Targeting epigenetic vulnerabilities in PI3K/AKT-driven triple-negative breast cancer

Abstract

Breast cancer is one of the most frequently diagnosed malignancies in women, with approximately one in eight women developing the disease in their lifetime. Triple-negative breast cancer (TNBC) is the most aggressive subtype, characterized by increased metastatic potential and high rates of relapse. Current standard-of-care treatments for TNBC rely largely on cytotoxic chemotherapy, underscoring the urgent need for the development novel targeted therapeutic strategies. Recurrent genetic alterations in components of the PI3K/AKT signaling pathway occur across all breast cancer subtypes, with PI3K/AKT signaling playing a critical role in driving tumorigenesis in TNBC. Although multiple PI3K/AKT pathway inhibitors have been approved for use in estrogen receptor positive (ER+) breast cancer in combination with endocrine therapy, these agents have failed to demonstrate efficacy in phase III clinical trials in TNBC. This highlights the need to identify mechanistically informed combination strategies that improve the efficacy of PI3K/AKT inhibitors. In this thesis, we identify the bromodomain and extra-terminal domain (BET) family member BRD2 as a co-targetable vulnerability in breast cancer when combined with PI3K or AKT inhibition. BET proteins (BRD2, BRD3, BRD4, and BRDT) function as epigenetic readers that recruit transcriptional machinery to acetylated chromatin, thereby regulating gene expression. Dysregulation of these factors contribute to multiple pathologies including cancer, immune disorders, and metabolic diseases. While BET proteins have emerged as promising therapeutic targets in cancer, most studies to date have focused predominantly on BRD4, and the distinct functional roles of BRD2 remain incompletely understood. Here, we demonstrate that both pan-BET inhibition (BETi) and BRD2 knockout synergizes strongly with PI3K pathway inhibitors across multiple cancer lineages. BRD2 knockout significantly suppresses TNBC growth in vitro and in mouse xenograft models, while inducing activation of key signaling and cellular stress response pathways. Additionally, combined BET and PI3K inhibition induces synergistic cell death in breast cancer patient-derived organoids (PDOs). Mechanistically, we demonstrate that BRD2 is regulated by phosphorylation at serine 37 (Ser37) by the mitogen- and stress-activated kinase (MSK) and ribosomal S6 kinase (RSK). Phosphorylation of BRD2 at Ser37 modulates cell growth and gene transcription, thereby establishing a link between oncogenic signaling and epigenetic regulation in breast cancer. Collectively, these findings define a rational, mechanism-based therapeutic strategy that exploits BRD2 as a collateral vulnerability to PI3K pathway inhibition in breast cancer. The discovery of an MSK/RSK-regulated phosphorylation on BRD2 further provides a molecular framework for overcoming adaptive resistance and achieving durable, targeted therapeutic approaches in this aggressive disease.