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Fully computational design of PAM-relaxed Staphylococcus aureus Cas9 with expanded targeting capability using UniDesign.
CRISPR-Cas9 nucleases have transformed genome engineering, yet their application is often constrained by protospacer-adjacent motif (PAM) requirements. Staphylococcus aureus Cas9 (SaCas9) is particularly attractive for in vivo applications due to its compact size; however, its NNGRRT PAM limits targetable genomic sites. Here, we report KRH, a SaCas9 variant designed entirely from the wild-type enzyme through a fully computational point-mutation design workflow, UniDesign, without additional experimental optimization. As expected, KRH efficiently recognizes an expanded NNNRRT PAM and exhibits substantially enhanced editing efficiency at non-canonical PAM sites, with improvements of up to 116-fold over the wild type. KRH achieves genome- and base-editing efficiencies comparable to, or exceeding, those of the well-known evolution-derived KKH variant. Computational modeling by UniDesign provides a mechanistic explanation for the PAM relaxation observed in both KRH and KKH, with structural and energetic analyses revealing that KRH relaxes PAM specificity by fine-tuning the balance between sequence-specific interactions with PAM bases and nonspecific contacts with the DNA backbone. Beyond its practical utility, KRH demonstrates that computational design can identify a minimal set of mutations sufficient to remodel the PAM interface while preserving high nuclease activity. This approach recapitulates-and in some cases surpasses-the performance of evolution-derived variants, offering a scalable strategy for high-throughput Cas9 engineering. Overall, these results establish KRH as a blueprint for rationally engineered, PAM-relaxed nucleases and underscore the power of computational design to accelerate next-generation genome editing.
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Deep learning models simultaneously trained on multiple datasets improve base-editing activity prediction.
CRISPR-derived base editors (BE) enable precise single nucleotide substitution without introducing double-stranded DNA breaks. Apart from the base editing enzymes, efficient base editing strongly depends on both the CRISPR guide RNA (gRNA) efficiency and the edited position. Here, we show that the accuracy of BE gRNA design can be significantly improved by generating more data and by introducing deep neural networks trained on multiple different datasets simultaneously. Generating ~20,000 gRNAs for A•T to G•C and C•G to T•A conversions, we present such deep learning models, which also allow users to do dataset-aware predictions. The methods are available online and as stand-alone software.
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Off-target effects of base editors: what we know and how we can reduce it.
The recently discovered CRISPR-Cas9 modification, base editors (BEs), is considered as one of the most promising tools for correcting disease-causing mutations in humans, since it allows point substitutions to be edited without generating double-stranded DNA breaks, and, therefore, with a significant decrease in non-specific activity. Until recently, this method was considered the safest, but at the same time, it is quite effective. However, recent studies of non-specific activity of BEs revealed that some of them lead to the formation of a huge number of off-targets in both DNA and RNA, occurring due to the nature of the Cas9-fused proteins used. In this review article, we have considered and combined data from numerous studies about the most commonly used and more described in detail APOBEC-based BEs and Target-AID version of CBE, as well as ABE7 and ABE8 with their basic modifications into TadA to improve BEs' specificity. In our opinion, modern advances in molecular genetics make it possible to dramatically reduce the off-target activity of base editors due to introducing mutations into the domains of deaminases or inhibition of Cas9 by anti-CRISPR proteins, which returns BEs to the leading position in genome editing technologies.
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Expanding C-T base editing toolkit with diversified cytidine deaminases.
Base editing tools for cytosine to thymine (C-T) conversion enable genome manipulation at single base-pair resolution with high efficiency. Available base editors (BEs) for C-T conversion (CBEs) have restricted editing scopes and nonnegligible off-target effects, which limit their applications. Here, by screening diversified lamprey cytidine deaminases, we establish various CBEs with expanded and diversified editing scopes, which could be further refined by various fusing strategies, fusing at either N-terminus or C-terminus of nCas9. Furthermore, off-target analysis reveals that several CBEs display improved fidelity. Our study expands the toolkits for C-T conversion, serves as guidance for appropriate choice and offers a framework for benchmarking future improvement of base editing tools.
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PAM-flexible SpCas9 variants expand the targeting scope for porcine genome editing and cellular disease modeling.
BACKGROUND: CRISPR-Cas-mediated gene editing has revolutionized life sciences, yet the targeting scope of the widely used SpCas9 is limited by its strict requirement for the NGG protospacer adjacent motif (PAM). To overcome this limitation, PAM-flexible SpCas9 variants have been developed and characterized in multiple species; however, their potential in pigs (an important biomedical model for humans) remains unexplored. Here, we systematically evaluated the editing performance of three PAM-flexible SpCas9 variants (SpRY, SpG, and SpCas9-NG) and their derived base editors in porcine fetal fibroblasts (PFFs). RESULTS: Profiling across 228 target sites revealed that SpRY exhibits nearly PAM-less activity, with significantly higher editing efficiency at NRN (15.82%, R = A/G) than at NYN PAMs (5.75%, Y = C/T). SpG and SpCas9-NG preferentially targeted NGN PAMs, achieving mean efficiencies of 14.81% and 16.33%, respectively. PAM‑flexible cytosine base editors (CBEs) mediated efficient C:G‑to‑T:A conversion, with mean efficiencies of 12.01% for SpRY‑BE4max (NNN PAMs), 15.43% for SpG‑BE4max (NGN PAMs), and 18.39% for SpCas9‑NG‑BE4max (NGN PAMs). Similarly, PAM‑flexible adenine base editors (ABEs) mediated efficient A:T‑to‑G:C conversion, with mean efficiencies of 15.66% for SpRY‑ABE8e (NNN PAMs), 24.16% for SpG‑ABE8e (NGN PAMs), and 20.50% for SpCas9‑NG‑ABE8e (NGN PAMs). By exploiting this expanded targeting scope, we successfully introduced 16 pathogenic single‑nucleotide variants (SNVs) at NRN PAM sites in the porcine genome, with editing efficiencies reaching up to 40.68% for CBEs and 61.76% for ABEs. CONCLUSIONS: PAM-flexible SpCas9 variants and their derived base editors greatly expand the targeting scope for porcine genome engineering, thereby substantially broadening the applicability potential of CRISPR-Cas-mediated genome editing tools in porcine genetic improvement and disease model generation.
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Miniature CRISPR-Cas12f1-Mediated Single-Nucleotide Microbial Genome Editing Using 3'-Truncated sgRNA.
The CRISPR-Cas system has been used as a convenient tool for genome editing because the nuclease that cuts the target DNA and the guide RNA that recognizes the target are separated into modules. Cas12f1, which has a smaller size than that of other Cas nucleases, is easily loaded into vectors and is emerging as a new genome editing tool. In this study, AsCas12f1 was used to negatively select only Escherichia coli cells obtained by oligonucleotide-directed genome editing. Although double-, triple-, and quadruple-base substitutions were accurately and efficiently performed in the genome, the performance of single-base editing was poor. To resolve this limitation, we serially truncated the 3'-end of sgRNAs and determined the maximal truncation required to maintain the target DNA cleavage activity of Cas12f1. Negative selection of single-nucleotide-edited cells was efficiently performed with the maximally 3'-truncated sgRNA-Cas12f1 complex in vivo. Moreover, Sanger sequencing showed that the accuracy of single-nucleotide substitution, insertion, and deletion in the microbial genome was improved. These results demonstrated that a truncated sgRNA approach could be widely used for accurate CRISPR-mediated genome editing.
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High-efficiency plastome base editing in rice with TAL cytosine deaminase.
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CRISPR/Cas9-mediated base-editing system efficiently generates gain-of-function mutations in Arabidopsis.
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Manipulating and studying gene function in human pluripotent stem cell models.
Human pluripotent stem cells (hPSCs) are uniquely suited to study human development and disease and promise to revolutionize regenerative medicine. These applications rely on robust methods to manipulate gene function in hPSC models. This comprehensive review aims to both empower scientists approaching the field and update experienced stem cell biologists. We begin by highlighting challenges with manipulating gene expression in hPSCs and their differentiated derivatives, and relevant solutions (transfection, transduction, transposition, and genomic safe harbor editing). We then outline how to perform robust constitutive or inducible loss-, gain-, and change-of-function experiments in hPSCs models, both using historical methods (RNA interference, transgenesis, and homologous recombination) and modern programmable nucleases (particularly CRISPR/Cas9 and its derivatives, i.e., CRISPR interference, activation, base editing, and prime editing). We further describe extension of these approaches for arrayed or pooled functional studies, including emerging single-cell genomic methods, and the related design and analytical bioinformatic tools. Finally, we suggest some directions for future advancements in all of these areas. Mastering the combination of these transformative technologies will empower unprecedented advances in human biology and medicine.
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The Solanum tuberosum GBSSI gene: a target for assessing gene and base editing in tetraploid potato.
The StGBSSI gene was successfully and precisely edited in the tetraploid potato using gene and base-editing strategies, leading to plants with impaired amylose biosynthesis. Genome editing has recently become a method of choice for basic research and functional genomics, and holds great potential for molecular plant-breeding applications. The powerful CRISPR-Cas9 system that typically produces double-strand DNA breaks is mainly used to generate knockout mutants. Recently, the development of base editors has broadened the scope of genome editing, allowing precise and efficient nucleotide substitutions. In this study, we produced mutants in two cultivated elite cultivars of the tetraploid potato (Solanum tuberosum) using stable or transient expression of the CRISPR-Cas9 components to knock out the amylose-producing StGBSSI gene. We set up a rapid, highly sensitive and cost-effective screening strategy based on high-resolution melting analysis followed by direct Sanger sequencing and trace chromatogram analysis. Most mutations consisted of small indels, but unwanted insertions of plasmid DNA were also observed. We successfully created tetra-allelic mutants with impaired amylose biosynthesis, confirming the loss of function of the StGBSSI protein. The second main objective of this work was to demonstrate the proof of concept of CRISPR-Cas9 base editing in the tetraploid potato by targeting two loci encoding catalytic motifs of the StGBSSI enzyme. Using a cytidine base editor (CBE), we efficiently and precisely induced DNA substitutions in the KTGGL-encoding locus, leading to discrete variation in the amino acid sequence and generating a loss-of-function allele. The successful application of base editing in the tetraploid potato opens up new avenues for genome engineering in this species.
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