Equivocal (HER2 IHC 2+) breast carcinomas: gene-protein assay testing reveals association between genetic heterogeneity, individual cell amplification status and potential treatment benefits.

AIMS: Genetic heterogeneity can pose a challenge to identifying eligible cases for targeted therapy in the human epidermal growth factor receptor 2 (HER2) immunohistochemistry (IHC) 2+ breast carcinoma group. In this study, we characterized this subset of tumors according to clinicopathological parameters.

METHODS AND RESULTS: We assessed 1000 tumor cells per case and recorded the number of HER2 and chromosome enumeration probe 17 (CEP17) copies using gene-protein assay slides. HER2 status was determined based on ASCO/CAP 2013 guidelines. Tumors with 5% to 50% of cancer cells with amplification were considered to be heterogeneous, whereas those with >50% were considered to be non-heterogeneous. In a study cohort of 110 HER2 IHC 2+ carcinomas, 93 (84.5%) were non-amplified, 12 (10.9%) were amplified, and 5 (4.5%) were ISH equivocal. All of the HER2-amplified and 2 of ISH-equivocal cases (12.7%) corresponded to non-heterogeneous tumors, with highly significant differences evident in the average HER2/CEP17 ratio (p=0.0002) and the proportion of cells with HER2 >6 copies (p<0.0001) compared with heterogeneous lesions. NST grade 3 and HER2-amplified carcinomas average HER2/CEP17 ratio correlated with an increased number of cells with HER2/CEP17 ≥2.0 (p<0.014). Triple-negative CEP17 polysomic carcinomas showed increased metastatic capacity (p=0.003) compared with other tumor types.

CONCLUSION: Non-heterogeneous HER2 IHC 2+ tumors tend to be HER2-amplified. Adding the percentage of cells with HER2 >6 copies to the average HER2/CEP17 ratio may facilitate assessment of amplification status in ISH-equivocal cases. The proportion of cells with HER2/CEP17≥2.0 contributes information about the actual average HER2/CEP17 ratio, depending on tumor type. This article is protected by copyright. All rights reserved.