Stem cells, often lauded as future cures for numerous diseases, are immature "master" cells that can be programmed to become many kinds of tissue, but experts have known for a long time that stem cells harvested from embryos can turn cancerous and have been linked to leukaemias and breast cancer.
Two research teams have now found that stem cells from adults can also turn into cancer, which until now was not known. It has been widely assumed that adult stem cells, taken from bone marrow for example, do not form cancers.
A Spanish team at the Universidad Autónoma de Madrid, looked at human stem cells that had been extracted from fat tissue. The stem cells were grown for up to eight months and underwent between 90 and 140 divisions to multiply in number during this time. The researchers found that when they were transplanted into animals, the oldest stem cells formed cancers.
Danish researchers from the Institute of Cancer Biology suggest the reason these stem cells become cancerous is connected to them being replicated too many times. The Danish team found that cells that became cancerous started to make an enzyme, called telomerase, which enables them to continue dividing long after they would normally stop.
The researchers in Spain say this risk is at this stage only a theory, because the stem cells treatments currently being tested in humans uses cells that are only briefly grown outside the body, however, treatments often require vast numbers of stem cells that have to be grown from very small numbers. They suggest that stem cells stored for years in banks might not be safe to use.
Antonio Bernad and colleagues suggest that the cut-off point should be around 60 divisions or generations and recommend more research be done.