Tissue- and time-directed electroporation of CAS9 protein-gRNA complexes in vivo yields efficient multigene knockout for studying gene function in regeneration

Sall4 regulates downstream patterning genes during limb regeneration

Many salamanders can completely regenerate a fully functional limb. Limb regeneration is a carefully coordinated process involving several defined stages. One key event during the regeneration process is the patterning of the blastema to inform cells of what they must differentiate into. Although it is known that many genes involved in the initial development of the limb are re-used during regeneration, the exact molecular circuitry involved in this process is not fully understood. Several large-scale transcriptional profiling studies of axolotl limb regeneration have identified many transcription factors that are up-regulated after limb amputation. Sall4 is a transcription factor that has been identified to play essential roles in maintaining cells in an undifferentiated state during development and also plays a unique role in limb development. Inactivation of Sall4 during limb bud development results in defects in anterior-posterior patterning of the limb. Sall4 has been found to be up-regulated during limb regeneration in both Xenopus and salamanders, but to date it function has been untested. We confirmed that Sall4 is up-regulated during limb regeneration in the axolotl using qRT-PCR and identified that it is present in the skin cells and also in cells within the blastema. Using CRISPR technology we microinjected gRNAs specific for Sall4 complexed with cas9 protein into the blastema to specifically knockout Sall4 in blastema cells only. This resulted in limb regenerate defects, including missing digits, fusion of digit elements, and defects in the radius and ulna. This suggests that during regeneration Sall4 may play a similar role in regulating the specification of anterior-proximal skeletal elements.

Tail and Spinal Cord Regeneration in Urodelean Amphibians

Urodelean amphibians can regenerate the tail and the spinal cord (SC) and maintain this ability throughout their life. This clearly distinguishes these animals from mammals. The phenomenon of tail and SC regeneration is based on the capability of cells involved in regeneration to dedifferentiate, enter the cell cycle, and change their (or return to the pre-existing) phenotype during de novo organ formation. The second critical aspect of the successful tail and SC regeneration is the mutual molecular regulation by tissues, of which the SC and the apical wound epidermis are the leaders. Molecular regulatory systems include signaling pathways components, inflammatory factors, ECM molecules, ROS, hormones, neurotransmitters, HSPs, transcriptional and epigenetic factors, etc. The control, carried out by regulatory networks on the feedback principle, recruits the mechanisms used in embryogenesis and accompanies all stages of organ regeneration, from the moment of damage to the completion of morphogenesis and patterning of all its structures. The late regeneration stages and the effects of external factors on them have been poorly studied. A new model for addressing this issue is herein proposed. The data summarized in the review contribute to understanding a wide range of fundamentally important issues in the regenerative biology of tissues and organs in vertebrates including humans.

Revolutionising healing: Gene Editing's breakthrough against sickle cell disease

Recent advancements in gene editing illuminate new potential therapeutic approaches for Sickle Cell Disease (SCD), a debilitating monogenic disorder caused by a point mutation in the β-globin gene. Despite the availability of several FDA-approved medications for symptomatic relief, allogeneic hematopoietic stem cell transplantation (HSCT) remains the sole curative option, underscoring a persistent need for novel treatments. This review delves into the growing field of gene editing, particularly the extensive research focused on curing haemoglobinopathies like SCD. We examine the use of techniques such as CRISPR-Cas9 and homology-directed repair, base editing, and prime editing to either correct the pathogenic variant into a non-pathogenic or wild-type one or augment fetal haemoglobin (HbF) production. The article elucidates ways to optimize these tools for efficacious gene editing with minimal off-target effects and offers insights into their effective delivery into cells. Furthermore, we explore clinical trials involving alternative SCD treatment strategies, such as LentiGlobin therapy and autologous HSCT, distilling the current findings. This review consolidates vital information for the clinical translation of gene editing for SCD, providing strategic insights for investigators eager to further the development of gene editing for SCD.

A Practical Guide for CRISPR-Cas9-Induced Mutations in Axolotls

Clustered regularly interspaced short palindromic repeats (CRISPR) is a powerful tool that enables editing of the axolotl genome. In this chapter, we will cover how to retrieve gene sequences, confirm annotation, design CRISPR targets, analyze indels, and screen for Crispant axolotls. This is a comprehensive guide on how to use CRISPR on your favorite gene and gain insights into its function.

Establishing an Efficient Electroporation-Based Method to Manipulate Target Gene Expression in the Axolotl Brain

The tetrapod salamander species axolotl (Ambystoma mexicanum) is capable of regenerating injured brain. For better understanding the mechanisms of brain regeneration, it is very necessary to establish a rapid and efficient gain-of-function and loss-of-function approaches to study gene function in the axolotl brain. Here, we establish and optimize an electroporation-based method to overexpress or knockout/knockdown target gene in ependymal glial cells (EGCs) in the axolotl telencephalon. By orientating the electrodes, we were able to achieve specific expression of EGFP in EGCs located in dorsal, ventral, medial, or lateral ventricular zones. We then studied the role of Cdc42 in brain regeneration by introducing Cdc42 into EGCs through electroporation, followed by brain injury. Our findings showed that overexpression of Cdc42 in EGCs did not significantly affect EGC proliferation and production of newly born neurons, but it disrupted their apical polarity, as indicated by the loss of the ZO-1 tight junction marker. This disruption led to a ventricular accumulation of newly born neurons, which are failed to migrate into the neuronal layer where they could mature, thus resulted in a delayed brain regeneration phenotype. Furthermore, when electroporating CAS9-gRNA protein complexes against TnC (Tenascin-C) into EGCs of the brain, we achieved an efficient knockdown of TnC. In the electroporation-targeted area, TnC expression is dramatically reduced at both mRNA and protein levels. Overall, this study established a rapid and efficient electroporation-based gene manipulation approach allowing for investigation of gene function in the process of axolotl brain regeneration.

Applying a Knock-In Strategy to Create Reporter-Tagged Knockout Alleles in Axolotls

Tetrapod species axolotls exhibit the powerful capacity to fully regenerate their tail and limbs upon injury, hence serving as an excellent model organism in regenerative biology research. Developing proper molecular and genetic tools in axolotls is an absolute necessity for deep dissection of tissue regeneration mechanisms. Previously, CRISPR-/Cas9-based knockout and targeted gene knock-in approaches have been established in axolotls, allowing genetically deciphering gene function, labeling, and tracing particular types of cells. Here, we further extend the CRISPR/Cas9 technology application and describe a method to create reporter-tagged knockout allele in axolotls. This method combines gene knockout and knock-in and achieves loss of function of a given gene and simultaneous labeling of cells expressing this particular gene, that allows identification, tracking of the "knocking out" cells. Our method offers a useful gene function analysis tool to the field of axolotl developmental and regenerative research.

CRISPR/Cas systems: Delivery and application in gene therapy

The CRISPR/Cas systems in prokaryotes such as bacteria and archaea are the adaptive immune system to prevent infection from viruses, phages, or other foreign substances. When viruses or phages first invade the bacteria, Cas proteins recognize and cut the DNA from viruses or phages into short fragments that will be integrated into the CRISPR array. Once bacteria are invaded again, the modified CRISPR and Cas proteins react quickly to cut DNA at the specified target location, protecting the host. Due to its high efficiency, versatility, and simplicity, the CRISPR/Cas system has become one of the most popular gene editing technologies. In this review, we briefly introduce the CRISPR/Cas systems, focus on several delivery methods including physical delivery, viral vector delivery, and non-viral vector delivery, and the applications of disease therapy. Finally, some problems in CRISPR/Cas9 technology have been proposed, such as the off-target effects, the efficiency of DNA repair mechanisms, and delivery of CRISPR/Cas system safely and efficiently to the target location.

Regulation of stem cell identity by miR-200a during spinal cord regeneration

Axolotls are an important model organism for multiple types of regeneration, including functional spinal cord regeneration. Remarkably, axolotls can repair their spinal cord after a small lesion injury and can also regenerate their entire tail following amputation. Several classical signaling pathways that are used during development are reactivated during regeneration, but how this is regulated remains a mystery. We have previously identified miR-200a as a key factor that promotes successful spinal cord regeneration. Here, using RNA-seq analysis, we discovered that the inhibition of miR-200a results in an upregulation of the classical mesodermal marker brachyury in spinal cord cells after injury. However, these cells still express the neural stem cell marker sox2. In vivo cell tracking allowed us to determine that these cells can give rise to cells of both the neural and mesoderm lineage. Additionally, we found that miR-200a can directly regulate brachyury via a seed sequence in the 3′UTR of the gene. Our data indicate that miR-200a represses mesodermal cell fate after a small lesion injury in the spinal cord when only glial cells and neurons need to be replaced.

Summary: Axolotl spinal cord cells have the potential to form cells of the ectoderm and mesoderm depending on the extent of the injury they are responding to.

The Axolotl's journey to the modern molecular era

The salamander Ambystoma mexicanum, commonly called "the axolotl" has a long, illustrious history as a model organism, perhaps with one of the longest track records as a laboratory-bred vertebrate, yet it also holds a prominent place among the emerging model organisms. Or rather it is by now an "emerged" model organism, boasting a full cohort molecular genetic tools that allows an expanding community of researchers in the field to explore the remarkable traits of this animal including regeneration, at cellular and molecular precision-which had been a dream for researchers over the years. This chapter describes the journey to this status, that could be helpful for those developing their respective animal or plant models.

Editing SOX Genes by CRISPR-Cas: Current Insights and Future Perspectives

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and its associated proteins (Cas) is an adaptive immune system in archaea and most bacteria. By repurposing these systems for use in eukaryote cells, a substantial revolution has arisen in the genome engineering field. In recent years, CRISPR-Cas technology was rapidly developed and different types of DNA or RNA sequence editors, gene activator or repressor, and epigenome modulators established. The versatility and feasibility of CRISPR-Cas technology has introduced this system as the most suitable tool for discovering and studying the mechanism of specific genes and also for generating appropriate cell and animal models. SOX genes play crucial roles in development processes and stemness. To elucidate the exact roles of SOX factors and their partners in tissue hemostasis and cell regeneration, generating appropriate in vitro and in vivo models is crucial. In line with these premises, CRISPR-Cas technology is a promising tool for studying different family members of SOX transcription factors. In this review, we aim to highlight the importance of CRISPR-Cas and summarize the applications of this novel, promising technology in studying and decoding the function of different members of the SOX gene family.

Spatiotemporal control of cell cycle acceleration during axolotl spinal cord regeneration

Axolotls are uniquely able to resolve spinal cord injuries, but little is known about the mechanisms underlying spinal cord regeneration. We previously found that tail amputation leads to reactivation of a developmental-like program in spinal cord ependymal cells (Rodrigo Albors et al., 2015), characterized by a high-proliferation zone emerging 4 days post-amputation (Rost et al., 2016). What underlies this spatiotemporal pattern of cell proliferation, however, remained unknown. Here, we use modeling, tightly linked to experimental data, to demonstrate that this regenerative response is consistent with a signal that recruits ependymal cells during ~85 hours after amputation within ~830 μm of the injury. We adapted Fluorescent Ubiquitination-based Cell Cycle Indicator (FUCCI) technology to axolotls (AxFUCCI) to visualize cell cycles in vivo. AxFUCCI axolotls confirmed the predicted appearance time and size of the injury-induced recruitment zone and revealed cell cycle synchrony between ependymal cells. Our modeling and imaging move us closer to understanding bona fide spinal cord regeneration.

The use of transgenics in the laboratory axolotl

The ability to generate transgenic animals sparked a wave of research committed to implementing such technology in a wide variety of model organisms. Building a solid base of ubiquitous and tissue-specific reporter lines has set the stage for later interrogations of individual cells or genetic elements. Compared to other widely used model organisms such as mice, zebrafish and fruit flies, there are only a few transgenic lines available in the laboratory axolotl (Ambystoma mexicanum), although their number is steadily expanding. In this review, we discuss a brief history of the transgenic methodologies in axolotl and their advantages and disadvantages. Next, we discuss available transgenic lines and insights we have been able to glean from them. Finally, we list challenges when developing transgenic axolotl, and where further work is needed in order to improve their standing as both a developmental and regenerative model.

Salamanders: The molecular basis of tissue regeneration and its relevance to human disease

Salamanders are recognized for their ability to regenerate a broad range of tissues. They have also have been used for hundreds of years for classical developmental biology studies because of their large accessible embryos. The range of tissues these animals can regenerate is fascinating, from full limbs to parts of the brain or heart, a potential that is missing in humans. Many promising research efforts are working to decipher the molecular blueprints shared across the organisms that naturally have the capacity to regenerate different tissues and organs. Salamanders are an excellent example of a vertebrate that can functionally regenerate a wide range of tissue types. In this review, we outline some of the significant insights that have been made that are aiding in understanding the cellular and molecular mechanisms of tissue regeneration in salamanders and discuss why salamanders are a worthy model in which to study regenerative biology and how this may benefit research fields like regenerative medicine to develop therapies for humans in the future.

Towards comparative analyses of salamander limb regeneration

Among tetrapods, only salamanders can regenerate their limbs and tails throughout life. This amazing regenerative ability has attracted the attention of scientists for hundreds of years. Now that large, salamander genomes are beginning to be sequenced for the first time, omics tools and approaches can be used to integrate new perspectives into the study of tissue regeneration. Here we argue the need to move beyond the primary salamander models to investigate regeneration in other species. Salamanders at first glance come across as a phylogenetically conservative group that has not diverged greatly from their ancestors. While salamanders do present ancestral characteristics of basal tetrapods, including the ability to regenerate limbs, data from fossils and data from studies that have tested for species differences suggest there may be considerable variation in how salamanders develop and regenerate their limbs. We review the case for expanded studies of salamander tissue regeneration and identify questions and approaches that are most likely to reveal commonalities and differences in regeneration among species. We also address challenges that confront such an initiative, some of which are regulatory and not scientific. The time is right to gain evolutionary perspective about mechanisms of tissue regeneration from comparative studies of salamander species.

Delivery of CRISPR/Cas systems for cancer gene therapy and immunotherapy

The clustered, regularly interspaced, short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems are efficient and versatile gene editing tools, which offer enormous potential to treat cancer by editing genome, transcriptome or epigenome of tumor cells and/or immune cells. A large body of works have been done with CRISPR/Cas systems for genetic modification, and 16 clinical trials were conducted to treat cancer by ex vivo or in vivo gene editing approaches. Now, promising preclinical works have begun using CRISPR/Cas systems in vivo. However, efficient and safe delivery of CRISPR/Cas systems in vivo is still a critical challenge for their clinical applications. This article summarizes delivery of CRISPR/Cas systems by physical methods, viral vectors and non-viral vectors for cancer gene therapy and immunotherapy. The prospects for the development of physical methods, viral vectors and non-viral vectors for delivery of CRISPR/Cas systems are reviewed, and promising advances in cancer treatment using CRISPR/Cas systems are discussed.

Walking with Salamanders: From Molecules to Biorobotics

How do four-legged animals adapt their locomotion to the environment? How do central and peripheral mechanisms interact within the spinal cord to produce adaptive locomotion and how is locomotion recovered when spinal circuits are perturbed? Salamanders are the only tetrapods that regenerate voluntary locomotion after full spinal transection. Given their evolutionary position, they provide a unique opportunity to bridge discoveries made in fish and mammalian models. Genetic dissection of salamander neural circuits is becoming feasible with new methods for precise manipulation, elimination, and visualisation of cells. These approaches can be combined with classical tools in neuroscience and with modelling and a robotic environment. We propose that salamanders provide a blueprint of the function, evolution, and regeneration of tetrapod locomotor circuits.

Midkine is a dual regulator of wound epidermis development and inflammation during the initiation of limb regeneration

Formation of a specialized wound epidermis is required to initiate salamander limb regeneration. Yet little is known about the roles of the early wound epidermis during the initiation of regeneration and the mechanisms governing its development into the apical epithelial cap (AEC), a signaling structure necessary for outgrowth and patterning of the regenerate. Here, we elucidate the functions of the early wound epidermis, and further reveal midkine (mk) as a dual regulator of both AEC development and inflammation during the initiation of axolotl limb regeneration. Through loss- and gain-of-function experiments, we demonstrate that mk acts as both a critical survival signal to control the expansion and function of the early wound epidermis and an anti-inflammatory cytokine to resolve early injury-induced inflammation. Altogether, these findings unveil one of the first identified regulators of AEC development and provide fundamental insights into early wound epidermis function, development, and the initiation of limb regeneration.

A Comparative Perspective on Brain Regeneration in Amphibians and Teleost Fish

Regeneration of lost cells in the central nervous system, especially the brain, is present to varying degrees in different species. In mammals, neuronal cell death often leads to glial cell hypertrophy, restricted proliferation, and formation of a gliotic scar, which prevents neuronal regeneration. Conversely, amphibians such as frogs and salamanders and teleost fish possess the astonishing capacity to regenerate lost cells in several regions of their brains. While frogs lose their regenerative abilities after metamorphosis, teleost fish and salamanders are known to possess regenerative competence even throughout adulthood. In the last decades, substantial progress has been made in our understanding of the cellular and molecular mechanisms of brain regeneration in amphibians and fish. But how similar are the means of brain regeneration in these different species? In this review, we provide an overview of common and distinct aspects of brain regeneration in frog, salamander, and teleost fish species: from the origin of regenerated cells to the functional recovery of behaviors.

Spinal Cord Regeneration in Amphibians: A Historical Perspective

In some vertebrates, a grave injury to the central nervous system (CNS) results in functional restoration, rather than in permanent incapacitation. Understanding how these animals mount a regenerative response by activating resident CNS stem cell populations is of critical importance in regenerative biology. Amphibians are of a particular interest in the field because the regenerative ability is present throughout life in urodele species, but in anuran species it is lost during development. Studying amphibians, who transition from a regenerative to a nonregenerative state, could give insight into the loss of ability to recover from CNS damage in mammals. Here, we highlight the current knowledge of spinal cord regeneration across vertebrates and identify commonalities and differences in spinal cord regeneration between amphibians.

Insights into Electroreceptor Development and Evolution from Molecular Comparisons with Hair Cells

The vertebrate lateral line system comprises a mechanosensory division, with neuromasts containing hair cells that detect local water movement ("distant touch"); and an electrosensory division, with electrosensory organs that detect the weak, low-frequency electric fields surrounding other animals in water (primarily used for hunting). The entire lateral line system was lost in the amniote lineage with the transition to fully terrestrial life; the electrosensory division was lost independently in several lineages, including the ancestors of frogs and of teleost fishes. (Electroreception with different characteristics subsequently evolved independently within two teleost lineages.) Recent gene expression studies in a non-teleost actinopterygian fish suggest that electroreceptor ribbon synapses employ the same transmission mechanisms as hair cell ribbon synapses, and show that developing electrosensory organs express transcription factors essential for hair cell development, including Atoh1 and Pou4f3. Previous hypotheses for electroreceptor evolution suggest either that electroreceptors and hair cells evolved independently in the vertebrate ancestor from a common ciliated secondary cell, or that electroreceptors evolved from hair cells. The close developmental and putative physiological similarities implied by the gene expression data support the latter hypothesis, i.e., that electroreceptors evolved in the vertebrate ancestor as a "sister cell-type" to lateral line hair cells.

A Regeneration Toolkit

The ability of animals to replace injured body parts has been a subject of fascination for centuries. The emerging importance of regenerative medicine has reinvigorated investigations of innate tissue regeneration, and the development of powerful genetic tools has fueled discoveries into how tissue regeneration occurs. Here, we present an overview of the armamentarium employed to probe regeneration in vertebrates, highlighting areas where further methodology advancement will deepen mechanistic findings.

Application and optimization of CRISPR-Cas9-mediated genome engineering in axolotl (Ambystoma mexicanum)

Genomic manipulation is essential to the use of model organisms to understand development, regeneration and adult physiology. The axolotl (Ambystoma mexicanum), a type of salamander, exhibits an unparalleled regenerative capability in a spectrum of complex tissues and organs, and therefore serves as a powerful animal model for dissecting mechanisms of regeneration. We describe here an optimized stepwise protocol to create genetically modified axolotls using the CRISPR-Cas9 system. The protocol, which takes 7-8 weeks to complete, describes generation of targeted gene knockouts and knock-ins and includes site-specific integration of large targeting constructs. The direct use of purified CAS9-NLS (CAS9 containing a C-terminal nuclear localization signal) protein allows the prompt formation of guide RNA (gRNA)-CAS9-NLS ribonucleoprotein (RNP) complexes, which accelerates the creation of double-strand breaks (DSBs) at targeted genomic loci in single-cell-stage axolotl eggs. With this protocol, a substantial number of F0 individuals harboring a homozygous-type frameshift mutation can be obtained, allowing phenotype analysis in this generation. In the presence of targeting constructs, insertions of exogenous genes into targeted axolotl genomic loci can be achieved at efficiencies of up to 15% in a non-homologous end joining (NHEJ) manner. Our protocol bypasses the long generation time of axolotls and allows direct functional analysis in F0 genetically manipulated axolotls. This protocol can be potentially applied to other animal models, especially to organisms with a well-characterized transcriptome but lacking a well-characterized genome.

Nerves and Proliferation of Progenitor Cells in Limb Regeneration

Nerves, in conjunction with the apical epidermal cap (AEC), play an important role in the proliferation of the mesenchymal progenitor cells comprising the blastema of regenerating urodele amphibian limbs. Reinnervation after amputation requires factors supplied by the forming blastema, and neurotrophic factors must be present at or above a quantitative threshold for mitosis of the blastema cells. The AEC forms independently of nerves, but requires nerves to be maintained. Urodele limb buds are independent of nerves for regeneration, but innervation imposes a regenerative requirement for nerve factors on their cells as they differentiate. There are three main ideas on the functional relationship between nerves, AEC, and blastema cells: (1) nerves and AEC produce factors with different roles in maintaining progenitor status and mitosis; (2) the AEC produces the factors that promote blastema cell mitosis, but requires nerves to express them; (3) blastema cells, nerves, and AEC all produce the same factor(s) that additively attain the required threshold for mitosis.

Pseudotyped baculovirus is an effective gene expression tool for studying molecular function during axolotl limb regeneration

Axolotls can regenerate complex structures through recruitment and remodeling of cells within mature tissues. Accessing the underlying mechanisms at a molecular resolution is crucial to understand how injury triggers regeneration and how it proceeds. However, gene transformation in adult tissues can be challenging. Here we characterize the use of pseudotyped baculovirus (BV) as an effective gene transfer method both for cells within mature limb tissue and within the blastema. These cells remain competent to participate in regeneration after transduction. We further characterize the effectiveness of BV for gene overexpression studies by overexpressing Shh in the blastema, which yields a high penetrance of classic polydactyly phenotypes. Overall, our work establishes BV as a powerful tool to access gene function in axolotl limb regeneration.

Efficient gene knockin in axolotl and its use to test the role of satellite cells in limb regeneration

Salamanders exhibit extensive regenerative capacities and serve as a unique model in regeneration research. However, due to the lack of targeted gene knockin approaches, it has been difficult to label and manipulate some of the cell populations that are crucial for understanding the mechanisms underlying regeneration. Here we have established highly efficient gene knockin approaches in the axolotl (Ambystoma mexicanum) based on the CRISPR/Cas9 technology. Using a homology-independent method, we successfully inserted both the Cherry reporter gene and a larger membrane-tagged Cherry-ERT2-Cre-ERT2 (∼5-kb) cassette into axolotl Sox2 and Pax7 genomic loci. Depending on the size of the DNA fragments for integration, 5-15% of the F0 transgenic axolotl are positive for the transgene. Using these techniques, we have labeled and traced the PAX7-positive satellite cells as a major source contributing to myogenesis during axolotl limb regeneration. Our work brings a key genetic tool to molecular and cellular studies of axolotl regeneration.

Salamander spinal cord regeneration: The ultimate positive control in vertebrate spinal cord regeneration

Repairing injured tissues / organs is one of the major challenges for the maintenance of proper organ function in adulthood. In mammals, the central nervous system including the spinal cord, once established during embryonic development, has very limited capacity to regenerate. In contrast, salamanders such as axolotls can fully regenerate the injured spinal cord, making this a very powerful vertebrate model system for studying this process. Here we discuss the cellular and molecular requirements for spinal cord regeneration in the axolotl. The recent development of tools to test molecular function, including CRISPR-mediated gene editing, has lead to the identification of key players involved in the cell response to injury that ultimately leads to outgrowth of neural stem cells that are competent to replay the process of spinal cord development to replace the damaged/missing tissue.

Delivery strategies of the CRISPR-Cas9 gene-editing system for therapeutic applications

The CRISPR-Cas9 genome-editing system is a part of the adaptive immune system in archaea and bacteria to defend against invasive nucleic acids from phages and plasmids. The single guide RNA (sgRNA) of the system recognizes its target sequence in the genome, and the Cas9 nuclease of the system acts as a pair of scissors to cleave the double strands of DNA. Since its discovery, CRISPR-Cas9 has become the most robust platform for genome engineering in eukaryotic cells. Recently, the CRISPR-Cas9 system has triggered enormous interest in therapeutic applications. CRISPR-Cas9 can be applied to correct disease-causing gene mutations or engineer T cells for cancer immunotherapy. The first clinical trial using the CRISPR-Cas9 technology was conducted in 2016. Despite the great promise of the CRISPR-Cas9 technology, several challenges remain to be tackled before its successful applications for human patients. The greatest challenge is the safe and efficient delivery of the CRISPR-Cas9 genome-editing system to target cells in human body. In this review, we will introduce the molecular mechanism and different strategies to edit genes using the CRISPR-Cas9 system. We will then highlight the current systems that have been developed to deliver CRISPR-Cas9 in vitro and in vivo for various therapeutic purposes.