Scientists have long known that as human embryonic stem (ES) cells age, they develop genetic mutations that could limit their medical usefulness. Now a study has offered the first close-up look at just what happens over time to these cells. Scientists say the findings bolster the argument that federally funded researchers need access to fresh lines and should not have to rely just on those derived before 9 August 2001 (ScienceNOW, 10 August 2001:).
The age of a cell line is defined by its "passage," or the amount of time it takes for the cells to double, which may be anything from hours to a few days. An international team led by researchers at Johns Hopkins University in Baltimore, Maryland, compared early-passage and late-passage versions of nine human ES cells lines obtained from labs around the world. Using microarrays that enabled them to examine 100,000 sites on the genome, they looked at the cells' nuclear DNA, mitochondrial DNA, and methylation patterns (which give clues to whether a gene has been turned on or off).
The team found that most of the late-passage cell batches, which were up to 3 years older than the early versions, showed genetic and methylation changes, some of which resemble those seen in cancerous cells. Compared with the early-passage versions, five of the nine late-passage cell lines had either extra or fewer copies of at least one section of DNA. Two developed changes in their mitochondrial DNA sequences, and all nine exhibited a shift in methylation of at least one of three specific genes.
The researchers, who describe their findings online 4 September in Nature Genetics, admit that they don't know just how the changes affect cell function. But they are "potentially deleterious," says co-author Anirban Maitra, a pathologist at Johns Hopkins, and underline the need for close monitoring of stem cell cultures as they age.
Johns Hopkins stem cell research John Gearhart, who did not participate in the study, says the findings strengthen the case for making more lines available to researchers because it shows that "changes are occurring even under ideal [cell] culture conditions." Stem cell researcher Peter Andrews of the University of Sheffield in the United Kingdom adds that the next step is to find out what the genetic changes signify--for example, whether they affect the cells' ability to differentiate.