A high-throughput screen identifying sequence and promiscuity characteristics of the loxP spacer region in Cre-mediated recombination

Delivering large genes using adeno-associated virus and the CRE-lox DNA recombination system

Adeno-associated virus (AAV) is a safe and efficient gene delivery vehicle for gene therapies. However, its relatively small packaging capacity limits its use as a gene transfer vector. Here, we describe a strategy to deliver large genes that exceed the AAV's packaging capacity using up to four AAV vectors and the CRE-lox DNA recombination system. We devised novel lox sites by combining non-compatible and reaction equilibrium-modifying lox site variants. These lox sites facilitate sequence-specific and near-unidirectional recombination of AAV vector genomes, enabling efficient reconstitution of up to 16 kb of therapeutic genes in a pre-determined configuration. Using this strategy, we have developed AAV gene therapy vectors to deliver IFT140, PCDH15, CEP290, and CDH23 and demonstrate efficient production of full-length proteins in cultured mammalian cells and mouse retinas. Notably, AAV-IFT140 gene therapy vectors ameliorated retinal degeneration and preserved visual functions in an IFT140-associated retinitis pigmentosa mouse model. The CRE-lox approach described here provides a simple, flexible, and effective platform for generating AAV gene therapy vectors beyond AAV's packaging capacity.

Dynamics in Cre-loxP site-specific recombination

Cre recombinase is a phage-derived enzyme that has found utility for precise manipulation of DNA sequences. Cre recognizes and recombines pairs of loxP sequences characterized by an inverted repeat and asymmetric spacer. Cre cleaves and religates its DNA targets such that error-prone repair pathways are not required to generate intact DNA products. Major obstacles to broader applications are lack of knowledge of how Cre recognizes its targets, and how its activity is controlled. The picture emerging from high resolution methods is that the dynamic properties of both the enzyme and its DNA target are important determinants of its activity in both sequence recognition and DNA cleavage. Improved understanding of the role of dynamics in the key steps along the pathway of Cre-loxP recombination should significantly advance our ability to both redirect Cre to new sequences and to control its DNA cleavage activity in the test tube and in cells.

Engineering spacer specificity of the Cre/loxP system

Translational research on the Cre/loxP recombination system focuses on enhancing its specificity by modifying Cre/DNA interactions. Despite extensive efforts, the exact mechanisms governing Cre discrimination between substrates remains elusive. Cre recognizes 13 bp inverted repeats, initiating recombination in the 8 bp spacer region. While literature suggests that efficient recombination proceeds between lox sites with non-loxP spacer sequences when both lox sites have matching spacers, experimental validation for this assumption is lacking. To fill this gap, we investigated target site variations of identical pairs of the loxP 8 bp spacer region, screening 6000 unique loxP-like sequences. Approximately 84% of these sites exhibited efficient recombination, affirming the plasticity of spacer sequences for catalysis. However, certain spacers negatively impacted recombination, emphasizing sequence dependence. Directed evolution of Cre on inefficiently recombined spacers not only yielded recombinases with enhanced activity but also mutants with reprogrammed selective activity. Mutations altering spacer specificity were identified, and molecular modelling and dynamics simulations were used to investigate the possible mechanisms behind the specificity switch. Our findings highlight the potential to fine-tune site-specific recombinases for spacer sequence specificity, offering a novel concept to enhance the applied properties of designer-recombinases for genome engineering applications.

Redefining the bacteriophage mv4 site-specific recombination system and the sequence specificity of its attB and core-attP sites

Through their involvement in the integration and excision of a large number of mobile genetic elements, such as phages and integrative and conjugative elements (ICEs), site-specific recombination systems based on heterobivalent tyrosine recombinases play a major role in genome dynamics and evolution. However, despite hundreds of these systems having been identified in genome databases, very few have been described in detail, with none from phages that infect Bacillota (formerly Firmicutes). In this study, we reanalyzed the recombination module of Lactobacillus delbrueckii subsp. bulgaricus phage mv4, previously considered atypical compared with classical systems. Our results reveal that mv4 integrase is a 369 aa protein with all the structural hallmarks of recombinases from the Tn916 family and that it cooperatively interacts with its recombination sites. Using randomized DNA libraries, NGS sequencing, and other molecular approaches, we show that the 21-bp core-attP and attB sites have structural similarities to classical systems only if considering the nucleotide degeneracy, with two 7-bp inverted regions corresponding to mv4Int core-binding sites surrounding a 7-bp strand-exchange region. We also examined the different compositional constraints in the core-binding regions, which define the sequence space of permissible recombination sites.

Delivering large genes using adeno-associated virus and the CRE-lox DNA recombination system

Adeno-associated virus (AAV) is a safe and efficient gene delivery vehicle for gene therapies. However, its relatively small packaging capacity limits its use as a gene transfer vector. Here, we describe a strategy to deliver large genes that exceed the AAV's packaging capacity using up to four AAV vectors and the CRE-lox DNA recombination system. We devised novel lox sites by combining non-compatible and reaction equilibrium-modifying lox site variants. These lox sites facilitate sequence-specific and near-unidirectional recombination of AAV vector genomes, enabling efficient reconstitution of up to 16 kb of therapeutic genes in a pre-determined configuration. Using this strategy, we have developed AAV gene therapy vectors to deliver IFT140 , PCDH15 , CEP290 , and CDH23 and demonstrate efficient production of full-length proteins in cultured mammalian cells and mouse retinas. Notably, this approach significantly surpasses the trans-splicing and split-intein-based reconstitution methods in efficiency, requiring lower doses, minimizing or eliminating the production of truncated protein products, and offering flexibility in selecting splitting positions. The CRE-lox approach described here provides a simple and effective platform for producing AAV gene therapy vectors beyond AAV's packaging capacity.

Orthogonal LoxPsym sites allow multiplexed site-specific recombination in prokaryotic and eukaryotic hosts

Site-specific recombinases such as the Cre-LoxP system are routinely used for genome engineering in both prokaryotes and eukaryotes. Importantly, recombinases complement the CRISPR-Cas toolbox and provide the additional benefit of high-efficiency DNA editing without generating toxic DNA double-strand breaks, allowing multiple recombination events at the same time. However, only a handful of independent, orthogonal recombination systems are available, limiting their use in more complex applications that require multiple specific recombination events, such as metabolic engineering and genetic circuits. To address this shortcoming, we develop 63 symmetrical LoxP variants and test 1192 pairwise combinations to determine their cross-reactivity and specificity upon Cre activation. Ultimately, we establish a set of 16 orthogonal LoxPsym variants and demonstrate their use for multiplexed genome engineering in both prokaryotes (E. coli) and eukaryotes (S. cerevisiae and Z. mays). Together, this work yields a significant expansion of the Cre-LoxP toolbox for genome editing, metabolic engineering and other controlled recombination events, and provides insights into the Cre-LoxP recombination process.

Viral vector-mediated transgene delivery with novel recombinase systems for targeting neuronal populations defined by multiple features

A comprehensive understanding of neuronal diversity and connectivity is essential for understanding the anatomical and cellular mechanisms that underlie functional contributions. With the advent of single-cell analysis, growing information regarding molecular profiles leads to the identification of more heterogeneous cell types. Therefore, the need for additional orthogonal recombinase systems is increasingly apparent, as heterogeneous tissues can be further partitioned into increasing numbers of specific cell types defined by multiple features. Critically, new recombinase systems should work together with pre-existing systems without cross-reactivity in vivo. Here, we introduce novel site-specific recombinase systems based on ΦC31 bacteriophage recombinase for labeling multiple cell types simultaneously and a novel viral strategy for versatile and robust intersectional expression of any transgene. Together, our system will help researchers specifically target different cell types with multiple features in the same animal.

Pioneering DNA assembling techniques and their applications in eukaryotic microalgae

Assembling DNA fragments is a fundamental manipulation of cloning microalgal genes and carrying out microalgal synthetic biological studies. From the earliest DNA recombination to current trait and metabolic pathway engineering, we are always accompanied by homology-based DNA assembling. The improvement and modification of pioneering DNA assembling techniques and the combinational applications of the available assembling techniques have diversified and complicated the literature environment and aggravated our identification of the core and pioneering methodologies. Identifying the core assembling methodologies and using them appropriately and flourishing them even are important for researchers. A group of microalgae have been evolving as the models for both industrial applications and biological studies. DNA assembling requires researchers to know the methods available and their improvements and evolvements. In this review, we summarized the pioneering (core; leading) DNA assembling techniques developed previously, extended these techniques to their modifications, improvements and their combinations, and highlighted their applications in eukaryotic microalgae. We predicted that the gene(s) will be assembled into a functional cluster (e.g., those involving in a metabolic pathway, and stacked on normal microalgal chromosomes, their artificial episomes and looming artificial chromosomes. It should be particularly pointed out that the techniques mentioned in this review are classified according to the strategy used to assemble the final construct.

Cre-loxP-mediated genetic lineage tracing: Unraveling cell fate and origin in the developing heart

The Cre-loxP-mediated genetic lineage tracing system is essential for constructing the fate mapping of single-cell progeny or cell populations. Understanding the structural hierarchy of cardiac progenitor cells facilitates unraveling cell fate and origin issues in cardiac development. Several prospective Cre-loxP-based lineage-tracing systems have been used to analyze precisely the fate determination and developmental characteristics of endocardial cells (ECs), epicardial cells, and cardiomyocytes. Therefore, emerging lineage-tracing techniques advance the study of cardiovascular-related cellular plasticity. In this review, we illustrate the principles and methods of the emerging Cre-loxP-based genetic lineage tracing technology for trajectory monitoring of distinct cell lineages in the heart. The comprehensive demonstration of the differentiation process of single-cell progeny using genetic lineage tracing technology has made outstanding contributions to cardiac development and homeostasis, providing new therapeutic strategies for tissue regeneration in congenital and cardiovascular diseases (CVDs).

The role of replication-induced chromosomal copy numbers in spatio-temporal gene regulation and evolutionary chromosome plasticity

For a coherent response to environmental changes, bacterial evolution has formed a complex transcriptional regulatory system comprising classical DNA binding proteins sigma factors and modulation of DNA topology. In this study, we investigate replication-induced gene copy numbers - a regulatory concept that is unlike the others not based on modulation of promoter activity but on replication dynamics. We show that a large fraction of genes are predominantly affected by transient copy numbers and identify cellular functions and central pathways governed by this mechanism in Escherichia coli. Furthermore, we show quantitatively that the previously observed spatio-temporal expression pattern between different growth phases mainly emerges from transient chromosomal copy numbers. We extend the analysis to the plant pathogen Dickeya dadantii and the biotechnologically relevant organism Vibrio natriegens. The analysis reveals a connection between growth phase dependent gene expression and evolutionary gene migration in these species. A further extension to the bacterial kingdom indicates that chromosome evolution is governed by growth rate related transient copy numbers.

Evaluation of a library of loxP variants with a wide range of recombination efficiencies by Cre

Sparse labeling of individual cells is an important approach in neuroscience and many other fields of research. Various methods have been developed to sparsely label only a small population of cells; however, there is no simple and reproducible strategy for managing the probability of sparse labeling at desired levels. Here, we aimed to develop a novel methodology based on the Cre-lox system to regulate sparseness at desired levels, and we purely analyzed cleavage efficiencies of loxP mutants by Cre. We hypothesized that mutations in the loxP sequence reduce the recognition efficiency by Cre, which enables the regulation of the sparseness level of gene expression. In this research, we mutagenized the loxP sequence and analyzed a library of loxP variants. We evaluated more than 1000 mutant loxP sequences, including mutants with reduced excision efficiencies by Cre ranging from 0.51% to 59%. This result suggests that these mutant loxP sequences can be useful in regulating the sparseness of genetic labeling at desired levels.

Mechanisms of Cre recombinase synaptic complex assembly and activation illuminated by Cryo-EM

Cre recombinase selectively recognizes DNA and prevents non-specific DNA cleavage through an orchestrated series of assembly intermediates. Cre recombines two loxP DNA sequences featuring a pair of palindromic recombinase binding elements and an asymmetric spacer region, by assembly of a tetrameric synaptic complex, cleavage of an opposing pair of strands, and formation of a Holliday junction intermediate. We used Cre and loxP variants to isolate the monomeric Cre-loxP (54 kDa), dimeric Cre2-loxP (110 kDa), and tetrameric Cre4-loxP2 assembly intermediates, and determined their structures using cryo-EM to resolutions of 3.9, 4.5 and 3.2 Å, respectively. Progressive and asymmetric bending of the spacer region along the assembly pathway enables formation of increasingly intimate interfaces between Cre protomers and illuminates the structural bases of biased loxP strand cleavage order and half-the-sites activity. Application of 3D variability analysis to the tetramer data reveals constrained conformational sampling along the pathway between protomer activation and Holliday junction isomerization. These findings underscore the importance of protein and DNA flexibility in Cre-mediated site selection, controlled activation of alternating protomers, the basis for biased strand cleavage order, and recombination efficiency. Such considerations may advance development of site-specific recombinases for use in gene editing applications.

Lox'd in translation: contradictions in the nomenclature surrounding common lox-site mutants and their implications in experiments

The Cre-Lox system is a highly versatile and powerful DNA recombinase mechanism, mainly used in genetic engineering to insert or remove desired DNA sequences. It is widely utilized across multiple fields of biology, with applications ranging from plants, to mammals, to microbes. A key feature of this system is its ability to allow recombination between mutant lox sites. Two of the most commonly used mutant sites are named lox66 and lox71, which recombine to create a functionally inactive double mutant lox72 site. However, a large portion of the published literature has incorrectly annotated these mutant lox sites, which in turn can lead to difficulties in replication of methods, design of proper vectors and confusion over the proper nomenclature. Here, we demonstrate common errors in annotations, the impacts they can have on experimental viability, and a standardized naming convention. We also show an example of how this incorrect annotation can induce toxic effects in bacteria that lack optimal DNA repair systems, exemplified by Mycoplasma pneumoniae.

MicroRNAs Regulating Autophagy in Neurodegeneration

Social and economic impacts of neurodegenerative diseases (NDs) become more prominent in our constantly aging population. Currently, due to the lack of knowledge about the aetiology of most NDs, only symptomatic treatment is available for patients. Hence, researchers and clinicians are in need of solid studies on pathological mechanisms of NDs. Autophagy promotes degradation of pathogenic proteins in NDs, while microRNAs post-transcriptionally regulate multiple signalling networks including autophagy. This chapter will critically discuss current research advancements in the area of microRNAs regulating autophagy in NDs. Moreover, we will introduce basic strategies and techniques used in microRNA research. Delineation of the mechanisms contributing to NDs will result in development of better approaches for their early diagnosis and effective treatment.

Budding yeast as a factory to engineer partial and complete microbial genomes

Yeast cells have long been used as hosts to propagate exogenous DNA. Recent progress in genome editing opens new avenues in synthetic biology. These developments allow the efficient engineering of microbial genomes in Saccharomyces cerevisiae that can then be rescued to yield modified bacteria/viruses. Recent examples show that the ability to quickly synthesize, assemble, and/or modify viral and bacterial genomes may be a critical factor to respond to emerging pathogens. However, this process has some limitations. DNA molecules much larger than two megabase pairs are complex to clone, bacterial genomes have proven to be difficult to rescue, and the dual-use potential of these technologies must be carefully considered. Regardless, the use of yeast as a factory has enormous appeal for biological applications.

Toward Shortened the Time-to-Market for Biopharmaceutical Proteins: Improved Fab Protein Expression Stability Using the Cre/lox System in a Multi-Use Clonal Cell Line

Stable gene integration and rapid selection of high-expressing clones are important when developing biopharmaceutical systems to produce a protein of interest. According to regulatory guidelines, the final production clones should be stable through multiple cell generations. To achieve long-term stable expression of Fab genes via recombinase-mediated cassette exchange (RMCE), we modified mutual configurations of the lox sequences. By inversion of the spacer orientation, we avoided the loss of the integrated gene after several dozen cycles of cell division. This feature also prevents reversible transgene integration. Although the RMCE allows us to generate transgenic lines rapidly relative to current methods, it remains difficult to obtain stable industrial cell lines for long-term culturing and for the initial development stage. In this study, we present an approach to shortening the timeline for therapeutic protein development. Our approach provides easy access to the same clonal cell line in the initial development phase, and also for the production of biopharmaceutical proteins.

Design and Derivation of Multi-Reporter Pluripotent Stem Cell Lines via CRISPR/Cas9n-Mediated Homology-Directed Repair

During the past decade, RNA-guided Cas9 nuclease from microbial clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) has become a powerful tool for gene editing of human pluripotent stem cells (PSCs). Using paired CRISPR/Cas9 nickases (CRISPR/Cas9n) it is furthermore possible to reduce off-target effects that may typically occur with traditional CRISPR/Cas9 systems while maintaining high on-target efficiencies. With this technology and a well-designed homology-directed repair vector (HDR), we are now able to integrate transgenes into specific gene loci of PSCs in an allele conserving way. In this protocol we describe CRISPR/Cas9n design and homology directed repair vector design, transfection of human pluripotent stem cells and selection and expansion of generated cell clones. © 2020 The Authors. Basic Protocol 1: Repair template design and CRISPR/Cas9n construction Basic Protocol 2: Transfection of human pluripotent stem cells by electroporation Basic Protocol 3: Genotyping of generated cell clones.

Cre-mediated, loxP independent sequential recombination of a tripartite transcriptional stop cassette allows for partial read-through transcription

One of the widely used applications of the popular Cre-loxP method for targeted recombination is the permanent activation of marker genes, such as reporter genes or antibiotic resistance genes, by excision of a preceding transcriptional stop signal. The STOP cassette consists of three identical SV40-derived poly(A) signal repeats and is flanked by two loxP sites. We found that in addition to complete loxP-mediated recombination, limiting levels of the Cre recombinase also cause incomplete recombination of the STOP cassette. Partial recombination leads to the loss of only one or two of the three identical poly(A) repeats with recombination breakpoints always precisely matching the end/start of each poly(A) signal repeat without any relevant similarity to the canonical or known cryptic loxP sequences, suggesting that this type of Cre-mediated recombination is loxP-independent. Incomplete deletion of the STOP cassette results in partial read-through transcription, explaining at least some of the variability often observed in marker gene expression from an otherwise identical locus.

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.

Molecular tools to create new strains for mosquito sexing and vector control

Vector control programs based on population reduction by matings with mass-released sterile insects require the release of only male mosquitoes, as the release of females, even if sterile, would increase the number of biting and potentially disease-transmitting individuals. While small-scale releases demonstrated the applicability of sterile males releases to control the yellow fever mosquito Aedes aegypti, large-scale programs for mosquitoes are currently prevented by the lack of efficient sexing systems in any of the vector species.Different approaches of sexing are pursued, including classical genetic and mechanical methods of sex separation. Another strategy is the development of transgenic sexing systems. Such systems already exist in other insect pests. Genome modification tools could be used to apply similar strategies to mosquitoes. Three major tools to modify mosquito genomes are currently used: transposable elements, site-specific recombination systems, and genome editing via TALEN or CRISPR/Cas. All three can serve the purpose of developing sexing systems and vector control strains in mosquitoes in two ways: first, via their use in basic research. A better understanding of mosquito biology, including the sex-determining pathways and the involved genes can greatly facilitate the development of sexing strains. Moreover, basic research can help to identify other regulatory elements and genes potentially useful for the construction of transgenic sexing systems. Second, these genome modification tools can be used to apply the gained knowledge to build and test mosquito sexing strains for vector control.

Genome Editing in Model Strain Myxococcus xanthus DK1622 by a Site-Specific Cre/loxP Recombination System

Myxococcus xanthus DK1622 is a rich source of novel secondary metabolites, and it is often used as an expression host of exogenous biosynthetic gene clusters. However, the frequency of obtaining large genome-deletion variants by using traditional strategies is low, and progenies generated by homologous recombination contain irregular deletions. The present study aims to develop an efficient genome-engineering system for this bacterium based on the Cre/loxP system. We first verified the functionality of the native cre system that was integrated into the chromosome with an inducible promoter P_cuoA. Then we assayed the deletion frequency of 8-bp-spacer-sequence mutants in loxP by Cre recombinase which was expressed by suicide vector pBJ113 or self-replicative vector pZJY41. It was found that higher guanine content in a spacer sequence had higher deletion frequency, and the self-replicative vector was more suitable for the Cre/loxP system, probably due to the leaky expression of inducible promoter P_cuoA. We also inspected the effects of different antibiotics and the native or synthetic cre gene. Polymerase chain reaction (PCR) and sequencing of new genome joints confirmed that the Cre/loxP system was able to delete a 466 kb fragment in M. xanthus. This Cre/loxP-mediated recombination could serve as an alternative genetic manipulation method.

Unrestrained markerless trait stacking in Nannochloropsis gaditana through combined genome editing and marker recycling technologies

Stacking traits in microalgae is limited by a lack of robust genome modification tools and selectable marker availability. This presents a key hurdle in developing strains for renewable products including biofuels. Here, we overcome these limitations by combining inducible Cre recombinase with constitutive Cas9 nuclease expression in the industrial strain, Nannochloropsis gaditana. With this system, we demonstrate marker- and reporter-free recapitulation of an important lipid productivity trait. In addition, we generate a strain harboring seven-gene knockouts within the photosystem antennae encoding genes. The combined use of relatively mature (Cre) and emerging (CAS9) genome modification technologies can thus accelerate the pace of industrial strain development and facilitate basic research into functionally redundant gene families.

Robust molecular tool kits in model and industrial microalgae are key to efficient targeted manipulation of endogenous and foreign genes in the nuclear genome for basic research and, as importantly, for the development of algal strains to produce renewable products such as biofuels. While Cas9-mediated gene knockout has been demonstrated in a small number of algal species with varying efficiency, the ability to stack traits or generate knockout mutations in two or more loci are often severely limited by selectable agent availability. This poses a critical hurdle in developing production strains, which require stacking of multiple traits, or in probing functionally redundant gene families. Here, we combine Cas9 genome editing with an inducible Cre recombinase in the industrial alga Nannochloropsis gaditana to generate a strain, NgCas9⁺Cre⁺, in which the potentially unlimited stacking of knockouts and addition of new genes is readily achievable. Cre-mediated marker recycling is first demonstrated in the removal of the selectable marker and GFP reporter transgenes associated with the Cas9/Cre construct in NgCas9⁺Cre⁺. Next, we show the proof-of-concept generation of a markerless knockout in a gene encoding an acyl-CoA oxidase (Aco1), as well as the markerless recapitulation of a 2-kb insert in the ZnCys gene 5′-UTR, which results in a doubling of wild-type lipid productivity. Finally, through an industrially oriented process, we generate mutants that exhibit up to ∼50% reduction in photosynthetic antennae size by markerless knockout of seven genes in the large light-harvesting complex gene family.

Engineering altered protein-DNA recognition specificity

Protein engineering is used to generate novel protein folds and assemblages, to impart new properties and functions onto existing proteins, and to enhance our understanding of principles that govern protein structure. While such approaches can be employed to reprogram protein-protein interactions, modifying protein-DNA interactions is more difficult. This may be related to the structural features of protein-DNA interfaces, which display more charged groups, directional hydrogen bonds, ordered solvent molecules and counterions than comparable protein interfaces. Nevertheless, progress has been made in the redesign of protein-DNA specificity, much of it driven by the development of engineered enzymes for genome modification. Here, we summarize the creation of novel DNA specificities for zinc finger proteins, meganucleases, TAL effectors, recombinases and restriction endonucleases. The ease of re-engineering each system is related both to the modularity of the protein and the extent to which the proteins have evolved to be capable of readily modifying their recognition specificities in response to natural selection. The development of engineered DNA binding proteins that display an ideal combination of activity, specificity, deliverability, and outcomes is not a fully solved problem, however each of the current platforms offers unique advantages, offset by behaviors and properties requiring further study and development.

Rapid pathway prototyping and engineering using in vitro and in vivo synthetic genome SCRaMbLE-in methods

Exogenous pathway optimization and chassis engineering are two crucial methods for heterologous pathway expression. The two methods are normally carried out step-wise and in a trial-and-error manner. Here we report a recombinase-based combinatorial method (termed "SCRaMbLE-in") to tackle both challenges simultaneously. SCRaMbLE-in includes an in vitro recombinase toolkit to rapidly prototype and diversify gene expression at the pathway level and an in vivo genome reshuffling system to integrate assembled pathways into the synthetic yeast genome while combinatorially causing massive genome rearrangements in the host chassis. A set of loxP mutant pairs was identified to maximize the efficiency of the in vitro diversification. Exemplar pathways of β-carotene and violacein were successfully assembled, diversified, and integrated using this SCRaMbLE-in method. High-throughput sequencing was performed on selected engineered strains to reveal the resulting genotype-to-phenotype relationships. The SCRaMbLE-in method proves to be a rapid, efficient, and universal method to fast track the cycle of engineering biology.

Vector and Cell Line Engineering Technologies Toward Recombinant Protein Expression in Mammalian Cell Lines

The rapid growth of global biopharmaceutical market in the recent years has been a good indication of its significance in biotechnology industry. During a long period of time in recombinant protein production from 1980s, optimizations in both upstream and downstream processes were launched. In this regard, one of the most promising strategies is expression vector engineering technology based on incorporation of DNA opening elements found in the chromatin border regions of vectors as well as targeting gene integration. Along with these approaches, cell line engineering has revealed convenient outcomes in isolating high-producing clones. According to the fact that more than 50% of the approved therapeutic proteins is being manufactured in mammalian cell lines, in this review, we focus on several approaches and developments in vector and cell line engineering technologies in mammalian cell culture.

Application of the Cre/lox System to Construct Auxotrophic Markers for Quantitative Genetic Analyses in Fusarium graminearum

The bacteriophage P1 Cre/lox system has been utilized in diverse fungi for marker recycling and exchange, generation of targeted chromosome translocations, and targeted deletion of interstitial chromosome segments. Here we show the application of this tool in the wheat and maize pathogen, Fusarium graminearum. We explored three different ways to introduce Cre into strains with floxed genes, namely transformation with an episomal or integrative plasmid (pLC28), fusion of protoplasts of strains carrying floxed genes with strains expressing Cre by forcing heterokaryons, and crosses between strains with floxed genes and strains expressing Cre to isolate progeny in which the target genes had been deleted during the cross. We used this system for the construction of strains bearing auxotrophic markers that were generated by gene replacement with positively selectable markers followed by Cre-mediated marker excision. In addition, updated protocols for transformation and crosses for F. graminearum are provided. In combination, strains and tools developed here add to the arsenal of methods that can be used to carry out molecular genetics with F. graminearum.

Cloning-free genome engineering in Sinorhizobium meliloti advances applications of Cre/loxP site-specific recombination

The soil-dwelling α-proteobacterium Sinorhizobium meliloti serves as model for studies of symbiotic nitrogen fixation, a highly important process in sustainable agriculture. Here, we report advancements of the genetic toolbox accelerating genome editing in S. meliloti. The hsdMSR operon encodes a type-I restriction-modification (R-M) system. Transformation of S. meliloti is counteracted by the restriction endonuclease HsdR degrading DNA which lacks the appropriate methylation pattern. We provide a stable S. meliloti hsdR deletion mutant showing enhanced transformation with Escherichia coli-derived plasmid DNA and demonstrate that using an E. coli plasmid donor, expressing S. meliloti methyl transferase genes, is an alternative strategy of increasing the transformation efficiency of S. meliloti. Furthermore, we devise a novel cloning-free genome editing (CFGE) method for S. meliloti, Agrobacterium tumefaciens and Xanthomonas campestris, and demonstrate the applicability of this method for intricate applications of the Cre/lox recombination system in S. meliloti. An enhanced Cre/lox system, allowing for serial deletions of large genomic regions, was established. An assay of lox spacer mutants identified a set of lox sites mediating specific recombination. The availability of several non-promiscuous Cre recognition sites enables simultaneous specific Cre/lox recombination events. CFGE combined with Cre/lox recombination is put forward as powerful approach for targeted genome editing, involving serial steps of manipulation to expedite the genetic accessibility of S. meliloti as chassis.

Gene stacking by recombinases

Efficient methods of stacking genes into plant genomes are needed to expedite transfer of multigenic traits to crop varieties of diverse ecosystems. Over two decades of research has identified several DNA recombinases that carryout efficient cis and trans recombination between the recombination sites artificially introduced into the plant chromosome. The specificity and efficiency of recombinases make them extremely attractive for genome engineering. In plant biotechnology, recombinases have mostly been used for removing selectable marker genes and have rarely been extended to more complex applications. The reversibility of recombination, a property of the tyrosine family of recombinases, does not lend itself to gene stacking approaches that involve rounds of transformation for integrating genes into the engineered sites. However, recent developments in the field of recombinases have overcome these challenges and paved the way for gene stacking. Some of the key advancements include the application of unidirectional recombination systems, modification of recombination sites and transgene site modifications to allow repeated site-specific integrations into the selected site. Gene stacking is relevant to agriculturally important crops, many of which are difficult to transform; therefore, development of high-efficiency gene stacking systems will be important for its application on agronomically important crops, and their elite varieties. Recombinases, by virtue of their specificity and efficiency in plant cells, emerge as powerful tools for a variety of applications including gene stacking.

Novel Heterotypic Rox Sites for Combinatorial Dre Recombination Strategies

Site-specific recombinases (SSRs) such as Cre are widely used in gene targeting and genetic approaches for cell labeling and manipulation. They mediate DNA strand exchange between two DNA molecules at dedicated recognition sites. Precise understanding of the Cre recombination mechanism, including the role of individual base pairs in its loxP target site, guided the generation of mutant lox sites that specifically recombine with themselves but not with the wild type loxP. This has led to the development of a variety of combinatorial Cre-dependent genetic strategies, such as multicolor reporters, irreversible inversions, or recombination-mediated cassette exchange. Dre, a Cre-related phage integrase that recognizes roxP sites, does not cross-react with the Cre-loxP system, but has similar recombination efficiency. We have previously described intersectional genetic strategies combining Dre and Cre. We now report a mutagenesis screen aimed at identifying roxP base pairs critical for self-recognition. We describe several rox variant sites that are incompatible with roxP, but are able to efficiently recombine with themselves in either purified systems or bacterial and eukaryotic tissue culture systems. These newly identified rox sites are not recognized by Cre, thus enabling potential combinatorial strategies involving Cre, Dre, and target loci including multiple loxP and roxP variants.

Genome engineering: Drosophila melanogaster and beyond

A central challenge in investigating biological phenomena is the development of techniques to modify genomic DNA with nucleotide precision that can be transmitted through the germ line. Recent years have brought a boon in these technologies, now collectively known as genome engineering. Defined genomic manipulations at the nucleotide level enable a variety of reverse engineering paradigms, providing new opportunities to interrogate diverse biological functions. These genetic modifications include controlled removal, insertion, and substitution of genetic fragments, both small and large. Small fragments up to a few kilobases (e.g., single nucleotide mutations, small deletions, or gene tagging at single or multiple gene loci) to large fragments up to megabase resolution can be manipulated at single loci to create deletions, duplications, inversions, or translocations of substantial sections of whole chromosome arms. A specialized substitution of chromosomal portions that presumably are functionally orthologous between different organisms through syntenic replacement, can provide proof of evolutionary conservation between regulatory sequences. Large transgenes containing endogenous or synthetic DNA can be integrated at defined genomic locations, permitting an alternative proof of evolutionary conservation, and sophisticated transgenes can be used to interrogate biological phenomena. Precision engineering can additionally be used to manipulate the genomes of organelles (e.g., mitochondria). Novel genome engineering paradigms are often accelerated in existing, easily genetically tractable model organisms, primarily because these paradigms can be integrated in a rigorous, existing technology foundation. The Drosophila melanogaster fly model is ideal for these types of studies. Due to its small genome size, having just four chromosomes, the vast amount of cutting-edge genetic technologies, and its short life-cycle and inexpensive maintenance requirements, the fly is exceptionally amenable to complex genetic analysis using advanced genome engineering. Thus, highly sophisticated methods developed in the fly model can be used in nearly any sequenced organism. Here, we summarize different ways to perform precise inheritable genome engineering using integrases, recombinases, and DNA nucleases in the D. melanogaster. For further resources related to this article, please visit the WIREs website.

Cre recombinase activity is inhibited in vivo but not ex vivo by a mutation in the asymmetric spacer region of the distal loxP site

The cre/loxP recombination system is a valuable tool used to generate tissue specific genomic rearrangements in mouse models. The deletion of a region of interest flanked by two loxP sites is accomplished by the recombinase (cre) enzyme, which binds to the inverted repeat segments of two loxP sites and recognition of a conserved TA sequence in the asymmetric central spacer region "ATAACTTCGTATA -NNNTANNN-TATACGAAGTTAT. In vivo, we found that a single T to C mutation at position 4 of the central spacer region in the distal (3') loxP site, completely inhibited the recombination reaction in two conditional mouse models. These mice were generated using a mitochondrial methionyl-tRNA formyltransferase (Mtfmt) gene targeted construct and cre transgene under the control of tissue-specific promoters: calcium/calmodulin-dependent kinase II alpha (Camk2a-cre) and myosin light polypeptide 1 (Myl1-cre). Surprisingly, transient transfection of a plasmid expressing cre in dermal fibroblasts derived from the same mutant floxed Mtfmt((loxP/loxP)) mice line, successfully deleted the region of interest. This study demonstrates the sequence specificity required in vivo, the possibility of bypassing this limitation by expressing high levels of cre recombinase ex vivo and raises concerns related to the quality control of large scale production of gene targeted constructs and mice. genesis 53:695-700, 2015. © 2015 Wiley Periodicals, Inc.

Per aspera ad astra: When harmful chromosomal translocations become a plus value in genetic evolution. Lessons from Saccharomyces cerevisiae

In this review we will focus on chromosomal translocations (either spontaneous or induced) in budding yeast. Indeed, very few organisms tolerate so well aneuploidy like Saccharomyces, allowing in depth studies on chromosomal numerical aberrations. Many wild type strains naturally develop chromosomal rearrangements while adapting to different environmental conditions. Translocations, in particular, are valuable not only because they naturally drive species evolution, but because they might allow the artificial generation of new strains that can be optimized for industrial purposes. In this area, several methodologies to artificially trigger chromosomal translocations have been conceived in the past years, such as the chromosomal fragmentation vector (CFV) technique, the Cre-loxP procedure, the FLP/FRT recombination method and, recently, the bridge - induced translocation (BIT) system. An overview of the methodologies to generate chromosomal translocations in yeast will be presented and discussed considering advantages and drawbacks of each technology, focusing in particular on the recent BIT system. Translocants are important for clinical studies because translocated yeast cells resemble cancer cells from morphological and physiological points of view and because the translocation event ensues in a transcriptional de-regulation with a subsequent multi-factorial genetic adaptation to new, selective environmental conditions. The phenomenon of post-translocational adaptation (PTA) is discussed, providing some new unpublished data and proposing the hypothesis that translocations may drive evolution through adaptive genetic selection.

In vivo generation of DNA sequence diversity for cellular barcoding

Heterogeneity is a ubiquitous feature of biological systems. A complete understanding of such systems requires a method for uniquely identifying and tracking individual components and their interactions with each other. We have developed a novel method of uniquely tagging individual cells in vivo with a genetic 'barcode' that can be recovered by DNA sequencing. Our method is a two-component system comprised of a genetic barcode cassette whose fragments are shuffled by Rci, a site-specific DNA invertase. The system is highly scalable, with the potential to generate theoretical diversities in the billions. We demonstrate the feasibility of this technique in Escherichia coli. Currently, this method could be employed to track the dynamics of populations of microbes through various bottlenecks. Advances of this method should prove useful in tracking interactions of cells within a network, and/or heterogeneity within complex biological samples.

Genome-scale engineering for systems and synthetic biology

This review provides an overview of methodologies and technologies enabling genome-scale engineering, focusing on the design, construction, and testing of modified genomes in a variety of organisms. Future applications for systems and synthetic biology are discussed.

Genome-modification technologies enable the rational engineering and perturbation of biological systems. Historically, these methods have been limited to gene insertions or mutations at random or at a few pre-defined locations across the genome. The handful of methods capable of targetedgene editing suffered from low efficiencies, significant labor costs, or both. Recent advances have dramatically expanded our ability to engineer cells in a directed and combinatorial manner. Here, we review current technologies and methodologies for genome-scale engineering, discuss the prospects for extending efficient genome modification to new hosts, and explore the implications of continued advances toward the development of flexibly programmable chasses, novel biochemistries, and safer organismal and ecological engineering.

Identifying Distal cis-acting Gene-Regulatory Sequences by Expressing BACs Functionalized with loxP-Tn10 Transposons in Zebrafish

Bacterial Artificial Chromosomes (BACs) are large pieces of DNA from the chromosomes of organisms propagated faithfully in bacteria as large extra-chromosomal plasmids. Expression of genes contained in BACs can be monitored after functionalizing the BAC DNA with reporter genes and other sequences that allow stable maintenance and propagation of the DNA in the new host organism. The DNA in BACs can be altered within its bacterial host in several ways. Here we discuss one such approach, using Tn10 mini-transposons, to introduce exogenous sequences into BACs for a variety of purposes. The largely random insertions of Tn10 transposons carrying lox sites have been used to position mammalian cell-selectable antibiotic resistance genes, enhancer-traps and inverted repeat ends of the vertebrate transposon Tol2 precisely at the ends of the genomic DNA insert in BACs. These modified BACs are suitable for expression in zebrafish or mouse, and have been used to functionally identify important long-range gene regulatory sequences in both species. Enhancer-trapping using BACs should prove uniquely useful in analyzing multiple discontinuous DNA domains that act in concert to regulate expression of a gene, and is not limited by genome accessibility issues of traditional enhancer-trapping methods.

Mutants of Cre recombinase with improved accuracy

Despite rapid advances in genome engineering technologies, inserting genes into precise locations in the human genome remains an outstanding problem. It has been suggested that site-specific recombinases can be adapted towards use as transgene delivery vectors. The specificity of recombinases can be altered either with directed evolution or via fusions to modular DNA-binding domains. Unfortunately, both wild-type and altered variants often have detectable activities at off-target sites. Here we use bacterial selections to identify mutations in the dimerization surface of Cre recombinase (R32V, R32M and 303GVSdup) that improve the accuracy of recombination. The mutants are functional in bacteria, in human cells and in vitro (except for 303GVSdup, which we did not purify), and have improved selectivity against both model off-target sites and the entire E. coli genome. We propose that destabilizing binding cooperativity may be a general strategy for improving the accuracy of dimeric DNA-binding proteins.

Generating libraries of iTol2-end insertions at BAC ends using loxP and lox511 Tn10 transposons

BACKGROUND

Bacterial Artificial Chromosomes (BACs) have been widely used as transgenes in vertebrate model systems such as mice and zebrafish, for a variety of studies. BAC transgenesis has been a powerful tool to study the function of the genome, and gene regulation by distal cis-regulatory elements. Recently, BAC transgenesis in both mice and zebrafish was further facilitated by development of the transposon-mediated method using the Tol2 element. Tol2 ends, in the inverted orientation and flanking a 1 kb spacer DNA (iTol2), were introduced into the BAC DNA within the bacterial host using recombination of homologous sequences. Here we describe experiments designed to determine if a simpler and more flexible system could modify BACs so that they would be suitable for transgenesis into zebrafish or mouse embryos using the Tol2 transposase.

RESULTS

A new technique was developed to introduce recognition sequences for the Tol2 transposase into BACs in E. coli using the Tn10 transposon vector system. We constructed pTnloxP-iTol2kan and pTnlox511-iTol2kan to introduce the loxP or lox511 site and iTol2 cassette, containing the Tol2 cis-sequences in the inverted orientation, into BACs that have loxP and lox511 sites flanking genomic DNA inserts by Tn10-mediated transposition. The procedure enables rapid generation of a large collection of BACs ready for transgenesis with the iTol2 cassette at the new end of a progressively truncated genomic insert via lox-Cre recombination. The iTol2 ends are efficiently recognized by the Tol2 transposase, and the BACs readily integrate into zebrafish chromosomes.

CONCLUSION

The new technology described here can rapidly introduce iTol2 ends at a BAC end of choice, and simultaneously generate a large collection of BACs with progressive deletions of the genomic DNA from that end in a single experiment. This procedure should be applicable to a wider variety of BACs containing lox sites flanking the genomic DNA insert, including those with sequence repeats. The libraries of iTol2 inserted BACs with truncations from an end should facilitate studies on the impact of distal cis-regulatory sequences on gene function, as well as standard BAC transgenesis with precisely trimmed genes in zebrafish or mouse embryos using Tol2 transposition.

Site-specific integration of transgene targeting an endogenous lox-like site in early mouse embryos

Functional lox-like sequences have been identified within the yeast and mammalian genome. These hetero-specific lox sites also allow Cre recombinase to specifically target efficient integration of exogenous DNA into the endogenous pseudo-lox (ψlox) sequences that occur naturally in the host genome using a Cre/loxP integrative recombination system. We investigated whether the Cre/ψlox system is useful for site-specific integration of transgenes and for improving the production efficiency of transgenic animals. This is the first report on Cre-mediated integrative recombination targeting an endogenous lox-like sequence termed pseudo-loxm5 (ψloxm5) in early mouse embryos. We characterized the Cre/ψloxm5 system in embryonic environment. Cre-expressing plasmid and a transgene (CMV/LacZ gene) flanked by ψloxm5 and ψloxcorem5 sites were co-microinjected into the pronucleus of fertilized mouse oocytes. The injected eggs were transferred into foster mothers, and the recombination products were investigated. The results show that the ψloxm5 site is an active substrate for Cre-mediated recombination in the mouse embryonic environment. The transgenesis efficiency was up to 27% (6/22). The site-specific integration of the transgene into the endogenous ψloxm5 site was found in 50 % of the transgenic pups. Our findings demonstrated that the Cre/ψloxm5 integrative recombination system is an efficient and simple strategy for targeting an endogenous lox-like site in mammalian embryos.

Cre recombinase-mediated site-specific modification of a cellular genome using an integrase-defective retroviral vector

Retroviral integrase is an enzyme responsible for the integration of retroviruses. A single mutation in the integrase core domain can severely compromise its integration ability, leading to the accumulation of circular retroviral cDNA in the nuclei of infected cells. We therefore attempted to use those cDNA as substrates for Cre recombinase to perform a recombinase-mediated cassette exchange (RMCE), thereby targeting retroviral vectors to a predetermined site. An expression unit containing a promoter, an ATG codon and marker genes (hygromycin resistance gene and red fluorescent protein gene) flanked by wild-type and mutant loxP sites was first introduced into cellular chromosome to build founder cell lines. We then constructed another plasmid for the production of integrase-defective retroviral vectors (IDRV), which contains an ATG-deficient neomycin resistance gene and green fluorescent protein gene, flanked by a compatible pair of loxPs. After providing founder cells with Cre and infecting with IDRV later, effective RMCE occurred, resulting in the appearance of G418-resistant colonies and a change in the color of fluorescence from red to green. Southern blot and PCR analyses on selected clones further confirmed site-specific recombination. The successful substitution of the original viral integration machinery with a non-viral mechanism could expand the application of retroviral vectors.

Replacing the wild type loxP site in BACs from the public domain with lox66 using a lox66 transposon

Background

Chromatin adjoining the site of integration of a transgene affects expression and renders comparisons of closely related transgenes, such as those derived from a BAC deletion series retrofitted with enhancer-traps, unreliable. Gene targeting to a pre-determined site on the chromosome is likely to alleviate the problem.

Findings

A general procedure to replace the loxP site located at one end of genomic DNA inserts in BACs with lox66 is described. Truncating insert DNA from the loxP end with a Tn10 transposon carrying a lox66 site simultaneously substitutes the loxP with a lox66 sequence. The replacement occurs with high stringency, and the procedure should be applicable to all BACs in the public domain. Cre recombination of loxP with lox66 or lox71 was found to be as efficient as another loxP site during phage P1 transduction of small plasmids containing those sites. However the end-deletion of insert DNA in BACs using a lox66 transposon occurred at no more than 20% the efficiency observed with a loxP transposon. Differences in the ability of Cre protein available at different stages of the P1 life cycle to recombine identical versus non-identical lox-sites is likely responsible for this discrepancy. A possible mechanism to explain these findings is discussed.

Conclusions

The loxP/lox66 replacement procedure should allow targeting BACs to a pre-positioned lox71 site in zebrafish chromosomes; a system where homologous recombination-mediated "knock-in" technology is unavailable.

Transgenesis upgrades for Drosophila melanogaster

Drosophila melanogaster is a highly attractive model system for the study of numerous biological questions pertaining to development, genetics, cell biology, neuroscience and disease. Until recently, our ability to manipulate flies genetically relied heavily on the transposon-mediated integration of DNA into fly embryos. However, in recent years significant improvements have been made to the transgenic techniques available in this organism, particularly with respect to integrating DNA at specific sites in the genome. These new approaches will greatly facilitate the structure-function analyses of Drosophila genes, will enhance the ease and speed with which flies can be manipulated, and should advance our understanding of biological processes during normal development and disease.