The Frog Prince: a reconstructed transposon from Rana pipiens with high transpositional activity in vertebrate cells

Heterologous survey of 130 DNA transposons in human cells highlights their functional divergence and expands the genome engineering toolbox

Experimental studies on DNA transposable elements (TEs) have been limited in scale, leading to a lack of understanding of the factors influencing transposition activity, evolutionary dynamics, and application potential as genome engineering tools. We predicted 130 active DNA TEs from 102 metazoan genomes and evaluated their activity in human cells. We identified 40 active (integration-competent) TEs, surpassing the cumulative number (20) of TEs found previously. With this unified comparative data, we found that the Tc1/mariner superfamily exhibits elevated activity, potentially explaining their pervasive horizontal transfers. Further functional characterization of TEs revealed additional divergence in features such as insertion bias. Remarkably, in CAR-T therapy for hematological and solid tumors, Mariner2_AG (MAG), the most active DNA TE identified, largely outperformed two widely used vectors, the lentiviral vector and the TE-based vector SB100X. Overall, this study highlights the varied transposition features and evolutionary dynamics of DNA TEs and increases the TE toolbox diversity.

Ancestral genome reconstruction enhances transposable element annotation by identifying degenerate integrants

Growing evidence indicates that transposable elements (TEs) play important roles in evolution by providing genomes with coding and non-coding sequences. Identification of TE-derived functional elements, however, has relied on TE annotations in individual species, which limits its scope to relatively intact TE sequences. Here, we report a novel approach to uncover previously unannotated degenerate TEs (degTEs) by probing multiple ancestral genomes reconstructed from hundreds of species. We applied this method to the human genome and achieved a 10.8% increase in coverage over the most recent annotation. Further, we discovered that degTEs contribute to various cis-regulatory elements and transcription factor binding sites, including those of a known TE-controlling family, the KRAB zinc-finger proteins. We also report unannotated chimeric transcripts between degTEs and human genes expressed in embryos. This study provides a novel methodology and a freely available resource that will facilitate the investigation of TE co-option events on a full scale.

Artificial optimization of bamboo Ppmar2 transposase and host factors effects on Ppmar2 transposition in yeast

Mariner-like elements (MLEs) are promising tools for gene cloning, gene expression, and gene tagging. We have characterized two MLE transposons from moso bamboo, Ppmar1 and Ppmar2. Ppmar2, is smaller in size and has higher natural activities, thus making it a more potential genomic tool compared to Ppmar1. Using a two-component system consisting of a transposase expression cassette and a non-autonomous transposon cotransformed in yeast, we investigated the transposition activity of Ppmar2 and created hyperactive transposases. Five out of 19 amino acid mutations in Ppmar2 outperformed the wild-type in terms of catalytic activities, especially with the S347R mutant having 6.7-fold higher transposition activity. Moreover, 36 yeast mutants with single-gene deletion were chosen to screen the effects of the host factors on Ppmar2NA transposition. Compared to the control strain (his3Δ), the mobility of Ppmar2 was greatly increased in 9 mutants and dramatically decreased in 7 mutants. The transposition ability in the efm1Δ mutant was 15-fold higher than in the control, while it was lowered to 1/66 in the rtt10Δ mutant. Transcriptomic analysis exhibited that EFM1 defection led to the significantly impaired DDR2, HSP70 expression and dramatically boosted JEN1 expression, whereas RTT10 defection resulted in significantly suppressed expression of UTP20, RPA190 and RRP5. Protein methylation, chromatin and RNA transcription may affect the Ppmar2NA transposition efficiency in yeast. Overall, the findings provided evidence for transposition regulation and offered an alternative genomic tool for moso bamboo and other plants.

Functional Characterization of the N-Terminal Disordered Region of the piggyBac Transposase

The piggyBac DNA transposon is an active element initially isolated from the cabbage looper moth, but members of this superfamily are also present in most eukaryotic evolutionary lineages. The functionally important regions of the transposase are well described. There is an RNase H-like fold containing the DDD motif responsible for the catalytic DNA cleavage and joining reactions and a C-terminal cysteine-rich domain important for interaction with the transposon DNA. However, the protein also contains a ~100 amino acid long N-terminal disordered region (NTDR) whose function is currently unknown. Here we show that deletion of the NTDR significantly impairs piggyBac transposition, although the extent of decrease is strongly cell-type specific. Moreover, replacing the NTDR with scrambled but similarly disordered sequences did not rescue transposase activity, indicating the importance of sequence conservation. Cell-based transposon excision and integration assays reveal that the excision step is more severely affected by NTDR deletion. Finally, bioinformatic analyses indicated that the NTDR is specific for the piggyBac superfamily and is also present in domesticated, transposase-derived proteins incapable of catalyzing transposition. Our results indicate an essential role of the NTDR in the “fine-tuning” of transposition and its significance in the functions of piggyBac-originated co-opted genes.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Accelerating and de-risking CMC development with transposon-derived manufacturing cell lines

The development of highly productive, genetically stable manufacturing cell lines is on the critical path to IND filing for protein-based biologic drugs. Here, we describe the Leap-In Transposase® platform, a novel transposon-based mammalian (e.g., Chinese hamster ovary) cell line development system that produces high-titer stable pools with productivity and product quality attributes that are highly comparable to clones that are subsequently derived therefrom. The productivity distributions of clones are strongly biased toward high producers, and genetic and expression stability is consistently high. By avoiding the poor integration rates, concatemer formation, detrimental transgene recombination, low average expression level, unpredictable product quality, and inconsistent genetic stability characteristic of nonhomologous recombination methods, Leap-In provides several opportunities to de-risk programs early and reduce timelines and resources.

Contemporary Transposon Tools: A Review and Guide through Mechanisms and Applications of Sleeping Beauty, piggyBac and Tol2 for Genome Engineering

Transposons are mobile genetic elements evolved to execute highly efficient integration of their genes into the genomes of their host cells. These natural DNA transfer vehicles have been harnessed as experimental tools for stably introducing a wide variety of foreign DNA sequences, including selectable marker genes, reporters, shRNA expression cassettes, mutagenic gene trap cassettes, and therapeutic gene constructs into the genomes of target cells in a regulated and highly efficient manner. Given that transposon components are typically supplied as naked nucleic acids (DNA and RNA) or recombinant protein, their use is simple, safe, and economically competitive. Thus, transposons enable several avenues for genome manipulations in vertebrates, including transgenesis for the generation of transgenic cells in tissue culture comprising the generation of pluripotent stem cells, the production of germline-transgenic animals for basic and applied research, forward genetic screens for functional gene annotation in model species and therapy of genetic disorders in humans. This review describes the molecular mechanisms involved in transposition reactions of the three most widely used transposon systems currently available (Sleeping Beauty, piggyBac, and Tol2), and discusses the various parameters and considerations pertinent to their experimental use, highlighting the state-of-the-art in transposon technology in diverse genetic applications.

Divergent evolution profiles of DD37D and DD39D families of Tc1/mariner transposons in eukaryotes

DNA transposons play a significant role in shaping the size and structure of eukaryotic genomes. The Tc1/mariner transposons are the most diverse and widely distributed superfamily of DNA transposons and the structure and distribution of several Tc1/mariner families, such as DD35E/TR, DD36E/IC, DD37E/TRT, and DD41D/VS, have been well studied. Nonetheless, a greater understanding of the structure and diversity of Tc1/mariner transposons will provide insight into the evolutionary history of eukaryotic genomes. Here, we conducted further analysis of DD37D/maT and DD39D (named Guest, GT), which were identified by the specific catalytic domains DD37D and DD39D. Most transposons of the maT family have a total length of approximately 1.3 kb and harbor a single open reading frame encoding a ~ 346 amino acid (range 302-398 aa) transposase protein, flanked by short terminal inverted repeats (TIRs) (13-48 base pairs, bp). In contrast, GTs transposons were longer (2.0-5.8 kb), encoded a transposase protein of ~400 aa (range 140-592 aa), and were flanked by short TIRs (19-41 bp). Several conserved motifs, including two helix-turn-helix (HTH) motifs, a GRPR (GRKR) motif, a nuclear localization sequence, and a DDD domain, were also identified in maT and GT transposases. Phylogenetic analyses of the DDD domain showed that the maT and GT families each belong to a monophyletic clade and appear to be closely related to DD41D/VS and DD34D/mariner. In addition, maTs are mainly distributed in invertebrates (144 species), whereas GTs are mainly distributed in land plants through a small number of GTs are present in Chromista and animals. Sequence identity and phylogenetic analysis revealed that horizontal transfer (HT) events of maT and GT might occur between kingdoms and phyla of eukaryotes; however, pairwise distance comparisons between host genes and transposons indicated that HT events involving maTs might be less frequent between invertebrate species and HT events involving GTs may be less frequent between land plant species. Overall, the DD37D/maT and DD39D/GT families display significantly different distribution and tend to be identified in more ancient evolutionary families. The discovery of intact transposases, perfect TIRs, and target site duplications (TSD) of maTs and GTs illustrates that the DD37D/maT and DD39D/GT families may be active. Together, these findings improve our understanding of the diversity of Tc1/mariner transposons and their impact on eukaryotic genome evolution.

A native, highly active Tc1/mariner transposon from zebrafish (ZB) offers an efficient genetic manipulation tool for vertebrates

New genetic tools and strategies are currently under development to facilitate functional genomics analyses. Here, we describe an active member of the Tc1/mariner transposon superfamily, named ZB, which invaded the zebrafish genome very recently. ZB exhibits high activity in vertebrate cells, in the range of those of the widely used transposons piggyBac (PB), Sleeping Beauty (SB) and Tol2. ZB has a similar structural organization and target site sequence preference to SB, but a different integration profile with respect to genome-wide preference among mammalian functional annotation features. Namely, ZB displays a preference for integration into transcriptional regulatory regions of genes. Accordingly, we demonstrate the utility of ZB for enhancer trapping in zebrafish embryos and in the mouse germline. These results indicate that ZB may be a powerful tool for genetic manipulation in vertebrate model species.

Traveler, a New DD35E Family of Tc1/Mariner Transposons, Invaded Vertebrates Very Recently

The discovery of new members of the Tc1/mariner superfamily of transposons is expected based on the increasing availability of genome sequencing data. Here, we identified a new DD35E family termed Traveler (TR). Phylogenetic analyses of its DDE domain and full-length transposase showed that, although TR formed a monophyletic clade, it exhibited the highest sequence identity and closest phylogenetic relationship with DD34E/Tc1. This family displayed a very restricted taxonomic distribution in the animal kingdom and was only detected in ray-finned fish, anura, and squamata, including 91 vertebrate species. The structural organization of TRs was highly conserved across different classes of animals. Most intact TR transposons had a length of ∼1.5 kb (range 1,072-2,191 bp) and harbored a single open reading frame encoding a transposase of ∼340 aa (range 304-350 aa) flanked by two short-terminal inverted repeats (13-68 bp). Several conserved motifs, including two helix-turn-helix motifs, a GRPR motif, a nuclear localization sequence, and a DDE domain, were also identified in TR transposases. This study also demonstrated the presence of horizontal transfer events of TRs in vertebrates, whereas the average sequence identities and the evolutionary dynamics of TR elements across species and clusters strongly indicated that the TR family invaded the vertebrate lineage very recently and that some of these elements may be currently active, combining the intact TR copies in multiple lineages of vertebrates. These data will contribute to the understanding of the evolutionary history of Tc1/mariner transposons and that of their hosts.

Intruder (DD38E), a recently evolved sibling family of DD34E/Tc1 transposons in animals

BACKGROUND

A family of Tc1/mariner transposons with a characteristic DD38E triad of catalytic amino acid residues, named Intruder (IT), was previously discovered in sturgeon genomes, but their evolutionary landscapes remain largely unknown.

RESULTS

Here, we comprehensively investigated the evolutionary profiles of ITs, and evaluated their cut-and-paste activities in cells. ITs exhibited a narrow taxonomic distribution pattern in the animal kingdom, with invasions into two invertebrate phyla (Arthropoda and Cnidaria) and three vertebrate lineages (Actinopterygii, Agnatha, and Anura): very similar to that of the DD36E/IC family. Some animal orders and species seem to be more hospitable to Tc1/mariner transposons, one order of Amphibia and seven Actinopterygian orders are the most common orders with horizontal transfer events and have been invaded by all four families (DD38E/IT, DD35E/TR, DD36E/IC and DD37E/TRT) of Tc1/mariner transposons, and eight Actinopterygii species were identified as the major hosts of these families. Intact ITs have a total length of 1.5-1.7 kb containing a transposase gene flanked by terminal inverted repeats (TIRs). The phylogenetic tree and sequence identity showed that IT transposases were most closely related to DD34E/Tc1. ITs have been involved in multiple events of horizontal transfer in vertebrates and have invaded most lineages recently (< 5 million years ago) based on insertion age analysis. Accordingly, ITs presented high average sequence identity (86-95%) across most vertebrate species, suggesting that some are putatively active. ITs can transpose in human HeLa cells, and the transposition efficiency of consensus TIRs was higher than that of the TIRs of natural isolates.

CONCLUSIONS

We conclude that DD38E/IT originated from DD34E/Tc1 and can be detected in two invertebrate phyla (Arthropoda and Cnidaria), and in three vertebrate lineages (Actinopterygii, Agnatha and Anura). IT has experienced multiple HT events in animals, dominated by recent amplifications in most species and has high identity among vertebrate taxa. Our reconstructed IT transposon vector designed according to the sequence from the "cat" genome showed high cut-and-paste activity. The data suggest that IT has been acquired recently and is active in many species. This study is meaningful for understanding the evolution of the Tc1/mariner superfamily members and their hosts.

Evolution of pogo, a separate superfamily of IS630-Tc1-mariner transposons, revealing recurrent domestication events in vertebrates

BACKGROUND

Tc1/mariner and Zator, as two superfamilies of IS630-Tc1-mariner (ITm) group, have been well-defined. However, the molecular evolution and domestication of pogo transposons, once designated as an important family of the Tc1/mariner superfamily, are still poorly understood.

RESULTS

Here, phylogenetic analysis show that pogo transposases, together with Tc1/mariner, DD34E/Gambol, and Zator transposases form four distinct monophyletic clades with high bootstrap supports (> = 74%), suggesting that they are separate superfamilies of ITm group. The pogo superfamily represents high diversity with six distinct families (Passer, Tigger, pogoR, Lemi, Mover, and Fot/Fot-like) and wide distribution with an expansion spanning across all the kingdoms of eukaryotes. It shows widespread occurrences in animals and fungi, but restricted taxonomic distribution in land plants. It has invaded almost all lineages of animals-even mammals-and has been domesticated repeatedly in vertebrates, with 12 genes, including centromere-associated protein B (CENPB), CENPB DNA-binding domain containing 1 (CENPBD1), Jrk helix-turn-helix protein (JRK), JRK like (JRKL), pogo transposable element derived with KRAB domain (POGK), and with ZNF domain (POGZ), and Tigger transposable element-derived 2 to 7 (TIGD2-7), deduced as originating from this superfamily. Two of them (JRKL and TIGD2) seem to have been co-domesticated, and the others represent independent domestication events. Four genes (TIGD3, TIGD4, TIGD5, and POGZ) tend to represent ancient domestications in vertebrates, while the others only emerge in mammals and seem to be domesticated recently. Significant structural variations including target site duplication (TSD) types and the DDE triad signatures (DD29-56D) were observed for pogo transposons. Most domesticated genes are derived from the complete transposase genes; but CENPB, POGK, and POGZ are chimeric genes fused with additional functional domains.

CONCLUSIONS

This is the first report to systematically reveal the evolutionary profiles of the pogo transposons, suggesting that pogo and Tc1/Mariner are two separate superfamilies of ITm group, and demonstrating the repeated domestications of pogo in vertebrates. These data indicate that pogo transposons have played important roles in shaping the genome and gene evolution of fungi and animals. This study expands our understanding of the diversity of pogo transposons and updates the classification of ITm group.

Phylogenetic analysis of the Tc1/mariner superfamily reveals the unexplored diversity of pogo-like elements

Background

Tc1/mariner transposons are widespread DNA transposable elements (TEs) that have made important contributions to the evolution of host genomic complexity in metazoans. However, the evolution and diversity of the Tc1/mariner superfamily remains poorly understood. Following recent developments in genome sequencing and the availability of a wealth of new genomes, Tc1/mariner TEs have been identified in many new taxa across the eukaryotic tree of life. To date, the majority of studies focussing on Tc1/mariner elements have considered only a single host lineage or just a small number of host lineages. Thus, much remains to be learnt about the evolution of Tc1/mariner TEs by performing analyses that consider elements that originate from across host diversity.

Results

We mined the non-redundant database of NCBI using BLASTp searches, with transposase sequences from a diverse set of reference Tc1/mariner elements as queries. A total of 5158 Tc1/mariner elements were retrieved and used to reconstruct evolutionary relationships within the superfamily. The resulting phylogeny is well resolved and includes several new groups of Tc1/mariner elements. In particular, we identify a new family of plant-genome restricted Tc1/mariner elements, which we call PlantMar. We also show that the pogo family is much larger and more diverse than previously appreciated, and we review evidence for a potential revision of its status to become a separate superfamily.

Conclusions

Our study provides an overview of Tc1-mariner phylogeny and summarises the impressive diversity of Tc1-mariner TEs among sequenced eukaryotes. Tc1/mariner TEs are successful in a wide range of eukaryotes, especially unikonts (the taxonomic supergroup containing Amoebozoa, Opisthokonta, Breviatea, and Apusomonadida). In particular, ecdysozoa, and especially arthropods, emerge as important hosts for Tc1/mariner elements (except the PlantMar family). Meanwhile, the pogo family, which is by far the largest Tc1/mariner family, also includes many elements from fungal and chordate genomes. Moreover, there is evidence of the repeated exaptation of pogo elements in vertebrates, including humans, in addition to the well-known example of CENP-B. Collectively, our findings provide a considerable advancement in understanding of Tc1/mariner elements, and more generally they suggest that much work remains to improve understanding of the diversity and evolution of DNA TEs.

Incomer, a DD36E family of Tc1/mariner transposons newly discovered in animals

Background

The Tc1/mariner superfamily might represent the most diverse and widely distributed group of DNA transposons. Several families have been identified; however, exploring the diversity of this superfamily and updating its classification is still ongoing in the life sciences.

Results

Here we identified a new family of Tc1/mariner transposons, named Incomer (IC), which is close to, but distinct from the known family DD34E/Tc1. ICs have a total length of about 1.2 kb, and harbor a single open reading frame encoding a ~ 346 amino acid transposase with a DD36E motif and flanked by short terminal inverted repeats (TIRs) (22–32 base pairs, bp). This family is absent from prokaryotes, and is mainly distributed among vertebrates (141 species of four classes), including Agnatha (one species of jawless fish), Actinopterygii (132 species of ray-finned fish), Amphibia (four species of frogs), and Mammalia (four species of bats), but have a restricted distribution in invertebrates (four species in Insecta and nine in Arachnida). All ICs in bats (Myotis lucifugus, Eptesicus fuscus, Myotis davidii, and Myotis brandtii) are present as truncated copies in these genomes, and most of them are flanked by relatively long TIRs (51–126 bp). High copy numbers of miniature inverted-repeat transposable elements (MITEs) derived from ICs were also identified in bat genomes. Phylogenetic analysis revealed that ICs are more closely related to DD34E/Tc1 than to other families of Tc1/mariner (e.g., DD34D/mariner and DD × D/pogo), and can be classified into four distinct clusters. The host and IC phylogenies and pairwise distance comparisons between RAG1 genes and all consensus sequences of ICs support the idea that multiple episodes of horizontal transfer (HT) of ICs have occurred in vertebrates. In addition, the discovery of intact transposases, perfect TIRs and target site duplications of ICs suggests that this family may still be active in Insecta, Arachnida, frogs, and fish.

Conclusions

Exploring the diversity of Tc1/mariner transposons and revealing their evolutionary profiles will help provide a better understanding of the evolution of DNA transposons and their impact on genomic evolution. Here, a newly discovered family (DD36E/Incomer) of Tc1/mariner transposons is described in animals. It displays a similar structural organization and close relationship with the known DD34E/Tc1 elements, but has a relatively narrow distribution, indicating that DD36E/IC might have originated from the DD34E/Tc1 family. Our data also support the hypothesis of horizontal transfer of IC in vertebrates, even invading one lineage of mammals (bats). This study expands our understanding of the diversity of Tc1/mariner transposons and updates the classification of this superfamily.

Affinities of Terminal Inverted Repeats to DNA Binding Domain of Transposase Affect the Transposition Activity of Bamboo Ppmar2 Mariner-Like Element

Mariner-like elements (MLE) are a super-family of DNA transposons widespread in animal and plant genomes. Based on their transposition characteristics, such as random insertions and high-frequency heterogeneous transpositions, several MLEs have been developed to be used as tools in gene tagging and gene therapy. Two active MLEs, Ppmar1 and Ppmar2, have previously been identified in moso bamboo (Phyllostachys edulis). Both of these have a preferential insertion affinity to AT-rich region and their insertion sites are close to random in the host genome. In Ppmar2 element, we studied the affinities of terminal inverted repeats (TIRs) to DNA binding domain (DBD) and their influence on the transposition activity. We could identify two putative boxes in the TIRs which play a significant role in defining the TIR's affinities to the DBD. Seven mutated TIRs were constructed, differing in affinities based on similarities with those of other plant MLEs. Gel mobility shift assays showed that the TIR mutants with mutation sites G669A-C671A had significantly higher affinities than the mutants with mutation sites C657T-A660T. The high-affinity TIRs indicated that their transposition frequency was 1.5-2.0 times higher than that of the wild type TIRs in yeast transposition assays. The MLE mutants with low-affinity TIRs had relatively lower transposition frequency from that of wild types. We conclude that TIR affinity to DBD significantly affects the transposition activity of Ppmar2. The mutant MLEs highly active TIRs constructed in this study can be used as a tool for bamboo genetic studies.

Early heading 7 interacts with DTH8, and regulates flowering time in rice

EH7/Ghd7 interacts with DTH8, and regulates heading date by controlling the expression of Ehd1 in rice. Heading date, or flowering time, an important agronomic trait, influences regional adaptability and yield of crops. Many genes related to heading date in rice have been identified, and a preliminary regulatory network has been established, but the relationships between proteins involved are poorly understood. We identified a flowering suppressor EH7 (Early heading 7) that represses flowering under long-day (LD) conditions. The eh7 allele caused earlier heading, shorter plant height and less grain per main panicle than did the wild type (WT), but the tiller number and 1000-grain weight were not significantly affected under natural long-day conditions. Biochemical assays showed that EH7 interacts with DTH8. Quantitative PCR showed that EH7 inhibited heading date by downregulating the expression of Ehd1, Hd3a and RFT1. We propose that EH7 interacts with DTH8 to control flowering time by regulating the expression of Ehd1, Hd3a and RFT1.

Transposons As Tools for Functional Genomics in Vertebrate Models

Genetic tools and mutagenesis strategies based on transposable elements are currently under development with a vision to link primary DNA sequence information to gene functions in vertebrate models. By virtue of their inherent capacity to insert into DNA, transposons can be developed into powerful tools for chromosomal manipulations. Transposon-based forward mutagenesis screens have numerous advantages including high throughput, easy identification of mutated alleles, and providing insight into genetic networks and pathways based on phenotypes. For example, the Sleeping Beauty transposon has become highly instrumental to induce tumors in experimental animals in a tissue-specific manner with the aim of uncovering the genetic basis of diverse cancers. Here, we describe a battery of mutagenic cassettes that can be applied in conjunction with transposon vectors to mutagenize genes, and highlight versatile experimental strategies for the generation of engineered chromosomes for loss-of-function as well as gain-of-function mutagenesis for functional gene annotation in vertebrate models, including zebrafish, mice, and rats.

Characterization of autonomous families of Tc1/mariner transposons in neoteleost genomes

We report the comprehensive analysis of Tc1/mariner transposons in six species of neoteleost (cod, tetraodon, fugu, medaka, stickleback, and tilapia) for which draft sequences are available. In total, 33 Tc1/mariner families were identified in these neoteleost genomes, with 3-7 families in each species. Thirty of these are in full length and designed as autonomous families, and were classified into the DD34E (Tc1) and DD×D (pogo) groups. The DD34E (Tc1) group was further classified into five clusters (Passport-like, SB-like, Frog Prince-like, Minos-like, and Bari-like). Within the genomes of cod, tetraodon, fugu, and stickleback, the Tc1/mariner DNA transposons exhibit very low proliferation with <1% of genome. In contrast, medaka and tilapia display high accumulation of Tc1/mariner transposons with 2.91% and 5.09% of genome coverages, respectively. Divergence analysis revealed that most identified Tc1/mariner transposons have undergone one round of recent accumulation, followed by a decrease in activity. One family in stickleback (Tc1_6_Ga) exhibits a very recent and strong expansion, which suggests that this element is a very young invader and putatively active. The structural organization of these Tc1/mariner elements is also described. Generally, the Tc1/mariner transposons display a high diversity and varied abundance in the neoteleost genomes with current and recent activity.

Transposition of the bamboo Mariner-like element Ppmar1 in yeast

The moso bamboo genome contains the two structurally intact and thus potentially functional mariner-like elements Ppmar1 and Ppmar2. Both elements contain perfect terminal inverted repeats (TIRs) and a full-length intact transposase gene. Here we investigated whether Ppmar1 is functional in yeast (Saccharomyces cerevisiae). We have designed a two-component system consisting of a transposase expression cassette and a non-autonomous transposon on two separate plasmids. We demonstrate that the Ppmar1 transposase Pptpase1 catalyses excision of the non-autonomous Ppmar1NA element from the plasmid and reintegration at TA dinucleotide sequences in the yeast chromosomes. In addition, we generated 14 hyperactive Ppmar1 transposase variants by systematic single amino acid substitutions. The most active transposase variant, S171A, induces 10-fold more frequent Ppmar1NA excisions in yeast than the wild type transposase. The Ppmar1 transposon is a promising tool for insertion mutagenesis in moso bamboo and may be used in other plants as an alternative to the established transposon tagging systems.

MDTE DB: a database for microRNAs derived from Transposable element

MicroRNAs are crucial regulators of gene expression at post-transcriptional level. Understanding origin and evolution of miRNAs and their functions. Transposable elements (TEs) provide a natural mechanism for the origin of new miRNAs derived from TEs (MDTEs) were collected to contruct a database named MDTE database (MDTE DB) for storing, searching and analyzing MDTEs. The database proveds a convenient source for studying the origin and evolution of miRNAs.

DNA Transposition at Work

DNA transposons are defined segments of DNA that are able to move from one genomic location to another. Movement is facilitated by one or more proteins, called the transposase, typically encoded by the mobile element itself. Here, we first provide an overview of the classification of such mobile elements in a variety of organisms. From a mechanistic perspective, we have focused on one particular group of DNA transposons that encode a transposase with a DD(E/D) catalytic domain that is topologically similar to RNase H. For these, a number of three-dimensional structures of transpososomes (transposase-nucleic acid complexes) are available, and we use these to describe the basics of their mechanisms. The DD(E/D) group, in addition to being the largest and most common among all DNA transposases, is the one whose members have been used for a wide variety of genomic applications. Therefore, a second focus of the article is to provide a nonexhaustive overview of transposon applications. Although several non-transposon-based approaches to site-directed genome modifications have emerged in the past decade, transposon-based applications are highly relevant when integration specificity is not sought. In fact, for many applications, the almost-perfect randomness and high frequency of integration make transposon-based approaches indispensable.

Sleeping Beauty transposon system for genetic etiological research and gene therapy of cancers

Carcinogenesis is etiologically associated with somatic mutations of critical genes. Recently, a number of somatic mutations and key molecules have been found to be involved in functional networks affecting cancer progression. Suitable animal models are required to validate cancer-promoting or -inhibiting capacities of these mutants and molecules. Sleeping Beauty transposon system consists of a transposon that carries gene(s) of interest and a transposase that recognizes, excises, and reinserts genes in given location of the genome. It can create both gain-of-function and loss-of-function mutations, thus being frequently chosen to investigate the etiological mechanisms and gene therapy for cancers in animal models. In this review, we summarized current advances of Sleeping Beauty transposon system in revealing molecular mechanism of cancers and improving gene therapy. Understanding molecular mechanisms by which driver mutations contribute to carcinogenesis and metastasis may pave the way for the development of innovative prophylactic and therapeutic strategies against malignant diseases.

Tc1-like Transposase Thm3 of Silver Carp (Hypophthalmichthys molitrix) Can Mediate Gene Transposition in the Genome of Blunt Snout Bream (Megalobrama amblycephala)

Tc1-like transposons consist of an inverted repeat sequence flanking a transposase gene that exhibits similarity to the mobile DNA element, Tc1, of the nematode, Caenorhabditis elegans. They are widely distributed within vertebrate genomes including teleost fish; however, few active Tc1-like transposases have been discovered. In this study, 17 Tc1-like transposon sequences were isolated from 10 freshwater fish species belonging to the families Cyprinidae, Adrianichthyidae, Cichlidae, and Salmonidae. We conducted phylogenetic analyses of these sequences using previously isolated Tc1-like transposases and report that 16 of these elements comprise a new subfamily of Tc1-like transposons. In particular, we show that one transposon, Thm3 from silver carp (Hypophthalmichthys molitrix; Cyprinidae), can encode a 335-aa transposase with apparently intact domains, containing three to five copies in its genome. We then coinjected donor plasmids harboring 367 bp of the left end and 230 bp of the right end of the nonautonomous silver carp Thm1 cis-element along with capped Thm3 transposase RNA into the embryos of blunt snout bream (Megalobrama amblycephala; one- to two-cell embryos). This experiment revealed that the average integration rate could reach 50.6% in adult fish. Within the blunt snout bream genome, the TA dinucleotide direct repeat, which is the signature of Tc1-like family of transposons, was created adjacent to both ends of Thm1 at the integration sites. Our results indicate that the silver carp Thm3 transposase can mediate gene insertion by transposition within the genome of blunt snout bream genome, and that this occurs with a TA position preference.

Excision efficiency is not strongly coupled to transgenic rate: cell type-dependent transposition efficiency of sleeping beauty and piggyBac DNA transposons

The Sleeping Beauty (SB) and piggyBac (PB) DNA transposons represent an emerging new gene delivery technology, potentially suitable for human gene therapy applications. Previous studies pointed to important differences between these transposon systems, depending on the cell types examined and the methodologies applied. However, efficiencies cannot always be compared because of differences in applications. In addition, "overproduction inhibition," a phenomenon believed to be a characteristic of DNA transposons, can remarkably reduce the overall transgenic rate, emphasizing the importance of transposase dose applied. Therefore, because of lack of comprehensive analysis, researchers are forced to optimize the technology for their own "in-house" platforms. In this study, we investigated the transposition of several SB (SB11, SB32, SB100X) and PB (mPB and hyPB) variants in various cell types at three levels: comparing the excision efficiency of the reaction by real-time PCR, testing the overall transgenic rate by detecting cells with stable integrations, and determining the average copy number when using different transposon systems and conditions. We concluded that high excision activity is not always followed by a higher transgenic rate, as exemplified by the hyperactive transposases, indicating that the excision and the integration steps of transposition are not strongly coupled as previously thought. In general, all levels of transposition show remarkable differences depending on the transposase used and cell lines examined, being the least efficient in human embryonic stem cells (hESCs). In spite of the comparably low activity in those special cell types, the hyperactive SB100X and hyPB systems could be used in hESCs with similar transgenic efficiency and with reasonably low (2-3) transgene copy numbers, indicating their potential applicability for gene therapy purposes in the future.

Efficient transposition of the youngest miniature inverted repeat transposable element family of yellow fever mosquito in yeast

Miniature inverted repeat transposable elements (MITEs) are often the most numerous DNA transposons in plant and animal genomes. The dramatic amplification of MITE families during evolution is puzzling, because the transposase sources for the vast majority of MITE families are unknown. The yellow fever mosquito genome contains > 220-Mb MITE sequences; however, transposition activity has not been demonstrated for any of the MITE families. The Gnome elements are the youngest MITE family in this genome, with at least 116 identical copies. To test whether the putative autonomous element Ozma is capable of mobilizing Gnome and its two sibling MITEs, analyses were performed in a yeast transposition assay system. Whereas the wild-type transposase resulted in very low transposition activity, mutations in the region containing a putative nuclear export signal motif resulted in a dramatic (at least 4160-fold) increase in transposition frequency. We have also demonstrated that each residue of the novel DD37E motif is required for the activity of the Ozma transposase. Footprint sequences left at the donor sites suggest that the transposase may cleave between the second and the third nucleotides from the 5' ends of the elements. The excised elements reinsert specifically at dinucleotide 'TA', ~ 55% of them in yeast genes. The elements described in this article could potentially be useful as genetic tools for genetic manipulation of mosquitoes.

The Drosophila mojavensis Bari3 transposon: distribution and functional characterization

BACKGROUND

Bari-like transposons belong to the Tc1-mariner superfamily, and they have been identified in several genomes of the Drosophila genus. This transposon's family has been used as paradigm to investigate the complex dynamics underlying the persistence and structural evolution of transposable elements (TEs) within a genome. Three structural Bari variants have been identified so far and can be distinguished based on the organization of their terminal inverted repeats. Bari3 is the last discovered member of this family identified in Drosophila mojavensis, a recently emerged species of the Repleta group of the genus Drosophila.

RESULTS

We studied the insertion pattern of Bari3 in different D. mojavensis populations and found evidence of recent transposition activity. Analysis of the transposase domains unveiled the presence of a functional nuclear localization signal, as well as a functional binding domain. Using luciferase-based assays, we investigated the promoter activity of Bari3 as well as the interaction of its transposase with its left terminus. The results suggest that Bari3 is transposition-competent. Finally we demonstrated transposase transcript processing when the transposase gene is overexpressed in vivo and in vitro.

CONCLUSIONS

Bari3 displays very similar structural and functional features with its close relative, Bari1. Our results strongly suggest that Bari3 is an independent element that has generated genomic diversity in D. mojavensis. It can autonomously transcribe its transposase gene, which in turn can localize in the nucleus and bind the terminal inverted repeats of the transposon. Nevertheless, the identification of an unpredicted spliced form of the Bari3 transposase transcript allows us to hypothesize a control mechanism of its mobility based on mRNA processing. These results will aid the studies on the Bari family of transposons, which is intriguing for its widespread diffusion in Drosophilids coupled with a structural diversity generated during the evolution of Bari-like elements in their host genomes.

Biochemical characterization and comparison of two closely related active mariner transposases

Most DNA transposons move from one genomic location to another by a cut-and-paste mechanism and are useful tools for genomic manipulations. Short inverted repeat (IR) DNA sequences marking each end of the transposon are recognized by a DNA transposase (encoded by the transposon itself). This enzyme cleaves the transposon ends and integrates them at a new genomic location. We report here a comparison of the biophysical and biochemical properties of two closely related and active mariner/Tc1 family DNA transposases: Mboumar-9 and Mos1. We compared the in vitro cleavage activities of the enzymes on their own IR sequences, as well as cross-recognition of their inverted repeat sequences. We found that, like Mos1, untagged recombinant Mboumar-9 transposase is a dimer and forms a stable complex with inverted repeat DNA in the presence of Mg²⁺ ions. Mboumar-9 transposase cleaves its inverted repeat DNA in the manner observed for Mos1 transposase. There was minimal cross-recognition of IR sequences between Mos1 and Mboumar-9 transposases, despite these enzymes having 68% identical amino acid sequences. Transposases sharing common biophysical and biochemical properties, but retaining recognition specificity toward their own IR, are a promising platform for the design of chimeric transposases with predicted and improved sequence recognition.

DNA transposon-based gene vehicles - scenes from an evolutionary drive

DNA transposons are primitive genetic elements which have colonized living organisms from plants to bacteria and mammals. Through evolution such parasitic elements have shaped their host genomes by replicating and relocating between chromosomal loci in processes catalyzed by the transposase proteins encoded by the elements themselves. DNA transposable elements are constantly adapting to life in the genome, and self-suppressive regulation as well as defensive host mechanisms may assist in buffering ‘cut-and-paste’ DNA mobilization until accumulating mutations will eventually restrict events of transposition. With the reconstructed Sleeping Beauty DNA transposon as a powerful engine, a growing list of transposable elements with activity in human cells have moved into biomedical experimentation and preclinical therapy as versatile vehicles for delivery and genomic insertion of transgenes. In this review, we aim to link the mechanisms that drive transposon evolution with the realities and potential challenges we are facing when adapting DNA transposons for gene transfer. We argue that DNA transposon-derived vectors may carry inherent, and potentially limiting, traits of their mother elements. By understanding in detail the evolutionary journey of transposons, from host colonization to element multiplication and inactivation, we may better exploit the potential of distinct transposable elements. Hence, parallel efforts to investigate and develop distinct, but potent, transposon-based vector systems will benefit the broad applications of gene transfer. Insight and clever optimization have shaped new DNA transposon vectors, which recently debuted in the first DNA transposon-based clinical trial. Learning from an evolutionary drive may help us create gene vehicles that are safer, more efficient, and less prone for suppression and inactivation.

Functional characterization of the human mariner transposon Hsmar2

DNA transposons are mobile elements with the ability to mobilize and transport genetic information between different chromosomal loci. Unfortunately, most transposons copies are currently inactivated, little is known about mariner elements in humans despite their role in the evolution of the human genome, even though the Hsmar2 transposon is associated to hotspots for homologous recombination involved in human genetic disorders as Charcot–Marie–Tooth, Prader-Willi/Angelman, and Williams syndromes. This manuscript describes the functional characterization of the human HSMAR2 transposase generated from fossil sequences and shows that the native HSMAR2 is active in human cells, but also in bacteria, with an efficiency similar to other mariner elements. We observe that the sub-cellular localization of HSMAR2 is dependent on the host cell type, and is cytotoxic when overexpressed in HeLa cells. Finally, we also demonstrate that the binding of HSMAR2 to its own ITRs is specific, and that the excision reaction leaves non-canonical footprints both in bacteria and eukaryotic cells.

Regulation of DNA transposition by CpG methylation and chromatin structure in human cells

Background

The activity of transposable elements can be regulated by different means. DNA CpG methylation is known to decrease or inhibit transpositional activity of diverse transposons. However, very surprisingly, it was previously shown that CpG methylation of the Sleeping Beauty (SB) transposon significantly enhanced transposition in mouse embryonic stem cells.

Results

In order to investigate the unexpected response of SB transposition to CpG methylation, related transposons from the Tc1/mariner superfamily, that is, Tc1, Himar1, Hsmar1, Frog Prince (FP) and Minos were tested to see how transposition was affected by CpG methylation. A significant increase of >20-fold in transposition of SB, FP and Minos was seen, whereas Tc1, Himar1 and Hsmar1 showed no difference in transposition upon CpG-methylation. The terminal inverted repeats (TIRs) of the SB, FP and Minos elements share a common structure, in which each TIR contains two functionally important binding sites for the transposase (termed the IR/DR structure). The group of IR/DR elements showed increased excision after CpG methylation compared to untreated transposon donor plasmids. We found that de novo CpG methylation is not required for transposition. A mutated FP donor plasmid with depleted CpG sites in both TIRs was as efficient in transposition as the wild-type transposon, indicating that CpG sites inside the TIRs are not responsible for altered binding of factors potentially modulating transposition. By using an in vivo one-hybrid DNA-binding assay in cultured human cells we found that CpG methylation had no appreciable effect on the affinity of SB transposase to its binding sites. However, chromatin immunoprecipitation indicated that CpG-methylated transposon donor plasmids are associated with a condensed chromatin structure characterized by trimethylated histone H3K9. Finally, DNA compaction by protamine was found to enhance SB transposition.

Conclusions

We have shown that DNA CpG methylation upregulates transposition of IR/DR elements in the Tc1/mariner superfamily. CpG methylation provokes the formation of a tight chromatin structure at the transposon DNA, likely aiding the formation of a catalytically active complex by facilitating synapsis of sites bound by the transposase.

Identification of multiple binding sites for the THAP domain of the Galileo transposase in the long terminal inverted-repeats

Galileo is a DNA transposon responsible for the generation of several chromosomal inversions in Drosophila. In contrast to other members of the P-element superfamily, it has unusually long terminal inverted-repeats (TIRs) that resemble those of Foldback elements. To investigate the function of the long TIRs we derived consensus and ancestral sequences for the Galileo transposase in three species of Drosophilids. Following gene synthesis, we expressed and purified their constituent THAP domains and tested their binding activity towards the respective Galileo TIRs. DNase I footprinting located the most proximal DNA binding site about 70 bp from the transposon end. Using this sequence we identified further binding sites in the tandem repeats that are found within the long TIRs. This suggests that the synaptic complex between Galileo ends may be a complicated structure containing higher-order multimers of the transposase. We also attempted to reconstitute Galileo transposition in Drosophila embryos but no events were detected. Thus, although the limited numbers of Galileo copies in each genome were sufficient to provide functional consensus sequences for the THAP domains, they do not specify a fully active transposase. Since the THAP recognition sequence is short, and will occur many times in a large genome, it seems likely that the multiple binding sites within the long, internally repetitive, TIRs of Galileo and other Foldback-like elements may provide the transposase with its binding specificity.

Comparative analysis of the recently discovered hAT transposon TcBuster in human cells

Background

Transposons are useful tools for creating transgenic organisms, insertional mutagenesis, and genome engineering. TcBuster, a novel hAT-family transposon system derived from the red flour beetle Tribolium castaneum, was shown to be highly active in previous studies in insect embryoes.

Methodology/Principal Findings

We tested TcBuster for its activity in human embryonic kidney 293 (HEK-293) cells. Excision footprints obtained from HEK-293 cells contained small insertions and deletions consistent with a hAT-type repair mechanism of hairpin formation and non-homologous end-joining. Genome-wide analysis of 23,417 piggyBac, 30,303 Sleeping Beauty, and 27,985 TcBuster integrations in HEK-293 cells revealed a uniquely different integration pattern when compared to other transposon systems with regards to genomic elements. TcBuster experimental conditions were optimized to assay TcBuster activity in HEK-293 cells by colony assay selection for a neomycin-containing transposon. Increasing transposon plasmid increased the number of colonies, whereas gene transfer activity dependent on codon-optimized transposase plasmid peaked at 100 ng with decreased colonies at the highest doses of transposase DNA. Expression of the related human proteins Buster1, Buster3, and SCAND3 in HEK-293 cells did not result in genomic integration of the TcBuster transposon. TcBuster, Tol2, and piggyBac were compared directly at different ratios of transposon to transposase and found to be approximately comparable while having their own ratio preferences.

Conclusions/Significance

TcBuster was found to be highly active in mammalian HEK-293 cells and represents a promising tool for mammalian genome engineering.

RNA interference is responsible for reduction of transgene expression after Sleeping Beauty transposase mediated somatic integration

Background

Integrating non-viral vectors based on transposable elements are widely used for genetically engineering mammalian cells in functional genomics and therapeutic gene transfer. For the Sleeping Beauty (SB) transposase system it was demonstrated that convergent transcription driven by the SB transposase inverted repeats (IRs) in eukaryotic cells occurs after somatic integration. This could lead to formation of double-stranded RNAs potentially presenting targets for the RNA interference (RNAi) machinery and subsequently resulting into silencing of the transgene. Therefore, we aimed at investigating transgene expression upon transposition under RNA interference knockdown conditions.

Principal Findings

To establish RNAi knockdown cell lines we took advantage of the P19 protein, which is derived from the tomato bushy stunt virus. P19 binds and inhibits 21 nucleotides long, small-interfering RNAs and was shown to sufficiently suppress RNAi. We found that transgene expression upon SB mediated transposition was enhanced, resulting into a 3.2-fold increased amount of colony forming units (CFU) after transposition. In contrast, if the transgene cassette is insulated from the influence of chromosomal position effects by the chicken-derived cHS4 insulating sequences or when applying the Forg Prince transposon system, that displays only negligible transcriptional activity, similar numbers of CFUs were obtained.

Conclusion

In summary, we provide evidence for the first time that after somatic integration transposon derived transgene expression is regulated by the endogenous RNAi machinery. In the future this finding will help to further improve the molecular design of the SB transposase vector system.

Goldfish transposase Tgf2 presumably from recent horizontal transfer is active

Hobo/Activator/Tam3 (hAT) superfamily transposons occur in plants and animals and play a role in genomic evolution. Certain hAT transposons are active and have been developed as incisive genetic tools. Active vertebrate elements are rarely discovered; however, Tgf2 transposon was recently discovered in goldfish (Carassius auratus). Here, we found that the endogenous Tgf2 element can transpose in goldfish genome. Seven different goldfish mRNA transcripts, encoding three lengths of Tgf2 transposase, were identified. Tgf2 transposase mRNA was detected in goldfish embryos, mainly in epithelial cells; levels were high in ovaries and mature eggs and in all adult tissues tested. Endogenous Tgf2 transposase mRNA is active in mature eggs and can mediate high rates of transposition (>30%) when injected with donor plasmids harboring a Tgf2 cis-element. When donor plasmid was coinjected with capped Tgf2 transposase mRNA, the insertion rate reached >90% at 1 yr. Nonautonomous copies of the Tgf2 transposon with large-fragment deletions and low levels of point mutations were also detected in common goldfish. Phylogenetic analysis indicates the taxonomic distribution of Tgf2 in goldfish is not due to vertical inheritance. We propose that the goldfish Tgf2 transposon originated by recent horizontal transfer and maintains a highly native activity.

Bioinformatics and genomic analysis of transposable elements in eukaryotic genomes

A major portion of most eukaryotic genomes are transposable elements (TEs). During evolution, TEs have introduced profound changes to genome size, structure, and function. As integral parts of genomes, the dynamic presence of TEs will continue to be a major force in reshaping genomes. Early computational analyses of TEs in genome sequences focused on filtering out "junk" sequences to facilitate gene annotation. When the high abundance and diversity of TEs in eukaryotic genomes were recognized, these early efforts transformed into the systematic genome-wide categorization and classification of TEs. The availability of genomic sequence data reversed the classical genetic approaches to discovering new TE families and superfamilies. Curated TE databases and their accurate annotation of genome sequences in turn facilitated the studies on TEs in a number of frontiers including: (1) TE-mediated changes of genome size and structure, (2) the influence of TEs on genome and gene functions, (3) TE regulation by host, (4) the evolution of TEs and their population dynamics, and (5) genomic scale studies of TE activity. Bioinformatics and genomic approaches have become an integral part of large-scale studies on TEs to extract information with pure in silico analyses or to assist wet lab experimental studies. The current revolution in genome sequencing technology facilitates further progress in the existing frontiers of research and emergence of new initiatives. The rapid generation of large-sequence datasets at record low costs on a routine basis is challenging the computing industry on storage capacity and manipulation speed and the bioinformatics community for improvement in algorithms and their implementations.

Genetic aspects of cell line development from a synthetic biology perspective

Animal cells can be regarded as factories for the production of relevant proteins. The advances described in this chapter towards the development of cell lines with higher productivity capacities, certain metabolic and proliferation properties, reduced apoptosis and other features must be regarded in an integrative perspective. The systematic application of systems biology approaches in combination with a synthetic arsenal for targeted modification of endogenous networks are proposed to lead towards the achievement of a predictable and technologically advanced cell system with high biotechnological impact.

Xenopus transgenics: methods using transposons

The generation of transgenic animals is an essential tool for many genetic strategies. DNA "cut-and-paste" transposon systems can be used to efficiently modify the Xenopus genome. The DNA transposon substrate, harbored on a circularized plasmid, is co-injected into fertilized Xenopus embryos at the one-cell stage together with mRNA encoding the cognate transposase enzyme. The cellular machinery rapidly translates the exogenous mRNA to produce active transposase enzyme that catalyzes excision of the transposon substrate from the plasmid and stable integration into the genomic DNA.

The Sleeping Beauty transposon system for clinical applications

Extensive efforts have been made to establish efficient and safe gene delivery protocols that could meet demanding expectations of a successful gene therapy. The Sleeping Beauty (SB) transposon system combines simplicity and inexpensive manufacture offered by plasmid-based vector formulation with integrative features exhibited by some viral vectors. Activated after over ten million years of silent genomic existence, the SB transposable element entered the 21st century as a potent technology for a broad range of applications in genome engineering, including gene therapy. Beneficially for gene therapy purposes, the SB system has been demonstrated to enable persistent expression of therapeutic genes followed by restoration of homeostasis in a variety of disease models. Importantly, this non-viral gene delivery vehicle is postulated to constitute a relatively safe vector system, because it lacks a preference for inserting into transcription units and their upstream regulatory regions, thereby minimizing genotoxic risks that might be associated with vector integration. Further evolution and wide, comprehensive preclinical testing of the SB transposon system in the context of several disease models is expected to further refine this valuable technology matched by enhanced biosafety towards disease treatment.

Genome walking in eukaryotes

Genome walking is a molecular procedure for the direct identification of nucleotide sequences from purified genomes. The only requirement is the availability of a known nucleotide sequence from which to start. Several genome walking methods have been developed in the last 20 years, with continuous improvements added to the first basic strategies, including the recent coupling with next generation sequencing technologies. This review focuses on the use of genome walking strategies in several aspects of the study of eukaryotic genomes. In a first part, the analysis of the numerous strategies available is reported. The technical aspects involved in genome walking are particularly intriguing, also because they represent the synthesis of the talent, the fantasy and the intelligence of several scientists. Applications in which genome walking can be employed are systematically examined in the second part of the review, showing the large potentiality of this technique, including not only the simple identification of nucleotide sequences but also the analysis of large collections of mutants obtained from the insertion of DNA of viral origin, transposons and transfer DNA (T-DNA) constructs. The enormous amount of data obtained indicates that genome walking, with its large range of applicability, multiplicity of strategies and recent developments, will continue to have much to offer for the rapid identification of unknown sequences in several fields of genomic research.

Sleeping Beauty transposon mutagenesis in rat spermatogonial stem cells

We describe an experimental approach for generating mutant alleles in rat spermatogonial stem cells (SSCs) using Sleeping Beauty (SB) transposon-mediated insertional mutagenesis. The protocol is based on mobilization of mutagenic gene-trap transposons from transfected plasmid vectors into the genomes of cultured stem cells. Cells with transposon insertions in expressed genes are selected on the basis of activation of an antibiotic-resistance gene encoded by the transposon. These gene-trap clones are transplanted into the testes of recipient males (either as monoclonal or polyclonal libraries); crossing of these founders with wild-type females allows the insertions to be passed to F(1) progeny. This simple, economic and user-friendly methodological pipeline enables screens for functional gene annotation in the rat, with applicability in other vertebrate models where germ line-competent stem cells have been established. The complete protocol from transfection of SSCs to the genotyping of heterozygous F(1) offspring that harbor genomic SB gene-trap insertions takes 5-6 months.

Endogenous transposases affect differently Sleeping Beauty and Frog Prince transposons in fish cells

Fish cells stably expressing exogenous genes have potential applications in the production of fish recombinant proteins, gene-function studies, gene-trapping, and the production of transgenic fish. However, expression of a gene of interest after random integration may be difficult to predict or control. In the past decade, major contributions have been made in vertebrate-gene transfer, by using tools derived from DNA transposons. Among them, the Sleeping Beauty (SB) and Frog Prince (FP) transposons, derived, respectively, from fish and frog genomes, mediate transposition in a large variety of cells, although with different efficiency. This study was aimed at assessing the activities of the SB and the FP transposases in fish cell lines from genetically distant species (CHSE-214, RTG-2, BF-2, EPC, and SAF-1). Their transpositional ability was evaluated by the plasmid-based excision assay, the colony formation assay, and the footprint patterns. The results reveal that while both transposases are active in all cell lines, the transposition rates and the precision of the transposition are overall higher with FP than SB. Our results also indicated a key role of cell-specific host factors in transposition, which was associated with the presence of Tc1-like endogenous transposases; this effect was more accentuated in the two salmonid cell lines transfected with SB. This result agrees with previous studies supporting the use of transposons in heterologous organisms to prevent from genomic instability and from impeding the precise activity of the exogenous transposase.

Avoiding cytotoxicity of transposases by dose-controlled mRNA delivery

The Sleeping Beauty (SB) transposase and its newly developed hyperactive variant, SB100X, are of increasing interest for genome modification in experimental models and gene therapy. The potential cytotoxicity of transposases requires careful assessment, considering that residual integration events of transposase expression vectors delivered by physicochemical transfection or episomal retroviral vectors may lead to permanent transposase expression and resulting uncontrollable transposition. Comparing retrovirus-based approaches for delivery of mRNA, episomal DNA or integrating DNA, we found that conventional SB transposase, SB100X and a newly developed codon-optimized SB100Xo may trigger premitotic arrest and apoptosis. Cell stress induced by continued SB overexpression was self-limiting due to the induction of cell death, which occurred even in the absence of a co-transfected transposable element. The cytotoxic effects of SB transposase were strictly dose dependent and heralded by induction of p53 and c-Jun. Inactivating mutations in SB's catalytic domain could not abrogate cytotoxicity, suggesting a mechanism independent of DNA cleavage activity. An improved approach of retrovirus particle-mediated mRNA transfer allowed transient and dose-controlled expression of SB100X, supported efficient transposition and prevented cytotoxicity. Transposase-mediated gene transfer can thus be tuned to maintain high efficiency in the absence of overt cell damage.

Human pluripotent stem cells in pharmacological and toxicological screening: new perspectives for personalized medicine

Human stem cells provide an important novel tool for generating in vitro pharmacological and toxicological test systems. In the development of new targeted therapies, as well as in critical safety issues, including hepato-, neuro- and cardio-toxicity, animal-based tests are mostly unsatisfactory, whereas the use of in vitro model systems is limited by the unavailability of relevant human tissues. Human embryonic stem cell lines may fill this gap and offer an advantage over primary cultures as well as tissue-derived (adult) stem cells. Human embryonic stem cells represent an unlimited source for the production of differentiated somatic progenies and allow various stable genetic manipulations. As a new opening in personalized medicine test systems, the generation of induced pluripotent stem cell lines and their derivatives can provide patient- and disease-specific cellular assays for drug development and safety assessments. This article reviews promising human stem cell applications in pharmacological and toxicological screenings, focusing on the implications for personalized medicine.

Effective generation of transgenic reporter and gene trap lines of the medaka (Oryzias latipes) using the Ac/Ds transposon system

In model teleost fishes like the medaka and the zebrafish many genes which have been identified in genome sequencing projects await their functional characterization. Techniques for the effective generation of transgenic animals are a prerequisite for this challenging task, and, due to their transparency, fish offer the possibility to combine the use of fluorescent proteins and developmental analysis in vivo. Here we describe the application of the Ac/Ds transposon system to generate transgenic medaka reporter and gene trap lines. We determined a germline transmission rate of 30% in our experiments using constructs ranging in size from 1.8 to 6 kilobase pairs. The genomic integration site of the Ds-elements can be easily identified which is an important feature for gene trap mutagenesis experiments and similar approaches. We constructed gene trap vectors with functional elements of medaka sequences that produce in frame fusions of the endogenous sequence to EGFP. These vectors mimic endogenous expression of the trapped allele in transgenic animals and are capable to interfere with the expression of the wild type allele in the homozygous individuals.