Ancient jumping DNA found napping in fish has been revived and is being used to identify cancer genes in mice. But the benefits of this aptly named ‘Sleeping Beauty’ system could reach far beyond cancer.
Transposable elements are discrete pieces of DNA that can jump around in the genome of a living organism. These elements were initially discovered in corn through the Nobel-prize-winning work of Barbara McClintock1, and they have been found in organisms throughout the tree of life. For each DNA transposon, a corresponding protein, called a transposase, mediates the jumping. One such transposon/transposase duo, aptly named Sleeping Beauty (SB), was found latent in the genome of salmonid fish. Its DNA sequence had mutated to the point where it no longer jumped, but rather slumbered as inactive ‘junk DNA’. The extinct SB jumping functions have been resurrected2, and in this issue Dupuy et al. (page 221)3 and Collier et al. (page 272)4 present improvements to this technology and exploit the system to identify genes involved in cancer.
Many agents, including chemicals, radiation and viruses, are routinely used to disrupt genes randomly, with the aim of identifying gene functions and the diseases associated with them. But it can be extremely difficult to find where a random mutation has been introduced, requiring huge amounts of sequencing to pinpoint each tiny change. Transposable elements are powerful reagents in this regard because their sequence is known, so when they mutate genes they provide a ‘tag’ that pinpoints their location in the vast sea of genomic DNA. Tagging genes by insertion is not a new idea, and geneticists working on many organisms use transposable elements. They have been little used in mice, however, because known transposons hop around the mouse genome very infrequently, yielding few new mutations.
The two groups surmounted this problem in two ways. Collier et al.4 engineered an SB transposon (T2/Onc) to have the ability to enhance or disrupt genes. They used this in a strain of mice that produce the SB transposase in all their cells and which have mutations that make them especially susceptible to cancer (Fig. 1). Dupuy et al.3 redesigned the transposon to be smaller (T2/Onc2), and created a mouse strain whose cells contained increased amounts of transposase, leading to higher mutation rates. A major advantage of the SB system over viral or previously used transposon systems is that, because the transposon comes from fish, it is distinct from the large number of native transposons present5, making it easy to find the exact site(s) in the genome where SB integrates.
The authors used their system to search for genes that cause cancer, as genes identified in mouse cancer are often also perturbed in human cancer. To understand how SB can identify cancer genes, imagine the genome as a book of instructions on how cells work. SB is like a small phrase or set of directions that can hop into, and potentially alter, any instruction in the book. Sometimes, inserting the phrase will cause insignificant changes, but if SB alters instructions for key processes, such as cell proliferation or cell death, the cells can grow and divide beyond their normal potential and become cancerous. The authors designed the SB transposon to disrupt gene function in two ways. If it inserts in a gene, it will truncate the protein encoded by that gene, usually destroying its function. This will identify genes that help to protect against cancer (tumour-suppressor genes). If the transposon inserts near a gene, it causes an increase in the gene product, allowing cancer-promoting genes (oncogenes) to be identified.
By isolating the sites of SB insertion in tumours, the groups tagged genes that are known to be important in the development of cancer and those likely to be involved in the disease that had not previously been associated with it. Dupuy et al.3 also demonstrated networks of genes that interact to cause cancer. In addition, Collier et al.4 show, using animals that harbour cancer-predisposing mutations, that the SB system can tag genes in a solid tumour called a sarcoma. This tumour can involve various tissue types, including neural cells and connective tissue cells.
Cancer is rarely caused by the mutation of a single gene; rather, perturbations of several genes tend to cooperate to cause the disease6. Genetic pathways involved in the development of leukaemia have been dissected by tagging with mouse leukaemia retroviruses7,8, but gene networks in other tumour types are less well studied. The development of new treatment strategies would ideally require information on all the genes involved in common and devastating cancers such as breast, colon, prostate and lung cancer, and the SB system seems a promising way to provide this.
The technology is likely to be very powerful, because the transposase can be designed to be expressed selectively in a specific cell type or developmental stage, so that transposition will occur only in those cells or at that time. Many cancer-associated genes are disrupted in only one particular cancer, and the selective SB technique can be used to locate these. Furthermore, the ability to limit transposase expression will enable the system to be turned on to make mutations and then turned off to stabilize them. It also allows for the controlled excision of the transposons, so that mutations can be reversed.
At present, however, the ability to restrict gene expression to each type of tissue-specific stem cell is limited9. (Tissue-specific stem cells are the immature cells that give rise to specialized tissues, where cancer mutations are most likely to occur.) Further research into stem-cell-restricted gene expression will expand the application of the SB system.
The SB system of genome manipulation and mutation will be valuable in organisms other than mice, and has applications outside cancer. It can be used in human cells, or in any organism that is a model for human disease. The transposon can be engineered to deliver any DNA cargo to many locations in the genome. This has remarkable potential for discovering the genes associated with diseases that have a genetic component (heart disease, diabetes, birth defects, and many more). It might also provide therapeutic gene delivery and large-scale genome modification. With the genome sequences for many organisms nearly complete, the utility of this beautiful tag for DNA mutations is infinite.
McClintock, B. Proc. Natl Acad. Sci. USA 36, 344–355 (1950).
Ivics, Z., Hackett, P. B., Plasterk, R. H. & Izsvak, Z. Cell 91, 501–510 (1997).
Dupuy, A. J., Akagi, K., Largaespada, D. A., Copeland, N. G. & Jenkins, N. A. Nature 436, 221–226 (2005).
Collier, L. S., Carlson, C. M., Ravimohan, S., Dupuy, A. J. & Largaespada, D. A. Nature 436, 272–276 (2005).
Brosius, J. Bioinformatics 19 (Suppl. 2), ii35 (2003).
Vogelstein, B. & Kinzler, K. W. Trends Genet. 9, 138–141 (1993).
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Hansen, G. M., Skapura, D. & Justice, M. J. Genome Res. 10, 237–243 (2000).
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