The progress of science is paved with stories of high hopes and heartbreaks. But in a busy lab at the University of Rochester the two extremes have met in one dazzling yet devastating experiment.
Researchers there have for the first time essentially cured rats of a Parkinson's-like disease using human embryonic stem cells. But 10 weeks into the trial, they discovered brain tumours had begun to grow in every animal treated.
"Here we have this method that works so well to reverse the symptoms of Parkinson's," said lead investigator Steven Goldman, "But no matter how you look at it, it's an expanding mass and that's bad news."
None of the cells growing out of control were cancerous tumours. But as Dr. Goldman pointed out, "In the brain, nothing's benign."
The work, published today in the journal of Nature Medicine, is a sobering setback for plans to use stem cells from human embryos to grow tissues for human transplant.
"My hopes are still high, but I think this injects real caution," said Dr. Goldman, who spent four years on the experiment and a 23-year career building up to it. "Some folks are portraying this as imminently useful and it's not. There's still a lot that has to be sorted out."
By definition, human embryonic stem cells have the almost mythical, immortal power to grow and divide indefinitely as they become the various tissues that make up the body. As a result, scientists have always known that any stem cell therapy could result in an uncontrolled growth of cells that could give rise to cancer.
But that risk has remained largely theoretical since there have been few attempts to transplant tissues grown from stem cells into live animals. The work is difficult, expensive and tricky to pull off when several countries -- Canada included -- have enacted tough laws that limit research on stem cells from human embryos.
"A lot of the representations of stem-cell research have resulted from the initial excitement and speculation of what can be achieved. But we're still in that early stage, we haven't seen real clinical breakthroughs," said Tim Caulfield, director of the Health Law Institute at the University of Alberta. "This [experiment] shows the incredible potential of the field, but it also sheds a more realistic light on the near-future potential."
For Mick Bhatia, scientific director of the Cancer and Stem Cell Research Institute at McMaster University, it's a bit of déjà vu. In 2004, he succeeded in growing human blood cells from embryonic stem cells but found transplanting them into mice wasn't simple.
"I pushed the program back," Dr. Bhatia said. "We need to do more on the basic biology."
Still, few scientific fields are hotter than stem cells as researchers everywhere investigate the possibility of using them to grow replacement parts -- cardiac cells for heart patients or islet cells for diabetics.
Parkinson's has been considered one of the prime candidates for a stem-cell therapy because just a single cell type is needed -- one that produces dopamine.
Neurons that make dopamine are crucial for movement and degenerate in people with Parkinson's, often leaving them stiff, unable to control their physical gestures and suffering from tremors.
Twenty years worth of studies have tried treating Parkinson's with dopamine-cell transplants, in rodents, primates and people. In the 1990s, after a Swedish study found some benefit to patients who received dopamine-cell transplants from aborted fetal tissue, large human trials began in both Canada and the United States.
"These failed," Dr. Goldman said. "It made things worse; patients suffered movement abnormalities." In part, he explained, this was because the transplants contained all sorts of cells. Less than 10 per cent of the cells, and in some cases, less than 1 per cent, produced dopamine.
