Cell Reboot

Through a genetic trick, it is now possible to create "reprogrammed" stem cells that have been stripped of potentially cancer-causing genes.

The reprogramming technique was originally developed using retroviruses to insert four genes into the chromosomes of mouse and human skin cells. Together, these genes turned back the cells' developmental clock. The resulting "induced pluripotent stem cells", or iPS cells, had essentially been turned into embryonic stem cells, able to turn into any of the body's tissues. Unlike previous sources of ESCs, the cells had not required the use of a human embryo, so gets around ethical concerns. This method created 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.

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 put the reprogramming genes into adenoviruses, which live for only a short while in a cell and don't normally jump into its chromosomes. The technique worked in mouse liver cells - but was very inefficient. Currently, 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 "transposable element" known as piggyBac. 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 "TTAA". The two teams, operating out of the University of Edinburgh and Mount Sinai Hospital in Toronto, removed the gene for piggyBac's 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's retroviruses. The key advance, however, is that a flaw in piggyBac's 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. "Not even a partial sequence was left. This is an elegant method...It allows you to seamlessly excise the genes."