The combined gene protein assay (GPA) can simultaneously assess HER2 gene copy number and protein on a single slide using bright-field microscopy.
GPA was compared with a fluorescence in situ hybridization (FISH) assay on 50 invasive breast carcinomas with a 2+ score on immunohistochemistry (IHC).
The cases were categorized into positive, equivocal, or negative for HER2 gene amplification using the 2013 American Society of Clinical Oncology/College of American Pathologists criteria. This resulted in 82% agreement (41 of 50) between FISH and GPA. In addition, 25 known IHC 3+ breast carcinomas analyzed by GPA showed protein overexpression and clusters of HER2 gene consistent with unequivocal amplification, and 22 known IHC-negative cases were negative for HER2 gene amplification by GPA.
Although GPA is an alternative to both IHC and FISH, it may be an unnecessary test for IHC 0/1+/3+ cases. The clinical utility of GPA appears to be similar to other in situ hybridization assays (ie, adjudicator of HER2 status for IHC 2+ cases).