WASHINGTON -- It may be impossible to clone humans because primate cells do not divide properly, a "pothole in the process" that creates chromosomes too abnormal for a pregnancy to begin, scientists say.
The findings by the University of Pittsburgh researchers come five months after a cult group claimed to have cloned a person. Those claims have not been verified.
The scientists, reporting Thursday in the journal Science, were trying to understand why efforts to clone a monkey had been unsuccessful. They found that, from the very start, the cloned primate cells did not divide properly.
Humans are primates.
"Most people in the cloning field will be surprised by this," said lead researcher Gerald Schatten. "This work demonstrates there's a pothole in the process. We now know the depth and breadth of the pothole, and we're designing strategies to get around" it.
Dozens of animal clones -- including cows, pigs, mice, goats and a cat -- have been born since Dolly the sheep became the first new being created from an adult cell in 1997. But it still is a very uncertain field. Many are stillborn and some survive only with severe defects.
Thus far, attempts to clone monkeys -- far closer genetically to people -- using the Dolly technique have failed. Cloning experts fear that it would be dangerous to attempt human cloning because of those failures, as well as the birth defects in animals that have been cloned.
In addition to the unsubstantiated claims by the cult group, a doctor who separately is pursuing human cloning has reported in an Internet journal preliminary data on an early stage cloned human embryo, but with no chromosome information.
To clone, scientists harvest an unfertilized egg from a female donor, removing the genetic material and replacing it with new DNA from an adult cell of the animal to be cloned. An electric shock coaxes it into dividing. If all goes well, the egg grows into an embryo that can be implanted into a surrogate mother.
It took 277 attempts before Dolly was born. Schatten's group tried even longer to clone a rhesus monkey -- 724 eggs that yielded only 33 embryos and not a single pregnancy.
For cells to properly divide, chromosomes must duplicate themselves and precisely line up along a zipper-like structure called a spindle. Once the chromosomes are in place, the spindle helps the cell pull apart into two. During human reproduction, if the chromosomes do not split properly, defects such as Down syndrome result, or the pregnancy fails.
Schatten wondered if chromosome abnormalities were behind failed monkey clonings. Indeed, inside cloned monkey cells, the Pittsburgh researchers discovered deformed spindles and chaotic chromosome numbers.
Eggs harbor proteins that act as molecular motors that are key to spindle formation. In primates, those proteins are so tightly bound to the egg's DNA that cloning's first step of DNA removal pulls them out, too, dooming hope of later pregnancy, Schatten said.
In other mammals, enough spindle-forming proteins float in the egg's remaining fluid for reproduction to occur, he said.
The discovery is very important, said Dr. Duane Kraemer, a successful cloner of nonprimates at Texas A&M University.
"The fact that they don't get pregnancies at all is suggestive that there is something different going on there than with other species," he said. "It points to a potential problem that may have to be solved before the next advance can be made."
It is not just bad news for reproductive cloning. It also means the related field of therapeutic cloning -- using embryonic stem cells to grow customized tissues for medical treatment -- may prove harder, too, Schatten said. However, if 95 percent of cells growing in a lab dish have abnormal chromosomes, the remaining good 5 percent still could be used, he added.
His lab is exploring a way to overcome the problem, by combining cloning with old-fashioned egg fertilization. The sperm-and-egg joining jump-starts spindle formation. Schatten then pulls out that sperm and egg DNA, leaving just the clone DNA in the now-growing monkey cells.
"The value of this for deriving embryonic stem cells is going to be very attractive," Schatten said.
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