HER3-EGFR score to predict clinical outcomes in triple-negative breast cancer.

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11612 Background: Limited preclinical evidence suggests that the ErbB family member HER3 may have prognostic value in TNBC. However, HER3 is a pseudokinase that cannot homodimerize, so in order to signal it must bind to other ErbB family members such as HER2 or EGFR. EGFR is frequently overexpressed in TNBC; consequently, it may be necessary to consider HER3 levels in the context of EGFR levels in TNBC to derive clinically meaningful insights. Methods: Towards this end, we tested the prognostic value of a combined immunohistochemical HER3-EGFR score (the sum of the individual H-scores, with the median used as a cutpoint) in a multi-institutional study of n = 510 TNBC patients using Cox proportional hazards regression. We also compared the HER3-EGFR-high and low groups in terms of 105 immunohistochemical biomarkers using Mann-Whitney U tests as well as Ingenuity canonical pathways using gene expression data from RNA-seq. Results: Among chemotherapy-treated TNBC patients, high HER3-EGFR score conferred a 2.30-fold increased risk of dying from breast cancer and a 1.78-fold increased risk of distant metastasis (p = 0.006 and p = 0.041, respectively) after adjusting for age and stage. Individual HER3 and EGFR H-scores were not associated with outcomes in simple or multivariable models. We also found that tumors from chemotherapy-treated TNBC patients with high HER3-EGFR scores exhibited higher immunohistochemical expression of luminal cytokeratins, DNA damage response proteins, and P-cadherin compared with tumors from chemotherapy-treated TNBC patients with low scores (q < 0.25). The top canonical pathway whose components were overexpressed in HER3-EGFR-high TNBCs was Hepatic Fibrosis (p = 0.008), which is linked to distant metastasis, and the top upstream regulator was HNF4A (p = 0.012), a transcription factor for ERBB3 with isoforms that promote liver and gut tumorigenesis. Conclusions: Collectively, our study reveals that HER3-EGFR score may identify chemotherapy-treated TNBC patients at increased risk for distant metastases and death whose tumors may be characterized by fibrotic processes. Our immunohistochemical test thus identifies high-risk TNBCs who may benefit from agents that inhibit HER3-EGFR signaling.