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DeepCRISTL: deep transfer learning to predict CRISPR/Cas9 on-target editing efficiency in specific cellular contexts.
MOTIVATION: CRISPR/Cas9 technology has been revolutionizing the field of gene editing. Guide RNAs (gRNAs) enable Cas9 proteins to target specific genomic loci for editing. However, editing efficiency varies between gRNAs and so computational methods were developed to predict editing efficiency for any gRNA of interest. High-throughput datasets of Cas9 editing efficiencies were produced to train machine-learning models to predict editing efficiency. However, these high-throughput datasets have a low correlation with functional and endogenous datasets, which are too small to train accurate machine-learning models on. RESULTS: We developed DeepCRISTL, a deep-learning model to predict the editing efficiency in a specific cellular context. DeepCRISTL takes advantage of high-throughput datasets to learn general patterns of gRNA editing efficiency and then fine-tunes the model on functional or endogenous data to fit a specific cellular context. We tested two state-of-the-art models trained on high-throughput datasets for editing efficiency prediction, our newly improved DeepHF and CRISPRon, combined with various transfer-learning approaches. The combination of CRISPRon and fine-tuning all model weights was the overall best performer. DeepCRISTL outperformed state-of-the-art methods in predicting editing efficiency in a specific cellular context on functional and endogenous datasets. Using saliency maps, we identified and compared the important features learned by DeepCRISTL across cellular contexts. We believe DeepCRISTL will improve prediction performance in many other CRISPR/Cas9 editing contexts by leveraging transfer learning to utilize both high-throughput datasets and smaller and more biologically relevant datasets. AVAILABILITY AND IMPLEMENTATION: DeepCRISTL is available via https://github.com/OrensteinLab/DeepCRISTL.
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Non-viral delivery of genome-editing nucleases for gene therapy.
Manipulating the genetic makeup of mammalian cells using programmable nuclease-based genome-editing technology has recently evolved into a powerful avenue that holds great potential for treating genetic disorders. There are four types of genome-editing nucleases, including meganucleases, zinc finger nucleases, transcription activator-like effector nucleases and clustered, regularly interspaced, short palindromic repeat-associated nucleases such as Cas9. These nucleases have been harnessed to introduce precise and specific changes of the genome sequence at virtually any genome locus of interest. The therapeutic relevance of these genome-editing technologies, however, is challenged by the safe and efficient delivery of nuclease into targeted cells. Herein, we summarize recent advances that have been made on non-viral delivery of genome-editing nucleases. In particular, we focus on non-viral delivery of Cas9/sgRNA ribonucleoproteins for genome editing. In addition, the future direction for developing non-viral delivery of programmable nucleases for genome editing is discussed.
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Evaluation of the correlation between nuclear localization levels and genome editing efficiencies of Cas12a fused with nuclear localization signals.
Genome editing technology using the CRISPR-Cas system is attracting much attention not only as a promising experimental tool for analysis of genome functions, but also as a novel therapeutic approach for genetic disorders. Among the various types of Cas proteins, Cas12a is expected to be a promising gene editing tool due to its unique properties, including low off-target effects. As Cas proteins are of prokaryotic origin, they need to be fused with appropriate localization signals to perform their function in eukaryotic cells. Cas12a proteins fused with a nuclear localization signal (NLS) have been developed so far, but the relation between the nuclear localization activity and the genome editing efficiency has not been fully elucidated. Here, utilizing two Cas12a orthologs, AsCas12a and LbCas12a, with various number of NLSs derived from various origins, we revealed that the improved nuclear localization resulted in increased genome editing efficiencies when expressed using adenovirus (Ad) vector in cultured cells. However, when they were expressed in mouse liver, the improvement of the nuclear localization activity was not necessarily required to achieve the maximum genome editing efficiency four weeks after Ad vector administration. These data indicated that the optimized NLS modification of Cas12a proteins in vitro situations differed from that in vivo.
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Gene Therapy Approaches to Human Immunodeficiency Virus and Other Infectious Diseases.
Advances in gene therapy technologies, particularly in gene editing, are suggesting new avenues for the treatment of human immunodeficiency virus and other infectious diseases. This article outlines recent developments in antiviral gene therapies, including those based on the disruption of entry receptors or that target viral genomes using targeted nucleases, such as the CRISPR/Cas9 system. In addition, new ways to express circulating antiviral factors, such as antibodies, and approaches to harness and engineer the immune system to provide an antiviral effect that is not naturally achieved are described.
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Anolis Lizards as a Model System for Studies of Gene Function in Reptile Development and Evolution.
Anolis lizards are an ecologically diverse group that includes more than 400 described species. These reptiles have been the subject of wide-ranging studies, from speciation and convergent evolution to climate adaptation and tail regeneration. While CRISPR-based gene editing has tremendous potential to reveal new insights into these and other aspects of Anolis biology, the reproductive biology of these reptiles has presented significant barriers to gene editing. Here, we briefly summarize gene editing approaches in vertebrates and discuss some of the major challenges associated with the performance of gene editing in anoles. We then introduce a recently established surgical procedure that enables the injection of CRISPR-Cas into the developing oocytes of female lizards. This approach circumvents the need to manipulate early-stage embryos and permits the production of gene-edited anoles. This method has recently been successfully adapted for use in other reptiles, suggesting that it may be effective in a wide range of species and will broadly enable studies of gene function in reptiles.
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Female iPSC X chromosome inactivation erosion and its transcriptomic effects during CRISPR gene editing and neural differentiation.
Human induced pluripotent stem cells (hiPSCs), together with CRISPR editing, are commonly used for studying neurodevelopmental disorders. Female iPSCs gradually lose the expression of the noncoding X chromosome inactive-specific transcript (XIST), leading to random X chromosome inactivation (XCI) erosion that affects both X-linked and autosomal gene expression. However, the effects of CRISPR editing and neural differentiation on XCI erosion and how this may confound a real-world analysis of differentially expressed genes (DEGs) are poorly understood. Here, we analyzed hundreds of RNA sequencing (RNA-seq) samples of CRISPR-edited iPSC lines for 66 genes and the matched single-neuron RNA-seq data. We found variable XCI in CRISPR-edited female iPSCs, which was largely preserved in iPSC-derived neurons and predominantly influenced X-linked genes. Mechanistically, XIST mediates allele-specific epigenetic regulation of both X-linked and autosomal genes, with the former showing stronger allele-specific expression (ASE) bias that affects neurodevelopmental genes. These results have significant implications in hiPSC-based modeling of neurodevelopmental disorders.
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Plenary Symposia.
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T cell engineering by targeted small extracellular vesicles delivering CRISPR/Cas9 system for PD-1 knock-out.
Among the emerging cancer therapies, immunotherapy using T cell engineering stands out as one of the most promising approaches. While the clustered regularly interspaced palindromic repeats (CRISPR/Cas) system is widely recognized as one of the most common gene-editing system tools, the main challenge for in vivo delivery applications remains the lack of an efficient and safe intracellular carrier. Small extracellular vesicles (s-EVs), as natural nanoparticles with low immunogenicity, emerge as a promising candidate for delivering the CRISPR/Cas system. Two guide RNAs (gRNAs) targeting the programmed cell death (PD-1) gene were cloned into the PX-459 vector, and these CRISPR/Cas9 plasmids were loaded into targeted s-EVs by electroporation. Subsequently, T cells were exposed to these s-EVs, and the disruption to the PD-1 gene was examined using flow cytometry and sequencing. Flow cytometry results indicated that newly produced targeted s-EVs exhibited a higher uptake by T cells compared to non-targeted s-EVs. The successful loading of PX-459 plasmids into targeted s-EVs and their functional delivery to T cells were confirmed. Subsequent carboxyfluorescein succinimidyl ester (CFSE) staining and enzyme-linked immunosorbent assay (ELISA) analyses demonstrated that the knockout (KO) of the PD-1 gene in T cells significantly enhanced both their cytotoxicity and cytokine release compared to untreated T cells. Targeted s-EVs were used in the current study as a means to deliver the CRISPR/Cas9 system into T cells, enabling genetic modification of target cells. This approach holds the potential to improve and simplify immunotherapy. This study demonstrates a novel approach to genetic modification in T cells using targeted s-EVs containing the CRISPR/Cas9 gene-editing system. KEY POINTS: •CRISPR/Cas9 is a kind of powerful gene editing system in cancer therapeutics. •s-EVs can be applied as a biological carrier for CRISPR/Cas9 system. • T cell engineering-based immunotherapy is a promising form of anticancer therapy.
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A cyclical marker system enables indefinite series of oligonucleotide-directed gene editing in Chlamydomonas reinhardtii.
CRISPR/Cas9 gene editing in the model green alga Chlamydomonas reinhardtii relies on the use of selective marker genes to enrich for nonselectable target mutations. This becomes challenging when many sequential modifications are required in a single-cell line, as useful markers are limited. Here, we demonstrate a cyclical selection process which only requires a single marker gene to identify an almost infinite sequential series of CRISPR-based target gene modifications. We used the NIA1 (Nit1, NR; nitrate reductase) gene as the selectable marker in this study. In the forward stage of the cycle, a stop codon was engineered into the NIA1 gene at the CRISPR target location. Cells retaining the wild-type NIA1 gene were killed by chlorate, while NIA1 knockout mutants survived. In the reverse phase of the cycle, the stop codon engineered into the NIA1 gene during the forward phase was edited back to the wild-type sequence. Using nitrate as the sole nitrogen source, only the reverted wild-type cells survived. By using CRISPR to specifically deactivate and reactivate the NIA1 gene, a marker system was established that flipped back and forth between chlorate- and auxotrophic (nitrate)-based selection. This provided a scarless cyclical marker system that enabled an indefinite series of CRISPR edits in other, nonselectable genes. We demonstrate that this "Sequential CRISPR via Recycling Endogenous Auxotrophic Markers (SCREAM)" technology enables an essentially limitless series of genetic modifications to be introduced into a single-cell lineage of C. reinhardtii in a fast and efficient manner to complete complex genetic engineering.
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An improved CRISPR-Cas9 protein-based method for knocking out insect Sf9 cell genes.
Insect cells are one of the uprising expression systems in the biopharmaceutical industry to produce vaccines and gene therapy vectors, but cell line development has been limited by the lack of established genetic engineering tools and genomic characterization. CRISPR-Cas9 has arisen as a powerful tool for gene editing but has seen little application in insect cells. In this work, a gene editing pipeline for the delivery of a ribonucleoprotein (RNP) complex comprised of a guide RNA and the enzyme Cas9 to insect Sf9 cells was implemented and then applied to knockout caspase initiator Sf-Dronc, aiming at alleviating cell apoptosis during an infection process. The resulting engineered cell lines were characterized as per their phenotype and production of three different product modalities. Utilizing the established workflow, a knockout rate of 68% was achieved with the implemented protocol (vs. the 12% presumed efficiency of a previously reported system) when targeting the fdl gene. When applied to Sf-Dronc, mutants containing deletions in several alleles of the host genome were identified and confirmed by next-generation sequencing. Generated clones exhibited higher apoptosis resistance and delayed onset of cell viability drop following infection with baculovirus. While Sf-Dronc deletion was shown to have negligible impact on the production of rAAV and PfRipr5, production of iVLPS showed an > twofold increase over wild-type Sf9. Overall, this study showcases the successful implementation of an efficient CRISPR-Cas9 pipeline, further leveraging the usage of genetic engineering in insect Sf9 cells towards the development of enhanced cell hosts for biopharmaceutical production. KEY POINTS: • Implementation of an efficient CRISPR-Cas9 RNP complex delivery strategy to insect cells. • Establishment of the genome editing pipeline demonstrated through Sf-Dronc knockout, resulting in increased apoptosis resistance and delayed loss of viability upon baculovirus infection. • Sf-Dronc deletion led to over a twofold increase in the production of influenza VLPs compared to wild-type Sf9 cells.
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