Abstract 4245: Targeting ERalpha disorder-function mechanisms in ER-positive breast cancer

Background: The estrogen receptor (ER or ERalpha) is commonly activated by phosphorylation at serine 118 (Ser118) within its intrinsically disordered N-terminal transactivation domain (NTD), enabling estrogen-independent function in breast cancer. While kinase inhibitors targeting this modification are in clinical trials, understanding the molecular mechanism of activation is essential for developing direct therapeutic strategies. Methods: We combined small-angle X-ray scattering and NMR spectroscopy with functional assays to characterize conformational changes in the ER-NTD. We then conducted activity-guided screening to identify small molecules directly targeting the disordered NTD. Results: Ser118 phosphorylation triggers an unexpected conformational expansion by disrupting specific hydrophobic clusters between aromatic-rich regions within the NTD - a mechanism extending beyond simple electrostatic effects (Nature 2024, in press). Leveraging this insight, we developed selective small molecules that directly bind the disordered NTD and potently inhibit growth of hormone-independent breast cancer models. Conclusions: Our work establishes both the mechanistic basis of ER regulation and a therapeutic framework for targeting intrinsically disordered regions in treatment-resistant breast cancer. These findings open a new avenue for first-in-class drugs that modulate protein disorder in cancer. Citation Format: Zhanwen Du, Shuqi Luo, Sichun Yang. Targeting ERalpha disorder-function mechanisms in ER-positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4245.