WASHINGTON, Aug. 15 (UPI) -- Although the recent claim by Chinese scientists that they created a cloned human embryo using rabbit cells made headlines around the world, cloning experts have many reasons to doubt the assertion.
The skepticism stems from the fact that other groups have tried similar cross-species cloning experiments without success. Also, key data that would have substantiated the Chinese claim have not been provided. And researchers in China have gained a reputation for making bold claims about cloning and stem cells that, all too often, other scientists are unable to verify.
As described in Cell Research, an obscure journal published by the Chinese Academy of Sciences, a group led by Huizhen Sheng of Shanghai Second Medical University inserted the nucleus or DNA material from human skin cells into an egg cell from a rabbit. The resultant rabbit-human hybrid developed into a human embryo from which Sheng's group claims to have obtained embryonic stem cells.
The manuscript of the study has been circulating among the scientific community for more than two years but the top-tier scientific journals refused to publish it. Prestigious journals, such as Science, Nature and the Proceedings of the National Academy of Sciences, declined to publish the study because their reviewers -- scientific experts who advise journals on a study's merit prior to publication -- found it lacked standard data that should have been included. Requests by the reviewers for additional data from the Chinese researchers apparently went unfulfilled.
Sheng could not be reached by UPI for comment by presstime, but she was quoted in a news article in Nature as saying, "It still may take a while for people to accept the work. But the scientific community has the right to question the details of the work and we have a responsibility to respond to them."
Many scientists familiar with this line of research found Sheng's claim hard to accept.
"It sounds a little too good to be true," said Dr. Robert Lanza, vice president of medical and scientific development at Advanced Cell Technology of Worcester, Mass., the only group to have produced a cloned human embryo legitimately (using human egg cells, not a rabbit's).
"This flies in the face of existing scientific knowledge," Lanza told United Press International. His company has attempted to use rabbit egg cells to clone primates, a group that includes humans as well as their closest living relatives -- monkeys and apes -- and found it to be impossible, he said.
"There are curious aspects to the data (and) I'm sure the entire scientific community will remain skeptical until it's reproduced," said Dr. George Daley, a stem cell researcher at the Whitehead Institute in Cambridge, Mass.
If it is authentic, the study would represent an exciting breakthrough because it would permit studying the phenomena of reprogramming -- converting an adult cell back to its infancy or embryonic stage, which could either be used to produce embryonic stem cells or directed to develop into a specific type of tissue, such as brain or heart. The technique also could prove useful as a conservation tool because scientists could then use rabbit eggs to clone endangered or extinct animals.
Embryonic stem cells are valued for their ability to become any tissue in the body and scientists think one day they could be harnessed as treatments for various diseases. The resultant stem cells from a rabbit-human hybrid probably would not have any direct value in medical treatments due to concerns they could contain rabbit diseases or pathogens and unwittingly transmit them to people. But it would represent "an enormous experimental advantage" for studying reprogramming, Daley said.
Pro-life and anti-abortion groups object to embryonic stem cell research because it requires the destruction of human embryos. However, if researchers could learn how to reprogram adult cells they could retain all the potential medical benefits and perhaps avoid the moral quandary involved. "That's the Holy Grail ... and this may be an important research tool for getting us there," Daley said.
However, scientists remain doubtful the research will turn out to be valid. Lanza noted other groups have attempted similar experiments -- combining rabbit egg cells and DNA from other species -- and have been unsuccessful. In addition, several scientists who heard about the study before it was published attempted to replicate it in their own labs and none of the experiments was successful, he said.
Advanced Cell has done extensive work in the field of cross-species cloning, using the technique to clone two endangered species successfully. One of the main technical barriers is that the egg cell carries DNA from a component of cells called the mitochondria, which has its own genetic material that is separate and distinct from the DNA in the nucleus. Mitochondrial DNA in the egg cell can negatively interact with the DNA of the species attempting to be cloned. This generally is not a problem in species that are closely related but it wreaks havoc in species that are evolutionarily distant.
"When you try these cross-species experiments as soon as the evolutionary divergence gets too great cells cease to divide," Lanza said. "When you try combinations that are even slightly outside the family (of closely related species), you get this blockage."
The general rule is if species are evolutionarily separated by more than 8-million to 18-million years, it is next to impossible, he said. Rabbits and humans fall well outside of that range, having diverged more than 80 million years ago.
Even if the Chinese researchers did manage to overcome the barrier of evolutionary separation and actually produce a human embryo, scientists still have reason to doubt claims they obtained embryonic stem cells.
They did not conduct the necessary experiments to show they had extracted true embryonic stem cells, Daley said. To be authentic stem cells, it would be necessary to show the cells were immortal or could divide indefinitely, and that they could give rise to all the different tissues in the body. Such proof -- which involves relatively straightforward and standard procedures -- was not included in the manuscript, he said.
In addition, some scientists in the cloning field are wary of any research coming from China because a number of other manuscripts purporting bold advances in stem cell and cloning technology originated there, although most never have been verified.
"There are lots of claims and rumors coming out of the mainland that are unsubstantiated," one researcher familiar with the Chinese research told UPI. "There are a number of manuscripts that have circulated that have never gotten published" because journals and reviewers doubted their authenticity, said the researcher, who requested anonymity.
Still, both Lanza and Daley do not rule out the possibility the rabbit-human hybrid experiment could be valid.
The inherent problems and past difficulties with such experiments do not "mean it can't be done, it just means there's a lot of skepticism," Lanza said.
Daley agreed, noting breakthroughs in areas where others have previously failed happen often in science. On the other hand, "science is also full of initial claims of success that don't ultimately hold up to scrutiny," he said. "My sense is if this is as easy as the authors claim, other groups will quickly reproduce it."