Abstract PO3-25-03: Can intratumoral microRNA-125a levels be a prognostic biomarker of a ER-positive breast cancer patients？

Abstract Background: MicroRNA-125a (miR-125a) has been shown to function as tumor suppressive miRNA by pre-clinical in vitro studies. However, to the best of our knowledge, there has been no study that investigated the clinical relevance of miR-125a in breast cancer patients. We hypothesized that miR-125a expression in breast cancer associate with its aggressiveness and can be a prognostic biomarker. Material and Methods: Non-tumorigenic epithelial cell line, MCF-10A and breast cancer cell lines, MCF-7, T47D, and MDA-MB231, were used to analyze the expression levels of miR-125a as well as cell viability. The data of 2042 patients were extracted from publicly available databases, The Molecular Taxonomy of Breast Cancer International Consortium (METARBRIC), The Cancer Genome Atlas (TCGA), and GSE57897. These databases included clinicopathological and survival data as well as transcriptome data. The survival analysis, gene set enrichment analysis (GSEA) were conducted comparing miR-125a high expressing and low expressing tumors, divided by the median cutoff. The association between the miR-125a expression and tumor immune microenvironment was assessed by xCell. Results: The expression levels of miR-125a were suppressed in breast cancer cell lines, MCF-7, T47D, and MDA-MB231compared with MCF-10A. This was concordant with human samples. The expression levels of miR-125a were lower in tumors compared with normal breast tissues in both TCGA and GSE57897 cohorts. These results were in agreement with the notion that it is a tumor suppressive miRNA (p< 0.001 and p< 0.001, respectively). ER-positive/HER2-negative (ER+/HER2-) showed the highest miR-125a expression among the subtypes in TCGA (p< 0.001). MiR-125a expression was not associated with cancer staging in any of the subtypes in neither TCGA nor METABRIC cohorts. To our surprise, high miR-125a tumors demonstrated worse disease free survival (DFS), disease specific survival (DSS), and overall survival (OS) compared with low expressing in ER+/HER2- breast cancer patients (p=0.008, p=0.005, and p=0.037) which was not the case for the other subtypes in METABRIC cohort. Interestingly, miR-125a expression was associated with Nottingham histological grade only in ER+/HER2- among the subtypes (p< 0.001). Also, high miR-125a tumors significantly demonstrated higher expression of MKI67, one of the most commonly used parameters for cell proliferation, correlated with miR-125a expression in ER+/HER2- (p=0.02), but not in other subtypes. Furthermore, high miR-125a tumors enriched four out of five cell proliferation-related gene sets in Hallmark collection, such as E2F Targets, G2M Checkpoint, Mitotic Spindle, and MYC Targets V2 in ER+/HER2- subtype, but not in TNBC. This was also the case in immune-related gene sets; interferon-alpha response and interferon-gamma response that enriched to high miR-125a tumors in ER+/HER2-, but not in TNBC. Anti-tumor immune cells, CD8 cells, T-helper type 1 cells, and M1 macrophages, were all elevated in high miR-125a tumors in ER+/HER2- subtype (all p< 0.03), but none in TNBC. Also, pro-cancerous immune cell such as T-helper type 2 cells were similarly highly infiltrated with high miR-125a tumors only in ER+/HER2- subtype (p=0.026). Given its association with the survival outcome, we cannot help but speculate that miR-125a expression is parallel with highly proliferative ER+/HER2- breast cancer, but its tumor suppressive effect is not enough to improve survival outcome. Conclusion: High miR-125a levels in tumors were associated with aggressive biology and worse survival, thus it can be a candidate for a prognostic biomarker in ER+/HER2- patients. Citation Format: Yoshihisa Tokumaru, Masanori Oshi, Mai Okawa, Saki Ando, Yoshimi Niwa, Ryutaro Mori, Kazuaki Takabe, Manabu Futamura. Can intratumoral microRNA-125a levels be a prognostic biomarker of a ER-positive breast cancer patients？ [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 PO3-25-03.