Through a neat genetic trick, it is now possible to create 鈥渞eprogrammed鈥� stem cells that have been stripped of potentially cancer-causing genes. This latest advance boosts hopes of one day using these cells to treat devastating human diseases.
The reprogramming technique was originally developed by Shinya Yamanaka of Kyoto University in Japan, who used retroviruses to insert four genes into the chromosomes of mouse and human skin cells.
Together, these genes turned back the cells鈥� developmental clock. The resulting 鈥渋nduced pluripotent stem cells鈥�, or iPS cells, had essentially been turned into embryonic stem cells, able to turn into any of the body鈥檚 tissues. Unlike previous sources of ESCs, the cells had not required the use of a human embryo, so gets around ethical concerns.
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But Yamanaka鈥檚 method creates a two-pronged risk of cancer. Not only can retroviruses jump into and disrupt genes that normally hold cancer in check, but the genes used for reprogramming can themselves trigger tumours.
Cut and paste
Until now, most scientists trying to create safer iPS cells have tried to avoid inserting the reprogramming genes into the chromosomes in the first place.
Last year, for instance, a team led by of the Massachusetts General Hospital in Boston put the reprogramming genes into adenoviruses, which live for only a short while in a cell and don鈥檛 normally jump into its chromosomes. The technique worked in mouse liver cells 鈥� but was very inefficient.
Now two groups of researchers have taken a different approach, using a system that puts the genes into chromosomes, but allows them to subsequently be removed.
Rather than using viruses, the researchers instead turned to a 鈥�transposable element鈥� known as . Originally found in insects, piggyBac is a parasitic stretch of DNA that can move around the genome, cutting and pasting itself into a chromosome where the genetic sequence reads 鈥淭TAA鈥�.
The two teams, led by of the University of Edinburgh, UK, and of the Samuel Lunenfeld Research Institute at Mount Sinai Hospital in Toronto, Canada, removed the gene for piggyBac鈥榮 cut-and-paste enzyme and instead packed the element with the four reprogramming genes.
When the altered piggyBac was then put into mouse and human skin cells together with a separate loop of DNA bearing the gene for the cut-and-paste enzyme, it jumped into the chromosomes and turned them into iPS cells with about the same efficiency as Yamanaka鈥檚 retroviruses.
Gene eraser
The key advance, however, is that a flaw in piggyBac鈥榮 machinery makes it possible to remove the element and its cargo of potentially cancer-triggering genes after it has triggered reprogramming.
Each time piggyBac leaps out of the genome, it has only a 40% chance of reinserting itself. By taking the iPS cells and reactivating the element, the researchers were able to get piggyBac to jump out of the chromosomes and then screen for those cells in which it had failed to jump back in again. This left them with chromosomes free of the dangerous reprogramming genes. 鈥淣ot even a partial sequence was left,鈥� says Nagy.
鈥淭his is an elegant method,鈥� says who heads the Institute for Regeneration Medicine at the University of California, San Francisco. 鈥淚t allows you to seamlessly excise the genes.鈥�
Whether the method catches on may depend on rival researchers鈥� success in attempts to devise simpler methods that don鈥檛 require the reprogramming genes. For instance, of the Scripps Research Institute in La Jolla, California, is experimenting with reprogramming using small-molecule drugs.
Journal reference: (DOI: 10.1038/nature07863 and DOI: 10.1038/nature07864)
