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Evidence of Existence?

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Cancer researchers have theorized that a small population of cells, dubbed cancer stem cells, drives tumor growth. Recent studies have made the theory more plausible, and now, says Nature News' Monya Baker, three new studies track what may be cancer stem cells forming tumors in the brain, the skin, and the gut. "Their results support the ideas that a small subset of cells drives tumor growth and that curing cancer may require those cells to be eliminated," Baker adds. The University of Texas Southwestern Medical Center's Luis Parada, who led the glioblastoma study published in Nature, tells Baker that if these findings hold up, it may change the way therapeutics are developed and evaluated for efficacy. "Instead of testing whether a therapy shrinks a tumor, for instance, researchers would assess whether it kills the right sorts of cell," Baker says.

Until now, those skeptical of the cancer stem cell theory have pointed out that the only way to find and observe these cells is to sort them out from other cells in cancer biopsies, a process that may change the behavior of the cells, Baker says. But all three new studies address this issue. Parada's team used a genetic marker to label glioblastoma cancer stem cells, and at the Hubrecht Institute in Utrecht, Hans Clevers and his team — whose study is published in Science — engineered mice to carry a gene that would make cause cells fluoresce into one of four colors when they come in contact with different tumor cell types. And for the skin study, also published in Nature, Cédric Blanpain and his group at the Free University of Brussels labeled individual tumor cells, without specifically targeting cancer stem cells.

Parada's experiment showed that unlabeled cells originated from labeled predecessors and that unlabeled cells could be killed with chemotherapy, after which the tumors returned, Baker says, while Clevers' project — which ended up with many single-color tumors — suggested that each tumor arose from a single cell type. The skin study, meanwhile, showed two patterns of cell division and pointed to a subtype of cells that consistently produced new cells, she adds.

"The next step, the three groups say, is figuring out how the cells tracked in these experiments relate to putative cancer stem cells identified by years of transplantation studies," Baker says. "Researchers are already busy hunting for ways to kill these cells; now they have more tools to tell whether such a strategy will work."

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