Cancer concern over stem cells

Stem cells can turn cancerous if they are grown for too long in the laboratory, two new studies show. A report in New Scientist highlights two articles which show that adult stem cells can become immortal, a key feature of cancer cells. The scientists stress that current trials using injections of bone marrow stem cells should be safe, since these cells are only briefly grown outside the body. However, they say that more research needs to be done to establish a safe time limit for growing stem cells destined to be used in therapies.

Many scientists believe that stem cells derived from very early human embryos - the body's 'master' cells capable of growing into any type of tissue - could lead to new treatments for a range of diseases. However, it has long been known from animal studies that such embryo stem cells (ES cells) have the potential to form cancers called teratomas when injected into the body.  Now, a team at the Autonomous University of Madrid in Spain has shown that adult stem cells could also form cancers, if they are grown outside the body for a long time.

The scientists grew human mesenchymal stem cells, extracted from fat cells, for up to eight months in the laboratory. During this time, the cells divided and multiplied between 90 and 140 times. When injected into animals, the cells  became cancerous, the team reported in the journal Cancer Research. However, current clinical trials using injections of bone marrow cells to treat heart muscle damage should be safe, since these stem cells are only grown outside the body for a short time.

Stem cells probably become cancerous when they switch on production of a protein called telomerase to repair the cells' telomeres - protective pieces of DNA at the ends chromosomes, which usually shorten every time a cell divides and copies its genetic information. This mechanism acts as a biological 'clock', limiting the number of times a cell can divide and multiply. However, telomerase overrides this clock, by restoring telomeres to their original length. In a second study,  also published in Cancer Research, a team of Danish researchers found that artificially switching on telomerase production in mesenchymal stem cells eventually turns them into immortal cancer cells.

The scientists, based at the University Hospital of Odense, say that although their experiment is an artificial situation, switching on telomerase is 'clearly sufficient for them to acquire the ability to become tumorigenic'. Team leader Jorge Burns says that 'we need to know where to draw the line between safe and unsafe expansion of cells even in non-telomerase-producing cells'. Chris Higgins, of the UK's Medical Research Council Clinical Science Centre, said that stem cell lines maintained and developed in stem cell banks are currently only used for research purposes. 'Stem cells can potentially be of enormous benefit for clinical treatment but as with the development of all drugs and therapies, there are safety issues that need to be investigated and resolved', he said.