Overview about the transposon – based technology

Transposition is a new approach to genome manipulation. Transposable elements are DNA segments with the peculiar capability to move about the genome. Transposon-based genetic strategies are applied for the transgenesis of somatic or germ-line cells, for insertional mutagenesis, both loss or gain of function and for non viral DNA transfer into cells in cell-based clinical applications. Transposons have been identified in all organism and have been distinguished in two classes in relation of their mechanism of action. The mobility of class I elements, also named as retro-transposons, work through RNA intermediate and encode for one nucleic binding protein and an enzyme that acts as endonuclease and retro transcriptase: endonuclease generates a nick into the target DNA and retro transcriptase starts the reverse transcription of the RNA of the transposon from nicked DNA.

Class II transposable elements are simpler than those of class I and their genome, flanked by two inverted terminal repeats, encodes for a transposase protein that allows the excision of transposon and the insertion into DNA target. In this case, the transposition process could be easily controlled by separating the transposase from the transposable DNA. Several kind of transposons are now available and are used as a vector to manipulate the genomes: the first studies were performed on C. Elegans and Drosophila and only since 1997 when has been demonstrated the re-activation of the Sleeping Beauty transposon system, has been employed also in vertebrates and mammals. Parameters that have to be considered during the choice of transposon as a vector are the size of DNA that could be moved and the integration site of preference. The capacity of moving large DNA fragments varies between the species of transposons, Sleeping Beauty transposon is inhibited by large size fragments, by contrast piggyBac transposons are more tolerant to large size without reducing their efficiency. The insertion point of transposons is non-random, but occurs in hot regions of the genome, for instance intron or transcription unit, that are characteristic of each type of transposon. The integration site preference is important for the application of transposon vectors: for example, mutagenesis screening could be more efficient by using elements that tend to land in genes, while human gene therapy protocols could require vectors showing the least preference to target gene. Transposons are used to perform recessive genetic screens in embryonic stem cells and in germ-line in vivo and to transfer DNA into stem cells and oocytes and embryos. In the next future, the transposon- based technology could be applied on germ-line transgenesis of laboratory animals and in larger species, like sheep and pig. Furthermore, gene transfer could be improved into therapeutically relevant primary cells, including stem cells, allowing the implementation of ex vivo and in vivo therapies. In conclusion, transposon approaches seem to be really promising for future advances of science.