BRD8 is a therapeutic vulnerability for overcoming resistance to dual ER/HER2 blockade therapy in HR+/HER2+ breast cancer [scRNA-seq]

Hormone receptor (HR)-positive, HER2-positive breast cancers are resistant to endocrine and anti-HER2 therapies due to crosstalk between estrogen receptor (ER) and HER2. However, how anti-HER2 agents activate ER as a mechanism of resistance remains unknown. Using single-cell RNA sequencing, we identified Bromodomain Containing Protein 8 (BRD8) as a major mediator of ER activation in response to neratinib, a HER2 tyrosine kinase inhibitor. BRD8 expression was rapidly induced by various anti-HER2 agents (neratinib, lapatinib and trastuzumab) and its expression positively correlates with ER. BRD8 regulates both ER-dependent and -independent growth promoting pathways. Moreover, BRD8 ablation re-sensitizes fulvestrant- or neratinib-resistant HR+/HER2+ cells to neratinib, suggesting that combinatorial targeting of BRD8 and HER2 attenuates signal crosstalk and possibly overcomes treatment resistance to dual anti-ER/HER2 blockade therapy. This work identifies BRD8 as not only a central hub for ER signaling activation upon anti-HER2 treatment, but also a druggable vulnerability for treating HR+/HER2+ breast cancer.