For all their promise, stem cells harbor a dark side: Tumors
It started with a worst-case scenario. An Israeli boy was born with a rare disease called ataxia telangiectasia, which consumes parts of the brain and can cause paralysis. Anxious to halt the disease before the damage potentially turned lethal, the boy's family sent him to a clinic in Moscow that gave him injections of neural stem cells from fetuses — once when he was 9 and then again when he was 10 and 12.
A year after the boy's final treatment, disaster struck. He began complaining of headaches, and scans revealed that he had developed two nonmalignant tumors, one on his brain stem and the other on his spinal cord. Although he eventually underwent successful surgery to have the spinal-cord tumor removed, the case forced experts to reexamine the inherent risks of stem-cell treatment, particularly the tendency of transplanted stem cells to divide out of control. In a report on the Moscow debacle, Ninette Amariglio of the Sheba Medical Center in Israel wrote that mixing stem cells from multiple fetuses with growth-promoting compounds "may have created a high-risk situation where abnormal growth of more than one cell occurred."
Other kinds of stem cells could have similar effects in humans if they are not properly directed to evolve into distinct tissue types, warns Jack Kessler of the Feinberg Neuroscience Institute. "If you take a human embryonic stem cell and transplant it into a person without the cell being differentiated, it will cause a tumor, period. We have to remove all possibility of that happening."
Making sure tumors don't develop in stem-cell recipients is a tall order, requiring researchers to finely control the growth of the transplanted stem cells. Too little control, and the cells divide willy-nilly; too much, and they lose their regenerative capacity. Researchers have recently learned more about the biological factors that control stem-cell proliferation — last year, Jurgen Knoblich of the Institute of Molecular Biotechnology isolated several cell proteins that can regulate or halt cell division. Still, the difficulty of striking the elusive cellular balance between stability and clinical effectiveness helps explain why we haven't seen more stem-cell therapies in human trials yet. "I know there's a lot of public pressure to demonstrate that stem-cell science is producing something," says Marius Wernig, who directs a laboratory at the Stanford University School of Medicine, "but things can really backfire if we proceed too quickly."single page