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REPORTS: SCIENCE & HEALTH |
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Tuesday 23
September 2003
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Human
Cloning 'Currently Impossible'
NewScientist.com news service
A newly discovered quirk of primate cell biology suggests that monkeys - and humans - are impossible to clone from adult cells using current techniques. The finding directly contradicts by Clonaid, a company started by a UFO cult, to have created several cloned babies.
"There's a molecular obstacle that stops the technology from working in primates," says Gerald Schatten, at the University of Pittsburgh School of Medicine in Pennsylvania. "Charlatans who claim they have cloned humans clearly don't understand the biology."
Unlike other species in which adult animals have been successfully cloned, Schatten's team found that the eggs of rhesus monkeys are robbed of a key set of proteins during the cloning procedure. The same appears to be true for human cells.
That loss causes genetic chaos in cloned monkey embryos, with chromosomes distributed almost at random. As a result, the embryos look fine at an early stage, but are completely incapable of further development.
"It's an interesting part of the puzzle of why primates have been so difficult to clone," says Robert Lanza of Advanced Cell Technology, a Massachusetts-based biotech company that has cloned very early stage human embryos for therapeutic research.
"Gallery of horrors"
Schatten's group want to clone monkeys to assist in the study of human diseases. The key technology is called somatic cell nuclear transfer (SCNT), where a cell from the adult animal to be cloned is fused with an egg stripped of its own nuclear DNA.
Other researchers had cloned sheep, cows, mice, goats, pigs, rabbits and a cat, so Schatten was confident monkeys could be cloned too. But despite producing perfect-looking monkey embryos using SCNT, none developed further.
New Scientist reported concerns about cloned monkey embryos in December 2001, when one of Schatten's colleagues described them as a "gallery of horrors".
The new study of 716 rhesus monkey embryos revealed the same chromosomal chaos. Some of the embryos' cells contained double the normal number of chromosomes, others had odd combinations and some had none at all. And Schatten's team have now discovered why.
Lost direction
On a hunch they examined the cells' spindles, structures that guide chromosomes into daughter cells as the embryo divides. The researchers found that SCNT primate embryos lacked at least two proteins required for proper spindle function, leaving the chromosomes to distribute randomly throughout the embryo.
These proteins turn out to be tightly linked to the chromosomes in the monkey's eggs, which are removed in one of the first steps of the nuclear transfer process. Further, unpublished work by Schatten's group and others has shown the same is true for human cells.
In contrast, mice and cows have extra copies of these proteins floating around to help out the cloned embryo. Schatten jokes: "It's almost like God in her wisdom said go ahead and clone cows and sheep, but if you clone a human I'm going to paralyse the egg."
Embryonic cells
The discovery is important, says Lanza, but there may be other important factors. Although attempts to clone a monkey by SCNT using adult cells have all failed, two animals were cloned in 1997 by embryonic cell nuclear transfer, which Schatten reports also creates the damaging spindle defect.
Furthermore, even trivial differences, such as a slight changes to reagents, can turn success into failure when cloning other species, says Lanza.
Schatten intends to test his spindle idea by using a different cloning technique. He will allow the egg's chromosomes to remain in the embryo until after the donor cell has been fused, so the spindle proteins can migrate to new locations. He already has preliminary evidence that proper spindles then form, suggesting primate cloning could perhaps be feasible.
But he warns against any attempt at human cloning, given the high rate of abortions, neonatal deaths and health problems in clones. "I hope this natural obstacle affords us time to make responsible and enforceable legislation to prevent anyone attempting human reproductive cloning," he says.
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