Ideal delivery systems for gene therapy should be safe and efficient. Non-viral DNA transposons and retro-transposons have been successfully applied in mammalian gene transfer in vitro. For example, the Sleeping Beauty (SB) system has been utilized to generate transgenic mouse models. A more versatile tool developed recently for mutagenesis is the PiggyBac (PB). Compared to the SB system, PB is precisely excised, leaving the integration sites unaltered. Thus, PB has a high transposition efficiency, stability, and mutagenic potential in most mammalian cell lines.
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The Technology and Mechanism
PB is a transposon system that can be utilized to integrate transgenes into host cells. The technology is an efficient genetic manipulation tool that overcomes SB’s limitations of genetic footprint mutations after excision and local hopping (Kim & Pyykko, 2011). PB, which comes from the cabbage looper moth, also has superior transposon activity and site-selectivity. Structurally, PB comprises inverted repeats (transposase gene) 2,475 bp long (Kim & Pyykko, 2011). Two terminal repeats flank the PB element at 13bp and 19bp.
The expressed transposase promotes an accurate excision of PB to mediate gene transfer in mammalian cells. The least number of PB sequences required for optimal transposition from a plasmid to a genome is 55bp (Kim & Pyykko, 2011). However, the lowest terminal repeats may vary between insect cells. Thus, the molecular structure of the flanking and transposase gene affects the transposition efficiency of PB.
PB integrates transgenes into host chromosomes through a transposase-mediated cut-and-paste process. According to Kim and Pyykko (2011), PB works by first nicking of the 3’ ends of the PB, which exposes 3’OHs that subsequently attach the “complementary strand into the flanking donor DNA” (p. 303). As a result, hairpins are formed on the 3’ ends, which are then released. Ligation of the 5’TTAA region overhanging the donor sequences creates a precise TTAA sequence. The repair of the single-stranded gaps of the inserted gene completes the integration process.
For this PB-mediated gene transfer to happen, the vector must contain a PB transposase open reading frame or a helper plasmid that expresses this enzyme (Kim & Pyykko, 2011). The function of the encoded protein is to precisely cut and insert the gene into the target genome. Besides the helper plasmid, the actual enzyme or mRNA molecules copied from the transposase gene can play this role. This second approach reduces the risk of genotoxicity. Compared to other transposons, PB shows a high frequency of precise excisions.
Utilization in Research
Presently, PB has versatile applications given its high transposition efficiency compared to other transposons. It is a useful delivery method for genes in vitro. The PB system can also deliver larger transgenes of up to 14 kb in mouse models. It has also been shown to integrate 18-kb inserts in human embryonic stem cells (Kim & Pyykko, 2011). The high transposition efficiency and larger size of the transgene inserted make the PB system a potentially useful technology for gene therapy in vitro. On the other hand, SB systems involve overexpression of transposase, which leads to loss of enzymatic activity. This problem does not affect PB. Thus, a vector with transposase and transposon elements can be created, resulting in a doubling of transposition activity.
PB has been shown to mediate a parallel integration of multiple transposons at the same time in mammalian cells. In one transfection experiment, up to four different genetic elements were inserted in human cell lines (Kim & Pyykko, 2011). This technology yields stable transgenic cells useful in downstream physiological analyses. Thus, PB systems have potential applications in developing mutant cell lines for drug discovery. Multiple recombinant proteins produced by these models can be tested for their efficacy in treating multifactorial genetic disorders.
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In medicine, the initial application of PB was in gene transfer in different invertebrates. However, in recent years, it has been applied in germ-line transfection and transgenesis in vertebrates. PB was first used to transform the Mediterranean fruit fly in the 1990s (Kim & Pyykko, 2011). Many insect species were later transfected successfully using this technology. This tool has also facilitated the genetic transformation of vertebrates, including human and chicken cell lines in vitro. A non-medical application of PB has been the transgenesis of silkworms to produce superior quality silk.
PB systems have been applied not only in gene therapy research but also in producing mutagenic and transgenic cell lines and animal models. An example is a mutagenesis in Drosophila melanogaster. PB was used to produce deletions and duplications in key genes that confer alcohol tolerance and metabolism (Kim & Pyykko, 2011). The research using mutant fruit flies will help us understand how ethanol tolerance develops in humans.
Several examples of transgenic organisms exist. In mosquitoes, PB was used to transform germ cells to help characterize functional proteins produced by the highly virulent Plasmodium falciparum strains carried by the Anopheles species (Kim & Pyykko, 2011). The parasites cause malaria, a leading killer in many parts of the world. Therefore, PB can facilitate functional genomics to uncover Plasmodium biology for the development of effective therapeutic agents for this disease. Another transgenic invertebrate created using PB is the human blood fluke that causes schistosomiasis. A PB transposon inserted in this parasite’s genome was found to reduce its pathogenicity. Thus, using this technology, heritable transgenesis to reduce the risk of infection is possible.
Invertebrate organisms, chicken embryos have been transformed to study the process of embryogenesis and neurogenesis. PB allows large DNA fragments to be inserted into the genome. The transgenic chicks express the transgenes, allowing neural development at the embryonic stage to be studied. In humans, PB has been applied in producing transgenic antigen-specific T cells that have shown potential in the treatment of cancers such as lymphoma and other malignancies (Kim & Pyykko, 2011). PB is an efficient delivery system that causes less genotoxicity in target cell lines than lentiviral vectors. Pluripotent human cells have also been developed using PB systems to transpose embryonic fibroblasts (Kim & Pyykko, 2011). Thus, this transfection method can be useful in stem cell research. It has also been applied in developing transgenic cell lines that produce virus-like particles with the potential as a vaccine for the human immunodeficiency virus. Recently, this technology was used to transform swine tissue cells.
Gene therapy applications of PB systems exploit its efficiency as a delivery system of large-sized transgenes. It is an important experimental tool for studying ovarian carcinoma treatments in mice. Using PB and the polyethyleneimine approach, the number of cancerous cells was greatly reduced in murine ovaries (Kim & Pyykko, 2011). The prolonged-expression of transfected genes ensures a sustained mutagenic effect on recipient cells.
Improvements or Modifications of the Technology
The high transposition frequency and efficiency associated with PB make it an ideal delivery system for producing stable transgenic and mutant cell lines for research. However, this system has some limitations. Though PB is specific to the TTAA sequence, its integration of transgenes in target genomes is rather random (Kim & Pyykko, 2011). Thus, it cannot insert genes tintoa a particular chromosome or site. Transposons are integrated into any region with the TTAA sequence. Addressing this drawback will improve this technology. Site-specific insertion can be attained by enhancing the transposase’s binding capacity to specific target DNA regions. Using DNA-binding domains (DBDs) that bind to unique sequences of the chromosome can help resolve this problem.
PB-based vectors also preferentially insert transgenes in the transcription start sites. A possible suggestion is to use chimeric PB systems joined to DBDs to ensure specificity to pre-determined sites. Another drawback that needs to be addressed to improve PB’s transposition efficiency is remobilization. In this case, a gene inserted in its correct site is moved to a new location. To prevent this process, a post-transposition degradation of transposases is necessary. By eliminating this enzyme from the cells, stable transgenic or mutant cells can be established.
PB has many advantages over other gene delivery systems, including SB. It can deliver a large-sized genetic material more efficiently to create stable transgenic cell lines for research. The mechanism of action essentially involves a cut-and-paste approach. PB is characterized by high site selectivity, making it an efficient system for targeted DNA manipulation. However, specific drawbacks, such as random integration of genes to any TTAA sequences and preferential insertion of transgenes at transcription sites, must be addressed first before using this tool.
Kim, A. & Pyykko, I. (2011). Size matters: Versatile use of PiggyBac transposons as a genetic manipulation tool. Molecular and Cellular Biology, 354, 301–309. Web.