Researchers have found a marker on head and neck tumor cells that indicates which cells are capable of fueling the cancer’s growth. The finding is the first evidence of cancer stem cells in head and neck tumors.
Cancer stem cells are the small number of cancer cells that replicate to drive tumor growth. Researchers believe current cancer treatments sometimes fail because they are not attacking the cancer stem cells. By identifying the stem cells, researchers can then develop drugs to target and kill these cells.
“Our treatment results for head and neck cancer are not as good as we’d like them to be. A lot of people still die of head and neck cancer. This finding will impact our understanding of head and neck cancer, and we hope it will lead to treatments that will be more effective,” says study author Mark Prince, M.D., assistant professor of otolaryngology at the University of Michigan Medical School and section chief of otolaryngology at the VA Ann Arbor Healthcare System.
Results of the study appear in the Jan. 16 issue of the Proceedings of the National Academy of Sciences.
Researchers at the U-M Comprehensive Cancer Center and Stanford University School of Medicine took tumor samples from patients undergoing surgery for head and neck squamous cell carcinoma, including cancers of the tongue, larynx, throat and sinus. Cells from the samples were separated based on whether they expressed a marker on their surface called CD44. The sorted cells were then implanted into immune-deficient mice to grow tumors.
The cells that expressed CD44 were able to grow new tumors, while the cells that did not express CD44 did not grow new tumors. The tumors that grew were found to be identical to the original tumors and to contain cells that expressed CD44 as well as cells that did not express the marker. This ability to both self-renew and produce different types of cells is a hallmark of stem cells.
Stem cells have been identified in several other cancer types, including breast, brain, central nervous system and colon cancers, as well as leukemia. U-M researchers in 2003 were the first to report the existence of stem cells in a solid tumor, finding them in breast cancer. CD44 was also found to play a role in breast cancer stem cells.
“We know CD44 is important in breast cancer and now in head and neck cancer, so it might be important in other cancer types. This work gives more evidence that the cancer stem cell theory is valid,” Prince says.
That theory suggests that a small subpopulation of cancer cells are the critical cells in cancer growth and progression, and the key to treating it is to kill the stem cells. It’s a radically different model than current treatment approaches, which are designed to shrink the tumor by killing as many cells as possible. Researchers suspect cancer recurs because the treatments are not killing the stem cells.
The current finding in head and neck tumors does not pinpoint the exact stem cells, the researchers believe, but rather narrows down the field. The percent of cells within a tumor expressing CD44 varied from one sample to the next, with one sample composed of as high as 40 percent of these cells. Studies in other cancer types have found the stem cell population to be smaller than 5 percent.
“The CD44-positive cells contain the tumorigenic cells, but we don’t think that’s a pure population of cancer stem cells. We still need to drill down further to find the subpopulation of those cells that is the pure version,” Prince says.