The intra-tumor heterogeneity of ER and HER2 expression in patients with ER-positive and HER2-positive breast cancer.

e12550 Background: ER- positive and HER2-positive (double-positive) breast cancers are known to display significant heterogeneity; however, traditional immunohistochemistry (IHC) only reflects inter-tumor heterogeneity. Studies are needed to demonstrate the intra-tumor heterogeneity of ER and HER2 expression and to further explore its impact. Methods: The retrospective study included three double-positive breast cancer cohorts. Data from a total of 141 patients from The Cancer Genome Atlas (TCGA) and 104 patients from the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) were used to investigate the molecular characteristics of double-positive breast cancers at the tissue level via somatic mutation and RNA-seq analyses, while 43 patients treated at our hospital were selected to analyze ER and HER2 expression and localization at a single-cell level using multiple IF. The rH/R was proposed to reflect ER and HER2 expression profiles based on multiple IF or RNA-seq results, as follows: rH/E = HER2+ cell percentage / (ER+ cell percentage + 1) or rH/E = ERBB2 expression quantity / (ESR1 expression quantity + 1). Results: In the first two cohorts, we found that the HER2-enriched subtype displayed unique molecular characteristics, including increased TP53, ERBB3, and PI3KCA mutation rates and the abnormal expression of important signaling pathways (e.g., G2/M cell cycle checkpoint and epithelial mesenchymal transition); The rH/E was a significant index for diagnosing HER2-enriched patients (area under the receiver operating characteristic curve [AUC] = 0.92 and 0.75, both P < 0.0001). In the third cohort, we found that 86 % of the patients contained all four tumor cell types (ER+HER2+, ER+HER2-, ER-HER2+, and ER-HER2- cells); The rH/E was an independent risk factor for recurrence (hazard ratio [HR] = 2.63, P = 0.02) and patients with a rH/E ≥ 1.5 had a significantly shorter 5-year disease-free survival (DFS = 67 % vs. 92 %, P = 0.046). Conclusions: The significant intra- and inter-tumor heterogeneity of double-positive breast cancers is closely related to the prognosis of patients, which is of great significance for precision treatment.