HER2 Part 4: How doctors determine if you're HER2 positive
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Remember what's happening in the cancer cells of patients with an overabundance of HER2: 1) there's amplification of the part of chromosome 17 containing the HER2 gene which creates more copies of the HER2 gene (DNA), 2) as a result there is an increase in the amount of HER2 mRNA produced, 3) the HER2 mRNAs make more HER2 proteins, 4) these HER2 proteins go to the cell membranes and there they have to pair up with each other with other HER family members to send a grow signal into the cell.
The two standard tests look at two different points in this process: the amount of HER2 protein on the cell surface or the amount of HER2 gene due to amplification.
These two tests are known by acronyms.
IHC (immunohistochemisty, and usually a commercial test called HercepTest) looks at the amount of HER2 protein.
FISH (fluorescence in situ hybridization, and usually either the PathVysion test or the HER2 FISH pharmDx test) looks at the amount of HER2 DNA (genes).
Normally IHC (HercepTest) is used as the primary test. A biopsy sample is sent to a pathology lab. Slices of the sample are stained with special antibody-based reagents that bind to HER2 molecules on cells. The stained slice is put under a microscope. A histopathologist looks at the level of staining and assigns a value based on the amount of staining seen from 0 to 3+. Zero and 1+ are considered HER2 negative. A value of 3+ is considered HER2 positive. A value of 2+ (weakly staining) is considered "equivocal," or in other words, unclear. (See image, the HER2 protein on the cells stains orange-brown).
In situations where the IHC result is equivocal, they then look at a different cell characteristic -- DNA amplification -- using FISH. Again they use a biopsy sample. In FISH, DNA is stained with fluorescently-labeled DNA (Note: there is a newer technique called CISH -- commercial test name SPoT-Light -- which uses something called chromogenic staining instead of fluorescent staining, but these are essentially equivalent).
Anyway, with FISH, HER2 genes stain red. A region of chromosome 17 that is never amplified is stained green. Using a microscope, the ratio of red to green fluorescence is determined. The higher the ratio, the higher the degree of HER2 gene amplification. Information about the amount of HER2 gene amplification is used to resolve an equivocal 2+ IHC test result.
The concerns about the accuracy of HER2 testing (and thus the appropriate use of Herceptin and other HER2 targeted drugs) have to do standardization, accuracy and reproducibility of these tests. Both are subjective and only semi-quantitative. It isn't always easy to grade IHC stained cells to begin with and, on top of that, artifacts due to lab technique can sometimes impact staining.
In 2007 the major medical associations in this field stated in their published guidelines that standardization of IHC and FISH was major problem. A study in a reputable journal in 2002 estimated that 20% of HER2 testing might be inaccurate. Other studies generally found that variations between different laboratories caused a 10-20% disagreement in results between IHC and FISH. And a study in 2006 found that agreement in test results between local and centralized laboratories for IHC was just 75-82%, and for FISH was 88%. (References supplied upon request.)
The issue of HER2 testing accuracy remains. This is a topic we'll be digging into deeper in future blogs.