In May 2001, Israeli parents of a nine-year old boy with a crippling disease that left him wheelchair-bound took their child to see doctors in Moscow. In a highly experimental procedure that was presumably unavailable in their home country, those doctors injected fetal stem cells into various regions of his brain.

The boy’s parents—they aren’t named in a report describing the case in this week’s PLoS Medicine—must have been desperate. The nine-year old suffered from ataxia-telangiectasia, a childhood disease that causes degeneration of parts of the brain that control muscle movements and speech. The symptoms include slurred speech, poor balance, impaired immune function, and the appearance of red spider veins called telangiectasias in the eyes, ears or cheeks.

There are no treatments for the disorder and the prognosis is dim; patients usually only make it into their teens or early twenties, according to the National Institute of Neurological Disorders and Stroke. While it's unclear exactly what the Russian doctors were trying to achieve (the researchers who wrote the case report were not involved in the stem cell therapy), they must have been hoping that the injected cells would restore some function in his brain, or at least slow the disease progression. The boy went back for injections in 2002 and 2004, although it's not clear from the report whether his condition improved as a result.

Then he was diagnosed with a brain tumor in 2005. That tumor, it turns out, grew out of the stem cells, obtained from at least two aborted fetuses, used in his brain.

The tumor was benign, doctors safely removed it, and it has gradually been growing back since the surgery. But this is the first-known case of a brain tumor caused by a brain stem cell therapy, according to the report—a phenomenon scientists have predicted in the pages of Scientific American and elsewhere. The theory is that because these stem cells are fetal cells, they are designed to proliferate and give rise to new tissue, which means they have the potential to produce tumors. The case, write the authors of this week’s case study, should serve as a warning that more research is needed to gauge the safety of these novel therapies.

Other stem cell experts echo their concerns and worry that scientists don't yet understand exactly how stem cells used in such treatments behave once inside the body. Treating neurological disorders with stem cells from fetal brains is a "great scientific goal to pursue," but there is simply not enough evidence from animal studies, let alone human studies, to prove it is safe or effective for treating these diseases in children, says Sean Savitz, a neurologist at the University of Texas Medical School at Houston.

Savitz has just begun enrolling patients in a study on treating adult stroke victims with their own—adult—stem cells. The intent of the boy's treatment must have been to use these fetal stem cells to regenerate tissue lost in certain areas of the brain, Savitz speculates, but he adds, "we don't have a full understanding of how [brainlike] stem cells can generate different cells in the brain."

Savitz says that the stem cells used in his trial are not likely to cause cancer because they are adult cells taken from bone marrow that die once they have accomplished their mission of repairing brain tissue. In their study, Savitz and his colleagues will remove cells from the bone marrow of patients 24 to 72 hours after they suffer a stroke, isolate hundreds of millions of stem cells from that marrow and then re-inject the stem cells into the bloodstream.

Once inside the body, the stem cells will migrate to the brain and promote new blood vessel growth, reduce inflammation, and rescue neurons at risk of dying, Savitz hopes. And once they have done their job, they will basically commit suicide—unlike fetal neural cells, which tend to set up camp and proliferate, setting the stage for possible tumor formation, he explains.

"The best case scenario is that we determine that there are no serious adverse events associated with this approach," Savitz says cautiously. Although a small trial like this could (and hopefully will) yield benefits to the patients involved, Savitz notes, establishing safety is the objective.

For more on the potential stem cell–cancer link, see Stem Cells: The Real Culprits in Cancer?, Stem Cells Cause Cancer, and The World's First Neural Stem Cell Transplant.

Image: Spider veins, or "telangiectasias" in the eyes of a patient with ataxia-telangiectasia. ©National Eye Institute through Wikimedia Commons