Abstract PS2-11-05: Nuclear HER2-ER crosstalk in HR+/HER2+ breast cancer

Abstract Approximately half of all HER2+ breast cancers are hormone receptor positive (HR+) and express estrogen and/or progesterone receptors (ER, PR). These HR+/HER2+ tumors are less responsive to neoadjuvant anti-HER2 therapy and exhibit lower rates of pathologic complete response (pCR). While HER2 primarily functions as a transmembrane receptor that mediates cytoplasmic signaling, a subpopulation of HER2 resides in the nucleus. Prior studies have shown that chromatin-associated HER2 can serve as a transcriptional co-regulator, and its presence in tamoxifen-treated, ER+ (non-HER2-amplified) breast cancer correlates with shorter disease-free survival. We therefore hypothesized that nuclear HER2 may reprogram ER function in HR+/HER2+ breast cancer, thereby contributing to reduced pCR rates. In multiple HR+/HER2+ cell line models, low baseline levels of nuclear HER2 are rapidly increased following treatment with the monoclonal antibodies trastuzumab and pertuzumab (TP). While TP does not alter nuclear ER levels, it enhances ER recruitment to promoter and enhancer regions of TFF1 and PGR (two classical ER target genes) and upregulates their mRNA levels in HR+/HER2+, but not in ER+ only breast cancer cells. To define the global impact of TP treatment on the ER cistrome, we performed Cleavage Under Targets & Release Using Nuclease (CUT&RUN) for ER in the presence and absence of TP in the HR+/HER2+ cell line BT474. TP markedly expands the ER cistrome, promoting ER binding to novel sites on genomic regions enriched for cut-like homeobox 2 (CUX2) and goosecoid (GSC) motifs. These TP-induced ER binding sites are enriched in genes involved in the Wnt, NOD, and receptor for advanced glycation end-products (RAGE) signaling pathways. Expression of a 39-gene signature of unique genes from these pathways that exhibit increased ER binding is associated with reduced distant metastasis-free and overall survival (DMFS, OS) in HER2+ breast cancer, but not in ER+ or triple negative breast cancer in the METABRIC cohort. In publicly available data from 151 HER2+ breast cancers across several clinical studies, expression of this signature is also significantly increased in patients who did not achieve pCR following anti-HER2 neoadjuvant therapy. Finally, multispectral immunofluorescence staining of an in-house tissue microarray (TMA) of 50 HER2+ breast cancers (18 HER2+ only, 32 HR+/HER2+) shows significantly higher nuclear HER2 intensity in HR+/HER2+ vs HER2+ only, with the strongest nuclear HER2 signal observed in cells positive for both ER and Ki67. Together, these data suggest that TP induces reprogramming of the ER cistrome via nuclear HER2, and that these events may ultimately play a role in reducing the rate of pCR in HR+/HER2+ breast cancer. Ongoing CUT&RUN analyses of primary and recurrent HR+/HER2+ tumors will further define the ER cistrome in clinical samples. Citation Format: S. Tam, S. Bahnassy, L. Jin, D. Kung, R. Rahhal, S. M. Swain, R. B. Riggins. Nuclear HER2-ER crosstalk in HR+/HER2+ breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-11-05.