Abstract PS3-13-20: Fibroblastic mutant trp53 in the stroma promotes mammary tumor development through enhanced paracrine factor secretion

Abstract Tumor progression critically depends on stromal contributions, particularly from fibroblasts that regulate growth factors, angiogenesis, immunity, and ECM remodeling. While TP53 is the most frequently mutated tumor suppressor and its cell-autonomous roles have been extensively studied, emerging evidence shows its stromal functions actively shape tumor-stroma crosstalk. However, the molecular mechanisms by which mutant stromal TP53 influences tumor development remain undefined. This study examined whether fibroblast-specific expression of p53R172H influences HER2/neu-driven mammary tumorigenesis. We generated a female mouse cohort with genotypes MMTV-neu; Fsp-Cre; Trp53 wm-R172H/+ (NP) and MMTV-neu; Fsp-Cre or Trp53 wm-R172H/+ (N) as controls. In NP mice, Cre recombinase drove recombination of the conditional Trp53 wm-R172H/+ allele, coverting wild-type p53 to mutant p53R172H specifically in fibroblasts. NP mice developed mammary tumors significantly earlier, with tumor-free survival of 520 days compared to 620 days in controls (p = 0.0007). Although tumor burden and metastatic frequency were unchanged, RNA sequencing revealed 855 upregulated and 1,927 downregulated genes unique to NP tumors. Gene Set Enrichment Analysis indicated activation of multiple pathways, notably PI3K/AKT/mTOR. In addition, transcriptomic profiling of NP versus N mammary glands showed elevated expression of secretory proteins SAA1, SAA2, and THBS4, previously linked to PI3K/AKT-driven cell growth. In vitro assays using synthetic peptides of these proteins and HER2/neu+ mammary tumor cell lines confirmed that SAA1, SAA2, and THBS4 promote tumor cell proliferation and migration. Analysis of TCGA and METABRIC datasets further demonstrated increased SAA1 and SAA2 expression in TP53-mutant human breast cancers, supporting their relevance in patients. Together, these findings reveal that fibroblastic mutant p53 accelerates breast tumorigenesis through paracrine signaling that activates PI3K/AKT/mTOR. These results highlight stromal mutant p53-mediated tumor-stroma crosstalk as a critical and potential therapeutic target for breast cancer biology. Citation Format: B. Liu, G. Chau, G. Lozano, S. Xiong, J. McDaniel, D. Chachad, C. Johnson, A. Williams-Villalobo, H. Chen, Y. Zhang, Y. Qi, X. Su, A. K. El-Naggar. Fibroblastic mutant trp53 in the stroma promotes mammary tumor development through enhanced paracrine factor secretion abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-13-20.