Although only this month renowned stem cell expert Alan Trounson, of Monash University in Melbourne, told the media that transplanted embryonic stem cells do not cause tumours in animal tests, he appears to be wrong. A fast-tracked on-line article in Nature Medicine by scientists at the University of Rochester Medical Centre in New York brings deeply disappointing news for supporters of embryonic stem cells and therapeutic cloning.

Steven Goldman and his colleagues experimented with rats which had a condition mimicking Parkinson’s disease. When treated with mature neurons which had been created from human embryonic stem cells, they recovered all their lost motor function. So far, so good.

However, ten weeks after the transplant, the rats’ brains were examined. To the dismay of the researchers, only about one-fifth of the transplanted cells were now producing dopamine (the chemical lacking in patients with Parkinson’s). The rest had begun dividing uncontrollably into benign brain tumours.

Although a stem cell scientist quoted in the journal Science, Evan Snyder, of Burnham Institute of Medical Research, declared that further fine-tuning of the research will eventually find "the recipe to make it perfect", the language of the article was more pessimistic. "These promising results were unfortunately accompanied by several potentially disturbing outcomes," it said. First, the number of neurons producing dopamine fell sharply after one month, even though they had been "highly enriched" and differentiated cells. Second, "their persistent, uncontrolled and grossly homogeneous expansion over a 10-week span before the animals were killed nonetheless suggested graft-associated tumorigenesis". And it seemed clear, too, that the problem was not contamination by undifferentiated embryonic stem cells. The very cells which were supposed to cure caused the tumours.

The experiment provokes many questions, as the researchers do not know what would happen to the cells after 10 days. But they urge caution for therapeutic applications, "given the phenotypic instability and potential for undifferentiated donor cell expansion".

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Michael Cook

Michael Cook edits BioEdge, a bioethics newsletter, and MercatorNet, an on-line magazine whose focus is human dignity. He writes from Sydney, Australia.

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