PITTSBURGH - After analyzing hundreds of their failed attempts to clone rhesus monkeys, a team of Pittsburgh researchers has found a biological mechanism that they say makes it nearly impossible to clone humans and other primates using the techniques that produced Dolly the sheep.

In primates, removing the nucleus of an egg cell - the first step in the process used to clone Dolly - also removes proteins that are essential to proper cell division, said Gerald Schatten, director of the Pittsburgh Development Center at Magee-Womens Research Institute.

Without these proteins, subsequent cell divisions result in "a car wreck of chromosomes," he said, with genetic material divided haphazardly between the new cells, eventually causing the embryo to die.

"The barrier that we've encountered is certainly a nail in the coffin of reproductive cloning with the technology we have available," Dr. Schatten said. It may be possible one day to make genetic copies of primates using still-unproven methods, he acknowledged, but human reproductive cloning does not appear to be imminent.

The findings by Dr. Schatten, Calvin Simerly, Christopher Navara and their colleagues are being published today in the journal Science.

"I think the findings ... provide a basis for understanding why there has been no documented success to date in achieving normal embryonic development after replacing the genome of a human egg with that of a [nonreproductive] cell," said Roger Pedersen, who attempted human nuclear transfer experiments at the University of California, San Francisco, before relocating two years ago to Cambridge University in England.

The findings also could affect Congress, which has rushed to ban all forms of human cloning following claims late last year by a sect called the Raelians that they had produced a cloned human baby. The Raelians have yet to provide evidence for their claim.

"Now [members of Congress] don't have to fear that cloning is right around the corner," said Ronald Cole-Turner, an ethicist at the Pittsburgh Theological Seminary.

Speculation about human clones was all the rage in 1997, when Scottish researchers announced Dolly, the first genetic copy of an adult mammal to be produced by a method known as somatic cell nuclear transfer. Scientists rushed to use the technique to produce genetic copies of mice, goats, and cows, among other animals, but no one has yet produced a cloned primate. Dr. Schatten emphasized that he has no interest in producing cloned human babies. "I am 10,000 percent in favor of responsible legislation that no one ever attempt human reproductive cloning," he said. "At its core, it's unsafe," he said, noting that most cloning attempts are unsuccessful or result in fatal deformities.

Dr. Schatten's lab has pursued monkey cloning both as a way to develop identical monkeys with human diseases that could be used in medical testing, and to determine whether nuclear transfer could be used to produce embryos that could be a source of embryonic stem cells for medical therapies.

The nuclear transfer technique involves removing the gene-containing nucleus from an unfertilized egg cell and replacing it with the nucleus of an adult somatic cell, such as a skin cell.

In the experiments on Dolly and other cloned mammals, the transferred nuclei then would develop into embryos, although many of the embryos created this way did not survive, and the cloned mammals that were born often had shortened life spans and other problems.

But Dr. Schatten's group couldn't even get that far with the rhesus cloning. To analyze the failures, Mr. Simerly and Mr. Navara added fluorescent tags to cellular components and studied the cells microscopically.

They found that removing the egg nucleus also removed so-called motor proteins. These proteins play a key role during cell division, when the cell's chromosomes are copied. The chromosomes, which carry all of the cell's genes, must be divvied up evenly before a cell splits so that each of the resulting cells has a full genetic complement.

Normally, two tiny bundles of hollow tubes position themselves on either side of this chromosome soup. The motor proteins help the chromosomes climb up the tubes, one copy to either side. The cell then divides.

But after nuclear transfer in primates, so many of these proteins are missing that the chromosomes can't properly separate themselves. In sheep, by contrast, the motor proteins are distributed differently through the egg, Dr. Schatten said, so enough remain to permit normal cell division. Another complicating factor, Dr. Schatten suggested, is that the sperm appears to be the preferred source of the tube bundles.

One way around this roadblock, Dr. Schatten said, may be to begin the process with a fertilized egg, add the nucleus from the adult somatic cell, and only then remove the original egg and sperm nuclei. His lab is now studying this method.

Even if reproductive cloning is impossible, Dr. Schatten said it may still be possible - but difficult - to use nuclear transplantation as a way of producing embryonic stem cells.

Even with the problems his group discovered in cell division, a small percentage of cells created this way will, by chance, have a normal complement of chromosomes and might survive long enough in a cell culture to produce stem cells. Stem cells are precursor cells that can develop into different types of body cells, from neurons to muscle cells to blood cells. Donald Kennedy, editor of Science, said the Schatten paper "is a nice piece of science," but cautioned against reading too much into it. "I think it would be a mistake to say, 'Aha! This proves that human cloning won't work,' " he said.

Don Wolf, senior scientist at the Oregon National Primate Research Center, where Dr. Schatten worked until coming to Pittsburgh two years ago, said he hasn't given up his efforts to use somatic cell nuclear transfer to produce monkeys.

"Dr. Schatten may be quite correct in his assessments of motor protein deficits," Mr. Wolf said. But he noted that motor proteins didn't seem to be a problem in 1997, when Mr. Wolf used nuclear transfer to produce two noncloned monkeys, using the nuclei of embryos rather than of somatic cells. "I still remain convinced that reproductive cloning in primates is possible, albeit at low efficiency."

And Panos Zavos, a Kentucky researcher who is working with Italian fertility doctor Severino Antinori, announced earlier this week in an online journal, Reproductive BioMedicine Online, that his team has created the first human cloned embryo for reproductive purposes. Working in an overseas lab, the team allowed the embryo to develop in culture for four days before freezing it for future testing.

But in the same issue, Azim Surani of the Wellcome Institute at Cambridge University complained that Mr. Zavos' report was too sketchy to be useful.

The Block News Alliance consists of The Blade and the Pittsburgh Post-Gazette.

First Published April 11, 2003, 11:47 a.m.