Scientists have found potentially definitive evidence that cloning is far too unsafe to be used in human reproduction, should it ever be viewed as ethically acceptable in the future.
Hidden genetic defects were found in otherwise healthy cloned animals in a study that could fatally undermine the arguments for loosening controls on the most controversial area of reproductive technology.
The research could explain why a huge proportion of cloned animals are either stillborn or suffer from congenital defects, and points to the presence of an underlying genetic flaw in all clones. Professor Ian Wilmut, the British scientist who cloned Dolly the sheep, said last night that the research represented a serious blow to people such as Sevorino Antinori, the Italian doctor who has said that he wants to clone a baby.
"It surely adds yet more evidence that there should be a moratorium against copying people. How can anybody take the risk of cloning a baby when the outcome is unpredictable?" Professor Wilmut said.
The study was done by scientists drawn from two leading laboratories in America. They found that cloned mice that were healthy in all outward respects carried a high "burden" of genetic abnormalities, which could shorten their lives.
The cloning process has also been shown to cause a higher-than-normal incidence of birth defects. Other lambs cloned with Dolly, for example, either died in the womb or were born with serious imperfections.
Now, a team led by Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, has found that healthy, cloned mice possess a hidden "instability" of their genes, which is not present in normal mice. The instability causes some genes to work, or to be "expressed", at abnormal levels, probably as a result of the cloning process bypassing the normal way that chromosomes from two parents work together when a sexually produced embryo is created by the fusion of sperm and egg.
Despite this instability, many of the cloned embryos survived to adulthood suggesting that mice and other mammals – including humans – are surprisingly tolerant of such genetic aberrations.
"This suggests that even apparently normal clones may have subtle aberrations of gene expression that are not easily detected in the cloned animal," Professor Jaenisch said.
The study, published in the journal Science, suggests that reproductive cloning causes unavoidable fundamental flaws, a finding that has surprised the researchers themselves, who included Ryuzo Yanagimachi of the University of Hawaii, the first person to clone adult mice.
David Humpherys, a member of the research team, said that by tagging certain genes, the scientists found that cloning appeared to upset a phenomenon known as "genomic imprinting", where the genes on the chromosomes from one of the parents are switched on or off.
"The big concern is that there would be some underlying problem that you can't see at birth or that there are other problems you can't even assess in mice, such as cognitive problems," Mr Humpherys said. "It seems very unwise to attempt this sort of cloning on humans."
Dr Yanagimachi was hailed in 1998 when he led a team of American, Japanese, Italian and British scientists who succeeded in producing a colony of 22 cloned mice.
The achievement, which some scientists believed was a biological impossibility, was expected to lead to new cancer therapies, improvements in agriculture and in the production of pharmaceutical drugs.Reuse content