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Recent Progress and Future Prospective in HBV Cure by CRISPR/Cas.
Hepatitis B virus (HBV) infection remains an important issue of global public health. Although current antiviral therapy has dramatically reduced the mortality and morbidity of chronic hepatitis B (CHB), it fails to cure it. Rebound viremia often occurs after stopping antiviral therapy. Persistent HBV covalently closed circular DNA (cccDNA) and integrated DNA under antiviral therapy form the major barrier to eradication of HBV infection. CRISPR-mediated genome editing has emerged as a promising therapeutic approach to specifically destroy persistent HBV genomes, both cccDNA and integrated DNA, for HBV cure. However, the cleavage of integrated HBV DNA by CRISPR-Cas9 will cause double-strand break (DSB) of host genome, raising a serious safety concern about genome instability and carcinogenesis. The newly developed CRISPR-derived base editors (BEs), which fuse a catalytically disabled nuclease with a nucleobase deaminase enzyme, can be used to permanently inactivate HBV genome by introducing irreversible point mutations for generation of premature stop codons without DSBs of host genome. Although promising, CRISPR-mediated base editing still faces daunting challenges before its clinical application, including the base-editing efficacy, the off-target effect, the difficulty in finding conserved target HBV sequences, and in vivo delivery efficiency. Several strategies have been adopted to optimize the efficiency and specificity of CRISPR-BEs and to improve in vivo delivery efficacy through novel viral and non-viral delivery approaches. Particularly, the non-viral delivery of Cas9 mRNA and ribonucleoprotein by lipid nanoparticles exhibits attractive potential for liver-targeted delivery in clinical. Along with all progress above, the CRISPR-mediated gene therapy will ultimately achieve HBV cure.
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Sequence-specific prediction of the efficiencies of adenine and cytosine base editors.
Base editors, including adenine base editors (ABEs)1 and cytosine base editors (CBEs)2,3, are widely used to induce point mutations. However, determining whether a specific nucleotide in its genomic context can be edited requires time-consuming experiments. Furthermore, when the editable window contains multiple target nucleotides, various genotypic products can be generated. To develop computational tools to predict base-editing efficiency and outcome product frequencies, we first evaluated the efficiencies of an ABE and a CBE and the outcome product frequencies at 13,504 and 14,157 target sequences, respectively, in human cells. We found that there were only modest asymmetric correlations between the activities of the base editors and Cas9 at the same targets. Using deep-learning-based computational modeling, we built tools to predict the efficiencies and outcome frequencies of ABE- and CBE-directed editing at any target sequence, with Pearson correlations ranging from 0.50 to 0.95. These tools and results will facilitate modeling and therapeutic correction of genetic diseases by base editing.
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Boosting targeted genome editing using the hei-tag.
Precise, targeted genome editing by CRISPR/Cas9 is key for basic research and translational approaches in model and non-model systems. While active in all species tested so far, editing efficiencies still leave room for improvement. The bacterial Cas9 needs to be efficiently shuttled into the nucleus as attempted by fusion with nuclear localization signals (NLSs). Additional peptide tags such as FLAG- or myc-tags are usually added for immediate detection or straightforward purification. Immediate activity is usually granted by administration of preassembled protein/RNA complexes. We present the 'hei-tag (high efficiency-tag)' which boosts the activity of CRISPR/Cas genome editing tools already when supplied as mRNA. The addition of the hei-tag, a myc-tag coupled to an optimized NLS via a flexible linker, to Cas9 or a C-to-T (cytosine-to-thymine) base editor dramatically enhances the respective targeting efficiency. This results in an increase in bi-allelic editing, yet reduction of allele variance, indicating an immediate activity even at early developmental stages. The hei-tag boost is active in model systems ranging from fish to mammals, including tissue culture applications. The simple addition of the hei-tag allows to instantly upgrade existing and potentially highly adapted systems as well as to establish novel highly efficient tools immediately applicable at the mRNA level.
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Targeting fidelity of adenine and cytosine base editors in mouse embryos.
Base editing directly converts a target base pair into a different base pair in the genome of living cells without introducing double-stranded DNA breaks. While cytosine base editors (CBE) and adenine base editors (ABE) are used to install and correct point mutations in a wide range of organisms, the extent and distribution of off-target edits in mammalian embryos have not been studied in detail. We analyze on-target and proximal off-target editing at 13 loci by a variety of CBEs and ABE in more than 430 alleles generated from mouse zygotic injections using newly generated and published sequencing data. ABE predominantly generates anticipated A•T-to-G•C edits. Among CBEs, SaBE3 and BE4, result in the highest frequencies of anticipated C•G-to-T•A products relative to editing byproducts. Together, these findings highlight the remarkable fidelity of ABE in mouse embryos and identify preferred CBE variants when fidelity in vivo is critical.
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CRISPR-Cas systems for genome editing of mammalian cells.
In the past decade, ZFNs and TALENs have been used for targeted genome engineering and have gained scientific attention. It has demonstrated huge potential for gene knockout, knock-in, and indels in desired locations of genomes to understand molecular mechanism of diseases and also discover therapy. However, both the genome engineering techniques are still suffering from design, screening and validation in cell and higher organisms. CRISPR-Cas9 is a rapid, simple, specific, and versatile technology and it has been applied in many organisms including mammalian cells. CRISPR-Cas9 has been used for animal models to modify animal cells for understanding human disease for novel drug discovery and therapy. Additionally, base editing has also been discussed herewith for conversion of C/G-to-T/A or A/T-to-G/C without DNA cleavage or donor DNA templates for correcting mutations or altering gene functions. In this chapter, we highlight CRISPR-Cas9 and base editing for desired genome editing in mammalian cells for a better understanding of molecular mechanisms, and biotechnological and therapeutic applications.
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Enhanced genome editing with a Streptococcus equinus Cas9.
A large number of SpCas9 orthologs has been computationally identified, but their genome editing potential remains largely unknown. In this study, a GFP-activation assay was used to screen a panel of 18 SpCas9 orthologs, ten of which demonstrated activity in human cells. Notably, these orthologs had a preference for purine-rich PAM sequences. Four of the tested orthologs displayed enhanced specificity compared to SpCas9. Of particular interest is SeqCas9, which recognizes a simple NNG PAM and displays activity and specificity comparable to SpCas9-HF1. In addition, SeqCas9 exhibits superior base editing efficiency compared to SpCas9-NG and SpCas9-NRRH at multiple endogenous loci. This research sheds light on the diversity of SpCas9 orthologs and their potential for specific and efficient genome editing, especially in cases involving base editing.
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A novel EYS c.6192-1G>A variant presents ideal base editing therapeutic opportunities.
PURPOSE: Causal variants in the EYS gene are a major cause of retinitis pigmentosa (RP). However, treatment development for EYS-RP has been hindered due to the large size of the coding sequence and transcriptional complexity of the mRNA. Recently developed adenine base editors (ABEs), a form of CRISPR gene editing, offer another method to develop treatment for genetic diseases caused by G>A point mutations. MATERIALS AND METHODS: This is a retrospective report describing a 73-year-old female with RP, who underwent clinical examination and retinal imaging. Genetic testing included sequencing of 111 known retinal genes and in silico prediction of pathogenicity of the identified variants. Availability and safety of ABE-mediated approaches to correct the patient variant were analyzed. RESULTS: Clinical evaluation revealed moderately advanced retinitis pigmentosa which had become symptomatic at 35 years of age. Genetic testing revealed a likely pathogenic homozygous EYS c.6192-1 G>A mutation, disrupting a canonical splice acceptor site. As a result, exon skipping with related frameshift deletion and introduction of a premature stop-codon in the forming mRNA is likely, leading to significantly truncated protein or total abolition of translated EYS. Multiple ABE approaches to correct the variant were detected. CONCLUSIONS: In summary, we report a novel splice site variant in EYS in a patient with classical signs of RP. Further research is needed to develop safe and efficient treatment options for EYS-associated RP.
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Next-generation T cell immunotherapies engineered with CRISPR base and prime editing: challenges and opportunities.
T cells can be reprogrammed with transgenic antigen recognition receptors, including chimeric antigen receptors and T cell receptors, to selectively recognize and kill cancer cells. Such adoptive T cell therapies are effective in patients with certain haematological cancers but challenges persist, including primary and secondary resistance, a lack of efficacy in patients with solid tumours, a narrow range of targetable antigens, and time-consuming and complex manufacturing processes. CRISPR-based genome editing is a potent strategy to enhance cellular immunotherapies. Conventional CRISPR-Cas9 systems are useful for gene editing, transgene knock-in or gene knockout but can result in undesired editing outcomes, including translocations and chromosomal truncations. Base editing and prime editing technologies constitute a new generation of CRISPR platforms and enable highly precise and programmable installation of defined nucleotide variants in primary T cells. Owing to their high precision and versatility, base editing and prime editing systems, hereafter collectively referred to as CRISPR 2.0, are advancing to become the new standard for precision-engineering of cellular immunotherapies. CRISPR 2.0 can be used to augment immune cell function, broaden the spectrum of targetable antigens and facilitate streamlined production of T cell therapies. Notably, CRISPR 2.0 is reaching clinical maturity, with multiple clinical trials of CRISPR 2.0-modified cellular therapies currently ongoing. In this Review, we discuss emerging CRISPR 2.0 technologies and their progress towards clinical translation, highlighting challenges and opportunities, and describe strategies for the use of CRISPR 2.0 to advance cellular immunotherapy for haematological malignancies and solid tumours in the future.
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Directed-evolution mutations enhance DNA-binding affinity and protein stability of the adenine base editor ABE8e.
Adenine base editors (ABEs), consisting of CRISPR Cas nickase and deaminase, can chemically convert the A:T base pair to G:C. ABE8e, an evolved variant of the base editor ABE7.10, contains eight directed evolution mutations in its deaminase TadA8e that significantly increase its base editing activity. However, the functional implications of these mutations remain unclear. Here, we combined molecular dynamics (MD) simulations and experimental measurements to investigate the role of the directed-evolution mutations in the base editing catalysis. MD simulations showed that the DNA-binding affinity of TadA8e is higher than that of the original deaminase TadA7.10 in ABE7.10 and is mainly driven by electrostatic interactions. The directed-evolution mutations increase the positive charge density in the DNA-binding region, thereby enhancing the electrostatic attraction of TadA8e to DNA. We identified R111, N119 and N167 as the key mutations for the enhanced DNA binding and confirmed them by microscale thermophoresis (MST) and in vivo reversion mutation experiments. Unexpectedly, we also found that the directed mutations improved the thermal stability of TadA8e by ~ 12 °C (Tm, melting temperature) and that of ABE8e by ~ 9 °C, respectively. Our results demonstrate that the directed-evolution mutations improve the substrate-binding ability and protein stability of ABE8e, thus providing a rational basis for further editing optimisation of the system.
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Predicting adenine base editing efficiencies in different cellular contexts by deep learning.
BACKGROUND: Adenine base editors (ABEs) enable the conversion of A•T to G•C base pairs. Since the sequence of the target locus influences base editing efficiency, efforts have been made to develop computational models that can predict base editing outcomes based on the targeted sequence. However, these models were trained on base editing datasets generated in cell lines and their predictive power for base editing in primary cells in vivo remains uncertain. RESULTS: In this study, we conduct base editing screens using SpRY-ABEmax and SpRY-ABE8e to target 2,195 pathogenic mutations with a total of 12,000 guide RNAs in cell lines and in the murine liver. We observe strong correlations between in vitro datasets generated by ABE-mRNA electroporation into HEK293T cells and in vivo datasets generated by adeno-associated virus (AAV)- or lipid nanoparticle (LNP)-mediated nucleoside-modified mRNA delivery (Spearman R = 0.83-0.92). We subsequently develop BEDICT2.0, a deep learning model that predicts adenine base editing efficiencies with high accuracy in cell lines (R = 0.60-0.94) and in the liver (R = 0.62-0.81). CONCLUSIONS: In conclusion, our work confirms that adenine base editing holds considerable potential for correcting a large fraction of pathogenic mutations. We also provide BEDICT2.0 - a robust computational model that helps identify sgRNA-ABE combinations capable of achieving high on-target editing with minimal bystander effects in both in vitro and in vivo settings.
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