The news that human skin cells had been converted into stem cells hit the world headlines in late 2007. Hailed by the experts as the Holy Grail, this discovery has revived both hope for new medical therapies and controversy over the legitimacy of research using embryonic stem cells.
But how did the researchers manage to rejuvenate skin cells to make them so akin to embryonic stem cells? Although the techniques employed by the two teams were similar, they differed in a number of aspects.
Four rejuvenating genes
Shinya Yamanaka’s team, from the University of Kyoto in Japan, worked on skin cells from the face of a 36-year-old woman. After collecting these fibroblasts, the scientists injected them with genes coding for four transcription factors. This genetic manipulation enabled them to activate certain genes in the skin cells that are normally active only at the embryonic development stage. This tricks the fibroblasts into believing that they are unspecialised embryonic stem cells. The means used by the Japanese team to trick the skin cells were: Oct3/4 and Klf4, two transcription factors involved in maintaining the pluripotency of stem cells (i.e. their ability to differentiate into any type of foetal or adult cell); Sox2, a protein present in embryonic stem cells; and c-Myc, which is essential for cell growth and division as well as for slowing cell differentiation.
Shinya Yamanaka and his team succeeded in producing a stem cell line from 5 000 skin cells. “Although such efficacy may seem low, it means that multiple lines of pluripotent stem cells can be obtained from a single 10-centi - metre sample,” explains Shinya Yamanaka. The results of this study were published in the journal Cell (1) in November 2007.
Across the Pacific, the recipe was a little different. While James Thomson’s team also used genes coding for four transcription factors, they did not use the same candidate proteins as the Japanese team. Although the University of Wisconsin-Madison researchers also used Oct3/4 and Sox2, they chose NANOG and LIN28 for their other two transcription factors. Another difference between the two teams was that Thomson’s worked on cells from the foreskin of a newborn and succeeded in rejuvenating one cell in every 10 000. Their results were published in the journal Science(2).
So, researchers in both Japan and the United States have managed to reprogramme cells that are already fully differentiated. Both teams used a retrovirus to introduce selected genes into skin cells.
These two techniques make it possible to produce stem cells that contain the patient’s own genetic information. This would be a significant advantage in avoiding the risk of rejection in transplant patients. However, there is one point in favour of the American technique. “The big difference between the two teams is that the Japanese researchers used c-Myc, which is a cancer gene,” explains Daniel Brison, co-Director of the North West Embryonic Stem Cell Centre at the Universityof Manchester (UK). “As the Americans did not use c-Myc, their cells are clinically more advantageous.”
What’s the situation on using embryos?
The stem cells obtained from reprogramming human adult cells are called “induced pluripotent stem cells” (iPS). Their basic physical, genetic and biological properties are very similar to those of embryonic stem cells (ES cells). “However, years of research are needed before we know whether they are as effective as embryonic stem cells or for certain differences to become apparent. Embryonic stem cells have been studied for more than 20 years in mice, and 10 years in humans,” says Daniel Brison.
Although the results of these recent iPS studies are of enormous biological and clinical interest, the research has also triggered an ethical debate. What is the advantage of such a technique over current techniques? While some people present iPS as the solution to many ethical issues posed by embryonic stem cells and therapeutic cloning, not all the experts are in agreement. “Stem cells that have been reprogrammed from adult cells are more interesting from every standpoint”, argues Jonas Frisén from the Department of Cell and Molecular Biology (CMB) of Sweden’s Karolinska Institutet. “The technique doesnot require the use of embryos and produces cells that are genetically identical to the patient’s own cells.”
North West Embryonic Stem Cell Centre co-Director, Daniel Brison, is less convinced of the advantages of iPS: “The embryos used to produce stem cells are surplus embryos creatresearch ed for in vitro fertilisation treatments. They are doomed in any case. Embryonic stem cells are still the most natural source of pluripotent cells because they are able to form a heart, muscles, brain tissues and so on, which adult skin cells are not supposed to do.” Daniel Brison also believes that, even though therapeutic cloning has not yet been tested in humans, it is a more natural method than iPS. “The egg is the normal environment in which the nucleus is programmed to be pluripotent. Although therapeutic cloning is unnatural in that it involves placing an adult cell nucleus into an egg to allow it to be reprogrammed, it is still a more natural method than one using iPS, which are genetically modified cells.” Daniel Brison even predicts a public outcry against iPS-derived therapies, as happened with genetically modified food.
Scotsman, Ian Wilmut, who cloned Dolly the sheep in 1997, expresses a contrasting view. He finds the new technique highly promising and has decided to abandon his research on embryo cloning to devote himself to iPS, in the belief that it heralds “a new era” for biology.
Sparking a whole new ethical debate…
One of the chief advantages of reprogramming skin cells is its simplicity. Any standard laboratory can manipulate the four genes needed to make skin cells regress into stem cells. Furthermore, skin cells are much easier to collect than embryos.
The researchers who made this extraordinary but controversial discovery are cautious, however. “The research has only just begun and we have very little idea how these cells function”, explains James Thomson, leader of the American research team, who still considers embryonic stem cells to be the research gold standard. Japanese lead researcher, Shinya Yamanaka, believes that it will take at least one year to prove the safety of the new technique.
Although the two research teams’ results have been welcomed because they do not use embryos, they have triggered a whole new ethical debate. Indeed, some scientists are already pointing to the possibility of scientific misconduct in the use of this technique. They claim that, in theory, a single person’s DNA could be used to create both eggs and spermatozoa from iPS. In practice this could wreak havoc with the reproductive process.
No legislation on the subject yet exists. A spokesperson for the British Human Fertilisation and Embryology Authority (HFEA) told the Daily Telegraph newspaper that legally it is a “grey area”. In an interview published by leading French newspaper, Le Figaro, the chairman of France’s National Ethics Advisory Committee for the Life Sciences and Health (CCNE) played down the risk of scientific misconduct. Legislation to ban this practice is expected very soon.
While it is wise to foresee the risks of new practices, it would appear difficult to create gametes (sex cells) from iPS. Indeed, sex cells can be formed only in their niche (ovarian follicles in women and seminiferous tubules in men). Furthermore, as they mature, gametes undergo meiotic reduction. This process specific to sex cells results in cells with half the number of chromosomes. So it is pure fantasy to imagine that gametes could be produced from iPS without going through meiosis!
Source... http://ec.europa.eu/research/research-eu/