Human ES Cells Evolve in Culture

To identify such candidate genes, Andrews and his colleagues organized a worldwide collaborative effort to compare the genomes of as many hESC cell lines as possible. “Individual studies are generally not big enough to have enough power,” said Louise Laurent of the University of California, San Diego, School of Medicine, who was not involved in the study. The new study, she said, “is an example of what the ISCI is good at—bringing together a lot of collaborators and putting together a large-scale project.”

In total, the genomes of 125 independent hESC lines from 38 laboratories in 19 different countries were analyzed. A number of commonly amplified and deleted regions were identified, but only one—an amplification on the long arm of chromosome 20—occurred with sufficient frequency and was of a small enough size to identify a single candidate gene.

The gene, BCL2L1, encodes a protein that inhibits cell death. Cell lines with the amplified BCL2L1 region had a growth advantage in culture, as did cells in which BCL2L1 itself was artificially overexpressed—suggesting BCL2L1 is under selective pressure in hESCs in culture.

Such genetic changes to hESCs might appear to put the kibosh on plans for their use in patients. However, Andrews insisted that the finding was “not a disaster.” For one thing, the amplification of BCL2L1, like other changes, was more common in hESC that had been cultured for a long time, indicating that younger cultured cells may contain fewer genetic differences. Furthermore, the majority of the cell lines analyzed remained at least karyotypically normal—that is, they did not have large-scale changes.

“What it says from a safety and application point of view is that we can grow cells and we can expect them to remain genetically normal, but we have to monitor them,” Andrews said. “Because if you grow the cells for sufficient time there is a likelihood that they will acquire changes, and that these changes will be non-random.” Should an hESC cell line develop the BCL2L1 region amplification, for example, it might be time to discard them. “It is a phenomenon that will happen, but it is manageable,” he said.

Laurent’s view was equally tempered. “These sorts of studies are really valuable because people need to go into potential stem cell therapies with their eyes open,” she said. “However, the caveat is that for none of these [abnormal genetic sites] has a direct functional role been shown. It’s going to take a lot more work before we can definitively say whether they are a bad sign or not.”

The International Stem Cell Initiative, “Screening ethnically diverse human embryonic stem cells identifies a chromosome 20 minimal amplicon conferring growth advantage,” Nature Biotechnology, doi:10.1038/nbt.2051, 2011.