Transposon Tc1 of the nematode Caenorhabditis elegans jumps in human cells

Sleeping Beauty transposition: from biology to applications

Sleeping Beauty (SB) is the first synthetic DNA transposon that was shown to be active in a wide variety of species. Here, we review studies from the last two decades addressing both basic biology and applications of this transposon. We discuss how host-transposon interaction modulates transposition at different steps of the transposition reaction. We also discuss how the transposon was translated for gene delivery and gene discovery purposes. We critically review the system in clinical, pre-clinical and non-clinical settings as a non-viral gene delivery tool in comparison with viral technologies. We also discuss emerging SB-based hybrid vectors aimed at combining the attractive safety features of the transposon with effective viral delivery. The success of the SB-based technology can be fundamentally attributed to being able to insert fairly randomly into genomic regions that allow stable long-term expression of the delivered transgene cassette. SB has emerged as an efficient and economical toolkit for safe and efficient gene delivery for medical applications.

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.

Mariner transposons as genetic tools in vertebrate cells

Transposable elements (TEs) are being investigated as potential molecular tools in genetic engineering, for use in procedures such as transgenesis and insertional mutagenesis. Naturally active and reconstructed active TEs are both being studied to develop non-viral delivery vehicles. To date, the active elements being used include three Mariner-Like Elements (MLEs). We review below the studies that have investigated the ability of these MLEs to insert a transgene in vertebrate cells.

The Tol1 element of the medaka fish, a member of the hAT transposable element family, jumps in Caenorhabditis elegans

Tol1 is a DNA-based transposable element residing in the genome of the medaka fish Oryzias latipes, and has been proven to be transposed in various vertebrate species, including mammals. This element belongs to the hAT (hobo/Activator/Tam3) transposable element family, whose members are distributed in a wide range of organisms. It is thus possible that Tol1 is mobile in organisms other than vertebrates. We here show that transposition of this element occurs in the nematode Caenorhabditis elegans. A donor plasmid containing a Tol1 element and a helper plasmid carrying the transposase gene were delivered into gonad cells and, after several generations of culturing, were recovered from worms. PCR analysis of the donor plasmid, using primers that encompassed the Tol1 element, revealed excision of the Tol1 portion from the plasmid. Analysis of genomic DNA of the worms by the inverse PCR method provided evidence that Tol1 had been integrated into the C. elegans chromosomes. Vertebrates and C. elegans are phylogenetically distantly related organisms in that the former are deuterostomes and the latter a protostome animal. Our results indicate (1) the transposition reaction of the Tol1 element requires, besides the transposase, no factors from host cells, or (2) the host factors, even if required, are those that are common to protostomes and deuterostomes. The results also have significance for the development of a gene transfer vector and other biotechnology tools for C. elegans.

Sequence analysis and quantification of transposase cDNAs of transposon TCp3.2 in Cydia pomonella larvae

The Tc1-like transposable element TCp3.2 was previously found to be horizontally transferred from the genome of Cydia pomonella to the C. pomonella granulovirus (CpGV). In this study, the transcription of transposase genes of endogenous TCp3.2 copies in the insect host genome was investigated. Cloning and sequencing of cDNAs prepared from TCp3.2 transposase transcripts resulted in the identification of a 199-bp-long intron. Sequence heterogeneities among different cDNA clones suggested that multiple copies of the transposase are transcribed, but that a part of these copies encode a defective transposase. The actin gene of C. pomonella was cloned and sequenced, and used to standardise quantitative real time PCR on prepared cDNA of the TCp3.2 transposase. Comparison of cDNA levels of TCp3.2 transposase prepared from mock and CpGV-infected C. pomonella larvae did not provide evidence that CpGV infection influenced the transcription level of TCp3.2 transposase.

RNA as a source of transposase for Sleeping Beauty-mediated gene insertion and expression in somatic cells and tissues

Sleeping Beauty (SB) is a DNA transposon capable of mediating gene insertion and long-term expression in vertebrate cells when co-delivered with a source of transposase. In all previous reports of SB-mediated gene insertion in somatic cells, the transposase component has been provided by expression of a co-delivered DNA molecule that has the potential for integration into the host cell genome. Integration and continued expression of a gene encoding SB transposase could be problematic if it led to transposon re-mobilization and reintegration. We addressed this potential problem by supplying the transposase-encoding molecule in the form of mRNA. We show that transposase-encoding mRNA can effectively mediate transposition in vitro in HT1080 cells and in vivo in mouse liver following co-delivery with a recoverable transposon or with a luciferase transposon. We conclude that in vitro-transcribed mRNA can be used as an effective source of transposase for SB-mediated transposition in mammalian cells and tissues.

Counterselection and co-delivery of transposon and transposase functions for Sleeping Beauty-mediated transposition in cultured mammalian cells

Sleeping Beauty (SB) is a gene-insertion system reconstructed from transposon sequences found in teleost fish and is capable of mediating the transposition of DNA sequences from transfected plasmids into the chromosomes of vertebrate cell populations. The SB system consists of a transposon, made up of a gene of interest flanked by transposon inverted repeats, and a source of transposase. Here we carried out a series of studies to further characterize SB-mediated transposition as a tool for gene transfer to chromosomes and ultimately for human gene therapy. Transfection of mouse 3T3 cells, HeLa cells, and human A549 lung carcinoma cells with a transposon containing the neomycin phosphotransferase (NEO) gene resulted in a several-fold increase in drug-resistant colony formation when co-transfected with a plasmid expressing the SB transposase. A transposon containing a methotrexate-resistant dihydrofolate reductase gene was also found to confer an increased frequency of methotrexate-resistant colony formation when co-transfected with SB transposase-encoding plasmid. A plasmid containing a herpes simplex virus thymidine kinase gene as well as a transposon containing a NEO gene was used for counterselection against random recombinants (NEO+TK+) in medium containing G418 plus ganciclovir. Effective counterselection required a recovery period of 5 days after transfection before shifting into medium containing ganciclovir to allow time for transiently expressed thymidine kinase activity to subside in cells not stably transfected. Southern analysis of clonal isolates indicated a shift from random recombination events toward transposition events when clones were isolated in medium containing ganciclovir as well as G418. We found that including both transposon and transposase functions on the same plasmid substantially increased the stable gene transfer frequency in Huh7 human hepatoma cells. The results from these experiments contribute technical and conceptual insight into the process of transposition in mammalian cells, and into the optimal provision of transposon and transposase functions that may be applicable to gene therapy studies.

Synthetic, self-assembly ABCD nanoparticles; a structural paradigm for viable synthetic non-viral vectors

Gene therapy research is still in trouble owing to a paucity of acceptable vector systems to deliver nucleic acids to patients for therapy. Viral vectors are efficient but may be too dangerous. Synthetic non-viral vectors are inherently safer but are currently not efficient enough to be clinically viable. The solution for gene therapy lies with improved synthetic non-viral vectors systems. This review is focused on synthetic cationic liposome/micelle-based non-viral vector systems and is a critical review written to illustrate the increasing importance of chemistry in gene therapy research. This review should be of primary interest to synthetic chemists and biomedical researchers keen to appreciate emerging technologies, but also to biological scientists who remain to be convinced about the relevance of chemistry to biology.

Insertional mutagenesis in mice: new perspectives and tools

Insertional mutagenesis has been at the core of functional genomics in many species. In the mouse, improved vectors and methodologies allow easier genome-wide and phenotype-driven insertional mutagenesis screens. The ability to generate homozygous diploid mutations in mouse embryonic stem cells allows prescreening for specific null phenotypes prior to in vivo analysis. In addition, the discovery of active transposable elements in vertebrates, and their development as genetic tools, has led to in vivo forward insertional mutagenesis screens in the mouse. These new technologies will greatly contribute to the speed and ease with which we achieve complete functional annotation of the mouse genome.

Sleeping Beauty Transposon‐Mediated Gene Therapy for Prolonged Expression

The Sleeping Beauty (SB) transposon system represents a new vector for non-viral gene transfer that melds advantages of viruses and other forms of naked DNA transfer. The transposon itself is comprised of two inverted terminal repeats of about 340 base pairs each. The SB system directs precise transfer of specific constructs from a donor plasmid into a mammalian chromosome. The excision of the transposon from a donor plasmid and integration into a chromosomal site is mediated by Sleeping Beauty transposase, which can be delivered to cells vita its gene or its mRNA. As a result of its integration in chromosomes, and its lack of viral sequences that are often detected by poorly understood cellular defense mechanisms, a gene in a chromosomally integrated transposon can be expressed over the lifetime of a cell. SB transposons integrate nearly randomly into chromosomes at TA-dinucleotide base pairs although the sequences flanking the TAs can influence the probability of integration at a given site. Although random integration of vectors into human genomes is often thought to raise significant safety issues, evidence to date does not indicate that random insertions of SB transposons represent risks that are equal to those of viral vectors. Here we review the activities of the SB system in mice used as a model for human gene therapy, methods of delivery of the SB system, and its efficacy in ameliorating disorders that model human disease.

CemaT1 is an active transposon within the Caenorhabditis elegans genome

The maT clade of transposons is a group of transposable elements intermediate in sequence and predicted protein structure to mariner and Tc transposons, with a distribution thus far limited to a few invertebrate species. In the nematode Caenorhabditis elegans, there are eight copies of CemaT1 that are predicted to encode a functional transposase, with five copies being >99% identical. We present evidence, based on searches of publicly available databases and on PCR-based mobility assays, that the CemaT1 transposase is expressed in C. elegans and that the CemaT transposons are capable of excising in both somatic and germline tissues. We also show that the frequency of CemaT1 excisions within the genome of the N2 strain of C. elegans is comparable to that of the Tc1 transposon. However, unlike Tc transposons in mutator strains of C. elegans, maT transposons do not exhibit increased frequencies of mobility, suggesting that maT is not regulated by the same factors that control Tc activity in these strains. Finally, we show that CemaT1 transposons are capable of precise transpositions as well as orientation inversions at some loci, and thereby become members of an increasing number of identified active transposons within the C. elegans genome.

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

Members of the Tc1/mariner superfamily of transposable elements isolated from vertebrates are transpositionally inactive due to the accumulation of mutations in their transposase genes. A novel open reading frame-trapping method was used to isolate uninterrupted transposase coding regions from the genome of the frog species Rana pipiens. The isolated clones were approximately 90% identical to a predicted transposase gene sequence from Xenopus laevis, but contained an unpredicted, approximately 180 bp region encoding the N-terminus of the putative transposase. None of these native genes was found to be active. Therefore, a consensus sequence of the transposase gene was derived. This engineered transposase and the transposon inverted repeats together constitute the components of a novel transposon system that we named Frog Prince (FP). FP has only approximately 50% sequence similarity to Sleeping Beauty (SB), and catalyzes efficient cut-and-paste transposition in fish, amphibian and mammalian cell lines. We demonstrate high-efficiency gene trapping in human cells using FP transposition. FP is the most efficient DNA-based transposon from vertebrates described to date, and shows approximately 70% higher activity in zebrafish cells than SB. Frog Prince can greatly extend our possibilities for genetic analyses in vertebrates.

Generating and manipulating transgenic animals using transposable elements

Transposable elements, or transposons, have played a significant role in the history of biological research. They have had a major influence on the structure of genomes during evolution, they can cause mutations, and their study led to the concept of so-called "selfish DNA". In addition, transposons have been manipulated as useful gene transfer vectors. While primarily restricted to use in invertebrates, prokaryotes, and plants, it is now clear that transposon technology and biology are just as relevant to the study of vertebrate species. Multiple transposons now have been shown to be active in vertebrates and they can be used for germline transgenesis, somatic cell transgenesis/gene therapy, and random germline insertional mutagenesis. The sophistication of these applications and the number of active elements are likely to increase over the next several years. This review covers the vertebrate-active retrotransposons and transposons that have been well studied and adapted for use as gene transfer agents. General considerations and predictions about the future utility of transposon technology are discussed.

The medaka fish Tol2 transposable element can undergo excision in human and mouse cells

Tol2 is an active DNA-based transposable element identified in the medaka fish, Oryzias latipes. Originating from a vertebrate and belonging to the hAT ( hobo/ Activator/ Tam3) transposable element family, featuring a wide distribution among organisms, Tol2 would be expected to be active if introduced into mammals. We, therefore, examined if excision, one part of the transposition reaction, can occur in human and mouse culture cells. A Tol2 clone was introduced into cells and, after incubation, recovered. PCR and sequencing analysis provided evidence for precise and near precise excision in these cells. Tol2 can thus be expected to serve as a material for developing a gene transfer vector and other genetic tools applicable to mammals. It was also suggested that an intact Tol2 element could retain autonomy as a transposable element in mammalian cells.

Heterologous transposition in Ustilago maydis

The phytopathogenic basidiomycete Ustilago maydis has become a model system for the analysis of plant-pathogen interactions. The genome sequence of this organism will soon be available, increasing the need for techniques to analyse gene function on a broad basis. We describe a heterologous transposition system for U. maydis that is based on the Caenorhabditis transposon Tc1, which is known to function independently of host factors and to be active in evolutionarily distant species. We have established a nitrate reductase based two-component counterselection system to screen for Tc1 transposition. The element was shown to be functional and transposed to several different locations in the genome of U. maydis. The insertion pattern observed was consistent with the proposed general mechanism of Tc1/mariner integration and constitutes a proof of principle for the first heterologous transposition system in a basidiomycete species. By mapping the insertion site context to known genomic sequences, Tc1 insertion events were shown to occur on different chromosomes, but exhibit a preference for non-coding regions. Only 20% of the insertions were found in putative open reading frames. The establishment of this system will permit efficient gene tagging in U. maydis and possibly also in other fungi.

In vivo correction of murine tyrosinemia type I by DNA-mediated transposition

Gene therapy applications of naked DNA constructs for genetic disorders have been limited because of lack of permanent transgene expression. This limitation, however, can be overcome by the Sleeping Beauty (SB) transposable element, which can achieve permanent transgene expression through genomic integration from plasmid DNA. To date, only one example of an in vivo gene therapy application of this system has been reported. In this report, we have further defined the activity of the SB transposon in vivo by analyzing the expression and integration of a fumarylacetoacetate hydrolase (FAH) transposon in FAH-deficient mice. In this model, stably corrected FAH(+) hepatocytes are clonally selected and stable integration events can therefore be quantified and characterized at the molecular level. Herein, we demonstrate that SB-transposon-transfected hepatocytes can support significant repopulation of the liver, resulting in long-lasting correction of the FAH-deficiency phenotype. A single, combined injection of an FAH-expressing transposon plasmid and a transposase expression construct resulted in stable FAH expression in approximately 1% of transfected hepatocytes. The average transposon copy number was determined to be approximately 1/diploid genome and expression was not silenced during serial transplantation. Molecular analysis indicated that high-efficiency DNA-mediated transposition into the mouse genome was strictly dependent on the expression of wild-type transposase.

Design of a nonviral vector for site-selective, efficient integration into the human genome

Gene therapy in eukaryotes has met many obstacles. Research into the design of suitable nonviral vectors has been slow. To our knowledge, no nonviral vector has been proposed that allows for the possibility of highly efficient, site-selective integration into the genome of mammalian cells. On the basis of prior studies investigating the components necessary for transposon, retrovirus-like retrotransposon, and retroviral integration, we propose a nonviral system that would potentially allow for site-selective, efficient integration into the mammalian genome. Transposons have been developed that can transform a variety of cell lines. For example, the Sleeping Beauty transposon (SB) can transform a wide range of vertebrate cells from fish to human, and it mediates stable integration and long-term transgene expression in mice. However, the efficiency of transposition varies significantly among cell lines, suggesting the possible involvement of host factors in SB transposition. Here, we propose the use of a chimeric transposase (i.e., transposase-host DNA binding domain) to bypass the potential requirement of a host DNA-directing factor (or factors) for efficient, site-selective integration. We also discuss another potential method of docking the transposon-based vector adjacent to the host DNA, utilizing repetitive sequences for homologous recombination to promote efficient site-selective integration, as well as other site-selective nonviral approaches.

Structure-function analysis of the inverted terminal repeats of the sleeping beauty transposon

Translocation of Sleeping Beauty (SB) transposon requires specific binding of SB transposase to inverted terminal repeats (ITRs) of about 230 bp at each end of the transposon, which is followed by a cut-and-paste transfer of the transposon into a target DNA sequence. The ITRs contain two imperfect direct repeats (DRs) of about 32 bp. The outer DRs are at the extreme ends of the transposon whereas the inner DRs are located inside the transposon, 165-166 bp from the outer DRs. Here we investigated the roles of the DR elements in transposition. Although there is a core transposase-binding sequence common to all of the DRs, additional adjacent sequences are required for transposition and these sequences vary in the different DRs. As a result, SB transposase binds less tightly to the outer DRs than to the inner DRs. Two DRs are required in each ITR for transposition but they are not interchangeable for efficient transposition. Each DR appears to have a distinctive role in transposition. The spacing and sequence between the DR elements in an ITR affect transposition rates, suggesting a constrained geometry is involved in the interactions of SB transposase molecules in order to achieve precise mobilization. Transposons are flanked by TA dinucleotide base-pairs that are important for excision; elimination of the TA motif on one side of the transposon significantly reduces transposition while loss of TAs on both flanks of the transposon abolishes transposition. These findings have led to the construction of a more advanced transposon that should be useful in gene transfer and insertional mutagenesis in vertebrates.

In vivo transposition of Minos, a Drosophila mobile element, in mammalian tissues

Transposable elements have been used widely in the past 20 years for gene transfer and insertional mutagenesis in Drosophila. Transposon-based technology for gene manipulation and genomic analysis currently is being adopted for vertebrates. We tested the ability of Minos, a DNA transposon from Drosophila hydei, to transpose in mouse tissues. Two transgenic mouse lines were crossed, one expressing Minos transposase in lymphocytes under the control of the CD2 promoter/locus control region and another carrying a nonautonomous Minos transposon. Only mice containing both transgenes show excision of the transposon and transposition into new chromosomal sites in thymus and spleen cells. In addition, expression of Minos transposase in embryonic fibroblast cell lines derived from a transposon-carrying transgenic mouse resulted in excision of the transposon. These results are a first step toward a reversible insertional mutagenesis system in the mouse, opening the way to develop powerful technologies for functional genomic analysis in mammals.

A family of Tc1-like transposons from the genomes of fishes and frogs: evidence for horizontal transmission

Tc1-like transposons are very widely distributed within the genomes of animal species. They consist of an inverted repeat sequence flanking a transposase gene with homology to the mobile DNA element, Tc1 of the nematode Caenorhabditis elegans. These elements seem particularly to infest the genomes of fish and amphibian species where they can account for 1% of the total genome. However, all vertebrate Tc1-like elements isolated so far are non-functional in that they contain multiple frameshifts within their transposase coding regions. Here I describe a Tc1-like transposon (PPTN) from the genome of a marine flatfish species (Pleuronectes platessa) which bears conserved inverted repeats flanking an apparently intact transposase gene. Closely related, although degenerate, Tc1-like transposons were also isolated from the genomes of Atlantic salmon (SSTN, Salmo salar) and frog (RTTN, Rana temporaria). Consensual nucleic acid sequences were derived by comparing several individual isolates from each species and conceptual amino acid sequences were thence derived for their transposases. Phylogenetic analysis of these sequences with previously isolated Tc1-like transposases shows that the elements from plaice, salmon and frog comprise a new subfamily of Tc1-like transposons. Each member is distinct in that it is not found in the genomes of the other species tested. Plaice genomes contain about 300 copies of PPTN, salmon 1200 copies of SSTN and frog genomes about 500 copies of RTTN. The presence of these closely related elements in the genomes of fish and frog species, representing evolutionary lines, which diverged more than 400 million years ago, is not consistent with a vertical transmission model for their distributions.

Regulated transposition of a fish transposon in the mouse germ line

Tc1/mariner elements are able to transpose in species other than the host from which they were isolated. As potential vectors for insertional mutagenesis and transgenesis of the mouse, these cut-and-paste transposons were tested for their ability to transpose in the mouse germ line. First, the levels of activity of several Tc1/mariner elements in mammalian cells were compared; the reconstructed fish transposon Sleeping Beauty (SB) was found to be an order of magnitude more efficient than the other tested transposons. SB then was introduced into the mouse germ line as a two-component system: one transgene for the expression of the transposase in the male germ line and a second transgene carrying a modified transposon. In 20% of the progeny of double transgenic male mice the transposon had jumped from the original chromosomal position into another locus. Analysis of the integration sites shows that these jumps indeed occurred through the action of SB transposase, and that SB has a strong preference for intrachromosomal transposition. Analysis of the excision sites suggests that double-strand breaks in haploid spermatids are repaired via nonhomologous end joining. The SB system may be a powerful tool for transposon mutagenesis of the mouse germ line.

A novel repeated sequence DNA originated from a Tc1-like transposon in water green frog Rana esculenta

We have identified and characterized a highly repetitive family, called R.e./Tc1 in the genome of the green water frog Rana esculenta. This family consists of tandemly repeated sequences, localized at the centromeric regions of chromosomes as shown by Southern blot and 'in situ' hybridization. The repeat unit contains a residue of a Tc1-like transposon by Haematobia irritans fly, bordered by two short direct repeats of 9 bp. Tc1 remnant lays near a sequence identical to Homo sapiens Werner syndrome gene stretch. These sequence data suggest that R.e./Tc1 element was probably originated from a transposition event and a duplication via DNA mechanism of the R.e./Tc1 unit that could give rise to the observed tandem array.

Towards integrating vectors for gene therapy: expression of functional bacteriophage MuA and MuB proteins in mammalian cells

Bacteriophage Mu has one of the best studied, most efficient and largest transposition machineries of the prokaryotic world. To harness this attractive integration machinery for use in mammalian cells, we cloned the coding sequences of the phage factors MuA and MuB in a eukaryotic expression cassette and fused them to a FLAG epitope and a SV40-derived nuclear localization signal. We demonstrate that these N-terminal extensions were sufficient to target the Mu proteins to the nucleus, while their function in Escherichia coli was not impeded. In vivo transposition in mammalian cells was analysed by co-transfection of the MuA and MuB expression vectors with a donor construct, which contained a miniMu transposon carrying a Hygromycin-resistance marker (Hyg(R)). In all co-transfections, a significant but moderate (up to 2.7-fold) increase in Hyg(R) colonies was obtained if compared with control experiments in which the MuA vector was omitted. To study whether the increased efficiency was the result of bona fide Mu transposition, integrated vector copies were cloned from 43 monoclonal and one polyclonal cell lines. However, in none of these clones, the junction between the vector and the chromosomal DNA was localized precisely at the border of the Att sites. From our data we conclude that expression of MuA and MuB increases the integration of miniMu vectors in mammalian cells, but that this increase is not the result of bona fide Mu-induced transposition.

Genome-wide insertional mutagenesis in human cells by the Drosophila mobile element Minos

The development of efficient non-viral methodologies for genome-wide insertional mutagenesis and gene tagging in mammalian cells is highly desirable for functional genomic analysis. Here we describe transposon mediated mutagenesis (TRAMM), using naked DNA vectors based on the Drosophila hydei transposable element Minos. By simple transfections of plasmid Minos vectors in HeLa cells, we have achieved high frequency generation of cell lines, each containing one or more stable chromosomal integrations. The Minos-derived vectors insert in different locations in the mammalian genome. Genome-wide mutagenesis in HeLa cells was demonstrated by using a Minos transposon containing a lacZ-neo gene-trap fusion to generate a HeLa cell library of at least 10(5) transposon insertions in active genes. Multiple gene traps for six out of 12 active genes were detected in this library. Possible applications of Minos-based TRAMM in functional genomics are discussed.

Hybrid vector designs to control the delivery, fate and expression of transgenes

One of the greatest challenges to gene therapy is the targetting of gene delivery selectively to the sites of disease and regulation of transgene expression without adverse effects. Ultimately, the successful realization of these goals is dependent upon improvements in vector design. Over the years, viral vector design has progressed from various types of replication-defective viral mutants to replication-conditioned viruses and, more recently, to 'gutted' and hybrid vectors, which have, respectively, eliminated expression of non-relevant or toxic viral genes and incorporated desired elements of different viruses so as to increase the efficacy of gene delivery in vivo. This review will focus on the different viral and cellular elements which have been incorporated into virus vectors to: improve transduction efficiencies; alter the entry specificity of virions; control the fate of transgenes in the host cells; and regulate transgene expression.

Mobile elements and the human genome

Genomic DNA is often thought of as the stable template of heredity, largely dormant and unchanging, apart from perhaps the occasional point mutation. But it has become increasingly clear that DNA is dynamic rather than static, being subjected to rearrangements, insertions and deletions. Much of this plasticity can be attributed to transposable elements and their genomic relatives.

Purification of the Caenorhabditis elegans transposase Tc1A refolded during gel filtration chromatography

Full-length recombinant transposase Tc1A from Caenorhabditis elegans (343 amino acids) expressed in Escherichia coli BL21 in inclusion bodies has been purified in a high yield in a soluble form. The procedure includes denaturation of the inclusion bodies followed by refolding of the Tc1A protein by gel filtration. This last step is absolutely crucial to give a high yield of soluble and active protein since it allows the physical separation of the aggregates from intermediates that give rise to correctly refolded protein. This step is very sensitive to the concentration of protein. Good yields of refolded protein are obtained by refolding 2 to 12 mg of denatured protein. The other purification steps involve the initial use of gel filtration under denaturing conditions and a final step of ion-exchange chromatography. Biological activity of the purified protein was confirmed in an in vitro transposon excision assay and its DNA-binding capacity by UV crosslinking. This new Tc1A purification procedure gives a yield of 12-16 mg/liter E. coli culture, in a form suitable for crystallization studies.

Mobility assays confirm the broad host-range activity of the Minos transposable element and validate new transformation tools

Fast and reliable methods for assessing the mobility of the transposable element Minos have been developed. These methods are based on the detection of excision and insertion of Minos transposons from and into plasmids which are co-introduced into cells. Excision is detected by polymerase chain reaction (PCR) with appropriate primers. Transposition is assayed by marker rescue in Escherichia coli, using a transposon plasmid that carries a tetracycline resistance gene and a target plasmid carrying a gene that can be selected against in E. coli. Using both assays, Minos was shown to transpose in Drosophila melanogaster cells and embryos, and in cultured cells of a mosquito, Aedes aegypti, and a lepidopteran, Spodoptera frugiperda. In all cases, mobility was dependent on the presence of exogenously supplied transposase, and both excision and transposition were precise. The results indicate that Minos can transpose in heterologous insect species with comparable efficiencies and therefore has the potential to be used as a transgenesis vector for diverse species.

Somatic integration and long-term transgene expression in normal and haemophilic mice using a DNA transposon system

The development of non-viral gene-transfer technologies that can support stable chromosomal integration and persistent gene expression in vivo is desirable. Here we describe the successful use of transposon technology for the nonhomologous insertion of foreign genes into the genomes of adult mammals using naked DNA. We show that the Sleeping Beauty transposase can efficiently insert transposon DNA into the mouse genome in approximately 5-6% of transfected mouse liver cells. Chromosomal transposition resulted in long-term expression (>5 months) of human blood coagulation factor IX at levels that were therapeutic in a mouse model of haemophilia B. Our results establish DNA-mediated transposition as a new genetic tool for mammals, and provide new strategies to improve existing non-viral and viral vectors for human gene therapy applications.

cis and trans factors affecting Mos1 mariner evolution and transposition in vitro, and its potential for functional genomics

Mos1 and other mariner / Tc1 transposons move horizon-tally during evolution, and when transplanted into heterologous species can transpose in organisms ranging from prokaryotes to protozoans and vertebrates. To further develop the Drosophila Mos1 mariner system as a genetic tool and to probe mechanisms affecting the regulation of transposition activity, we developed an in vitro system for Mos1 transposition using purified transposase and selectable Mos1 derivatives. Transposition frequencies of nearly 10(-3)/target DNA molecule were obtained, and insertions occurred at TA dinucleotides with little other sequence specificity. Mos1 elements containing only the 28 bp terminal inverted repeats were inactive in vitro, while elements containing a few additional internal bases were fully active, establishing the minimal cis -acting requirements for transposition. With increasing transposase the transposition frequency increased to a plateau value, in contrast to the predictions of the protein over-expression inhibition model and to that found recently with a reconstructed Himar1 transposase. This difference between the 'natural' Mos1 and 'reconstructed' Himar1 transposases suggests an evolutionary path for down-regulation of mariner transposition following its introduction into a naïve population. The establishment of the cis and trans requirements for optimal mariner transposition in vitro provides key data for the creation of vectors for in vitro mutagenesis, and will facilitate the development of in vivo systems for mariner transposition.

Interspersed repeats and other mementos of transposable elements in mammalian genomes

The bulk of the human genome is ultimately derived from transposable elements. Observations in the past year lead to some new and surprising ideas on functions and consequences of these elements and their remnants in our genome. The many new examples of human genes derived from single transposon insertions highlight the large contribution of selfish DNA to genomic evolution.

Resident aliens: the Tc1/mariner superfamily of transposable elements

Transgenic technology is currently applied to several animal species of agricultural or medical importance, such as fish, cattle, mosquitos and parasitic worms. However, the repertoire of genetic tools used for molecular analyses of mice and Drosophila is not always applicable to other species. For example, while retroviral enhancer-trap experiments in mice can be based on embryonic stem (ES) cell technology, this is not currently an option with other animals. Similarly, the germline transformation of Drosophila depends on the use of the P-element transposon, which does not jump in other genera. This article analyses the main characteristics of Tc1/mariner transposable elements, examines some of the factors that have contributed to their evolutionary success, and describes their potential, as well as their limitations, for transgenesis and insertional mutagenesis in diverse animals.