The difference in miR-21 expression levels between invasive and non-invasive breast cancers emphasizes its role in breast cancer invasion

MicroRNA-21 (miR-21) overexpression is characteristic for various types of tumors, but it is still unknown whether its expression levels differ between invasive and non-invasive breast carcinomas. The main goal of the study was to determine the difference in miR-21 expression among normal tissue, non-invasive, invasive with non-invasive component, and pure invasive breast cancer samples, to explain its potential role and significance in breast cancer invasiveness. The second goal was to propose miR-21 as molecular marker of breast cancer invasiveness and potential target for future anti-miR therapies for the prevention of invasion and metastasis. In order to reveal the role of miR-21 in breast cancer invasiveness, we measured miR-21 expression levels in 44 breast cancer and four normal samples by stem-loop real-time RT-PCR using TaqMan technology. Relative expression levels of miR-21 were significantly higher in invasive than in other groups (P = 0.002) and significantly higher in invasive compared with invasive with non-invasive component group in histological (P = 0.043) and nuclear grade 2 (P = 0.036), estrogen-receptor-positive (ER+) (P = 0.006), progesterone-receptor-positive (PR+) (P = 0.008), ER+ PR+ (P = 0.007), and proliferation index (Ki-67) <= 20 % (P = 0.036) tumors. Our findings suggest that miR-21 could be independent molecular marker of breast cancer invasiveness and potential target for future anti-miR therapies for the prevention of invasion and metastasis.