Abstract 264: A novel gene therapy approach for treating breast cancer through the overexpression of mutant estrogen receptor.

Abstract Background: Endocrine therapy (ET) is the primary treatment for hormone receptor-positive (HR+) breast cancer, but resistance inevitably develops, leading to endocrine-resistant breast cancer. While agents like tamoxifen and aromatase inhibitors improve outcomes, resistance mechanisms remain significant challenges. This suggests a complex role for estrogen receptor (ER) signaling. We hypothesized that maintaining a non-functional, mutant ER (mER) could overcome this resistance. Methods: We synthesized a full-length ER (ESR1) gene with a mutation in the DNA-binding domain (DBD). To ensure targeted delivery, the mER-encoding plasmid was encapsulated within poly(lactic-co-glycolic acid) (PLGA) nanoparticles. These nanoparticles were surface-functionalized with 4-carboxyl-17beta-estradiol, an ER ligand, to facilitate active targeting to ER-positive breast cancer cells. The effects of mER overexpression were investigated in vitro using HER2-ER+PR+ MCF7 and T47D cell lines, as well as their tamoxifen-resistant (TamR) counterparts. In vivo antitumoral effects were assessed using xenograft mouse models. Results: The ER-targeted PLGA nanoparticles effectively delivered the mER plasmid to ER-expressing breast cancer cells in vitro and in vivo. Overexpression of mER did not activate estrogen-responsive genes and exerted a potent antiproliferative effect in both MCF7 and T47D cells. Notably, mER overexpression maintained its strong antitumoral activity in tamoxifen-resistant (TamR) MCF7 and T47D cells. Mechanistically, mER overexpression led to a reduction in wild-type ER (wtER) levels and suppressed DNA replication-related gene pathways. In vivo, mER treatment significantly inhibited the growth of both standard and tamoxifen-resistant tumors. Furthermore, a synergistic antitumoral effect was observed when mER was combined with tamoxifen. Conclusions: Overexpression of a mutant ER with a non-functional DBD, delivered via an ER-targeted nanoparticle system, represents a novel therapeutic strategy for advanced HR+ breast cancer, including endocrine-resistant and tamoxifen-resistant disease. By suppressing wtER, disrupting DNA replication, and maintaining activity in resistant models, mER effectively inhibits tumor growth and overcomes therapeutic resistance. Citation Format: Seung-hwan Jeong, Seokbong Hong, Hyeong Dong Yuk, Cheol Kwak. A novel gene therapy approach for treating breast cancer through the overexpression of mutant estrogen receptor abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 264.