WASHINGTON, July 6 (AFP) - Scientists Friday issued a new warning about the dangers of human cloning, saying cloned lab animals were found to harbor serious but outwardly undetectable genetic abnormalities.
Researchers at the Whitehead Institute for Biomedical Research and the University of Hawaii reported they had found the first evidence that gene functioning in clones may be prey to subtle flaws.
The finding backs suspicions in many scientific quarters that reproductive cloning is not only inefficient but also potentially dangerous to the cloned creature.
The researchers, Rudolf Jaenisch and Ryuzo Yanagimachi, sought to find out why cloning in animals results in so many catastrophic failures, such as false pregnancies, early death and chronic overgrowth.
They grew mouse clones from embryonic stem cells, the "master" cells that develop into all the body's different tissues.
The team then noted surprising variations in the functioning of structural genes -- genes whose expression varies according to its donor and which switched on or off by chemical tags.
The biggest differences occurred in the placenta, kidneys, heart and liver of the cloned mice.
The problem appears to lie in with the donor embryonic stem cells, which can be extremely unstable in culture. As they divide, these cells lose the tags that tell a gene to be either turned on or off during development.
In spite of the instability, some clones reach adulthood, suggesting that mammalian development is surprisingly tolerant to aberrant gene regulation.
"This suggests that even apparently normal clones may have subtle aberrations of gene expression that are not easily detected in the cloned animal," said Jaenisch.
Their research was published Friday in the US weekly Science.
The cloning procedure consists of replacing the genetic material-bearing nucleus of an egg with the nucleus of an adult cell or of an embryonic stem cell.
In theory, the egg resets the developmental clock of the nucleus back to a state compatible with early embryonic growth and gives rise to a new organism that is genetically identical to the donor cell.
Up to now, the researchers believed the nucleus of embryonic stem cells required less reprogramming than the nucleus of an adult cell, which has already committed to a certain function.
At present, five mammal species have been cloned -- sheep, mice, goats, cows and pigs -- but the success rate is just three to five percent.
The study was published amid a reported crackdown by US federal investigators into an illegal effort to clone a human being by a religious sect called the Raelians.
The group's chief scientist, French national Brigitte Boisselier, along with Italian gynecologist Severino Antinori and US doctor Richard Seed lead an international consortium that has announced plans to clone a 10-month-old baby that died during heart surgery.
On June 20, Britain's most eminent scientific body, the Royal Society, demanded a worldwide moratorium on human cloning, which it branded "unethical, dangerous and irresponsible".