The researchers used two Affymetrix GeneChip microarrays to scan the stem cell genome at a level of detail never before possible. They found previously undetectable mutations in stem cells that had been grown in the laboratory for dozens of generations.
Mutations occur all the time as cells grow, but inside the body, mutated cells with harmful effects are often cleared out by the immune system, whereas in the laboratory culture dish, where there is no immune system, mutated cells can grow uncontrolled like a cancer.
Over time, any cell line grown in the laboratory, including stem cells, can develop mutations that are compounded over many generations. The danger is that scientists may think they are studying stem cells identical to those present in the human body, but in reality the laboratory-grown cells have mutated, and any discoveries may not accurately reflect what is happening in people.
The cell lines discovered as mutant in this study are part of the limited number of existing human embryonic stem cell lines (hESC) approved by President Bush for federally-funded research in August 2001. At the time, scientists estimated that more than 60 genetically distinct stem cell lines existed, but today only 22 are available for purchase in federally-supported research.