Stromal expression of miR-21 identifies high-risk group in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is an aggressive subtype defined by the lack of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 expression. Expression of miR-21, an oncomiR, is frequently altered and may be distinctly expressed in the tumor stroma. Because tumor lesions are a complex mixture of cell types, we hypothesized that analysis of miR-21 expression at single-cell resolution could provide more accurate information to assess disease recurrence risk and BC-related death. We implemented a fully automated, tissue slide-based assay to detect miR-21 expression in 988 patients with BC. The miR-21(High) group exhibited shorter recurrence-free survival [hazard ratio (HR), 1.71; P < 0.001] and BC-specific survival (HR, 1.96; P < 0.001) in multivariate regression analyses. When tumor compartment and levels of miR-21 expression were considered, significant associations with poor clinical outcome were detected exclusively in tumor epithelia from estrogen receptor- and/or progesterone receptor-positive human epidermal growth factor receptor 2-negative cases [recurrence-free survival: HR, 3.67 (P = 0.006); BC-specific survival: HR, 5.13 (P = 0.002)] and in tumor stroma from TNBC cases [recurrence-free survival: HR, 2.59 (P = 0.013); BC-specific survival: HR, 3.37 (P = 0.003)]. These findings suggest that the context of altered miR-21 expression provides clinically relevant information. Importantly, miR-21 expression was predominantly up-regulated and potentially prognostic in the tumor stroma of TNBC.