Abstract P2-11-15: Focal ESR1 gene amplification is an independent prognostic marker in postmenopausal patients with endocrine-responsive early breast cancer

Abstract Purpose: Estrogen receptor (ER) expression predicts response to endocrine therapy and is a prognostic biomarker. However, no ER-associated biomarker is able to identify patients with particularly favorable outcome among ER+ tumors. We investigated the value of ESR1 amplification in predicting long-term outcome in tamoxifen-treated postmenopausal women with early breast cancer with and without nodal involvement. Patients and Methods: 394 patients with ER+ breast cancer (235 node-negative, 159 patients node- positive), who had received adjuvant tamoxifen for 5 years in the prospective randomized ABCSG-06 trial, and in whom FFPE tumor tissue was available, were included in this analysis. ER alpha immunoreactivity was evaluated using the Allred score, while ESR1 gene amplification was evaluated by FISH analysis. Results: Focal ESR1 amplification was detected in 187 (47%) tumors, and was associated with a favorable outcome. After a median follow-up of 10 years, women with focal ESR1 amplification had a significantly longer distant recurrence-free survival (DRFS; adjusted HR 0.48; 95% CI 0.26-0.91; p=0.02) and breast cancer-specific survival (BCSS; adjusted HR 0.47; CI 0.27-0.80; p=0.01) compared to women without ESR1 amplification. The association between ESR1 amplification and improved DRFS and BCSS was only found in node-positive tumors, but not in nodal-negative tumors. ER alpha immunoreactivity, evaluated by Allred score, correlated significantly with focal ESR1 amplification (p< 0∙0001; Chi-squared test), but without prognostic significance. Conclusion: We suggest focal ESR1 amplification as predictor of improved long-term outcome in postmenopausal women with node-positive ER+ early breast cancer. Citation Format: Christian F. Singer, Frederik Holst, Stefan Steurer, Eike Burandt, Sigurd Lax, Raimund Jakesz, Margaretha Rudas, Herbert Stöger, Richard Greil, Guido Sauter, Martin Filipits, Ronald Simon, Michael Gnant. Focal ESR1 gene amplification is an independent prognostic marker in postmenopausal patients with endocrine-responsive early breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-11-15.