Long-term culture of human embryonic stem (hES) cells can cause them to gain or lose large sections of chromosomes, report two papers in Nature Biotechnology. This instability can lessen the reproducibility and reliability of experimental results, and, by raising the specter of cancer, could hinder the clinical application of stem cells.

Checking cell lines in practice is not always easy, experimentally or logistically, says Anselme Perrier of The Institute for Stem Cell Therapy in Evry, France. He and his colleagues discovered that long-term culture of five hES cell lines resulted in a genomic amplification of the 20q.11.21 locus in four cases. “We discovered this mutation during routine quality control” says Perrier, “and it was happening too frequently for it to simply be an artefact”. Investigating the locus, the team found that it contained 23 genes, many of which have roles in proliferation and cell survival, hinting that this amplification may give cells a selective advantage and therefore become more prevalent over time.1 “We will have to check cell lines more stringently from now on, and be careful not to use hES cells that have had too many passages,” says Perrier, “we don't want cells that are one step closer to cancer”.

In a complementary study, Dr Claudia Spits of Vrije Universiteit Brussel in Belgium examined 17 different hES cell lines with her colleagues and discovered the same amplification in five cases, along with a derivative of chromosome 18 in three cell lines and a number of trisomies and monosomies as well.2 The deletion of part of chromosome 18 led to rapid increase of cell growth, indicating that there may be a tumour supressor in that area. “It's still an early stage” says Spits, who intends to look further at chromosome 18. “The potentially oncogenic genes that lie in areas that are amplified or duplicated are not well characterised yet, but they have been found in a number of cancers.”

“These are both interesting studies,” says Martin Pera, Professor of Cell and Neurobiology at the University of Southern California. “They clearly show that recurring abnormalities occur, not infrequently, in a number of different cell lines. There must be something giving these cells a growth advantage” he added, noting that the amplified region of chromosome 20 has been found in tumours. Additionally, amplifications of arms or entire copies of chromosomes 12 and 17 have also been reported.3

“For therapeutic purposes, this is not good news,” says Spits. Some cell lines are difficult to differentiate into one cell type while others are easily inducible and this may have something to do with chromosome abnormalities, she adds. Perrier, Spits and Pera are all of the opinion that the research community may not be as aware of these abberations as they should be, but this is likely to change in the future, they say. “The challenge is going to be understanding the biological significance of any mutations,” says Pera. “I expect we'll discover more abberations in the future so we need efficient tests to identify common changes and then we need to know which mutations are biologically important”. While stressing that increased monitoring will certainly be required within the hES cell research community, particularly for any clinical applications, Perrier feels that chromosome instability won't affect hES research too much; “This is just another problem that we will need to deal with,” he says.

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