Two new studies have cast further doubt on the ability of blood stem cells to turn into heart cells, even though several clinical trials based on this promising new treatment are currently underway. Researchers at Stanford University, California, and the University of Washington in Seattle have failed to duplicate the findings of work carried out three years ago. In 2001, scientists at New York Medical College in Valhalla, New York, reported that in mice, bone marrow stem cells injected into damaged heart muscle can grow into new heart cells. However, the new research, reported online in the journal Nature, suggests that blood stem cells can only produce new blood cells.
There is some evidence that adult stem cells present in bone marrow can produce other body tissues, as well as different types of blood cells. But it is not known exactly how this happens - it could be that the cells are fusing with existing cells, rather than transforming into them. Some scientists have warned that human trials using blood stem cells to treat damaged tissues should be postponed, until more is known about the processes involved. However, several preliminary human trials show that blood stem cells from bone marrow can successfully repair heart attack damage. Other studies have highlighted potential problems with this approach, and now it seems that the underlying science is once again being called into question.
The Stanford University scientists carried out 145 bone marrow cell transplants in normal and injured adult mouse hearts, and tracked their fate using 'reporter genes'. They found no evidence that any of the cells turned into heart cells, and say their results 'raise a cautionary note' for clinical trials using blood stem cells to treat heart muscle damage. Researchers at the University of Washington obtained similar results: 'We could not detect a single cell in the heart that had come from the injected cells' said team leader Charles E Murry.
Piero Anversa, who lead the team that carried out the 2001 study, says he stands by his original findings, adding that 'many clinical trials demonstrate beneficial effects in real patients'. It could be that the varying results are down to the different methods used by laboratories to purify the stem cells present in bone marrow, or to track their progress in the body. And Robert Robbins, leader of the Stanford team, says that blood stem cells could still help repair hearts, even if they do not actually turn into heart cells. 'Maybe these cells don't need to differentiate', he said, adding that the transplanted stem cells may help by recruiting new blood vessels to the damaged area. Researchers in Robbins' laboratory are also looking at other types of stem cell to use in heart treatments, including embryonic and muscle stem cells.