Advancing nutritional quality in oilseed crops through genome editing: a comprehensive review

Targeting DNA With Fingers and TALENs.

Transcription activator-like effector (TALE) nuclease (TALEN) is widely used as a tool in genome editing. The DNA binding part of TALEN consists of a tandem array of TAL-repeats that form a right-handed superhelix. Each TAL-repeat recognises a specific base by the repeat variable diresidue (RVD) at positions 12 and 13. TALEN comprising the TAL-repeats with periodic mutations to residues at positions 4 and 32 (non-RVD sites) in each repeat (VT-TALE) exhibits increased efficacy in genome editing compared with a counterpart without the mutations (CT-TALE). The molecular basis for the elevated efficacy is unknown. In this report, comparison of the physicochemical properties between CT- and VT-TALEs revealed that VT-TALE has a larger amplitude motion along the superhelical axis (superhelical motion) compared with CT-TALE. The greater superhelical motion in VT-TALE enabled more TAL-repeats to engage in the target sequence recognition compared with CT-TALE. The extended sequence recognition by the TAL-repeats improves site specificity with limiting the spatial distribution of FokI domains to facilitate their dimerization at the desired site. Molecular dynamics simulations revealed that the non-RVD mutations alter inter-repeat hydrogen bonding to amplify the superhelical motion of VT-TALE. The TALEN activity is associated with the inter-repeat hydrogen bonding among the TAL repeats.

Genetically modified animals such as knockout mice are essential for elucidating in vivo gene functions and identifying genetic contributions to the molecular pathophysiology of human diseases. For the past two decades, knockout mice have been created via embryonic stem (ES) cell-based gene targeting, a time-consuming, laborious, inefficient, and expensive process. The rapid emergence of targeted genome editing technologies is drastically revolutionizing this situation. Genome editing mediated by transcription activator-like effector (TALE) nucleases (TALENs), one of the popular genome editing tools, is a simple and powerful gene-targeting technology. With its extremely high efficiency, the mouse genome can be manipulated directly in fertilized eggs without any targeting vector or selection steps by a process called in vivo genome editing. TALEN-mediated in vivo genome editing provides an exciting opportunity for simple, convenient, and ultra-rapid production of precisely targeted knockout and knockin mice. Using this technology, researchers can freely and routinely manipulate mouse genomes and accelerate in vivo functional genomic research.

Creation of gene-specific rice mutants by AvrXa23-based TALENs

Although noncancerous immortalized cell lines have been developed by introducing genes into human and murine somatic cells, such cell lines have not been available in large domesticated animals like pigs. For immortalizing porcine cells, primary porcine fetal fibroblasts were isolated and cultured using the human telomerase reverse transcriptase (hTERT) gene. After selecting cells with neomycin for 2 weeks, outgrowing colonized cells were picked up and subcultured for expansion. Immortalized cells were cultured for more than 9 months without changing their doubling time (~24 hours) or their diameter (< 20 µm) while control cells became replicatively senescent during the same period. Even a single cell expanded to confluence in 100 mm dishes. Furthermore, to knockout the CMAH gene, designed plasmids encoding a transcription activator-like effector nuclease (TALENs) pairs were transfected into the immortalized cells. Each single colony was analyzed by the mutation-sensitive T7 endonuclease I assay, fluorescent PCR, and dideoxy sequencing to obtain three independent clonal populations of cells that contained biallelic modifications. One CMAH knockout clone was chosen and used for somatic cell nuclear transfer. Cloned embryos developed to the blastocyst stage. In conclusion, we demonstrated that immortalized porcine fibroblasts were successfully established using the human hTERT gene, and the TALENs enabled biallelic gene disruptions in these immortalized cells.

Recently, transcription activator-like effector nucleases (TALENs) have emerged as a highly effective tool for genomic editing. A pair of TALENs binds to two DNA recognition sites separated by a spacer sequence, and the dimerized FokI nucleases at the C terminal then cleave DNA in the spacer. Because of its modular design and capacity to precisely target almost any desired genomic locus, TALENs is a technology that can revolutionize the entire biomedical research field. Currently, for genomic editing in cultured cells, two plasmids encoding a pair of TALENs are co-transfected, followed by limited dilution to isolate cell colonies with the intended genomic manipulation. However, uncertain transfection efficiency becomes a bottleneck, especially in hard-to-transfect cells, reducing the overall efficiency of genome editing. We have developed a robust TALENs system in which each TALEN plasmid also encodes a fluorescence protein. Thus, cells transfected with both TALEN plasmids, a prerequisite for genomic editing, can be isolated by fluorescence-activated cell sorting. Our improved TALENs system can be applied to all cultured cells to achieve highly efficient genomic editing. Furthermore, an optimized procedure for genomic editing using TALENs is also presented. We expect our system to be widely adapted by the scientific community. Citation Format: Siliang Zhang, Yuanxi Feng, Xin Huang. A robust TALENs system for highly efficient mammalian genome editing. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3482. doi:10.1158/1538-7445.AM2014-3482

Gene Editing on Center Stage

BackgroundGenome of an organism has always fascinated life scientists. With the discovery of restriction endonucleases, scientists were able to make targeted manipulations (knockouts) in any gene sequence of any organism, by the technique popularly known as genome engineering. Though there is a range of genome editing tools, but this era of genome editing is dominated by the CRISPR/Cas9 tool due to its ease of design and handling. But, when it comes to clinical applications, CRISPR is not usually preferred. In this review, we will elaborate on the structural and functional role of designer nucleases with emphasis on TALENs and CRISPR/Cas9 genome editing system. We will also present the unique features of TALENs and limitations of CRISPRs which makes TALENs a better genome editing tool than CRISPRs. Main bodyGenome editing is a robust technology used to make target specific DNA modifications in the genome of any organism. With the discovery of robust programmable endonucleases-based designer gene manipulating tools such as meganucleases (MN), zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats associated protein (CRISPR/Cas9), the research in this field has experienced a tremendous acceleration giving rise to a modern era of genome editing with better precision and specificity. Though, CRISPR-Cas9 platform has successfully gained more attention in the scientific world, TALENs and ZFNs are unique in their own ways. Apart from high-specificity, TALENs are proven to target the mitochondrial DNA (mito-TALEN), where gRNA of CRISPR is difficult to import. This review talks about genome editing goals fulfilled by TALENs and drawbacks of CRISPRs. ConclusionsThis review provides significant insights into the pros and cons of the two most popular genome editing tools TALENs and CRISPRs. This mini review suggests that, TALENs provides novel opportunities in the field of therapeutics being highly specific and sensitive toward DNA modifications. In this article, we will briefly explore the special features of TALENs that makes this tool indispensable in the field of synthetic biology. This mini review provides great perspective in providing true guidance to the researchers working in the field of trait improvement via genome editing.

Recently, transcription activator–like effector nucleases (TALENs) have emerged as a highly effective tool for genomic editing. A pair of TALENs binds to two DNA recognition sites separated by a spacer sequence, and the dimerized FokI nucleases at the C terminal then cleave DNA in the spacer. Because of its modular design and capacity to precisely target almost any desired genomic locus, TALEN is a technology that can revolutionize the entire biomedical research field. Currently, for genomic editing in cultured cells, two plasmids encoding a pair of TALENs are co-transfected, followed by limited dilution to isolate cell colonies with the intended genomic manipulation. However, uncertain transfection efficiency becomes a bottleneck, especially in hard-to-transfect cells, reducing the overall efficiency of genome editing. We have developed a robust TALENs system in which each TALEN plasmid also encodes a fluorescence protein. Thus, cells transfected with both TALEN plasmids, a prerequisite for genomic editing, can be isolated by fluorescence-activated cell sorting. Our improved TALENs system can be applied to all cultured cells to achieve highly efficient genomic editing. Furthermore, an optimized procedure for genomic editing using TALENs is also presented. We expect our system to be widely adopted by the scientific community.

Transcription activator-like effector (TALE) nucleases (TALENs) have recently emerged as a revolutionary genome editing tool in many different organisms and cell types. The site-specific chromosomal double-strand breaks introduced by TALENs significantly increase the efficiency of genomic modification. The modular nature of the TALE central repeat domains enables researchers to tailor DNA recognition specificity with ease and target essentially any desired DNA sequence. Here, we comprehensively review the development of TALEN technology in terms of scaffold optimization, DNA recognition, and repeat array assembly. In addition, we provide some perspectives on the future development of this technology.

Alongside other aspects of agriculture, plant breeding is pivotal to securing crop yields necessary to meet the growing demands for human food and animal feed. In addition to the important targets of yield, nutritional quality and resilience to abiotic stresses, breeding for resistance to pests and diseases will become even more critical as the availability of plant protection products is further diminished by regulation and/or the lack of new active ingredients coming to market (Chapman 2014) and by consumer preferences for fewer inputs. In this regard, it is fortuitous and timely that future crop genetic improvement will be aided by several developments in plant breeding that are underpinned by the massive increase in DNA sequence information flowing into databases from new methods of reading DNA that were developed in the mid 2000's (Moorthie et al. 2011). A stark example of this revolution in sequencing speed is seen in the progress of rice genomics where, after about three years work, the first draft genomic sequence of rice was published in 2002 (Goff et al. 2002; Yu et al. 2002). Yet, little over ten years later, IRRI published the genomic sequence of 3000 different rice varieties (Alexandrov 2015). This step‐change in sequencing and bioinformatics resulted in the generation of massive data sets that can be mined to give information about the genetic location and function of specific alleles which in turn can be used to inform and facilitate crop improvement via conventional breeding methods or via a spectrum of rapidly evolving molecular breeding technologies and concepts of synthetic biology. Two such technologies close to commercialization are genome editing and interorganism silencing which are discussed below. However, the plant breeder faces significant challenges to fully integrate these novel approaches to produce new varieties because of ambiguities and uncertainties surrounding risk assessment and regulation.

Human pluripotent stem cells (hPSCs) offer unprecedented opportunities to study cellular differentiation and model human diseases. The ability to precisely modify any genomic sequence holds the key to realizing the full potential of hPSCs. Thanks to the rapid development of novel genome editing technologies driven by the enormous interest in the hPSC field, genome editing in hPSCs has evolved from being a daunting task a few years ago to a routine procedure in most laboratories. Here, we provide an overview of the mainstream genome editing tools, including zinc finger nucleases, transcription activator-like effector nucleases, clustered regularly interspaced short palindromic repeat/CAS9 RNA-guided nucleases, and helper-dependent adenoviral vectors. We discuss the features and limitations of these technologies, as well as how these factors influence the utility of these tools in basic research and therapies.

Detailed analysis of targeted gene mutations caused by the Platinum-Fungal TALENs in Aspergillus oryzae RIB40 strain and a ligD disruptant

Secondary metabolites, including alkaloids, flavonoids, and tannins, are crucial for human health, agriculture, and ecosystem functioning. Their synthesis is often species-specific, influenced by both genetic and environmental factors. The increasing demand for these compounds across various industries highlights the need for advancements in plant breeding and biotechnological approaches. Transcription activator-like effector nucleases (TALENs) have emerged as a powerful tool for precise genome editing, offering significant potential for enhancing the synthesis of secondary metabolites in plants. However, while plant genome editing technologies have advanced significantly, the application of TALENs in improving secondary metabolite production and expanding genetic diversity remains underexplored. Therefore, this review aims to provide a comprehensive analysis of TALEN-mediated genome editing in plants, focusing on their role in enhancing secondary metabolite biosynthetic pathways and improving genetic diversity. The mechanisms underlying TALENs are examined, including their ability to target specific genes involved in the synthesis of bioactive compounds, highlighting comparisons with other genome editing tools such as CRISPR/Cas9. This review further highlights key applications in medicinal plants, particularly the modification of pathways responsible for alkaloids, flavonoids, terpenoids, and phenolic compounds. Furthermore, the role of TALENs in inducing genetic variation, improving stress tolerance, and facilitating hybridization in plant breeding programs is highlighted. Recent advances, challenges, and limitations associated with using TALENs for enhancing secondary metabolite production are critically evaluated. In this review, gaps in current research are identified, particularly regarding the integration of TALENs with multi-omics technologies and synthetic biology approaches. The findings suggest that while underutilized, TALENs offer sustainable strategies for producing high-value secondary metabolites in medicinal plants. Future research should focus on optimizing TALEN systems for commercial applications and integrating them with advanced biotechnological platforms to enhance the yield and resilience of medicinal plants.

Genome editing (also known as targeted mutation) has promise for molecular breeding. Compared with the CRISPR/Cas9 system, the transcription activator-like effector nucleases (TALENs) have likely a lesser off-target rate in genome editing. Both a rapid test system for the functionality of designed TALENs and an effective delivery system for introducing the TALENs to plants are critical for successful target mutation. TALENs have usually been tested in protoplasts or introduced to plants with viral vectors. However, plant regeneration from protoplast culture can generate extensive somatic variation. Viral vectors are not always available, and plants edited by these vectors usually require virus elimination. Here, we used a non-viral, Agrobacterium-mediated transient expression approach, to serve both rapid test and effective delivery of TALENs into two vegetatively propagated potato cultivars, Solanum tuberosum Russet Burbank and Shepody. Two TALENs with different molecular weights (22 and 27 aa-repeat modules) were expressed to target two endogenous genes (starch branching enzyme and an acid invertase) by Agrobacterium-mediated infiltration (agroinfiltration) into leaves of potato plants. The infiltrated leaf DNA was analyzed using restriction site loss assay and subsequent DNA sequencing. Deep sequencing of these tetraploid cultivars was also conducted to determine the zygosity at the targeted chromosomal loci. TALENs, with different molecular weights, successfully agroinfiltrated and induced mutations at both targeted loci.