The road less traveled: strategies to enhance the frequency of homology-directed repair (HDR) for increased efficiency of CRISPR/Cas-mediated transgenesis

Gene therapy for CNS disorders: modalities, delivery and translational challenges

Gene therapy is emerging as a powerful tool to modulate abnormal gene expression, a hallmark of most CNS disorders. The transformative potentials of recently approved gene therapies for the treatment of spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS) and active cerebral adrenoleukodystrophy are encouraging further development of this approach. However, most attempts to translate gene therapy to the clinic have failed to make it to market. There is an urgent need not only to tailor the genes that are targeted to the pathology of interest but to also address delivery challenges and thereby maximize the utility of genetic tools. In this Review, we provide an overview of gene therapy modalities for CNS diseases, emphasizing the interconnectedness of different delivery strategies and routes of administration. Important gaps in understanding that could accelerate the clinical translatability of CNS genetic interventions are addressed, and we present lessons learned from failed clinical trials that may guide the future development of gene therapies for the treatment and management of CNS disorders.

CRISPR/Cas Technology: The Unique Synthetic Biology Genome-Editing Tool Shifting the Paradigm in Viral Diagnostics, Defense, and Therapeutics

The emergence of the COVID-19 pandemic has starkly exposed our significantly limited ability to promptly identify and respond to emergent biological threats. Consequently, there is an urgent need to advance biotechnological methods for addressing both known and unforeseen biological hazards. Recently, the CRISPR/Cas system has revolutionized genetic engineering, enabling precise and efficient synthetic biology applications. Therefore, this review aims to provide a comprehensive introduction to the fundamental principles underlying the CRISPR/Cas system and assess the advantages and limitations of various CRISPR/Cas-based techniques applicable to the detection of, defense against, and treatment of viral infections. These techniques include viral diagnostics, the development of antiviral vaccines, B cell engineering for antibody production, viral activation/interference, and epigenetic modifications. Furthermore, this review delves into the challenges and bioethical considerations associated with use of the CRISPR/Cas system. With the continuous evolution of technology, the CRISPR/Cas system holds considerable promise for addressing both existing and unforeseen biological threats.

Single-step generation of homozygous knockout/knock-in individuals in an extremotolerant parthenogenetic tardigrade using DIPA-CRISPR

Tardigrades are small aquatic invertebrates known for their remarkable tolerance to diverse extreme stresses. To elucidate the in vivo mechanisms underlying this extraordinary resilience, methods for genetically manipulating tardigrades have long been desired. Despite our prior success in somatic cell gene editing by microinjecting Cas9 ribonucleoproteins (RNPs) into the body cavity of tardigrades, the generation of gene-edited individuals remained elusive. In this study, employing an extremotolerant parthenogenetic tardigrade species, Ramazzottius varieornatus, we established conditions that led to the generation of gene-edited tardigrade individuals. Drawing inspiration from the direct parental CRISPR (DIPA-CRISPR) technique employed in several insects, we simply injected a concentrated Cas9 RNP solution into the body cavity of parental females shortly before their initial oviposition. This approach yielded gene-edited G0 progeny. Notably, only a single allele was predominantly detected at the target locus for each G0 individual, indicative of homozygous mutations. By co-injecting single-stranded oligodeoxynucleotides (ssODNs) with Cas9 RNPs, we achieved the generation of homozygously knocked-in G0 progeny, and these edited alleles were inherited by G1/G2 progeny. This is the first example of heritable gene editing in the entire phylum of Tardigrada. This establishment of a straightforward method for generating homozygous knockout/knock-in individuals not only facilitates in vivo analyses of the molecular mechanisms underpinning extreme tolerance, but also opens up avenues for exploring various topics, including Evo-Devo, in tardigrades.

Genome editing and its role in vaccine, diagnosis, and therapeutic advancement

Genome editing involves precise modification of specific nucleotides in the genome using nucleases like CRISPR/Cas, ZFN, or TALEN, leading to increased efficiency of homologous recombination (HR) for gene editing, and it can result in gene disruption events via non-homologous end joining (NHEJ) or homology-driven repair (HDR). Genome editing, particularly CRISPR-Cas9, revolutionizes vaccine development by enabling precise modifications of pathogen genomes, leading to enhanced vaccine efficacy and safety. It allows for tailored antigen optimization, improved vector design, and deeper insights into host genes' impact on vaccine responses, ultimately enhancing vaccine development and manufacturing processes. This review highlights different types of genome editing methods, their associated risks, approaches to overcome the shortcomings, and the diverse roles of genome editing.

Efficient multiple gene knockout in Colletotrichum higginsianum via CRISPR/Cas9 ribonucleoprotein and URA3-based marker recycling

Colletotrichum higginsianum is a hemibiotrophic pathogen that causes anthracnose disease on crucifer hosts, including Arabidopsis thaliana. Despite the availability of genomic and transcriptomic information and the ability to transform both organisms, identifying C. higginsianum genes involved in virulence has been challenging due to recalcitrance to gene targeting and redundancy of virulence factors. To overcome these obstacles, we developed an efficient method for multiple gene disruption in C. higginsianum by combining CRISPR/Cas9 and a URA3-based marker recycling system. Our method significantly increased the efficiency of gene knockout via homologous recombination by introducing genomic DNA double-strand breaks. We demonstrated the applicability of the URA3-based marker recycling system for multiple gene targeting in the same strain. Using our technology, we successfully targeted two melanin biosynthesis genes, SCD1 and PKS1, which resulted in deficiency in melanization and loss of pathogenicity in the mutants. Our findings demonstrate the effectiveness of our methods in analysing virulence factors in C. higginsianum, thus accelerating research on plant-fungus interactions.

Homology-Directed-Repair-Based Genome Editing in HSPCs for the Treatment of Inborn Errors of Immunity and Blood Disorders

Genome engineering via targeted nucleases, specifically CRISPR-Cas9, has revolutionized the field of gene therapy research, providing a potential treatment for diseases of the blood and immune system. While numerous genome editing techniques have been used, CRISPR-Cas9 homology-directed repair (HDR)-mediated editing represents a promising method for the site-specific insertion of large transgenes for gene knock-in or gene correction. Alternative methods, such as lentiviral/gammaretroviral gene addition, gene knock-out via non-homologous end joining (NHEJ)-mediated editing, and base or prime editing, have shown great promise for clinical applications, yet all possess significant drawbacks when applied in the treatment of patients suffering from inborn errors of immunity or blood system disorders. This review aims to highlight the transformational benefits of HDR-mediated gene therapy and possible solutions for the existing problems holding the methodology back. Together, we aim to help bring HDR-based gene therapy in CD34⁺ hematopoietic stem progenitor cells (HSPCs) from the lab bench to the bedside.

High-Efficiency CRISPR/Cas9-Mediated Correction of a Homozygous Mutation in Achromatopsia-Patient-Derived iPSCs

Achromatopsia is an autosomal recessive disorder, in which cone photoreceptors undergo progressive degeneration, causing color blindness and poor visual acuity, among other significant eye affectations. It belongs to a group of inherited retinal dystrophies that currently have no treatment. Although functional improvements have been reported in several ongoing gene therapy studies, more efforts and research should be carried out to enhance their clinical application. In recent years, genome editing has arisen as one of the most promising tools for personalized medicine. In this study, we aimed to correct a homozygous PDE6C pathogenic variant in hiPSCs derived from a patient affected by achromatopsia through CRISPR/Cas9 and TALENs technologies. Here, we demonstrate high efficiency in gene editing by CRISPR/Cas9 but not with TALENs approximation. Despite a few of the edited clones displaying heterozygous on-target defects, the proportion of corrected clones with a potentially restored wild-type PDE6C protein was more than half of the total clones analyzed. In addition, none of them presented off-target aberrations. These results significantly contribute to advances in single-nucleotide gene editing and the development of future strategies for the treatment of achromatopsia.

Epigenetic marks for mitigating abiotic stresses in plants

Abiotic stressors are one of the major factors affecting agricultural output. Plants have evolved adaptive systems to respond appropriately to various environmental cues. These responses can be accomplished by modulating or fine-tuning genetic and epigenetic regulatory mechanisms. Understanding the response of plants' molecular features to abiotic stress is a priority in the current period of continued environmental changes. Epigenetic modifications are necessary that control gene expression by changing chromatin status and recruiting various transcription regulators. The present study summarized the current knowledge on epigenetic modifications concerning plant responses to various environmental stressors. The functional relevance of epigenetic marks in regulating stress tolerance has been revealed, and epigenetic changes impact the effector genes. This study looks at the epigenetic mechanisms that govern plant abiotic stress responses, especially DNA methylation, histone methylation/acetylation, chromatin remodeling, and various metabolites. Plant breeders will benefit from a thorough understanding of these processes to create alternative crop improvement approaches. Genome editing with clustered regularly interspaced short palindromic repeat/CRISPR-associated proteins (CRISPR/Cas) provides genetic tools to make agricultural genetic engineering more sustainable and publicly acceptable.

CRISPR-Cas technology a new era in genomic engineering

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CRISPR-Cas systems offer a flexible and easy-to-use molecular platform to precisely modify and control organisms' genomes in a variety of fields, from agricultural biotechnology to therapeutics.

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With CRISPR technology, crop genomes can be precisely edited in a shorter and more efficient approach compared to traditional breeding or classic mutagenesis.

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CRISPR-Cas system can be used to manage the fermentation process by addressing phage resistance, antimicrobial activity, and genome editing.

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CRISPR-Cas technology has opened up a new era in gene therapy and other therapeutic fields and given hope to thousands of patients with genetic diseases.

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Anti-CRISPR molecules are powerful tools for regulating the CRISPR-Cas systems.

The CRISPR-Cas systems have offered a flexible, easy-to-use platform to precisely modify and control the genomes of organisms in various fields, ranging from agricultural biotechnology to therapeutics. This system is extensively used in the study of infectious, progressive, and life-threatening genetic diseases for the improvement of quality and quantity of major crops and in the development of sustainable methods for the generation of biofuels. As CRISPR-Cas technology continues to evolve, it is becoming more controllable and precise with the addition of molecular regulators, which will provide benefits for everyone and save many lives. Studies on the constant growth of CRISPR technology are important due to its rapid development. In this paper, we present the current applications and progress of CRISPR-Cas genome editing systems in several fields of research, we further highlight the applications of anti-CRISPR molecules to regulate CRISPR-Cas gene editing systems, and we discuss ethical considerations in CRISPR-Cas applications.

Enhancing HR Frequency for Precise Genome Editing in Plants

Gene-editing tools, such as Zinc-fingers, TALENs, and CRISPR-Cas, have fostered a new frontier in the genetic improvement of plants across the tree of life. In eukaryotes, genome editing occurs primarily through two DNA repair pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). NHEJ is the primary mechanism in higher plants, but it is unpredictable and often results in undesired mutations, frameshift insertions, and deletions. Homology-directed repair (HDR), which proceeds through HR, is typically the preferred editing method by genetic engineers. HR-mediated gene editing can enable error-free editing by incorporating a sequence provided by a donor template. However, the low frequency of native HR in plants is a barrier to attaining efficient plant genome engineering. This review summarizes various strategies implemented to increase the frequency of HDR in plant cells. Such strategies include methods for targeting double-strand DNA breaks, optimizing donor sequences, altering plant DNA repair machinery, and environmental factors shown to influence HR frequency in plants. Through the use and further refinement of these methods, HR-based gene editing may one day be commonplace in plants, as it is in other systems.

An Insight into Modern Targeted Genome-Editing Technologies with a Special Focus on CRISPR/Cas9 and its Applications

Genome-editing technology has enabled scientists to make changes in model organisms' DNA at the genomic level to get biotechnologically important products from them. Most commonly employed technologies for this purpose are transcription activator like effector nucleases (TALENs), homing-endonucleases or meganucleases, zinc finger nucleases (ZFNs), and clustered regularly interspaced short palindromic repeats (CRISPR) associated protein 9 (Cas9). Among these tools, CRISPR/Cas9 is most preferred because it's easy to use, has a small mutation rate, has great effectiveness, low cost of development, and decreased rate of advancement. CRISPR/Cas9 has a lot of applications in plants, animals, humans, and microbes. It also has applications in many fields such as horticulture, cancer, food biotechnology, and targeted human genome treatments. CRISPR technology has shown great potential for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic to provide early and easy detection methods, possible treatment, and vaccine development. In the present review, genome-editing tools with their basic assembly and features have been discussed. Exceptional notice has been paid to CRISPR technology on basis of its structure and significant applications in humans, plants, animals, and microbes such as bacteria, viruses, and fungi. The review has also shed a little light on current CRISPR challenges and future perspectives.

Development and Application of CRISPR-Cas Based Tools

Abundant CRISPR-Cas systems in nature provide us with unlimited valuable resources to develop a variety of versatile tools, which are powerful weapons in biological discovery and disease treatment. Here, we systematically review the development of CRISPR-Cas based tools from DNA nuclease to RNA nuclease, from nuclease dependent-tools to nucleic acid recognition dependent-tools. Also, considering the limitations and challenges of current CRISPR-Cas based tools, we discuss the potential directions for development of novel CRISPR toolkits in the future.

Therapeutic potentials of CRISPR-Cas genome editing technology in human viral infections

Viral infections are a common cause of morbidity worldwide. The emergence of Coronavirus Disease 2019 (COVID-19) has led to more attention to viral infections and finding novel therapeutics. The CRISPR-Cas9 system has been recently proposed as a potential therapeutic tool for the treatment of viral diseases. Here, we review the research progress in the use of CRISPR-Cas technology for treating viral infections, as well as the strategies for improving the delivery of this gene-editing tool in vivo. Key challenges that hinder the widespread clinical application of CRISPR-Cas9 technology are also discussed, and several possible directions for future research are proposed.

Strategies for High-Efficiency Mutation Using the CRISPR/Cas System

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated systems have revolutionized traditional gene-editing tools and are a significant tool for ameliorating gene defects. Characterized by high target specificity, extraordinary efficiency, and cost-effectiveness, CRISPR/Cas systems have displayed tremendous potential for genetic manipulation in almost any organism and cell type. Despite their numerous advantages, however, CRISPR/Cas systems have some inherent limitations, such as off-target effects, unsatisfactory efficiency of delivery, and unwanted adverse effects, thereby resulting in a desire to explore approaches to address these issues. Strategies for improving the efficiency of CRISPR/Cas-induced mutations, such as reducing off-target effects, improving the design and modification of sgRNA, optimizing the editing time and the temperature, choice of delivery system, and enrichment of sgRNA, are comprehensively described in this review. Additionally, several newly emerging approaches, including the use of Cas variants, anti-CRISPR proteins, and mutant enrichment, are discussed in detail. Furthermore, the authors provide a deep analysis of the current challenges in the utilization of CRISPR/Cas systems and the future applications of CRISPR/Cas systems in various scenarios. This review not only serves as a reference for improving the maturity of CRISPR/Cas systems but also supplies practical guidance for expanding the applicability of this technology.

Chicken blastoderms and primordial germ cells possess a higher expression of DNA repair genes and lower expression of apoptosis genes to preserve their genome stability

DNA is susceptible to damage by various sources. When the DNA is damaged, the cell repairs the damage through an appropriate DNA repair pathway. When the cell fails to repair DNA damage, apoptosis is initiated. Although several genes are involved in five major DNA repair pathways and two major apoptosis pathways, a comprehensive understanding of those gene expression is not well-understood in chicken tissues. We performed whole-transcriptome sequencing (WTS) analysis in the chicken embryonic fibroblasts (CEFs), stage X blastoderms, and primordial germ cells (PGCs) to uncover this deficiency. Stage X blastoderms mostly consist of undifferentiated progenitor (pluripotent) cells that have the potency to differentiate into all cell types. PGCs are also undifferentiated progenitor cells that later differentiate into male and female germ cells. CEFs are differentiated and abundant somatic cells. Through WTS analysis, we identified that the DNA repair pathway genes were expressed more highly in blastoderms and high in PGCs than CEFs. Besides, the apoptosis pathway genes were expressed low in blastoderms and PGCs than CEFs. We have also examined the WTS-based expression profiling of candidate pluripotency regulating genes due to the conserved properties of blastoderms and PGCs. In the results, a limited number of pluripotency genes, especially the core transcriptional network, were detected higher in both blastoderms and PGCs than CEFs. Next, we treated the CEFs, blastoderm cells, and PGCs with hydrogen peroxide (H₂O₂) for 1 h to induce DNA damage. Then, the H₂O₂ treated cells were incubated in fresh media for 3–12 h to observe DNA repair. Subsequent analyses in treated cells found that blastoderm cells and PGCs were more likely to undergo apoptosis along with the loss of pluripotency and less likely to undergo DNA repair, contrasting with CEFs. These properties of blastoderms and PGCs should be necessary to preserve genome stability during the development of early embryos and germ cells, respectively.

Precise genome editing across kingdoms of life using retron-derived DNA

Exogenous DNA can be a template to precisely edit a cell’s genome. However, the delivery of in vitro-produced DNA to target cells can be inefficient, and low abundance of template DNA may underlie the low rate of precise editing. One potential tool to produce template DNA inside cells is a retron, a bacterial retroelement involved in phage defense. However, little effort has been directed at optimizing retrons to produce designed sequences. Here, we identify modifications to the retron non-coding RNA that result in more abundant reverse transcribed DNA. By testing architectures of the retron operon that enable efficient reverse transcription, we find that gains in DNA production are portable from prokaryotic to eukaryotic cells and result in more efficient genome editing. Finally, we show that retron RT-DNA can be used to precisely edit cultured human cells. These experiments provide a general framework to produce DNA using retrons for genome modification.

The Advance of CRISPR-Cas9-Based and NIR/CRISPR-Cas9-Based Imaging System

The study of different genes, chromosomes and the spatiotemporal relationship between them is of great significance in the field of biomedicine. CRISPR-Cas9 has become the most widely used gene editing tool due to its excellent targeting ability. In recent years, a series of advanced imaging technologies based on Cas9 have been reported, providing fast and convenient tools for studying the sites location of genome, RNA, and chromatin. At the same time, a variety of CRISPR-Cas9-based imaging systems have been developed, which are widely used in real-time multi-site imaging in vivo. In this review, we summarized the component and mechanism of CRISPR-Cas9 system, overviewed the NIR imaging and the application of NIR fluorophores in the delivery of CRISPR-Cas9, and highlighted advances of the CRISPR-Cas9-based imaging system. In addition, we also discussed the challenges and potential solutions of CRISPR-Cas9-based imaging methods, and looked forward to the development trend of the field.

Targeted insertion of large genetic payloads using cas directed LINE-1 reverse transcriptase

A difficult genome editing goal is the site-specific insertion of large genetic constructs. Here we describe the GENEWRITE system, where site-specific targetable activity of Cas endonucleases is coupled with the reverse transcriptase activity of the ORF2p protein of the human retrotransposon LINE-1. This is accomplished by providing two RNAs: a guide RNA targeting Cas endonuclease activity and an appropriately designed payload RNA encoding the desired insertion. Using E. coli as a simple platform for development and deployment, we show that with proper payload design and co-expression of helper proteins, GENEWRITE can enable insertion of large genetic payloads to precise locations, although with off-target effects, using the described approach. Based upon these results, we describe a potential strategy for implementation of GENEWRITE in more complex systems.

A Bioluminescence Resonance Energy Transfer-Based Reporter System: Characterization and Applications

Genome editing strategies and DNA repair research need powerful analytical tools. We generated a bioluminescence resonance energy transfer (BRET)-based reporter for the quantification of indel frequencies induced by DNA repair. The BRET reporter, expressed as a single molecule, consists of a mutated Renilla reniformis luciferase domain and a GFP2 domain separated by a shuttle-cloning box for the integration of any given endonuclease target sequence. The luciferase activity acts both as energy donor and as the internal standard, while the loss of GFP2 fluorescence acts as a reporter for the out-of-frame sequence alterations that result from the DNA repair via the non-homologous end joining/microhomology-mediated end joining DNA repair pathways of the endonuclease-mediated DNA double-strand break. This results in a decrease of the fluorescence/luminescence ratio. Employing this reporter in different experimental scenarios, using different cell lines and diseases targeted, we quantified the influence of both protein knockdown of DNA repair pathways as well as guide RNA mismatches on CRISPR-mediated nuclease activity and subsequent repair based on mutagenic repair on the reporter. In conclusion, we demonstrated this BRET-based reporter to be a robust and sensitive analytical tool for assessment of variety of different genome editing-based approaches.

From Descriptive to Functional Genomics of Leukemias Focusing on Genome Engineering Techniques

Genome engineering makes the precise manipulation of DNA sequences possible in a cell. Therefore, it is essential for understanding gene function. Meganucleases were the start of genome engineering, and it continued with the discovery of Zinc finger nucleases (ZFNs), followed by Transcription activator-like effector nucleases (TALENs). They can generate double-strand breaks at a desired target site in the genome, and therefore can be used to knock in mutations or knock out genes in the same way. Years later, genome engineering was transformed by the discovery of clustered regularly interspaced short palindromic repeats (CRISPR). Implementation of CRISPR systems involves recognition guided by RNA and the precise cleaving of DNA molecules. This property proves its utility in epigenetics and genome engineering. CRISPR has been and is being continuously successfully used to model mutations in leukemic cell lines and control gene expression. Furthermore, it is used to identify targets and discover drugs for immune therapies. The descriptive and functional genomics of leukemias is discussed in this study, with an emphasis on genome engineering methods. The CRISPR/Cas9 system's challenges, viewpoints, limits, and solutions are also explored.

Homology-directed gene-editing approaches for hematopoietic stem and progenitor cell gene therapy

The advent of next-generation genome engineering tools like CRISPR-Cas9 has transformed the field of gene therapy, rendering targeted treatment for several incurable diseases. Hematopoietic stem and progenitor cells (HSPCs) continue to be the ideal target cells for gene manipulation due to their long-term repopulation potential. Among the gene manipulation strategies such as lentiviral gene augmentation, non-homologous end joining (NHEJ)-mediated gene editing, base editing and prime editing, only the homology-directed repair (HDR)-mediated gene editing provides the option of inserting a large transgene under its endogenous promoter or any desired locus. In addition, HDR-mediated gene editing can be applied for the gene knock-out, correction of point mutations and introduction of beneficial mutations. HSPC gene therapy studies involving lentiviral vectors and NHEJ-based gene-editing studies have exhibited substantial clinical progress. However, studies involving HDR-mediated HSPC gene editing have not yet progressed to the clinical testing. This suggests the existence of unique challenges in exploiting HDR pathway for HSPC gene therapy. Our review summarizes the mechanism, recent progresses, challenges, and the scope of HDR-based gene editing for the HSPC gene therapy.

Progress in Gene-Editing Technology of Zebrafish

As a vertebrate model, zebrafish (Danio rerio) plays a vital role in the field of life sciences. Recently, gene-editing technology has become increasingly innovative, significantly promoting scientific research on zebrafish. However, the implementation of these methods in a reasonable and accurate manner to achieve efficient gene-editing remains challenging. In this review, we systematically summarize the development and latest progress in zebrafish gene-editing technology. Specifically, we outline trends in double-strand break-free genome modification and the prospective applications of fixed-point orientation transformation of any base at any location through a multi-method approach.

Small molecule inhibition of ATM kinase increases CRISPR-Cas9 1-bp insertion frequency

Mutational outcomes following CRISPR-Cas9-nuclease cutting in mammalian cells have recently been shown to be predictable and, in certain cases, skewed toward single genotypes. However, the ability to control these outcomes remains limited, especially for 1-bp insertions, a common and therapeutically relevant class of repair outcomes. Here, through a small molecule screen, we identify the ATM kinase inhibitor KU-60019 as a compound capable of reproducibly increasing the fraction of 1-bp insertions relative to other Cas9 repair outcomes. Small molecule or genetic ATM inhibition increases 1-bp insertion outcome fraction across three human and mouse cell lines, two Cas9 species, and dozens of target sites, although concomitantly reducing the fraction of edited alleles. Notably, KU-60019 increases the relative frequency of 1-bp insertions to over 80% of edited alleles at several native human genomic loci and improves the efficiency of correction for pathogenic 1-bp deletion variants. The ability to increase 1-bp insertion frequency adds another dimension to precise template-free Cas9-nuclease genome editing.

The mutational outcome of CRISPR-Cas9 editing can be both predictable and targeted. Here the authors show that ATM inhibitor KU-60019 increases 1 bp insertions at the targeted locus.

Approaches to Enhance Precise CRISPR/Cas9-Mediated Genome Editing

Modification of the human genome has immense potential for preventing or treating disease. Modern genome editing techniques based on CRISPR/Cas9 show great promise for altering disease-relevant genes. The efficacy of precision editing at CRISPR/Cas9-induced double-strand breaks is dependent on the relative activities of nuclear DNA repair pathways, including the homology-directed repair and error-prone non-homologous end-joining pathways. The competition between multiple DNA repair pathways generates mosaic and/or therapeutically undesirable editing outcomes. Importantly, genetic models have validated key DNA repair pathways as druggable targets for increasing editing efficacy. In this review, we highlight approaches that can be used to achieve the desired genome modification, including the latest progress using small molecule modulators and engineered CRISPR/Cas proteins to enhance precision editing.

CRISPR-Cas systems: Challenges and future prospects

The advancement gained over the past couple of decades in clustered regularly interspaced short palindromic repeats and CRISPR associated proteins (CRISPR-Cas) systems have revolutionized the field of synthetic biology, therapeutics, diagnostics and metabolic engineering. The technique has enabled the process of genome editing to be very precise, rapid, cost-effective and highly efficient which were the downfalls for the previously debuted zinc finger nucleases (ZFN) and transcription activator-like effector nucleases (TALEN) technologies. However, despite its great potential, challenges including off-target activity, method of delivery, ethical and regulatory issues still remain unresolved for the CRISPR-Cas systems. In this chapter, we present and point out the obstacles faced in implementation of the CRISPR-Cas system along with its future prospects.

CRISPR Gene-Editing Models Geared Toward Therapy for Hereditary and Developmental Neurological Disorders

Hereditary or developmental neurological disorders (HNDs or DNDs) affect the quality of life and contribute to the high mortality rates among neonates. Most HNDs are incurable, and the search for new and effective treatments is hampered by challenges peculiar to the human brain, which is guarded by the near-impervious blood-brain barrier. Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR), a gene-editing tool repurposed from bacterial defense systems against viruses, has been touted by some as a panacea for genetic diseases. CRISPR has expedited the research into HNDs, enabling the generation of in vitro and in vivo models to simulate the changes in human physiology caused by genetic variation. In this review, we describe the basic principles and workings of CRISPR and the modifications that have been made to broaden its applications. Then, we review important CRISPR-based studies that have opened new doors to the treatment of HNDs such as fragile X syndrome and Down syndrome. We also discuss how CRISPR can be used to generate research models to examine the effects of genetic variation and caffeine therapy on the developing brain. Several drawbacks of CRISPR may preclude its use at the clinics, particularly the vulnerability of neuronal cells to the adverse effect of gene editing, and the inefficiency of CRISPR delivery into the brain. In concluding the review, we offer some suggestions for enhancing the gene-editing efficacy of CRISPR and how it may be morphed into safe and effective therapy for HNDs and other brain disorders.

De Novo Generation of Murine and Human MADR Recipient Cell Lines for Locus-Specific, Stable Integration of Transgenic Elements

Mosaic analysis by dual recombinase-mediated cassette exchange (MADR) is a technology that allows stable and locus-specific integration of transgenic elements into recipient cells carrying loxP and FRT sites. Nevertheless, most cell lines lack these recombination-specific sites. This protocol describes a method to introduce the minimum requirements into cells, leading to the generation of de novo primary MADR recipient cells or MADR "Proxy" cells. These cell lines allow the combinatorial use of a wide range of transgenic elements through MADR. For complete details on the use and execution of this protocol, please refer to Kim et al. (2019).

The development of genome editing tools as powerful techniques with versatile applications in biotechnology and medicine: CRISPR/Cas9, ZnF and TALE nucleases, RNA interference, and Cre/loxP

The huge progress in whole genome sequencing (genomic revolution) methods including next generation sequencing (NGS) techniques allows one to obtain data on genome sequences of all organisms, ranging from bacteria to plants to mammals, within hours to days (era of whole genome/exome sequencing) (Goodwin et al. in Nat Rev Genet 17:333–351, 2016; Levy and Myers in Annu Rev Genomics Hum Genet 17:95–115, 2016; Giani et al. in Comput Struct Biotechnol J 18:9–19, 2020). Today, within the era of functional genomics the highest goal is to transfer this huge amount of sequencing data into information of functional and clinical relevance (genome annotation project). The World Health Organization (WHO) estimates that more than 10,000 diseases in humans are monogenic, i.e., that these diseases are caused by mutations within single genes (Jackson et al. in Essays Biochem 62:643–723, 2018). NGS technologies are continuously improving while our knowledge on genetic mutations driving the development of diseases is also still emerging (Giani et al. in Comput Struct Biotechnol J 18:9–19, 2020). It would be desirable to have tools that allow one to correct these genetic mutations, so-called genome editing tools. Apart from applications in biotechnology, medicine, and agriculture, it is still not concisely understood in basic science how genotype influences phenotype. Firstly, the Cre/loxP system and RNA-based technologies for gene knockout or knockdown are explained. Secondly, zinc-finger (ZnF) nucleases and transcription activator-like effector nucleases (TALENs) are discussed as targeted genome editing systems. Thirdly, CRISPR/Cas is presented including outline of the discovery and mechanisms of this adaptive immune system in bacteria and archaea, structure and function of CRISPR/Cas9 and its application as a tool for genomic editing. Current developments and applications of CRISPR/Cas9 are discussed. Moreover, limitations and drawbacks of the CRISPR/Cas system are presented and questions on ethical concerns connected to application of genome editing tools are discussed.

CRISPR/Cas gene therapy

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated enzyme (Cas) is a naturally occurring genome editing tool adopted from the prokaryotic adaptive immune defense system. Currently, CRISPR/Cas9-based genome editing has been becoming one of the most promising tools for treating human genetic diseases, including cardiovascular diseases, neuro-disorders, and cancers. As the quick modification of the CRISPR/Cas9 system, including delivery system, CRISPR/Cas9-based gene therapy has been extensively studied in preclinic and clinic treatments. CRISPR/Cas genome editing is also a robust tool to create animal genetic models for studying and treating human genetic disorders, particularly diseases associated with point mutations. However, significant challenges also remain before CRISPR/Cas technology can be routinely employed in the clinic for treating different genetic diseases, which include toxicity and immune response of treated cells to CRISPR/Cas component, highly throughput delivery method, and potential off-target impact. The off-target effect is one of the major concerns for CRISPR/Cas9 gene therapy, more research should be focused on limiting this impact by designing high specific gRNAs and using high specificity of Cas enzymes. Modifying the CRISPR/Cas9 delivery method not only targets a specific tissue/cell but also potentially limits the off-target impact.

Methods Favoring Homology-Directed Repair Choice in Response to CRISPR/Cas9 Induced-Double Strand Breaks

Precise gene editing is-or will soon be-in clinical use for several diseases, and more applications are under development. The programmable nuclease Cas9, directed by a single-guide RNA (sgRNA), can introduce double-strand breaks (DSBs) in target sites of genomic DNA, which constitutes the initial step of gene editing using this novel technology. In mammals, two pathways dominate the repair of the DSBs-nonhomologous end joining (NHEJ) and homology-directed repair (HDR)-and the outcome of gene editing mainly depends on the choice between these two repair pathways. Although HDR is attractive for its high fidelity, the choice of repair pathway is biased in a biological context. Mammalian cells preferentially employ NHEJ over HDR through several mechanisms: NHEJ is active throughout the cell cycle, whereas HDR is restricted to S/G2 phases; NHEJ is faster than HDR; and NHEJ suppresses the HDR process. This suggests that definitive control of outcome of the programmed DNA lesioning could be achieved through manipulating the choice of cellular repair pathway. In this review, we summarize the DSB repair pathways, the mechanisms involved in choice selection based on DNA resection, and make progress in the research investigating strategies that favor Cas9-mediated HDR based on the manipulation of repair pathway choice to increase the frequency of HDR in mammalian cells. The remaining problems in improving HDR efficiency are also discussed. This review should facilitate the development of CRISPR/Cas9 technology to achieve more precise gene editing.

Variability in Genome Editing Outcomes: Challenges for Research Reproducibility and Clinical Safety

Genome editing tools have already revolutionized biomedical research and are also expected to have an important impact in the clinic. However, their extensive use in research has revealed much unpredictability, both off and on target, in the outcome of their application. We discuss the challenges associated with this unpredictability, both for research and in the clinic. For the former, an extensive validation of the model is essential. For the latter, potential unpredicted activity does not preclude the use of these tools but requires that molecular evidence to underpin the relevant risk:benefit evaluation is available. Safe and successful clinical application will also depend on the mode of delivery and the cellular context.

Challenges associated with homologous directed repair using CRISPR-Cas9 and TALEN to edit the DMD genetic mutation in canine Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene that abolish the expression of dystrophin protein. Dogs with the genetic homologue, golden retriever muscular dystrophy dog (GRMD), have a splice site mutation that leads to skipping of exon 7 and a stop codon in the DMD transcript. Gene editing via homology-directed repair (HDR) has been used in the mdx mouse model of DMD but not in GRMD. In this study, we used clustered regularly interspaced short palindromic repeats (CRISPR) and transcription activator-like effector nucleases (TALEN) to restore dystrophin expression via HDR in myoblasts/myotubes and later via intramuscular injection of GRMD dogs. In vitro, DNA and RNA were successfully corrected but dystrophin protein was not translated. With intramuscular injection of two different guide arms, sgRNA A and B, there was mRNA expression and Sanger sequencing confirmed inclusion of exon 7 for all treatments. On Western blot analysis, protein expression of up to 6% of normal levels was seen in two dogs injected with sgRNA B and up to 16% of normal in one dog treated with sgRNA A. TALEN did not restore any dystrophin expression. While there were no adverse effects, clear benefits were not seen on histopathologic analysis, immunofluorescence microscopy, and force measurements. Based on these results, methods must be modified to increase the efficiency of HDR-mediated gene repair and protein expression.

Efficient knock-in at the chicken ovalbumin locus using adenovirus as a CRISPR/Cas9 delivery system

In this study, efficient knock-in (KI) of human epidermal growth factor (hEGF) cDNA at the ovalbumin (OV) locus in cultured chicken cells was achieved using adenovirus as a delivery for CRISPR/Cas9 elements and optimizing donor vector construction. The strategy of recruiting donor DNA to the insertion site further improved the KI efficiency. The inserted hEGF cDNA can expressed in primary oviduct cells and secreted hEGF promoted proliferation of Hela cells. Moreover, we achieved efficient KI in blastoderm cells without altering their induction in vitro and obtained germline chimeric KI chicken embryos by transplanting KI blastoderm cells as well as injecting adenovirus directly, in vivo. Our results provided an efficient KI method for chicken cells and embryos, and lay the foundation for more convenient production of KI chicken at the OV locus, which will promote the development of oviduct-specific bioreactor.

Ku complex suppresses recombination in the absence of MRX activity during budding yeast meiosis

During meiosis, programmed double-strand breaks (DSBs) are repaired via recombination pathways that are required for faithful chromosomal segregation and genetic diversity. In meiotic progression, the non-homologous end joining (NHEJ) pathway is suppressed and instead meiotic recombination initiated by nucleolytic resection of DSB ends is the major pathway employed. This requires diverse recombinase proteins and regulatory factors involved in the formation of crossovers (COs) and non-crossovers (NCOs). In mitosis, spontaneous DSBs occurring at the G1 phase are predominantly repaired via NHEJ, mediating the joining of DNA ends. The Ku complex binds to these DSB ends, inhibiting additional DSB resection and mediating end joining with Dnl4, Lif1, and Nej1, which join the Ku complex and DSB ends. Here, we report the role of the Ku complex in DSB repair using a physical analysis of recombination in Saccharomyces cerevisiae during meiosis. We found that the Ku complex is not essential for meiotic progression, DSB formation, joint molecule formation, or CO/NCO formation during normal meiosis. Surprisingly, in the absence of the Ku complex and functional Mre11-Rad50-Xrs2 (MRX) complex, a large portion of meiotic DSBs was repaired via the recombination pathway to form COs and NCOs. Our data suggested that Ku complex prevents meiotic recombination in the elimination of MRX activity.

The giant titin: how to evaluate its role in cardiomyopathies

Titin, the largest protein known, has attracted a lot of interest in the cardiovascular field in recent years, since the discovery that truncating variants in titin are commonly found in patients with dilated cardiomyopathy. This review will discuss the contribution of variants in titin to inherited cardiac conditions (cardiomyopathies) and how model systems, such as animals and cellular systems, can help to provide insights into underlying disease mechanisms. It will also give an outlook onto exciting technological developments, such as in the field of CRISPR, which may facilitate future research on titin variants and their contributions to cardiomyopathies.

A CRISPR/Cas9 based polymeric nanoparticles to treat/inhibit microbial infections

The latest breakthrough towards the adequate and decisive methods of gene editing tools provided by CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeat/CRISPR Associated System), has been repurposed into a tool for genetically engineering eukaryotic cells and now considered as the major innovation in gene-related disorders. Nanotechnology has provided an alternate way to overcome the conventional problems where methods to deliver therapeutic agents have failed. The use of nanotechnology has the potential to safe-side the CRISPR/Cas9 components delivery by using customized polymeric nanoparticles for safety and efficacy. The pairing of two (CRISPR/Cas9 and nanotechnology) has the potential for opening new avenues in therapeutic use. In this review, we will discuss the most recent advances in developing nanoparticle-based CRISPR/Cas9 gene editing cargo delivery with a focus on several polymeric nanoparticles including fabrication proposals to combat microbial infections.