Gene Protein Assay(GPA): Unveiling Tumor Heterogeneity in HER2/neu Positive Breast Cancer

Authors: Dr. Tanuja Shet, Dr. Aditi Rathi

Introduction: HER2/neu gene amplification on Fluorescence in-situ hybridization (FISH) is defined as average HER2neu gene signals > 4 and HER2:CEP17 ratio of > 2, in breast cancer. However, with tumor heterogeneity interpretation of these cut-offs is challenging and some cases test as borderline. We attempted a study analyzing Gene protein assay (GPA) which combines HER2/neu immunohistochemistry (IHC), and D-DISH (dual color dual in-situ hybridization) assay to help resolve this issue.

Methods: A total of 31 cases reported as HER2/neu amplified with intermixed tumor heterogeneity on FISH were taken for this study. The original HER2/neu count was between 4 to 6 and ratio near 2 in most cases. GPA was done using 4B5 HER2/neu IHC and Roche/Ventana D-DISH kit on Ventana Ultra machine by doing IHC first followed by D-DISH.

Result: On GPA, 3/31 cases were reported as non-amplified, and the rest were amplified. Advantage of GPA was that it helped count D-DISH as per membrane staining and identified more tumor heterogeneity in contrast to FISH in six cases. The results of the remaining cases were the same.