HER2 Gene Protein Assay Is Useful to Determine HER2 Status and Evaluate HER2 Heterogeneity in HER2 Equivocal Breast Cancer

Objectives: Approximately 15% of breast cancers show equivocal human epidermal growth factor receptor 2 (HER2) results on HER2 immunohistochemistry (IHC) and are reflexed for fluorescence in situ hybridization (FISH). However, some cases remain equivocal. In this study, we evaluated these double-equivocal cases by using a novel gene protein assay (GPA), which can simultaneously assess HER2 gene copy number and protein on a single slide using bright-field microscopy. Methods: GPA was performed on 42 HER2 IHC and FISH double-equivocal cases. Results: GPA was negative for amplification in 28 cases, equivocal in three cases, and positive in 11 cases. The GPA results showed excellent concordance with either repeat FISH using a chromosome 17 centromere probe or FISH using an alternative probe. Furthermore, HER2 heterogeneity was identified in three of 11 GPA-positive cases. Conclusions: HER2 GPA performs accurately and is very useful to determine HER2 status in HER2 IHC and FISH double-equivocal breast cancer cases and identify HER2 heterogeneity.