Web-based design and analysis tools for CRISPR base editing

Optimized dual-AAV base editor delivery system with enhanced editing efficiency and virion production titer

Base editors (BEs) are a promising tool for precise base conversion in human cells and animals, while the adeno-associated virus (AAV) is the major vector for human gene therapy. However, the size of the DNA cassette required for BE expression exceeds the 4.7 kb packing capacity of the AAV vector, making dual-AAV approaches based on trans-splicing intein necessary. Even with this approach, current split DNA cassettes are still larger than the AAV packing limit, posing a challenge for cellular production of AAV. Moreover, some split strategies yield variable editing results and target coverage. To address these limitations, 25 different split sets for BE4max and A3A-BE4max were tested at two target sites respectively, with splitting sites ranging from 493rd to 517th amino acids on the Cas9 peptide. Fortunately, the best Cas9 split site was identified between His511 and Ser512 and the arrangement of the AAV expression cassette was further manipulated to create evenly distributed CBE and ABE intein systems within 4.7 kb. These novel dual-AAV systems, designated 4.6AAV-CBE and 4.7AAV-ABE, were found to have base editing efficiencies similar to wild-type BEs, with a narrower editing window than the current 573 split system. Notably, 4.6AAV-CBE yield a higher AAV production titer, up to 2.1-fold in AAV-N and 1.5-fold in AAV-C, compared to the split-573BE system, likely due to the reduction of DNA cassette size within the AAV packaging capacity. Moreover, after packaging and infecting cells with AAV-N and AAV-C at the same volume and number of cells, the multiplicities of infection (MOI) and editing efficiency of 4.6 AAV-CBE were both higher than those of the split-573BE system. This study present advanced dual-AAV systems for ABE and CBE delivery, establishing a basis for safe and efficient BE therapies.

Bystander editing by adenine base editors impairs vision restoration in a mouse model of Leber congenital amaurosis

Base editors (BEs) have emerged as a powerful tool for gene correction with high activity. However, bystander base editing, a byproduct of BEs, presents challenges for precise editing. Here, we investigated the effects of bystander edits on phenotypic restoration in the context of Leber congenital amaurosis (LCA), a hereditary retinal disorder, as a therapeutic model. We observed that in retinal degeneration 12 (rd12) of LCA model mice, the highest editing activity version of an adenine base editors (ABEs), ABE8e, generated substantial bystander editing, resulting in missense mutations despite RPE65 expression, preventing restoration of visual function. Through AlphaFold-based mutational scanning and molecular dynamics simulations, we identified that the ABE8e-driven L43P mutation disrupts RPE65 structure and function. Our findings underscore the need for more stringent requirements in developing precise BEs for future clinical applications.

BEscreen: a versatile toolkit to design base editing libraries

Base editing enables the high-throughput screening of genetic variants for phenotypic effects. Base editing screens require the design of single guide RNA (sgRNA) libraries to enable either gene- or variant-centric approaches. While computational tools supporting the design of sgRNAs exist, no solution offers versatile and scalable library design enabling all major use cases. Here, we introduce BEscreen, a comprehensive base editing guide design tool provided as a web server (bescreen.ostendorflab.org) and as a command line tool. BEscreen provides variant-, gene-, and region-centric modes to accommodate various screening approaches. The variant mode accepts genomic coordinates, amino acid changes, or rsIDs as input. The gene mode designs near-saturation libraries covering the entire coding sequence of given genes or transcripts, and the region mode designs all possible guides for given genomic regions. BEscreen enables selection of guides by biological consequence, it features comprehensive customization of base editor characteristics, and it offers optional annotation using Ensembl's Variant Effect Predictor. In sum, BEscreen is a highly versatile tool to design base editing screens for a wide range of use cases with seamless scalability from individual variants to large, near-saturation libraries.

CRISPR-BEasy: a free web-based service for designing sgRNA tiling libraries for CRISPR-dependent base editing screens

CRISPR-dependent base editing (BE) enables the modeling and correction of genetic mutations at single-base resolution. Base editing screens, where point mutations are queried en masse, are powerful tools to systematically draw genotype-phenotype associations and characterise the function of genes and other genomic elements. However, the lack of user-friendly web-based tools for designing base editing screens can hinder broad technology adoption. Here, we introduce CRISPR-BEasy (https://crispr-beasy.cerc-genomic-medicine.ca), a free, automated web-based server that streamlines the creation of single guide (sg)RNA tiling libraries for base editing screens. Researchers can provide their genes or genomic features of interest, their base editors of choice, and target sequences to act as positive and negative controls. The server designs and annotates sgRNA libraries by integrating custom code with publicly available tools such as crisprVerse and Ensembl's Variant Effect Predictor. CRISPR-BEasy provides downloadable results, including sgRNA on/off-target scores, predicted mutational outcomes per base editor, and intuitive interactive visualizations for data quality assessment. CRISPR-BEasy also provides a separate tool that assembles sgRNA libraries into oligonucleotides for cloning following the detailed protocol documented in the searchable web server manual. Together, CRISPR-BEasy ensures the seamless design of cloning-ready sgRNA libraries, seeking to democratise access to base editing screening technologies.

The 2SP Site Mutation in the Bovine Natural Resistance-Associated Macrophage 1 Promoter Exhibits Antituberculosis Potential

Gene-edited cattle overexpressing natural resistance-associated macrophage 1 (NRAMP1) have demonstrated enhanced resistance to tuberculosis (TB). However, introducing synthetic sequences and selection markers may pose potential risks. The endogenous editing of target gene promoters could effectively mitigate these risks. To date, no available mutation sites in the bovine NRAMP1 promoter have been identified to enhance host resistance to TB. In this study, we identified a unique mutation editing site, designated as 2SP, within the bovine NRAMP1 promoter, using bioinformatics analysis and dual luciferase assays. The mutation at the 2SP site specifically increased NRAMP1 promoter activity by 2.3-fold after Mycobacterium tuberculosis H37Ra infection, without modifying promoter activity in non-infected groups. By using base editing techniques, an endogenously edited THP-1 cell line with a mutation at the homologous region of the 2SP site was generated, without introducing screening markers. In H37Ra infection experiments, the edited THP-1 cells specifically upregulated NRAMP1 expression and significantly inhibited H37Ra proliferation, while maintaining baseline NRAMP1 expression levels in the absence of infection. In this research, we identified a novel mutation site and provided a fundamental reference for the development of gene-edited cattle with enhanced resistance to TB.

CRISPR-dependent base editing as a therapeutic strategy for rare monogenic disorders

Rare monogenic disorders are caused by mutations in single genes and have an incidence rate of less than 0.5%. Due to their low prevalence, these diseases often attract limited research and commercial interest, leading to significant unmet medical needs. In a therapeutic landscape where treatments are targeted to manage symptoms, gene editing therapy emerges as a promising approach to craft curative and lasting treatments for these patients, often referred to as "one-and-done" therapeutics. CRISPR-dependent base editing enables the precise correction of genetic mutations by direct modification of DNA bases without creating potentially deleterious DNA double-strand breaks. Base editors combine a nickase version of Cas9 with cytosine or adenine deaminases to convert C·G to T·A and A·T to G·C, respectively. Together, cytosine (CBE) and adenine (ABE) base editors can theoretically correct ∼95% of pathogenic transition mutations cataloged in ClinVar. This mini-review explores the application of base editing as a therapeutic approach for rare monogenic disorders. It provides an overview of the state of gene therapies and a comprehensive compilation of preclinical studies using base editing to treat rare monogenic disorders. Key considerations for designing base editing-driven therapeutics are summarized in a user-friendly guide for researchers interested in applying this technology to a specific rare monogenic disorder. Finally, we discuss the prospects and challenges for bench-to-bedside translation of base editing therapies for rare monogenic disorders.

Engineering a photoactivatable A-to-I RNA base editor for gene therapy in vivo

Tunable and reversible regulation of exogenous and endogenous gene expression would be useful for improving the safety and efficacy of gene therapy. Current chemically inducible systems are limited by the rapid diffusion and extended metabolism of small molecules, and associated side effects. Here we develop a photoactivatable RNA adenosine base editor (PA-rABE) by harnessing a compact Cas13 variant and a split ADAR2 deaminase fused with the Magnets system, which is activated through blue-light-induced dimerization. PA-rABE achieves highly efficient editing on endogenous RNA with minimal bystander editing and off-target effects. By editing a phosphorylation site of the endogenous CTNNB1 gene, PA-rABE stabilizes the β-catenin protein and activates Wnt signaling in vivo. Using adeno-associated virus vectors to deliver PA-rABE along with an hF9 variant containing a premature termination codon, we show amelioration of clotting defects in hemophilia B mice upon illumination. In summary, PA-rABE offers a controlled RNA base-editing technology for diverse biomedical applications, enabling reversible and spatiotemporally specific modulation.

Cas9-mediated gene-editing frequency in microalgae is doubled by harnessing the interaction between importin α and phytopathogenic NLSs

Pathogen-derived nuclear localization signals (NLSs) enable vigorous nuclear invasion in the host by the virulence proteins harboring them. Herein, inspired by the robust nuclear import mechanism, we show that NLSs originating from the plant infection-associated Agrobacterium proteins VirD2 and VirE2 can be incorporated into the Cas9 system as efficient nuclear delivery enhancers, thereby improving the low gene-editing frequency in a model microalga, Chlamydomonas reinhardtii, caused by poor nuclear localization of the bulky nuclease. Prior to evaluation of the NLSs, IPA1 (Cre04.g215850) was first defined in the alga as the nuclear import-related importin alpha (Impα) that serves as a counterpart adaptor protein of the NLSs, based on extensive in silico analyses considering the protein's sequence, tertiary folding behavior, and structural basis when interacting with a well-studied SV40TAg NLS. Through precursive affinity explorations, we reproducibly found that the NLSs mediated the binding between the Cas9 and Impα with nM affinities and visually confirmed that the fusion of the NLSs strictly localized the peptide-bearing cargoes in the microalgal nucleus without compensating for their cleavage ability. When employed in a real-world application, the VirD2 NLS increases the mutation frequency (~1.12 × 10-5) over 2.4-fold compared to an archetypal SV40TAg NLS (~0.46 × 10-5) when fused with Cas9. We demonstrate the cross-species versatility of the Impα-dependent strategy by successfully applying it to an industrial alga, Chlorella Sp. HS2. This work, focused on affinity augmentation, provides insights into increasing the frequency of gene editing, which can be advantageously used in programmable mutagenesis with broad applicability.

BES-Designer: A Web Tool to Design Guide RNAs for Base Editing to Simplify Library

CRISPR/Cas base editors offer precise conversion of single nucleotides without inducing double-strand breaks. This technology finds extensive applications in gene therapy, gene function analysis, and other domains. However, a crucial challenge lies in selecting the appropriate guide RNAs (gRNAs) for base editing. Although various gRNAs design tools exist, creating a simplified base-editing library with diverse protospacer adjacent motifs (PAM) sequences for gRNAs screening remains a challenge. We present a user-friendly web tool, BES-Designer ( https://bes-designer.aielab.net ), for gRNAs design based on base editors, aimed at streamlining the creation of a base-editing library. BES-Designer incorporates our proposed rules for target sequence simplification, helping researchers narrow down the scope of biological experiments in the lab. It allows users to design target sequences with various PAMs and editing types simultaneously, and prioritize them in the simplified base-editing library. This tool has been experimentally proven to achieve a 30% simplification efficiency on the base-editing-library.

Circular Vectors as an efficient, fully synthetic, cell-free approach for preparing small circular DNA as a plasmid substitute for guide RNA expression in CRISPR-Cas9 genome editing

Robust expression of guide RNA (gRNA) is essential for successful implementation of CRISPR-Cas9 genome-editing methods. The gRNA components, such as an RNA polymerase promoter followed by the gRNA coding sequence and an RNA polymerase terminator sequence, and the Cas9 protein are expressed either via an all-in-one plasmid or separate dedicated plasmids. The preparation of such plasmids involves a laborious multi-day process of DNA assembly, bacterial cloning, validation, purification and sequencing. Our Circular Vector (CV) protocol introduces an efficient, fully synthetic, cell-free approach for preparing gRNA expression templates suitable for transfection, marking a significant advancement over traditional plasmid-based approaches. This protocol consists of the circularization and purification of linear double-stranded DNA (dsDNA) containing gRNA expression elements into compact, bacterial-backbone-free circular DNA expression vectors in as little as 3 h. We provide a guide to the design of the dsDNA template coding for gRNA elements for CRISPR-Cas9 base and prime editing, along with step-by-step instructions for the efficient preparation of gRNA-expressing CVs. In addition to rapid preparation, CVs created via this protocol offer several key advantages: a compact size, absence of a bacterial backbone, absence of bacterial endotoxins and no contamination by bacterial RNA or DNA fragments. These features make gRNA-expressing CVs a superior choice over plasmid-based gRNA expression templates.

Generation of Inheritable A-to-G Transitions Using Adenine Base Editing and NG-PAM Cas9 in Arabidopsis thaliana

Towards precise genome editing, base editors have been developed by fusing catalytically compromised Cas9 with deaminase components, mediating C-to-T (cytosine base editors) or A-to-G (adenine base editors) transition. We developed a set of vectors consisting of a 5'-NG-3' PAM-recognizing variant of SpCas9 with adenosine deaminases TadA7.10 or TadA8e. Using a phenotype-based screen in Arabidopsis thaliana targeting multiple PDS3 intron splice sites, we achieved up to 81% somatic A-to-G editing in primary transformants at a splice acceptor site with NGG PAM, while 35% was achieved for the same target adenine with NGA PAM. Among tested vectors, pECNUS4 (Addgene #184887), carrying TadA8e, showed the highest adenine base editor (ABE) efficiency. With pECNUS4, we recreated a naturally occurring allele of DANGEROUS MIX3 (DM3) in two generations, transgene-free, for NGC PAM. We also simultaneously base-edited four redundant DM1/SSI4 homologs, encoding nucleotide-binding leucine-rich repeat (NLR) proteins, using a single gRNA with NGA PAM targeting the conserved yet functionally crucial P-loop motif of NLR proteins. We found fixation of A-to-G in three NLR genes for all three possible adenine sites within base-editing window 3-9, as the edited genes segregate in T2. Multigene targeting succeeded in rescuing the previously reported autoimmune phenotype in two generations. Mediating desired ABE on seven NLR genes simultaneously was successful as well; above 77% editing was achieved in six of the seven possible targets in a T1 plant, with the remaining having a moderately high (32%) editing. ABE application to specifically inactivate functional motifs is anticipated to expedite the discovery of novel roles for proteins. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

CRISPR-Cas9-based approaches for genetic analysis and epistatic interaction studies in Coxiella burnetii

Coxiella burnetii is an obligate intracellular bacterial pathogen that replicates to high numbers in an acidified lysosome-derived vacuole. Intracellular replication requires the Dot/Icm type IVB secretion system, which translocates over 100 different effector proteins into the host cell. Screens employing random transposon mutagenesis have identified several C. burnetii effectors that play an important role in intracellular replication; however, the difficulty in conducting directed mutagenesis has been a barrier to the systematic analysis of effector mutants and to the construction of double mutants to assess epistatic interactions between effectors. Here, two CRISPR-Cas9 technology-based approaches were developed to study C. burnetii phenotypes resulting from targeted gene disruptions. CRISPRi was used to silence gene expression and demonstrated that silencing of effectors or Dot/Icm system components resulted in phenotypes similar to those of transposon insertion mutants. A CRISPR-Cas9-mediated cytosine base editing protocol was developed to generate targeted loss-of-function mutants through the introduction of premature stop codons into C. burnetii genes. Cytosine base editing successfully generated double mutants in a single step. A double mutant deficient in both cig57 and cig2 had a robust and additive intracellular replication defect when compared to either single mutant, which is consistent with Cig57 and Cig2 functioning in independent pathways that both contribute to a vacuole that supports C. burnetii replication. Thus, CRISPR-Cas9-based technologies expand the genetic toolbox for C. burnetii and will facilitate genetic studies aimed at investigating the mechanisms this pathogen uses to replicate inside host cells.

IMPORTANCE

Understanding the genetic mechanisms that enable C. burnetii to replicate in mammalian host cells has been hampered by the difficulty in making directed mutations. Here, a reliable and efficient system for generating targeted loss-of-function mutations in C. burnetii using a CRISPR-Cas9-assisted base editing approach is described. This technology was applied to make double mutants in C. burnetii that enabled the genetic analysis of two genes that play independent roles in promoting the formation of vacuoles that support intracellular replication. This advance will accelerate the discovery of mechanisms important for C. burnetii host infection and disease.

KOnezumi-AID: Automation Software for Efficient Multiplex Gene Knockout Using Target-AID

With the groundbreaking advancements in genome editing technologies, particularly CRISPR-Cas9, creating knockout mutants has become highly efficient. However, the CRISPR-Cas9 system introduces DNA double-strand breaks, increasing the risk of chromosomal rearrangements and posing a major obstacle to simultaneous multiple gene knockout. Base-editing systems, such as Target-AID, are safe alternatives for precise base modifications without requiring DNA double-strand breaks, serving as promising solutions for existing challenges. Nevertheless, the absence of adequate tools to support Target-AID-based gene knockout highlights the need for a comprehensive system to design guide RNAs (gRNAs) for the simultaneous knockout of multiple genes. Here, we aimed to develop KOnezumi-AID, a command-line tool for gRNA design for Target-AID-mediated genome editing. KOnezumi-AID facilitates gene knockout by inducing the premature termination codons or promoting exon skipping, thereby generating experiment-ready gRNA designs for mouse and human genomes. Additionally, KOnezumi-AID exhibits batch processing capacity, enabling rapid and precise gRNA design for large-scale genome editing, including CRISPR screening. In summary, KOnezumi-AID is an efficient and user-friendly tool for gRNA design, streamlining genome editing workflows and advancing gene knockout research.

Engineering CjCas9 for Efficient Base Editing and Prime Editing

The CRISPR-Cas9 system has been applied for clinical applications of gene therapy. Most CRISPR-based gene therapies are derived from Streptococcus pyogenes Cas9, which is challenging to package into a single adeno-associated virus vector and limits its clinical applications. Campylobacter jejuni Cas9 (CjCas9) is one of the smallest Cas9 proteins. CjCas9-mediated base editing (CjBE) efficiency varies across genomic sites, while CjCas9-mediated prime editing (CjPE) efficiency is less than 5% on average. Here we developed enhanced cytosine base editors (enCjCBEs) and adenine base editors (enCjABEs) by engineered CjCas9P47K. We demonstrated the robust C-to-T conversion (70% on average) by enCjCBE or A-to-G conversion (76% on average) by enCjABE. Meanwhile, we applied the CjCas9P47K variant to generate enhanced CjPE (enCjPE), which increases the editing efficiency 17-fold at the PRNP site over wild-type CjPE. Fusing nonspecific DNA binding protein Sso7d to enCjCas9 and MS2 stem-loop RNA aptamer to the 3-terminal of cognate pegRNA resulted in 12% editing efficiency on average with a 24-fold increase over wild-type CjPE, and we termed it SsenCjPE. The SsenCjPE can also be combined with hMLH1dn to further increase the editing efficiency and MMLV RTaseΔRnH to reduce size. Finally, we introduced an additional mutation D829R into SsenCjPE and generated SsenCjPE-M2 with a 61-fold increase of PE efficiency over wild-type at the PRNP site. In summary, enCjBEs, SsenCjPEs, or SsenCjPE-M2 are compact Cas9-derived BE or prime editors in biological research or biomedical applications.

Lipid nanoparticle-mediated base-editing of the Hao1 gene achieves sustainable primary hyperoxaluria type 1 therapy in rats

Primary hyperoxaluria type 1 (PH1) is a severe hereditary disease, leading to the accumulation of oxalate in multiple organs, particularly the kidney. Hydroxyacid oxidase 1 (HAO1), a pivotal gene involved in oxalate production, is an approved target for the treatment of PH1. In this study, we demonstrated the discovery of several novel therapeutic sites of the Hao1 gene and the efficient editing of Hao1 c.290-2 A in vivo with lipid nanoparticles (LNP) delivered adenine base editing (ABE) mRNA. A single infusion of LNP-ABE resulted in a near-complete knockout of Hao1 in the liver, leading to the sustainable normalization of urinary oxalate (for at least 6 months) and complete rescue of the patho-physiology in PH1 rats. Additionally, a significant correlation between Hao1 editing efficiency and urinary oxalate levels was observed and over 60% Hao1 editing efficiency was required to achieve the normalization of urinary oxalate in PH1 rats. These findings suggest that the LNP-mediated base-editing of Hao1 c.290-2 A is an efficient and safe approach to PH1 therapy, highlighting its potential utility in clinical settings.

Versatile and efficient mammalian genome editing with Type I-C CRISPR System of Desulfovibrio vulgaris

CRISPR-Cas tools for mammalian genome editing typically rely on single Cas9 or Cas12a proteins. While type I CRISPR systems in Class I may offer greater specificity and versatility, they are not well-developed for genome editing. Here, we present an alternative type I-C CRISPR system from Desulfovibrio vulgaris (Dvu) for efficient and precise genome editing in mammalian cells and animals. We optimized the Dvu type I-C editing complex to generate precise deletions at multiple loci in various cell lines and pig primary fibroblast cells using a paired PAM-in crRNA strategy. These edited pig cells can serve as donors for generating transgenic cloned piglets. The Dvu type I-C editor also enabled precise large fragment replacements with homology-directed repair. Additionally, we adapted the Dvu-Cascade effector for cytosine and adenine base editing, developing Dvu-CBE and Dvu-ABE systems. These systems efficiently induced C-to-T and A-to-G substitutions in human genes without double-strand breaks. Off-target analysis confirmed the high specificity of the Dvu type I-C editor. Our findings demonstrate the Dvu type I-C editor's potential for diverse mammalian genome editing applications, including deletions, fragment replacement, and base editing, with high efficiency and specificity for biomedicine and agriculture.

Bystander base editing interferes with visual function restoration in Leber congenital amaurosis

Base editors (BEs) have emerged as a powerful tool for gene correction with high activity. However, bystander base editing, a byproduct of BEs, presents challenges for precise editing. Here, we investigated the effects of bystander edits on phenotypic restoration in the context of Leber congenital amaurosis (LCA), a hereditary retinal disorder, as a therapeutic model. We observed that in rd12 of LCA model mice, the highest editing activity version of an adenine base editors (ABEs), ABE8e, generated substantial bystander editing, resulting in missense mutations despite RPE65 expression, preventing restoration of visual function. Through AlphaFold-based mutational scanning and molecular dynamics simulations, we identified that the ABE8e-driven L43P mutation disrupts RPE65 structure and function. Our findings underscore the need for more stringent requirements in developing precise BEs for future clinical applications.

Glycosylase-based base editors for efficient T-to-G and C-to-G editing in mammalian cells

Base editors show promise for treating human genetic diseases, but most current systems use deaminases, which cause off-target effects and are limited in editing type. In this study, we constructed deaminase-free base editors for cytosine (DAF-CBE) and thymine (DAF-TBE), which contain only a cytosine-DNA or a thymine-DNA glycosylase (CDG/TDG) variant, respectively, tethered to a Cas9 nickase. Multiple rounds of mutagenesis by directed evolution in Escherichia coli generated two variants with enhanced base-converting activity-CDG-nCas9 and TDG-nCas9-with efficiencies of up to 58.7% for C-to-A and 54.3% for T-to-A. DAF-BEs achieve C-to-G/T-to-G editing in mammalian cells with minimal Cas9-dependent and Cas9-independent off-target effects as well as minimal RNA off-target effects. Additional engineering resulted in DAF-CBE2/DAF-TBE2, which exhibit altered editing windows from the 5' end to the middle of the protospacer and increased C-to-G/T-to-G editing efficiency of 3.5-fold and 1.2-fold, respectively. Compared to prime editing or CGBEs, DAF-BEs expand conversion types of base editors with similar efficiencies, smaller sizes and lower off-target effects.

Protocol for establishing knockout cell clones by deletion of a large gene fragment using CRISPR-Cas9 with multiple guide RNAs

Genome editing is a powerful tool for establishing gene knockout or mutant cell lines. Here, we present a protocol for establishing knockout cell clones by deletion of large gene fragments using CRISPR-Cas9 with multiple guide RNAs. We describe steps for designing guide RNAs, cloning them into CRISPR-Cas9 vectors, cell seeding, transfection into cultured cells, clonal selection, and screening assays. This protocol can delete gene regions over 100 kbp, including GC-rich domains, and is applicable to various cell lines. For complete details on the use and execution of this protocol, please refer to Saito et al.,1 Saito and Endo et al.,2 and Higashi et al.3.

Discovery of novel disease-causing mutation in SSBP1 and its correction using adenine base editor to improve mitochondrial function

Mutations in nuclear genes regulating mitochondrial DNA (mtDNA) replication are associated with mtDNA depletion syndromes. Using whole-genome sequencing, we identified a heterozygous mutation (c.272G>A:p.Arg91Gln) in single-stranded DNA-binding protein 1 (SSBP1), a crucial protein involved in mtDNA replisome. The proband manifested symptoms including sensorineural deafness, congenital cataract, optic atrophy, macular dystrophy, and myopathy. This mutation impeded multimer formation and DNA-binding affinity, leading to reduced efficiency of mtDNA replication, altered mitochondria dynamics, and compromised mitochondrial function. To correct this mutation, we tested two adenine base editor (ABE) variants on patient-derived fibroblasts. One variant, NG-Cas9-based ABE8e (NG-ABE8e), showed higher editing efficacy (≤30%) and enhanced mitochondrial replication and function, despite off-target editing frequencies; however, risks from bystander editing were limited due to silent mutations and off-target sites in non-translated regions. The other variant, NG-Cas9-based ABE8eWQ (NG-ABE8eWQ), had a safer therapeutic profile with very few off-target effects, but this came at the cost of lower editing efficacy (≤10% editing). Despite this, NG-ABE8eWQ-edited cells still restored replication and improved mtDNA copy number, which in turn recovery of compromised mitochondrial function. Taken together, base editing-based gene therapies may be a promising treatment for mitochondrial diseases, including those associated with SSBP1 mutations.

Engineering Oncogenic Hotspot Mutations on SF3B1 via CRISPR-Directed PRECIS Mutagenesis

SF3B1 is the most recurrently mutated RNA splicing gene in cancer. However, research of its pathogenic role has been hindered by a lack of disease-relevant cell line models. Here, our study compared four genome engineering platforms to establish SF3B1 mutant cell lines: CRISPR-Cas9 editing, AAV homology-directed repair editing, base editing (ABEmax, ABE8e), and prime editing (PE2, PE3, PE5max). We showed that prime editing via PE5max achieved the most efficient SF3B1 K700E editing across a wide range of cell lines. Our approach was further refined by coupling prime editing with a fluorescent reporter that leverages a SF3B1 mutation-responsive synthetic intron to mark successfully edited cells. By applying this approach, called prime editing coupled intron-assisted selection (PRECIS), we introduced the K700E hotspot mutation into two chronic lymphocytic leukemia cell lines, HG-3 and MEC-1. We demonstrated that our PRECIS-engineered cells faithfully recapitulate known mutant SF3B1 phenotypes, including altered splicing, copy number variations, and cell-growth defect. Moreover, we discovered that the SF3B1 mutation can cause the loss of Y chromosome in chronic lymphocytic leukemia. Our results showcase that PRECIS is an efficient and generalizable method for engineering genetically faithful SF3B1 mutant models. Our approach provides new insights on the role of SF3B1 mutation in cancer and enables the generation of SF3B1 mutant cell lines in relevant cellular context.

SIGNIFICANCE

This study developed an approach that can reliably and efficiently engineer SF3B1 mutation into different cellular contexts, thereby revealing novel roles of SF3B1 mutation in driving aberrant splicing, clonal evolution, and genome instability.

Engineering APOBEC3A deaminase for highly accurate and efficient base editing

Cytosine base editors (CBEs) are effective tools for introducing C-to-T base conversions, but their clinical applications are limited by off-target and bystander effects. Through structure-guided engineering of human APOBEC3A (A3A) deaminase, we developed highly accurate A3A-CBE (haA3A-CBE) variants that efficiently generate C-to-T conversion with a narrow editing window and near-background level of DNA and RNA off-target activity, irrespective of methylation status and sequence context. The engineered deaminase domains are compatible with PAM-relaxed SpCas9-NG variant, enabling accurate correction of pathogenic mutations in homopolymeric cytosine sites through flexible positioning of the single-guide RNAs. Dual adeno-associated virus delivery of one haA3A-CBE variant to a mouse model of tyrosinemia induced up to 58.1% editing in liver tissues with minimal bystander editing, which was further reduced through single dose of lipid nanoparticle-based messenger RNA delivery of haA3A-CBEs. These results highlight the tremendous promise of haA3A-CBEs for precise genome editing to treat human diseases.

Engineering IscB to develop highly efficient miniature editing tools in mammalian cells and embryos

The IscB proteins, as the ancestors of Cas9 endonuclease, hold great promise due to their small size and potential for diverse genome editing. However, their activity in mammalian cells is unsatisfactory. By introducing three residual substitutions in IscB, we observed an average 7.5-fold increase in activity. Through fusing a sequence-non-specific DNA-binding protein domain, the eIscB-D variant achieved higher editing efficiency, with a maximum of 91.3%. Moreover, engineered ωRNA was generated with a 20% reduction in length and slightly increased efficiency. The engineered eIscB-D/eωRNA system showed an average 20.2-fold increase in activity compared with the original IscB. Furthermore, we successfully adapted eIscB-D for highly efficient cytosine and adenine base editing. Notably, eIscB-D is highly active in mouse cell lines and embryos, enabling the efficient generation of disease models through mRNA/ωRNA injection. Our study suggests that these miniature genome-editing tools have great potential for diverse applications.

DNA base editing corrects common hemophilia A mutations and restores factor VIII expression in in vitro and ex vivo models

BACKGROUND

Replacement and nonreplacement therapies effectively control bleeding in hemophilia A (HA) but imply lifelong interventions. Authorized gene addition therapy could provide a cure but still poses questions on durability. FVIIIgene correction would definitively restore factor (F)VIII production, as shown in animal models through nuclease-mediated homologous recombination (HR). However, low efficiency and potential off-target double-strand break still limit HR translatability.

OBJECTIVES

To correct common model single point mutations leading to severe HA through the recently developed double-strand break/HR-independent base editing (BE) and prime editing (PE) approaches.

METHODS

Screening for efficacy of BE/PE systems in HEK293T cells transiently expressing FVIII variants and validation at DNA (sequencing) and protein (enzyme-linked immunosorbent assay; activated partial thromboplastin time) level in stable clones. Evaluation of rescue in engineered blood outgrowth endothelial cells by lentiviral-mediated delivery of BE.

RESULTS

Transient assays identified the best-performing BE/PE systems for each variant, with the highest rescue of FVIII expression (up to 25% of wild-type recombinant FVIII) for the p.R2166∗ and p.R2228Q mutations. In stable clones, we demonstrated that the mutation reversion on DNA (∼24%) was consistent with the rescue of FVIII secretion and activity of 20% to 30%. The lentiviral-mediated delivery of the selected BE systems was attempted in engineered blood outgrowth endothelial cells harboring the p.R2166∗ and p.R2228Q variants, which led to an appreciable and dose-dependent rescue of secreted functional FVIII.

CONCLUSION

Overall data provide the first proof-of-concept for effective BE/PE-mediated correction of HA-causing mutations, which encourage studies in mouse models to develop a personalized cure for large cohorts of patients through a single intervention.

CRISPR Base Editing to Create Potential Charcot-Marie-Tooth Disease Models with High Editing Efficiency: Human Induced Pluripotent Stem Cell Harboring SH3TC2 Variants

Human induced pluripotent stem cells (hiPSCs) represent a powerful tool to investigate neuropathological disorders in which the cells of interest are inaccessible, such as in the Charcot-Marie-Tooth disease (CMT), the most common inherited peripheral neuropathy. Developing appropriate cellular models becomes crucial in order to both study the disease's pathophysiology and test new therapeutic approaches. The generation of hiPS cellular models for disorders caused by a single nucleotide variation has been significantly improved following the development of CRISPR-based editing tools. In this study, we efficiently and quickly generated, by CRISPR editing, the two first hiPSCs cellular models carrying alterations involved in CMT4C, also called AR-CMTde-SH3TC2. This subtype of CMT is associated with alterations in the SH3TC2 gene and represents the most prevalent form of autosomal recessive demyelinating CMT. We aimed to develop models for two different SH3TC2 nonsense variants, c.211C>T, p.Gln71* and the most common AR-CMTde-SH3TC2 alteration, c.2860C>T, p.Arg954*. First, in order to determine the best CRISPR strategy to adopt on hiPSCs, we first tested a variety of sgRNAs combined with a selection of recent base editors using the conveniently cultivable and transfectable HEK-293T cell line. The chosen CRISPR base-editing strategy was then applied to hiPSCs derived from healthy individuals to generate isogenic CMT disease models with up to 93% editing efficiency. For point mutation generation, we first recommend to test your strategies on alternative cell line such as HEK-293T before hiPSCs to evaluate a variety of sgRNA-BE combinations, thus boosting the chance of achieving edited cellular clones with the hard-to-culture and to transfect hiPSCs.

In vivo adenine base editing rescues adrenoleukodystrophy in a humanized mouse model

X-linked adrenoleukodystrophy (ALD), an inherited neurometabolic disorder caused by mutations in ABCD1, which encodes the peroxisomal ABC transporter, mainly affects the brain, spinal cord, adrenal glands, and testes. In ALD patients, very-long-chain fatty acids (VLCFAs) fail to enter the peroxisome and undergo subsequent β-oxidation, resulting in their accumulation in the body. It has not been tested whether in vivo base editing or prime editing can be harnessed to ameliorate ALD. We developed a humanized mouse model of ALD by inserting a human cDNA containing the pathogenic variant into the mouse Abcd1 locus. The humanized ALD model showed increased levels of VLCFAs. To correct the mutation, we tested both base editing and prime editing and found that base editing using ABE8e(V106W) could correct the mutation in patient-derived fibroblasts at an efficiency of 7.4%. Adeno-associated virus (AAV)-mediated systemic delivery of NG-ABE8e(V106W) enabled robust correction of the pathogenic variant in the mouse brain (correction efficiency: ∼5.5%), spinal cord (∼5.1%), and adrenal gland (∼2%), leading to a significant reduction in the plasma levels of C26:0/C22:0. This established humanized mouse model and the successful correction of the pathogenic variant using a base editor serve as a significant step toward treating human ALD disease.

CRISPR Tools for Engineering Prokaryotic Systems: Recent Advances and New Applications

In the past decades, the broad selection of CRISPR-Cas systems has revolutionized biotechnology by enabling multimodal genetic manipulation in diverse organisms. Rooted in a molecular engineering perspective, we recapitulate the different CRISPR components and how they can be designed for specific genetic engineering applications. We first introduce the repertoire of Cas proteins and tethered effectors used to program new biological functions through gene editing and gene regulation. We review current guide RNA (gRNA) design strategies and computational tools and how CRISPR-based genetic circuits can be constructed through regulated gRNA expression. Then, we present recent advances in CRISPR-based biosensing, bioproduction, and biotherapeutics across in vitro and in vivo prokaryotic systems. Finally, we discuss forthcoming applications in prokaryotic CRISPR technology that will transform synthetic biology principles in the near future.

Post-translational modification-centric base editor screens to assess phosphorylation site functionality in high throughput

Signaling pathways that drive gene expression are typically depicted as having a dozen or so landmark phosphorylation and transcriptional events. In reality, thousands of dynamic post-translational modifications (PTMs) orchestrate nearly every cellular function, and we lack technologies to find causal links between these vast biochemical pathways and genetic circuits at scale. Here we describe the high-throughput, functional assessment of phosphorylation sites through the development of PTM-centric base editing coupled to phenotypic screens, directed by temporally resolved phosphoproteomics. Using T cell activation as a model, we observe hundreds of unstudied phosphorylation sites that modulate NFAT transcriptional activity. We identify the phosphorylation-mediated nuclear localization of PHLPP1, which promotes NFAT but inhibits NFκB activity. We also find that specific phosphosite mutants can alter gene expression in subtle yet distinct patterns, demonstrating the potential for fine-tuning transcriptional responses. Overall, base editor screening of PTM sites provides a powerful platform to dissect PTM function within signaling pathways.

Enhancing genome editing in hPSCs through dual inhibition of DNA damage response and repair pathways

Precise genome editing is crucial for establishing isogenic human disease models and ex vivo stem cell therapy from the patient-derived hPSCs. Unlike Cas9-mediated knock-in, cytosine base editor and prime editor achieve the desirable gene correction without inducing DNA double strand breaks. However, hPSCs possess highly active DNA repair pathways and are particularly susceptible to p53-dependent cell death. These unique characteristics impede the efficiency of gene editing in hPSCs. Here, we demonstrate that dual inhibition of p53-mediated cell death and distinct activation of the DNA damage repair system upon DNA damage by cytosine base editor or prime editor additively enhanced editing efficiency in hPSCs. The BE4stem system comprised of p53DD, a dominant negative p53, and three UNG inhibitor, engineered to specifically diminish base excision repair, improves cytosine base editor efficiency in hPSCs. Addition of dominant negative MLH1 to inhibit mismatch repair activity and p53DD in the conventional prime editor system also significantly enhances prime editor efficiency in hPSCs. Thus, combined inhibition of the distinct cellular cascades engaged in hPSCs upon gene editing could significantly enhance precise genome editing in these cells.

Revolutionising healing: Gene Editing's breakthrough against sickle cell disease

Recent advancements in gene editing illuminate new potential therapeutic approaches for Sickle Cell Disease (SCD), a debilitating monogenic disorder caused by a point mutation in the β-globin gene. Despite the availability of several FDA-approved medications for symptomatic relief, allogeneic hematopoietic stem cell transplantation (HSCT) remains the sole curative option, underscoring a persistent need for novel treatments. This review delves into the growing field of gene editing, particularly the extensive research focused on curing haemoglobinopathies like SCD. We examine the use of techniques such as CRISPR-Cas9 and homology-directed repair, base editing, and prime editing to either correct the pathogenic variant into a non-pathogenic or wild-type one or augment fetal haemoglobin (HbF) production. The article elucidates ways to optimize these tools for efficacious gene editing with minimal off-target effects and offers insights into their effective delivery into cells. Furthermore, we explore clinical trials involving alternative SCD treatment strategies, such as LentiGlobin therapy and autologous HSCT, distilling the current findings. This review consolidates vital information for the clinical translation of gene editing for SCD, providing strategic insights for investigators eager to further the development of gene editing for SCD.

Genetics of cystogenesis in base-edited human organoids reveal therapeutic strategies for polycystic kidney disease

In polycystic kidney disease (PKD), microscopic tubules expand into macroscopic cysts. Among the world's most common genetic disorders, PKD is inherited via heterozygous loss-of-function mutations but is theorized to require additional loss of function. To test this, we establish human pluripotent stem cells in allelic series representing four common nonsense mutations, using CRISPR base editing. When differentiated into kidney organoids, homozygous mutants spontaneously form cysts, whereas heterozygous mutants (original or base corrected) express no phenotype. Using these, we identify eukaryotic ribosomal selective glycosides (ERSGs) as PKD therapeutics enabling ribosomal readthrough of these same nonsense mutations. Two different ERSGs not only prevent cyst initiation but also limit growth of pre-formed cysts by partially restoring polycystin expression. Furthermore, glycosides accumulate in cyst epithelia in organoids and mice. Our findings define the human polycystin threshold as a surmountable drug target for pharmacological or gene therapy interventions, with relevance for understanding disease mechanisms and future clinical trials.

Adenine transversion editors enable precise, efficient A•T-to-C•G base editing in mammalian cells and embryos

Base editors have substantial promise in basic research and as therapeutic agents for the correction of pathogenic mutations. The development of adenine transversion editors has posed a particular challenge. Here we report a class of base editors that enable efficient adenine transversion, including precise A•T-to-C•G editing. We found that a fusion of mouse alkyladenine DNA glycosylase (mAAG) with nickase Cas9 and deaminase TadA-8e catalyzed adenosine transversion in specific sequence contexts. Laboratory evolution of mAAG significantly increased A-to-C/T conversion efficiency up to 73% and expanded the targeting scope. Further engineering yielded adenine-to-cytosine base editors (ACBEs), including a high-accuracy ACBE-Q variant, that precisely install A-to-C transversions with minimal Cas9-independent off-targeting effects. ACBEs mediated high-efficiency installation or correction of five pathogenic mutations in mouse embryos and human cell lines. Founder mice showed 44-56% average A-to-C edits and allelic frequencies of up to 100%. Adenosine transversion editors substantially expand the capabilities and possible applications of base editing technology.

Highly efficient biallelic correction of homozygous COL7A1 mutation using ABE8e adenine base editor

Recessive dystrophic epidermolysis bullosa (RDEB) is a genetic skin disorder characterized by severe skin blistering. With no cure at present, we have pursued a gene repair approach using base editing of COL7A1. Following on from our previous report of highly efficient base editing using the novel base editor, ABE8e, here we explore the use of lipid nanoparticle (LNP)-based delivery systems instead of electroporation to effect base editing in RDEB fibroblasts and correct a missense G > A mutation. We demonstrate increasingly high editing efficiencies, averaging 90%, which vary with the ABE8e dosage. We then demonstrate the viability of topical delivery of mRNA encapsulated in an LNP. Our work underscores the translational potential of this therapeutic route for DEB.

Harnessing non-Watson-Crick's base pairing to enhance CRISPR effectors cleavage activities and enable gene editing in mammalian cells

Genomic DNA of the cyanophage S-2L virus is composed of 2-aminoadenine (Z), thymine (T), guanine (G), and cytosine (C), forming the genetic alphabet ZTGC, which violates Watson-Crick base pairing rules. The Z-base has an extra amino group on the two position that allows the formation of a third hydrogen bond with thymine in DNA strands. Here, we explored and expanded applications of this non-Watson-Crick base pairing in protein expression and gene editing. Both ZTGC-DNA (Z-DNA) and ZUGC-RNA (Z-RNA) produced in vitro show detectable compatibility and can be decoded in mammalian cells, including Homo sapiens cells. Z-crRNA can guide CRISPR-effectors SpCas9 and LbCas12a to cleave specific DNA through non-Watson-Crick base pairing and boost cleavage activities compared to A-crRNA. Z-crRNA can also allow for efficient gene and base editing in human cells. Together, our results help pave the way for potential strategies for optimizing DNA or RNA payloads for gene editing therapeutics and give insights to understanding the natural Z-DNA genome.

Protocol for arrayed gRNA screening by base editors in mammalian cell lines using lentiviral system

Base editing, a CRISPR-based genome engineering technique, enables precise single-nucleotide modifications while minimizing double-strand breaks. Here, we present a protocol for arrayed mutagenesis using base editors to identify regulatory elements within the gamma-globin locus. We describe steps for guide RNA (gRNA) cloning into lentiviral vectors, establishing stable cell lines with base editor expression, transducing gRNAs, and assessing editing efficiency. This protocol can be applied to diverse genomic regions and cell lines for arrayed screening, facilitating genetic research, and target discovery. For complete details on the use and execution of this protocol, please refer to Ravi et al. (2022)1.

Directed evolution rice genes with randomly multiplexed sgRNAs assembly of base editors

CRISPR-based directed evolution is an effective breeding biotechnology to improve agronomic traits in plants. However, its gene diversification is still limited using individual single guide RNA. We described here a multiplexed orthogonal base editor (MoBE), and a randomly multiplexed sgRNAs assembly strategy to maximize gene diversification. MoBE could induce efficiently orthogonal ABE (<36.6%), CBE (<36.0%), and A&CBE (<37.6%) on different targets, while the sgRNA assembling strategy randomized base editing events on various targets. With respective 130 and 84 targets from each strand of the 34th exon of rice acetyl-coenzyme A carboxylase (OsACC), we observed the target-scaffold combination types up to 27 294 in randomly dual and randomly triple sgRNA libraries. We further performed directed evolution of OsACC using MoBE and randomly dual sgRNA libraries in rice, and obtained single or linked mutations of stronger herbicide resistance. These strategies are useful for in situ directed evolution of functional genes and may accelerate trait improvement in rice.

Proteome-wide base editor screens to assess phosphorylation site functionality in high-throughput

Signaling pathways that drive gene expression are typically depicted as having a dozen or so landmark phosphorylation and transcriptional events. In reality, thousands of dynamic post-translational modifications (PTMs) orchestrate nearly every cellular function, and we lack technologies to find causal links between these vast biochemical pathways and genetic circuits at scale. Here, we describe "signaling-to-transcription network" mapping through the development of PTM-centric base editing coupled to phenotypic screens, directed by temporally-resolved phosphoproteomics. Using T cell activation as a model, we observe hundreds of unstudied phosphorylation sites that modulate NFAT transcriptional activity. We identify the phosphorylation-mediated nuclear localization of the phosphatase PHLPP1 which promotes NFAT but inhibits NFκB activity. We also find that specific phosphosite mutants can alter gene expression in subtle yet distinct patterns, demonstrating the potential for fine-tuning transcriptional responses. Overall, base editor screening of PTM sites provides a powerful platform to dissect PTM function within signaling pathways.

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.

Split complementation of base editors to minimize off-target edits

Base editors (BEs) empower the efficient installation of beneficial or corrective point mutations in crop and human genomes. However, conventional BEs can induce unpredictable guide RNA (gRNA)-independent off-target edits in the genome and transcriptome due to spurious activities of BE-enclosing deaminases, and current improvements mostly rely on deaminase-specific mutagenesis or exogenous regulators. Here we developed a split deaminase for safe editing (SAFE) system applicable to BEs containing distinct cytidine or adenosine deaminases, with no need of external regulators. In SAFE, a BE was properly split at a deaminase domain embedded inside a Cas9 nickase, simultaneously fragmenting and deactivating both the deaminase and the Cas9 nickase. The gRNA-conditioned BE reassembly conferred robust on-target editing in plant, human and yeast cells, while minimizing both gRNA-independent and gRNA-dependent off-target DNA/RNA edits. SAFE also substantially increased product purity by eliminating indels. Altogether, SAFE provides a generalizable solution for BEs to suppress off-target editing and improve on-target performance.

High-Efficiency Multiplexed Cytosine Base Editors for Natural Product Synthesis in Yarrowia lipolytica

Yarrowia lipolytica is an industrial host with a high fatty acid flux. Even though CRISPR-based tools have accelerated its metabolic engineering, there remains a need to develop tools for rapid multiplexed strain engineering to accelerate the design-build-test-learn cycle. Base editors have the potential to perform high-efficiency multiplexed gene editing because they do not depend upon double-stranded DNA breaks. Here, we identified that base editors are less toxic than CRISPR-Cas9 for multiplexed gene editing. We increased the editing efficiency by removing the extra nucleotides between tRNA and gRNA and increasing the base editor and gRNA copy number in a Ku70 deficient strain. We achieved five multiplexed gene editing in the ΔKu70 strain at 42% efficiency. Initially, we were unsuccessful at performing multiplexed base editing in NHEJ competent strain; however, we increased the editing efficiency by using a co-selection approach to enrich base editing events. Base editor-mediated canavanine gene (CAN1) knockout provided resistance to the import of canavanine, which enriched the base editing in other unrelated genetic loci. We performed multiplexed editing of up to three genes at 40% efficiency in the Po1f strain through the CAN1 co-selection approach. Finally, we demonstrated the application of multiplexed cytosine base editor for rapid multigene knockout to increase naringenin production by 2-fold from glucose or glycerol as a carbon source.

Nonviral base editing of KCNJ13 mutation preserves vision in a model of inherited retinal channelopathy

Clinical genome editing is emerging for rare disease treatment, but one of the major limitations is the targeting of CRISPR editors' delivery. We delivered base editors to the retinal pigmented epithelium (RPE) in the mouse eye using silica nanocapsules (SNCs) as a treatment for retinal degeneration. Leber congenital amaurosis type 16 (LCA16) is a rare pediatric blindness caused by point mutations in the KCNJ13 gene, a loss of function inwardly rectifying potassium channel (Kir7.1) in the RPE. SNCs carrying adenine base editor 8e (ABE8e) mRNA and sgRNA precisely and efficiently corrected the KCNJ13W53X/W53X mutation. Editing in both patient fibroblasts (47%) and human induced pluripotent stem cell-derived RPE (LCA16-iPSC-RPE) (17%) showed minimal off-target editing. We detected functional Kir7.1 channels in the edited LCA16-iPSC-RPE. In the LCA16 mouse model (Kcnj13W53X/+ΔR), RPE cells targeted SNC delivery of ABE8e mRNA preserved normal vision, measured by full-field electroretinogram (ERG). Moreover, multifocal ERG confirmed the topographic measure of electrical activity primarily originating from the edited retinal area at the injection site. Preserved retina structure after treatment was established by optical coherence tomography (OCT). This preclinical validation of targeted ion channel functional rescue, a challenge for pharmacological and genomic interventions, reinforced the effectiveness of nonviral genome-editing therapy for rare inherited disorders.

CRISPR single base-editing: in silico predictions to variant clonal cell lines

Engineering single base edits using CRISPR technology including specific deaminases and single-guide RNA (sgRNA) is a rapidly evolving field. Different types of base edits can be constructed, with cytidine base editors (CBEs) facilitating transition of C-to-T variants, adenine base editors (ABEs) enabling transition of A-to-G variants, C-to-G transversion base editors (CGBEs) and recently adenine transversion editors (AYBE) that create A-to-C and A-to-T variants. The base-editing machine learning algorithm BE-Hive predicts which sgRNA and base editor combinations have the strongest likelihood of achieving desired base edits. We have used BE-Hive and TP53 mutation data from The Cancer Genome Atlas (TCGA) ovarian cancer cohort to predict which mutations can be engineered, or reverted to wild-type (WT) sequence, using CBEs, ABEs or CGBEs. We have developed and automated a ranking system to assist in selecting optimally designed sgRNA that considers the presence of a suitable protospacer adjacent motif (PAM), the frequency of predicted bystander edits, editing efficiency and target base change. We have generated single constructs containing ABE or CBE editing machinery, an sgRNA cloning backbone and an enhanced green fluorescent protein tag (EGFP), removing the need for co-transfection of multiple plasmids. We have tested our ranking system and new plasmid constructs to engineer the p53 mutants Y220C, R282W and R248Q into WT p53 cells and shown that these mutants cannot activate four p53 target genes, mimicking the behaviour of endogenous p53 mutations. This field will continue to rapidly progress, requiring new strategies such as we propose to ensure desired base-editing outcomes.

Corneal gene therapy: Structural and mechanistic understanding

Cornea, a dome-shaped and transparent front part of the eye, affords 2/3rd refraction and barrier functions. Globally, corneal diseases are the leading cause of vision impairment. Loss of corneal function including opacification involve the complex crosstalk and perturbation between a variety of cytokines, chemokines and growth factors generated by corneal keratocytes, epithelial cells, lacrimal tissues, nerves, and immune cells. Conventional small-molecule drugs can treat mild-to-moderate traumatic corneal pathology but requires frequent application and often fails to treat severe pathologies. The corneal transplant surgery is a standard of care to restore vision in patients. However, declining availability and rising demand of donor corneas are major concerns to maintain ophthalmic care. Thus, the development of efficient and safe nonsurgical methods to cure corneal disorders and restore vision in vivo is highly desired. Gene-based therapy has huge potential to cure corneal blindness. To achieve a nonimmunogenic, safe and sustained therapeutic response, the selection of a relevant genes, gene editing methods and suitable delivery vectors are vital. This article describes corneal structural and functional features, mechanistic understanding of gene therapy vectors, gene editing methods, gene delivery tools, and status of gene therapy for treating corneal disorders, diseases, and genetic dystrophies.

CRISPR/Cas9-based gene activation and base editing in Populus

The genus Populus has long been used for environmental, agroforestry and industrial applications worldwide. Today Populus is also recognized as a desirable crop for biofuel production and a model tree for physiological and ecological research. As such, various modern biotechnologies, including CRISPR/Cas9-based techniques, have been actively applied to Populus for genetic and genomic improvements for traits such as increased growth rate and tailored lignin composition. However, CRISPR/Cas9 has been primarily used as the active Cas9 form to create knockouts in the hybrid poplar clone "717-1B4" (P. tremula x P. alba clone INRA 717-1B4). Alternative CRISPR/Cas9-based technologies, e.g. those involving modified Cas9 for gene activation and base editing, have not been evaluated in most Populus species for their efficacy. Here we employed a deactivated Cas9 (dCas9)-based CRISPR activation (CRISPRa) technique to fine-tune the expression of two target genes, TPX2 and LecRLK-G which play important roles in plant growth and defense response, in hybrid poplar clone "717-1B4" and poplar clone "WV94" (P. deltoides "WV94"), respectively. We observed that CRISPRa resulted in 1.2-fold to 7.0-fold increase in target gene expression through transient expression in protoplasts and Agrobacterium-mediated stable transformation, demonstrating the effectiveness of dCas9-based CRISPRa system in Populus. In addition, we applied Cas9 nickase (nCas9)-based cytosine base editor (CBE) to precisely introduce premature stop codons via C-to-T conversion, with an efficiency of 13%-14%, in the target gene PLATZ which encodes a transcription factor involved in plant fungal pathogen response in hybrid poplar clone "717-1B4". Overall, we showcase the successful application of CRISPR/Cas-based technologies in gene expression regulation and precise gene engineering in two Populus species, facilitating the adoption of emerging genome editing tools in woody species.

Re-engineering the adenine deaminase TadA-8e for efficient and specific CRISPR-based cytosine base editing

Cytosine base editors (CBEs) efficiently generate precise C·G-to-T·A base conversions, but the activation-induced cytidine deaminase/apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (AID/APOBEC) protein family deaminase component induces considerable off-target effects and indels. To explore unnatural cytosine deaminases, we repurpose the adenine deaminase TadA-8e for cytosine conversion. The introduction of an N46L variant in TadA-8e eliminates its adenine deaminase activity and results in a TadA-8e-derived C-to-G base editor (Td-CGBE) capable of highly efficient and precise C·G-to-G·C editing. Through fusion with uracil glycosylase inhibitors and further introduction of additional variants, a series of Td-CBEs was obtained either with a high activity similar to that of BE4max or with higher precision compared to other reported accurate CBEs. Td-CGBE/Td-CBEs show very low indel effects and a background level of Cas9-dependent or Cas9-independent DNA/RNA off-target editing. Moreover, Td-CGBE/Td-CBEs are more efficient in generating accurate edits in homopolymeric cytosine sites in cells or mouse embryos, suggesting their accuracy and safety for gene therapy and other applications.

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.

Improving adenine and dual base editors through introduction of TadA-8e and Rad51DBD

Base editors, including dual base editors, are innovative techniques for efficient base conversions in genomic DNA. However, the low efficiency of A-to-G base conversion at positions proximal to the protospacer adjacent motif (PAM) and the A/C simultaneous conversion of the dual base editor hinder their broad applications. In this study, through fusion of ABE8e with Rad51 DNA-binding domain, we generate a hyperactive ABE (hyABE) which offers improved A-to-G editing efficiency at the region (A₁₀-A₁₅) proximal to the PAM, with 1.2- to 7-fold improvement compared to ABE8e. Similarly, we develop optimized dual base editors (eA&C-BEmax and hyA&C-BEmax) with markedly improved simultaneous A/C conversion efficiency (1.2-fold and 1.5-fold improvement, respectively) compared to A&C-BEmax in human cells. Moreover, these optimized base editors catalyze efficiently nucleotide conversions in zebrafish embryos to mirror human syndrome or in human cells to potentially treat genetic diseases, indicating their great potential in broad applications for disease modeling and gene therapy.

Enhancement of Gene Editing and Base Editing with Therapeutic Ribonucleoproteins through In Vivo Delivery Based on Absorptive Silica Nanoconstruct

Key to the widespread and secure application of genome editing tools is the safe and effective delivery of multiple components of ribonucleoproteins (RNPs) into single cells, which remains a biological barrier to their clinical application. To overcome this issue, a robust RNP delivery platform based on a biocompatible sponge-like silica nanoconstruct (SN) for storing and directly delivering therapeutic RNPs, including Cas9 nuclease RNP (Cas9-RNP) and base editor RNP (BE-RNP) is designed. Compared with commercialized material such as lipid-based methods, up to 50-fold gene deletion and 10-fold base substitution efficiency is obtained with a low off-target efficiency by targeting various cells and genes. In particular, gene correction is successfully induced by SN-based delivery through intravenous injection in an in vivo solid-tumor model and through subretinal injection in mouse eye. Moreover, because of its low toxicity and high biodegradability, SN has negligible effect on cellular function of organs. As the engineered SN can overcome practical challenges associated with therapeutic RNP application, it is strongly expected this platform to be a modular RNPs delivery system, facilitating in vivo gene deletion and editing.

Genetically engineered zebrafish as models of skeletal development and regeneration

Zebrafish (Danio rerio) are aquatic vertebrates with significant homology to their terrestrial counterparts. While zebrafish have a centuries-long track record in developmental and regenerative biology, their utility has grown exponentially with the onset of modern genetics. This is exemplified in studies focused on skeletal development and repair. Herein, the numerous contributions of zebrafish to our understanding of the basic science of cartilage, bone, tendon/ligament, and other skeletal tissues are described, with a particular focus on applications to development and regeneration. We summarize the genetic strengths that have made the zebrafish a powerful model to understand skeletal biology. We also highlight the large body of existing tools and techniques available to understand skeletal development and repair in the zebrafish and introduce emerging methods that will aid in novel discoveries in skeletal biology. Finally, we review the unique contributions of zebrafish to our understanding of regeneration and highlight diverse routes of repair in different contexts of injury. We conclude that zebrafish will continue to fill a niche of increasing breadth and depth in the study of basic cellular mechanisms of skeletal biology.

Large-scale genome editing in plants: approaches, applications, and future perspectives

As a powerful genome editing technology, CRISPR/Cas is revolutionizing both fundamental research and crop breeding, and has now evolved into large-scale editing tools that are efficient, simple, and programmable. With such CRISPR screening technologies, the numbers of genome-edited crops are rapidly increasing. Here, we describe the general workflow of a CRISPR screen in plants, including the selection of appropriate editors, genome-wide guide RNA design, pooled library construction, massive transformation, and high-throughput genotyping. We also discuss applications for the screening of candidate genes, the optimization of spatiotemporal expression, the evolution of protein activities, and the establishment of genome-wide libraries of knockout mutant. After considering the current challenges and limitations, we finally envision a virus-mediated strategy to improve CRISPR screens.