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Targeted deletion of rice retrotransposon Tos17 via CRISPR/Cas9.
A successful example of transposon deletion via CRISPR/Cas9-mediated genome editing suggests a novel alternative approach to plant breeding. Transposition of transposable elements (TEs) can affect adjacent genes, leading to changes in genetic traits. Expression levels and patterns, splicing and epigenetic status, and function of genes located in, or near, the inserted/excised locus can be affected. Artificial modification of loci adjacent to TEs, or TEs themselves, by genome editing could mimic the translocation of TEs that occurs in nature, suggesting that it might be possible to produce novel plants by modification of TEs via genome editing. To our knowledge, there are no reports thus far of modification of TEs by genome editing in plants. In this study, we performed targeted deletion of the Tos17 retrotransposon, which is flanked at both ends by long terminal repeat (LTR) sequences, via genome editing in rice. We succeeded in targeted mutagenesis of the LTR, and targeted deletion between LTRs, in calli transformed with CRISPR/Cas9 vectors for the Tos17 LTR. Moreover, we also successfully obtained regenerated plants derived from transformed calli and plants homozygous for lacking Tos17 in the next generation. Taken together, our results demonstrate successful deletion of the Tos17 retrotransposon from the rice genome by targeted mutagenesis using CRISPR/Cas9. We believe that this strategy could be applied to other TEs in many plant species, providing a rapid breeding technology as an alternative means to re-activate expression of agronomically important genes that have been inactivated by TE insertion, especially in plants such as fruit trees, in which it is difficult to maintain parental agronomical traits by cross-breeding due to high heterozygosity.
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CRISPR/dCas9 platforms in plants: strategies and applications beyond genome editing.
Clustered regularly interspaced short palindromic repeat (CRISPR) and Cas9-associated protein systems provide a powerful genetic manipulation tool that can drive plant research forward. Nuclease-dead Cas9 (dCas9) is an enzymatically inactive mutant of Cas9 in which its endonuclease activity is non-functional. The applications of CRISPR/dCas9 have expanded and diversified in recent years. Originally, dCas9 was used as a CRISPR/Cas9 re-engineering tool that enables targeted expression of any gene or multiple genes through recruitment of transcriptional effector domains without introducing irreversible DNA-damaging mutations. Subsequent applications have made use of its ability to recruit modifying enzymes and reporter proteins to DNA target sites. In this paper, the most recent progress in the applications of CRISPR/dCas9 in plants, which include gene activation and repression, epigenome editing, modulation of chromatin topology, live-cell chromatin imaging and DNA-free genetic modification, will be reviewed. The associated strategies for exploiting the CRISPR/dCas9 system for crop improvement with a dimer of the future of the CRISPR/dCas9 system in the functional genomics of crops and the development of traits will be briefly discussed.
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CRISPR/Cas9 mediated sequential editing of genes critical for ookinete motility in Plasmodium yoelii.
CRISPR/Cas9 has been successfully adapted for gene editing in malaria parasites including Plasmodium falciparum and Plasmodium yoelii. However, the reported methods were limited to editing one gene at a time. In practice, it is often desired to modify multiple genetic loci in a parasite genome. Here we describe a CRISPR/Cas9 mediated genome editing method that allows successive modification of more than one gene in the genome of P. yoelii using an improved single-vector system (pYCm) we developed previously. Drug resistant genes encoding human dihydrofolate reductase (hDHFR) and a yeast bifunctional protein (yFCU), with cytosine deaminase (CD) and uridyl phosphoribosyl transferase (UPRT) activities in the plasmid, allowed sequential positive (pyrimethamine, Pyr) and negative (5-fluorocytosine, 5FC) selections and generation of transgenic parasites free of the episomal plasmid after genetic modification. Using this system, we were able to efficiently tag a gene of interest (Pyp28) and subsequently disrupted two genes (Pyctrp and Pycdpk3) that are individually critical for ookinete motility. Disruption of the genes either eliminated (Pyctrp) or greatly reduced (Pycdpk3) ookinete forward motility in matrigel in vitro and completely blocked oocyst development in mosquito midgut. The method will greatly facilitate studies of parasite gene function, development, and disease pathogenesis.
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Generation of Genetically Engineered Embryos Using Gene Editing and Somatic Cell Nuclear Transfer for Production of Sheep Models of Human Disease.
Large animal models are valuable tools for investigating human disease. Sheep, pigs, and goats often better recapitulate the anatomy and physiology of human organs and the complexity of human disease, thereby enhancing their clinical relevance compared to rodents. CRISPR-Cas9 and somatic cell nuclear transfer (SCNT) enable the generation of large animal models with greater precision, versatility, and genetic uniformity. The primary benefit of this approach, compared with zygote microinjection, is the ability to confirm in vitro whether the desired genetic modification and potential off-target mutations are present in gene-edited cells prior to animal production. Moreover, SCNT eliminates the chance of genetic mosaicism, which frequently results from zygote microinjection. Here, we describe the generation of gene-edited ovine cells through non-homologous end-joining (NHEJ) and homology-directed repair (HDR), followed by the production of cloned embryos carrying the mutations of interest. Genetic modifications are introduced by transfecting cultured somatic cells, typically fetal fibroblasts, with the CRISPR-Cas9 system. Mutation efficiency in pooled cells is assessed by polymerase chain reaction (PCR) and Sanger sequencing of edited genes and analyzed using Tracking of Indels by DEcomposition (TIDE)/Tracking of Insertions, Deletions, and Recombination events (TIDER) software. Limiting dilution of the pooled cells is performed to obtain single-cell-derived colonies, which are screened by PCR and DNA sequencing of edited genes. Donor cells with the edit(s) of interest are subsequently expanded and used for the generation of embryos by SCNT. After limiting dilution and cell screening, 22/114 (19.3%) of colonies modified through NHEJ contained knockout (KO) mutations and 4/56 (7.1%) of colonies modified with HDR contained the F508del mutation. A total of 370 genetically modified embryos were created from four colonies. These methods are successfully used for precise gene editing in fetal fibroblasts and generation of genetically engineered embryos to produce ovine models of human disease.
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SelectRepair Knockout: Efficient PTC-Free Gene Knockout Through Selectable Homology-Directed DNA Repair.
Generating nonessential gene knockouts using CRISPR/Cas9 technology is becoming increasingly common in biological research. In a typical workflow, the Cas9 endonuclease is used to induce a DNA double-strand break that relies on nonhomologous end-joining (NHEJ) to introduce a premature termination codon (PTC) in the target gene. The goal is to isolate clones in which the gene produces PTC-containing mRNA transcripts that are degraded via nonsense-mediated mRNA decay (NMD) to cause loss of gene function. Unfortunately, this approach is laborious, and not all PTCs trigger NMD. More importantly, mounting evidence suggest that PTC mutations can also result in a transcriptional adaptation response that can mask the effects of a PTC-mediated gene knockout. In this chapter, we present a PTC-free gene knockout strategy that implements homology-directed DNA repair (HDR) with selectable markers to substantially reduce the complexity of the screening and validation of genome edits in cells containing more than one gene copy as in the case of the commonly used hypotriploid HEK293 cell line. We describe how to obtain a complete knockout of the Ligase IV protein (LIG4) and provide considerations for the application of this SelectRepair Knockout method to other genes.
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Gene Editing and Rett Syndrome: Does It Make the Cut?
Rett syndrome (RTT) is a rare neurogenetic disorder caused by pathogenic variants of the Methyl CpG binding protein 2 (MECP2) gene. The RTT is characterized by apparent normal early development followed by regression of communicative and fine motor skills. Comorbidities include epilepsy, severe cognitive impairment, and autonomic and motor dysfunction. Despite almost 60 clinical trials and the promise of a gene therapy, no cure has yet emerged with treatment remaining symptomatic. Advances in understanding RTT has provided insight into the complexity and exquisite control of MECP2 expression, where loss of expression leads to RTT and overexpression leads to MECP2 duplication syndrome. Therapy development requires regulated expression that matches the spatiotemporal endogenous expression of MECP2 in the brain. Gene editing has revolutionized gene therapy and promises an exciting strategy for many incurable monogenic disorders, including RTT, by editing the native locus and retaining endogenous gene expression. Here, we review the literature on the currently available editing technologies and discuss their limitations and applicability to the treatment of RTT.
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In Silico Design and Characterization of a Rationally Engineered Cas12j2 Gene Editing System for the Treatment of HPV-Associated Cancers.
CRISPR-Cas9 systems have enabled unprecedented advances in genome engineering, particularly in developing treatments for human diseases, like cancer. Despite potential applications, limitations of Cas9 include its relatively large size and strict targeting requirements. Cas12j2, a variant ofCasΦ-2, shows promise for overcoming these limitations. However, its effectiveness in mammalian cells remains relatively unexplored. This study sought to develop an optimized CRISPR-Cas12j2 system for targeted knockout of the E6 oncogene in HPV-associated cancers. A combination of computational tools (ColabFold, CCTop, Cas-OFFinder, HADDOCK2.4, and Amber for Molecular Dynamics) was utilized to investigate the impact of engineered modifications on structural integrity and gRNA binding of Cas12j2 fusion constructs, in potential intracellular conditions. Cas12j2_F2, a Cas12j2 variant designed and evaluated in this study, behaves similarly to the wild-type Cas12j2 structure in terms of RMSD/RMSF profiles, compact Rg values, and minimal electrostatic perturbation. The computationally validated Cas12j2 variant was incorporated into a custom expression vector, co-expressing the engineered construct along with a dual gRNA for packaging into a viral vector for targeted knockout of HPV-associated cancers. This study provides a structural and computational foundation for the rational design of Cas12j2 fusion constructs with enhanced stability and functionality, supporting their potential application for precise genome editing in mammalian cells.
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Pac-Man like DNA cut-and-paste tool allows larger gene edits.
The CRISPR-Cas toolbox allows genetic manipulation of cultured cells, plants and animals on the basis of simpler RNA-guided DNA recognition. It has provided breakthrough scientific opportunities to engineer desirable traits, cure genetic diseases and enable point-of-care diagnostics. A recent study by Joseph Bondy-Denomy and colleagues further equips this toolbox to cut larger chunks of DNA from a cell’s genome.
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CRISPR-Cas9-driven antigen conversion of clinically relevant blood group systems.
The common practice of blood transfusion entirely relies on blood donations from the population. Ensuring blood group compatibility between a donor and a recipient is paramount to prevent critical adverse reactions. Finding compatible blood can be challenging given the high diversity of blood group antigens, especially for chronically transfused patients at higher risk of alloimmunization owing to repeated exposures to foreign RBCs. In addition, due to the immunogenicity of the ABO blood group and the highly polymorphic nature of the Rhesus (Rh) system, they both remain of prime importance in transfusion medicine. Cultured red blood cells (cRBCs) may eventually provide an alternative for blood donations-at least in some circumstances. Combining cRBCs with blood group gene editing could broaden transfusion accessibility by making antigen expression compatible with rare phenotypes, thus meeting the needs of more patients. Starting from mobilized, erythroid-primed hematopoietic stem and progenitor cells (HSPCs), we used virus- and selection-free, CRISPR-Cas9-mediated knockouts to produce erythroid cells devoid of AB and Rh antigen. The approach yielded almost complete conversion to O- and RhNull phenotypes, as determined by standard hemagglutination and flow cytometry analyses. Combined with robust cRBC protocols, these clinically relevant phenotypic changes could eventually expand the accessibility of blood transfusion for specific and unmet clinical needs.
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Hidden Outcomes of CRISPR-Cas9 Genome Editing Revealed by Targeted Sequencing in Isogenic Human iPSC Clones.
Advances in CRISPR-Cas9-based genome editing have simplified the generation of isogenic induced pluripotent stem cell (iPSC) pairs differing by a single genetic perturbation, minimizing interindividual variability in disease modeling. However, imprecise editing can cause unintended mutations that distort genotype-phenotype interpretation. Here, we investigated the molecular consequences of CRISPR-Cas9 editing in human iPSCs carrying a tandem duplication of the STAG2 locus. Using hybridization capture-based targeted next-generation sequencing of genomic regions surrounding Cas9-induced double-strand breaks, we identified inversions, plasmid DNA integrations, and complex structural rearrangements that had been missed by conventional PCR-based genotyping. The main advance of our study is the application of a practical hybridization capture-based workflow for resolving such hidden editing outcomes during the generation of isogenic iPSC models. In the selected clones subjected to detailed analysis, targeted sequencing enabled us to revise the initial interpretation of editing outcomes and resolve discrepancies between genotypic and transcriptomic data. Our results demonstrate that complex on-target alterations can remain undetected by conventional PCR-based genotyping and show that hybridization capture-based targeted sequencing can serve as a practical quality-control approach for the characterization of edited iPSC clones.
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