CRISPRMatch: An Automatic Calculation and Visualization Tool for High-throughput CRISPR Genome-editing Data Analysis

SuperDecode: An integrated toolkit for analyzing mutations induced by genome editing

Genome editing using CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein) or other systems has become a cornerstone of numerous biological and applied research fields. However, detecting the resulting mutations by analyzing sequencing data remains time consuming and inefficient. In response to this issue, we designed SuperDecode, an integrated software toolkit for analyzing editing outcomes using a range of sequencing strategies. SuperDecode comprises three modules, DSDecodeMS, HiDecode, and LaDecode, each designed to automatically decode mutations from Sanger, high-throughput short-read, and long-read sequencing data, respectively, from targeted PCR amplicons. By leveraging specific strategies for constructing sequencing libraries of pooled multiple amplicons, HiDecode and LaDecode facilitate large-scale identification of mutations induced by single or multiplex target-site editing in a cost-effective manner. We demonstrate the efficacy of SuperDecode by analyzing mutations produced using different genome editing tools (CRISPR/Cas, base editing, and prime editing) in different materials (diploid and tetraploid rice and protoplasts), underscoring its versatility in decoding genome editing outcomes across different applications. Furthermore, this toolkit can be used to analyze other genetic variations, as exemplified by its ability to estimate the C-to-U editing rate of the cellular RNA of a mitochondrial gene. SuperDecode offers both a standalone software package and a web-based version, ensuring its easy access and broad compatibility across diverse computer systems. Thus, SuperDecode provides a comprehensive platform for analyzing a wide array of mutations, advancing the utility of genome editing for scientific research and genetic engineering.

A comprehensive all-in-one CRISPR toolbox for large-scale screens in plants

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated nuclease (Cas) technologies facilitate routine genome engineering of one or a few genes at a time. However, large-scale CRISPR screens with guide RNA libraries remain challenging in plants. Here, we have developed a comprehensive all-in-one CRISPR toolbox for Cas9-based genome editing, cytosine base editing, adenine base editing (ABE), Cas12a-based genome editing and ABE, and CRISPR-Act3.0-based gene activation in both monocot and dicot plants. We evaluated all-in-one T-DNA expression vectors in rice (Oryza sativa, monocot) and tomato (Solanum lycopersicum, dicot) protoplasts, demonstrating their broad and reliable applicability. To showcase the applications of these vectors in CRISPR screens, we constructed guide RNA (gRNA) pools for testing in rice protoplasts, establishing a high-throughput approach to select high-activity gRNAs. Additionally, we demonstrated the efficacy of sgRNA library screening for targeted mutagenesis of ACETOLACTATE SYNTHASE in rice, recovering novel candidate alleles for herbicide resistance. Furthermore, we carried out a CRISPR activation screen in Arabidopsis thaliana, rapidly identifying potent gRNAs for FLOWERING LOCUS T activation that confer an early-flowering phenotype. This toolbox contains 61 versatile all-in-one vectors encompassing nearly all commonly used CRISPR technologies. It will facilitate large-scale genetic screens for loss-of-function or gain-of-function studies, presenting numerous promising applications in plants.

Enhanced Genome Editing Activity with Novel Chimeric ScCas9 Variants in Rice

The Streptococcus canis Cas9 protein (ScCas9) recognizes the NNG protospacer adjacent motif (PAM), offering a wider range of targets than that offered by the commonly used S. pyogenes Cas9 protein (SpCas9). However, both ScCas9 and its evolved Sc++ variant still exhibit low genome editing efficiency in plants, particularly at the less preferred NTG and NCG PAM targets. In this study, a chimeric SpcRN++ variant is engineered by grafting the recognition (REC) domain of SpCas9 into the Sc++ variant, incorporating the R221K/N394K mutations, and retaining the positively charged loop of S. anginosus Cas9. The SpcRN++ variant exhibits a higher genome editing capacity and wider target range than the Sc++ variant in rice protoplasts and stable transgenic plants. Further evidence indicates that nSpcRN++-based A3A/Y130F and TadA8e exhibit enhanced cytosine and adenine editing efficiency in plants. Finally, herbicide-resistant rice germplasms are produced by targeting the OsACC gene using nSpcRN++-based adenine base editors. These results demonstrate that SpcRN++ is a powerful tool for genome editing in plants, and this integrative protein engineering strategy holds promise for engineering other Cas9 proteins.

PAM-relaxed and temperature-tolerant CRISPR-Mb3Cas12a single transcript unit systems for efficient singular and multiplexed genome editing in rice, maize, and tomato

Class 2 Type V-A CRISPR-Cas (Cas12a) nucleases are powerful genome editing tools, particularly effective in A/T-rich genomic regions, complementing the widely used CRISPR-Cas9 in plants. To enhance the utility of Cas12a, we investigate three Cas12a orthologs-Mb3Cas12a, PrCas12a, and HkCas12a-in plants. Protospacer adjacent motif (PAM) requirements, editing efficiencies, and editing profiles are compared in rice. Among these orthologs, Mb3Cas12a exhibits high editing efficiency at target sites with a simpler, relaxed TTV PAM which is less restrictive than the canonical TTTV PAM of LbCas12a and AsCas12a. To optimize Mb3Cas12a, we develop an efficient single transcription unit (STU) system by refining the linker between Mb3Cas12a and CRISPR RNA (crRNA), nuclear localization signal (NLS), and direct repeat (DR). This optimized system enables precise genome editing in rice, particularly for fine-tuning target gene expression by editing promoter regions. Further, we introduced Arginine (R) substitutions at Aspartic acid (D) 172, Asparagine (N) 573, and Lysine (K) 579 of Mb3Cas12a, creating two temperature-tolerant variants: Mb3Cas12a-R (D172R) and Mb3Cas12a-RRR (D172R/N573R/K579R). These variants demonstrate significantly improved editing efficiency at lower temperatures (22 °C and 28 °C) in rice cells, with Mb3Cas12a-RRR showing the best performance. We extend this approach by developing efficient Mb3Cas12a-RRR STU systems in maize and tomato, achieving biallelic mutants targeting single or multiple genes in T0 lines cultivated at 28 °C and 25 °C, respectively. This study significantly expands Cas12a's targeting capabilities in plant genome editing, providing valuable tools for future research and practical applications.

Versatile plant genome engineering using anti-CRISPR-Cas12a systems

CRISPR-Cas12a genome engineering systems have been widely used in plant research and crop breeding. To date, the performance and use of anti-CRISPR-Cas12a systems have not been fully established in plants. Here, we conduct in silico analysis to identify putative anti-CRISPR systems for Cas12a. These putative anti-CRISPR proteins, along with known anti-CRISPR proteins, are assessed for their ability to inhibit Cas12a cleavage activity in vivo and in planta. Among all anti-CRISPR proteins tested, AcrVA1 shows robust inhibition of Mb2Cas12a and LbCas12a in E. coli. Further tests show that AcrVA1 inhibits LbCas12a mediated genome editing in rice protoplasts and stable transgenic lines. Impressively, co-expression of AcrVA1 mitigates off-target effects by CRISPR-LbCas12a, as revealed by whole genome sequencing. In addition, transgenic plants expressing AcrVA1 exhibit different levels of inhibition to LbCas12a mediated genome editing, representing a novel way of fine-tuning genome editing efficiency. By controlling temporal and spatial expression of AcrVA1, we show that inducible and tissue specific genome editing can be achieved in plants. Furthermore, we demonstrate that AcrVA1 also inhibits LbCas12a-based CRISPR activation (CRISPRa) and based on this principle we build logic gates to turn on and off target genes in plant cells. Together, we have established an efficient anti-CRISPR-Cas12a system in plants and demonstrate its versatile applications in mitigating off-target effects, fine-tuning genome editing efficiency, achieving spatial-temporal control of genome editing, and generating synthetic logic gates for controlling target gene expression in plant cells.

Expanding plant genome editing scope and profiles with CRISPR-FrCas9 systems targeting palindromic TA sites

CRISPR-Cas9 is widely used for genome editing, but its PAM sequence requirements limit its efficiency. In this study, we explore Faecalibaculum rodentium Cas9 (FrCas9) for plant genome editing, especially in rice. FrCas9 recognizes a concise 5'-NNTA-3' PAM, targeting more abundant palindromic TA sites in plant genomes than the 5'-NGG-3' PAM sites of the most popular SpCas9. FrCas9 shows cleavage activities at all tested 5'-NNTA-3' PAM sites with editing outcomes sharing the same characteristics of a typical CRISPR-Cas9 system. FrCas9 induces high-efficiency targeted mutagenesis in stable rice lines, readily generating biallelic mutants with expected phenotypes. We augment FrCas9's ability to generate larger deletions through fusion with the exonuclease, TREX2. TREX2-FrCas9 generates much larger deletions than FrCas9 without compromise in editing efficiency. We demonstrate TREX2-FrCas9 as an efficient tool for genetic knockout of a microRNA gene. Furthermore, FrCas9-derived cytosine base editors (CBEs) and adenine base editors (ABE) are developed to produce targeted C-to-T and A-to-G base edits in rice plants. Whole-genome sequencing-based off-target analysis suggests that FrCas9 is a highly specific nuclease. Expression of TREX2-FrCas9 in plants, however, causes detectable guide RNA-independent off-target mutations, mostly as single nucleotide variants (SNVs). Together, we have established an efficient CRISPR-FrCas9 system for targeted mutagenesis, large deletions, C-to-T base editing, and A-to-G base editing in plants. The simple palindromic TA motif in the PAM makes the CRISPR-FrCas9 system a promising tool for genome editing in plants with an expanded targeting scope.

High performance TadA-8e derived cytosine and dual base editors with undetectable off-target effects in plants

Cytosine base editors (CBEs) and adenine base editors (ABEs) enable precise C-to-T and A-to-G edits. Recently, ABE8e, derived from TadA-8e, enhances A-to-G edits in mammalian cells and plants. Interestingly, TadA-8e can also be evolved to confer C-to-T editing. This study compares engineered CBEs derived from TadA-8e in rice and tomato cells, identifying TadCBEa, TadCBEd, and TadCBEd_V106W as efficient CBEs with high purity and a narrow editing window. A dual base editor, TadDE, promotes simultaneous C-to-T and A-to-G editing. Multiplexed base editing with TadCBEa and TadDE is demonstrated in transgenic rice, with no off-target effects detected by whole genome and transcriptome sequencing, indicating high specificity. Finally, two crop engineering applications using TadDE are shown: introducing herbicide resistance alleles in OsALS and creating synonymous mutations in OsSPL14 to resist OsMIR156-mediated degradation. Together, this study presents TadA-8e derived CBEs and a dual base editor as valuable additions to the plant editing toolbox.

Genome editing in rice and tomato with a small Un1Cas12f1 nuclease

The clustered regularly interspaced short palindromic repeats (CRISPR) systems have been demonstrated to be the foremost compelling genetic tools for manipulating prokaryotic and eukaryotic genomes. Despite the robustness and versatility of Cas9 and Cas12a/b nucleases in mammalian cells and plants, their large protein sizes may hinder downstream applications. Therefore, investigating compact CRISPR nucleases will unlock numerous genome editing and delivery challenges that constrain genetic engineering and crop development. In this study, we assessed the archaeal miniature Un1Cas12f1 type-V CRISPR nuclease for genome editing in rice and tomato protoplasts. By adopting the reengineered guide RNA modifications ge4.1 and comparing polymerase II (Pol II) and polymerase III (Pol III) promoters, we demonstrated uncultured archaeon Cas12f1 (Un1Cas12f1) genome editing efficacy in rice and tomato protoplasts. We characterized the protospacer adjacent motif (PAM) requirements and mutation profiles of Un1Cas12f1 in both plant species. Interestingly, we found that Pol III promoters, not Pol II promoters, led to higher genome editing efficiency when they were used to drive guide RNA expression. Unlike in mammalian cells, the engineered Un1Cas12f1-RRA variant did not perform better than the wild-type Un1Cas12f1 nuclease, suggesting continued protein engineering and other innovative approaches are needed to further improve Un1Cas12f1 genome editing in plants.

CRISPR/Cas9-gene editing approaches in plant breeding

CRISPR/Cas9 gene editing system is recently developed robust genome editing technology for accelerating plant breeding. Various modifications of this editing system have been established for adaptability in plant varieties as well as for its improved efficiency and portability. This review provides an in-depth look at the various strategies for synthesizing gRNAs for efficient delivery in plant cells, including chemical synthesis and in vitro transcription. It also covers traditional analytical tools and emerging developments in detection methods to analyze CRISPR/Cas9 mediated mutation in plant breeding. Additionally, the review outlines the various analytical tools which are used to detect and analyze CRISPR/Cas9 mediated mutations, such as next-generation sequencing, restriction enzyme analysis, and southern blotting. Finally, the review discusses emerging detection methods, including digital PCR and qPCR. Hence, CRISPR/Cas9 has great potential for transforming agriculture and opening avenues for new advancements in the system for gene editing in plants.

Hs1Cas12a and Ev1Cas12a confer efficient genome editing in plants

Cas12a, also known as Cpf1, is a highly versatile CRISPR-Cas enzyme that has been widely used in genome editing. Unlike its well-known counterpart, Cas9, Cas12a has unique features that make it a highly efficient genome editing tool at AT-rich genomic regions. To enrich the CRISPR-Cas12a plant genome editing toolbox, we explored 17 novel Cas12a orthologs for their genome editing capabilities in plants. Out of them, Ev1Cas12a and Hs1Cas12a showed efficient multiplexed genome editing in rice and tomato protoplasts. Notably, Hs1Cas12a exhibited greater tolerance to lower temperatures. Moreover, Hs1Cas12a generated up to 87.5% biallelic editing in rice T0 plants. Both Ev1Cas12a and Hs1Cas12a achieved effective editing in poplar T0 plants, with up to 100% of plants edited, albeit with high chimerism. Taken together, the efficient genome editing demonstrated by Ev1Cas12a and Hs1Cas12a in both monocot and dicot plants highlights their potential as promising genome editing tools in plant species and beyond.

Efficient plant genome engineering using a probiotic sourced CRISPR-Cas9 system

Among CRISPR-Cas genome editing systems, Streptococcus pyogenes Cas9 (SpCas9), sourced from a human pathogen, is the most widely used. Here, through in silico data mining, we have established an efficient plant genome engineering system using CRISPR-Cas9 from probiotic Lactobacillus rhamnosus. We have confirmed the predicted 5'-NGAAA-3' PAM via a bacterial PAM depletion assay and showcased its exceptional editing efficiency in rice, wheat, tomato, and Larix cells, surpassing LbCas12a, SpCas9-NG, and SpRY when targeting the identical sequences. In stable rice lines, LrCas9 facilitates multiplexed gene knockout through coding sequence editing and achieves gene knockdown via targeted promoter deletion, demonstrating high specificity. We have also developed LrCas9-derived cytosine and adenine base editors, expanding base editing capabilities. Finally, by harnessing LrCas9's A/T-rich PAM targeting preference, we have created efficient CRISPR interference and activation systems in plants. Together, our work establishes CRISPR-LrCas9 as an efficient and user-friendly genome engineering tool for diverse applications in crops and beyond.

Genome- and transcriptome-wide off-target analyses of a high-efficiency adenine base editor in tomato

Adenine base editors (ABEs) are valuable, precise genome editing tools in plants. In recent years, the highly promising ADENINE BASE EDITOR8e (ABE8e) was reported for efficient A-to-G editing. However, compared to monocots, comprehensive off-target analyses for ABE8e are lacking in dicots. To determine the occurrence of off-target effects in tomato (Solanum lycopersicum), we assessed ABE8e and a high-fidelity version, ABE8e-HF, at 2 independent target sites in protoplasts, as well as stable T0 lines. Since ABE8e demonstrated higher on-target efficiency than ABE8e-HF in tomato protoplasts, we focused on ABE8e for off-target analyses in T0 lines. We conducted whole-genome sequencing (WGS) of wild-type (WT) tomato plants, green fluorescent protein (GFP)-expressing T0 lines, ABE8e-no-gRNA control T0 lines, and edited T0 lines. No guide RNA (gRNA)-dependent off-target edits were detected. Our data showed an average of approximately 1,200 to 1,500 single-nucleotide variations (SNVs) in either GFP control plants or base-edited plants. Also, no specific enrichment of A-to-G mutations were found in base-edited plants. We also conducted RNA sequencing (RNA-seq) of the same 6 base-edited and 3 GFP control T0 plants. On average, approximately 150 RNA-level SNVs were discovered per plant for either base-edited or GFP controls. Furthermore, we did not find enrichment of a TA motif on mutated adenine in the genomes and transcriptomes in base-edited tomato plants, as opposed to the recent discovery in rice (Oryza sativa). Hence, we could not find evidence for genome- and transcriptome-wide off-target effects by ABE8e in tomato.

A Digital PCR Method Based on Highly Specific Taq for Detecting Gene Editing and Mutations

Digital PCR (dPCR) has great potential for assessing gene editing or gene mutation due to its ability to independently inspect each DNA template in parallel. However, current dPCR methods use a fluorescence-labeled probe to detect gene variation events, and their ability to distinguish variated sequences from the wild-type sequence is limited by the probe's tolerance to mismatch. To address this, we have developed a novel dPCR method that uses a primer instead of a probe to sense gene variation. The enhanced Taq DNA polymerase in the PCR system has a high mismatch sensitivity, which enables our dPCR method to distinguish gene mutations from wild-type sequences. Compared to current dPCR methods, our method shows superior precision in assessing gene editing efficiency and single-base DNA mutation. This presents a promising opportunity to advance gene editing research and rare gene mutation detection.

Massively parallel CRISPR off-target detection enables rapid off-target prediction model building

BACKGROUND

CRISPR (clustered regularly interspaced short palindromic repeats) genome editing holds tremendous potential in clinical translation. However, the off-target effect has always been a major concern.

METHODS

Here, we have developed a novel sensitive and specific off-target detection method, AID-seq (adaptor-mediated off-target identification by sequencing), that can comprehensively and faithfully detect the low-frequency off targets generated by different CRISPR nucleases (including Cas9 and Cas12a).

FINDINGS

Based on AID-seq, we developed a pooled strategy to simultaneously identify the on/off targets of multiple gRNAs, as well as using mixed human and human papillomavirus (HPV) genomes to screen the most efficient and safe targets from 416 HPV gRNA candidates for antiviral therapy. Moreover, we used the pooled strategy with 2,069 single-guide RNAs (sgRNAs) at a pool size of about 500 to profile the properties of our newly discovered CRISPR, FrCas9. Importantly, we successfully built an off-target detection model using these off-target data via the CRISPR-Net deep learning method (area under the receiver operating characteristic curve [AUROC] = 0.97, area under the precision recall curve [AUPRC] = 0.29).

CONCLUSIONS

To our knowledge, AID-seq is the most sensitive and specific in vitro off-target detection method to date. And the pooled AID-seq strategy can be used as a rapid and high-throughput platform to select the best sgRNAs and characterize the properties of new CRISPRs.

FUNDING

This work was supported by The National Natural Science Foundation of China (grant nos. 32171465 and 82102392), the General Program of Natural Science Foundation of Guangdong Province of China (grant no. 2021A1515012438), Guangdong Basic and Applied Basic Research Foundation (grant no. 2020A1515110170), and the National Ten Thousand Plan-Young Top Talents of China (grant no. 80000-41180002).

CRISPR-GRANT: a cross-platform graphical analysis tool for high-throughput CRISPR-based genome editing evaluation

BACKGROUD

CRISPR/Cas is an efficient genome editing system that has been widely used for functional genetic studies and exhibits high potential in biomedical translational applications. Indel analysis has thus become one of the most common practices in the lab to evaluate DNA editing events generated by CRISPR/Cas. Several indel analysis tools have been reported, however, it is often required that users have certain bioinformatics training and basic command-line processing capability.

RESULTS

Here, we developed CRISPR-GRANT, a stand-alone graphical CRISPR indel analysis tool, which could be easily installed for multi-platforms, including Linux, Windows, and macOS. CRISPR-GRANT offered a straightforward GUI by simple click-and-run for genome editing analysis of single or pooled amplicons and one-step analysis for whole-genome sequencing without the need of data pre-processing, making it ideal for novice lab scientists. Moreover, it also exhibited shorter run-time compared with tools currently available.

CONCLUSION

Therefore, CRISPR-GRANT is a valuable addition to the current CRISPR toolkits that significantly lower the barrier for wet-lab researchers to conduct indel analysis from large NGS datasets. CRISPR-GRANT binaries are freely available for Linux (above Ubuntu 16.04), macOS (above High Sierra 10.13) and Windows (above Windows 7) at https://github.com/fuhuancheng/CRISPR-GRANT . CRISPR-GRANT source code is licensed under the GPLv3 license and free to download and use.

Boosting genome editing efficiency in human cells and plants with novel LbCas12a variants

BACKGROUND

Cas12a (formerly known as Cpf1), the class II type V CRISPR nuclease, has been widely used for genome editing in mammalian cells and plants due to its distinct characteristics from Cas9. Despite being one of the most robust Cas12a nucleases, LbCas12a in general is less efficient than SpCas9 for genome editing in human cells, animals, and plants.

RESULTS

To improve the editing efficiency of LbCas12a, we conduct saturation mutagenesis in E. coli and identify 1977 positive point mutations of LbCas12a. We selectively assess the editing efficiency of 56 LbCas12a variants in human cells, identifying an optimal LbCas12a variant (RVQ: G146R/R182V/E795Q) with the most robust editing activity. We further test LbCas12a-RV, LbCas12a-RRV, and LbCas12a-RVQ in plants and find LbCas12a-RV has robust editing activity in rice and tomato protoplasts. Interestingly, LbCas12a-RRV, resulting from the stacking of RV and D156R, displays improved editing efficiency in stably transformed rice and poplar plants, leading to up to 100% editing efficiency in T0 plants of both plant species. Moreover, this high-efficiency editing occurs even at the non-canonical TTV PAM sites.

CONCLUSIONS

Our results demonstrate that LbCas12a-RVQ is a powerful tool for genome editing in human cells while LbCas12a-RRV confers robust genome editing in plants. Our study reveals the tremendous potential of these LbCas12a variants for advancing precision genome editing applications across a wide range of organisms.

CRISPR-Combo-mediated orthogonal genome editing and transcriptional activation for plant breeding

CRISPR-Cas nuclease systems, base editors, and CRISPR activation have greatly advanced plant genome engineering. However, the combinatorial approaches for multiplexed orthogonal genome editing and transcriptional regulation were previously unexploited in plants. We have recently established a single Cas9 protein-based CRISPR-Combo platform, enabling efficient multiplexed orthogonal genome editing (double-strand break-mediated genome editing or base editing) and transcriptional activation in plants via engineering the single guide RNA (sgRNA) structure. Here, we provide step-by-step instructions for constructing CRISPR-Combo systems for speed breeding of transgene-free, genome-edited Arabidopsis plants and enhancing rice regeneration with more heritable targeted mutations in a hormone-free manner. We also provide guidance on designing efficient sgRNA, Agrobacterium-mediated transformation of Arabidopsis and rice, rice regeneration without exogenous plant hormones, gene editing evaluation and visual identification of transgene-free Arabidopsis plants with high editing activity. With the use of this protocol, it takes ~2 weeks to establish the CRISPR-Combo systems, 4 months to obtain transgene-free genome-edited Arabidopsis plants and 4 months to obtain rice plants with enrichment of heritable targeted mutations by hormone-free tissue culture.

On- and Off-Target Analyses of CRISPR-Cas12b Genome Editing Systems in Rice

The CRISPR-associated Cas12b system is the third most efficient CRISPR tool for targeted genome editing in plants after Cas9 and Cas12a. Although the genome editing ability of AaCas12b has been previously investigated in rice, its off-target effects in plants are largely not known. In this study, we first engineered single-guide RNA (sgRNA) complexes with various RNA scaffolds to enhance editing frequency. We targeted EPIDERMAL PATTERNING FACTOR LIKE 9 (OsEPFL9) and GRAIN SIZE 3 (OsGS3) genes with GTTG and ATTC protospacer adjacent motifs, respectively. The use of two Alicyclobacillus acidoterrestris scaffolds (Aac and Aa1.2) significantly increased the frequency of targeted mutagenesis. Next, we performed whole-genome sequencing (WGS) of stably transformed T₀ rice plants to assess off-target mutations. WGS analysis revealed background mutations in both coding and noncoding regions with no evidence of sgRNA-dependent off-target activity in edited genomes. We also showed Mendelian segregation of insertion and deletion (indel) mutations in T₁ generation. In conclusion, both Aac and Aa1.2 scaffolds provided precise and heritable genome editing in rice.

Editing the genome of common cereals (Rice and Wheat): techniques, applications, and industrial aspects

Gene editing techniques have made a significant contribution to the development of better crops. Gene editing enables precise changes in the genome of crops, which can introduce new possibilities for altering the crops' traits. Since the last three decades, various gene editing techniques such as meganucleases, zinc finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), and clustered regularly interspersed short palindromic repeats (CRISPR)/Cas (CRISPR-associated proteins) have been discovered. In this review, we discuss various gene editing techniques and their applications to common cereals. Further, we elucidate the future of gene-edited crops, their regulatory features, and industrial aspects globally. To achieve this, we perform a comprehensive literature survey using databases such as PubMed, Web of Science, SCOPUS, Google Scholar etc. For the literature search, we used keywords such as gene editing, crop genome modification, CRISPR/Cas, ZFN, TALEN, meganucleases etc. With the advent of the CRISPR/Cas technology in the last decade, the future of gene editing has transitioned into a new dimension. The functionality of CRISPR/Cas in both DNA and RNA has increased through the use of various Cas enzymes and their orthologs. Constant research efforts in this direction have improved the gene editing process for crops by minimizing its off-target effects. Scientists also use computational tools, which help them to design experiments and analyze the results of gene editing experiments in advance. Gene editing has diverse potential applications. In the future, gene editing will open new avenues for solving more agricultural issues and boosting crop production, which may have great industrial prospects.

Genome-wide analyses of PAM-relaxed Cas9 genome editors reveal substantial off-target effects by ABE8e in rice

PAM-relaxed Cas9 nucleases, cytosine base editors and adenine base editors are promising tools for precise genome editing in plants. However, their genome-wide off-target effects are largely unexplored. Here, we conduct whole-genome sequencing (WGS) analyses of transgenic plants edited by xCas9, Cas9-NGv1, Cas9-NG, SpRY, nCas9-NG-PmCDA1, nSpRY-PmCDA1 and nSpRY-ABE8e in rice. Our results reveal that Cas9 nuclease and base editors, when coupled with the same guide RNA (gRNA), prefer distinct gRNA-dependent off-target sites. De novo generated gRNAs by SpRY editors lead to additional, but insubstantial, off-target mutations. Strikingly, ABE8e results in ~500 genome-wide A-to-G off-target mutations at TA motif sites per transgenic plant. ABE8e's preference for the TA motif is also observed at the target sites. Finally, we investigate the timeline and mechanism of somaclonal variation due to tissue culture, which chiefly contributes to the background mutations. This study provides a comprehensive understanding on the scale and mechanisms of off-target and background mutations occurring during PAM-relaxed genome editing in plants.

Resensitization of Fosfomycin-Resistant Escherichia coli Using the CRISPR System

Antimicrobial resistance is a public health burden with worldwide impacts and was recently identified as one of the major causes of death in 2019. Fosfomycin is an antibiotic commonly used to treat urinary tract infections, and resistance to it in Enterobacteriaceae is mainly due to the metalloenzyme FosA3 encoded by the fosA3 gene. In this work, we adapted a CRISPR-Cas9 system named pRE-FOSA3 to restore the sensitivity of a fosA3+ Escherichia coli strain. The fosA3+ E. coli strain was generated by transforming synthetic fosA3 into a nonpathogenic E. coli TOP10. To mediate the fosA3 disruption, two guide RNAs (gRNAs) were selected that used conserved regions within the fosA3 sequence of more than 700 fosA3+ E. coli isolates, and the resensitization plasmid pRE-FOSA3 was assembled by cloning the gRNA into pCas9. gRNA_195 exhibited 100% efficiency in resensitizing the bacteria to fosfomycin. Additionally, the edited strain lost the ampicillin resistance encoded in the same plasmid containing the synthetic fosA3 gene, despite not being the CRISPR-Cas9 target, indicating plasmid clearance. The in vitro analysis presented here points to a path that can be explored to assist the development of effective alternative methods of treatment against fosA3+ bacteria.

Highly efficient CRISPR systems for loss-of-function and gain-of-function research in pear calli

CRISPR/Cas systems have been widely used for genome engineering in many plant species. However, their potentials have remained largely untapped in fruit crops, particularly in pear, due to the high levels of genomic heterozygosity and difficulties in tissue culture and stable transformation. To date, only a few reports on the application of the CRISPR/Cas9 system in pear have been documented, and have shown very low editing efficiency. Here we report a highly efficient CRISPR toolbox for loss-of-function and gain-of-function research in pear. We compared four different CRISPR/Cas9 expression systems for loss-of-function analysis and identified a potent system that showed nearly 100% editing efficiency for multi-site mutagenesis. To expand the targeting scope, we further tested different CRISPR/Cas12a and Cas12b systems in pear for the first time, albeit with low editing efficiency. In addition, we established a CRISPR activation (CRISPRa) system for multiplexed gene activation in pear calli for gain-of-function analysis. Furthermore, we successfully engineered the anthocyanin and lignin biosynthesis pathways using both CRISPR/Cas9 and CRISPRa systems in pear calli. Taking these results together, we have built a highly efficient CRISPR toolbox for genome editing and gene regulation, paving the way for functional genomics studies as well as molecular breeding in pear.

Non-homologous dsODN increases the mutagenic effects of CRISPR-Cas9 to disrupt oncogene E7 in HPV positive cells

Genome editing tools targeting high-risk human papillomavirus (HPV) oncogene could be a promising therapeutic strategy for the treatment of HPV-related cervical cancer. We aimed to improve the editing efficiency and detect off-target effects concurrently for the clinical translation strategy by using CRISPR-Cas9 system co-transfected with 34nt non-homologous double-stranded oligodeoxynucleotide (dsODN). We firstly tested this strategy on targeting the Green Fluorescent Protein (GFP) gene, of which the expression is easily observed. Our results showed that the GFP+ cells were significantly decreased when using GFP-sgRNAs with dsODN, compared to using GFP-sgRNAs without donors. By PCR and Sanger sequencing, we verified the dsODN integration into the break sites of the GFP gene. And by amplicon sequencing, we observed that the indels% of the targeted site on the GFP gene was increased by using GFP-sgRNAs with dsODN. Next, we went on to target the HPV18 E7 oncogene by using single E7-sgRNA and multiplexed E7-sgRNAs respectively. Whenever using single sgRNA or multiplexed sgRNAs, the mRNA expression of HPV18 E7 oncogene was significantly decreased when adding E7-sgRNAs with dsODN, compared to E7-sgRNAs without donor. And the indels% of the targeted sites on the HPV18 E7 gene was markedly increased by adding dsODN with E7-sgRNAs. Finally, we performed GUIDE-Seq to verify that the integrated dsODN could serve as the marker to detect off-target effects in using single or multiplexed two sgRNAs. And we detected fewer on-target reads and off-target sites in multiplexes compared to the single sgRNAs when targeting the GFP and the HPV18 E7 genes. Together, CRISPR-Cas9 system co-transfected with 34nt dsODN concurrently improved the editing efficiency and monitored off-target effects, which might provide new insights in the treatment of HPV infections and related cervical cancer.

Boosting plant genome editing with a versatile CRISPR-Combo system

CRISPR-Cas9, its derived base editors and CRISPR activation systems have greatly aided genome engineering in plants. However, these systems are mostly used separately, leaving their combinational potential largely untapped. Here we develop a versatile CRISPR-Combo platform, based on a single Cas9 protein, for simultaneous genome editing (targeted mutagenesis or base editing) and gene activation in plants. We showcase the powerful applications of CRISPR-Combo for boosting plant genome editing. First, CRISPR-Combo is used to shorten the plant life cycle and reduce the efforts in screening transgene-free genome-edited plants by activation of a florigen gene in Arabidopsis. Next, we demonstrate accelerated regeneration and propagation of genome-edited plants by activation of morphogenic genes in poplar. Furthermore, we apply CRISPR-Combo to achieve rice regeneration without exogenous plant hormones, which is established as a new method to predominately enrich heritable targeted mutations. In conclusion, CRISPR-Combo is a versatile genome engineering tool with promising applications in crop breeding.

Expanding the targeting scope of FokI-dCas nuclease systems with SpRY and Mb2Cas12a

CRISPR-Cas9 and Cas12a are widely used sequence specific nucleases (SSNs) for genome editing. The nuclease domains of Cas proteins can induce DNA double strand breaks upon RNA guided DNA targeting. Zinc finger nucleases (ZFNs) and Transcription Activator-Like Effector Nucleases (TALENs) have been popular SSNs prior to CRISPR. Both ZFNs and TALENs are based on reconstitution of two monomers with each consisting of a DNA binding domain and a FokI nuclease domain. Inspired by the configuration of ZFNs and TALENs, dimeric FokI-dCas9 systems were previously demonstrated in human cells. Such configuration, based on a pair of guide RNAs (gRNAs), offers great improvement on targeting specificity. To expand the targeting scope of dimeric FokI-dCas systems, we explored the PAM (protospacer adjacent motif)-less SpRY Cas9 variant and the PAM-relaxed Mb2Cas12a system. We showed in rice cells that FokI-dSpRY had more robust editing efficiency than a paired SpRY nickase system. Furthermore, we developed a dimeric FokI-dMb2Cas12a system that displayed comparable editing activity to Mb2Cas12a nuclease in rice cells. Finally, we developed a single-chain FokI-FokI-dMb2Cas12a system that cuts DNA outside its targeting sequence, which could be useful for many versatile applications. Together, this work greatly expanded the FokI based CRISPR-Cas systems for genome editing. This article is protected by copyright. All rights reserved.

FrCas9 is a CRISPR/Cas9 system with high editing efficiency and fidelity

Genome editing technologies hold tremendous potential in biomedical research and drug development. Therefore, it is imperative to discover gene editing tools with superior cutting efficiency, good fidelity, and fewer genomic restrictions. Here, we report a CRISPR/Cas9 from Faecalibaculum rodentium, which is characterized by a simple PAM (5'-NNTA-3') and a guide RNA length of 21-22 bp. We find that FrCas9 could achieve comparable efficiency and specificity to SpCas9. Interestingly, the PAM of FrCas9 presents a palindromic sequence, which greatly expands its targeting scope. Due to the PAM sequence, FrCas9 possesses double editing-windows for base editor and could directly target the TATA-box in eukaryotic promoters for TATA-box related diseases. Together, our results broaden the understanding of CRISPR/Cas-mediated genome engineering and establish FrCas9 as a safe and efficient platform for wide applications in research, biotechnology and therapeutics.

CRISPR-BETS: a base-editing design tool for generating stop codons

Cytosine base editors (CBEs) can install a predefined stop codon at the target site, representing a more predictable and neater method for creating genetic knockouts without altering the genome size. Due to the enhanced predictability of the editing outcomes, it is also more efficient to obtain homozygous mutants in the first generation. With the recent advancement of CBEs on improved editing activity, purify and specificity in plants and animals, base editing has become a more appealing technology for generating knockouts. However, there is a lack of design tools that can aid the adoption of CBEs for achieving such a purpose, especially in plants. Here, we developed a user-friendly design tool named CRISPR-BETS (base editing to stop), which helps with guide RNA (gRNA) design for introducing stop codons in the protein-coding genes of interest. We demonstrated in rice and tomato that CRISPR-BETS is easy-to-use, and its generated gRNAs are highly specific and efficient for generating stop codons and obtaining homozygous knockout lines. While we tailored the tool for the plant research community, CRISPR-BETS can also serve non-plant species.

A Review: Computational Approaches to Design sgRNA of CRISPR-Cas9

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Clustered regularly interspaced short palindromic repeats along with CRISPR-associated protein mechanisms preserve the memory of previous experiences with DNA invaders, in particular spacers that are embedded in CRISPR arrays between coordinate repeats. There has been a fast progression in the comprehension of this immune system and its implementations; however, there are numerous points of view that anticipate explanations to make the field an energetic research zone. The efficiency of CRISPR-Cas depends upon well-considered single guide RNA; for this purpose, many bioinformatics methods and tools are created to support the design of greatly active and precise single guide RNA. Insilico single guide RNA architecture is a crucial point for effective gene editing by means of the CRISPR technique. Persistent attempts have been made to improve in-silico single guide RNA formulation having great on-target effectiveness and decreased off-target effects. This review offers a summary of the CRISPR computational tools to help different researchers pick a specific tool for their work according to pros and cons, along with new thoughts to make new computational tools to overcome all existing limitations.

The comparison of ZFNs, TALENs, and SpCas9 by GUIDE-seq in HPV-targeted gene therapy

Zinc-finger nucleases (ZFNs), transcription activator-like endonucleases (TALENs), and CRISPR-associated Cas9 endonucleases are three major generations of genome editing tools. However, no parallel comparison about the efficiencies and off-target activity of the three nucleases has been reported, which is critical for the final clinical decision. We for the first time developed the genome-wide unbiased identification of double-stranded breaks enabled by sequencing (GUIDE-seq) method in ZFNs and TALENs with novel bioinformatics algorithms to evaluate the off-targets. By targeting human papillomavirus 16 (HPV16), we compared the performance of ZFNs, TALENs, and SpCas9 in vivo. Our data showed that ZFNs with similar targets could generate distinct massive off-targets (287–1,856), and the specificity could be reversely correlated with the counts of middle “G” in zinc finger proteins (ZFPs). We also compared the TALENs with different N-terminal domains (wild-type [WT]/αN/βN) and G recognition modules (NN/NH) and found the design (αN or NN) to improve the efficiency of TALEN inevitably increased off-targets. Finally, our results showed that SpCas9 was more efficient and specific than ZFNs and TALENs. Specifically, SpCas9 had fewer off-target counts in URR (SpCas9, n = 0; TALEN, n = 1; ZFN, n = 287), E6 (SpCas9, n = 0; TALEN, n = 7), and E7 (SpCas9, n = 4; TALEN, n = 36). Taken together, we suggest that for HPV gene therapies, SpCas9 is a more efficient and safer genome editing tool. Our off-target data could be used to improve the design of ZFNs and TALENs, and the universal in vivo off-target detection pipeline for three generations of artificial nucleases provided useful tools for genome engineering-based gene therapy.

Improved plant cytosine base editors with high editing activity, purity, and specificity

Cytosine base editors (CBEs) are great additions to the expanding genome editing toolbox. To improve C-to-T base editing in plants, we first compared seven cytidine deaminases in the BE3-like configuration in rice. We found A3A/Y130F-CBE_V01 resulted in the highest C-to-T base editing efficiency in both rice and Arabidopsis. Furthermore, we demonstrated this A3A/Y130F cytidine deaminase could be used to improve iSpyMacCas9-mediated C-to-T base editing at A-rich PAMs. To showcase its applications, we first applied A3A/Y130F-CBE_V01 for multiplexed editing to generate microRNA-resistant mRNA transcripts as well as pre-mature stop codons in multiple seed trait genes. In addition, we harnessed A3A/Y130F-CBE_V01 for efficient artificial evolution of novel ALS and EPSPS alleles which conferred herbicide resistance in rice. To further improve C-to-T base editing, multiple CBE_V02, CBE_V03 and CBE_V04 systems were developed and tested in rice protoplasts. The CBE_V04 systems were found to have improved editing activity and purity with focal recruitment of more uracil DNA glycosylase inhibitors (UGIs) by the engineered single guide RNA 2.0 scaffold. Finally, we used whole-genome sequencing (WGS) to compare six CBE_V01 systems and four CBE_V04 systems for genome-wide off-target effects in rice. Different levels of cytidine deaminase-dependent and sgRNA-independent off-target effects were indeed revealed by WGS among edited lines by these CBE systems. We also investigated genome-wide sgRNA-dependent off-target effects by different CBEs in rice. This comprehensive study compared 21 different CBE systems, and benchmarked PmCDA1-CBE_V04 and A3A/Y130F-CBE_V04 as next-generation plant CBEs with high editing efficiency, purity, and specificity.

Exploring C-To-G Base Editing in Rice, Tomato, and Poplar

As a precise genome editing technology, base editing is broadly used in both basic and applied plant research. Cytosine base editors (CBEs) and adenine base editors (ABEs) represent the two commonly used base editor types that mediate C-to-T and A-to-G base transition changes at the target sites, respectively. To date, no transversion base editors have been described in plants. Here, we assessed three C-to-G base editors (CGBEs) for targeting sequences with SpCas9's canonical NGG protospacer adjacent motifs (PAMs) as well as three PAM-less SpRY-based CGBEs for targeting sequences with relaxed PAM requirements. The analyses in rice and tomato protoplasts showed that these CGBEs could make C-to-G conversions at the target sites, and they preferentially edited the C6 position in the 20-nucleotide target sequence. C-to-T edits, insertions and deletions (indels) were major byproducts induced by these CGBEs in the protoplast systems. Further assessment of these CGBEs in stably transformed rice and poplar plants revealed the preference for editing of non-GC sites, and C-to-T edits are major byproducts. Successful C-to-G editing in stably transgenic rice plants was achieved by rXRCC1-based CGBEs with monoallelic editing efficiencies up to 38% in T0 lines. The UNG-rAPOBEC1 (R33A)-based CGBE resulted in successful C-to-G editing in polar, with monoallelic editing efficiencies up to 6.25% in T0 lines. Overall, this study revealed that different CGBEs have different preference on preferred editing sequence context, which could be influenced by cell cycles, DNA repair pathways, and plant species.

Expanding the scope of plant genome engineering with Cas12a orthologs and highly multiplexable editing systems

CRISPR-Cas12a is a promising genome editing system for targeting AT-rich genomic regions. Comprehensive genome engineering requires simultaneous targeting of multiple genes at defined locations. Here, to expand the targeting scope of Cas12a, we screen nine Cas12a orthologs that have not been demonstrated in plants, and identify six, ErCas12a, Lb5Cas12a, BsCas12a, Mb2Cas12a, TsCas12a and MbCas12a, that possess high editing activity in rice. Among them, Mb2Cas12a stands out with high editing efficiency and tolerance to low temperature. An engineered Mb2Cas12a-RVRR variant enables editing with more relaxed PAM requirements in rice, yielding two times higher genome coverage than the wild type SpCas9. To enable large-scale genome engineering, we compare 12 multiplexed Cas12a systems and identify a potent system that exhibits nearly 100% biallelic editing efficiency with the ability to target as many as 16 sites in rice. This is the highest level of multiplex edits in plants to date using Cas12a. Two compact single transcript unit CRISPR-Cas12a interference systems are also developed for multi-gene repression in rice and Arabidopsis. This study greatly expands the targeting scope of Cas12a for crop genome engineering.

Patents and technology transfer in CRISPR technology

CRISPR technology has revolutionized biological research in the last decade and many academic institutions and companies have patented CRISPR systems and applications. Several patents have been filed for various applications of CRISPR in different industries such as agriculture, synthetic biology, bio-nanotechnology and precision medicine. Despite tremendous pressure on the technology transfer teams, several startups and spin-out companies are already using CRISPR technologies for commercial applications. In this chapter, we discuss the different CRISPR nucleases and their applications. Secondly, we detail our current opinion and perspective on the CRISPR patent and technology landscape for non-mammalian systems. We present two case-studies on CRISPR diagnostics companies, SHERLOCK and Mammoth Biosciences, who are currently at the forefront of establishing diagnostics platforms for coronavirus (SARS-CoV-2) detection. Finally, our chapter identifies future advancements and possible challenges that CRISPR technology might face in non-mammalian systems.

PAM-less plant genome editing using a CRISPR-SpRY toolbox

The rapid development of the CRISPR-Cas9, -Cas12a and -Cas12b genome editing systems has greatly fuelled basic and translational plant research^1-6. DNA targeting by these Cas nucleases is restricted by their preferred protospacer adjacent motifs (PAMs). The PAM requirement for the most popular Streptococcus pyogenes Cas9 (SpCas9) is NGG (N = A, T, C, G)⁷, limiting its targeting scope to GC-rich regions. Here, we demonstrate genome editing at relaxed PAM sites in rice (a monocot) and the Dahurian larch (a coniferous tree), using an engineered SpRY Cas9 variant⁸. Highly efficient targeted mutagenesis can be readily achieved by SpRY at relaxed PAM sites in the Dahurian larch protoplasts and in rice transgenic lines through non-homologous end joining (NHEJ). Furthermore, an SpRY-based cytosine base editor was developed and demonstrated by directed evolution of new herbicide resistant OsALS alleles in rice. Similarly, a highly active SpRY adenine base editor was developed based on ABE8e (ref. ⁹) and SpRY-ABE8e was able to target relaxed PAM sites in rice plants, achieving up to 79% editing efficiency with high product purity. Thus, the SpRY toolbox breaks a PAM restriction barrier in plant genome engineering by enabling DNA editing in a PAM-less fashion. Evidence was also provided for secondary off-target effects by de novo generated single guide RNAs (sgRNAs) due to SpRY-mediated transfer DNA self-editing, which calls for more sophisticated programmes for designing highly specific sgRNAs when implementing the SpRY genome editing toolbox.

In silico Method in CRISPR/Cas System: An Expedite and Powerful Booster

The CRISPR/Cas system has stood in the center of attention in the last few years as a revolutionary gene editing tool with a wide application to investigate gene functions. However, the labor-intensive workflow requires a sophisticated pre-experimental and post-experimental analysis, thus becoming one of the hindrances for the further popularization of practical applications. Recently, the increasing emergence and advancement of the in silico methods play a formidable role to support and boost experimental work. However, various tools based on distinctive design principles and frameworks harbor unique characteristics that are likely to confuse users about how to choose the most appropriate one for their purpose. In this review, we will present a comprehensive overview and comparisons on the in silico methods from the aspects of CRISPR/Cas system identification, guide RNA design, and post-experimental assistance. Furthermore, we establish the hypotheses in light of the new trends around the technical optimization and hope to provide significant clues for future tools development.

The Improvement of CRISPR-Cas9 System With Ubiquitin-Associated Domain Fusion for Efficient Plant Genome Editing

Genome editing technology represented by CRISPR-Cas9 had been widely used in many biological fields such as gene function analysis, gene therapy, and crop improvement. However, in the face of the complexity of the eukaryotic genome, the CRISPR-Cas9 genome editing tools have shown an unstable editing efficiency with large variability at different target sites. It was important to further improve the editing efficiency of the CRISPR-Cas9 system among the whole genome. In this study, based on the previous single transcription unit genome editing system (STU-SpCas9), using the ubiquitin-associated domain (UBA) to enhance the stability of Cas9 protein, we constructed three Cas9-UBA fusion systems (SpCas9-SD01, SpCas9-SD02, and SpCas9-SD03). Four different target sites of rice OsPDS, OsDEP1 and OsROC5 genes were chosen to evaluate the genome editing efficiency in rice protoplasts and stable transformed rice plants. The results showed that the fusion of UBA domains did not affect the cleavage mode of Cas9 protein, and effectively increase the editing efficiency of STU-SpCas9 at the target sites. This new CRISPR-Cas9-UBA system provided a new strategy and tool for improving the genome editing efficiency of CRISPR-Cas9 in plants.

CRISPR-Cas12b enables efficient plant genome engineering

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas12b is a newly emerged genome engineering system. Here, we compared Cas12b from Alicyclobacillus acidoterrestris (Aac), Alicyclobacillus acidiphilus (Aa), Bacillus thermoamylovorans (Bth) and Bacillus hisashii (Bh) for genome engineering in rice, an important crop. We found AaCas12b was more efficient than AacCas12b and BthCas12b for targeted mutagenesis, which was further demonstrated in multiplexed genome editing. We also engineered the Cas12b systems for targeted transcriptional repression and activation. Our work establishes Cas12b as the third promising CRISPR system, after Cas9 and Cas12a, for plant genome engineering.

Intron-Based Single Transcript Unit CRISPR Systems for Plant Genome Editing

BACKGROUND

Expression of either Cas9 or Cas12a and guide RNAs by a single Polymerase II (Pol II) promoter represents a compact CRISPR expression system and has many advantages for different applications. In order to make this system routine in plant biology, engineering efforts are needed for developing and optimizing such single transcript unit (STU) systems for plant genome editing.

RESULTS

To develop novel intron-based STU (iSTU) CRISPR system (STU CRISPR 3.0), we first evaluated three introns from three plant species for carrying guide RNAs by using an enhanced green fluorescence protein (eGFP) system in rice. After validation of proper intron slicing, we inserted these gRNA-containing introns into the open reading frames (ORFs) of Cas9 and Cas12a for testing their genome editing capability. Different guide RNA processing strategies have been tested for Cas9 and Cas12a. We demonstrated singular genome editing and multiplexed genome editing with these iSTU-Cas9 and iSTU-Cas12a systems.

CONCLUSION

We developed multiple iSTU-CRISPR/Cas9 and Cas12a systems for plant genome editing. Our results shed light on potential directions for further improvement of the iSTU systems.

Accurate Detection and Evaluation of the Gene-Editing Frequency in Plants Using Droplet Digital PCR

Gene-editing techniques are becoming powerful tools for modifying target genes in organisms. Although several methods have been reported that detect mutations at targeted loci induced by the CRISPR/Cas system in different organisms, they are semiquantitative and have difficulty in the detection of mutants in processed food samples containing low initial concentrations of DNA and may not accurately quantify editing frequency, especially at very low frequencies in a complex polyploid plant genome. In this study, we developed a duplexed dPCR-based method for the detection and evaluation of gene-editing frequencies in plants. We described the design, performance, accurate quantification, and comparison with other detection systems. The results show that the dPCR-based method is sensitive to different kinds of gene-editing mutations induced by gene-editing. Moreover, the method is applicable to polyploid plants and processed food samples containing low initial concentrations of DNA. Compared with qPCR and NGS-based methods, the dPCR method has a lower limit of detection (LOD) of the editing frequency and a better relationship with the expected editing frequency in detecting the edited region of gene-edited rice samples. Taken together, the duplexed dPCR assay is accurate and precise, and it will be a powerful tool for the detection and evaluation of gene-editing frequencies in plants in gene-editing technology.

A qPCR method for genome editing efficiency determination and single-cell clone screening in human cells

CRISPR/Cas9 technology has been widely used for targeted genome modification both in vivo and in vitro. However, an effective method for evaluating genome editing efficiency and screening single-cell clones for desired modification is still lacking. Here, we developed this real time PCR method based on the sensitivity of Taq DNA polymerase to nucleotide mismatch at primer 3' end during initiating DNA replication. Applications to CRISPR gRNAs targeting EMX1, DYRK1A and HOXB13 genes in Lenti-X 293 T cells exhibited comprehensive advantages. Just in one-round qPCR analysis using genomic DNA from cells underwent CRISPR/Cas9 or BE4 treatments, the genome editing efficiency could be determined accurately and quickly, for indel, HDR as well as base editing. When applied to single-cell clone screening, the genotype of each cell colony could also be determined accurately. This method defined a rigorous and practical way in quantify genome editing events.

Computational approaches for effective CRISPR guide RNA design and evaluation

The Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/ CRISPR-associated (Cas) system has emerged as the main technology for gene editing. Successful editing by CRISPR requires an appropriate Cas protein and guide RNA. However, low cleavage efficiency and off-target effects hamper the development and application of CRISPR/Cas systems. To predict cleavage efficiency and specificity, numerous computational approaches have been developed for scoring guide RNAs. Most scores are empirical or trained by experimental datasets, and scores are implemented using various computational methods. Herein, we discuss these approaches, focusing mainly on the features or computational methods they utilise. Furthermore, we summarise these tools and give some suggestions for their usage. We also recommend three versatile web-based tools with user-friendly interfaces and preferable functions. The review provides a comprehensive and up-to-date overview of computational approaches for guide RNA design that could help users to select the optimal tools for their research.

SWPepNovo: An Efficient De Novo Peptide Sequencing Tool for Large-scale MS/MS Spectra Analysis

Tandem mass spectrometry (MS/MS)-based de novo peptide sequencing is a powerful method for high-throughput protein analysis. However, the explosively increasing size of MS/MS spectra dataset inevitably and exponentially raises the computational demand of existing de novo peptide sequencing methods, which is an issue urgently to be solved in computational biology. This paper introduces an efficient tool based on SW26010 many-core processor, namely SWPepNovo, to process the large-scale peptide MS/MS spectra using a parallel peptide spectrum matches (PSMs) algorithm. Our design employs a two-level parallelization mechanism: (1) the task-level parallelism between MPEs using MPI based on a data transformation method and a dynamic feedback task scheduling algorithm, (2) the thread-level parallelism across CPEs using asynchronous task transfer and multithreading. Moreover, three optimization strategies, including vectorization, double buffering and memory access optimizations, have been employed to overcome both the compute-bound and the memory-bound bottlenecks in the parallel PSMs algorithm. The results of experiments conducted on multiple spectra datasets demonstrate the performance of SWPepNovo against three state-of-the-art tools for peptide sequencing, including PepNovo+, PEAKS and DeepNovo-DIA. The SWPepNovo also shows high scalability in experiments on extremely large datasets sized up to 11.22 GB. The software and the parameter settings are available at https://github.com/ChuangLi99/SWPepNovo.

Improving Plant Genome Editing with High-Fidelity xCas9 and Non-canonical PAM-Targeting Cas9-NG

Two recently engineered SpCas9 variants, namely xCas9 and Cas9-NG, show promising potential in improving targeting specificity and broadening the targeting range. In this study, we evaluated these Cas9 variants in the model and crop plant, rice. We first tested xCas9-3.7, the most effective xCas9 variant in mammalian cells, for targeted mutagenesis at 16 possible NGN PAM (protospacer adjacent motif) combinations in duplicates. xCas9 exhibited nearly equivalent editing efficiency to wild-type Cas9 (Cas9-WT) at most canonical NGG PAM sites tested, whereas it showed limited activity at non-canonical NGH (H = A, C, T) PAM sites. High editing efficiency of xCas9 at NGG PAMs was further demonstrated with C to T base editing by both rAPOBEC1 and PmCDA1 cytidine deaminases. With mismatched sgRNAs, we found that xCas9 had improved targeting specificity over the Cas9-WT. Furthermore, we tested two Cas9-NG variants, Cas9-NGv1 and Cas9-NG, for targeting NGN PAMs. Both Cas9-NG variants showed higher editing efficiency at most non-canonical NG PAM sites tested, and enabled much more efficient editing than xCas9 at AT-rich PAM sites such as GAT, GAA, and CAA. Nevertheless, we found that Cas9-NG variants showed significant reduced activity at the canonical NGG PAM sites. In stable transgenic rice lines, we demonstrated that Cas9-NG had much higher editing efficiency than Cas9-NGv1 and xCas9 at NG PAM sites. To expand the base-editing scope, we developed an efficient C to T base-editing system by making fusion of Cas9-NG nickase (D10A version), PmCDA1, and UGI. Taken together, our work benchmarked xCas9 as a high-fidelity nuclease for targeting canonical NGG PAMs and Cas9-NG as a preferred variant for targeting relaxed PAMs for plant genome editing.

Single transcript unit CRISPR 2.0 systems for robust Cas9 and Cas12a mediated plant genome editing

CRISPR-Cas9 and Cas12a are two powerful genome editing systems. Expression of CRISPR in plants is typically achieved with a mixed dual promoter system, in which Cas protein is expressed by a Pol II promoter and a guide RNA is expressed by a species-specific Pol III promoter such as U6 or U3. To achieve coordinated expression and compact vector packaging, it is desirable to express both CRISPR components under a single Pol II promoter. Previously, we demonstrated a first-generation single transcript unit (STU)-Cas9 system, STU-Cas9-RZ, which is based on hammerhead ribozyme for processing single guide RNAs (sgRNAs). In this study, we developed two new STU-Cas9 systems and one STU-Cas12a system for applications in plants, collectively called the STU CRISPR 2.0 systems. We demonstrated these systems for genome editing in rice with both transient expression and stable transgenesis. The two STU-Cas9 2.0 systems process the sgRNAs with Csy4 ribonuclease and endogenous tRNA processing system respectively. Both STU-Cas9-Csy4 and STU-Cas9-tRNA systems showed more robust genome editing efficiencies than our first-generation STU-Cas9-RZ system and the conventional mixed dual promoter system. We further applied the STU-Cas9-tRNA system to compare two C to T base editing systems based on rAPOBEC1 and PmCDA1 cytidine deaminases. The results suggest STU-based PmCDA1 base editor system is highly efficient in rice. The STU-Cas12a system, based on Cas12a' self-processing of a CRISPR RNA (crRNA) array, was also developed and demonstrated for expression of a single crRNA and four crRNAs. Altogether, our STU CRISPR 2.0 systems further expanded the CRISPR toolbox for plant genome editing and other applications.

Application of CRISPR-Cas12a temperature sensitivity for improved genome editing in rice, maize, and Arabidopsis

BACKGROUND

CRISPR-Cas12a (formerly Cpf1) is an RNA-guided endonuclease with distinct features that have expanded genome editing capabilities. Cas12a-mediated genome editing is temperature sensitive in plants, but a lack of a comprehensive understanding on Cas12a temperature sensitivity in plant cells has hampered effective application of Cas12a nucleases in plant genome editing.

RESULTS

We compared AsCas12a, FnCas12a, and LbCas12a for their editing efficiencies and non-homologous end joining (NHEJ) repair profiles at four different temperatures in rice. We found that AsCas12a is more sensitive to temperature and that it requires a temperature of over 28 °C for high activity. Each Cas12a nuclease exhibited distinct indel mutation profiles which were not affected by temperatures. For the first time, we successfully applied AsCas12a for generating rice mutants with high frequencies up to 93% among T0 lines. We next pursued editing in the dicot model plant Arabidopsis, for which Cas12a-based genome editing has not been previously demonstrated. While LbCas12a barely showed any editing activity at 22 °C, its editing activity was rescued by growing the transgenic plants at 29 °C. With an early high-temperature treatment regime, we successfully achieved germline editing at the two target genes, GL2 and TT4, in Arabidopsis transgenic lines. We then used high-temperature treatment to improve Cas12a-mediated genome editing in maize. By growing LbCas12a T0 maize lines at 28 °C, we obtained Cas12a-edited mutants at frequencies up to 100% in the T1 generation. Finally, we demonstrated DNA binding of Cas12a was not abolished at lower temperatures by using a dCas12a-SRDX-based transcriptional repression system in Arabidopsis.

CONCLUSION

Our study demonstrates the use of high-temperature regimes to achieve high editing efficiencies with Cas12a systems in rice, Arabidopsis, and maize and sheds light on the mechanism of temperature sensitivity for Cas12a in plants.

Bidirectional Promoter-Based CRISPR-Cas9 Systems for Plant Genome Editing

CRISPR-Cas systems can be expressed in multiple ways, with different capabilities regarding tissue-specific expression, efficiency, and expression levels. Thus far, three expression strategies have been demonstrated in plants: mixed dual promoter systems, dual Pol II promoter systems, and single transcript unit (STU) systems. We explored a fourth strategy to express CRISPR-Cas9 in the model and crop plant, rice, where a bidirectional promoter (BiP) is used to express Cas9 and single guide RNA (sgRNA) in opposite directions. We first tested an engineered BiP system based on double-mini 35S promoter and an Arabidopsis enhancer, which resulted in 20.7% and 52.9% genome editing efficiencies at two target sites in T0 stable transgenic rice plants. We further improved the BiP system drastically by using a rice endogenous BiP, OsBiP1. The endogenous BiP expression system had higher expression strength and led to 75.9-93.3% genome editing efficiencies in rice T0 generation, when the sgRNAs were processed by either tRNA or Csy4. We provided a proof-of-concept study of applying BiP systems for expressing two-component CRISPR-Cas9 genome editing reagents in rice. Our work could promote future research and adoption of BiP systems for CRISPR-Cas-based genome engineering in plants.

A large-scale whole-genome sequencing analysis reveals highly specific genome editing by both Cas9 and Cpf1 (Cas12a) nucleases in rice

BACKGROUND

Targeting specificity has been a barrier to applying genome editing systems in functional genomics, precise medicine and plant breeding. In plants, only limited studies have used whole-genome sequencing (WGS) to test off-target effects of Cas9. The cause of numerous discovered mutations is still controversial. Furthermore, WGS-based off-target analysis of Cpf1 (Cas12a) has not been reported in any higher organism to date.

RESULTS

We conduct a WGS analysis of 34 plants edited by Cas9 and 15 plants edited by Cpf1 in T0 and T1 generations along with 20 diverse control plants in rice. The sequencing depths range from 45× to 105× with read mapping rates above 96%. Our results clearly show that most mutations in edited plants are created by the tissue culture process, which causes approximately 102 to 148 single nucleotide variations (SNVs) and approximately 32 to 83 insertions/deletions (indels) per plant. Among 12 Cas9 single guide RNAs (sgRNAs) and three Cpf1 CRISPR RNAs (crRNAs) assessed by WGS, only one Cas9 sgRNA resulted in off-target mutations in T0 lines at sites predicted by computer programs. Moreover, we cannot find evidence for bona fide off-target mutations due to continued expression of Cas9 or Cpf1 with guide RNAs in T1 generation.

CONCLUSIONS

Our comprehensive and rigorous analysis of WGS data across multiple sample types suggests both Cas9 and Cpf1 nucleases are very specific in generating targeted DNA modifications and off-targeting can be avoided by designing guide RNAs with high specificity.

Special issue on Computational Resources and Methods in Biological Sciences

This special issue covers a wide range of topics in computational biology, such as database construction, sequence analysis and function prediction with machine learning methods, disease-related diagnosis, drug-target and drug discovery, and electronic health record system construction.