Stem Cells Might Cause Cancer

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Like a plate of poisoned cookies from Grandma, cancer could be coming from an unlikely place. Increasingly, some research is pointing to stem cells, usually thought of as a promising disease cure-all.

The term "stem cells" covers any cells capable of perpetually growing more of themselves. Most often, people refer to pluripotent, or embryonic, stem cells, which have the ability to become any cell in the body. But there are also adult stem cells, which are more limited in the cells they can create.

Now, according to some researchers, there are also tumor stem cells.

"They aren't the same thing as regular stem cells," said Dr. Allan Mufson, chief of Cancer Immunology/Hematology Branch Division of Cancer Biology at the National Cancer Institute. "But there seems to be a small population of cells within tumors that are responsible for keeping the tumor going. They're the only cells that can give rise to new tumors."

According to the tumor stem cell theory, the problem with common cancer treatments (chemotherapy and radiation) is that they focus on the whole tumor, when it's only the rare tumor stem cells that really matter. Doctors tend to use large doses of potentially deadly medications that weaken patients and aren't specifically aimed at killing tumor stem cells, increasing the risk that they'll be missed and the cancer will grow back.

This new theory of cancer is being studied, but questions still remain. One of the big ones: Where do tumor stem cells come from? Dr. John Kersey, a researcher at the University of Minnesota's Masonic Cancer Center, thinks he's found an answer: Tumor stem cells may be damaged versions of normal, potentially life-saving adult stem cells.

Adult stem cells come in two flavors: The highly specialized progenitor stem cells and an immature variety that are more flexible. For instance, a progenitor cell might only be able to grow white blood cells, while the immature adult stem cell could grow several different cells in the circulatory system. Researchers are still debating which type becomes a tumor stem cell, but Kersey's findings, detailed in the May issue of the journal Cancer Cell, suggest it is the immature adult stem cells, at least for certain types of leukemia.

To figure that out, Kersey and his team grew mice whose adult stem cells, both progenitor and immature, contained a gene that causes leukemia. Both types were then separated out and injected into healthy mice. The mice who got progenitor cells didn't get leukemia. The ones who got the immature adult stem cells did.

While this doesn't completely prove stem cells are the culprit behind leukemia, it does go a long way towards showing that low doses of cancer genes can transform a stem cell from something that creates life to something that takes it, Mufson said. Cancer researchers say that figuring out where tumor stem cells come from is the first step in turning them back into useful tools for health. If damaged stem cells really are the building blocks behind tumors, doctors might be able to figure out a way to target those cells, or even just a part of them, leading to safer and more effective cancer treatments.

"This is exciting stuff," Kersey said. "Understanding this will be essential to developing specific treatments for specific cancers, based on the part of the cancer that's actually growing."

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