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A motif preferred adenine base editor with minimal bystander and off-targets editing.
47% of hereditable diseases are caused by single C•G-to-T•A base conversions, which means efficient A-to-G base editing tools (ABEs) have great potential for the treatment of these diseases. However, the existing efficient ABEs, while catalyzing targeted A-to-G conversion, cause high A or C bystander editing and off-target events, which poses safety concerns for their clinical applications. To overcome this shortcoming, we have developed ABE8e-YA (ABE8e with TadA-8e A48E) for efficient and accurate editing of As in YA motifs with YAY > YAR (Y = T or C, R = A or G) hierarchy through structure-oriented rational design. Compared with ABE3.1, which is currently the only ABE version with a YAC motif preference, ABE8e-YA exhibits an average A-to-G editing efficiency improvement of an up to 3.1-fold increase in the indicated YA motif while maintaining reduced bystander C editing and minimized DNA or RNA off-targets. Additionally, we demonstrate that ABE8e-YA efficiently and precisely corrects pathogenic mutations in human cells, suggesting its high suitability for addressing 9.3% of pathogenic point mutations, higher than that of ABE8e and ABE9. Moreover, by using ABE8e-YA, we efficiently and precisely generate hypocholesterolemia and tail-loss mouse models mimicking human-associated disease, as well as performed in vivo mouse proprotein convertase subtilisin/kexin type 9 (Pcsk9) base editing for hypercholesterolemia gene therapy. Together these data indicate its great potential in broad applications for disease modeling and gene therapy.
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Precision base-editing of the cryptic 3' acceptor site to correct the RNA splicing defect of β654-thalassemia.
β-Thalassemia is one of the most common inherited disorders worldwide and is caused by mutations affecting β-globin production. β654 mutation (IVS2-654, C > T) is one of the most frequently occurring β-thalassemia alleles in Han Chinese population, which activates a cryptic 3' splice site and leads to aberrant RNA splicing. Previous studies demonstrated that direct base editing of the IVS2-654 mutation is challenging because suitable single-guide RNAs (sgRNAs) cannot effectively target this site. Here we investigated an alternative therapeutic strategy by targeting the mutation-activated cryptic 3' splice acceptor site, rather than the disease-causing mutation itself. We first introduced base substitutions into the cryptic splice acceptor site in a β654-thalassemia mouse model using CRISPR-Cas9-mediated homology-directed repair, which restored normal RNA splicing and validated the therapeutic rationale of this approach. We then generated base-edited β654 mice by microinjecting Td-CBEmax mRNA together with sgRNAs targeting the cryptic splice acceptor site into one-cell embryos. Base editing was successfully achieved in 78% of live-born β654 mice, of which 86% produced correctly spliced β-globin transcripts. Restoration of normal RNA splicing was accompanied by marked improvement of hematological parameters and tissue pathology in most base-edited founder mice and their offspring compared with non-edited β654 mice. Together, these findings demonstrate that mutation-activated cryptic splice sites are therapeutically actionable targets for precision base editing and provide proof of concept for a precise and effective strategy to correct aberrant RNA splicing in β654-thalassemia.
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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.
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In vivo prime editing rescues alternating hemiplegia of childhood in mice.
Alternating hemiplegia of childhood (AHC) is a neurodevelopmental disorder with no disease-modifying treatment. Mutations in ATP1A3, encoding an Na+/K+ ATPase subunit, cause 70% of AHC cases. Here, we present prime editing (PE) and base editing (BE) strategies to correct ATP1A3 and Atp1a3 mutations in human cells and in two AHC mouse models. We used PE and BE to correct five prevalent ATP1A3 mutations with 43%-90% efficiency. AAV9-mediated in vivo PE corrects Atp1a3 D801N and E815K in the CNS of two AHC mouse models, yielding up to 48% DNA correction and 73% mRNA correction in bulk brain cortex. In vivo PE rescued clinically relevant phenotypes, including restoration of ATPase activity; amelioration of paroxysmal spells, motor defects, and cognition deficits; and dramatic extension of animal lifespan. This work suggests a potential one-time PE treatment for AHC and establishes the ability of PE to rescue a neurological disease in animals.
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SpRY-mediated screens facilitate functional dissection of non-coding sequences at single-base resolution.
CRISPR mutagenesis screens conducted with SpCas9 and other nucleases have identified certain cis-regulatory elements and genetic variants but at a limited resolution due to the absence of protospacer adjacent motif (PAM) sequences. Here, leveraging the broad targeting scope of the near-PAMless SpRY variant, we have demonstrated that saturated SpRY mutagenesis and base editing screens can faithfully identify functional regulatory elements and essential genetic variants for target gene expression at single-base resolution. We further extended this methodology to investigate a genome-wide association study (GWAS) locus at 10q22.1 associated with a red blood cell trait, where we identified potential enhancers regulating HK1 gene expression, despite not all of these enhancers exhibiting typical chromatin signatures. More importantly, our saturated base editing screens pinpoint multiple causal variants within this locus that would otherwise be missed by Bayesian statistical fine-mapping. Our approach is generally applicable to functional interrogation of all non-coding genomic elements while complementing other high-coverage CRISPR screens.
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Base editing screens define the genetic landscape of cancer drug resistance mechanisms.
Drug resistance is a principal limitation to the long-term efficacy of cancer therapies. Cancer genome sequencing can retrospectively delineate the genetic basis of drug resistance, but this requires large numbers of post-treatment samples to nominate causal variants. Here we prospectively identify genetic mechanisms of resistance to ten oncology drugs from CRISPR base editing mutagenesis screens in four cancer cell lines using a guide RNA library predicted to install 32,476 variants in 11 cancer genes. We identify four functional classes of protein variants modulating drug sensitivity and use single-cell transcriptomics to reveal how these variants operate through distinct mechanisms, including eliciting a drug-addicted cell state. We identify variants that can be targeted with alternative inhibitors to overcome resistance and functionally validate an epidermal growth factor receptor (EGFR) variant that sensitizes lung cancer cells to EGFR inhibitors. Our variant-to-function map has implications for patient stratification, therapy combinations and drug scheduling in cancer treatment.
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Advances and prospects of large DNA fragment editing in plants.
Structural variations drive plant genome evolution and shape agronomic traits. Manipulating structural variations has great potential to improve complex plant traits and enhance agricultural sustainability. Genome editing technologies have evolved from gene knockouts and base editing to the modification of short DNA fragments, and are now advancing towards the precise manipulation of large DNA fragments. This advancement facilitates targeted, large-scale genomic changes such as deletions, insertions, replacements, inversions, translocations and duplications. In this Review, we summarize recent advances in developing technologies for large DNA fragment editing and highlight their key applications in plants as well as their potential to accelerate crop improvement. Finally, we discuss the current challenges and future prospects for these technologies in plant science.
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Benchmarking of SpCas9 variants enables deeper base editor screens of BRCA1 and BCL2.
Numerous rationally-designed and directed-evolution variants of SpCas9 have been reported to expand the utility of CRISPR technology. Here, we assess the activity and specificity of WT-Cas9 and 10 SpCas9 variants by benchmarking their PAM preferences, on-target activity, and off-target susceptibility in cell culture assays with thousands of guides targeting endogenous genes. To enhance the coverage and thus utility of base editing screens, we demonstrate that the SpCas9-NG and SpG variants are compatible with both A > G and C > T base editors, more than tripling the number of guides and assayable residues. We demonstrate the performance of these technologies by screening for loss-of-function mutations in BRCA1 and Venetoclax-resistant mutations in BCL2, identifying both known and new mutations that alter function. We anticipate that the tools and methodologies described here will facilitate the investigation of genetic variants at a finer and deeper resolution for any locus of interest.
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Generation of a Hutchinson-Gilford progeria syndrome monkey model by base editing.
Many human genetic diseases, including Hutchinson-Gilford progeria syndrome (HGPS), are caused by single point mutations. HGPS is a rare disorder that causes premature aging and is usually caused by a de novo point mutation in the LMNA gene. Base editors (BEs) composed of a cytidine deaminase fused to CRISPR/Cas9 nickase are highly efficient at inducing C to T base conversions in a programmable manner and can be used to generate animal disease models with single amino-acid substitutions. Here, we generated the first HGPS monkey model by delivering a BE mRNA and guide RNA (gRNA) targeting the LMNA gene via microinjection into monkey zygotes. Five out of six newborn monkeys carried the mutation specifically at the target site. HGPS monkeys expressed the toxic form of lamin A, progerin, and recapitulated the typical HGPS phenotypes including growth retardation, bone alterations, and vascular abnormalities. Thus, this monkey model genetically and clinically mimics HGPS in humans, demonstrating that the BE system can efficiently and accurately generate patient-specific disease models in non-human primates.
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Efficient Correction of Oncogenic KRAS and TP53 Mutations through CRISPR Base Editing.
UNLABELLED: KRAS is the most frequently mutated oncogene in human cancer, and its activating mutations represent long-sought therapeutic targets. Programmable nucleases, particularly the CRISPR-Cas9 system, provide an attractive tool for genetically targeting KRAS mutations in cancer cells. Here, we show that cleavage of a panel of KRAS driver mutations suppresses growth in various human cancer cell lines, revealing their dependence on mutant KRAS. However, analysis of the remaining cell population after long-term Cas9 expression unmasked the occurence of oncogenic KRAS escape variants that were resistant to Cas9-cleavage. In contrast, the use of an adenine base editor to correct oncogenic KRAS mutations progressively depleted the targeted cells without the appearance of escape variants and allowed efficient and simultaneous correction of a cancer-associated TP53 mutation. Oncogenic KRAS and TP53 base editing was possible in patient-derived cancer organoids, suggesting that base editor approaches to correct oncogenic mutations could be developed for functional interrogation of vulnerabilities in a personalized manner for future precision oncology applications. SIGNIFICANCE: Repairing KRAS mutations with base editors can be used for providing a better understanding of RAS biology and may lay the foundation for improved treatments for KRAS-mutant cancers.
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