That raises the nightmare possibility of stem cell transplants that trigger tumors in patients. A February report in the PloS Medicine journal, for example, described the case if an Israeli child who received injections of "fetal neural stem cells" in a Russian clinic, triggering tumors.
At least four methods can make embryonic stem cell transplants safe for "regenerative medicine" if they are intrinsically cancer-tainted, Knoepfler suggests. The simplest way "may seem paradoxical: to not transplant stem cells at all into patients." Instead, in an approach that gained Food and Drug Administration approval for the first stem cell safety study in people in January, physicians differentiate the stem cells to the point that nearly every cell is a specialized organ tissue and then "sort" them to make sure no stem cells remain. "How few remaining stem cells are enough to cause concern about tumors? If the answer is 'zero,' then it may be difficult to achieve this goal because the reality is that the only pure cell population is that consisting of a single cell," he writes.
Alternately, stem cells transplant doctors could:
•Insert a "suicide" gene into stem cells that turns on after transplant and isn't passed on to their differentiated progeny.
•Use chemotherapy aimed at proteins found solely on the surface of stem cells, or genes that stems solely rely upon, an approach using the drug Rapamycin in a report in the April 28 Proceedings of the National Academy of Sciences journal.
•Treat the stem cells to remove their tumor-propensities before differentiating them into organ tissues. "Of course, at this time no such methodology exists and the notion of using stem cells directly for transplants would appear to be strongly out of favor with regulators," Knoepfler says.
All of these alternatives have problems, he adds, saying, "a much more open discussion and investigation of the tumorigenic nature of stem cells than has yet to occur, particularly that of (induced cells and human embryonic stem cells), will undoubtedly prove essential for the development of safe and effective regenerative medicine therapies."
Even cancer isn't the biggest question in stem cell science, says gene therapy researcher James Wilson of the University of Pennsylvania in the May 8 Science journal. In Obama's March announcement, "he took pains to temper Americans' hopes for quick fixes," Wilson writes. "Unfortunately, some stakeholders in (human embryonic stem cell) research have failed to exhibit the same restraint, effectively promising cures for Parkinson's disease, Alzheimer's disease, spinal cord injuries, diabetes, cancer, heart disease, multiple sclerosis, muscular dystrophy, macular degeneration, and hearing loss, to name a few."
Wilson has an interesting history, best known for the gene therapy experiment that killed 18-year-old Jesse Gelsinger in 1999, "in a trial that I led," he writes. Alongside success in treating inherited blindness and immune disorders, he notes that gene therapy has triggered cancer in patients, which has "derailed" the field. "It would be unfortunate if the field of (human embryonic stem cell) research missed this lesson from history and took a similar trajectory."