杏吧原创

CLONING doesn鈥檛 necessarily make cells grow old, and can even act as a
fountain of youth, say researchers who have been investigating a group of cloned
calves. The news comes a year after a study on Dolly the cloned sheep showed
that her cells appeared prematurely old. That finding raised fears that cloned
animals or future medical treatments based on cloned cells would turn out to be
short-lived.

Concerns over cellular ageing in clones centres around telomeres, regions of
DNA at the ends of chromosomes which shorten every time a cell divides.
Telomeres appear to shrink throughout our lives, and many researchers believe
that the symptoms of old age are caused in part by these shortened telomeres.
Dolly appears normal and healthy, but in last year鈥檚 study her telomeres were
found to be unusually short. It鈥檚 possible that this will cause her to age
prematurely.

Dolly鈥檚 predicament isn鈥檛 shared by six calves cloned by Advanced Cell
Technology (ACT) of Worcester, Massachusetts: their telomeres are significantly
longer than those in normal cows of the same age. 鈥淚 thought it might be
possible that there would be some maintenance of telomeres, but that we see
extension is unexpected,鈥� says Peter Lansdorp of the British Columbia Cancer
Research Agency in Vancouver, who collaborated with ACT.

Not only do cells from the clones have long telomeres, says Lansdorp, but
when grown in the lab they are able to divide more times than non-cloned cells.
In fact, non-cloned cells that were too old to continue dividing in the lab were
still able to produce healthy, 鈥測oung鈥� clones. Apparently, cloning can be a
rejuvenating process.

鈥淲hat is really surprising,鈥� says Robert Lanza, a vice president of ACT, 鈥渋s
not only were we able to clone healthy calves from cells at the end of their
lifespan, but the [cloned] animals had cells that appear to be younger than
their chronological age.鈥�

The researchers don鈥檛 know why their calves appear so different from Dolly.
They say it probably isn鈥檛 due to species differences. According to Lanza,
researchers who cloned cows using the same technique employed on Dolly also
ended up with animals that had shorter telomeres. 鈥淐learly it has to do with the
cloning technique and the donor cell used,鈥� says Lanza. Cells used to clone
Dolly were starved of nutrients to stop them at a particular stage in the cell
cycle, while ACT鈥檚 technique doesn鈥檛 starve donor cells.

The calf results will be a relief for medical researchers. They hope one day
to be able to grow cells in the lab that can replace damaged or dysfunctional
human tissue, or perhaps even whole organs. If these cells could be cloned from
the patient, they wouldn鈥檛 be rejected by the immune system. But if cloned cells
inevitably aged prematurely, they might deteriorate more quickly than
desired.

Jerry Yang, head of the University of Connecticut鈥檚 Transgenic Animal
Facility, believes the new study will change perceptions of cloning. He says
that medical ethicists often argue that cloning is dangerous and causes a
variety of problems. 鈥淥ne example they give is shortened telomeres,鈥� says Yang.
鈥淣ow the story will be changed.鈥�