Abstract PS3-02-06: Stromal-Fibroblastic remodelling and involution signatures in non-tumoral tissue of postpartum breast cancer patients: implications for breast cancer risk

Abstract Background: Breast cancer (BC) in young women under 45 is a growing public health concern, with around 50% of cases occurring in mothers within 5-10 years after childbirth. This entity, known as postpartum breast cancer (PPBC), is characterised by aggressive clinical behaviour and higher metastasis and mortality rates compared to age-matched BC patients. Despite its recognised poor prognosis, the molecular mechanisms driving PPBC remain poorly understood, and this knowledge gap limits the ability to identify women at higher risk and develop prevention strategies. This study aimed to characterize the transcriptomic alterations in both tumoral and histologically-normal adjacent breast tissue from PPBC patients, compared to non-PPBC and healthy controls, with a focus on identifying early risk signatures. Methods: RNA-seq analysis was performed on fresh breast tissue samples collected from 30 young women (aged ≤ 45 years) at Reina Sofia University Hospital of Córdoba: paired tumor and adjacent normal tissue from PPBC patients (n=12), tumor tissue from non-PPBC patients (n=8), and normal breast tissue from healthy nulliparous/parous women (n=10). Transcriptomic data were analysed for differential gene expression using DESeq2, and pathway enrichment analysis was performed using GSEA querying the Hallmarks (MSigDB) and 100 customized gene sets. Significantly enriched pathways were identified based on FDR q-values < 0.1. The differentially expressed genes (DEGs) were called based on the |Fold change|>1.5 and FDR p-value <0.05. The shared overexpressed genes in PPBC tumour and adjacent tissue versus healthy controls were evaluated as potential biomarkers using multivariate ROC analysis. Results: PPBC tumors exhibited a distinct, aggressive profile compared to non-PPBC patients with significant enrichment of cell cycle activation pathways (E2F, MYC targets and G2M checkpoint), reflecting high proliferative activity, as well as dysregulation of the WNT/β-catenin signalling pathway (FDR q-value < 0.1). Notably, adjacent histologically-normal tissue of PPBC patients also exhibited a unique transcriptomic profile, distinct from healthy breast tissue. This included enrichment of stromal-fibroblastic remodeling signatures, such as TGF-β signaling, epithelial-mesenchymal transition (EMT), and markers related with cancer-associated fibroblasts (CAFs) and endothelial cells. Interestingly, a robust post-lactational involution-like signature, characterized by upregulation of genes linked to mammary gland regression, late parity, and a transient postpartum immune state (FDR qvalue < 0.05). A set of 101 DEGs significantly overexpressed in both PPBC tumors and adjacent normal tissue compared with healthy women tissue, showed high discriminative ability (ROC AUC > 0.9). These genes were predominantly involved in pathways related to profibrotic and cytokine signaling, with top gene markers such as SOCS4, TMED7 and DYNLT3. Conclusions: Our findings reveal that molecular alterations in PPBC extend beyond the tumor itself, implicating the surrounding normal tissue in a tumor-promoting, stromal fibroblastic remodeling and post-lactational microenvironment. This transcriptomic profile supports the hypothesis that PPBC may develop within a mammary microenvironment already altered by post-gestational involution and stromal processes that could favour normal cells to malignancy. These insights highlight the significant potential for improving risk stratification and guiding early intervention strategies in postpartum women. Citation Format: J. de la Haba-Rodríguez, R. Peña-Enríquez, D. Ponferrada, R. Bautista-Moreno, S. Romero-Martín, E. Contreras, A. Díaz-Chacón, C. Morales, B. Rodríguez, P. Sanchez-Maurinho, A. Martinez-López, C. León, J. Cejas-Arjona, P. Notario-Fernández, P. Rioja-Torres, J. Raya-Povedano, M. Álvarez, S. Guil-Luna. Stromal-Fibroblastic remodelling and involution signatures in non-tumoral tissue of postpartum breast cancer patients: implications for breast cancer risk [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 PS3-02-06.