Abstract PR009: FMRP upregulation in cancer: Implicating FMRP-expressing cancer-associated fibroblasts in immune evasion

Abstract Our research has uncovered a novel and significant role for Fragile-X-mental-retardation-protein (FMRP) in cancer biology, extending its well-established functions beyond neuronal development. In a recent Science publication (Zeng et al., 2022), we demonstrated that elevated FMRP levels drive immune evasion in various murine and human tumors by modulating immune cell subsets, particularly through the suppression of effector T-cells. To further elucidate the mechanisms of FMRP-mediated immune evasion, we investigated its influence on the stromal component of the tumor microenvironment. Single-cell RNA sequencing of pancreatic ductal adenocarcinoma (PDAC) tumors revealed cancer-associated fibroblasts (CAFs) as the primary stromal cell subtype expressing FMRP. Given the crucial roles that CAFs play in tumor progression and therapeutic resistance, we explored whether FMRP expression contributes to the immunosuppressive characteristics of CAFs in PDAC and breast tumors. Our findings reveal that FMRP expression is markedly upregulated in CAFs from both breast and PDAC tumors, in contrast to its absence in naïve fibroblasts from healthy tissues. Knockdown of FMRP in CAFs resulted in substantial changes in their myofibroblastic and inflammatory properties, while overexpression of FMRP in naïve fibroblasts was sufficient to induce a CAF-like phenotype. Notably, FMRP overexpression led to significantly elevated inflammatory CAF (iCAF) and myofibroblastic CAF (myCAF) signatures, highlighting its role as a key pan-CAF regulator that influences both extracellular matrix (ECM) modulation and the secretory phenotype of CAFs. We utilized the full-body knockout (KO) mice model of the Fmr1 gene, which encodes FMRP, to further delineate the functional importance of FMRP expression in CAFs in stimulating an immunosuppressive program in vivo. Through bulk RNA sequencing, we developed an FMRP- associated gene signature for CAFs and applied it to human datasets, where we observed a pronounced increase in FMRP signatures in CAFs from human PDAC and breast tumors, highlighting its translational relevance. In conclusion, our research underscores the pivotal role of FMRP in cancer biology, impacting both the immune and stromal components of the tumor microenvironment. Targeting FMRP, particularly within CAFs, emerges as a promising therapeutic strategy to improve cancer treatment outcomes. Citation Format: Simge Yucel, Nadine Fournier, Mireia Blanco Gomez, Douglas Hanahan. FMRP upregulation in cancer: Implicating FMRP-expressing cancer-associated fibroblasts in immune evasion [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor-body Interactions: The Roles of Micro- and Macroenvironment in Cancer; 2024 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(22_Suppl):Abstract nr PR009.