WASHINGTON, Oct. 12 (UPI) -- A study in mice indicates that contrary to previous thinking, a type of adult stem cell derived from bone does not give rise to new heart and brain cells when transplanted into the body.
The finding that this type of stem cell might not be as medically beneficial as researchers had hoped likely will have a significant impact on the political debate over embryonic and adult stem cells.
Embryonic stem cells have a proven capacity to regenerate damaged tissue and offer the potential of treating diseases, such as Parkinson's and diabetes. Some groups, however, oppose the use of the cells because obtaining them involves destroying a human embryo.
These groups have called for a ban on the use of human embryonic stem cells and have argued adult stem cells, which can be obtained from various tissues in the body without harming a person, offer the same regenerative potential as the embryonic cells without the moral complications.
Previous studies, however, have hinted adult stem cells actually do not give rise to new cells when transplanted into the body, but merely fuse with existing cells. Based on this, many stem cell scientists had come to accept the idea the cells did not offer the same regenerative capacity as embryonic stem cells.
The new study confirms the previous observations of fusion and "draws into question" the idea the adult stem cells could be used in humans to repair damaged heart and brain cells, principal investigator Arturo Alvarez-Buylla and his co-authors concluded in their paper, which the journal Nature published online Sunday.
The study "casts great doubt on the idea that adult stem cells could be the equivalent of embryonic stem cells," Dr. David Scadden, director of the center for regenerative medicine at Massachusetts General Hospital, told United Press International. "That concept is generally, by and large, no longer accepted."
Dr. Robert Lanza, vice president of medical and scientific development at Advanced Cell Technology, of Worcester, Mass., which is developing medical treatments based on stem cells, said the "study should serve as a warning."
The research provides "the best evidence yet as to why we shouldn't abandon embryonic stem cell research," Lanza told UPI. "It would be foolish and premature for anyone to conclude that adult stem cells are just as good as embryonic stem cells -- and certainly that potential is going to vary from disease to disease, and from situation to situation."
Alvarez-Buylla, a professor of neurological surgery at the University of California in San Francisco, told UPI that although the findings dash hopes for the regenerative capacity of adult stem cells, they suggest the cells still might help repair damaged tissues in the body in other ways.
When the stem cells fused with a brain or heart cell in the mice, they took on the characteristics of the host cell. This suggests the brain and heart cells somehow reprogram certain genes in the stem cells, which could be a physiological mechanism used by the cells to incorporate components of foreign cells to avoid death, he said.
"There has to be some reprogramming going on somewhere (that) opens up new possibilities for brain repair and heart repair," Alvarez-Buylla said.
Dr. Irv Weissman, director of the Stanford University Cancer/Stem Cell Institute and one of world's leading experts on adult stem cells, doubted whether the fusion phenomena would be useful therapeutically.
"The fusion events are so rare that it is hard to believe that they represent a physiological mechanism for tissue repair," Weissman told UPI. "While one could continue to try to look for a therapeutic lining, conservative analyses of this and other (studies) ... should give those that would hype this finding pause."
Under normal circumstances, adult stem cells derived from the bone marrow give rise to components of blood. Several studies indicated the cells, when transplanted into the body, repair damaged hearts and blood vessels but it remained unclear whether the cells were transforming into new cell types.
To help clarify whether the adult stem cells were capable of regenerating damaged tissues, an international team of researchers led by Alvarez-Buylla harvested the cells and transplanted them into healthy mice that had their natural supply of the stem cells destroyed.
The researchers found no evidence the stem cells formed new cell types, Alvarez-Buylla said. Instead, the cells fused with existing heart, brain and liver cells, forming cells with two nuclei that appeared to function normally.
Fred Gage, a biologist at the Salk Institute for Biological Studies in San Diego, said the study confirms earlier work that detected fusion and suggests any "therapeutic benefit (of adult stem cells) will not be as a result of transdifferentiation" into new cell types.
Lanza noted the study "certainly doesn't rule out the use of adult stem cells to cure human disease (because) there is ample scientific evidence that adult stem cells can be used to repair damaged heart or brain tissue."
The benefit might not arise from the stem cells transforming into different cell types but "if it works, it works, regardless of the mechanism," he said.
Several groups that object to the use of embryonic stem cells were contacted by UPI to comment on this story but they did not respond.