Chimeric TALE recombinases with programmable DNA sequence specificity

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.

Recent advances in CRISPR-Cas9-based genome insertion technologies

Programmable genome insertion (or knock-in) is vital for both fundamental and translational research. The continuously expanding number of CRISPR-based genome insertion strategies demonstrates the ongoing development in this field. Common methods for site-specific genome insertion rely on cellular double-strand breaks repair pathways, such as homology-directed repair, non-homologous end-joining, and microhomology-mediated end joining. Recent advancements have further expanded the toolbox of programmable genome insertion techniques, including prime editing, integrase coupled with programmable nuclease, and CRISPR-associated transposon. These tools possess their own capabilities and limitations, promoting tremendous efforts to enhance editing efficiency, broaden targeting scope and improve editing specificity. In this review, we first summarize recent advances in programmable genome insertion techniques. We then elaborate on the cons and pros of each technique to assist researchers in making informed choices when using these tools. Finally, we identify opportunities for future improvements and applications in basic research and therapeutics.

Activation of recombinases at specific DNA loci by zinc-finger domain insertions

Recombinases have several potential advantages as genome editing tools compared to nucleases and other editing enzymes, but the process of engineering them to efficiently recombine predetermined DNA targets demands considerable investment of time and labor. Here we sought to harness zinc-finger DNA-binding domains (ZFDs) to program recombinase binding by developing fusions, in which ZFDs are inserted into recombinase coding sequences. By screening libraries of hybrid proteins, we optimized the insertion site, linker length, spacing and ZFD orientation and generated Cre-type recombinases that remain dormant unless the insertionally fused ZFD binds its target site placed in the vicinity of the recombinase binding site. The developed fusion improved targeted editing efficiencies of recombinases by four-fold and abolished measurable off-target activity in mammalian cells. The ZFD-dependent activity is transferable to a recombinase with relaxed specificity, providing the means for developing fully programmable recombinases. Our engineered recombinases provide improved genome editing tools with increased precision and efficiency.

Drag-and-drop genome insertion of large sequences without double-strand DNA cleavage using CRISPR-directed integrases

Programmable genome integration of large, diverse DNA cargo without DNA repair of exposed DNA double-strand breaks remains an unsolved challenge in genome editing. We present programmable addition via site-specific targeting elements (PASTE), which uses a CRISPR-Cas9 nickase fused to both a reverse transcriptase and serine integrase for targeted genomic recruitment and integration of desired payloads. We demonstrate integration of sequences as large as ~36 kilobases at multiple genomic loci across three human cell lines, primary T cells and non-dividing primary human hepatocytes. To augment PASTE, we discovered 25,614 serine integrases and cognate attachment sites from metagenomes and engineered orthologs with higher activity and shorter recognition sequences for efficient programmable integration. PASTE has editing efficiencies similar to or exceeding those of homology-directed repair and non-homologous end joining-based methods, with activity in non-dividing cells and in vivo with fewer detectable off-target events. PASTE expands the capabilities of genome editing by allowing large, multiplexed gene insertion without reliance on DNA repair pathways.

Structural basis for topological regulation of Tn3 resolvase

Site-specific DNA recombinases play a variety of biological roles, often related to the dissemination of antibiotic resistance, and are also useful synthetic biology tools. The simplest site-specific recombination systems will recombine any two cognate sites regardless of context. Other systems have evolved elaborate mechanisms, often sensing DNA topology, to ensure that only one of multiple possible recombination products is produced. The closely related resolvases from the Tn3 and γδ transposons have historically served as paradigms for the regulation of recombinase activity by DNA topology. However, despite many proposals, models of the multi-subunit protein-DNA complex (termed the synaptosome) that enforces this regulation have been unsatisfying due to a lack of experimental constraints and incomplete concordance with experimental data. Here, we present new structural and biochemical data that lead to a new, detailed model of the Tn3 synaptosome, and discuss how it harnesses DNA topology to regulate the enzymatic activity of the recombinase.

Emerging functions of mitochondria-encoded noncoding RNAs

Mitochondria, organelles that harbor their own circular genomes, are critical for energy production and homeostasis maintenance in eukaryotic cells. Recent studies discovered hundreds of mitochondria-encoded noncoding RNAs (mt-ncRNAs), including novel subtypes of mitochondria-encoded circular RNAs (mecciRNAs) and mitochondria-encoded double-stranded RNAs (mt-dsRNAs). Here, we discuss the emerging field of mt-ncRNAs by reviewing their expression patterns, biogenesis, metabolism, regulatory roles, and functional mechanisms. Many mt-ncRNAs have regulatory roles in cellular physiology, and some are associated with, or even act as, causal factors in human diseases. We also highlight developments in technologies and methodologies and further insights into future perspectives and challenges in studying these noncoding RNAs, as well as their potential biomedical applications.

CRISPR-based genome editing through the lens of DNA repair

Genome editing technologies operate by inducing site-specific DNA perturbations that are resolved by cellular DNA repair pathways. Products of genome editors include DNA breaks generated by CRISPR-associated nucleases, base modifications induced by base editors, DNA flaps created by prime editors, and integration intermediates formed by site-specific recombinases and transposases associated with CRISPR systems. Here, we discuss the cellular processes that repair CRISPR-generated DNA lesions and describe strategies to obtain desirable genomic changes through modulation of DNA repair pathways. Advances in our understanding of the DNA repair circuitry, in conjunction with the rapid development of innovative genome editing technologies, promise to greatly enhance our ability to improve food production, combat environmental pollution, develop cell-based therapies, and cure genetic and infectious diseases.

Gene Therapy for Cardiovascular Disease: Basic Research and Clinical Prospects

In recent years, the vital role of genetic factors in human diseases have been widely recognized by scholars with the deepening of life science research, accompanied by the rapid development of gene-editing technology. In early years, scientists used homologous recombination technology to establish gene knock-out and gene knock-in animal models, and then appeared the second-generation gene-editing technology zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) that relied on nucleic acid binding proteins and endonucleases and the third-generation gene-editing technology that functioned through protein-nucleic acids complexes-CRISPR/Cas9 system. This holds another promise for refractory diseases and genetic diseases. Cardiovascular disease (CVD) has always been the focus of clinical and basic research because of its high incidence and high disability rate, which seriously affects the long-term survival and quality of life of patients. Because some inherited cardiovascular diseases do not respond well to drug and surgical treatment, researchers are trying to use rapidly developing genetic techniques to develop initial attempts. However, significant obstacles to clinical application of gene therapy still exists, such as insufficient understanding of the nature of cardiovascular disease, limitations of genetic technology, or ethical concerns. This review mainly introduces the types and mechanisms of gene-editing techniques, ethical concerns of gene therapy, the application of gene therapy in atherosclerosis and inheritable cardiovascular diseases, in-stent restenosis, and delivering systems.

DNA methylation occurring in Cre-expressing cells inhibits loxP recombination and silences loxP-sandwiched genes

The low DNA recombination efficiency of site-specific recombinase systems in plants limits their application; however, the underlying mechanism is unknown. We evaluate the gene deletion performance of four recombinase systems (Cre/loxP, Flp/FRT, KD/KDRT and B3/B3RT) in tobacco where the recombinases are under the control of germline-specific promoters. We find that the expression of these recombinases results mostly in gene silencing rather than gene deletion. Using the Cre/loxP system as a model, we reveal that the region flanked by loxP sites (floxed) is hypermethylated, which prevents floxed genes from deletion while silencing the expression of the genes. We further show CG methylation alone in the recombinase binding element of the loxP site is unable to impede gene deletion; instead, CHH methylation in the crossover region is required to inhibit loxP recombination. Our study illustrates the important role of recombinase-induced DNA methylation in the inhibition of site-specific DNA recombination and uncovers the mechanism underlying recombinase-associated gene silence in plants.

Effective Downregulation of BCR-ABL Tumorigenicity by RNA Targeted CRISPR-Cas13a

AIM

To induce BCR-ABL gene silencing using CRISPR Cas13a.

BACKGROUND

CML is a clonal myeloproliferative disorder of pluripotent stem cells driven by a reciprocal translocation between chromosomes 9 and 22 forming a BCR-ABL fusion gene. Tyrosine- kinase inhibitor drugs like imatinib are the mainstay of treatment and cases resistant to these drugs have a poor prognosis in the absence of a compatible stem-cell donor. However with rapid advancements in gene-editing technologies most studies are now focusing on developing a translational model targeting single-gene disorders with a prospective permanent cure.

OBJECTIVE

To explore the potential application of the RNA targeting CRISPR-Cas13a system for effective knockdown of BCR-ABL fusion transcript in a CML cell line K562.

METHODS

CRISPR Cas13a crRNA was designed specific to the chimeric BCR-ABL gene and the system was transfected as a two-plasmid system into a CML cell line K562. The effects were enumerated by evaluating the expression levels of downstream genes dependent on the expression of the BCR-ABL gene. Also next-generation sequencing was used to ascertain the effects of CRISPR on the gene.

RESULTS

The CRISPR system was successfully able to lower the expression of downstream genes [pCRKL and pCRK] dependent on the activated BCR-ABL kinase signal by up-to 4.3 folds. The viability of the CRISPR-treated cells was also significantly lowered by 373.83-fold [p-value= 0.000891196]. The time-dependent kinetics also highlighted the significant in-vitro suppressive activity to last up to 8 weeks [p-value: 0.025]. As per the cDNA sequencing data from the Oxford MinION next-generation sequencer the CRISPR treated cells show 62.37% suspected cleaved reads.

CONCLUSION

These preliminary results highlight an excellent potential application of RNA targeting CRISPRs in Haematological neoplasms like CML and should pave the way for further research in this direction.

DNA-Targeted Metallodrugs: An Untapped Source of Artificial Gene Editing Technology

DNA binding metal complexes are synonymous with anticancer drug discovery. Given the array of structural and chemical reactivity properties available through careful design, metal complexes have been directed to bind nucleic acid structures through covalent or noncovalent binding modes. Several recognition modes - including crosslinking, intercalation, and oxidation - are central to the clinical success of broad-spectrum anticancer metallodrugs. However, recent progress in nucleic acid click chemistry coupled with advancement in our understanding of metal complex-nucleic acid interactions has opened up new avenues in genetic engineering and targeted therapies. Several of these applications are enabled by the hybridisation of oligonucleotide or polyamine probes to discrete metal complexes, which facilitate site-specific reactivity at the nucleic acid interface under the guidance of the probe. This Review focuses on recent advancements in hybrid design and, by way of an introduction to this topic, we provide a detailed overview of nucleic acid structures and metal complex-nucleic acid interactions. Our aim is to provide readers with an insight on the rational design of metal complexes with DNA recognition properties and an understanding of how the sequence-specific targeting of these interactions can be achieved for gene engineering applications.

The next frontier of oncotherapy: accomplishing clinical translation of oncolytic bacteria through genetic engineering

The development of a 'smart' drug capable of distinguishing tumor from host cells has been sought for centuries, but the microenvironment of solid tumors continues to confound therapeutics. Solid tumors present several challenges for current oncotherapeutics, including aberrant vascularization, hypoxia, necrosis, abnormally high pH and local immune suppression. While traditional chemotherapeutics are limited by such an environment, oncolytic microbes are drawn to it - having an innate ability to selectively infect, colonize and eradicate solid tumors. Development of an oncolytic species would represent a shift in the cancer therapeutic paradigm, with ramifications reaching from the medical into the socio-economic. Modern genetic engineering techniques could be implemented to customize 'Frankenstein' bacteria with advantageous characteristics from several species.

Expanding the Potential of Mammalian Genome Engineering via Targeted DNA Integration

Inserting custom designed DNA sequences into the mammalian genome plays an essential role in synthetic biology. In particular, the ability to introduce foreign DNA in a site-specific manner offers numerous advantages over random DNA integration. In this review, we focus on two mechanistically distinct systems that have been widely adopted for targeted DNA insertion in mammalian cells, the CRISPR/Cas9 system and site-specific recombinases. The CRISPR/Cas9 system has revolutionized the genome engineering field thanks to its high programmability and ease of use. However, due to its dependence on linearized DNA donor and endogenous cellular pathways to repair the induced double-strand break, CRISPR/Cas9-mediated DNA insertion still faces limitations such as small insert size, and undesired editing outcomes via error-prone repair pathways. In contrast, site-specific recombinases, in particular the Serine integrases, demonstrate large-cargo capability and no dependence on cellular repair pathways for DNA integration. Here we first describe recent advances in improving the overall efficacy of CRISPR/Cas9-based methods for DNA insertion. Moreover, we highlight the advantages of site-specific recombinases over CRISPR/Cas9 in the context of targeted DNA integration, with a special focus on the recent development of programmable recombinases. We conclude by discussing the importance of protein engineering to further expand the current toolkit for targeted DNA insertion in mammalian cells.

Harnessing the power of directed evolution to improve genome editing systems

The recent development of genome editing systems, such as zinc-finger nucleases, transcription activator-like effectors, CRISPR-Cas nucleases, and base editors has enabled the unprecedented capability to engineer the genomes of living cells. The ever-increasing demand for genome editors with improved accuracy, activity, and functionality has stimulated significant efforts to further engineer the genome editing systems. Directed evolution represents a promising strategy to improve the existing genome editing systems and enable new editing functions. Here, we review recent representative strategies to harness the power of directed evolution to improve genome editing systems, which have led to state-of-the-art genome editors that have significant implications for diverse applications in both laboratories and clinics.

Genome targeting by hybrid Flp-TAL recombinases

Genome engineering is a rapidly evolving field that benefits from the availability of different tools that can be used to perform genome manipulation tasks. We describe here the development of the Flp-TAL recombinases that can target genomic FRT-like sequences in their native chromosomal locations. Flp-TAL recombinases are hybrid enzymes that are composed of two functional modules: a variant of site-specific tyrosine recombinase Flp, which can have either narrow or broad target specificity, and the DNA-binding domain of the transcription activator-like effector, TAL. In Flp-TAL, the TAL module is responsible for delivering and stabilizing the Flp module onto the desired genomic FRT-like sequence where the Flp module mediates recombination. We demonstrate the functionality of the Flp-TAL recombinases by performing integration and deletion experiments in human HEK-293 cells. In the integration experiments we targeted a vector to three genomic FRT-like sequences located in the β-globin locus. In the deletion experiments we excised ~ 15 kilobases of DNA that contained a fragment of the integrated vector sequence and the neighboring genome sequence. On average, the efficiency of the integration and deletion reactions was about 0.1% and 20%, respectively.

RNA-Guided Recombinase-Cas9 Fusion Targets Genomic DNA Deletion and Integration

CRISPR-based technologies have become central to genome engineering. However, CRISPR-based editing strategies are dependent on the repair of DNA breaks via endogenous DNA repair mechanisms, which increases susceptibility to unwanted mutations. Here we complement Cas9 with a recombinase's functionality by fusing a hyperactive mutant resolvase from transposon Tn3, a member of serine recombinases, to a catalytically inactive Cas9, which we term integrase Cas9 (iCas9). We demonstrate iCas9 targets DNA deletion and integration. First, we validate iCas9's function in Saccharomyces cerevisiae using a genome-integrated reporter. Cooperative targeting by CRISPR RNAs at spacings of 22 or 40 bp enables iCas9-mediated recombination. Next, iCas9's ability to target DNA deletion and integration in human HEK293 cells is demonstrated using dual GFP-mCherry fluorescent reporter plasmid systems. Finally, we show that iCas9 is capable of targeting integration into a genomic reporter locus. We envision targeting and design concepts of iCas9 will contribute to genome engineering and synthetic biology.

Applications of genome editing technology in the targeted therapy of human diseases: mechanisms, advances and prospects

Based on engineered or bacterial nucleases, the development of genome editing technologies has opened up the possibility of directly targeting and modifying genomic sequences in almost all eukaryotic cells. Genome editing has extended our ability to elucidate the contribution of genetics to disease by promoting the creation of more accurate cellular and animal models of pathological processes and has begun to show extraordinary potential in a variety of fields, ranging from basic research to applied biotechnology and biomedical research. Recent progress in developing programmable nucleases, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeat (CRISPR)-Cas-associated nucleases, has greatly expedited the progress of gene editing from concept to clinical practice. Here, we review recent advances of the three major genome editing technologies (ZFNs, TALENs, and CRISPR/Cas9) and discuss the applications of their derivative reagents as gene editing tools in various human diseases and potential future therapies, focusing on eukaryotic cells and animal models. Finally, we provide an overview of the clinical trials applying genome editing platforms for disease treatment and some of the challenges in the implementation of this technology.

A Decade Decoded: Spies and Hackers in the History of TAL Effectors Research

Transcription activator-like effectors (TALEs) from the genus Xanthomonas are proteins with the remarkable ability to directly bind the promoters of genes in the plant host to induce their expression, which often helps bacterial colonization. Metaphorically, TALEs act as spies that infiltrate the plant disguised as high-ranking civilians (transcription factors) to trick the plant into activating weak points that allow an invasion. Current knowledge of how TALEs operate allows researchers to predict their activity (counterespionage) and exploit their function, engineering them to do our bidding (a Manchurian agent). This has been possible thanks particularly to the discovery of their DNA binding mechanism, which obeys specific amino acid-DNA correspondences (the TALE code). Here, we review the history of how researchers discovered the way these proteins work and what has changed in the ten years since the discovery of the code. Recommended music for reading this review can be found in the Supplemental Material.

High-resolution specificity profiling and off-target prediction for site-specific DNA recombinases

The development of site-specific recombinases (SSRs) as genome editing agents is limited by the difficulty of altering their native DNA specificities. Here we describe Rec-seq, a method for revealing the DNA specificity determinants and potential off-target substrates of SSRs in a comprehensive and unbiased manner. We applied Rec-seq to characterize the DNA specificity determinants of several natural and evolved SSRs including Cre, evolved variants of Cre, and other SSR family members. Rec-seq profiling of these enzymes and mutants thereof revealed previously uncharacterized SSR interactions, including specificity determinants not evident from SSR:DNA structures. Finally, we used Rec-seq specificity profiles to predict off-target substrates of Tre and Brec1 recombinases, including endogenous human genomic sequences, and confirmed their ability to recombine these off-target sequences in human cells. These findings establish Rec-seq as a high-resolution method for rapidly characterizing the DNA specificity of recombinases with single-nucleotide resolution, and for informing their further development.

The development of site-specific recombinases as genome editing tools is limited by the difficulty of altering their DNA sequence specificity. Here the authors present Rec-seq, a method for identifying specificity determinants and off-target substrates of recombinases in an unbiased manner.

Synthetic biology approaches for chromosomal integration of genes and pathways in industrial microbial systems

Industrial biotechnology is reliant on native pathway engineering or foreign pathway introduction for efficient biosynthesis of target products. Chromosomal integration, with intrinsic genetic stability, is an indispensable step for reliable expression of homologous or heterologous genes and pathways in large-scale and long-term fermentation. With advances in synthetic biology and CRISPR-based genome editing approaches, a wide variety of novel enabling technologies have been developed for single-step, markerless, multi-locus genomic integration of large biochemical pathways, which significantly facilitate microbial overproduction of chemicals, pharmaceuticals and other value-added biomolecules. Notably, the newly discovered homology-mediated end joining strategy could be widely applicable for high-efficiency genomic integration in a number of homologous recombination-deficient microbes. In this review, we explore the fundamental principles and characteristics of genomic integration, and highlight the development and applications of targeted integration approaches in the three representative industrial microbial systems, including Escherichia coli, actinomycetes and yeasts.

The Judgment of the CJEU of 25 July 2018 on Mutagenesis: Interpretation and Interim Legislative Proposal

The Judgment of 25 July 2018 of the Court of Justice of the European Union (CJEU) was optimistically awaited by breeders and supporters of agricultural biotechnology, but shortly after the press release advancing the Judgment, hope turned into frustration. Opinions on how to frame the New Breeding Techniques (NBT) in the context of Directive 2001/18/EC were issued before the Judgment, while proposals to assist the EU legislator to amend the regime driven by the Directive have been also provided afterwards by scientists and institutional bodies around the EU. However, they do not seem to have paid so much attention to the Judgment itself. This paper focuses on the Judgment. It finds out that while the impacts of the Judgment on the NBT might have been slightly overvalued, its potential negative effects on techniques of random mutagenesis and varieties breed through them have been generally underestimated if not absolutely overlooked. The analysis also shows that the Judgment does not preempt the possibility to exempt certain applications of some NBT from the scope of Directive 2001/18/EC, and, in fact, ODM, SDN1, and SDN2 might be, under certain conditions, easily exempted from its scope without the need of a deep legislative revolution nor even the amendment of Directive 2001/18/EC. As regards techniques of random mutagenesis and mutant varieties bred by means of those techniques, until action is taken by Member States (if finally taken), no real limitations upon them are to be feared. However, if Member States start to consider the path opened by the CJEU, then their regulation at an EU level should be readily explored in order to avoid further negative effects on plant breeding as well as on the free movement inside the EU of those varieties and the products thereof.

Development of Toolboxes for Precision Genome/Epigenome Editing and Imaging of Epigenetics

Zinc finger (ZF) proteins are composed of repeated ββα modules and coordinate a zinc ion. ZF domains recognizing specific DNA target sequences can be substituted for the binding domains of various DNA-modifying enzymes to create designer nucleases, recombinases, and methyltransferases with programmable sequence specificity. Enzymatic genome editing and modification can be applied to many fields of basic research and medicine. The recent development of new platforms using transcription activator-like effector (TALE) proteins or the CRISPR-Cas9 system has expanded the range of possibilities for genome-editing technologies. In addition, these DNA binding domains can also be utilized to build a toolbox for epigenetic controls by fusing them with protein- or DNA-modifying enzymes. Here, our research on epigenome editing including the development of artificial zinc finger recombinase (ZFR), split DNA methyltransferase, and fluorescence imaging of histone proteins by ZIP tag-probe system is introduced. Advances in the ZF, TALE, and CRISPR-Cas9 platforms have paved the way for the next generation of genome/epigenome engineering approaches.

The CRISPR/Cas9 system efficiently reverts the tumorigenic ability of BCR/ABL in vitro and in a xenograft model of chronic myeloid leukemia

CRISPR/Cas9 technology was used to abrogate p210 oncoprotein expression in the Boff-p210 cell line, a pro-B line derived from interlukin-3-dependent Baf/3, that shows IL-3-independence arising from the constitutive expression of BCR-ABL p210. Using this approach, pools of Boff-p210-edited cells and single edited cell-derived clones were obtained and functionally studied in vitro. The loss of p210 expression in Boff-p210 cells resulted in the loss of ability to grow in the absence of IL-3, as the Baf/3 parental line, showing significantly increased apoptosis levels. Notably, in a single edited cell-derived clone carrying a frame-shift mutation that prevents p210 oncoprotein expression, the effects were even more drastic, resulting in cell death. These edited cells were injected subcutaneously in immunosuppressed mice and tumor growth was followed for three weeks. BCR/ABL-edited cells developed smaller tumors than those originating from unedited Boff-p210 parental cells. Interestingly, the single edited cell-derived clone was unable to develop tumors, similar to what is observed with the parental Baf/3 cell line.

CRISPR/Cas9 genomic editing technology allows the ablation of the BCR/ABL fusion gene, causing an absence of oncoprotein expression, and blocking its tumorigenic effects in vitro and in the in vivo xenograft model of CML. The future application of this approach in in vivo models of CML will allow us to more accurately assess the value of CRISPR/Cas9 technology as a new therapeutic tool that overcomes resistance to the usual treatments for CML patients.

The Future of Multiplexed Eukaryotic Genome Engineering

Multiplex genome editing is the simultaneous introduction of multiple distinct modifications to a given genome. Though in its infancy, maturation of this field will facilitate powerful new biomedical research approaches and will enable a host of far-reaching biological engineering applications, including new therapeutic modalities and industrial applications, as well as "genome writing" and de-extinction efforts. In this Perspective, we focus on multiplex editing of large eukaryotic genomes. We describe the current state of multiplexed genome editing, the current limits of our ability to multiplex edits, and provide perspective on the many applications that fully realized multiplex editing technologies would enable in higher eukaryotic genomes. We offer a broad look at future directions, covering emergent CRISPR-based technologies, advances in intracellular delivery, and new DNA assembly approaches that may enable future genome editing on a massively multiplexed scale.

Site-specific chromosomal gene insertion: Flp recombinase versus Cas9 nuclease

Site-specific recombination systems like those based on the Flp recombinase proved themselves as efficient tools for cell line engineering. The recent emergence of designer nucleases, especially RNA guided endonucleases like Cas9, has considerably broadened the available toolbox for applications like targeted transgene insertions. Here we established a recombinase-mediated cassette exchange (RMCE) protocol for the fast and effective, drug-free isolation of recombinant cells. Distinct fluorescent protein patterns identified the recombination status of individual cells. In derivatives of a CHO master cell line the expression of the introduced transgene of interest could be dramatically increased almost 20-fold by subsequent deletion of the fluorescent protein gene that provided the initial isolation principle. The same master cell line was employed in a comparative analysis using CRISPR/Cas9 for transgene integration in identical loci. Even though the overall targeting efficacy was comparable, multi-loci targeting was considerably more effective for Cas9-mediated transgene insertion when compared to RMCE. While Cas9 is inherently more flexible, our results also alert to the risk of aberrant recombination events around the cut site. Together, this study points at the individual strengths in performance of both systems and provides guidance for their appropriate use.

Beyond editing to writing large genomes

Recent exponential advances in genome sequencing and engineering technologies have enabled an unprecedented level of interrogation into the impact of DNA variation (genotype) on cellular function (phenotype). Furthermore, these advances have also prompted realistic discussion of writing and radically re-writing complex genomes. In this Perspective, we detail the motivation for large-scale engineering, discuss the progress made from such projects in bacteria and yeast and describe how various genome-engineering technologies will contribute to this effort. Finally, we describe the features of an ideal platform and provide a roadmap to facilitate the efficient writing of large genomes.

Genome Engineering for Personalized Arthritis Therapeutics

Arthritis represents a family of complex joint pathologies responsible for the majority of musculoskeletal conditions. Nearly all diseases within this family, including osteoarthritis, rheumatoid arthritis, and juvenile idiopathic arthritis, are chronic conditions with few or no disease-modifying therapeutics available. Advances in genome engineering technology, most recently with CRISPR-Cas9, have revolutionized our ability to interrogate and validate genetic and epigenetic elements associated with chronic diseases such as arthritis. These technologies, together with cell reprogramming methods, including the use of induced pluripotent stem cells, provide a platform for human disease modeling. We summarize new evidence from genome-wide association studies and genomics that substantiates a genetic basis for arthritis pathogenesis. We also review the potential contributions of genome engineering in the development of new arthritis therapeutics.

Engineering and Application of Zinc Finger Proteins and TALEs for Biomedical Research

Engineered DNA-binding domains provide a powerful technology for numerous biomedical studies due to their ability to recognize specific DNA sequences. Zinc fingers (ZF) are one of the most common DNA-binding domains and have been extensively studied for a variety of applications, such as gene regulation, genome engineering and diagnostics. Another novel DNA-binding domain known as a transcriptional activator-like effector (TALE) has been more recently discovered, which has a previously undescribed DNA-binding mode. Due to their modular architecture and flexibility, TALEs have been rapidly developed into artificial gene targeting reagents. Here, we describe the methods used to design these DNA-binding proteins and their key applications in biomedical research.

A New Era of Genome Integration-Simply Cut and Paste!

Genome integration is a powerful tool in both basic and applied biological research. However, traditional genome integration, which is typically mediated by homologous recombination, has been constrained by low efficiencies and limited host range. In recent years, the emergence of homing endonucleases and programmable nucleases has greatly enhanced integration efficiencies and allowed alternative integration mechanisms such as nonhomologous end joining and microhomology-mediated end joining, enabling integration in hosts deficient in homologous recombination. In this review, we will highlight recent advances and breakthroughs in genome integration methods made possible by programmable nucleases, and their new applications in synthetic biology and metabolic engineering.

Visualization of aging-associated chromatin alterations with an engineered TALE system

Visualization of specific genomic loci in live cells is a prerequisite for the investigation of dynamic changes in chromatin architecture during diverse biological processes, such as cellular aging. However, current precision genomic imaging methods are hampered by the lack of fluorescent probes with high specificity and signal-to-noise contrast. We find that conventional transcription activator-like effectors (TALEs) tend to form protein aggregates, thereby compromising their performance in imaging applications. Through screening, we found that fusing thioredoxin with TALEs prevented aggregate formation, unlocking the full power of TALE-based genomic imaging. Using thioredoxin-fused TALEs (TTALEs), we achieved high-quality imaging at various genomic loci and observed aging-associated (epi) genomic alterations at telomeres and centromeres in human and mouse premature aging models. Importantly, we identified attrition of ribosomal DNA repeats as a molecular marker for human aging. Our study establishes a simple and robust imaging method for precisely monitoring chromatin dynamics in vitro and in vivo.

Large-scale design of robust genetic circuits with multiple inputs and outputs for mammalian cells

Genetic circuits engineered for mammalian cells often require extensive fine-tuning to perform their intended functions. To overcome this problem, we present a generalizable biocomputing platform that can engineer genetic circuits which function in human cells with minimal optimization. We used our Boolean Logic and Arithmetic through DNA Excision (BLADE) platform to build more than 100 multi-input-multi-output circuits. We devised a quantitative metric to evaluate the performance of the circuits in human embryonic kidney and Jurkat T cells. Of 113 circuits analysed, 109 functioned (96.5%) with the correct specified behavior without any optimization. We used our platform to build a three-input, two-output Full Adder and six-input, one-output Boolean Logic Look Up Table. We also used BLADE to design circuits with temporal small molecule-mediated inducible control and circuits that incorporate CRISPR/Cas9 to regulate endogenous mammalian genes.

Genome-Editing Technologies: Principles and Applications

Targeted nucleases have provided researchers with the ability to manipulate virtually any genomic sequence, enabling the facile creation of isogenic cell lines and animal models for the study of human disease, and promoting exciting new possibilities for human gene therapy. Here we review three foundational technologies-clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9), transcription activator-like effector nucleases (TALENs), and zinc-finger nucleases (ZFNs). We discuss the engineering advances that facilitated their development and highlight several achievements in genome engineering that were made possible by these tools. We also consider artificial transcription factors, illustrating how this technology can complement targeted nucleases for synthetic biology and gene therapy.

CRISPR-Cas9 System as a Versatile Tool for Genome Engineering in Human Cells

Targeted nucleases are influential instruments for intervening in genome revision with great accuracy. RNA-guided Cas9 nucleases produced from clustered regularly interspaced short palindromic repeats (CRISPR)-Cas systems have noticeably altered the means to modify the genomes of distinct organisms. They can be notably used to facilitate effective genome manipulation in eukaryotic cells by clearly detailing a 20-nt targeting sequence inside its directed RNA. We discuss the most recent advancements in the molecular basis of the type II CRISPR/Cas system and encapsulate applications and elements affecting its use in human cells. We also propose possible applications covering its uses ranging from basic science to implementation in the clinic.

A programmable Cas9-serine recombinase fusion protein that operates on DNA sequences in mammalian cells

We describe the development of ‘recCas9’, an RNA-programmed small serine recombinase that functions in mammalian cells. We fused a catalytically inactive dCas9 to the catalytic domain of Gin recombinase using an optimized fusion architecture. The resulting recCas9 system recombines DNA sites containing a minimal recombinase core site flanked by guide RNA-specified sequences. We show that these recombinases can operate on DNA sites in mammalian cells identical to genomic loci naturally found in the human genome in a manner that is dependent on the guide RNA sequences. DNA sequencing reveals that recCas9 catalyzes guide RNA-dependent recombination in human cells with an efficiency as high as 32% on plasmid substrates. Finally, we demonstrate that recCas9 expressed in human cells can catalyze in situ deletion between two genomic sites. Because recCas9 directly catalyzes recombination, it generates virtually no detectable indels or other stochastic DNA modification products. This work represents a step toward programmable, scarless genome editing in unmodified cells that is independent of endogenous cellular machinery or cell state. Current and future generations of recCas9 may facilitate targeted agricultural breeding, or the study and treatment of human genetic diseases.

Genomics Approaches For Improving Salinity Stress Tolerance in Crop Plants

Salinity is one of the major factors which reduces crop production worldwide. Plant responses to salinity are highly complex and involve a plethora of genes. Due to its multigenicity, it has been difficult to attain a complete understanding of how plants respond to salinity. Genomics has progressed tremendously over the past decade and has played a crucial role towards providing necessary knowledge for crop improvement. Through genomics, we have been able to identify and characterize the genes involved in salinity stress response, map out signaling pathways and ultimately utilize this information for improving the salinity tolerance of existing crops. The use of new tools, such as gene pyramiding, in genetic engineering and marker assisted breeding has tremendously enhanced our ability to generate stress tolerant crops. Genome editing technologies such as Zinc finger nucleases, TALENs and CRISPR/Cas9 also provide newer and faster avenues for plant biologists to generate precisely engineered crops.

Human Induced Pluripotent Stem Cells as a Platform for Personalized and Precision Cardiovascular Medicine

Human induced pluripotent stem cells (hiPSCs) have revolutionized the field of human disease modeling, with an enormous potential to serve as paradigm shifting platforms for preclinical trials, personalized clinical diagnosis, and drug treatment. In this review, we describe how hiPSCs could transition cardiac healthcare away from simple disease diagnosis to prediction and prevention, bridging the gap between basic and clinical research to bring the best science to every patient.

Development of an AAV9 coding for a 3XFLAG-TALEfrat#8-VP64 able to increase in vivo the human frataxin in YG8R mice

Artificially designed transcription activator-like effector (TALE) proteins fused to a transcription activation domain (TAD), such as VP64, are able to activate specific eukaryotic promoters. They thus provide a good tool for targeted gene regulation as a therapy. However, the efficacy of such an agent in vivo remains to be demonstrated as the majority of studies have been carried out in cell culture. We produced an adeno-associated virus 9 (AAV9) coding for a TALEfrat#8 containing 13 repeat variable diresidues able to bind to the proximal promoter of human frataxin (FXN) gene. This TALEfrat#8 was fused with a 3XFLAG at its N terminal and a VP64 TAD at its C terminal, and driven by a CAG promoter. This AAV9_3XFLAG-TALEfrat#8-VP64 was injected intraperitoneally to 9-day-old and 4-month-old YG8R mice. After 1 month, the heart, muscle and liver were removed and their FXN mRNA and FXN protein were analyzed. The results show that the AAV9_3XFLAG-TALEfrat#8-VP64 increased the FXN mRNA and FXN protein in the three organs studied. These results corroborate our previous in vitro studies in the FRDA human fibroblasts. Our study indicates that an AAV coding for a TALE protein coupled with a TAD may be used to increase gene expression in vivo as a possible treatment not only for FRDA but also for other haploinsufficiency diseases.

Engineering Delivery Vehicles for Genome Editing

The field of genome engineering has created new possibilities for gene therapy, including improved animal models of disease, engineered cell therapies, and in vivo gene repair. The most significant challenge for the clinical translation of genome engineering is the development of safe and effective delivery vehicles. A large body of work has applied genome engineering to genetic modification in vitro, and clinical trials have begun using cells modified by genome editing. Now, promising preclinical work is beginning to apply these tools in vivo. This article summarizes the development of genome engineering platforms, including meganucleases, zinc finger nucleases, TALENs, and CRISPR/Cas9, and their flexibility for precise genetic modifications. The prospects for the development of safe and effective viral and nonviral delivery vehicles for genome editing are reviewed, and promising advances in particular therapeutic applications are discussed.

Libraries of Synthetic TALE-Activated Promoters: Methods and Applications

The discovery of proteins with programmable DNA-binding specificities triggered a whole array of applications in synthetic biology, including genome editing, regulation of transcription, and epigenetic modifications. Among those, transcription activator-like effectors (TALEs) due to their natural function as transcription regulators, are especially well-suited for the development of orthogonal systems for the control of gene expression. We describe here the construction and testing of libraries of synthetic TALE-activated promoters which are under the control of a single TALE with a given DNA-binding specificity. These libraries consist of a fixed DNA-binding element for the TALE, a TATA box, and variable sequences of 19 bases upstream and 43 bases downstream of the DNA-binding element. These libraries were cloned using a Golden Gate cloning strategy making them usable as standard parts in a modular cloning system. The broad range of promoter activities detected and the versatility of these promoter libraries make them valuable tools for applications in the fine-tuning of expression in metabolic engineering projects or in the design and implementation of regulatory circuits.

Serine Resolvases

Serine resolvases are an interesting group of site-specific recombinases that, in their native contexts, resolve large fused replicons into smaller separated ones. Some resolvases are encoded by replicative transposons and resolve the transposition product, in which the donor and recipient molecules are fused, into separate replicons. Other resolvases are encoded by plasmids and function to resolve plasmid dimers into monomers. Both types are therefore involved in the spread and maintenance of antibiotic-resistance genes. Resolvases and the closely related invertases were the first serine recombinases to be studied in detail, and much of our understanding of the unusual strand exchange mechanism of serine recombinases is owed to those early studies. Resolvases and invertases have also served as paradigms for understanding how DNA topology can be harnessed to regulate enzyme activity. Finally, their relatively modular structure, combined with a wealth of structural and biochemical data, has made them good choices for engineering chimeric recombinases with designer specificity. This chapter focuses on the current understanding of serine resolvases, with a focus on the contributions of structural studies.

Site-specific recombinases: molecular machines for the Genetic Revolution

The fields of molecular genetics, biotechnology and synthetic biology are demanding ever more sophisticated molecular tools for programmed precise modification of cell genomic DNA and other DNA sequences. This review presents the current state of knowledge and development of one important group of DNA-modifying enzymes, the site-specific recombinases (SSRs). SSRs are Nature's 'molecular machines' for cut-and-paste editing of DNA molecules by inserting, deleting or inverting precisely defined DNA segments. We survey the SSRs that have been put to use, and the types of applications for which they are suitable. We also discuss problems associated with uses of SSRs, how these problems can be minimized, and how recombinases are being re-engineered for improved performance and novel applications.

Reactivation of Latent HIV-1 Expression by Engineered TALE Transcription Factors

The presence of replication-competent HIV-1 -which resides mainly in resting CD4+ T cells--is a major hurdle to its eradication. While pharmacological approaches have been useful for inducing the expression of this latent population of virus, they have been unable to purge HIV-1 from all its reservoirs. Additionally, many of these strategies have been associated with adverse effects, underscoring the need for alternative approaches capable of reactivating viral expression. Here we show that engineered transcriptional modulators based on customizable transcription activator-like effector (TALE) proteins can induce gene expression from the HIV-1 long terminal repeat promoter, and that combinations of TALE transcription factors can synergistically reactivate latent viral expression in cell line models of HIV-1 latency. We further show that complementing TALE transcription factors with Vorinostat, a histone deacetylase inhibitor, enhances HIV-1 expression in latency models. Collectively, these findings demonstrate that TALE transcription factors are a potentially effective alternative to current pharmacological routes for reactivating latent virus and that combining synthetic transcriptional activators with histone deacetylase inhibitors could lead to the development of improved therapies for latent HIV-1 infection.

The Development of TALE Nucleases for Biotechnology

The development of a facile genome engineering technology based on transcription activator-like effector nucleases (TALENs) has led to significant advances in diverse areas of science and medicine. In this review, we provide a broad overview of the development of TALENs and the use of this technology in basic science, biotechnology, and biomedical applications. This includes the discovery of DNA recognition by TALEs, engineering new TALE proteins to diverse targets, general advances in nuclease-based editing strategies, and challenges that are specific to various applications of the TALEN technology. We review examples of applying TALENs for studying gene function and regulation, generating disease models, and developing gene therapies. The current status of genome editing and future directions for other uses of these technologies are also discussed.