Is HER2 testing accurately identifying breast cancer patients who would benefit from targeted therapy?
What is HER2?
HER2 (also Her-2/neu or ErbB2) stands for Human Epidermal Growth Factor Receptor 2, and is one of a family of several receptor proteins. All normal breast cells contain the HER2 gene, which gives instructions to the cell on how to make the HER2 protein. The HER2 protein, also called a HER2 receptor, helps to send signals to the inside of the cell telling it to grow and divide. Breast cancer cells that are HER2-positive have extra copies of the HER2 gene and produce extra HER2 receptor proteins.
In HER2-postive breast cancer, there are extra copies of the HER2 gene (amplification) within the cells, leading to extra HER2 protein (overexpression) on the surface of the cells. Too much HER2 protein is thought to cause cancer cells to grow and divide more quickly. Overexpression of HER2 is found in 18-20 percent of all breast tumors (Seidenfeld 2008).
Trastuzumab (Herceptin®) and lapatinib (Tykerb®) specifically target the HER2 receptor. Accurate testing of HER2 is therefore important- for not only correctly identifying patients who would benefit from these drugs, but also identifying those who would not, and preventing unnecessary exposure to drugs that can have serious side effects.
The best way to test for HER2 has long been an issue of debate and controversy. At the heart of the debate is the issue of which specific testing method is better for the prediction of response to treatment. The two most commonly used methods of testing include:
immunohistochemistry (IHC), which looks for abnormalities in protein (receptor) overexpression, and
fluorescence in situ hybridization (FISH), which looks for abnormalities in gene amplification.
Gene amplification leads to high levels of protein (receptor) expression. In theory, therefore, the two testing methods are directed at different stages of the very same process.
One of the biggest problems encountered is a poor agreement between HER2 test results from local laboratories and those from large-volume central laboratories whether using either IHC or FISH methods. Several studies have confirmed these findings showing that a significant number of patients found to be strongly positive for HER2 in local laboratories were not confirmed as HER2+ when tested at a central laboratory (Paik 2002; Roche 2002).
IHC testing is relatively simple, does not require special equipment, and is the test usually performed in local laboratories. These tests can be completed relatively quickly and inexpensively. The slides that are examined under a microscope can be stored for long periods, so they can be reexamined if needed. However, testing procedures can vary widely from testing site to testing site. Another drawback is that the scoring system used to determine HER2 status is considered subjective. A pathologist looks for color change in the cells of a sample under a microscope. The pathologist determines the score for the test by judging the percentage and intensity of cells with color.
Unlike IHC testing, FISH requires specialized, expensive equipment which means it can’t be performed in all clinical laboratories. The advantage of FISH is a more objective scoring system compared to IHC. FISH testing is measuring the number of HER2 gene copies. The tumors are interpreted as “negative” or “positive” depending on the number of copies.
The Food and Drug Administration (FDA) approved two more tests recently, chromogenic in situ hybridization (CISH) and Inform Dual ISH, the latter of which was approved in June 2011. Similar to FISH, both of these tests measure the number of HER2 gene copies. However, CISH lacks some of the more sophisticated testing features of the FISH method. Because the technology is relatively new, there are some concerns about inexperience among those carrying out the test. However, CISH does not entail the specialized equipment required of FISH making it less expensive to administer. CISH is also generally quicker to administer. Like IHC, CISH samples can be stored for long periods. Also, as opposed to FISH, the recently approved Inform Dual ISH test allows lab staff to see the HER2 and chromosome 17 signals directly under a standard microscope and for longer periods of time.
The four types of tests—IHC, FISH, CISH, and Inform Dual ISH—are highly regarded for their ability to accurately identify abnormalities, but they are nonetheless subject to errors in specimen handling, testing procedures, and analyses.
Technical issues are only one part of the problem with HER2 testing overall. Gene amplification and its relationship to protein (receptor) levels are biologically complex. Debate continues over the meaning of test results in relation to disease and treatment outcomes.
At issue is the best strategy for results that are borderline or inconclusive. When results show that HER2 status is strongly negative or positive, clinical guidelines concerning the course of treatment are fairly certain, but when results are inconclusive the best course of treatment is not clear. In such cases the original test needs to be re-analyzed or repeated or a different test must be performed in an effort to obtain a more conclusive result.
Recognizing a need to address this issue, a National Comprehensive Cancer Network task force convened to make recommendations about the interpretation of test results (Carlson 2006). Similar guidelines were also published in the United Kingdom (Ellis 2004). But the development of guidelines has not resolved the issues. Debate continues over the testing methods and strategies, and the meaning of borderline results.