Her-2/neu, also called Human Epidermal Growth Factor Receptor-2, is a cellular receptor. A receptor is a protein that resides in the cell membrane itself. It actually spans the membrane and protrudes into the cell (called the intracellular domain) on one side and out of the cell on the other side (the extracellular domain). A variety of proteins found in the blood pass by the receptor, some may recognize it, and a smaller fraction will bind to it. Binding to the extracellular domain of the transmembrane receptor initiates a variety of signals within the cell including those commanding cell division, growth, and differentiation. These signals are only transmitted, however, once a protein has bound to its complementary receptor on the cell surface. The protein in the blood stream that binds and activates the Her-2/neu receptor is called Human Epidermal Growth Factor. When binding occurs, specific signals such as cell division, growth, transformation and differentiation, are transmitted from the extracellular domain inside the cell. This is part of a normal physiologic process. However, some cells can have too many Her-2/neu receptor proteins on their cell surface, and thus may receive too many signals. The multiple signals may result in aberrant cell function and even cancer.
The receptor protein Her-2/neu is encoded by the DNA found on the c-erb B-2 gene. Each cell is supposed to have two copies of a single gene (one inherited from each maternal and paternal inheritance). For unclear reasons, some cancer cells have multiple copies of the c-erb B-2 gene. This is known as gene amplification. Multiple copies of the gene lead to too many proteins being produced. This is known as protein overexpression. The significance of protein over production is still not entirely clear but it has become a marker for certain cancers. For example, it was noted that about one fifth to one third (20%-30%) of all breast cancers over produce this protein. Much research has been done to try to understand how these tumors may be different from other breast cancers. There are ongoing studies for Her-2/neu in disease prognosis, predicting response to treatment, and estimating disease recurrence. Data is somewhat confusing and sometimes contradictory, but overall, Her-2/neu positive breast cancers tend to be more aggressive in that they grow more quickly, may be resistant to some available chemotherapies, and are more likely to recur following treatment.
In order to diagnose Her-2/neu overexpression or c-erb B2 gene amplification, actual breast cancer tissue is required. When patients undergo a biopsy, the tissue is sent to the pathologist. In addition to histology (looking under the microscope to see what the cells actually look like), several additional tests are routinely completed. Testing for Her-2/neu overexpression has become the standard of care for breast cancer.
Immunohistochemistry (IHC) is the most widely available and least expensive test for Her-2/neu overexpression. The laboratory uses an antibody that recognizes the Her-2/neu cell surface receptor. These antibodies are specifically manufactured so that one end recognizes and binds to the receptor protein and the other end is labeled with a color or a dye that is easily detected under the microscope. The antibody is allowed to mix and bind with the cells for some time. The bound antibody will stay adherent to the tissue sample, but the unbound or excess antibody will be washed off. The pathologist can then look under the microscope and evaluate the intensity of the color or dye as an indication of the amount of Her-2/neu present on the cell surface. An IHC test is reported as a score of 0, 1+, 2+, and 3+. A score of 0 or 1+ implies very little bound antibody and therefore, likely very small amount of Her-2/neu. A score of 3+ is considered strongly positive with Her-2/neu overexpression. A score of 2+ is thought to be weakly positive, and, some studies indicate, may require further evaluation or testing. The IHC test is far from perfect as the quality varies from one laboratory to another. Different laboratories obtain antibodies from different manufactures and may have different affinities to the Her-2/neu. The IHC interpretation is also somewhat subjective as different labs have different reading systems in the criteria of what they call 2+ as opposed to 3+. Another reason for discrepancies in IHC reporting among reputable laboratories is tissue preparation. Cancer tissue preparation can affect the avidity with which the antibody may bind to Her-2/neu.
Fluorescence in situ hybridization, or FISH, is a more expensive and technically more difficult test that is also used to ascertain Her-2/neu overexpression and is often used to confirm a 2+ IHC score. The FISH test has the advantage of looking at each tumor cell individually. Probes tagged with fluorescent labels are used to identify the c-erb B2 genes themselves. If more than two fluorescent lights per cell are noted, then the cell is thought to over express Her-2/neu. Agreement among FISH laboratories is more reproducible; its disadvantage is that the expensive technology required is not often available in smaller clinics and rural hospitals.
In advanced disease, there may be Her-2/neu proteins in the blood. Blood tests may be done to try to discern the level of Her-2/neu. Rising levels may indicate disease progression, while a falling level may mean the opposite. Dr. Paul Zhan, an attending pathologist at the University of Pennsylvania, believes that could be a powerful office tool to follow disease progression; but the initial diagnosis should be made on biopsy proven tissue.
There are other tests used to diagnose Her 2/neu expression, but they are largely experimental. The PCR test, for example, has received much attention lately. Although it is considered to be a more sensitive test, Dr. Zhan notes that it is difficult to interpret. One needs to measure the Her-2/neu expression on the actual cancer tissue, not on adjacent normal tissue. When doing IHC or FISH, the pathologist can look under the microscope and see which cell is cancerous and which is not. The PCR test does not use whole cells, rather just the DNA from those cells. Thus, it is impossible to see if the DNA came from a cancer cell or a normal cell.
Her 2/neu overexpression is considered a poor prognostic factor in breast cancer. Overall, Her-2/neu strongly positive tumors tend to be a bit more aggressive and tend to recur following therapy. However, there is a new chemotherapy drug called trastuzumab, (Herceptin, Genentech), that was FDA approved (1998) for advanced stage Her-2/neu positive breast cancers only that is inappropriate for Her-2/neu negative tumors. For more details on Trastuzumab please see Her-2/neu Part II next week.
For more information, please see Endocrine therapy and chemotherapy for breast cancer and Her-2/neu overexpression: Science in Action: Part II.