Research Update
Is cloning the fountain of youth?
CMAJ 2000;162(13):1866
Scientists from the British Columbia Cancer Agency have discovered that cloned calves are younger, on the cellular level, than normal calves the same age (Science 2000;288:665-9). The finding contradicts a previous discovery that Dolly, the sheep cloned in Scotland 3 years ago, was biologically older than a normal lamb.
The startling discovery centres around telomeres, DNA threads that "tie up" the ends of chromosomes to protect the genetic coding information of the DNA. Telomeres generally become shorter with each cell division: the older the cell, the shorter the telomeres. When scientists examined the telomeres of Dolly, they discovered that the clone's cells were the same age as those of the 6-year-old sheep from which Dolly was cloned, meaning that Dolly was prematurely "old" and would probably not enjoy a normal lifespan.
However, the BC researchers have found that the telomeres in cloned cows are elongated compared with newborn and age-matched normal animals, meaning that the clones are more youthful than their normal counterparts. "Previously, it was thought that only the cells that produce sperm and eggs, as well as cancer cells, could elongate or maintain telomeres. But our findings clearly show that telomeres can be elongated by cloning as well," says Dr. Peter Lansdorp, senior scientist at the Terry Fox Laboratory.
Lansdorp proposes 2 possible explanations for the longer telomere length in the cows. "It may be that our notions about telomere maintenance in normal biology were wrong. We have evidence that telomeres are maintained in the male germ line, but in the female germ line, we actually never looked. It's possible that there is a need to restore telomere length because the female chromosomes have short telomeres. . . . Alternatively, there may be a safety mechanism, if the telomeres are too short in the fertilized egg."
Lansdorp noted that "the increase in telomere length in the cloned cells from cattle corresponds precisely to an increased ability of the cells to divide in Petri dishes. This research suggests that cloned cows might actually live longer than cows conceived naturally."
There is growing medical research on telomeres, because telomeres do not shorten in cancer cells, making them immortal. A promising approach to eventual cancer treatment in humans may be to inhibit telomerase, which appears to maintain telomeres, in order to reverse the immortality of tumour cells.
Meanwhile, in the burgeoning field of therapeutic cloning, scientists are trying to create embryonic stem cells that can be differentiated to grow replacement tissues or organs in the laboratory. Such differentiated cells already exist in laboratory mice, says Lansdorp. — Heather Kent, Vancouver
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