Stem Cells That Kill

Imagine a cell nestled happily in the human body and enjoying the best of all possible worlds. It is endowed with immortality, the remarkable ability to divide indefinitely. Each time it cleaves, it makes two daughter cells with different fates. One divides again and again and again, spawning hundreds of copies of itself before exhausting its powers of duplication and dying out. The other progeny is a bit more cunning, inheriting from its parent the gift of never-ending life. That cell resists the temptation to multiply and march to an inevitable death, choosing instead to divide only occasionally and, by doing so, live forever.

It is that cell that is suddenly sparking the interest of cancer researchers and molecular biologists around the world. Known as a cancer stem cell, it could be the culprit behind a malignant tumor's nasty habit of recurring year after year and popping up in distant parts of the body long after the primary growth is gone. Studies of that cell are helping scientists unravel some of cancer's deepest secrets and leading doctors closer to the ultimate goal of any cancer therapy--a cure. Think of the stem cell as a tumor's master print; as long as the original exists, copies can be made, and the disease can persist. But destroy the tumor at its source, and the abnormal cells can't survive. "This represents a conceptual revolution in cancer biology," says Dr. Robert Weinberg, a cancer-research pioneer at the Whitehead Institute in Cambridge, Mass. "This is going to explain the way a wide variety of human cancers originate and the way they grow." Says Dr. Jean-Pierre Issa, a leukemia researcher at MD Anderson Cancer Center in Houston, Texas: "If we are able to eradicate the cancer stem cell, we will be able to cure patients."

Those ideas are already changing the way doctors think about cancer. They are starting to set aside their decades-old obsession with reducing the bulk of a cancerous growth and appreciate instead that the vast majority of its mass is cellular noise, a distraction from the tiny percentage of cells--perhaps as few as 3% to 5%--that are the real culprits. At the latest meeting of the American Association for Cancer Research, researchers at City of Hope Cancer Center in Duarte, Calif., announced that they had isolated a group of stem cell--like cells in lungs that seed the abnormal growth of small-cell lung cancers. Scientists at Stanford University took the concept even further. They were able to isolate stem cells from breast-cancer tumors and identify a genetic signature that allowed them to predict the progression of the disease. "Everybody wants to talk about cancer stem cells now," says John Dick, a University of Toronto professor and one of the leading researchers in the field. "From funding agencies to institutions to scientists, people are recognizing that this is probably the game to be in."