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Recent advancements in exon-skipping therapies using antisense oligonucleotides and genome editing for the treatment of various muscular dystrophies.
Muscular dystrophy is a group of genetic disorders characterised by degeneration of muscles. Different forms of muscular dystrophy can show varying phenotypes with a wide range of age, severity and location of muscle deterioration. Many palliative care options are available for muscular dystrophy patients, but no curative treatment is available. Exon-skipping therapy aims to induce skipping of exons with disease-causing mutations and/or nearby exons to restore the reading frame, which results in an internally truncated, partially functional protein. In antisense-mediated exon-skipping synthetic antisense oligonucleotide binds to pre-mRNA to induce exon skipping. Recent advances in exon skipping have yielded promising results; the US Food and Drug Administration (FDA) approved eteplirsen (Exondys51) as the first exon-skipping drug for the treatment of Duchenne muscular dystrophy, and in vivo exon skipping has been demonstrated in animal models of dysferlinopathy, limb-girdle muscular dystrophy type 2C and congenital muscular dystrophy type 1A. Novel methods that induce exon skipping utilizing Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are also being developed where splice site mutations are created within the genome to induce exon skipping. Challenges remain as exon-skipping agents can have deleterious non-specific effects and different in-frame deletions show phenotypic variance. This article reviews the state of the art of exon skipping for treating muscular dystrophy and discusses challenges and future prospects.
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In Vitro CRISPR-Cas12a-Based Detection of Cancer-Associated TP53 Hotspot Mutations Beyond the crRNA Seed Region.
Cost-effective and time-efficient detection of oncogenic mutations supports improved presymptomatic cancer diagnostics and post-treatment disease monitoring. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas12a is an RNA-guided endonuclease that, upon protospacer adjacent motif (PAM)-dependent recognition of target DNA in cis, exhibits indiscriminate ssDNase activity in trans, which can be harnessed for diagnostics. TP53, one of the most frequently mutated tumor suppressor genes in cancer, displays recurring point mutations at so-called "hotspots." In this study, we optimized Cas12a-based assay conditions for in vitro detection of six TP53 hotspot mutations at the codon for p.R273, located outside the Cas12a seed region, and evaluated the specificities of four commercial Cas12a variants. We found that nonengineered LbCas12a significantly outperformed the other tested nucleases specifically in distinguishing mutant p.R273 codons in synthetic DNA, mock cell-free DNA, and tissue biopsies, despite the suboptimal PAM-distal positioning of the corresponding mutations. Future clinical Cas12a-based applications may include point-of-care tumor analysis, cost-effective mutation screening, and improved monitoring of individual cancer patients.
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Fitness effects of CRISPR endonucleases in Drosophila melanogaster populations.
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 provides a highly efficient and flexible genome editing technology with numerous potential applications ranging from gene therapy to population control. Some proposed applications involve the integration of CRISPR/Cas9 endonucleases into an organism's genome, which raises questions about potentially harmful effects to the transgenic individuals. One example for which this is particularly relevant are CRISPR-based gene drives conceived for the genetic alteration of entire populations. The performance of such drives can strongly depend on fitness costs experienced by drive carriers, yet relatively little is known about the magnitude and causes of these costs. Here, we assess the fitness effects of genomic CRISPR/Cas9 expression in Drosophila melanogaster cage populations by tracking allele frequencies of four different transgenic constructs that allow us to disentangle 'direct' fitness costs due to the integration, expression, and target-site activity of Cas9, from fitness costs due to potential off-target cleavage. Using a maximum likelihood framework, we find that a model with no direct fitness costs but moderate costs due to off-target effects fits our cage data best. Consistent with this, we do not observe fitness costs for a construct with Cas9HF1, a high-fidelity version of Cas9. We further demonstrate that using Cas9HF1 instead of standard Cas9 in a homing drive achieves similar drive conversion efficiency. These results suggest that gene drives should be designed with high-fidelity endonucleases and may have implications for other applications that involve genomic integration of CRISPR endonucleases.
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Concise Review: The Potential Use of Intestinal Stem Cells to Treat Patients with Intestinal Failure.
Intestinal failure is a rare life-threatening condition that results in the inability to maintain normal growth and hydration status by enteral nutrition alone. Although parenteral nutrition and whole organ allogeneic transplantation have improved the survival of these patients, current therapies are associated with a high risk for morbidity and mortality. Development of methods to propagate adult human intestinal stem cells (ISCs) and pluripotent stem cells raises the possibility of using stem cell-based therapy for patients with monogenic and polygenic forms of intestinal failure. Organoids have demonstrated the capacity to proliferate indefinitely and differentiate into the various cellular lineages of the gut. Genome-editing techniques, including the overexpression of the corrected form of the defective gene, or the use of CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 to selectively correct the monogenic disease-causing variant within the stem cell, make autologous ISC transplantation a feasible approach. However, numerous techniques still need to be further optimized, including more robust ex vivo ISC expansion, native ISC ablation, and engraftment protocols. Large-animal models can to be used to develop such techniques and protocols and to establish the safety of autologous ISC transplantation because outcomes in such models can be extrapolated more readily to humans. Stem Cells Translational Medicine 2017;6:666-676.
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Mesenchymal stromal cells improve the transplantation outcome of CRISPR-Cas9 gene-edited human HSPCs.
Mesenchymal stromal cells (MSCs) have been employed in vitro to support hematopoietic stem and progenitor cell (HSPC) expansion and in vivo to promote HSPC engraftment. Based on these studies, we developed an MSC-based co-culture system to optimize the transplantation outcome of clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 gene-edited (GE) human HSPCs. We show that bone marrow (BM)-MSCs produce several hematopoietic supportive and anti-inflammatory factors capable of alleviating the proliferation arrest and mitigating the apoptotic and inflammatory programs activated in GE-HSPCs, improving their expansion and clonogenic potential in vitro. The use of BM-MSCs resulted in superior human engraftment and increased clonal output of GE-HSPCs contributing to the early phase of hematological reconstitution in the peripheral blood of transplanted mice. In conclusion, our work poses the biological bases for a novel clinical use of BM-MSCs to promote engraftment of GE-HSPCs and improve their transplantation outcome.
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CRISPR-Cas editing technologies for viral-mediated gene therapies of human diseases: Mechanisms, progress, and challenges.
The gene therapy landscape has evolved substantially in recent years, beginning with the approval of the first adeno-associated virus-based gene therapy, Luxterna, in 2017. Since then, the US FDA has approved nearly 30 new viral gene therapy programs, with notable examples including Zolgensma, Spinraza, Hemgenix, Zynteglo, Lyfgenia, Kymriah, Skysona, and Tecelra. Remarkably, all these products rely on delivery via adeno-associated vectors (AAVs) and lentiviral vectors (LVs). Improvements in viral-mediated gene transfer efficiency and clinical-scale manufacturing, together with immense commercial interest, have greatly propelled the clinical adoption of gene therapy products. In recent years, clustered regularly interspaced short palindromic repeats (CRISPR) and its related Cas proteins (CRISPR-Cas) have made significant advances in gene therapy, offering next-generation approaches for curative gene editing to treat genetic diseases and disorders. In this review, we examine the range of these therapeutics and their viral carriers, focusing primarily on LVs and AAVs. We provide a snapshot of the current status of the field and highlight some of the current challenges in the clinical application of gene therapy, with particular emphasis on viral CRISPR-Cas-based technologies and their future potential.
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CRISPR-Based Genome Editing as a New Therapeutic Tool in Retinal Diseases.
Retinal diseases are the primary reasons for severe visual defects and irreversible blindness. Retinal diseases are also inherited and acquired. Both of them are caused by mutations in genes or disruptions in specific gene expression, which can be treated by gene-editing therapy. Clustered regularly interspaced short palindromic repeats (CRISPR-Cas9) system is a frontier of gene-editing tools with great potential for therapeutic applications in the ophthalmology field to modify abnormal genes and treat the genome or epigenome-related retinal diseases. The CRISPR system is able to edit and trim the gene include deletion, insertion, inhibition, activation, replacing, remodeling, epigenetic alteration, and modify the gene expression. CRISPR-based genome editing techniques have indicated the enormous potential to treat retinal diseases that previous treatment was not available for them. Also, recent CRISPR genome surgery experiments have shown the improvement of patient's vision who suffered from severe visual loss. In this article, we review the applications of the CRISPR-Cas9 system in human or animal models for treating retinal diseases such as retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), age-related macular degeneration (AMD), proliferative diabetic retinopathy (PDR), and proliferative vitreoretinopathy (PVR), then we survey limitations of CRISPR system for clinical therapy.
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Coordinated regulation using small-molecule drugs enables controlled therapeutic genome editing and enhanced genomic precision in situ.
Achieving precise temporal control over genome editing is essential for safety but remains a challenge, especially when using small-molecule drugs as external regulators over systems like clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated systems). Consequently, controlled therapeutic in situ editing that maintains both precision and efficacy has yet to be demonstrated. Here, we report the PRINCE system, in which nuclease proteins and guide RNAs are both inducible, to deliver programmable nucleases under control more effectively. PRINCE demonstrated temporal precision in human cell cultures over a 2-year period, even after stable genomic integration. The design principles of PRINCE were broadly applicable from CRISPR-Cas9 to a prime editor and also compact programmable nucleases, and the latter platform was named "Little Prince." Upon administration of drug inducers, Little Prince, delivered in a single adeno-associated virus vector in situ to humanized mouse models, ameliorated pathological phenotypes of hypercholesterolemia (average reductions of 45 and 47% in serum total cholesterol and low-density lipoprotein cholesterol, respectively) and neovascular age-related macular degeneration, with significantly reduced lesion size and leakage (P < 0.0001). Last, we demonstrated a consistent and marked reduction in off-target activity across the PRINCE and Little Prince systems in comparison with constitutive editors, with fewer off-target sites and substantially lower editing frequencies, irrespective of nuclease type, delivery method, or genomic target. These results position PRINCE and Little Prince as controlled genome editing platforms with potential for in vivo, particularly in situ, therapeutic applications.
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CRISPR/Cas9 Genome Engineering in Non-Conventional Oleaginous Yeasts: Applications, Challenges, and Prospects.
Given the biotechnological potential of yeast-derived oils for oleochemical production, genes encoding lipid metabolism enzymes are key targets for metabolic engineering. Genetic engineering tools such as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9, Transcription Activator-Like Effector Nucleases (TALENs), Zinc-Finger Nucleases (ZFNs), RNA interference (RNAi), and integrative plasmids can be used to modulate fatty acid biosynthesis and optimize lipid production. Among them, the CRISPR/Cas9 system, recognized for its simplicity and efficiency, has been deployed as a tool to create oleaginous yeast strains with high lipid productivity and features suitable for application in biorefineries. Species such as Cutaneotrichosporon oleaginosus, Rhodotorula toruloides, Candida spp., and Yarrowia lipolytica have already been engineered using CRISPR/Cas9 to enhance the production of fatty acids and their derivatives. However, designing and constructing an efficient CRISPR/Cas9 platform for oleaginous yeasts faces several hurdles, including low transformation efficiency, difficulties in expressing Cas9 and sgRNAs efficiently and consistently, the lack of well-characterized promoters, limited availability of PAM sequences, and poorly understood DNA repair mechanisms. Here, we address the application of the CRISPR/Cas9 system in oleaginous yeasts, laying out the challenges to developing efficient platforms and highlighting key trends in the field. We compare and discuss alternative CRISPR-Cas9 expression strategies to provide an overview of the current landscape and support the development of new approaches.
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DNA Targeting by a Minimal CRISPR RNA-Guided Cascade.
Bacteria employ surveillance complexes guided by CRISPR (clustered, regularly interspaced, short palindromic repeats) RNAs (crRNAs) to target foreign nucleic acids for destruction. Although most type I and type III CRISPR systems require four or more distinct proteins to form multi-subunit surveillance complexes, the type I-C systems use just three proteins to achieve crRNA maturation and double-stranded DNA target recognition. We show that each protein plays multiple functional and structural roles: Cas5c cleaves pre-crRNAs and recruits Cas7 to position the RNA guide for DNA binding and unwinding by Cas8c. Cryoelectron microscopy reconstructions of free and DNA-bound forms of the Cascade/I-C surveillance complex reveal conformational changes that enable R-loop formation with distinct positioning of each DNA strand. This streamlined type I-C system explains how CRISPR pathways can evolve compact structures that retain full functionality as RNA-guided DNA capture platforms.
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