Laboratory Testing for HER2/neu in Breast Carcinoma: An Evolving Strategy to Predict Response to Targeted Therapy

Nils M. Diaz, MD, Interdisciplinary Oncology Program at the H. Lee Moffitt Cancer Center & Research Institute at the University of South Florida, Tampa, Florida. Submitted July 1, 2001; accepted August 9, 2001

Cancer Control. 2001;8(5) 

In This Article

Testing for HER2/neu

Because anti-HER2/neu therapy benefits only patients with invasive breast carcinomas overexpressing HER2/neu, testing is used to identify those patients most likely to respond to anti-HER2/neu therapies. The potential side effects of Herceptin[6] and the cost of therapy increase the importance of identifying HER2/ neu overexpression. Given the latter considerations and the fact that the majority of patients with carcinoma overexpressing HER2/neu do not benefit from trastuzumab, testing may also be conceptualized as a mode of selecting patients who lack HER2/neu overexpression and thus should not be treated with Herceptin.

HER2/neu status in breast carcinomas can be determined by testing for (1) gene amplification by Southern blot, polymerase chain reaction, or fluorescence in situ hybridization (FISH), (2) mRNA using Northern blot, or (3) protein overexpression via enzyme linked immunosorbent assay, Western blot on cytosols, or immunohistochemistry (IHC).

The membrane localization of the protein forms the basis of IHC, the most commonly used method of testing for HER2/neu overexpression. The first IHC assay to be approved by the FDA as a response indication for Herceptin therapy, the HercepTest (Dako Corp, Carpinteria, Calif), uses a polyclonal antibody.[7] The anti-body approval was based on good concordance of its ability to detect HER2/neu overexpression with the monoclonal clinical trials assay antibodies used for determining eligibility in the initial trastuzumab clinical trials. Subsequently Pathway (Ventana Medical Systems, Tucson, Ariz), a monoclonal preparation (CB11), has also been approved by the FDA for the same response indication.

FISH, the other test currently utilized in clinical practice to select patients for Herceptin use, and IHC have distinct advantages and disadvantages. IHC is performed in more clinical laboratories, and it is less expensive and less labor intensive than FISH. FISH requires fluorescence microscopes rather than the light micro-scope used for routine microscopic evaluation by pathologists (chromogenic reagents under study may obviate this issue regarding in situ hybridization). Both methods test routinely processed surgical specimens (formalin fixed, paraffin-embedded tissue) and allow for analysis within individual cells. Because FISH tests for the gene rather than the HER2/neu protein tested for by IHC, FISH does not have the potential problem of antigen loss associated with formalin fixation. Reliable IHC, based on quality assurance and concordance studies with standard positive and negative results that correlate with either clinical outcome or with another IHC assay with documented correlation with outcome data, are required for this quantitative assay. IHC testing for HER2/neu should not be done when assay requirements, such as fixative type, are not met. Parenthetically, negative IHC results on archival material should be interpreted with caution because the negative result may be associated with prolonged fixation or storage considerations.

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