It seemed like the perfect target. An enzyme that gives cancer cells their everlasting ability to divide, but which is missing from most normal cells. Could this be the one weak spot common to all cancers, the route to a universal cure with no serious side effects? Discovered in the mid-1980s, this enzyme, called telomerase, generated a great deal of excitement among cancer researchers. Telomerase is largely missing from healthy cells, but 90 per cent of cancers churn it out by the bucketload. It gives them the ability to keep dividing long after they should have aged and died. Telomerase's discovery raised the hope that it could somehow be used to single out cancer cells from normal cells, and perhaps even strip them of their deadly immortality. In the early 1990s, telomerase was the focus of great optimism. But several major setbacks have been enough to scare away drug companies and convince some researchers that telomerase as a cancer therapy was a dead end. One of the biggest blows was the discovery that it takes months for a cancer cell to stop dividing after telomerase activity has ceased. For most cancer patients, that is simply too slow. But now comes a new twist. Researchers delving deeper into basic telomerase biology have uncovered evidence that this enzyme's role in cancer is a great deal more complex -and possibly even more critical - than originally thought. At the same time, there is growing excitement over recent experiments that have uncovered new and more efficient ways of killing cancer cells by exploiting telomerase. One approach, an anti-cancer vaccine that trains immune cells to zero in on cells making telomerase, already looks promising in clinical trials on cancer patients. As a result, some researchers are now quietly optimistic that telomerase may yet emerge as a powerful tool in the fight against cancer. "As our understanding of telomerase has become more sophisticated, its attractiveness as a target has not gone away," says Elizabeth Blackburn, co-discoverer of telomerase. "It's gotten stronger."
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