快猫短视频

CLONING does not have a very high success rate. Animals are often born sickly
or unusually large, and many do not live long. The reason, say scientists in
Scotland, may be just a few missing carbon atoms.

Lorraine Young of the Roslin Institute near Edinburgh and her colleagues have
shown that when sheep embryos are manipulated in the lab they can lose some of
the methyl (CH₃) groups attached to their genes. They say this small
change alters how actively the genes produce proteins which are key to the
animal鈥檚 survival.

The discovery may allow scientists to screen embryos produced by cloning and
other test-tube techniques, allowing faulty embryos to be eliminated before they
are implanted into a surrogate mother. 鈥淎 lot of us have been trying to nail
down what kills clones,鈥� says Mark Westhusin, who clones cattle at Texas A&M
University in College Station. 鈥淭his is absolutely a new twist that many people
are going to follow up on.鈥�

For every living success story, cloning generates many animals that die of
mysterious causes. Researchers at Roslin produced 277 cloned sheep embryos
before achieving a live birth with Dolly. Recently, Noah the gaur, cloned from a
member of this endangered ox-like species using a cow egg and a surrogate cow
mother, died of an infection soon after birth. Many experts believe Noah died
because of cloning-related health problems.

Some cloned animals grow huge, and often sickly, during gestation. Similar
problems can arise in embryos produced by other technologies that culture them
in test tubes. The Roslin researchers and others had previously found evidence
that unusual levels of proteins that regulate fetal growth could be partly to blame
(快猫短视频, 23 January 1999, p 15).

Production of some of these proteins is known to be affected by methyl groups
attached to the relevant genes, so they wondered if methylation was different in
enlarged animals. 鈥淲e and others have been investigating the idea,鈥� says Young.
鈥淗owever, this is the first proof of the theory.鈥�

Young and her colleagues studied sheep produced by artificial insemination
that suffered from what is called 鈥渓arge offspring syndrome鈥�. They found that
these LOS sheep often lacked all the methyl groups on the gene for a protein
called IGF2R, and that they produced 30 to 60 per cent less of the protein than
normal. IGF2R helps stop the fetus from growing too large. In control animals,
about 70 per cent of the same sites were methylated.

Young thinks that clones may suffer from an even greater array of defects
than these LOS animals because the embryos undergo greater manipulation and may
therefore lose more methyl groups from more genes.