Abstract PO3-13-07: MUC1-C INTEGRATES CHRONIC ACTIVATION OF INTERFERON PATHWAYS WITH CHROMATIN REMODELING IN TREATMENT RESISTANCE OF TRIPLE-NEGATIVE BREAST CANCER

Abstract Chronic inflammation is intimately linked to cancer progression and resistance to treatment. Triple-negative breast cancer (TNBC) is a recalcitrant malignancy largely unresponsive to cytotoxic, targeted and immunotherapeutic agents. The Mucin1 (MUC1) transmembrane heterodimeric protein, which is aberrantly expressed in TNBCs, evolved in mammals to provide protection of epithelia from the external environment. With loss of homeostasis, the (i) MUC1 N-terminal (MUC1-N) subunit is shed and released into serum as detected by the CA15-3 assay, and (ii) MUC1 C-terminal (MUC1-C) subunit is activated and thereby induces inflammatory, proliferative and remodeling signaling pathways associated with wound healing. However, prolonged MUC1-C activation in settings of chronic inflammation promotes cancer progression and treatment resistance. The SWI/SNF PBAF chromatin remodeling complex includes the polybromo-1 (PBRM1) subunit and drives DNA damage resistance and immune evasion in certain cancer cells through mechanisms that remain unclear. Recent studies in TNBCs have demonstrated that MUC1-C drives intrinsic activation of type II interferon (IFN) pathway that is linked to chronic inflammation and immune evasion. In extending this work, we have found that MUC1-C activates the type I IFN pathway, which contributes to genomic instability and DNA damage tolerance in TNBC cells. Importantly, our results further demonstrate that MUC1-C interacts with interferon regulatory factor 1 (IRF1) as a key regulatory node in integrating activation of the type I and II IFN pathways. Here, we report that MUC1-C is necessary for PBRM1 expression and that it forms a nuclear complex with PBRM1 in TNBC cells. Analysis of global transcriptional (RNA-seq) and chromatin accessibility (ATAC-seq) profiles further demonstrated that MUC1-C and PBRM1 drive STAT1 and IRF1 expression by increasing chromatin accessibility of promoter-like signatures (PLS) on their respective genes. We also found that MUC1-C, PBRM1, and IRF1 increase the expression and chromatin accessibility on PLSs of the (i) type II IFN pathway IDO1 and WARS genes and (ii) type I IFN pathway RIG-I, MDA5, cGAS, STING and ISG15 genes that collectively contribute to DNA damage resistance and immune evasion. In support of these results, targeting MUC1-C in wild-type BRCA1/2 TNBC cells enhanced carboplatin-induced DNA damage and loss of self- renewal capacity. In addition, MUC1-C was necessary for DNA damage resistance, self-renewal and tumorigenicity of olaparib-resistant BRCA1-mutant TNBC cells. Analysis of TNBC clinical samples corroborated that (i) MUC1 and PBRM1 are associated with decreased responsiveness to chemotherapy and (ii) MUC1-C expression is associated with the depletion of tumor-infiltrating lymphocytes. These findings demonstrate that MUC1-C activates PBRM1, and thereby chromatin remodeling of IFN-stimulated genes that promote chronic inflammation, DNA damage resistance, and immune evasion. These findings have identified MUC1-C as a potential target for the treatment of TNBC that have limited therapeutic options. To that end, the generation of MAb 3D1 against the MUC1-C extracellular domain provided an opportunity for the development of agents that target MUC1-C on the cancer cell surface. As one example, an allogeneic anti-MUC1-C CAR T cell using MAb 3D1 sequences is undergoing Phase I evaluation for the treatment of MUC1-C-expressing cancers (NCT05239143: P-MUC1C-ALLO1 Allogeneic CAR-T Cells in the Treatment of Subjects with Advanced or Metastatic Solid Tumors). In addition, anti-MUC1-C huMAb3D1-MMAE ADCs are under development by the NCI NExT Program for IND-enabling studies and performing early phase clinical trials in patients with TNBC. Citation Format: Nami Yamashita, Yoshihiro Morimoto, Atsushi Fushimi, Rehan Ahmad, Atrayee Bhattacharya, Yuka Inoue, Henry Withers, Geoffrey Shapiro, Tomoharu Yoshizumi, Takayuki Ueno, Shinji Ohno, Mark Long, Donald Kufe. MUC1-C INTEGRATES CHRONIC ACTIVATION OF INTERFERON PATHWAYS WITH CHROMATIN REMODELING IN TREATMENT RESISTANCE OF TRIPLE-NEGATIVE BREAST CANCER abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-13-07.