Testing for HER2 Overexpression in Breast Cancer

Testing for HER2 overexpression in breast cancer can affect treatment decisions.

Because HER2 status is important in understanding how aggressive a tumor might be—and how best to treat it—testing should be performed on the biopsy or surgery sample of all newly diagnosed invasive breast cancers and at the time of a recurrence, according to expert recommendations.

The Food and Drug Administration has approved two testing methods, known as IHC and FISH, to determine a breast cancer’s HER2 status. A patient’s tumor sample might be tested by either or both methods.

IHC, or immunohistochemistry, applies special antibodies that bind to the HER2 protein, along with a chemical detection method that stains the protein on the tumor cells. That staining can be evaluated under a microscope. An IHC score of 3+ (meaning intense, uniform staining of more than 30 percent of the invasive tumor cells’ membranes) indicates that a patient’s tumor is HER2-positive, while scores of 0 or 1+ designate the tumor is HER2-negative. A score of 2+, meanwhile, is considered borderline or “equivocal,” meaning that further testing should be conducted using the FISH method.

FISH, or fluorescent in situ hybridization, allows researchers using a special microscope to count the copies of the HER2 gene in tumor cells by flagging them with fluorescent pieces of genetic material that attach to the area of the chromosome that contains the HER2 gene. Using FISH, a tumor is deemed HER2-positive if more than six copies of the HER2 gene are detected per cell, or if more than 2.2 HER2 genes are counted for every copy of chromosome 17 (also known as the HER2 chromosome enumeration probe 17 [CEP17] ratio). A tumor is negative if less than four copies of the HER2 gene are counted per cell, or if the HER2/CEP17 ratio is less than 1.8. The result is borderline if the FISH count totals between four and six copies of the HER2 gene per cell, or between 1.8 and 2.2 HER2 genes per copy of chromosome 17. Borderline results should be followed up with counts performed in additional cells, by retesting using the FISH method or by testing with the IHC method, experts note.

Scientists have begun to explore whether a broader group of patients than those currently defined as HER2-positive could see gains from the targeted drugs.

To help ensure accuracy of test results, patients should ask their physicians whether the laboratory that performed the test is accredited according to American Society of Clinical Oncology/College of American Pathologists guidelines for testing.

Although the guidelines clearly define what is considered a HER2-positive versus a HER2- negative tumor, gray areas remain. Scientists have begun to explore whether a broader group of patients than those currently defined as HER2-positive could see gains from the targeted drugs.

For instance, a re-analysis of some tumor samples from one study of HER2-targeted therapy found that a portion of the tumors deemed HER2-positive were actually negative and that patients with these early-stage tumors seemed nevertheless to benefit from HER2-targeted therapy plus chemotherapy.

Another analysis of a previously conducted study found that women whose metastatic tumors were HER2-negative—but who had extra copies of chromosome 17—appeared to respond better to a combination of HER2- targeted therapy and chemotherapy than to chemotherapy alone. Other research is further identifying genetic and biochemical nuances that may better clarify when treatments that target HER2 will be most effective.

From "A Patient's Guide to HER2-Positive Breast Cancer," published in the Summer 2009 issue of CURE. Download the full guide here.