The fluorescent green parts of the mouse pups come from embryonic stem cells that were made from reprogrammed skin cells, without the need for eggs.
Credit: Rudolf Jaenisch/Nature
MELBOURNE: Four new studies in mice report advances in making cloned stem cells which - if successful in humans - would sidestep some of the major ethical barriers to their use.
Many people frown on the idea of cloning human beings, but cloning your own stem cells to repair failing organs is hailed as a great idea. This is the promise of so-called therapeutic cloning.
There are two ethical snags, however. One is that the technique requires the creation of a cloned embryo, which is later destroyed to obtain its stem cells. The other is that creating that clone embryo relies on women donating their unfertilised eggs – a procedure which exposes them to some health risks, however small.
Now, four new studies report innovative laboratory techniques that may overcome some of the ethical hurdles to therapeutic cloning by sidestepping the need to harvest unfertilised eggs from female donors, or by avoiding the need to use these eggs entirely.
No need to donate
The first technique, reported in the British journal Nature today by stem cell biologist Kevin Eggan at Harvard University in Boston, USA, clones embryos without needing unfertilised eggs. Instead it uses a resource that is commonly thrown away from IVF labs: faulty fertilised embryos.
To clone an embryo, researchers currently take the nucleus of a skin cell (which contains the DNA in chromosomes), and place it in an egg whose own chromosomes have been removed. Some of its 'Promethean factor' - like the Greek mythological figure with amazing regenerative powers - remains in the gutted egg to reprogram the skin cell to become an embryo.
When researchers first tried to clone, they used fertilised eggs but it never worked. Only unfertilised eggs seemed to contain the Promethean factor.
But Eggan's group cracked the mystery. One of the remarkable things about an unfertilised egg is that its chromosomes are not bagged in a nuclear membrane. Shortly after the sperm fertilises the egg, the nuclear membrane closes around the chromosomes. Eggan's group wondered if the Promethean factor might get scooped inside the nucleus and that's why gutted, fertilised eggs didn't work for cloning.
If that hunch was right, there was a second opportunity to get at the Promethean factor. As the fertilised egg approaches its first cell division, the nuclear membrane dissolves, and the Promethean factor might leak out like ooze from a burst soap bubble.
Searching for immortality
The hunch proved right. The researchers found that gutted fertilised mouse eggs would work for cloning embryos, as long as those eggs had already burst their nuclear membrane.
If the technique works in humans, scientists could avail themselves of tens of thousands of faulty fertilised eggs that are thrown away from IVF clinics each year – often because they carry more than one sperm. Eggan's group showed that these eggs are still useful for cloning.
Since women would not be required to donate their eggs for cloning, it would remove one major ethical barrier. But not all.
"It will still be problematic for those who are against the destruction of embryos. But my concern has always been [egg] provision, not the embryo. This is a tantalising method that is well worth persevering with," commented Alan Colman, chief scientific officer at biotechnology company ES Cell International in Singapore.
Nevertheless he points out that so far, cloning strategies that work in mice do not always work in humans. For instance while 'aged' eggs work for cloning mice, so far attempts to use similar eggs left over from IVF clinics have failed. "It's right to put up the warning signs", said Colman.
Stem cell mimics
Another three papers, also based on mouse research, herald the coming of a time when eggs and the creation of embryos will not be required at all to create cloned stem cells.
This research validates and advances last year's report by stem cell biologist Shinya Yamanaka's group at the University of Kyoto in Japan. In that report the Kyoto researchers made an educated guess as to what the Promethean factors that reprogram a skin cell to be an embryo would be. They found that when they infected a skin cell with a virus containing four genes, the skin cell started behaving similar to an embryonic stem cell.
But there were some differences. For instance, true embryonic stem cells can contribute to the formation of a live mouse ... and these couldn't.
Now three groups have refined the technique and produced what seem to be bone fide embryonic stem cells that can contribute to the birth of live mice. Two recent studies are published in Nature. These are from Rudolf Jaenisch's group at the Massachusetts Institute of Technology in Boston, as well as another from Yamanaka's group. A third published in the journal Cell: Stem Cell, is from researchers at Harvard.
Previously sceptical researchers are encouraged by the findings. "The fact that the original group and two others have all independently confirmed the original results means that we can have much more confidence that the results are meaningful – repetition is very good," said Robin Lovell-Badge, head of developmental genetics at Britain's National Institute for Medical Research in London.
But applying the technique to humans is not problem-free. For one thing, stem cells produced this way are prone to turn cancerous because one of the four genes transferred is the cancer-causing gene c-Myc.
And again the technique may not work for human cells. "There is still a long way to go," said Lovell-Badge.
