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Cloning may not always
completely reprogramme an egg cell the way sexual
reproduction does, which would explain why the process
fails so often, researchers have found.
While lawmakers around the world debate whether people
should be cloned, animal breeders are busy cloning prize
cattle, pigs and a range of other animals, including a
cat.
But the process fails more often than it works, and
animals often die at or near the time of birth. It has not
been entirely clear what goes wrong, but experts say
technique seems to be very important.
A team at the University of
Connecticut reported on Monday another possible explanation --
genes on the X chromosome often express, or turn on, improperly,
at least in cloned cattle.
Writing in the journal Nature Genetics, they said they looked at
the expression patterns of 10 genes on the X chromosomes in four
cloned cows that were born alive and four that died of
respiratory distress soon after they died. |
Dolly, the world's first
cloned animal, stands in her pen at the Roslin Institute
in Edinburgh in this February 23, 1997, file photo. |
They were all made by the same method -- somatic cell nuclear
transfer, which involves taking the nucleus out of an egg cell
and replacing it with the nucleus of an adult cell.
If done right, this starts the egg growing and dividing as if it
had been fertilised by a sperm.
The four heifers that lived were made using cumulus cells, cells
that nurture the eggs in the ovaries. Other researchers have
reported unusual success using these cells.
The four that died were made using skin cells.
Looking at which genes were turned on and off, which affects the
kind of tissue a particular cell will form, the researchers
found abnormal patterns in major organs such as the heart,
liver, brain, kidney and spleen of all the dead clones.
Live clones appeared to have normal gene expressions in their
blood and skin tissues. Organs were not sampled in the healthy
animals.
"Our study demonstrates that in clones, even though they can
develop to full term, many abnormalities in gene expression
exist which may be partially responsible for the developmental
abnormalities frequently observed, including death," Dr.
Xiangzhong Yang, who led the study, said in a statement.
They also think they explained why many cloned animals have
abnormal placentas -- which affects their size and survival.
Normally, only the mother's X chromosome is active in the
placenta, while the X chromosome inherited from the father is
switched off. But both versions were activated in the dead
clones.
"This finding may help to explain the commonly observed abnormal
placental problems for cloned animals," Yang said.
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