After lengthy periods of cultivation, embryonic stem cells accumulate genetic changes which can provoke human cancers, according to an article in the latest issue of Nature Genetics. This contradicts previous research and could be a major set-back for the cause of therapeutic cloning.

It seems that there must be something in the cell culture that turns on some genetic pathways, which in the body would normally be modulated,” says Aravinda Chakravarti, of Johns Hopkins University School of Medicine. “We don’t know why stem cells are so sensitive to mutations in vitro, or even if it is just stem cells that show such mutations over time, but certainly the medium in which they are cultured needs to be investigated,” he told New Scientist.

The researchers found serious genetic alterations in eight of the nine cell lines they cultured, including large DNA deletions and amplifications in certain regions of the genome over time. Dr Chakravarti suggests that stem cell lines should be transported quickly if they are to be used for cures. However, this may not be possible, as scientists envisage differentiating them into different cell types, which could be a lengthy process.