Breast cancer develops from normal cells within the breast that undergo genetic changes that cause them to become tumorigenic. A number of these changes are detectable by monitoring specific protein markers that are found specifically in breast cancer tissue and not in normal tissue or blood. It is important to determine which markers are present to direct a proper treatment course for this deadly disease.
Estrogen and Progesterone Receptors
Breast cancer tissue is routinely tested for levels of receptors for the hormones estrogen and progesterone. These hormones act as fuel for breast cancer cells and serve as drug targets for chemotherapy. The National Institutes of Health explain that breast cancers that contain the estrogen receptor are referred to as "ER positive" while progesterone-receptor containing tumors are called "PR positive." Tumors that contain these receptors tend to grow at a slower rate than tumors lacking such receptors. Tumors that have these receptors are also more susceptible to drugs that target ER and PR systems.
HER2
According to the Mayo Clinic, the HER2 gene is found in breast cancer tissue and signals the growth of breast cancer cells. HER2 levels are elevated in about 20 percent of breast cancers. This tumor marker is found in the breast tissue itself rather than in the blood. Breast cancers that express HER2 usually grow more rapidly than non-HER2 tumors. Once breast cancer is diagnosed, it is important to determine HER2 status as many therapies for breast cancer target the HER2 receptor.
CA 15-3 (Mucin-1, MUC1)
CA 15-3, also known as MUC1, is a glycoprotein expressed on the surface of many cancer cells, including breast cancer. According to the National Institutes of Health and the American Cancer Society, this marker is increased in the blood of advanced breast cancer patients as tumor cells travel through the blood stream to metastasize. Levels of CA 15-3 tend to decrease during successful therapy and are used to determine whether or not a treatment course is effective.
Carcinoembryonic antigen (CEA)
The American Cancer Society explains that CEA levels also increase during advanced breast cancer and this can be used to monitor treatment course.
