Supporters of research on embryonic stem cells have been cheered by a report in the New Scientist that old adult stem cells can also turn cancerous. Although ESCs easily turn into aggressive tumours called teratomas when injected into animals, it was generally thought that adult stem cells were more docile. However, scientists in Spain have published a study in the journal Cancer Research which says that when stem cells divide between 90 and 140 times, they can form cancers. A Danish group also published research in the same issue which appears to support the Spanish study.

The upshot of this research is that cells are safe to use until they begin manufacturing telomerase. Telomerase is an enzyme which protects the cells’ telomeres — genetic “fuses” which prompt cells to die off after a certain number of generations. The Spanish team suggests 60 generations as a provisional cut-off point.

The scientists stressed that the cancers came about in a very artificial situation, as usually adult stem cells are grown outside the body only for a short time. “In normal conditions in clinical applications we think the cells are pretty safe, but we must be careful,” says Antonio Bernad, of the Autonomous University of Madrid. “The key is not to grow them for too long.”

These results add to the simmering controversy amongst scientists about the potential uses of adult stem cells. Although there have been studies which suggest that their developmental clocks can be turned back, giving them more or less the same potential as ESCs, many embryonic stem cell scientists have told the media that they are sceptical. Some, like Larry Goldstein, of the Howard Hughes Medical Institute, told the Los Angeles Times that they haven’t had time to read the latest papers about the potential of adult stem cells. “I’m turning a blind eye to all of it,” says Dr Goldstein. “I figure if something really good happens, I’ll hear about it and go read about it.”

Three scientists have recently published peer-reviewed work which indicates that adult stem cells may be as malleable as ESCs. Catherine Verfaillie, of the University of Minnesota, is investigating bone marrow stem cells; Alan Mackay-Sim, of Griffith University in Brisbane, is looking at nerve cells in the nose; and Douglas Losardo, of Tufts University, found that bone marrow differentiated into nine kinds of tissue.

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