Stem cells have been the subject of worldwide scrutiny for years, yet they remain a puzzle. Although they carry the same DNA as regular body cells, they have a uniquely flexible identity that lets them develop into a wide variety of different tissues.
A team led by Joseph Ecker of the Salk Institute for Biological Studies in California recently made significant progress in discerning how stem cells keep their options open. He compared embryonic stem cells with cells from the lung called fibroblasts by analyzing their epigenetics—chemical changes that affect how genes behave without altering their DNA code. One major difference was in methylation, the way in which chemical structures called methyl groups cling to the rungs of DNA. In body cells, 99.98 percent of methylation occurs where the DNA sequence has a molecule of cytosine (C) adjacent to a guanine (G). But in stem cells, nearly a quarter of the methylation falls outside these areas.
The researchers also examined induced pluripotent stem cells, body cells that have been reprogrammed with the ability to grow into multiple kinds of tissue. The induced cells showed a pattern of methylation similar to that of embryonic stem cells, suggesting that the arrangement of methyl groups on DNA may be a key feature that determines the range of a cell’s possible future forms. Ecker hopes the epigenetic map will help researchers begin to piece together whether induced stem cells are truly as versatile as embryonic ones. “It provides a tool to begin to differentiate between these types of cells,” he says.