快猫短视频

THE row over whether clones are healthy or not has been stoked by the
discovery that some cloned animals really do die young.

When Dolly the sheep developed premature arthritis late last year, scientists
wondered whether all clones would grow old before their time. Now the first
controlled study of the lifespan of clones has come up with bad news. A batch of
cloned mice produced by Atsuo Ogura of the National Institute of Infectious
Diseases in Tokyo nearly all died earlier than their naturally bred cousins.

Cloners remove the nucleus from an egg and replace it with the nucleus of a
donor cell. Many of the resulting embryos don鈥檛 survive gestation. But some
researchers argue that the few survivors can develop into perfectly normal
animals. Last year, the Massachusetts company Advanced Cell Technology (ACT)
reported they had two dozen apparently healthy cloned cows.

But Ogura鈥檚 experiment suggests that some effects of cloning aren鈥檛 apparent
in the days or even years after birth, but only in the very long term. Whether
those creatures can be classified as 鈥渘ormal鈥� when they are young is
debatable.

The Japanese team cloned 12 male mice using nuclei from Sertoli cells taken
from the testes of adult mice. These clones were compared with seven males from
natural matings and six 鈥渢est-tube mice鈥� conceived by injecting sperm cells into
eggs. The young clones appeared active and healthy, and matched the control
animals in 14 out of 16 physiological measurements.

After 800 days, 10 of the 12 cloned animals were dead, while all but 3 of the
13 controls were still alive. The clones that died had suffered from pneumonia,
liver disease or cancer, and showed lower levels of antibody production,
suggesting they had an immune system defect. Ogura鈥檚 team is now trying to
pinpoint the precise cause of death.

Tony Perry of ACT points out that we don鈥檛 yet know whether clones from other
species such as cows or pigs die early. And just because an animal dies in old
age from cancer doesn鈥檛 mean it wasn鈥檛 healthy earlier in life, he points
out.

Even if clones do tend to die early, that won鈥檛 necessarily spell the end for
hopes of commercially cloned farm animals. If the illnesses that clones develop
are easily screened for and treated, or don鈥檛 occur until after the animal is
too old to be productive, cloned animals might still be useful.

But the defects Ogura found should still be taken seriously, says Willard
Eyestone, a cow cloner at the Virginia-Maryland College of Veterinary Medicine
and member of a panel drafting a report on cloning issues for the US Food and
Drug Administration. 鈥淲e鈥檇 be putting our heads in the sand if we didn鈥檛 take
heed from these data and be aware to look out for problems,鈥� he says.

Rudolf Jaenisch, a mouse cloner at MIT, adds that would-be human cloners
should take note. The Japanese results weaken their claim that defective human
embryos could be eliminated early by monitoring the fetus. 鈥淭hey probably won鈥檛
read this paper,鈥� says Jaenisch. 鈥淏ut they should.鈥�