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In Our Image: The Ethics of CRISPR Genome Editing.
The advent of genome editing technology promises to transform human health, livestock and agriculture, and to eradicate pest species. This transformative power demands urgent scrutiny and resolution of the ethical conflicts attached to the creation and release of engineered genomes. Here, I discuss the ethics surrounding the transformative CRISPR/Cas9-mediated genome editing technology in the contexts of human genome editing to eradicate genetic disease and of gene drive technology to eradicate animal vectors of human disease.
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An Update on the Application of CRISPR Technology in Clinical Practice.
The CRISPR/Cas system, an innovative gene-editing tool, is emerging as a promising technique for genome modifications. This straightforward technique was created based on the prokaryotic adaptive immune defense mechanism and employed in the studies on human diseases that proved enormous therapeutic potential. A genetically unique patient mutation in the process of gene therapy can be corrected by the CRISPR method to treat diseases that traditional methods were unable to cure. However, introduction of CRISPR/Cas9 into the clinic will be challenging because we still need to improve the technology's effectiveness, precision, and applications. In this review, we first describe the function and applications of the CRISPR-Cas9 system. We next delineate how this technology could be utilized for gene therapy of various human disorders, including cancer and infectious diseases and highlight the promising examples in the field. Finally, we document current challenges and the potential solutions to overcome these obstacles for the effective use of CRISPR-Cas9 in clinical practice.
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Optimizing the delivery of CRISPR/Cas9 ribonucleoproteins for efficient genome editing in bovine embryos.
To assist in establishing a streamlined and efficient workflow for generating gene-edited bovine embryos, we evaluated three transfection approaches for the delivery of CRISPR Cas9-sgRNA ribonucleoproteins into bovine zygotes: lipofection with Lipofectamine CRISPRMAX, and electroporation using either Neon or NEPA21 electroporation systems. Bos taurus prolactin receptor (PRLR) was used as the target gene. The PRLR editing outcomes were analysed by PCR genotyping and Sanger sequencing of individual embryos at day 8 post-fertilization. CRISPRMAX transfection generated up to 30 % PRLR-edited blastocysts (8 % homozygous deletion), without affecting the embryo cleavage (93 %) and blastocyst rate (39 %) relative to non-transfected controls. For both NEPA21 and Neon electroporation, we found that increasing the voltage, length and number of pulses resulted in enhanced gene editing efficiency but compromised embryo cleavage and blastocyst rates. NEPA21 electroporation with a commercial electroporation enhancer reagent produced up to 47.6 % transfected embryos with the PRLR deletion, but with decreased embryo cleavage (62 %) and blastocyst (18 %) rates. Combining NEPA21 electroporation with CRISPRMAX lipofection enhanced the gene editing efficiency to 50 % (23 % homozygous editing), with 64 % embryo cleavage rate and 18 % blastocyst rate. Notably, Neon electroporation resulted in 65.2 % PRLR-edited blastocysts with 21 % homozygous editing (50% cleavage rate and 10 % blastocyst rate). Additional studies may be necessary to further optimize electroporation parameters to achieve an optimal balance between embryo viability and gene editing efficiency. These outcomes will provide valuable insights for improving gene editing workflows for bovines and may help to promote and accelerate the widespread implementation of genome editing technology in livestock.
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Inhibition of CRISPR-Cas9 ribonucleoprotein complex assembly by anti-CRISPR AcrIIC2.
CRISPR-Cas adaptive immune systems function to protect bacteria from invasion by foreign genetic elements. The CRISPR-Cas9 system has been widely adopted as a powerful genome-editing tool, and phage-encoded inhibitors, known as anti-CRISPRs, offer a means of regulating its activity. Here, we report the crystal structures of anti-CRISPR protein AcrIIC2Nme alone and in complex with Nme1Cas9. We demonstrate that AcrIIC2Nme inhibits Cas9 through interactions with the positively charged bridge helix, thereby preventing sgRNA loading. In vivo phage plaque assays and in vitro DNA cleavage assays show that AcrIIC2Nme mediates its activity through a large electronegative surface. This work shows that anti-CRISPR activity can be mediated through the inhibition of Cas9 complex assembly.
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Engineered CRISPR-Cas9 nuclease with expanded targeting space.
The RNA-guided endonuclease Cas9 cleaves its target DNA and is a powerful genome-editing tool. However, the widely used Streptococcus pyogenes Cas9 enzyme (SpCas9) requires an NGG protospacer adjacent motif (PAM) for target recognition, thereby restricting the targetable genomic loci. Here, we report a rationally engineered SpCas9 variant (SpCas9-NG) that can recognize relaxed NG PAMs. The crystal structure revealed that the loss of the base-specific interaction with the third nucleobase is compensated by newly introduced non-base-specific interactions, thereby enabling the NG PAM recognition. We showed that SpCas9-NG induces indels at endogenous target sites bearing NG PAMs in human cells. Furthermore, we found that the fusion of SpCas9-NG and the activation-induced cytidine deaminase (AID) mediates the C-to-T conversion at target sites with NG PAMs in human cells.
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New approaches to moderate CRISPR-Cas9 activity: Addressing issues of cellular uptake and endosomal escape.
CRISPR-Cas9 is rapidly entering molecular biology and biomedicine as a promising gene-editing tool. A unique feature of CRISPR-Cas9 is a single-guide RNA directing a Cas9 nuclease toward its genomic target. Herein, we highlight new approaches for improving cellular uptake and endosomal escape of CRISPR-Cas9. As opposed to other recently published works, this review is focused on non-viral carriers as a means to facilitate the cellular uptake of CRISPR-Cas9 through endocytosis. The majority of non-viral carriers, such as gold nanoparticles, polymer nanoparticles, lipid nanoparticles, and nanoscale zeolitic imidazole frameworks, is developed with a focus toward optimizing the endosomal escape of CRISPR-Cas9 by taking advantage of the acidic environment in the late endosomes. Among the most broadly used methods for in vitro and ex vivo ribonucleotide protein transfection are electroporation and microinjection. Thus, other delivery formats are warranted for in vivo delivery of CRISPR-Cas9. Herein, we specifically revise the use of peptide and nanoparticle-based systems as platforms for CRISPR-Cas9 delivery in vivo. Finally, we highlight future perspectives of the CRISPR-Cas9 gene-editing tool and the prospects of using non-viral vectors to improve its bioavailability and therapeutic potential.
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CRISPR-Cas12a (Cpf1): A Versatile Tool in the Plant Genome Editing Tool Box for Agricultural Advancement.
Global population is predicted to approach 10 billion by 2050, an increase of over 2 billion from today. To meet the demands of growing, geographically and socio-economically diversified nations, we need to diversity and expand agricultural production. This expansion of agricultural productivity will need to occur under increasing biotic, and environmental constraints driven by climate change. Clustered regularly interspaced short palindromic repeats-site directed nucleases (CRISPR-SDN) and similar genome editing technologies will likely be key enablers to meet future agricultural needs. While the application of CRISPR-Cas9 mediated genome editing has led the way, the use of CRISPR-Cas12a is also increasing significantly for genome engineering of plants. The popularity of the CRISPR-Cas12a, the type V (class-II) system, is gaining momentum because of its versatility and simplified features. These include the use of a small guide RNA devoid of trans-activating crispr RNA, targeting of T-rich regions of the genome where Cas9 is not suitable for use, RNA processing capability facilitating simpler multiplexing, and its ability to generate double strand breaks (DSB) with staggered ends. Many monocot and dicot species have been successfully edited using this Cas12a system and further research is ongoing to improve its efficiency in plants, including improving the temperature stability of the Cas12a enzyme, identifying new variants of Cas12a or synthetically producing Cas12a with flexible PAM sequences. In this review we provide a comparative survey of CRISPR-Cas12a and Cas9, and provide a perspective on applications of CRISPR-Cas12 in agriculture.
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Agrobacterium tumefaciens-Mediated Plant Transformation and Gene Editing in Rice.
Bottlenecks in plant transformation and regeneration have slowed progress in applying CRISPR/Cas9-based genome editing for crop improvement. Rice (Oryza sativa L.) has highly efficient temperate japonica transformation protocols, along with reasonably efficient indica protocols using immature embryos. However, rapid and efficient protocols are not available for transformation and regeneration in tropical japonica varieties, even though they represent most of the rice production in the USA and South America, along with some regions in Asia. This chapter describes a protocol for CRISPR/Cas9 gene editing using Agrobacterium-mediated transformation for the tropical japonica rice cultivar Presidio leading to knock-out mutations in the phytoene desaturase (PDS) gene.
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Strategies to improve homology-based repair outcomes following CRISPR-based gene editing in mosquitoes: lessons in how to keep any repair disruptions local.
Programmable gene editing systems such as CRISPR-Cas have made mosquito genome engineering more practical and accessible, catalyzing the development of cutting-edge genetic methods of disease vector control. This progress, however, has been limited by the low efficiency of homology-directed repair (HDR)-based sequence integration at DNA double-strand breaks (DSBs) and a lack of understanding about DSB repair in mosquitoes. Innovative efforts to optimize HDR sequence integration by inhibiting non-homologous end joining or promoting HDR have been performed in mammalian systems, however many of these approaches have not been applied to mosquitoes. Here, we review some of the most relevant steps of DNA DSB repair choice and highlight promising approaches that influence this choice to enhance HDR in the context of mosquito gene editing.
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Gene Editing in Mouse Zygotes Using the CRISPR/Cas9 System.
Engineering of the mouse germline is a key technology in biomedical research for studying the function of genes in health and disease. Since the first knockout mouse was described in 1989, gene targeting was based on recombination of vector encoded sequences in mouse embryonic stem cell lines and their introduction into preimplantation embryos to obtain germline chimeric mice. This approach has been replaced in 2013 by the application of the RNA-guided CRISPR/Cas9 nuclease system, which is introduced into zygotes and directly creates targeted modifications in the mouse genome. Upon the introduction of Cas9 nuclease and guide RNAs into one-cell embryos, sequence-specific double-strand breaks are created that are highly recombinogenic and processed by DNA repair enzymes. Gene editing commonly refers to the diversity of DSB repair products that include imprecise deletions or precise sequence modifications copied from repair template molecules. Since gene editing can now be easily applied directly in mouse zygotes, it has rapidly become the standard procedure for generating genetically engineered mice. This article covers the design of guide RNAs, knockout and knockin alleles, options for donor delivery, preparation of reagents, microinjection or electroporation of zygotes, and the genotyping of pups derived from gene editing projects.
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