Four years after researchers in Scotland startled the world by announcing they had cloned a sheep named Dolly, scientists say evidence is mounting that creating healthy animals through cloning is more difficult than they had expected.
The clones that have been produced, they say, often have problems severe enough — developmental delays, heart defects, lung problems and malfunctioning immune systems — to give pause to anyone thinking of cloning a human being. In one example that seems like science fiction come true, some cloned mice that seemed normal, suddenly, as young adults, grew grotesquely fat.
It is not that one particular thing goes wrong or one specific aspect of development goes awry, researchers say. Rather, leading cloning experts and developmental biologists said in recent interviews that the cloning process seems to create random errors in the expression of individual genes. Those errors can produce any number of unpredictable problems, at any time in life.
Before Dolly's debut in 1997, scientists thought mammals could not be cloned. But now they have cloned not only sheep but also mice, cows, pigs and goats. With mice, they have even made clones of clones on down for six generations. Dolly, the first clone, has grown into a comfortable old age. And two infertility specialists recently announced that they want to clone humans.
Initial fears raised about clones — that they would age rapidly or develop cancer — turned out to be unfounded, scientists said. But as scientists gained more experience, and tried to discern why efforts so often end in failure, new questions about its safety have arisen. Fewer than 3 percent of all cloning efforts succeed.
In cloning, scientists slip a cell from an adult into an egg with its genetic material removed. The egg then reprograms the adult cell's genes so that they are ready to direct the development of an embryo, then a fetus, then a newborn that is genetically identical to the adult whose cell was used to start the process. No one knows how the egg reprograms an adult cell's genes, but that, scientists think, is the source of the cloning calamities that can occur.
The problem, they say, seems to be that an egg must do a task in minutes or hours that normally takes months or years. In the months it takes sperm to mature, their genes are being reprogrammed. The same thing happens in eggs, where over years they slowly mature in the ovaries. And this reprogramming must be perfect, scientists say, or individual genes can go amiss at any time in development or later life.
"With cloning, you are asking an egg to reprogram in minutes or, at most, in hours," said Dr. Rudolph Jaenisch, a professor of biology at the Whitehead Institute at the Massachusetts Institute of Technology. "That's where the major problem is," he said.
All the evidence so far, scientists say, indicates that the breathtakingly rapid reprogramming in cloning can introduce random errors into the clone's DNA, subtly altering individual genes with consequences that can halt embryo or fetal development, killing the clone. Or the gene alterations may be fatal soon after birth or lead to major medical problems later in life.
Some scientists say they shudder to think what might happen if human beings are cloned with today's techniques. While arguments over the ethics of human cloning have dominated the debate, these scientists say, the real issue is the likelihood that clones would have genetic abnormalities that could be fatal or subtle but devastating.
Until that problem is solved, they say, human cloning should be out of the question.
"It would be morally indefensible," said Dr. Brigid Hogan, a professor of cell biology at Vanderbilt University Medical Center in Nashville, Tenn., and an investigator with the Howard Hughes Medical Institute.
"It would be reckless and irresponsible," Jaenisch said. "What do you do with humans who are born with half a kidney or no immune system?"
And, he said, what about the real possibility of creating children who appear to be normal but whose genes for neurological development work improperly?
Scientists say they see what appear to be genetic problems almost every time they try to clone.
For example, some mouse clones grow fat, sometimes enormously obese, even though they are given exactly the same amount of food as otherwise identical mice that are not the products of cloning.
The fat mice seem fine until an age that would be the equivalent of 30 for a person, when their weight starts to soar, said Dr. Ryuzo Yanagimachi, a University of Hawaii researcher who first cloned these animals and has studied cloning's consequences in them.
Cloned mice also tend to have developmental abnormalities, taking longer to reach milestones like eye opening and ear twitching, Yanagimachi has found.
Cow clones are often born with enlarged hearts or lungs that do not develop properly, said Dr. Mark E. Westhusin, a cloning expert at Texas A&M University in College Station.
The genetic effects most often seem to be fatal at the very start of life, researchers say. With cattle, for example, 100 attempts to create a clone typically result in a single live calf, Westhusin said.
Cloning mice is more efficient, Yanagimachi said. But even then, only 2 percent to 3 percent of his attempts succeed.
"Cloned embryos have serious developmental and genetic problems," Yanagimachi said, which usually kill them before birth. Just after birth, he said, more die, usually of lung problems. He added that inbred strains are much harder to clone than hybrid strains of mice, which makes sense, he said. Inbred animals have much less genetic diversity and so less opportunity to bypass genetic errors than hybrid animals.