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CRISPR/Cas9-mediated base editors and their prospects for mitochondrial genome engineering.
Base editors are a type of double-stranded break (DSB)-free gene editing technology that has opened up new possibilities for precise manipulation of mitochondrial DNA (mtDNA). This includes cytosine and adenosine base editors and more recently guanosine base editors. Because of having low off-target and indel rates, there is a growing interest in developing and evolving this research field. Here, we provide a detailed update on DNA base editors. While base editing has widely been used for nuclear genome engineering, the growing interest in applying this technology to mitochondrial DNA has been faced with several challenges. While Cas9 protein has been shown to enter mitochondria, use of smaller Cas proteins, such as Cas12a, has higher import efficiency. However, sgRNA transfer into mitochondria is the most challenging step. sgRNA structure and ratio of Cas protein to sgRNA are both important factors for efficient sgRNA entry into mitochondria. In conclusion, while there are still several challenges to be addressed, ongoing research in this field holds the potential for new treatments and therapies for mitochondrial disorders.
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Creating large-scale genetic diversity in Arabidopsis via base editing-mediated deep artificial evolution.
BACKGROUND: Base editing is a powerful tool for artificial evolution to create allelic diversity and improve agronomic traits. However, the great evolutionary potential for every sgRNA target has been overlooked. And there is currently no high-throughput method for generating and characterizing as many changes in a single target as possible based on large mutant pools to permit rapid gene directed evolution in plants. RESULTS: In this study, we establish an efficient germline-specific evolution system to screen beneficial alleles in Arabidopsis which could be applied for crop improvement. This system is based on a strong egg cell-specific cytosine base editor and the large seed production of Arabidopsis, which enables each T1 plant with unedited wild type alleles to produce thousands of independent T2 mutant lines. It has the ability of creating a wide range of mutant lines, including those containing atypical base substitutions, and as well providing a space- and labor-saving way to store and screen the resulting mutant libraries. Using this system, we efficiently generate herbicide-resistant EPSPS, ALS, and HPPD variants that could be used in crop breeding. CONCLUSIONS: Here, we demonstrate the significant potential of base editing-mediated artificial evolution for each sgRNA target and devised an efficient system for conducting deep evolution to harness this potential.
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Optimizing linker length of base editors for precise crop breeding and gene therapy.
Base editing enables efficient nucleotide conversions without inducing DNA double-strand breaks (DSBs) or requiring exogenous donor DNA templates. However, its broader editing window often causes bystander editing, increasing the risk of unintended mutations. In this study, we find that linker length significantly influences the editing window, and base editors with a 7-amino-acid linker reduce bystander editing by an average of 54.4% across 13 endogenous genomic sites in both rice and human cell lines. We further develop an optimized strategy by modulating the linker length between various deaminases and Cas9 nickases, which effectively reduces bystander editing across multiple applications, including functional studies, precise crop breeding, and correction of pathogenic variants. Our work reveals that shortening the linker enhances the specificity of base editing, addressing a key safety concern for its agricultural and therapeutic applications.
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Production and Storage of High-Capacity Adenoviral Vectors for the Delivery of Advanced CRISPR Systems.
Genome editing based on engineered CRISPR systems is advancing rapidly, with the field increasingly moving toward approaches that avoid the induction of mutagenic double-stranded DNA breaks (e.g., RNA-programmable base editing, prime editing, and donor DNA transposition). These nuclease-free strategies often rely on large or multi-component molecular assemblies that can include gene-sized donor DNA substrates. There is, nonetheless, a paucity of vehicles capable of delivering such large and complex genome-editing components effectively and, ideally, in defined stoichiometric ratios. High-capacity adenoviral vector particles (AdVPs) offer an attractive set of features to address these challenges, including robust cell transduction levels regardless of mitotic status, exceptional payload capacity (up to ~36 kb), strict chromosomal nonintegrating character, and the complete absence of viral coding sequences. Hence, AdVPs can serve as biological nanoparticles suitable for the evaluation and application of next-generation CRISPR technologies in physiologically relevant cellular contexts, regardless of the size and number of the attendant tools. Here, after summarizing the key characteristics of earlier- and latest-generation adenoviral vector platforms, we describe protocols for producing AdVPs, including vectors that deliver multiplexing, prime-editing, and orthogonal nuclease constructs. Finally, we highlight important considerations for designing AdVP production reagents and validate a storage buffer that preserves AdVP functionality after repeated freeze-thaw cycles.
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Disease modeling by efficient genome editing using a near PAM-less base editor in vivo.
Base Editors are emerging as an innovative technology to introduce point mutations in complex genomes. So far, the requirement of an NGG Protospacer Adjacent Motif (PAM) at a suitable position often limits the base editing possibility to model human pathological mutations in animals. Here we show that, using the CBE4max-SpRY variant recognizing nearly all PAM sequences, we could introduce point mutations for the first time in an animal model with high efficiency, thus drastically increasing the base editing possibilities. With this near PAM-less base editor we could simultaneously mutate several genes and we developed a co-selection method to identify the most edited embryos based on a simple visual screening. Finally, we apply our method to create a zebrafish model for melanoma predisposition based on the simultaneous base editing of multiple genes. Altogether, our results considerably expand the Base Editor application to introduce human disease-causing mutations in zebrafish.
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[Design of Guide RNA for CRISPR/Cas Plant Genome Editing].
CRISPR/Cas technology of genome editing is a powerful tool for making targeted changes in the DNA of various organisms, including plants. The choice of the precise nucleotide sequence (protospacer) in the gene to be edited is important in the design of guide RNA, which can be carried out by specialized software. We review and compare all the known on-line and off-line resources for guide RNA design, with special attention paid to tools capable of searching for off-target edits sites in plant genomes. The use of Cas12a may be preferable to Cas9. Techniques allowing C→T and G→A base editing without DNA cleavage are discussed along with the basic requirements for the design of effective and highly specific guide RNAs. Ways for improving guide RNA design software are presented. We also discuss the lesser risks of off-target editing in plant genomes as opposed to animal genomes. Examples of edited plant genomes including those that do not lead to the creation of transgenic plants are reviewed.
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The CRISPR toolbox for the gram-positive model bacterium Bacillus subtilis.
CRISPR has revolutionized the way we engineer genomes. Its simplicity and modularity have enabled the development of a great number of tools to edit genomes and to control gene expression. This powerful technology was first adapted to Bacillus subtilis in 2016 and has been intensely upgraded since then. Many tools have been successfully developed to build a CRISPR toolbox for this Gram-positive model and important industrial chassis. The toolbox includes tools, such as double-strand and single-strand cutting CRISPR for point mutation, gene insertion, and gene deletion up to 38 kb. Moreover, catalytic dead Cas proteins have been used for base editing, as well as for the control of gene expression (CRISPRi and CRISPRa). Many of these tools have been used for multiplex CRISPR with the most successful one targeting up to six loci simultaneously for point mutation. However, tools for efficient multiplex CRISPR for other functionalities are still missing in the toolbox. CRISPR engineering has already resulted in efficient protein and metabolite-producing strains, demonstrating its great potential. In this review, we cover all the important additions made to the B. subtilis CRISPR toolbox since 2016, and strain developments fomented by the technology.
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Efficient base editing by RNA-guided cytidine base editors (CBEs) in pigs.
Cytidine base editors (CBEs) have been demonstrated to be useful for precisely inducing C:G-to-T:A base mutations in various organisms. In this study, we showed that the BE4-Gam system induced the targeted C-to-T base conversion in porcine blastocysts at an efficiency of 66.7-71.4% via the injection of a single sgRNA targeting a xeno-antigen-related gene and BE4-Gam mRNA. Furthermore, the efficiency of simultaneous three gene base conversion via the injection of three targeting sgRNAs and BE4-Gam mRNA into porcine parthenogenetic embryos was 18.1%. We also obtained beta-1,4-N-acetyl-galactosaminyl transferase 2, alpha-1,3-galactosyltransferase, and cytidine monophosphate-N-acetylneuraminic acid hydroxylase deficient pig by somatic cell nuclear transfer, which exhibited significantly decreased activity. In addition, a new CBE version (termed AncBE4max) was used to edit genes in blastocysts and porcine fibroblasts (PFFs) for the first time. While this new version demonstrated a three genes base-editing rate of 71.4% at the porcine GGTA1, B4galNT2, and CMAH loci, it increased the frequency of bystander edits, which ranged from 17.8 to 71.4%. In this study, we efficiently and precisely mutated bases in porcine blastocysts and PFFs using CBEs and successfully generated C-to-T and C-to-G mutations in pigs. These results suggest that CBEs provide a more simple and efficient method for improving economic traits, reducing the breeding cycle, and increasing disease tolerance in pigs, thus aiding in the development of human disease models.
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DNA topology regulates PAM-Cas9 interaction and DNA unwinding to enable near-PAMless cleavage by thermophilic Cas9.
CRISPR-Cas9-mediated genome editing depends on PAM recognition to initiate DNA unwinding. PAM mutations can abolish Cas9 binding and prohibit editing. Here, we identified a Cas9 from the thermophile Alicyclobacillus tengchongensis for which the PAM interaction can be robustly regulated by DNA topology. AtCas9 has a relaxed PAM of N4CNNN and N4RNNA (R = A/G) and is able to bind but not cleave targets with mutated PAMs. When PAM-mutated DNA was in underwound topology, AtCas9 exhibited enhanced binding affinity and high cleavage activity. Mechanistically, AtCas9 has a unique loop motif, which docked into the DNA major groove, and this interaction can be regulated by DNA topology. More importantly, AtCas9 showed near-PAMless editing of supercoiled plasmid in E. coli. In mammalian cells, AtCas9 exhibited broad PAM preference to edit plasmid with up to 72% efficiency and effective base editing at four endogenous loci, representing a potentially powerful tool for near-PAMless editing.
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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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