MicroRNA-21 Emerges as Key Prognostic Indicator After Breast Cancer Surgery

Background/Objective: MicroRNA-21 (miR-21) is one of the most widely studied oncogenic microRNAs and has been implicated in breast cancer progression, therapy resistance, and metastatic potential. However, its utility as a long-term prognostic biomarker in patients undergoing mastectomy remains insufficiently clarified. This study evaluated the prognostic significance of miR-21 expression in predicting overall and disease-free survival. Methods: A retrospective cohort of 426 breast cancer patients who underwent mastectomy between 2010 and 2017 was analyzed. Tumor miR-21 expression was measured using quantitative real-time PCR and categorized as high or low based on cohort-derived thresholds. Long-term outcomes were assessed over a median follow-up of 112 months. Kaplan–Meier survival curves, log-rank tests, and multivariable Cox proportional hazards models were used to estimate associations between miR-21 levels and survival outcomes. Results: High miR-21 expression was identified in 48.8% of cases. Patients with high miR-21 demonstrated significantly poorer overall survival (10-year OS: 61.4% vs. 82.7%; log-rank p < 0.001) and disease-free survival (10-year DFS: 54.9% vs. 78.3%; log-rank p < 0.001). In multivariable analysis, high miR-21 remained an independent predictor of decreased OS (HR = 2.18; 95% CI: 1.56–3.04) and DFS (HR = 2.44; 95% CI: 1.78–3.33). Conclusions: Elevated miR-21 expression is a significant independent biomarker of adverse long-term prognosis in breast cancer patients undergoing mastectomy. Integrating miR-21 into postoperative risk stratification may improve individualized management strategies.