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Tailoring Cas12a functionality with a user-friendly and versatile crRNA variant toolbox.
Cas12a, with its unique targeting and cleavage activity towards DNA, has been widely applied in gene editing and molecular diagnostics. However, there currently lacks an activity regulation strategy that combines flexibility and simplicity to adapt Cas12a to different demands across various application scenarios. In this study, we present a simple yet effective strategy, wherein we systematically mutate the crRNA direct repeat (DR) sequence to uncover a range of distinct crRNA mutants, which are then compiled into a crRNA toolbox to enable flexible regulation of Cas12a activity. By harnessing the complementarity and synergy between these mutants, we successfully enhance Cas12a performance across various application scenarios. Our crRNA toolbox enables fine-tuned control over expression levels, improves base editing accuracy, enhances transformation and editing efficiency in prokaryote homologous recombination-mediated gene editing, and facilitates rapid, accurate, one-pot, semi-quantitative nucleic acid diagnostics. In summary, the DR sequence mutation strategy provides simple, flexible, and diverse options for Cas12a activity regulation and functional optimization.
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Base editing reveals context-dependent regulation of adhesion, anoikis, and motility by BAP1 in renal cell models.
BAP1 is a tumor-suppressive deubiquitinase essential for DNA repair, and missense mutations in BAP1 are common in clear cell renal cell carcinoma (ccRCC). We previously showed that correction of the inactivating Glu31Lys mutation in KMRC-20 ccRCC cells using CRISPR/Cas9 base editing restored BAP1 function, reinstated anchorage dependence, and re-sensitized cells to anoikis. Here, we investigated whether disruption of Glu31 is sufficient to induce anchorage-independent growth and anoikis resistance in normal kidney epithelial cells. Using adenine base editing, we introduced an inactivating Glu31Gly mutation into HK-2 cells, generating two independent isogenic BAP1-mutant clones, and established a BAP1-knockout clone by CRISPR/Cas9 as an additional control. Glu31Gly mutants exhibited complete loss of BAP1 deubiquitinase activity and impaired UV-induced DNA damage repair, comparable to knockout cells. Despite the clear functional inactivation of BAP1, the Glu31Gly and knockout HK-2 cells neither acquired anchorage-independent growth nor anoikis resistance; instead, detached cells displayed increased apoptosis. In KMRC-20 cells, restoration of BAP1 enhanced both migration and invasion, whereas BAP1 inactivation or loss in HK-2 cells increased invasion but reduced migration, indicating distinct context-dependent roles for BAP1 in normal versus malignant renal cells. These findings demonstrate that BAP1 inactivation alone is insufficient to confer anchorage-independent survival in normal kidney epithelial cells and suggest that additional oncogenic alterations are required during kidney tumorigenesis. Our study further highlights the utility of precise base editing for dissecting the functional consequences of clinically relevant cancer mutations.
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TALEs, TALENs, and TALE Base Editors: From Plant Pathology to Biotechnology.
TALEs (transcription activator-like effectors) are an excellent example of how studying pathogen-host interactions can lead to significant biotechnology inventions. TALEs are bacterial effectors that are translocated into plant cells via a bacterial type III secretion system. Once inside the host cell, they are imported into the nucleus to bind specific promoters and induce expression of target genes, thereby supporting the bacterial infection. TALEs are found throughout many, but not all, Xanthomonas pathovars, which can be severe pathogens of different crops. The key feature of TALEs is their modular DNA-binding domain, which allows a simple evolutionary adaptation to novel DNA sequences as well as simple cloning of designer TALEs with desired DNA-binding specificity. Accordingly, TALE nucleases started the genome-editing revolution, and TALE base editors are the latest tools to efficiently edit chloroplast and mitochondrial genomes. We review recent advances in Xanthomonas genomics, synthesize current knowledge about naturally occurring TALEs, and highlight current roles of TALEs in genome editing and synthetic biology.
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Web-Based Base Editing Toolkits: BE-Designer and BE-Analyzer.
The CRISPR-Cas system is broadly used for genome editing because of its convenience and relatively low cost. However, the use of CRISPR nucleases to induce specific nucleotide changes in target DNA requires complex procedures and additional donor DNAs. Furthermore, CRISPR nuclease-mediated DNA cleavage at target sites frequently causes large deletions or genomic rearrangements. In contrast, base editors that consist of catalytically dead Cas9 (dCas9) or Cas9 nickase (nCas9) connected to a cytidine or a guanine deaminase can correct point mutations in the absence of additional donor DNA and without generating double-strand breaks (DSBs) in the target region. To design target sites and assess mutation ratios for cytosine and adenine base editors (CBEs and ABEs), we have developed web tools, named BE-Designer and BE-Analyzer. These tools are easy to use (such that tasks are accomplished by clicking on relevant buttons) and do not require a deep knowledge of bioinformatics.
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Small-molecule inhibitors of histone deacetylase improve CRISPR-based adenine base editing.
CRISPR-based base editors (BEs) are widely used to induce nucleotide substitutions in living cells and organisms without causing the damaging DNA double-strand breaks and DNA donor templates. Cytosine BEs that induce C:G to T:A conversion and adenine BEs that induce A:T to G:C conversion have been developed. Various attempts have been made to increase the efficiency of both BEs; however, their activities need to be improved for further applications. Here, we describe a fluorescent reporter-based drug screening platform to identify novel chemicals with the goal of improving adenine base editing efficiency. The reporter system revealed that histone deacetylase inhibitors, particularly romidepsin, enhanced base editing efficiencies by up to 4.9-fold by increasing the expression levels of proteins and target accessibility. The results support the use of romidepsin as a viable option to improve base editing efficiency in biomedical research and therapeutic genome engineering.
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Optimization of Prime Editing in Rice, Peanut, Chickpea, and Cowpea Protoplasts by Restoration of GFP Activity.
Precise editing of the plant genome has long been desired for functional genomic research and crop breeding. Prime editing is a newly developed precise editing technology based on CRISPR-Cas9, which uses an engineered reverse transcriptase (RT), a catalytically impaired Cas9 endonuclease (nCas9), and a prime editing guide RNA (pegRNA). In addition, prime editing has a wider range of editing types than base editing and can produce nearly all types of edits. Although prime editing was first established in human cells, it has recently been applied to plants. As a relatively new technique, optimization will be needed to increase the editing efficiency in different crops. In this study, we successfully edited a mutant GFP in rice, peanut, chickpea, and cowpea protoplasts. In rice, up to 16 times higher editing efficiency was achieved with a dual pegRNA than the single pegRNA containing vectors. Edited-mutant GFP protoplasts have also been obtained in peanut, chickpea, and cowpea after transformation with the dual pegRNA vectors, albeit with much lower editing efficiency than in rice, ranging from 0.2% to 0.5%. These initial results promise to expedite the application of prime editing in legume breeding programs to accelerate crop improvement.
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Analyzing the functional effects of DNA variants with gene editing.
Continual advancements in genomics have led to an ever-widening disparity between the rate of discovery of genetic variants and our current understanding of their functions and potential roles in disease. Systematic methods for phenotyping DNA variants are required to effectively translate genomics data into improved outcomes for patients with genetic diseases. To make the biggest impact, these approaches must be scalable and accurate, faithfully reflect disease biology, and define complex disease mechanisms. We compare current methods to analyze the function of variants in their endogenous DNA context using genome editing strategies, such as saturation genome editing, base editing and prime editing. We discuss how these technologies can be linked to high-content readouts to gain deep mechanistic insights into variant effects. Finally, we highlight key challenges that need to be addressed to bridge the genotype to phenotype gap, and ultimately improve the diagnosis and treatment of genetic diseases.
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Design and application of the transformer base editor in mammalian cells and mice.
Fusing apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like cytidine deaminase with catalytically impaired Cas proteins (e.g., nCas9 or dCas9) provides a novel gene-editing technology, base editing, that grants targeted base substitutions with high efficiency. However, genome-wide and transcriptome-wide off-target mutations are observed in base editing, which raises safety concerns regarding therapeutic applications. Previously, we developed a new base editing system, the transformer base editor (tBE), to induce efficient editing with no observable genome-wide or transcriptome-wide off-target mutations both in mammalian cells and in mice. Here we describe a detailed protocol for the design and application of the tBE. Steps for designing single-guide RNA (sgRNA) and helper sgRNA pairs, making constructs, determining the genome-wide and transcriptome-wide off-target mutations, producing the tBE-containing adeno-associated viruses, delivering adeno-associated viruses into mice and examining the in vivo editing effects are included in this protocol. High-precision base editing by the tBE can be completed within 2-3 weeks (in mammalian cells) or within 6-8 weeks (in mice), with sgRNA-helper sgRNA pairs. The whole process can be collaboratively accomplished by researchers using standard techniques from molecular biology, bioinformatics and mouse husbandry.
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An engineered U7 small nuclear RNA scaffold greatly increases ADAR-mediated programmable RNA base editing.
Custom RNA base editing exploiting the human Adenosine Deaminase Acting on RNA (ADAR) enzyme may enable therapeutic gene editing without DNA damage or use of foreign proteins. ADAR's adenosine-to-inosine (effectively A-to-G) deamination activity can be targeted to transcripts using an antisense guide RNA (gRNA), but efficacy is challenged by limits of in vivo delivery. Embedding gRNAs into a U7 small nuclear RNA (snRNA) framework greatly enhances RNA editing with endogenous ADAR, and a 750-plex single-cell mutagenesis screen further improved the framework. An optimized scaffold with a stronger synthetic U7 promoter enables 76% RNA editing in vitro from a single DNA construct per cell, and 75% editing in a Hurler syndrome mouse brain after one systemic AAV injection, surpassing circular gRNA approaches. The technology also improves published DMD exon-skipping designs 25-fold in differentiated myoblasts. Our engineered U7 framework represents a universal scaffold for ADAR-based RNA editing and other antisense RNA therapies.
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Genome scale analysis of pathogenic variants targetable for single base editing.
BACKGROUND: Single nucleotide variants account for approximately 90% of all known pathogenic variants responsible for human diseases. Recently discovered CRISPR/Cas9 base editors can correct individual nucleotides without cutting DNA and inducing double-stranded breaks. We aimed to find all possible pathogenic variants which can be efficiently targeted by any of the currently described base editors and to present them for further selection and development of targeted therapies. METHODS: ClinVar database (GRCh37_clinvar_20171203) was used to search and select mutations available for current single-base editing systems. We included only pathogenic and likely pathogenic variants for further analysis. For every potentially editable mutation we checked the presence of PAM. If a PAM was found, we analyzed the sequence to find possibility to edit only one nucleotide without changing neighboring nucleotides. The code of the script to search Clinvar database and to analyze the sequences was written in R and is available in the appendix. RESULTS: We analyzed 21 editing system currently reported in 9 publications. Every system has different working characteristics such as the editing window and PAM sequence. C > T base editors can precisely target 3196 mutations (46% of all pathogenic T > C variants), and A > G editors - 6900 mutations (34% of all pathogenic G > A variants). CONCLUSIONS: Protein engineering helps to develop new enzymes with a narrower window of base editors as well as using new Cas9 enzymes with different PAM sequences. But, even now the list of mutations which can be targeted with currently available systems is huge enough to choose and develop new targeted therapies.
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