杏吧原创

CLONED animals often have short lives or are grossly oversized. Now a team
from the Whitehead Institute for Biomedical Research in Cambridge,
Massachusetts, has worked out why. They say genes in the stem cells the animals
are made from get disrupted while they are growing in the lab.

This discovery further underlines how dangerous it would be to clone humans.
鈥淚t surely adds yet more evidence that there should be a universal moratorium
against copying people,鈥� says Ian Wilmut, who cloned Dolly the sheep at the
Roslin Institute near Edinburgh.

The Whitehead team was looking at gene patterns in embryonic stem cells from
mice. You can turn these cells into cloned mice or 鈥渃himeric鈥� mice via IVF. The
team spotted problems after studying six genes linked with embryonic
development. These genetic glitches happened when the embryonic stem cells were
being cultured in the lab鈥� not because of cloning or IVF.

The gene alterations, which occurred even though the cloned mice looked
normal, were related to a process called 鈥渋mprinting鈥�. Cells carry two copies of
every gene, one from each parent. Imprinting makes sure that in certain genes,
one of each pair is always switched off. The team found widely varying patterns
of imprinting in four of the six genes.

鈥淚t shows that cloners need to pay very careful attention to how they culture
cells,鈥� says team member Kevin Eggan. 鈥淒isruption of those genes in humans could
cause things like mental retardation.鈥� Eggan says that there鈥檚 no obvious way to
alter the culturing process to prevent the disruptions.