For the first time, biologists have reprogrammed adult mouse cells back to an embryonic state without using viruses that leave dangerous copies of cancer-causing genes in the cells鈥� chromosomes.
The work raises hope that the cells, known as 鈥渋nduced pluripotent stem cells鈥� or iPS cells, can be made safe for transplantation into people.
Shinya Yamanaka of Kyoto University in Japan, who pioneered the reprogramming technique, used retroviruses carrying four separate genes that effectively wipe the developmental slate of an adult cell.
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Now researchers led by of the Massachusetts General Hospital, Boston, have used the same four genes to create iPS cells, but carried instead by adenoviruses. These don鈥檛 normally integrate into the genome of cells that they infect and therefore present little risk of cancer.
The researchers initially tried to reprogramme mouse skin cells. 鈥淎t first, it didn鈥檛 work at all,鈥� says Hochedlinger. Then his team turned to liver cells, which seemed from previous reprogramming experiments to be easier to send back to an embryonic state. It worked, but much less efficiently than the standard iPS cell technique.
Yamanaka鈥檚 method typically produces one colony of iPS cells from every 1,000 to 10,000 adult cells. With the adenoviruses, the method was between 10 and 100 times less efficient 鈥� which will limit its practical use.
鈥淚t鈥檚 an exceptionally inefficient technique,鈥� observes , a stem cell biologist at the Burnham Institute for Medical Research in La Jolla, California.
Proof of principle
But researchers are encouraged by the basic finding that reprogramming can be achieved without using viruses that jump into the genome. 鈥淭he most important element is the proof of principle that it can be done,鈥� says , a stem-cell biologist at the Children鈥檚 Hospital Boston. 鈥淭he efficiency will improve over time.鈥�
Other teams are working on different techniques for creating iPS cells, which may turn out to be more efficient.
For instance, a team led by of the Scripps Research Institute in La Jolla is screening for small-molecule drugs that can substitute for the reprogramming genes.
Other teams are trying to introduce the proteins encoded by the genes directly into cells, while Yamanaka is experimenting with 鈥渕icroRNAs鈥� 鈥� snippets of RNA that help regulate gene activity.
Encouragingly, Hochedlinger鈥檚 cells do seem to be safer than conventional mouse iPS cells. When researchers create 鈥渃himeric鈥� mice by injecting iPS cells into early-stage mouse embryos, the resulting animals are unusually prone to cancer.
Although Hochedlinger has so far studied his chimeras only until 13 weeks of age, none have yet developed tumours. 鈥淲e鈥檙e still following up,鈥� Hochedlinger says.
Mystery chromosomes
One perplexing problem, however, is that three out of 13 of the iPS cell lines created by Hochedlinger鈥檚 team had a complete extra set of chromosomes, which has not been seen with iPS cells made using retroviruses. This could indicate that the adenoviruses are causing cells to fuse with one another.
鈥淚t raises the question of the mechanism and whether the cells are safe and normal,鈥� says Snyder.
Hochedlinger argues that cells with an extra set of chromosomes can be quickly identified and discarded. 鈥淪o far, in the characterisation we鈥檝e done of these cells, we did not see any other abnormalities,鈥� he says.
In addition to further investigating the safety issues, Hochedlinger鈥檚 team still has to show whether adenoviruses will be able to reprogram human cells, as well as those from mice.
Journal reference: (DOI: 10.1126/science.1162494)
