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Activity-based selection for enhanced base editor mutational scanning.
Base editing is a CRISPR-based technology that enables high-throughput, nucleotide-level functional interrogation of the genome that is essential for understanding the genetic basis of human disease and informing therapeutic development. Base editing screens have emerged as a powerful experimental approach, yet significant cell-to-cell variability in editing efficiency introduces noise that may obscure meaningful results. Here we develop a co-selection method that enriches for cells with high base editing activity, substantially increasing editing efficiency at a target locus. We evaluate this activity-based selection method against a traditional screening approach by tiling guide RNAs across TP53, demonstrating its enhanced capacity to pinpoint specific mutations and protein regions of functional importance. We anticipate that this modular selection method will enhance the resolution of base editing screens across many applications.
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Fast and Efficient Generation of Isogenic Induced Pluripotent Stem Cell Lines Using Adenine Base Editing.
Isogenic induced pluripotent stem cell (iPSC) lines are currently mostly created by homology directed repair evoked by a double-strand break (DSB) generated by CRISPR-Cas9. However, this process is in general lengthy and inefficient. This problem can be overcome, specifically for correction or insertion of transition mutations, by using base editing (BE). BE does not require DSB formation, hence avoiding creation of genomic off-target breaks and insertions and deletions, and as it is highly efficient, it also does not require integration of selection cassettes in the genome to enrich for edited cells. BE has been successfully used in many cell types as well as in some in vivo settings to correct or insert mutations, but very few studies have reported generation of isogenic iPSC lines using BE. Here, we describe a simple and fast workflow to generate isogenic iPSCs efficiently with a compound heterozygous or a homozygous Wolfram syndrome 1 (WFS1) mutation using adenine BE, without the need to include a genomic selection cassette and without off-target modifications. We demonstrated that correctly base-edited clones can be generated by screening only five cell clones in less than a month, provided that the mutation is positioned in a correct place with regards to the protospacer adjacent motif sequence and no putative bystander bases exist.
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In situ CRISPR-Cas9 base editing for the development of genetically engineered mouse models of breast cancer.
Genetically engineered mouse models (GEMMs) of cancer have proven to be of great value for basic and translational research. Although CRISPR-based gene disruption offers a fast-track approach for perturbing gene function and circumvents certain limitations of standard GEMM development, it does not provide a flexible platform for recapitulating clinically relevant missense mutations in vivo. To this end, we generated knock-in mice with Cre-conditional expression of a cytidine base editor and tested their utility for precise somatic engineering of missense mutations in key cancer drivers. Upon intraductal delivery of sgRNA-encoding vectors, we could install point mutations with high efficiency in one or multiple endogenous genes in situ and assess the effect of defined allelic variants on mammary tumorigenesis. While the system also produces bystander insertions and deletions that can stochastically be selected for when targeting a tumor suppressor gene, we could effectively recapitulate oncogenic nonsense mutations. We successfully applied this system in a model of triple-negative breast cancer, providing the proof of concept for extending this flexible somatic base editing platform to other tissues and tumor types.
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Targeting the hallmarks of ageing: Pharmacological challenges and breakthroughs in CRISPR delivery systems and future prospects.
CRISPR has emerged as a next-generation gene-editing tool with the potential to target the molecular pathways associated with ageing and related disorders. It functions through RNA-guided Cas nucleases, directing DNA cleavage and utilizing the native DNA repair machinery for genetic manipulations. Advances in CRISPR technology have significantly enhanced the precision and flexibility of techniques for genome editing. The enzyme Cas9's ability to cut DNA at exact site has revolutionized genome editing by enabling accurate modifications within living eukaryotic cells. This review critically examines recent developments in CRISPR-based technologies, including Cas9, Cas12, base editing, prime editing, and CRISPR-mediated gene regulation. It highlights their rising applications in ageing research, with more emphasis on neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The review also discusses the major pharmacological and translational challenges that currently limit clinical applications, including inefficient tissue-specific delivery, off-target genome editing, immunogenicity, manufacturing complexity, and long-term safety concerns. Also, recent progress in both, viral and non-viral delivery methods are critically evaluated, including adeno-associated viruses, lentivirus vectors, lipid nanoparticles, gold nanoparticles, exosomes, electroporation, and microinjection, is thoroughly discussed to highlight their therapeutic potential and translational limitations. Current studies indicate that CRISPR-based approaches have preclinical potential for targeting important hallmarks of ageing, particularly genomic instability, telomere attrition, and mitochondrial dysfunction. Other hallmarks of ageing, such as stem cell exhaustion, epigenetic modifications, and microbiome changes, are at earlier stages of development. Overall, this review describes future strategies for developing safe, precise, and clinically translatable CRISPR-based treatments to promote healthy ageing.
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Engineering domain-inlaid SaCas9 adenine base editors with reduced RNA off-targets and increased on-target DNA editing.
Precision genome engineering has dramatically advanced with the development of CRISPR/Cas base editing systems that include cytosine base editors and adenine base editors (ABEs). Herein, we compare the editing profile of circularly permuted and domain-inlaid Cas9 base editors, and find that on-target editing is largely maintained following their intradomain insertion, but that structural permutation of the ABE can affect differing RNA off-target events. With this insight, structure-guided design was used to engineer an SaCas9 ABE variant (microABE I744) that has dramatically improved on-target editing efficiency and a reduced RNA-off target footprint compared to current N-terminal linked SaCas9 ABE variants. This represents one of the smallest AAV-deliverable Cas9-ABEs available, which has been optimized for robust on-target activity and RNA-fidelity based upon its stereochemistry.
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Multiplex genome editing using a dCas9-cytidine deaminase fusion in Streptomyces.
CRISPR/Cas-mediated genome editing has greatly facilitated the study of gene function in Streptomyces. However, it could not be efficiently employed in streptomycetes with low homologous recombination (HR) ability. Here, a deaminase-assisted base editor dCas9-CDA-ULstr was developed in Streptomyces, which comprises the nuclease-deficient Cas9 (dCas9), the cytidine deaminase from Petromyzon marinus (PmCDA1), the uracil DNA glycosylase inhibitor (UGI) and the protein degradation tag (LVA tag). Using dCas9-CDA-ULstr, we achieved single-, double- and triple-point mutations (cytosine-to-thymine substitutions) at target sites in Streptomyces coelicolor with efficiency up to 100%, 60% and 20%, respectively. This base editor was also demonstrated to be highly efficient for base editing in the industrial strain, Streptomyces rapamycinicus, which produces the immunosuppressive agent rapamycin. Compared with base editors derived from the cytidine deaminase rAPOBEC1, the PmCDA1-assisted base editor dCas9-CDA-ULstr could edit cytosines preceded by guanosines with high efficiency, which is a great advantage for editing Streptomyces genomes (with high GC content). Collectively, the base editor dCas9-CDA-ULstr could be employed for efficient multiplex genome editing in Streptomyces. Since the dCas9-CDA-ULstr-based genome editing is independent of HR-mediated DNA repair, we believe this technology will greatly facilitate functional genome research and metabolic engineering in Streptomyces strains with weak HR ability.
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Base editing of key residues in the BCL11A-XL-specific zinc finger domains derepresses fetal globin expression.
BCL11A-XL directly binds and represses the fetal globin (HBG1/2) gene promoters, using 3 zinc-finger domains (ZnF4, ZnF5, and ZnF6), and is a potential target for β-hemoglobinopathy treatments. Disrupting BCL11A-XL results in derepression of fetal globin and high HbF, but also affects hematopoietic stem and progenitor cell (HSPC) engraftment and erythroid maturation. Intriguingly, neurodevelopmental patients with ZnF domain mutations have elevated HbF with normal hematological parameters. Inspired by this natural phenomenon, we used both CRISPR-Cas9 and base editing at specific ZnF domains and assessed the impacts on HbF production and hematopoietic differentiation. Generating indels in the various ZnF domains by CRISPR-Cas9 prevented the binding of BCL11A-XL to its site in the HBG1/2 promoters and elevated the HbF levels but affected normal hematopoiesis. Far fewer side effects were observed with base editing- for instance, erythroid maturation in vitro was near normal. However, we observed a modest reduction in HSPC engraftment and a complete loss of B cell development in vivo, presumably because current base editing is not capable of precisely recapitulating the mutations found in patients with BCL11A-XL-associated neurodevelopment disorders. Overall, our results reveal that disrupting different ZnF domains has different effects. Disrupting ZnF4 elevated HbF levels significantly while leaving many other erythroid target genes unaffected, and interestingly, disrupting ZnF6 also elevated HbF levels, which was unexpected because this region does not directly interact with the HBG1/2 promoters. This first structure/function analysis of ZnF4-6 provides important insights into the domains of BCL11A-XL that are required to repress fetal globin expression and provide framework for exploring the introduction of natural mutations that may enable the derepression of single gene while leaving other functions unaffected.
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Unlocking the secrets of ABEs: the molecular mechanism behind their specificity.
CRISPR-Cas, the bacterial immune systems, have transformed the field of genome editing by providing efficient, easily programmable, and accessible tools for targeted genome editing. DNA base editors (BE) are state-of-the-art CRISPR-based technology, allowing for targeted modifications of individual nucleobases within the genome. Among the BEs, adenine base editors (ABEs) have shown great potential due to their ability to convert A-to-G with high efficiency. However, current ABEs have limitations in terms of their specificity and targeting range. In this review, we provide an overview of the molecular mechanism of ABEs, with a focus on the mechanism of deoxyadenosine deamination by evolved tRNA-specific adenosine deaminase (TadA). We discuss how mutations and adjustments introduced via both directed evolution as well as rational design have improved ABE efficiency and specificity. This review offers insights into the molecular mechanism of ABEs, providing a roadmap for future developments in the precision genome editing field.
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Intravitreal adenine base editing of RS1 improves vision in a preclinical mouse model of retinoschisis.
Base editing offers high potential for treating genetic diseases, particularly those with limited treatment options. Retinoschisis, an X-linked retinal disease causing progressive vision loss, currently lacks effective therapies. We identified the c.422G>A (p.Arg141His) variant of the RS1 gene in six male patients with retinoschisis and generated a humanized mouse model harboring this variant, which mimicked the disease phenotype. By testing adenine base editors and single-guide RNAs, we identified an optimal combination of high editing efficiency and low bystander editing. Intravitreal injection of adeno-associated viral vectors encoding this adenine base editor achieved ∼40% editing efficiency in all retinal cells, restored retinal layer integrity, and preserved visual functions in 2-week-old male hemizygous mice. These mice exhibited retinal layer splitting at baseline, further validating the model. This study demonstrates a strategy for identifying effective base editing tools for clinical use through the preclinical evaluation of humanized mouse lines with patient-derived mutations and highlights their applicability in treating genetic diseases.
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Hybrid Cas12a Variants with Relaxed PAM Requirements Expand Genome Editing Compatibility.
Cas12a is a widely used programmable nuclease for genome editing across a variety of organisms, but its application is limited by its PAM recognition restriction. To alleviate these PAM constraints, protein engineering efforts have been applied to expand the PAM recognition range. In this study, we designed and constructed 990 synthetic hybrid Cas12a chimeras through domain shuffling and screened an efficient hybrid Cas12a (ehCas12a) that could recognize a broad range PAM of 5'-TYYN-3' (Y is T or C and N is A, T, C, or G). Furthermore, we constructed an ehCas12a variant, ehCas12a RRVR (T167R/N572R/K578V/N582R), with expanded PAM preference to 5'-TNYN, TWRV-3' (W is A or T, R is A or G, and V is A, C, or G), which can efficiently recognize -2* A/G PAMs that are barely recognized by Cas12a-type proteins and their mutants. Finally, we demonstrated that the DNase-inactivated ehCas12a RRVR base editor (dehCas12a RRVR-BE) was capable of targeting noncanonical PAMs in vivo and disease-related loci for potential therapeutic applications. Overall, our findings highlight the modular design and reconfiguration of Cas proteins for enhanced functionality.
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