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But telomeres weren't completely inert. One thing they almost always appeared to do was grow shorter. Each time a cell divided, the daughter cells it produced had a little less telomere to play with. Finally, when the cell reached its Hayflick limit of 100 or so replications, the telomere was reduced to a mere nub. At that point, the cell quit replicating. Once it did, researchers theorized, the genes previously covered by the telomere became exposed and active, producing proteins that triggered the tissue deterioration associated with aging.
While most every cell in the human body exhibited telomere loss, a few didn't. Among those spared were sperm and cancer cells--just the cells characterized by their ability to divide not just 100 times, but thousands.
The next step for scientists was obvious: study the cells with little or no replication limit and find out what mechanism kept their telomeres--and their lives--so long. In 1984 molecular biologists Carol Greider and Elizabeth Blackburn, then with the University of California, Berkeley, did just that. Working with a single-cell pond organism, they discovered a telomere-preserving enzyme they dubbed telomerase. Five years later, Gregg Morin at Yale University confirmed their work, identifying the same substance in cancer cells. In the Petri dish, the agent of eternal life had been found.
"The moment telomerase was discovered," says Hayflick, "it was clear that for immortal cells at least, this was a way to circumvent the inevitability of aging and dying." Telomerase has now been found in the precursor cells that give rise to human eggs, in the stem cells that give rise to blood cells and in up to 95% of cancer cells.
Since telomerase keeps these tenacious cells going, is it reasonable to assume that the same enzyme could be used artificially to help mortal cells--and the body itself--exceed their programmed life-span? At Geron Corp., a San Francisco-based biomedical firm, biologist Calvin Harley is trying to find out. Harley, who collaborated with Greider on her later telomere work, is looking for the genes that direct telomerase production, believing he might be able to manipulate them so that the spigot for the enzyme can be turned on and off at will. "I think we are going to see fundamental medicines for aging," Harley says. "With a pill, with cell therapy, I think we may be able to treat aging in very specific areas."
Of course, telomerase therapy has obvious problems. Dosing tissues with precisely the enzyme that helps turn healthy cells cancerous strikes many skeptics as less than a life-extending brainstorm, and even advocates of telomerase therapy don't pretend that such treatments could yet be considered safe. Moreover, how easy it would be to manipulate the telomerase gene in the first place is an open question, since merely locating it among the 100,000 or so we carry in each cell can be a mind-numbing job.