TAL nucleases (TALNs): hybrid proteins composed of TAL effectors and FokI DNA-cleavage domain

FLASH assembly of TALENs for high-throughput genome editing

Engineered transcription activator–like effector nucleases (TALENs) have shown promise as facile and broadly applicable genome editing tools. However, no publicly available high-throughput method for constructing TALENs has been published, and large-scale assessments of the success rate and targeting range of the technology remain lacking. Here we describe the fast ligation-based automatable solid-phase high-throughput (FLASH) system, a rapid and cost-effective method for large-scale assembly of TALENs. We tested 48 FLASH-assembled TALEN pairs in a human cell–based EGFP reporter system and found that all 48 possessed efficient gene-modification activities. We also used FLASH to assemble TALENs for 96 endogenous human genes implicated in cancer and/or epigenetic regulation and found that 84 pairs were able to efficiently introduce targeted alterations. Our results establish the robustness of TALEN technology and demonstrate that FLASH facilitates high-throughput genome editing at a scale not currently possible with other genome modification technologies.

Reverse genetic approaches in zebrafish

Zebrafish (Danio rerio) is a well-established vertebrate animal model. A comprehensive collection of reverse genetics tools has been developed for studying gene function in this useful organism. Morpholino is the most widely used reagent to knock down target gene expression post-transcriptionally. For a long time, targeted genome modification has been heavily relied on large-scale traditional forward genetic screens, such as ENU (N-ethyl-N-nitrosourea) mutagenesis derived TILLING (Targeting Induced Local Lesions IN Genomes) strategy and pseudo-typed retrovirus mediated insertional mutagenesis. Recently, engineered endonucleases, including ZFNs (zinc finger nucleases) and TALENs (transcription activator-like effector nucleases), provide new and efficient strategies to directly generate site-specific indel mutations by inducing double strand breaks in target genes. Here we summarize the major reverse genetic approaches for loss-of-function studies used and emerging in zebrafish, including strategies based on genome-wide mutagenesis and methods for site-specific gene targeting. Future directions and expectations will also be discussed.

A robust dual reporter system to visualize and quantify gene expression mediated by transcription activator-like effectors

Background

Transcription activator-like effectors (TALEs) are a class of naturally occurring transcription effectors that recognize specific DNA sequences and modulate gene expression. The modularity of TALEs DNA binding domain enables sequence-specific perturbation and offers broad applications in genetic and epigenetic studies. Although the efficient construction of TALEs has been established, robust functional tools to assess their functions remain lacking.

Results

We established a dual reporter system that was specifically designed for real-time monitoring and quantifying gene expression mediated by TALEs. We validated both sensitivity and specificity of this dual-reporter system in mammalian cells, and demonstrated that this dual reporter system is robust and potentially amenable to high throughput (HTP) applications.

Conclusion

We have designed, constructed and validated a novel dual reporter system for assessing TALE mediated gene regulations. This system offers a robust and easy-to- use tool for real-time monitoring and quantifying gene expression in mammalian cells.

Transcription activator-like effector proteins induce the expression of the frataxin gene

Genes encoding transcription activator-like effector (TALE) proteins may be engineered to target specific DNA sequences. TALEs fused with a transcription activator can be used to specifically induce the expression of a gene. This could lead to completely new therapies for several diseases. We have applied this potential therapeutic approach to Friedreich ataxia (FRDA), as an example. FRDA is due to reduced expression of frataxin because of elongation of a trinucleotide (GAA) repeat in intron 1. Our aim was to develop a potential treatment for FRDA by increasing the expression of the frataxin gene. We engineered 12 TALE genes (TALE(Frat)) encoding TALE(Frat) proteins, each specifically targeting different 14-bp DNA sequences within the proximal region of the human frataxin promoter. When the genes encoding these TALE(Frat) proteins were fused with a transcription activator, that is, four VP16 peptides (i.e., VP64), the resulting TALE(Frat)-VP64 proteins induced the expression of an mCherry reporter gene fused to a mini-cytomegalovirus promoter able to be activated by the insertion of the frataxin proximal promoter upstream to the minipromoter. These TALE(Frat)-VP64 proteins also increased, by 2- to 3-fold, frataxin gene expression (detected by qRT-PCR) in the cells. We conclude that TALE(Frat) proteins targeting the frataxin promoter may be used to increase the expression of frataxin mRNA and potentially could alleviate the symptoms of Friedreich ataxia. TALE methodology opens a new field of research, which could be used to develop TALE proteins to treat other diseases by inducing the expression of specific genes.

Neutrophil reverse migration becomes transparent with zebrafish

The precise control of neutrophil-mediated inflammation is critical for both host defense and the prevention of immunopathology. In vivo imaging studies in zebrafish, and more recently in mice, have made the novel observation that neutrophils leave a site of inflammation through a process called neutrophil reverse migration. The application of advanced imaging techniques to the genetically tractable, optically transparent zebrafish larvae was critical for these advances. Still, the mechanisms underlying neutrophil reverse migration and its effects on the resolution or priming of immune responses remain unclear. Here, we review the current knowledge of neutrophil reverse migration, its potential roles in host immunity, and the live imaging tools that make zebrafish a valuable model for increasing our knowledge of neutrophil behavior in vivo.

Recent advances in targeted genome engineering in mammalian systems

Targeted genome engineering enables researchers to disrupt, insert, or replace a genomic sequence precisely at a predetermined locus. One well-established technology to edit a mammalian genome is known as gene targeting, which is based on the homologous recombination (HR) mechanism. However, the low HR frequency in mammalian cells (except for mice) prevents its wide application. To address this limitation, a custom-designed nuclease is used to introduce a site-specific DNA double-strand break (DSB) on the chromosome and the subsequent repair of the DSB by the HR mechanism or the non-homologous end joining mechanism results in efficient targeted genome modifications. Engineered homing endonucleases (also called meganucleases), zinc finger nucleases, and transcription activator-like effector nucleases represent the three major classes of custom-designed nucleases that have been successfully applied in many different organisms for targeted genome engineering. This article reviews the recent developments of these genome engineering tools and highlights a few representative applications in mammalian systems. Recent advances in gene delivery strategies of these custom-designed nucleases are also briefly discussed.

Targeted DNA mutagenesis for the cure of chronic viral infections

Human immunodeficiency virus type 1 (HIV-1), hepatitis B virus (HBV), and herpes simplex virus (HSV) have been incurable to date because effective antiviral therapies target only replicating viruses and do not eradicate latently integrated or nonreplicating episomal viral genomes. Endonucleases that can target and cleave critical regions within latent viral genomes are currently in development. These enzymes are being engineered with high specificity such that off-target binding of cellular DNA will be absent or minimal. Imprecise nonhomologous-end-joining (NHEJ) DNA repair following repeated cleavage at the same critical site may permanently disrupt translation of essential viral proteins. We discuss the benefits and drawbacks of three types of DNA cleavage enzymes (zinc finger endonucleases, transcription activator-like [TAL] effector nucleases [TALENs], and homing endonucleases [also called meganucleases]), the development of delivery vectors for these enzymes, and potential obstacles for successful treatment of chronic viral infections. We then review issues regarding persistence of HIV-1, HBV, and HSV that are relevant to eradication with genome-altering approaches.

TAL Effector-Nucleotide Targeter (TALE-NT) 2.0: tools for TAL effector design and target prediction

Transcription activator-like (TAL) effectors are repeat-containing proteins used by plant pathogenic bacteria to manipulate host gene expression. Repeats are polymorphic and individually specify single nucleotides in the DNA target, with some degeneracy. A TAL effector-nucleotide binding code that links repeat type to specified nucleotide enables prediction of genomic binding sites for TAL effectors and customization of TAL effectors for use in DNA targeting, in particular as custom transcription factors for engineered gene regulation and as site-specific nucleases for genome editing. We have developed a suite of web-based tools called TAL Effector-Nucleotide Targeter 2.0 (TALE-NT 2.0; https://boglab.plp.iastate.edu/) that enables design of custom TAL effector repeat arrays for desired targets and prediction of TAL effector binding sites, ranked by likelihood, in a genome, promoterome or other sequence of interest. Search parameters can be set by the user to work with any TAL effector or TAL effector nuclease architecture. Applications range from designing highly specific DNA targeting tools and identifying potential off-target sites to predicting effector targets important in plant disease.

Highly efficient generation of heritable zebrafish gene mutations using homo- and heterodimeric TALENs

Transcription activator-like effector nucleases (TALENs) are powerful new research tools that enable targeted gene disruption in a wide variety of model organisms. Recent work has shown that TALENs can induce mutations in endogenous zebrafish genes, but to date only four genes have been altered, and larger-scale tests of the success rate, mutation efficiencies and germline transmission rates have not been described. Here, we constructed homodimeric TALENs to 10 different targets in various endogenous zebrafish genes and found that 7 nuclease pairs induced targeted indel mutations with high efficiencies ranging from 2 to 76%. We also tested obligate heterodimeric TALENs and found that these nucleases induce mutations with comparable or higher frequencies and have better toxicity profiles than their homodimeric counterparts. Importantly, mutations induced by both homodimeric and heterodimeric TALENs are passed efficiently through the germline, in some cases reaching 100% transmission. For one target gene sequence, we observed substantially reduced mutagenesis efficiency for a variant site bearing two mismatched nucleotides, raising the possibility that TALENs might be used to perform allele-specific gene disruption. Our results suggest that construction of one to two heterodimeric TALEN pairs for any given gene will, in most cases, enable researchers to rapidly generate knockout zebrafish.

From DNA to targeted therapeutics: bringing synthetic biology to the clinic

Synthetic biology aims to make biological engineering more scalable and predictable, lowering the cost and facilitating the translation of synthetic biological systems to practical applications. Increasingly sophisticated, rationally designed synthetic systems that are capable of complex functions pave the way to translational applications, including disease diagnostics and targeted therapeutics. Here, we provide an overview of recent developments in synthetic biology in the context of translational research and discuss challenges at the interface between synthetic biology and clinical medicine.

Retake the center stage--new development of rat genetics

The rat is a powerful model for the study of human physiology and diseases, and is preferred by physiologists, neuroscientists and toxicologists. However, the lack of robust genetic modification tools has severely limited the generation of rat genetic models over the last two decades. In the last few years, several gene-targeting strategies have been developed in rats using N-ethyl-N-nitrosourea (ENU), transposons, zinc-finger nucleases (ZFNs), bacterial artificial chromosome (BAC) mediated transgenesis, and recently established rat embryonic stem (ES) cells. The development and improvement of these approaches to genetic manipulation have created a bright future for the use of genetic rat models in investigations of gene function and human diseases. Here, we summarize the strategies used for rat genetic manipulation in current research. We also discuss BAC transgenesis as a potential tool in rat transgenic models.

Rapid and cost-effective gene targeting in rat embryonic stem cells by TALENs

The rat is the preferred animal model in many areas of biomedical research and drug development. Genetic manipulation in rats has lagged behind that in mice due to the lack of efficient gene targeting tools. Previously, we generated a knockout rat via conventional homologous recombination in rat embryonic stem (ES) cells. Here, we show that efficient gene targeting in rat ES cells can be achieved quickly through transcription activator-like effector nuclease (TALEN)-mediated DNA double-strand breaks. Using the Golden Gate cloning technique, we constructed a pair of TALEN targeting vectors for the gene of interest in 5 days. After gene transfection, the targeted rat ES cell colonies were isolated, screened, and confirmed by PCR without the need of drug selection. Our results suggest that TALEN-mediated gene targeting is a superior means of establishing genetically modified rat ES cell lines with high efficiency and short turnaround time.

Genome-enabled perspectives on the composition, evolution, and expression of virulence determinants in bacterial plant pathogens

Genome sequence analyses of bacterial plant pathogens are revealing important insights into the molecular determinants of pathogenicity and, through transcript characterization, responses to environmental conditions, evidence for small RNAs, and validation of uncharacterized genes. Genome comparison sheds further light on the processes impacting pathogen evolution and differences in gene repertoire among isolates contributing to niche specialization. Information derived from pathogen genome analysis is providing tools for use in diagnosis and interference with host-pathogen interactions for the purpose of disease control. However, the existing information infrastructure fails to adequately integrate the increasing numbers of sequence data sets, bioinformatic analyses, and experimental characterization, as required for effective systems-level analysis. Enhanced standardization of data formats at the point of publication is proposed as a possible solution.

Activation of the carbon concentrating mechanism by CO2 deprivation coincides with massive transcriptional restructuring in Chlamydomonas reinhardtii

A CO(2)-concentrating mechanism (CCM) is essential for the growth of most eukaryotic algae under ambient (392 ppm) and very low (<100 ppm) CO(2) concentrations. In this study, we used replicated deep mRNA sequencing and regulatory network reconstruction to capture a remarkable scope of changes in gene expression that occurs when Chlamydomonas reinhardtii cells are shifted from high to very low levels of CO(2) (≤100 ppm). CCM induction 30 to 180 min post-CO(2) deprivation coincides with statistically significant changes in the expression of an astonishing 38% (5884) of the 15,501 nonoverlapping C. reinhardtii genes. Of these genes, 1088 genes were induced and 3828 genes were downregulated by a log(2) factor of 2. The latter indicate a global reduction in photosynthesis, protein synthesis, and energy-related biochemical pathways. The magnitude of transcriptional rearrangement and its major patterns are robust as analyzed by three different statistical methods. De novo DNA motif discovery revealed new putative binding sites for Myeloid oncogene family transcription factors potentially involved in activating low CO(2)-induced genes. The (CA)(n) repeat (9 ≤ n ≤ 25) is present in 29% of upregulated genes but almost absent from promoters of downregulated genes. These discoveries open many avenues for new research.

Genome modifications in plant cells by custom-made restriction enzymes

Genome editing, i.e. the ability to mutagenize, insert, delete and replace sequences, in living cells is a powerful and highly desirable method that could potentially revolutionize plant basic research and applied biotechnology. Indeed, various research groups from academia and industry are in a race to devise methods and develop tools that will enable not only site-specific mutagenesis but also controlled foreign DNA integration and replacement of native and transgene sequences by foreign DNA, in living plant cells. In recent years, much of the progress seen in gene targeting in plant cells has been attributed to the development of zinc finger nucleases and other novel restriction enzymes for use as molecular DNA scissors. The induction of double-strand breaks at specific genomic locations by zinc finger nucleases and other novel restriction enzymes results in a wide variety of genetic changes, which range from gene addition to the replacement, deletion and site-specific mutagenesis of endogenous and heterologous genes in living plant cells. In this review, we discuss the principles and tools for restriction enzyme-mediated gene targeting in plant cells, as well as their current and prospective use for gene targeting in model and crop plants.

Concerted nicking of donor and chromosomal acceptor DNA promotes homology-directed gene targeting in human cells

The exchange of genetic information between donor and acceptor DNA molecules by homologous recombination (HR) depends on the cleavage of phosphodiester bonds. Although double-stranded and single-stranded DNA breaks (SSBs) have both been invoked as triggers of HR, until very recently the focus has been primarily on the former type of DNA lesions mainly due to the paucity of SSB-based recombination models. Here, to investigate the role of nicked DNA molecules as HR-initiating substrates in human somatic cells, we devised a homology-directed gene targeting system based on exogenous donor and chromosomal target DNA containing recognition sequences for the adeno-associated virus sequence- and strand-specific endonucleases Rep78 and Rep68. We found that HR is greatly fostered if a SSB is not only introduced in the chromosomal acceptor but also in the donor DNA template. Our data are consistent with HR models postulating the occurrence of SSBs or single-stranded gaps in both donor and acceptor molecules during the genetic exchange process. These findings can guide the development of improved HR-based genome editing strategies in which sequence- and strand-specific endonucleolytic cleavage of the chromosomal target site is combined with that of the targeting vector.

Rapid and highly efficient construction of TALE-based transcriptional regulators and nucleases for genome modification

Transcription activator-like effectors (TALEs) can be used as DNA-targeting modules by engineering their repeat domains to dictate user-selected sequence specificity. TALEs have been shown to function as site-specific transcriptional activators in a variety of cell types and organisms. TALE nucleases (TALENs), generated by fusing the FokI cleavage domain to TALE, have been used to create genomic double-strand breaks. The identity of the TALE repeat variable di-residues, their number, and their order dictate the DNA sequence specificity. Because TALE repeats are nearly identical, their assembly by cloning or even by synthesis is challenging and time consuming. Here, we report the development and use of a rapid and straightforward approach for the construction of designer TALE (dTALE) activators and nucleases with user-selected DNA target specificity. Using our plasmid set of 100 repeat modules, researchers can assemble repeat domains for any 14-nucleotide target sequence in one sequential restriction-ligation cloning step and in only 24 h. We generated several custom dTALEs and dTALENs with new target sequence specificities and validated their function by transient expression in tobacco leaves and in vitro DNA cleavage assays, respectively. Moreover, we developed a web tool, called idTALE, to facilitate the design of dTALENs and the identification of their genomic targets and potential off-targets in the genomes of several model species. Our dTALE repeat assembly approach along with the web tool idTALE will expedite genome-engineering applications in a variety of cell types and organisms including plants.

Advances in cell and gene-based therapies for cystic fibrosis lung disease

Cystic fibrosis (CF) is a disease characterized by airway infection, inflammation, remodeling, and obstruction that gradually destroy the lungs. Direct delivery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene to airway epithelia may offer advantages, as the tissue is accessible for topical delivery of vectors. Yet, physical and host immune barriers in the lung present challenges for successful gene transfer to the respiratory tract. Advances in gene transfer approaches, tissue engineering, and novel animal models are generating excitement within the CF research field. This review discusses current challenges and advancements in viral and nonviral vectors, cell-based therapies, and CF animal models.

When bacteria target the nucleus: the emerging family of nucleomodulins

The nucleus, at the heart of the eukaryotic cell, hosts and protects the genetic material, governs gene expression and regulates the whole cell physiology, including cell division. A growing number of studies indicate that various animal and plant pathogenic bacteria can deliver factors to this central organelle to subvert host defences by directly interfering with transcription, chromatin-remodelling, RNA splicing or DNA replication and repair. Such bacterial molecules entering the nucleus, which we propose to term 'nucleomodulins', use diverse strategies to hijack nuclear processes by targeting host DNA or an array of nuclear proteins. In some cases, bacteria can even enter the nucleus. These bacterial 'nuclear attacks' might have permanent genetic or long-term epigenetic effects on the host. Studying nucleomodulins and endonuclear bacteria can thus generate new insights into long-term impacts of infectious diseases and create novel tools for biotechnological applications and for deciphering the regulation of nuclear dynamics.

Optimized TAL effector nucleases (TALENs) for use in treatment of sickle cell disease

TAL effector nucleases (TALENs) represent a new class of artificial nucleases capable of cleaving long, specific target DNA sequences in vivo and are powerful tools for genome editing with potential therapeutic applications. Here we report a pair of custom-designed TALENs for targeted genetic correction of the sickle cell disease mutation in human cells, which represents an example of engineered TALENs capable of recognizing and cleaving a human disease-associated gene. By using a yeast reporter system, a systematic study was carried out to optimize TALEN architecture for maximal in vivo cleavage efficiency. In contrast to the previous reports, the engineered TALENs were capable of recognizing and cleaving target binding sites preceded by A, C or G. More importantly, the optimized TALENs efficiently cleaved a target sequence within the human β-globin (HBB) gene associated with sickle cell disease and increased the efficiency of targeted gene repair by >1000-fold in human cells. In addition, these TALENs showed no detectable cytotoxicity. These results demonstrate the potential of optimized TALENs as a powerful genome editing tool for therapeutic applications.

Targeted transcriptional repression using a chimeric TALE-SRDX repressor protein

Transcriptional activator-like effectors (TALEs) are proteins secreted by Xanthomonas bacteria when they infect plants. TALEs contain a modular DNA binding domain that can be easily engineered to bind any sequence of interest, and have been used to provide user-selected DNA-binding modules to generate chimeric nucleases and transcriptional activators in mammalian cells and plants. Here we report the use of TALEs to generate chimeric sequence-specific transcriptional repressors. The dHax3 TALE was used as a scaffold to provide a DNA-binding module fused to the EAR-repression domain (SRDX) to generate a chimeric repressor that targets the RD29A promoter. The dHax3.SRDX protein efficiently repressed the transcription of the RD29A::LUC transgene and endogenous RD29A gene in Arabidopsis. Genome wide expression profiling showed that the chimeric repressor also inhibited the expression of several other genes that contain the designer TALE-target sequence in their promoters. Our data suggest that TALEs can be used to generate chimeric repressors to specifically repress the transcription of genes of interest in plants. This sequence-specific transcriptional repression by direct on promoter effector technology is a powerful tool for functional genomics studies and biotechnological applications.

A transcription activator-like effector toolbox for genome engineering

Transcription activator-like effectors (TALEs) are a class of naturally occurring DNA-binding proteins found in the plant pathogen Xanthomonas sp. The DNA-binding domain of each TALE consists of tandem 34-amino acid repeat modules that can be rearranged according to a simple cipher to target new DNA sequences. Customized TALEs can be used for a wide variety of genome engineering applications, including transcriptional modulation and genome editing. Here we describe a toolbox for rapid construction of custom TALE transcription factors (TALE-TFs) and nucleases (TALENs) using a hierarchical ligation procedure. This toolbox facilitates affordable and rapid construction of custom TALE-TFs and TALENs within 1 week and can be easily scaled up to construct TALEs for multiple targets in parallel. We also provide details for testing the activity in mammalian cells of custom TALE-TFs and TALENs using quantitative reverse-transcription PCR and Surveyor nuclease, respectively. The TALE toolbox described here will enable a broad range of biological applications.

Double-strand break-induced targeted mutagenesis in plants

Double-strand breaks are very potent inducers of DNA recombination. There is no recombination between DNA molecules unless one or two DNA strands are broken. It has become feasible to introduce double-strand breaks at specific chromosomal loci by using dedicated, redesigned endonucleases with altered DNA-binding specificities. Such breaks are mainly repaired by error-prone nonhomologous recombination pathways in somatic cells, thus frequently producing mutations at the preselected chromosomal sites. Although the art and science of reengineering protein properties have been advancing quickly, an empirical validation of new endonucleases in a particular experimental environment is essential for successful targeted mutagenesis experiments. This chapter presents methods that were developed for a comprehensive evaluation of the DNA-binding and DNA-cutting activities of homing endonucleases in maize cells; however, they can be adopted for similar evaluation studies of other endonucleases and other plant species that are amenable for Agrobacterium-mediated transformation.

Precision editing of large animal genomes

Transgenic animals are an important source of protein and nutrition for most humans and will play key roles in satisfying the increasing demand for food in an ever-increasing world population. The past decade has experienced a revolution in the development of methods that permit the introduction of specific alterations to complex genomes. This precision will enhance genome-based improvement of farm animals for food production. Precision genetics also will enhance the development of therapeutic biomaterials and models of human disease as resources for the development of advanced patient therapies.

Vector systems for prenatal gene therapy: choosing vectors for different applications

This chapter gives a comparative review of the different vector systems applied to date in prenatal gene therapy experiments highlighting the need for versatility and choice for application in accordance with the actual aim of the study. It reviews the key characteristics of the four main gene therapy vector systems and gives examples for their successful application in prenatal gene therapy experiments.

Genome engineering with zinc-finger nucleases

Zinc-finger nucleases (ZFNs) are targetable DNA cleavage reagents that have been adopted as gene-targeting tools. ZFN-induced double-strand breaks are subject to cellular DNA repair processes that lead to both targeted mutagenesis and targeted gene replacement at remarkably high frequencies. This article briefly reviews the history of ZFN development and summarizes applications that have been made to genome editing in many different organisms and situations. Considerable progress has been made in methods for deriving zinc-finger sets for new genomic targets, but approaches to design and selection are still being perfected. An issue that needs more attention is the extent to which available mechanisms of double-strand break repair limit the scope and utility of ZFN-initiated events. The bright prospects for future applications of ZFNs, including human gene therapy, are discussed.

A novel TALE nuclease scaffold enables high genome editing activity in combination with low toxicity

Sequence-specific nucleases represent valuable tools for precision genome engineering. Traditionally, zinc-finger nucleases (ZFNs) and meganucleases have been used to specifically edit complex genomes. Recently, the DNA binding domains of transcription activator-like effectors (TALEs) from the bacterial pathogen Xanthomonas have been harnessed to direct nuclease domains to desired genomic loci. In this study, we tested a panel of truncation variants based on the TALE protein AvrBs4 to identify TALE nucleases (TALENs) with high DNA cleavage activity. The most favorable parameters for efficient DNA cleavage were determined in vitro and in cellular reporter assays. TALENs were designed to disrupt an EGFP marker gene and the human loci CCR5 and IL2RG. Gene editing was achieved in up to 45% of transfected cells. A side-by-side comparison with ZFNs showed similar gene disruption activities by TALENs but significantly reduced nuclease-associated cytotoxicities. Moreover, the CCR5-specific TALEN revealed only minimal off-target activity at the CCR2 locus as compared to the corresponding ZFN, suggesting that the TALEN platform enables the design of nucleases with single-nucleotide specificity. The combination of high nuclease activity with reduced cytotoxicity and the simple design process marks TALENs as a key technology platform for targeted modifications of complex genomes.

Towards artificial metallonucleases for gene therapy: recent advances and new perspectives

The process of DNA targeting or repair of mutated genes within the cell, induced by specifically positioned double-strand cleavage of DNA near the mutated sequence, can be applied for gene therapy of monogenic diseases. For this purpose, highly specific artificial metallonucleases are developed. They are expected to be important future tools of modern genetics. The present state of art and strategies of research are summarized, including protein engineering and artificial ‘chemical’ nucleases. From the results, we learn about the basic role of the metal ions and the various ligands, and about the DNA binding and cleavage mechanism. The results collected provide useful guidance for engineering highly controlled enzymes for use in gene therapy.

TAL effectors: customizable proteins for DNA targeting

Generating and applying new knowledge from the wealth of available genomic information is hindered, in part, by the difficulty of altering nucleotide sequences and expression of genes in living cells in a targeted fashion. Progress has been made in engineering DNA binding domains to direct proteins to particular sequences for mutagenesis or manipulation of transcription; however, achieving the requisite specificities has been challenging. Transcription activator-like (TAL) effectors of plant pathogenic bacteria contain a modular DNA binding domain that appears to overcome this challenge. Comprising tandem, polymorphic amino acid repeats that individually specify contiguous nucleotides in DNA, this domain is being deployed in DNA targeting for applications ranging from understanding gene function in model organisms to improving traits in crop plants to treating genetic disorders in people.

Creating highly specific nucleases by fusion of active restriction endonucleases and catalytically inactive homing endonucleases

Zinc-finger nucleases and TALE nucleases are produced by combining a specific DNA-binding module and a non-specific DNA-cleavage module, resulting in nucleases able to cleave DNA at a unique sequence. Here a new approach for creating highly specific nucleases was pursued by fusing a catalytically inactive variant of the homing endonuclease I-SceI, as DNA binding-module, to the type IIP restriction enzyme PvuII, as cleavage module. The fusion enzymes were designed to recognize a composite site comprising the recognition site of PvuII flanked by the recognition site of I-SceI. In order to reduce activity on PvuII sites lacking the flanking I-SceI sites, the enzymes were optimized so that the binding of I-SceI to its sites positions PvuII for cleavage of the composite site. This was achieved by optimization of the linker and by introducing amino acid substitutions in PvuII which decrease its activity or disturb its dimer interface. The most specific variant showed a more than 1000-fold preference for the addressed composite site over an unaddressed PvuII site. These results indicate that using a specific restriction enzyme, such as PvuII, as cleavage module, offers an alternative to the otherwise often used catalytic domain of FokI, which by itself does not contribute to the specificity of the engineered nuclease.

DNA nicks promote efficient and safe targeted gene correction

Targeted gene correction employs a site-specific DNA lesion to promote homologous recombination that eliminates mutation in a disease gene of interest. The double-strand break typically used to initiate correction can also result in genomic instability if deleterious repair occurs rather than gene correction, possibly compromising the safety of targeted gene correction. Here we show that single-strand breaks (nicks) and double-strand breaks both promote efficient gene correction. However, breaks promote high levels of inadvertent but heritable genomic alterations both locally and elsewhere in the genome, while nicks are accompanied by essentially no collateral local mutagenesis, and thus provide a safer approach to gene correction. Defining efficacy as the ratio of gene correction to local deletion, nicks initiate gene correction with 70-fold greater efficacy than do double-strand breaks (29.0±6.0% and 0.42±0.03%, respectively). Thus nicks initiate efficient gene correction, with limited local mutagenesis. These results have clear therapeutic implications, and should inform future design of meganucleases for targeted gene correction.

Carbon dioxide concentrating mechanism in Chlamydomonas reinhardtii: inorganic carbon transport and CO2 recapture

Many microalgae are capable of acclimating to CO(2) limited environments by operating a CO(2) concentrating mechanism (CCM), which is driven by various energy-coupled inorganic carbon (Ci; CO(2) and HCO(3)(-)) uptake systems. Chlamydomonas reinhardtii (hereafter, Chlamydomonas), a versatile genetic model organism, has been used for several decades to exemplify the active Ci transport in eukaryotic algae, but only recently have many molecular details behind these Ci uptake systems emerged. Recent advances in genetic and molecular approaches, combined with the genome sequencing of Chlamydomonas and several other eukaryotic algae have unraveled some unique characteristics associated with the Ci uptake mechanism and the Ci-recapture system in eukaryotic microalgae. Several good candidate genes for Ci transporters in Chlamydomonas have been identified, and a few specific gene products have been linked with the Ci uptake systems associated with the different acclimation states. This review will focus on the latest studies on characterization of functional components involved in the Ci uptake and the Ci-recapture in Chlamydomonas.

Modularly assembled designer TAL effector nucleases for targeted gene knockout and gene replacement in eukaryotes

Recent studies indicate that the DNA recognition domain of transcription activator-like (TAL) effectors can be combined with the nuclease domain of FokI restriction enzyme to produce TAL effector nucleases (TALENs) that, in pairs, bind adjacent DNA target sites and produce double-strand breaks between the target sequences, stimulating non-homologous end-joining and homologous recombination. Here, we exploit the four prevalent TAL repeats and their DNA recognition cipher to develop a 'modular assembly' method for rapid production of designer TALENs (dTALENs) that recognize unique DNA sequence up to 23 bases in any gene. We have used this approach to engineer 10 dTALENs to target specific loci in native yeast chromosomal genes. All dTALENs produced high rates of site-specific gene disruptions and created strains with expected mutant phenotypes. Moreover, dTALENs stimulated high rates (up to 34%) of gene replacement by homologous recombination. Finally, dTALENs caused no detectable cytotoxicity and minimal levels of undesired genetic mutations in the treated yeast strains. These studies expand the realm of verified TALEN activity from cultured human cells to an intact eukaryotic organism and suggest that low-cost, highly dependable dTALENs can assume a significant role for gene modifications of value in human and animal health, agriculture and industry.

Zinc-finger nucleases for somatic gene therapy: the next frontier

Zinc-finger nucleases (ZFNs) are a powerful tool that can be used to edit the human genome ad libitum. The technology has experienced remarkable development in the last few years with regard to both the target site specificity and the engineering platforms used to generate zinc-finger proteins. As a result, two phase I clinical trials aimed at knocking out the CCR5 receptor in T cells isolated from HIV patients to protect these lymphocytes from infection with the virus have been initiated. Moreover, ZFNs have been successfully employed to knockout or correct disease-related genes in human stem cells, including hematopoietic precursor cells and induced pluripotent stem cells. Targeted genome engineering approaches in multipotent and pluripotent stem cells hold great promise for future strategies geared toward correcting inborn mutations for personalized cell replacement therapies. This review describes how ZFNs have been applied to models of gene therapy, discusses the opportunities and the risks associated with this novel technology, and suggests future directions for their safe application in therapeutic genome engineering.

Improving immunity in crops: new tactics in an old game

Crop disease remains a major cause of yield loss and emerging diseases pose new threats to global food security. Despite the dearth of commercial development to date, progress in using our rapidly expanding knowledge of plant-pathogen interactions to invent new ways of controlling diseases in crops has been good. Many major resistance genes have now been shown to retain function when transferred between species, and evidence indicates that resistance genes are more effective when deployed in a background containing quantitative resistance traits. The EFR pattern-recognition receptor, present in only the Brassicaceae, functions to provide bacterial disease control in the Solanaceae. Knowledge of how transcription activator-like effectors bind DNA is leading to new methods for triggering disease resistance and broader applications in genome engineering.

Tapping natural reservoirs of homing endonucleases for targeted gene modification

Homing endonucleases mobilize their own genes by generating double-strand breaks at individual target sites within potential host DNA. Because of their high specificity, these proteins are used for "genome editing" in higher eukaryotes. However, alteration of homing endonuclease specificity is quite challenging. Here we describe the identification and phylogenetic analysis of over 200 naturally occurring LAGLIDADG homing endonucleases (LHEs). Biochemical and structural characterization of endonucleases from one clade within the phylogenetic tree demonstrates strong conservation of protein structure contrasted against highly diverged DNA target sites and indicates that a significant fraction of these proteins are sufficiently stable and active to serve as engineering scaffolds. This information was exploited to create a targeting enzyme to disrupt the endogenous monoamine oxidase B gene in human cells. The ubiquitous presence and diversity of LHEs described in this study may facilitate the creation of many tailored nucleases for genome editing.

Targeted mutagenesis of duplicated genes in soybean with zinc-finger nucleases

We performed targeted mutagenesis of a transgene and nine endogenous soybean (Glycine max) genes using zinc-finger nucleases (ZFNs). A suite of ZFNs were engineered by the recently described context-dependent assembly platform--a rapid, open-source method for generating zinc-finger arrays. Specific ZFNs targeting dicer-like (DCL) genes and other genes involved in RNA silencing were cloned into a vector under an estrogen-inducible promoter. A hairy-root transformation system was employed to investigate the efficiency of ZFN mutagenesis at each target locus. Transgenic roots exhibited somatic mutations localized at the ZFN target sites for seven out of nine targeted genes. We next introduced a ZFN into soybean via whole-plant transformation and generated independent mutations in the paralogous genes DCL4a and DCL4b. The dcl4b mutation showed efficient heritable transmission of the ZFN-induced mutation in the subsequent generation. These findings indicate that ZFN-based mutagenesis provides an efficient method for making mutations in duplicate genes that are otherwise difficult to study due to redundancy. We also developed a publicly accessible Web-based tool to identify sites suitable for engineering context-dependent assembly ZFNs in the soybean genome.

Transcriptional activators of human genes with programmable DNA-specificity

TAL (transcription activator-like) effectors are translocated by Xanthomonas bacteria into plant cells where they activate transcription of target genes. DNA target sequence recognition occurs in a unique mode involving a central domain of tandem repeats. Each repeat recognizes a single base pair in a contiguous DNA sequence and a pair of adjacent hypervariable amino acid residues per repeat specifies which base is bound. Rearranging the repeats allows the design of novel TAL proteins with predictable DNA-recognition specificities. TAL protein-based transcriptional activation in plant cells is mediated by a C-terminal activation domain (AD). Here, we created synthetic TAL proteins with designed repeat compositions using a novel modular cloning strategy termed “Golden TAL Technology”. Newly programmed TAL proteins were not only functional in plant cells, but also in human cells and activated targeted expression of exogenous as well as endogenous genes. Transcriptional activation in different human cell lines was markedly improved by replacing the TAL-AD with the VP16-AD of herpes simplex virus. The creation of TAL proteins with potentially any desired DNA-recognition specificity allows their versatile use in biotechnology.

Assembly of designer TAL effectors by Golden Gate cloning

Generation of customized DNA binding domains targeting unique sequences in complex genomes is crucial for many biotechnological applications. The recently described DNA binding domain of the transcription activator-like effectors (TALEs) from Xanthomonas consists of a series of repeats arranged in tandem, each repeat binding a nucleotide of the target sequence. We present here a strategy for engineering of TALE proteins with novel DNA binding specificities based on the 17.5 repeat-containing AvrBs3 TALE as a scaffold. For each of the 17 full repeats, four module types were generated, each with a distinct base preference. Using this set of 68 repeat modules, recognition domains for any 17 nucleotide DNA target sequence of choice can be constructed by assembling selected modules in a defined linear order. Assembly is performed in two successive one-pot cloning steps using the Golden Gate cloning method that allows seamless fusion of multiple DNA fragments. Applying this strategy, we assembled designer TALEs with new target specificities and tested their function in vivo.