Alu is particularly prolific, having originated in primates and expanding in a relatively short time to about 1 million copies per cell in humans.
The fact that roughly half of the human genome is made up of TEs, with a significant portion of them being L1 and Alu retrotransposons, raises an important question: What do all these jumping genes do, besides jump? Much of what a transposon does depends on where it lands. Landing inside a gene can result in a mutation , as was discovered when insertions of L1 into the factor VIII gene caused hemophilia Kazazian et al.
Similarly, a few years later, researchers found L1 in the APC genes in colon cancer cells but not in the APC genes in healthy cells in the same individuals. This confirms that L1 transposes in somatic cells in mammals, and that this element might play a causal role in disease development Miki et al.
Another example of transposon silencing involves plants in the genus Arabidopsis. Researchers who study these plants have found they contain more than 20 different mutator transposon sequences a type of transposon identified in maize.
In wild-type plants, these sequences are methylated , or silenced. However, in plants that are defective for one of the enzymes responsible for methylation, these transposons are transcribed. Moreover, several different mutant phenotypes have been explored in the methylation-deficient plants, and these phenotypes have been linked to transposon insertions Miura et al.
Based on studies such as these, scientists know that some TEs are epigenetically silenced; in recent years, however, researchers have begun to wonder whether certain TEs might themselves have a role in epigenetic silencing. It has taken decades for scientists to collect enough evidence to consider that maybe McClintock's speculation had a kernel of truth to it. RNAi is a naturally occurring mechanism that eukaryotes often use to regulate gene expression. Yang and Kazazian demonstrated that this results in homologous sequences that can hybridize, thereby forming a double-stranded RNA molecule that can serve as a substrate for RNAi.
Furthermore, when the investigators inhibited endogenous siRNA silencing mechanisms, they saw an increase in L1 transcripts, suggesting that transcription from L1 is indeed inhibited by siRNA. The fact that transposable elements do not always excise perfectly and can take genomic sequences along for the ride has also resulted in a phenomenon scientists call exon shuffling.
Exon shuffling results in the juxtaposition of two previously unrelated exons, usually by transposition, thereby potentially creating novel gene products Moran et al.
The ability of transposons to increase genetic diversity, together with the ability of the genome to inhibit most TE activity, results in a balance that makes transposable elements an important part of evolution and gene regulation in all organisms that carry these sequences. Feschotte, C. Plant transposable elements: Where genetics meets genomics.
Nature Reviews Genetics 3 , — link to article. Kazazian, H. Mobile elements: Drivers of genome evolution. Science , — doi The impact of L1 retrotransposons on the human genome. Nature Genetics 19 , 19—24 link to article. Haemophilia A resulting from de novo insertion of L1 sequences represents a novel mechanism for mutation in man.
Nature , — link to article. Koga, A. Vertebrate DNA transposon as a natural mutator: The medaka fish Tol2 element contributes to genetic variation without recognizable traces. Molecular Biology and Evolution 23 , — doi McLean, P.
McClintock, B. Mutable loci in maize. Carnegie Institution of Washington Yearbook 50 , — link to article. Miki, Y. Disruption of the APC gene by a retrotransposal insertion of L1 sequence in colon cancer.
Cancer Research 52 , — Miura, A. Mobilization of transposons by a mutation abolishing full DNA methylation in Arabidopsis. Moran, J. Exon shuffling by L1 retrotransposition. SanMiguel, P. Nested retrotransposons in the intergenic regions of the maize genome. Slotkin, R. Transposable elements and the epigenetic regulation of the genome. Nature Reviews Genetics 8 , — link to article. Yang, N. L1 retrotransposition is suppressed by endogenously encoded small interfering RNAs in human cultured cells.
Nature Structural and Molecular Biology 13 , — link to article. Restriction Enzymes. Genetic Mutation. Functions and Utility of Alu Jumping Genes.
Transposons: The Jumping Genes. DNA Transcription. What is a Gene? Colinearity and Transcription Units. Copy Number Variation. Copy Number Variation and Genetic Disease. Copy Number Variation and Human Disease. Tandem Repeats and Morphological Variation. Chemical Structure of RNA. TEs also make substantial contributions to non-protein coding functions of the cell.
They are major components of thousands of long non-coding RNAs in human and mouse genomes, often transcriptionally driven by retroviral LTRs [ ]. Some of these TE-driven lncRNAs appear to play important roles in the maintenance of stem cell pluripotency and other developmental processes [ , , , , ]. Many studies have demonstrated that TE sequences embedded within lncRNAs and mRNAs can directly modulate RNA stability, processing, or localization with important regulatory consequences [ , , , , ].
The myriad of mechanisms by which TEs contribute to coding and non-coding RNAs illustrate the multi-faceted interactions between these elements and their host. Cis-regulatory networks coordinate the transcription of multiple genes that function in concert to orchestrate entire pathways and complex biological processes. Indeed, TEs can disperse vast amounts of promoters and enhancers [ , , , , , ], transcription factor binding sites [ , , , , , ], insulator sequences [ , , ], and repressive elements [ , ] reviewed in [ ].
The varying coat colors of agouti mice provides a striking example of a host gene controlling coat color whose expression can be altered by the methylation levels of a TE upstream of its promoter [ , ]. In the oil palm, the methylation level of a TE that sits within a gene important for flowering ultimately controls whether or not the plants bear oil-rich fruit [ ].
An increasing number of studies support this model and suggest that TEs have provided the building blocks for the assembly and remodeling of cis-regulatory networks during evolution, including pathways underlying processes as diverse as pregnancy [ , ], stem cell pluripotency [ , , ], neocortex development [ ], innate immunity in mammals [ ], or the response to abiotic stress in maize [ ]. In this context, TEs are highly suitable agents to modify biological processes by creating novel cis-regulatory circuits and fine-tuning pre-existing networks.
TEs have been historically neglected and remain frequently ignored in genomic studies in part because of their repetitive nature, which poses a number of analytical challenges and often requires the use of specialized tools [ ]. As genomes can harbor thousands of copies of very similar TE sequences, uniqueness or, alternatively, repetitiveness of substrings within these regions need to be taken into consideration during both experimental design and analysis.
In some scenarios, it can be acceptable or even desirable to target many elements simultaneously [ ] or an entire TE family [ , , , , ]. Similarly, uniqueness and repetitiveness are important concepts to consider when aligning reads from next generation sequencing and analyzing TEs Fig. Various strategies exist to assign reads that could originate from multiple genomic locations: 1 mapping reads to consensus sequences of TE subfamilies [ ]; 2 mapping to the genome and keeping only uniquely-mapping reads [ , ]; 3 assigning multiple mapping reads at random between possible candidates [ ]; or 4 redistributing them according to various algorithms, such as maximum likelihood [ , ].
The choice is ultimately guided by the technique such as ChIP-seq and RNA-seq and the purpose of the analysis—is information about individual TE instances needed, or is a high-level tally of results for each subfamily sufficient?
Notably, these issues of uniqueness will differ substantially depending on the species studied and the presence or absence of recently, or currently, active TE families.
For example, mapping reads to TEs in the human genome will be less challenging than in the mouse genome given the more recent and mobile TE landscape of the latter species [ 36 ]. Finally, as sequencing technology and bioinformatics pipelines improve, notably with the increasing length of sequencing reads, many of the hurdles faced by earlier studies will be progressively removed [ ]. As potent insertional mutagens, TEs can have both positive and negative effects on host fitness, but it is likely that the majority of TE copies in any given species—and especially those such as humans with small effective population size—have reached fixation through genetic drift alone and are now largely neutral to their host.
When can we say that TEs have been co-opted for cellular function? To these critics, ignoring evolutionary definitions of function was a major misstep. This debate can be easily extended to include TEs. Today, the term is mostly used and abused by the media, but it has in fact deep roots in evolutionary biology [ ].
Regardless of the semantics, what evidence is needed to assign a TE with a function? Many TEs encode a wide range of biochemical activities that normally benefit their own propagation. For example, TEs often contain promoter or enhancer elements that highjack cellular RNA polymerases for transcription and autonomous elements encode proteins with various biochemical and enzymatic activities, all of which are necessary for the transposon to replicate. Do these activities make them functional? The vast differences in TEs between species make standard approaches to establish their regulatory roles particularly challenging [ ].
For example, intriguing studies on the impact of HERVs, in particular HERV-H, in stem cells and pluripotency [ , , ] must be interpreted using novel paradigms that do not invoke deep evolutionary conservation to imply function, as these particular ERVs are absent outside of great apes.
Evolutionary constraint can be measured at shorter time scales, including the population level, but this remains a statistically challenging task especially for non-coding sequences. Natural loss-of-function alleles may exist in the human population and their effect on fitness can be studied if their impact is apparent, but these are quite rare and do not allow systematic studies. It is possible to engineer genetic knockouts of a particular human TE locus to test its regulatory role but those are restricted to in-vitro systems, especially when the orthologous TE does not exist in the model species.
In this context, studying the impact of TEs in model species with powerful genome engineering tools and vast collections of mutants and other genetic resources, such as plants, fungi, and insects, will also continue to be extremely valuable. Finally, a growing consensus is urging more rigor when assigning cellular function to TEs, particularly for the fitness benefit of the host [ ].
Indeed, a TE displaying biochemical activity such as those bound by transcription factors or lying within open chromatin regions cannot be equated to a TE that shows evidence of purifying selection at the sequence level or, when genetically altered, result in a deleterious or dysfunctional phenotype. Recent advances in editing and manipulating the genome and the epigenome en masse yet with precision, including repetitive elements [ , , , , ], offer the promise for a systematic assessment of the functional significance of TEs.
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Convergent evolution of tRNA gene targeting preferences in compact genomes. Lynch M. The origins of genome architecture. Sunderland: Sinauer Associates; Google Scholar. Integration and fixation preferences of human and mouse endogenous retroviruses uncovered with functional data analysis.
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Curr Opin Virol. Horizontal transfer of retrotransposons between bivalves and other aquatic species of multiple phyla. Horizontal transfer of BovB and L1 retrotransposons in eukaryotes. Petrov DA. Mutational equilibrium model of genome size evolution. Theor Popul Biol. PubMed Article Google Scholar. Genome stability and evolution: attempting a holistic view.
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Pack-MULE transposable elements mediate gene evolution in plants. Cordaux R, Batzer MA. The impact of retrotransposons on human genome evolution. A solution to the C-value paradox and the function of junk DNA: the genome balance hypothesis. Mol Plant. Bennetzen JL, Wang H. The contributions of transposable elements to the structure, function, and evolution of plant genomes. Annu Rev Plant Biol. Mechanisms underlying structural variant formation in genomic disorders.
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If the cap fits, wear it: an overview of telomeric structures over evolution. Cell Mol Life Sci. Convergent domestication of pogo-like transposases into centromere-binding proteins in fission yeast and mammals.
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A trans-dominant form of gag restricts Ty1 retrotransposition and mediates copy number control. J Virol. Molaro A, Malik HS. Hide and seek: how chromatin-based pathways silence retroelements in the mammalian germline. Selective silencing of euchromatic L1s revealed by genome-wide screens for L1 regulators. Goodier JL. Restricting retrotransposons: a review.
An endosiRNA-based repression mechanism counteracts transposon activation during global dna demethylation in embryonic stem cells. Cell Stem Cell. Imbeault M, Trono D. Thus, TEs have played a significant role in genome evolution. However, from a strictly theoretical point of view, TEs can be considered as selfish DNA or junk DNA, and the existence of these elements in a genome represents the fight between selfish DNA to be perpetuated and the host to curtail their spread and its consequences.
As TEs make up a large percentage of genome volume, it is hypothesized that they have participated in changes of genome size during speciation and evolution, as reported in plants [ 6 ], Drosophila or primates [ 7 - 9 ]. The trigger s for TE-induced genome size increases is not clearly known, although it is thought that stress could be implicated in the amplification of TEs [ 10 ]. TEs are able to produce various genetic alterations upon insertion as a consequence of the transposition process insertions, excisions, duplications or translocations in the site of integration.
For example, DNA transposons can inactivate or alter the expression of genes by insertion within introns, exons or regulatory regions [ 11 - 15 ]. In addition, TEs can participate in the reorganization of a genome by the mobilization of non-transposon DNA [ 16 - 18 ] or by acting as recombination substrates. This recombination would occur by homology between two sequences of a transposon located in the same or different chromosomes, which could be the origin for several types of chromosome alterations [ 19 ].
Indeed, TEs can participate in the loss of genomic DNA by internal deletions [ 20 ] or other mechanisms [ 21 , 22 ]. The reduction in fitness suffered by the host due to transposition ultimately affects the transposon, since host survival is critical to perpetuation of the transposon.
Therefore, strategies have been developed by host and transposable elements to minimize the deleterious impact of transposition, and to reach equilibrium. For example, some transposons tend to insert in nonessential regions in the genome, such as heterochromatic regions [ 23 - 26 ], where insertions will likely have a minimal deleterious impact.
In addition, they might be active in the germ line or embryonic stage [ 27 - 29 ], where most deleterious mutations can be selected against during fecundation or development, allowing only non-deleterious or mildly deleterious insertions to pass to successive generations.
New insertions may also occur within an existing genomic insertion to generate an inactive transposon, or can undergo self-regulation by overproduction-inhibition see below.
On the other hand, host organisms have developed different mechanisms of defense against high rates of transposon activity, including DNA-methylation to reduce TE expression [ 30 - 33 ], several RNA interference mediated mechanisms [ 34 ] mainly in the germ line [ 35 , 36 ], or through the inactivation of transposon activity by the action of specific proteins [ 37 - 39 ]. A well-known example are RAG proteins, which participate in V D J recombination during antibody class switching, and exhibit a high similarity to DNA transposons, from which these proteins appear be derived [ 41 - 45 ].
Another example is the centromeric protein CENP-B, which seems to have originated from the pogo -like transposon [ 46 ]. The analogous human mariner Himar1 element has been incorporated into the SETMAR gene, which consists of the histone H3 methylase gene and the Himar1 transposase domain.
This gene is involved in the non-homologous end joining pathway of DNA repair, and has been shown to confer resistance to ionizing radiation [ 47 ]. Thus, we are likely underestimating the rate of domestication events in mammalian genomes.
LTR retrotransposons are similar in structure and life cycle to retroviruses, and their biology has been recently reviewed [ 50 ]. Additionally, the biology and impact of non-LTR retrotransposons in mammalian genomes has been reviewed extensively see [ 51 ], for a recent review as well as their potential use as mutagens in genomics [ 52 ]. Thus, no Class I TEs will be reviewed in this manuscript, although they posses some unique characteristics that may be very useful in genomics studies.
Classes of Transposable Elements TEs. DNA transposons Class II generally move by a cut-and-paste mechanism in which the transposon is excised from one location and reintegrated elsewhere. Most DNA transposons move through a non-replicative mechanism, although there are exceptions see below.
The transposase recognizes these TIRs to perform the excision of the transposon DNA body, which is inserted into a new genomic location see below for further details. Helitron and Maverick transposons belong to a different subclass Subclass II , since they are replicated and do not perform double-strand DNA breaks during their insertion see below. As an example, Miniature Inverted-repeat Transposable Elements MITEs are short bp DNA transposon-like elements present in large numbers in many eukaryotes, particularly plant species [ 53 , 54 ], and occasionally in bacteria [ 55 , 56 ].
Although they have TIRs and are flanked by TSDs, lack transposase coding potential and are thus presumably dependent on autonomous DNA transposons for their mobilization. In the following sections, we will describe and review several DNA transposon families, from their nature to their applications as genomic tools. Despite this fact, the vast majority harbor multiple inactivating mutations and only ten naturally occurring elements are known to be active: Tc1 and Tc3 from C.
Other motifs harbored by the transposase are the Nuclear Localization Signal NLS , indispensable for transposase transport through the nuclear membrane [ 78 ], and the WVPHEL linker motif, which might participate in the interaction between transposase monomers [ 78 ].
The mariner family is probably the most widely distributed family of transposons in nature, represented in such diverse taxa as fungi, ciliates, rotifers, insects, nematodes, plants, fish and mammals [ 57 - 59 ]. Transposition steps. Then, the transposon ends are brought together by both transposase monomers that form a dimer, generating the Paired-End Complex , and transposon excision takes places.
Finally, the transposase dimer recognises a TA dinucleotide, joins it, and forms the Target Capture Complex to carry out the insertion. Cut and paste reaction. Representation of cut-and-paste reaction in which the transposon is excised from one site and reintegrated at a TA target dinucleotide. Then, the host will repair the excision site.
If this repair is carried out by nonhomologous end-joining NHEJ , a transposon footprint is generated. The two transposase molecules interact and bring together the transposon ends to form the Paired-End Complex PEC generating a transposase dimer Fig. Therefore, the transposase selects a random TA where the transposon insertion will be carried out. None of the transposition steps described above require energy in the form of the cofactor ATP , since the necessary energy to form the phosphodiester bonds in the integration process comes from the cleavage reaction of target DNA exergonic reaction [ 87 - 89 ].
However, in some circumstances, it has been reported that the transposition efficiency can be affected by the cellular environment [ 92 ].
One possible pathway of DSB repair is homologous recombination HR , either using the homologous chromosome or the sister chromatid or a homologous sequence on the same chromosome as a template.
In the first case, the result is the regeneration of a new copy of the transposon [ 93 ]. In the second case, repair occurs by single-strand annealing, generating a deletion in the DNA flanking the excision site [ 93 ]. Transposition is potentially deleterious to the host as well as the transposon, whose replication and propagation depend on the survival of their host.
Thus, the development of ways to decrease the impact of transposition on host fitness is beneficial for both host and transposon. Some of the known strategies for transposon control are the following:. The transposase itself can act as a transposition inhibitor, as when it exceeds a threshold concentration, transposon activity is decreased. It has been suggested that transposase monomers could form inactive or less active oligomers, thus decreasing the activity of the transposition process [ 96 , 97 ].
When the copy number of these elements increases in the host genome, the production of transposase is also increased, and through OPI the mobilization of the transposon is reduced. It has been suggested that this is the result of selective pressure to reduce damage to the host genome [ 98 ].
In addition, inactive elements could produce inactive transposases that would impede the transposition of active elements, by OPI or by competition with the active transposases for TIRs. As two functional transposase molecules are necessary to perform transposition, inactive transposase proteins act as dominant negative inhibitors of transposition [ 96 , 99 ]. On the other hand, inactive elements with active TIRs can recruit active transposase to mediate their mobilization.
This phenomenon could explain the replacement of active elements by inactive elements, which seems to have occurred in many species during the course of evolution [ 53 ]. As mentioned above, the host can develop different mechanisms to decrease the activity of transposons. TEs are parasitic DNAs whose only function is to replicate and propagate themselves.
When a transposon invades a new host, it must colonize the germline genome to persist in the population. Then, it will increase in copy number [ ], and persists in the genome until, by vertical inactivation , all transposon copies become inactive and remain only as fossils.
These inactive elements may even disappear by genetic drift [ 98 ]. To escape this cycle, a transposon must invade a new species, or extends to multiple species. In other words, to ensure its survival, the transposon must pass to a new genome by Horizontal Transfer , and begin its life cycle again Fig.
The figure has been adapted from Miskey et al. Indeed, many cases of horizontal transfer between different hosts have been proposed for these elements. Examples include transfer between marine crustaceans [ ], between insects from different orders [ , ], and even between organisms from different phyla, as divergent as human and a parasitic nematode [ ]. However, it is not known how these elements are able to invade new genomes. Potential vectors that might be implicated in this horizontal transfer are external parasites, such as mites, which seems to be the vehicle for the horizontal transfer of P elements in Drosophila [ ], or internal parasites such as viruses [ ].
It is a synthetic transposable element reconstructed from defective copies of eight salmon species by reverse engineering [ 83 ]. SB is active in species ranging from protozoa to vertebrates, including frogs, fish, mice, rats or humans [ ]. The hyperactive version of SB, SBX, exhibits approximately a fold increase in efficiency when compared to the first generation of SB transposase, facilitating robust stable gene transfer in vertebrates [ ].
Therefore, SB represents a promising system for gene transfer in vertebrates somatic and germ line , embryonic stem cells, and many other cultured cell lines [ , ]. The SB transposase expression vector contains the SB transposase open reading frame ORF between a strong promoter ubiquitous or cell-type restricted and a poly A signal.
To achieve transposition of SB, the two components of the system are introduced in the host transfection in cell cultures, injection into fertilized eggs, injection in live animals, etc.
The SB system has been tested in several fish species, the frog Xenopus , rat, mouse and in cultured human cell lines [ , - ]. DNA-Transposon System. In humans, the SB transposon system was initially used in human T cells, resulting in stable gene transfer and expression of the reporter gene [ ].
The novel hyperactive SBX has been tested in primary human CDpositive hematopoietic stem cells, resulting in stable gene expression [ ]. Furthermore, transgenic mice have been generated by co-injecting the SB transposon vector with the SB transposase mRNA into fertilized oocytes, some of which gave rise to transgenic offspring [ ].
Additionally, SB has also been used in functional genetic screens in mammals for the identification of genes implicated in diseases such as cancer. SB is used to induce insertional mutagenesis, and candidate genes identified through the analysis of insertion sites in tumors vs control tissues in gain of function studies [ , ], reviewed in [ ].
Frog Prince was reconstructed from the Northern Leopard frog, Rana pipiens , and is characterized by the presence of bp-long TIRs flanking the transposase gene which harbors a DD34E catalytic domain, see above.
Frog Prince shows preference for intronic insertions, and is very efficient in gene trapping experiments conducted in tissue culture cells [ 73 ]. Furthermore, Frog Prince has been tested in zebrafish embryos and other cultured vertebrate cell lines [ 73 ]. Similarly, the transposon Minos , isolated from Drosophila hydei , is 1. This transposon has preference for genes, inserting mostly into introns, and has been tested in cultured human cells [ ], mouse tissues [ ] and the sea squirt Ciona intestinalis [ ].
Another example is Himar1 also with a DD34D transposase , reconstructed from Haematobia irritans , which has been used in screens to identify genes implicated in bacterial pathogenicity by insertional mutagenesis [ - ], and in cultured human cells [ ]. Related piggyBac transposable elements have been found in plants, fungi and animals, including humans [ ], although they are probably inactive due to mutation. The transposase from piggyBac has been optimized to generate a more active transposition system [ ].
This transposon has been used in such diverse organisms as protozoa, planaria, insects and mammals, including human cells [ - ]. It is an important tool to generate modified insects carrying lethality or sterility genes by transgenesis for plague control and thus pest control [ - ].
In mammals, the piggyBac system has been used for different applications, such as germline or somatic mutagenesis and gene therapy. It has been used to mediate gene transfer in human cells [ ] and recently to generate transgene-free induced pluripotent iPS stem cells from mouse cells [ ].
A member from this family widely used as a genetic tool is Tol2 , which was the first active autonomous transposon isolated in vertebrate species [ , ]. This element was identified in Medaka fish Oryzias latipes where it had generated a mutation in the tyrosinase gene, resulting in albino mutant fish. Tol2 is 4. It has also been engineered for improved efficiency to facilitate its use as a tool for enhancer trap screens in vertebrates to identify genes implicated in different functions and pathways [ - ].
This system has been used in different vertebrates such as zebrafish and Xenopus , chicken embryos, and cultured vertebrate cells, including human stem cells [ , - ]. In the following section, we will discuss the most useful characteristics of each DNA transposon as well as their known limitations. In contrast, transposon systems are inexpensive and easier to purify, and are non-inmunogenic [ - ].
In addition, they permit elimination of the transgene and, in some cases such as piggyBac , can be excised without leaving notable genetic alterations [ ]. Unfortunately, relative to viral systems, DNA transposons are less efficient for gene transfer. However, the efficiencies of newly developed transposon systems such as piggyBac and SBX are comparable to those of viruses [ , ].
Among the characteristics that distinguish DNA transposon systems as biotechnical tools, we highlight:. Transposon insertion efficiency can vary depending on the size of the gene to be transferred. In contrast, piggyBac or Tol2 transposons are more tolerant in their capacity for cargo.
In piggyBac, when the cargo approaches 9 kb the efficiency decreases in pronucleus-injected mice [ ], and in Tol2 the efficiency begins to drop off only when the cargo is higher than 10 kb [ ]. Integration site preference is an important consideration when choosing a transposon system for a given application. For example, piggyBac has preference for transcription units, with insertions primarily targeting introns [ ]. On the other hand, SB prefers heterochromatin over actively transcribed genes [ 26 , ], and when it does insert into genes, it prefers intronic sequences.
On the other hand, piggyBac inserts in its target TTAA without any other apparent requirements [ , ]. Thus, depending on the study, both SB and piggyBac can be useful systems. In the case of mutagenesis screens, it is preferable for the transposon to insert into genes, whereas gene therapy protocols require a system with less affinity for insertion within genes and, in general, low-risk chromosomal regions.
However, integration within heterochromatin as observed for SB, [ 26 ] has the disadvantage of typically producing low levels of transgene expression [ ]. In functional genomic studies, it is often desirable to inactive genes by insertional mutagenesis by transposons. If the transposon insertion takes place within an intron, splicing would likely render such an insertion irrelevant. To avoid this situation, a splice acceptor followed by the reporter gene and a poly A tail may be included in the transposon.
In this way, splicing is altered, leading to the fusion of the trapped gene and reporter gene downstream Fig. Thus, the trapped gene remains inactivated and the reporter gene is expressed. In sum, for insertional mutagenesis studies, both Tol2 and piggyBac are superior to SB, while for gene therapy SB is theoretically more secure than either Tol2 or piggyBac transposon systems. Gene Trap Transposons.
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