single-gene and multi-gene editing. Furthermore, based on the tRNA:gRNA platform, a CRISPR interference (CRISPRi) system was developed to construct an efficient dCas9-mediated gene expression regulation system for C. tropicalis. The CRISPRi system was employed to regulate the expression of the exogenous gene GFP3 (green fluorescent protein) and the endogenous gene ADE2 (phosphoribosylaminoimidazole carboxylase). Different regions of GFP3 and ADE2 were targeted with the gRNAs processed by the tRNAGly, and the transcription levels of GFP3 and ADE2 were successfully downregulated to 23.9% ± 4.1% and 38.0% ± 7.4%, respectively. The effects of the target regions on gene regulation were also investigated. Additionally, the regulation system was applied to silence ERG9 (squalene synthase) to enhance β-carotene biosynthesis in a metabolically modified C. tropicalis strain. The results suggest that the endogenous tRNAGly and the CRISPRi system have great potential for metabolic engineering of C. tropicalis. IMPORTANCE In the nonmodel yeast Candida tropicalis, a lack of available RNA polymerase type III (Pol III) promoters hindered the development of guide RNA (gRNA) expression platforms for the establishment of CRISPR-Cas-mediated genome editing and silencing strategies. Here, a tRNA:gRNA platform was constructed. We show that this platform allows efficient and precise expression and processing of different gRNAs from a single polycistronic gene capable of mediating multi-gene editing in combination with CRISPR-Cas9. Furthermore, in combination with dCas9, the tRNA:gRNA platform was efficiently used for silencing of exogenous and endogenous genes, representing the first CRISPR interference tool (CRISPRi) in C. tropicalis. Importantly, the established CRISPRi-tRNA:gRNA tool was also used for metabolic engineering by regulating β-carotene biosynthesis in C. tropicalis. The results suggest that the tRNA:gRNA platform and the CRISPRi system will further advance the application of the CRISPR-Cas-based editing and CRISPRi systems for metabolic engineering in C. tropicalis.

Improved loss-of-function CRISPR-Cas9 genome editing in human cells concomitant with inhibition of TGF-β signaling

Strategies to modulate cellular DNA repair pathways hold immense potential to enhance the efficiency of CRISPR-Cas9 genome editing platform. In the absence of a repair template, CRISPR-Cas9-induced DNA double-strand breaks are repaired by the endogenous cellular DNA repair pathways to generate loss-of-function edits. Here, we describe a reporter-based assay for expeditious measurement of loss-of-function editing by CRISPR-Cas9. An unbiased chemical screen performed using this assay enabled the identification of small molecules that promote loss-of-function editing. Iterative rounds of screens reveal Repsox, a TGF-β signaling inhibitor, as a CRISPR-Cas9 editing efficiency enhancer. Repsox invariably increased CRISPR-Cas9 editing in a panel of commonly used cell lines in biomedical research and primary cells. Furthermore, Repsox-mediated editing enhancement in primary human CD4+ T cells enabled the generation of HIV-1-resistant cells with high efficiency. This study demonstrates the potential of transiently targeting cellular pathways by small molecules to improve genome editing for research applications and is expected to benefit gene therapy efforts.

Multiplex base- and prime-editing with drive-and-process CRISPR arrays

Current base- and prime-editing technologies lack efficient strategies to edit multiple genomic loci simultaneously, limiting their applications in complex genomics and polygenic diseases. Here, we describe drive-and-process (DAP) CRISPR array architectures for multiplex base-editing (MBE) and multiplex prime-editing (MPE) in human cells. We leverage tRNA as the RNA polymerase III promoter to drive the expression of tandemly assembled tRNA-guide RNA (gRNA) arrays, of which the individual gRNAs are released by the cellular endogenous tRNA processing machinery. We engineer a 75-nt human cysteine tRNA (hCtRNA) for the DAP array, achieving up to 31-loci MBE and up to 3-loci MPE. By applying MBE or MPE elements for deliveries via adeno-associated virus (AAV) and lentivirus, we demonstrate simultaneous editing of multiple disease-relevant genomic loci. Our work streamlines the expression and processing of gRNAs on a single array and establishes efficient MBE and MPE strategies for biomedical research and therapeutic applications.

tReasure: R-based GUI package analyzing tRNA expression profiles from small RNA sequencing data

BACKGROUND

Recent deep sequencing technologies have proven to be valuable resources to gain insights into the expression profiles of diverse tRNAs. However, despite these technologies, the association of tRNAs with diverse diseases has not been explored in depth because analytical tools are lacking.

RESULTS

We developed a user-friendly tool, tRNA Expression Analysis Software Utilizing R for Easy use (tReasure), to analyze differentially expressed tRNAs (DEtRNAs) from deep sequencing data of small RNAs using R packages. tReasure can quantify individual mature tRNAs, isodecoders, and isoacceptors. By adopting stringent mapping strategies, tReasure supports the precise measurement of mature tRNA read counts. The whole analysis workflow for determining DEtRNAs (uploading FASTQ files, removing adapter sequences and poor-quality reads, mapping and quantifying tRNAs, filtering out low count tRNAs, determining DEtRNAs, and visualizing statistical analysis) can be performed with the tReasure package.

CONCLUSIONS

tReasure is an open-source software available for download at https://treasure.pmrc.re.kr and will be indispensable for users who have little experience with command-line software to explore the biological implication of tRNA expression.

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.

Multiplex Gene Tagging with CRISPR-Cas9 for Live-Cell Microscopy and Application to Study the Role of SARS-CoV-2 Proteins in Autophagy, Mitochondrial Dynamics, and Cell Growth

The lack of efficient tools to label multiple endogenous targets in cell lines without staining or fixation has limited our ability to track physiological and pathological changes in cells over time via live-cell studies. Here, we outline the FAST-HDR vector system to be used in combination with CRISPR-Cas9 to allow visual live-cell studies of up to three endogenous proteins within the same cell line. Our approach utilizes a novel set of advanced donor plasmids for homology-directed repair and a streamlined workflow optimized for microscopy-based cell screening to create genetically modified cell lines that do not require staining or fixation to accommodate microscopy-based studies. We validated this new methodology by developing two advanced cell lines with three fluorescent-labeled endogenous proteins that support high-content imaging without using antibodies or exogenous staining. We applied this technology to study seven severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2/COVID-19) viral proteins to understand better their effects on autophagy, mitochondrial dynamics, and cell growth. Using these two cell lines, we were able to identify the protein ORF3a successfully as a potent inhibitor of autophagy, inducer of mitochondrial relocalization, and a growth inhibitor, which highlights the effectiveness of live-cell studies using this technology.

Construction of a rAAV-SaCas9 system expressing eGFP and its application to improve muscle mass

An ideal rAAV gene editing system not only effectively edits genes at specific site, but also prevents the spread of the virus from occurring off-target or carcinogenic risks. This is important for gene editing research at specific site in vivo. We report a single rAAV containing SaCas9 and guide RNAs under the control of subtle EF1a and tRNA promoters. The capacity of rAAV was compressed, and the editing efficiency was similar to that of the classical Cas9 system in vitro and in vivo. And we inserted the sequence of the green fluorescent protein eGFP into rAAV. The number of cells infected with the rAAV and the region in which the rAAV spreads were known by the fluorescent expression of eGFP in cells. In addition, we demonstrated that myostatin gene in the thigh muscles of C57BL/10 mice was knocked out by the rAAV9-SaCas9 system to make muscle mass increased obviously. The protein eGFP into rAAV has significant implications for our indirect analysis of the editing efficiency of SaCas9 in the genome of the target tissue and reduces the harm caused by off-target editing and prevents other tissue mutations. The rAAV system has substantial potential in improving muscle mass and preventing muscle atrophy.

RNA Sequence and Structure Determinants of Pol III Transcriptional Termination in Human Cells

The precise mechanism of transcription termination of the eukaryotic RNA polymerase III (Pol III) has been a subject of considerable debate. Although previous studies have clearly shown that multiple uracils at the end of RNA transcripts are required for Pol III termination, the effects of upstream RNA secondary structure in the nascent transcript on transcriptional termination is still unclear. To address this, we developed an in cellulo Pol III transcription termination assay using the recently developed Tornado-Corn RNA aptamer system to create a Pol III-transcribed RNA that produces a detectable fluorescent signal when transcribed in human cells. To study the effects of RNA sequence and structure on Pol III termination, we systematically varied the sequence context upstream of the aptamer and identified sequence characteristics that enhance or diminish termination. For transcription from Pol III type 3 promoters, we found that only poly-U tracts longer than the average length found in the human genome efficiently terminate Pol III transcription without RNA secondary structure elements. We observed that RNA secondary structure elements placed in proximity to shorter poly-U tracts induced termination, and RNA secondary structure by itself was not sufficient to induce termination. For Pol III type 2 promoters, we found that the shorter poly-U tract lengths of 4 uracils were sufficient to induce termination. These findings demonstrate a key role for sequence and structural elements within Pol III-transcribed nascent RNA for efficient transcription termination, and demonstrate a generalizable assay for characterizing Pol III transcription in human cells.

Harnessing tRNA for Processing Ability and Promoter Activity

Transfer RNA (tRNA) and their associated production and processing machinery can be coopted as a versatile tool for the production of guide RNAs (gRNAs) for Cas9-based genome engineering. Using different tRNA variants enables the production of gRNAs at a variety of steady state levels. Furthermore, engineered tRNAs can be used to process gRNAs from Pol-II transcripts, thus enabling spatial/temporal control of gRNA expression. Here we describe the design, cloning, and testing of tRNA scaffolds for both Pol-III-driven expression of different levels of gRNAs, and for processing gRNAs from Pol-II transcripts.

CRISPR-Cas9 gene editing of hepatitis B virus in chronically infected humanized mice

Chronic hepatitis B virus (HBV) infection is a major public health problem. New treatment approaches are needed because current treatments do not target covalently closed circular DNA (cccDNA), the template for HBV replication, and rarely clear the virus. We harnessed adeno-associated virus (AAV) vectors and CRISPR-Staphylococcus aureus (Sa)Cas9 to edit the HBV genome in liver-humanized FRG mice chronically infected with HBV and receiving entecavir. Gene editing was detected in livers of five of eight HBV-specific AAV-SaCas9-treated mice, but not control mice, and mice with detectable HBV gene editing showed higher levels of SaCas9 delivery to HBV⁺ human hepatocytes than those without gene editing. HBV-specific AAV-SaCas9 therapy significantly improved survival of human hepatocytes, showed a trend toward decreasing total liver HBV DNA and cccDNA, and was well tolerated. This work provides evidence for the feasibility and safety of in vivo gene editing for chronic HBV infections, and it suggests that with further optimization, this approach may offer a plausible way to treat or even cure chronic HBV infections.

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.

Topologically Associated Domains Delineate Susceptibility to Somatic Hypermutation

Somatic hypermutation (SHM) introduces point mutations into immunoglobulin (Ig) genes but also causes mutations in other parts of the genome. We have used lentiviral SHM reporter vectors to identify regions of the genome that are susceptible (“hot”) and resistant (“cold”) to SHM, revealing that SHM susceptibility and resistance are often properties of entire topologically associated domains (TADs). Comparison of hot and cold TADs reveals that while levels of transcription are equivalent, hot TADs are enriched for the cohesin loader NIPBL, super-enhancers, markers of paused/stalled RNA polymerase 2, and multiple important B cell transcription factors. We demonstrate that at least some hot TADs contain enhancers that possess SHM targeting activity and that insertion of a strong Ig SHM-targeting element into a cold TAD renders it hot. Our findings lead to a model for SHM susceptibility involving the cooperative action of cis-acting SHM targeting elements and the dynamic and architectural properties of TADs.

Senigl et al. show that genome susceptibility to somatic hypermutation (SHM) is confined within topologically associated domains (TADs) and is linked to markers of strong enhancers and stalled transcription and high levels of the cohesin loader NIPBL. Insertion of an ectopic SHM targeting element renders an entire TAD susceptible to SHM.

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.

Improvement of muscular atrophy by AAV-SaCas9-mediated myostatin gene editing in aged mice

Muscle mass and area usually decrease with age, and this phenomenon is known as sarcopenia. This age-related atrophy correlates with insufficient levels of muscle cells differentiate and proliferate regulated by the TGF-β signaling pathway and the expression of E3s ubiquitin-protein ligase by the aged. Sarcopenia makes a huge impact on the aging society, because it has the characteristic of high incidence, extensive adverse effects and disease aggravation gradually. Guided by a single-guide RNA (sgRNA), Cas9 nuclease has been widely used in genome editing, opening up a new pathway for sarcopenia treatment. Here, we present two rAAV9 systems, pX601-AAV-CMV:SaCas9-U6:sgRNA and pX601-AAV-EF1α:SaCas9-tRNA_GLN: sgRNA, which edited myostatin efficiently. By delivering the two rAAV-SaCas9 targets to myostatin via intramuscular injection of aged mice, an increase in body weight and an increase in the number and area of myofibers were observed. Knockout of myostatin led to TGF-β signaling pathway changes, and increased MyoD, Pax7 and MyoG protein levels and increased the number of satellite cells to improve muscle cells differentiation. Moreover, knockout of myostatin prevented the atrophy of muscle cells through reduced Murf1 and MAFbx protein levels. We found that both rAAV-SaCas9 systems had gene editing efficiency, reducing the expression of myostatin by affecting the relevant signaling pathways, thereby altering the physiological status. We showed that myostatin has an important role in activating skeletal muscle proliferation and inhibiting muscular atrophy during aging. Thus, we propose that knockout of myostatin using the rAAV9-SaCas9 system has significant therapeutic potential in sarcopenia.

Translational efficiency across healthy and tumor tissues is proliferation-related

Different tissues express genes with particular codon usage and anticodon tRNA repertoires. However, the codon-anticodon co-adaptation in humans is not completely understood, nor is its effect on tissue-specific protein levels. Here, we first validated the accuracy of small RNA-seq for tRNA quantification across five human cell lines. We then analyzed the tRNA abundance of more than 8,000 tumor samples from TCGA, together with their paired mRNA-seq and proteomics data, to determine the Supply-to-Demand Adaptation. We thereby elucidate that the dynamic adaptation of the tRNA pool is largely related to the proliferative state across tissues. The distribution of such tRNA pools over the whole cellular translatome affects the subsequent translational efficiency, which functionally determines a condition-specific expression program both in healthy and tumor states. Furthermore, the aberrant translational efficiency of some codons in cancer, exemplified by ProCCA and GlyGGT, is associated with poor patient survival. The regulation of these tRNA profiles is partly explained by the tRNA gene copy numbers and their promoter DNA methylation.

CRISPR/Cas9-mediated modulation of splicing efficiency reveals short splicing isoform of Xist RNA is sufficient to induce X-chromosome inactivation

Alternative splicing of mRNA precursors results in multiple protein variants from a single gene and is critical for diverse cellular processes and development. Xist encodes a long noncoding RNA which is a central player to induce X-chromosome inactivation in female mammals and has two major splicing variants: long and short isoforms of Xist RNA. Although a differentiation-specific and a female-specific expression of Xist isoforms have been reported, the functional role of each Xist RNA isoform is largely unexplored. Using CRISPR/Cas9-mediated targeted modification of the 5' splice site in Xist intron 7, we create mutant female ES cell lines which dominantly express the long- or short-splicing isoform of Xist RNA from the inactive X-chromosome (Xi) upon differentiation. Successful execution of CRISPR/Cas-based splicing modulation indicates that our CRISPR/Cas-based targeted modification of splicing sites is a useful approach to study specific isoforms of a transcript generated by alternative splicing. Upon differentiation of splicing-mutant Xist female ES cells, we find that both long and short Xist isoforms can induce X-chromosome inactivation normally during ES cell differentiation, suggesting that the short splicing isoform of Xist RNA is sufficient to induce X-chromosome inactivation.

5S rRNA Promoter for Guide RNA Expression Enabled Highly Efficient CRISPR/Cas9 Genome Editing in Aspergillus niger

The CRISPR/Cas9 system is a revolutionary genome editing tool. However, in eukaryotes, search and optimization of a suitable promoter for guide RNA expression is a significant technical challenge. Here we used the industrially important fungus, Aspergillus niger, to demonstrate that the 5S rRNA gene, which is both highly conserved and efficiently expressed in eukaryotes, can be used as a guide RNA promoter. The gene editing system was established with 100% rates of precision gene modifications among dozens of transformants using short (40-bp) homologous donor DNA. This system was also applicable for generation of designer chromosomes, as evidenced by deletion of a 48 kb gene cluster required for biosynthesis of the mycotoxin fumonisin B1. Moreover, this system also facilitated simultaneous mutagenesis of multiple genes in A. niger. We anticipate that the use of the 5S rRNA gene as guide RNA promoter can broadly be applied for engineering highly efficient eukaryotic CRISPR/Cas9 toolkits. Additionally, the system reported here will enable development of designer chromosomes in model and industrially important fungi.

CRISPR/Cas: An intriguing genomic editing tool with prospects in treating neurodegenerative diseases

The CRISPR/Cas genome editing tool has led to a revolution in biological research. Its ability to target multiple genomic loci simultaneously allows its application in gene function and genomic manipulation studies. Its involvement in the sequence specific gene editing in different backgrounds has changed the scenario of treating genetic diseases. By unravelling the mysteries behind complex neuronal circuits, it not only paved way in understanding the pathogenesis of the disease but helped in the development of large animal models of different neuronal diseases; thereby opened the gateways of successfully treating different neuronal diseases. This review explored the possibility of using of CRISPR/Cas in engineering DNA at the embryonic stage, as well as during the functioning of different cell types in the brain, to delineate implications related to the use of this super-specialized genome editing tool to overcome various neurodegenerative diseases that arise as a result of genetic mutations.

Daisy-chain gene drives for the alteration of local populations

If they are able to spread in wild populations, CRISPR-based gene-drive elements would provide new ways to address ecological problems by altering the traits of wild organisms, but the potential for uncontrolled spread tremendously complicates ethical development and use. Here, we detail a self-exhausting form of CRISPR-based drive system comprising genetic elements arranged in a daisy chain such that each drives the next. "Daisy-drive" systems can locally duplicate any effect achievable by using an equivalent self-propagating drive system, but their capacity to spread is limited by the successive loss of nondriving elements from one end of the chain. Releasing daisy-drive organisms constituting a small fraction of the local wild population can drive a useful genetic element nearly to local fixation for a wide range of fitness parameters without self-propagating spread. We additionally report numerous highly active guide RNA sequences sharing minimal homology that may enable evolutionarily stable daisy drive as well as self-propagating CRISPR-based gene drive. Especially when combined with threshold dependence, daisy drives could simplify decision-making and promote ethical use by enabling local communities to decide whether, when, and how to alter local ecosystems.

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.

Toward genome editing in X-linked RP-development of a mouse model with specific treatment relevant features

Genome editing represents a powerful tool to treat inherited disorders. Highly specific endonucleases induce a DNA double strand break near the mutant site, which is subsequently repaired by cellular DNA repair mechanisms that involve the presence of a wild type template DNA. In vivo applications of this strategy are still rare, in part due to the absence of appropriate animal models carrying human disease mutations and knowledge of the efficient targeting of endonucleases. Here we report the generation and characterization of a new mouse model for X-linked retinitis pigmentosa (XLRP) carrying a point mutation in the mutational hotspot exon ORF15 of the RPGR gene as well as a recognition site for the homing endonuclease I-SceI. Presence of the genomic modifications was verified at the RNA and protein levels. The mutant protein was observed at low levels. Optical coherence tomography studies revealed a slowly progressive retinal degeneration with photoreceptor loss starting at 9 months of age, paralleling the onset of functional deficits as seen in the electroretinogram. Early changes to the outer retinal bands can be used as biomarker during treatment applications. We further show for the first time efficient targeting using the I-SceI enzyme at the genomic locus in a proof of concept in photoreceptors following adeno-associated virus mediated gene transfer in vivo. Taken together, our studies not only provide a human-XLRP disease model but also act as a platform to design genome editing technology for retinal degenerative diseases using the currently available endonucleases.

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.

Modular Ligation Extension of Guide RNA Operons (LEGO) for Multiplexed dCas9 Regulation of Metabolic Pathways in Saccharomyces cerevisiae

Metabolic engineering typically utilizes a suboptimal step-wise gene target optimization approach to parse a highly connected and regulated cellular metabolism. While the endonuclease-null CRISPR/Cas system has enabled gene expression perturbations without genetic modification, it has been mostly limited to small sets of gene targets in eukaryotes due to inefficient methods to assemble and express large sgRNA operons. In this work, we develop a TEF1p-tRNA expression system and demonstrate that the use of tRNAs as splicing elements flanking sgRNAs provides higher efficiency than both Pol III and ribozyme-based expression across a variety of single sgRNA and multiplexed contexts. Next, we devise and validate a scheme to allow modular construction of tRNA-sgRNA (TST) operons using an iterative Type IIs digestion/ligation extension approach, termed CRISPR-Ligation Extension of sgRNA Operons (LEGO). This approach enables facile construction of large TST operons. We demonstrate this utility by constructing a metabolic rewiring prototype for 2,3-butanediol production in 2 distinct yeast strain backgrounds. These results demonstrate that our approach can act as a surrogate for traditional genetic modification on a much shorter design-cycle timescale.

Live-Cell Imaging of Chromatin Condensation Dynamics by CRISPR

The spatiotemporal organization of chromatin plays central roles in cellular function. The ribosomal DNA (rDNA) chromatin undergoes dynamic structural changes during mitosis and stress. Here, we developed a CRISPR-based imaging system and tracked the condensation dynamics of rDNA chromatin in live yeast cells under glucose starvation. We found that acute glucose starvation triggers rapid condensation of rDNA. Time-lapse microscopy revealed two stages for rDNA condensation: a “primary stage,” when relaxed rDNA chromatin forms higher order loops or rings, and a “secondary stage,” when the rDNA rings further condense into compact clusters. Twisting of rDNA rings accompanies the secondary stage. The condensin complex, but not the cohesin complex, is required for efficient rDNA condensation in response to glucose starvation. Furthermore, we found that the DNA helicase Sgs1 is essential for the survival of cells expressing rDNA-bound dCas9, suggesting a role for helicases in facilitating DNA replication at dCas9-binding sites.

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A CRISPR-based imaging system allows tracking of rDNA condensation in single cells

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Glucose starvation triggers rDNA condensation in two prominent stages

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Condensin contributes to efficient rDNA condensation caused by glucose starvation

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Sgs1 helicase is required for normal rDNA replication at dCas9-binding sites

CRISPR/Cas9 mediated targeting of multiple genes in Dictyostelium

CRISPR/Cas9 has emerged in various organisms as a powerful technology for targeted gene knockout; however, no reports of editing the Dictyostelium genome efficiently using this system are available. We describe here the application of CRISPR/Cas9-mediated gene modification in Dictyostelium. The endogenous tRNA-processing system for expressing sgRNA was approximately 10 times more effective than the commonly used U6 promoter. The resulting sgRNA affected the sub-nuclear localisation of Cas9, indicating that the expression level of sgRNA was sufficiently high to form Cas9 and sgRNA complexes within the nucleus. The all-in-one vector containing Cas9 and sgRNA was transiently expressed to generate mutants in five PI3K genes. Mutation detective PCR revealed the mutagenesis frequency of the individual genes to be between 72.9% and 100%. We confirmed that all five targeting loci in the four independent clones had insertion/deletion mutations in their target sites. Thus, we show that the CRISPR/Cas9 system can be used in Dictyostelium cells to enable efficient genome editing of multiple genes. Since this system utilises transient expression of the all-in-one vector, it has the advantage that the drug resistance cassette is not integrated into the genome and simple vector construction, involving annealing two oligo-DNAs.

Rational Design of Mini-Cas9 for Transcriptional Activation

Nuclease dead Cas9 (dCas9) has been widely used for modulating gene expression by fusing with different activation or repression domains. However, delivery of the CRISPR/Cas system fused with various effector domains in a single adeno-associated virus (AAV) remains challenging due to the payload limit. Here, we engineered a set of downsized variants of Cas9 including Staphylococcus aureus Cas9 (SaCas9) that retained DNA binding activity by deleting conserved functional domains. We demonstrated that fusing FokI nuclease domain to the N-terminal of the minimal SaCas9 (mini-SaCas9) or to the middle of the split mini-SaCas9 can trigger efficient DNA cleavage. In addition, we constructed a set of compact transactivation domains based on the tripartite VPR activation domain and self-assembled arrays of split SpyTag:SpyCatch peptides, which are suitable for fusing to the mini-SaCas9. Lastly, we produced a single AAV containing the mini-SaCas9 fused with a downsized transactivation domain along with an optimized gRNA expression cassette, which showed efficient transactivation activity. Our results highlighted a practical approach to generate down-sized CRISPR/Cas9 and gene activation systems for in vivo applications.

Targeted Deletion of the USTA and UvSLT2 Genes Efficiently in Ustilaginoidea virens With the CRISPR-Cas9 System

Ustilaginoidea virens is the causal agent of rice false smut, one of the major fungal diseases of rice. However, there are only limited molecular studies with this important pathogen due to the lack of efficient approaches for generating targeted gene disruption mutants. In this study, we used the CRISPR-Cas9 system to efficiently generate mutants deleted of the USTA ustiloxin and UvSLT2 MAP kinase genes. Three gRNA spacers of USTA, UA01, UA13, and UA21, were expressed with the RNAP III promoter of Gln-tRNA. For all of them, the homologous gene replacement frequency was higher when the Cas9 and gRNA constructs were transformed into U. virens on the same vector than sequentially. UA01, the spacer with the highest on-target score, had the highest knockout frequency of 90%, which was over 200 times higher than that of Agrobacterium tumefaciens-mediated transformation (ATMT) for generating ustA mutants. None of these USTA spacers had predicted off-targets with 1 or 2-nt variations. For predicted off-targets with 3 or 4-nt variations, mutations were not detected in 10 ustA mutants generated with spacer UA13 or UA21, indicating a relatively low frequency of off-target mutations in U. virens. For UvSLT2, the homologous gene replacement frequency was 50% with CRISPR-Cas9, which also was significantly higher than that of ATMT. Whereas ustA mutants had no detectable phenotypes, Uvslt2 mutants were slightly reduced in growth rate and reduced over 70% in conidiation. Deletion of UvSLT2 also increased sensitivity to cell wall stresses but tolerance to hyperosmotic or oxidative stresses. Taken together, our results showed that the CRISPR-Cas9 system can be used as an efficient gene replacement or editing approach in U. virens and the UvSlt2 MAP kinase pathway has a conserved role in cell wall integrity.

Expression of short hairpin RNAs using the compact architecture of retroviral microRNA genes

Short hairpin RNAs (shRNAs) are effective in generating stable repression of gene expression. RNA polymerase III (RNAP III) type III promoters (U6 or H1) are typically used to drive shRNA expression. While useful for some knockdown applications, the robust expression of U6/H1-driven shRNAs can induce toxicity and generate heterogeneous small RNAs with undesirable off-target effects. Additionally, typical U6/H1 promoters encompass the majority of the ∼270 base pairs (bp) of vector space required for shRNA expression. This can limit the efficacy and/or number of delivery vector options, particularly when delivery of multiple gene/shRNA combinations is required. Here, we develop a compact shRNA (cshRNA) expression system based on retroviral microRNA (miRNA) gene architecture that uses RNAP III type II promoters. We demonstrate that cshRNAs coded from as little as 100 bps of total coding space can precisely generate small interfering RNAs (siRNAs) that are active in the RNA-induced silencing complex (RISC). We provide an algorithm with a user-friendly interface to design cshRNAs for desired target genes. This cshRNA expression system reduces the coding space required for shRNA expression by >2-fold as compared to the typical U6/H1 promoters, which may facilitate therapeutic RNAi applications where delivery vector space is limiting.

Application of the gene editing tool, CRISPR-Cas9, for treating neurodegenerative diseases

Increased accumulation of transcribed protein from the damaged DNA and reduced DNA repair capability contributes to numerous neurological diseases for which effective treatments are lacking. Gene editing techniques provide new hope for replacing defective genes and DNA associated with neurological diseases. With advancements in using such editing tools as zinc finger nucleases (ZFNs), meganucleases, and transcription activator-like effector nucleases (TALENs), etc., scientists are able to design DNA-binding proteins, which can make precise double-strand breaks (DSBs) at the target DNA. Recent developments with the CRISPR-Cas9 gene-editing technology has proven to be more precise and efficient when compared to most other gene-editing techniques. Two methods, non-homologous end joining (NHEJ) and homology-direct repair (HDR), are used in CRISPR-Cas9 system to efficiently excise the defective genes and incorporate exogenous DNA at the target site. In this review article, we provide an overview of the CRISPR-Cas9 methodology, including its molecular mechanism, with a focus on how in this gene-editing tool can be used to counteract certain genetic defects associated with neurological diseases. Detailed understanding of this new tool could help researchers design specific gene editing strategies to repair genetic disorders in selective neurological diseases.

Virus-host interplay in hepatitis B virus infection and epigenetic treatment strategies

Worldwide, chronic hepatitis B virus (HBV) infection is a major health problem and no cure exists. Importantly, hepatocyte intrusion by HBV particles results in a complex deregulation of both viral and host cellular genetic and epigenetic processes. Among the attempts to develop novel therapeutic approaches against HBV infection, several options targeting the epigenomic regulation of HBV replication are gaining attention. These include the experimental treatment with 'epidrugs'. Moreover, as a targeted approach, the principle of 'epigenetic editing' recently is being exploited to control viral replication. Silencing of HBV by specific rewriting of epigenetic marks might diminish viral replication, viremia, and infectivity, eventually controlling the disease and its complications. Additionally, epigenetic editing can be used as an experimental tool to increase our limited understanding regarding the role of epigenetic modifications in viral infections. Aiming for permanent epigenetic reprogramming of the viral genome without unspecific side effects, this breakthrough may pave the roads for an ambitious technological pursuit: to start designing a curative approach utilizing manipulative molecular therapies for viral infections in vivo.

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

Clustered regularly interspaced short palindromic repeat/Cas9 (CRISPR/Cas9) system has emerged in recent years as a highly efficient RNA-guided gene manipulation platform. Simultaneous editing or transcriptional activation/suppression of different genes becomes feasible with the co-delivery of multiple guide RNAs (gRNAs). Here, we report that multiple gRNAs linked with self-cleaving ribozymes and/or tRNA could be simultaneously expressed from a single U6 promoter to exert genome editing of dystrophin and myosin binding protein C3 in human and mouse cells. Moreover, this strategy allows the expression of multiple gRNAs for synergistic transcription activation of follistatin when used with catalytically inactive dCas9-VP64 or dCas9-p300core fusions. Finally, the gRNAs linked by the self-cleaving ribozymes and tRNA could be expressed from RNA polymerase type II (pol II) promoters such as generic CMV and muscle/heart-specific MHCK7. This is particularly useful for in vivo applications when the packaging capacity of recombinant adeno-associated virus is limited while tissue-specific delivery of gRNAs and Cas9 is desired. Taken together, this study provides a novel strategy to enable tissue-specific expression of more than one gRNAs for multiplex gene editing from a single pol II promoter.

CRISPR/Cas9 with single guide RNA expression driven by small tRNA promoters showed reduced editing efficiency compared to a U6 promoter

Multiplex genome engineering in vivo with CRISPR/Cas9 shows great promise as a potential therapeutic approach. The ability to incorporate multiple single guide RNA (sgRNA) cassettes together with Cas9 gene expression in one AAV vector could greatly enhance the efficiency. In a recent Method article, Mefferd and coworkers indicated that small tRNA promoters could be used to drive sgRNA expression to facilitate the construction of a more effective AAV vector. In contrast, we found that when targeting endogenous genomic loci, CRISPR/Cas9 with tRNA promoter-driven sgRNA expression showed much reduced genome editing activity, compared with significant cleavage with U6 promoter-driven sgRNA expression. Though the underlying mechanisms are still under investigation, our study suggests that the CRISPR/Cas9 system with tRNA promoter-driven sgRNA expression needs to be reevaluated before it can be used for therapeutic genome editing.

Engineering Delivery Vehicles for Genome Editing

The field of genome engineering has created new possibilities for gene therapy, including improved animal models of disease, engineered cell therapies, and in vivo gene repair. The most significant challenge for the clinical translation of genome engineering is the development of safe and effective delivery vehicles. A large body of work has applied genome engineering to genetic modification in vitro, and clinical trials have begun using cells modified by genome editing. Now, promising preclinical work is beginning to apply these tools in vivo. This article summarizes the development of genome engineering platforms, including meganucleases, zinc finger nucleases, TALENs, and CRISPR/Cas9, and their flexibility for precise genetic modifications. The prospects for the development of safe and effective viral and nonviral delivery vehicles for genome editing are reviewed, and promising advances in particular therapeutic applications are discussed.

Potential pitfalls of CRISPR/Cas9-mediated genome editing

Recently, a novel technique named the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas)9 system has been rapidly developed. This genome editing tool has improved our ability tremendously with respect to exploring the pathogenesis of diseases and correcting disease mutations, as well as phenotypes. With a short guide RNA, Cas9 can be precisely directed to target sites, and functions as an endonuclease to efficiently produce breaks in DNA double strands. Over the past 30 years, CRISPR has evolved from the 'curious sequences of unknown biological function' into a promising genome editing tool. As a result of the incessant development in the CRISPR/Cas9 system, Cas9 co-expressed with custom guide RNAs has been successfully used in a variety of cells and organisms. This genome editing technology can also be applied to synthetic biology, functional genomic screening, transcriptional modulation and gene therapy. However, although CRISPR/Cas9 has a broad range of action in science, there are several aspects that affect its efficiency and specificity, including Cas9 activity, target site selection and short guide RNA design, delivery methods, off-target effects and the incidence of homology-directed repair. In the present review, we highlight the factors that affect the utilization of CRISPR/Cas9, as well as possible strategies for handling any problems. Addressing these issues will allow us to take better advantage of this technique. In addition, we also review the history and rapid development of the CRISPR/Cas system from the time of its initial discovery in 2012.

In Vivo CRISPR/Cas9 Gene Editing Corrects Retinal Dystrophy in the S334ter-3 Rat Model of Autosomal Dominant Retinitis Pigmentosa

Reliable genome editing via Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/Cas9 may provide a means to correct inherited diseases in patients. As proof of principle, we show that CRISPR/Cas9 can be used in vivo to selectively ablate the rhodopsin gene carrying the dominant S334ter mutation (Rho(S334)) in rats that model severe autosomal dominant retinitis pigmentosa. A single subretinal injection of guide RNA/Cas9 plasmid in combination with electroporation generated allele-specific disruption of Rho(S334), which prevented retinal degeneration and improved visual function.

Optimization of a multiplex CRISPR/Cas system for use as an antiviral therapeutic

RNA-guided endonucleases or CRISPR/Cas systems have been widely employed for gene engineering/DNA editing applications, and have recently been used against a variety of dsDNA viruses as a potential therapeutic. However, in vivo delivery to specific tissue reservoirs using adeno-associated virus (AAV) vectors is problematic due to the large coding requirement for the principal effector commonly used in these applications, Streptococcus pyogenes (Spy) Cas9. Here we describe design of a minimal CRISPR/Cas system that is capable of multiplexing and can be packaged into a single AAV vector. This system consists of the small Type II Cas9 protein from Staphylococcus aureus (Sau) driven by a truncated CMV promoter/enhancer, and flanked 3' by a poly(A) addition signal, as well as two sgRNA expression cassettes driven by either U6 or ∼70-bp tRNA-derived Pol III promoters. Specific protocols for construction of these AAV vector scaffolds, shuttle cloning of their contents into AAV and lentiviral backbones, and a quantitative luciferase assay capable of screening for optimal sgRNAs, are detailed. These protocols can facilitate construction of AAV vectors that have optimal multiplexed sgRNA expression and function. These will have potential utility in multiplex applications, including in antiviral therapy in tissues chronically infected with a pathogenic DNA virus.

Characterization of Staphylococcus aureus Cas9: a smaller Cas9 for all-in-one adeno-associated virus delivery and paired nickase applications

Background

CRISPR-Cas systems have been broadly embraced as effective tools for genome engineering applications, with most studies to date utilizing the Streptococcus pyogenes Cas9. Here we characterize and manipulate the smaller, 1053 amino acid nuclease Staphylococcus aureus Cas9.

Results

We find that the S. aureus Cas9 recognizes an NNGRRT protospacer adjacent motif (PAM) and cleaves target DNA at high efficiency with a variety of guide RNA (gRNA) spacer lengths. When directed against genomic targets with mutually permissive NGGRRT PAMs, the S. pyogenes Cas9 and S. aureus Cas9 yield indels at comparable rates. We additionally show D10A and N580A paired nickase activity with S. aureus Cas9, and we further package it with two gRNAs in a single functional adeno-associated virus (AAV) vector. Finally, we assess comparative S. pyogenes and S. aureus Cas9 specificity using GUIDE-seq.

Conclusion

Our results reveal an S. aureus Cas9 that is effective for a variety of genome engineering purposes, including paired nickase approaches and all-in-one delivery of Cas9 and multiple gRNA expression cassettes with AAV vectors.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0817-8) contains supplementary material, which is available to authorized users.

Targeting hepatitis B virus cccDNA using CRISPR/Cas9

Despite the existence of an excellent prophylactic vaccine and the development of highly effective inhibitors of the viral polymerase, chronic hepatitis B virus (HBV) infection remains a major source of morbidity and mortality, especially in Africa and Asia. A significant problem is that, while polymerase inhibitors can effectively prevent the production of viral genomic DNA from pre-genomic RNA transcripts, they do not prevent the transcription and translation of viral mRNAs from the covalently closed circular DNA (cccDNA) templates present in the nuclei of infected cells. Moreover, because these cccDNAs are highly stable, chronic HBV infections are only very rarely cured by the use of polymerase inhibitors and these drugs clearly cannot entirely prevent the subsequent development of HBV-related morbidities such as cirrhosis and hepatocellular carcinoma. As a result, there has been considerable interest in the possibility of developing treatment approaches that directly target cccDNA for elimination. Here, we discuss recent publications that analyze the ability of the bacterial CRISPR/Cas DNA editing machinery to be repurposed as a tool for the specific cleavage and destruction of HBV cccDNAs in the nuclei of infected cells and consider which steps will be necessary to make CRISPR/Cas targeting of HBV DNA a clinically feasible approach to the treatment of chronic infections in humans. This article forms part of a symposium in Antiviral Research on "An unfinished story: from the discovery of the Australia antigen to the development of new curative therapies for hepatitis B."

A new age in functional genomics using CRISPR/Cas9 in arrayed library screening

CRISPR technology has rapidly changed the face of biological research, such that precise genome editing has now become routine for many labs within several years of its initial development. What makes CRISPR/Cas9 so revolutionary is the ability to target a protein (Cas9) to an exact genomic locus, through designing a specific short complementary nucleotide sequence, that together with a common scaffold sequence, constitute the guide RNA bridging the protein and the DNA. Wild-type Cas9 cleaves both DNA strands at its target sequence, but this protein can also be modified to exert many other functions. For instance, by attaching an activation domain to catalytically inactive Cas9 and targeting a promoter region, it is possible to stimulate the expression of a specific endogenous gene. In principle, any genomic region can be targeted, and recent efforts have successfully generated pooled guide RNA libraries for coding and regulatory regions of human, mouse and Drosophila genomes with high coverage, thus facilitating functional phenotypic screening. In this review, we will highlight recent developments in the area of CRISPR-based functional genomics and discuss potential future directions, with a special focus on mammalian cell systems and arrayed library screening.