MicroRNA-21 as a predictor and prognostic factor for trastuzumab therapy in human epidermal growth factor receptor 2-positive metastatic breast cancer

Breast cancer is the second most common cancer diagnosed worldwide. Human epidermal growth factor receptor 2 (HER2)-positive breast cancer represents about 20% to 30% of all breast cancers. Trastuzumab is used in the treatment of HER2-positive breast cancer. MicroRNA-21 (miR-21) is an oncomiR that acts by inhibiting many tumor-suppressor genes. We analyzed the relative expression levels of serum miR-21 in 20 HER2-positive metastatic breast cancer patients before and after 3 months of treatment with trastuzumab. miR-21 levels decreased with a high significant difference after trastuzumab therapy (P = 0.001). Although miR-21 expression levels were lower in responders than in nonresponders, the difference was not statistically significant ( P = 0.6). Our results demonstrated a significant negative correlation between its basal expression, expression levels after treatment, and time to progression ( P = 0.03 and 0.01, respectively). These results make miR-21 a potential prognostic factor for HER2-positive metastatic breast cancer patients. Additionally, it can be an interesting potential target in therapy using antisense oligonucleotides for miR-21.