Breast cancer is responsible for more than 40,000 deaths each year in the United States. Extensive research has focused on breast cancer treatment and significant advances have been made. However, metastatic breast cancer (breast cancer that spreads to other parts of the body) is still an incurable disease.

In their constant effort to find new treatment options for breast cancer, scientists often look to the cellular level to help them understand how cancer behaves in the body. At one time it was felt that virtually all cancer cells in a tumor had a more or less equal chance of rapidly dividing, spreading and producing new tumors elsewhere in the patient. Recent studies, however, have suggested that only a small number of cells actually do this. There are two possible explanations:

  1. Every cell within a tumor has the ability to produce new tumors but the probability of an individual cell doings so is small.
  2. Only a rare, distinct subset of cells has the ability to form new tumors, but this type of cell does so very efficiently.

In a study published in the Proceedings of the National Academy of Sciences , researchers found the second explanation to be correct. According to their research, solid tumors (like breast cancer) contain a distinct population of cells with an exclusive ability to form tumors in mice. In addition, they were able to identify exactly which cells these were by unique markings on their surfaces.

About the study

Scientists at the University of Michigan Medical School injected human breast tumor cells into immuno-compromised mice (mice whose immune systems were impaired or weakened). The breast cancer specimens were taken from nine breast cancer patients. Eight of the samples were drawn from metastatic tumors (those that had already spread to other parts of the body) and one from a primary breast tumor (cells from the original cancer in the breast).

The mice were studied for signs of tumor growth every week for six months. All nine samples led to the growth of tumors in the mice. Once a tumor had grown, tumor cells were harvested and injected into another mouse—a procedure called a passage. Using various techniques, researchers isolated the cancer cells that could initiate a tumor in the passage procedure from those cells that could not.

The findings

Once they identified which cells were the tumor-initiating cells based on their surface markers, the researchers realized as few as 100 of the tumor-initiating cells were able to form tumors in the mice, while tens of thousands of cells from the original tumor did not. In fact, these cells were still initiating tumors after two passages.

Interestingly, the tumor-initiating cells gave rise to both new tumor-initiating cells as well as non-tumor-initiating cancer cells. This suggests they may be like “cancer stem cells,” possessing the ability to make copies of themselves, as well as produce other types of cells.

Although these results are interesting, there are limitations to this study. First, the study was conducted on mice. Extrapolating animal-study derived data to human populations is often difficult and not a straightforward process. In addition, the sample size—nine patients—is small.

How does this affect you?

If future research confirms the findings of this study, the impact on cancer treatment could be enormous. Once researchers can reliably identify which cells in a tumor are the most dangerous, not only can they gain a better understanding of how these cells wreak their havoc, but they can develop new therapies that target this specific population of cancer cells.

Tumor-inducing cells have already been identified in blood cancers, like acute myeloid leukemia. This study lends evidence to the idea that other cancers may have identifiable tumor-inducing cells as well. If other research proves this to be true, it could be a promising lead toward an eventual cure for cancer.