Abstract PO1-03-09: Transcriptomic profiling of ER-negative and ER-low, HER2-negative breast cancer and its implications on immune regulation

Abstract Background: The recommended cut-off of 1% positive cells to define estrogen receptor (ER) negative status in breast cancer (BC) is highly debated. Few data exist on transcriptomic features of ER-low BC, suggesting no major differences vs ER-neg. Among HER2-negative BCs, we compared gene-expression profiling (including PAM50 intrinsic subtyping) according to ER status (ER-neg: 0% vs. ER-low: 1-9%) and assessed its impact on Tumor-infiltrating lymphocytes (TILs) levels. Methods: Across two cancer centers, stage I-III HER2-negative BCs FFPE samples were reviewed for ER status. All ER-low (ER 1-9%) samples were identified and a cohort of ER-neg BC samples, matched for age and stage, was also retrieved. A small cohort of pts with intermediate ER expression (ER-int: 10-50%) was included as a control cohort. Expression of 776 BC–related genes was evaluated by nCounter® (Breast Cancer 360TM Panel) and intrinsic molecular subtyping was determined using the PAM50 subtype predictor. A False Discovery Rate (FDR) corrected unpaired two-class SAM was used to identify genes differentially expressed in different subgroups. TILs were evaluated on archival H 5%); however, when the analysis was limited to PAM50 basal-like tumors (N=77) we only identified two genes that were downregulated in ER-low BCs (EDN1, PROM1) as compared to ER-neg BCs. On the contrary, significant differences in transcriptomic regulation were observed between ER-low and ER-int BCs, with ER-low BCs showing significantly higher expression of 53 genes and lower expression of 398 genes, as compared to ER-int (SAM analysis FDR 5%). We previously reported similar TIL levels in ER-low and ER-neg tumors. In both ER-low and ER-int BCs, PAM50 basal-like subtype was associated with significantly higher TILs: median TIL levels 20 (range 0-80) and 6 (range 1-40) for PAM50 Basal-like BCs versus other PAM50 subtypes respectively in ER-low samples (p 0.001); median TIL levels 53 (range 25-80) and 5 (range 0-10) for PAM50 Basal-like BCs versus other PAM50 subtypes respectively in ER-int samples (p=0.036). No significant difference in TIL levels according to PAM50 subtyping were observed in ER-neg tumors. No significant difference in mRNA levels for PD-1 (PDCD1) and PD-L1 (CD274) genes was observed between ER-low and ER-neg tumors. Conclusions: Among patients with HER2-neg BC, gene-expression profiling is similar in ER-neg (i.e., 1%) vs. ER-low tumors with a high prevalence of PAM50 Basal-like tumors and few differentially expressed genes. In both ER-low and ER-int HER2-neg BCs, PAM50 Basal-like subtype is also associated with higher TIL levels. Our results strongly indicate that HER2-neg/ER-low BCs and HER2-neg/ER-neg BCs are similar biological entities, thus further supporting the use of similar treatments in patients with early-stage ER-low and triple-negative BC, including immune checkpoint inhibitors. PAM50 intrinsic subtype distribution according to ER levels Citation Format: Gaia Griguolo, Claudio Vernieri, Davide Massa, Lorenzo Nicolè, Claudia Pinato, Federica Miglietta, Andrea Vingiani, Silvia Brich, Riccardo Lobefaro, Francesca Schiavi, Matteo Fassan, Giancarlo Pruneri, Valentina Guarneri, Maria Vittoria Dieci. Transcriptomic profiling of ER-negative and ER-low, HER2-negative breast cancer and its implications on immune regulation abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-03-09.