Micro-RNAs as clinical biomarkers and therapeutic targets in breast cancer: Quo vadis?

Breast cancer (BC) is the most frequent type of non skin cancer among women and a major leading cause of cancer-related deaths in Western countries. It is substantial to discover novel biomarkers with diagnostic, prognostic or predictive usefulness as well as therapeutic value for BC. Micro-RNAs (miRNAs) belong to a novel class of endogenous interfering RNAs that play a crucial role in post transcriptional gene silencing through mRNA targeting and, thus, are involved in many biological processes encompassing apoptosis, cell-cycle control, cell proliferation, DNA repair, immunity, metabolism, stress, aging, etc. MiRNAs exert their action mainly in a tumor suppressive or oncogenic manner. The specific aberrant expression patterns of miRNAs in BC that are detected with the use of high-throughput technologies reflect their key role in cancer initiation, progression, migration, invasion and metastasis. The detection of circulating extracellular miRNAs in plasma of BC patients may provide novel, non-invasive biomarkers in favor of BC diagnosis and prognosis and, at the same time, accumulating evidence has underscored the possible contribution of miRNAs as valuable biomarkers to predict response to chemotherapy or radiotherapy. Data from in vitro and in vivo studies on BC have revealed promising therapeutic approaches via miRNA delivery and miRNA inhibition. The purpose of this review is to explore the ontological role of miRNAs in BC etiopathogenesis as well as to highlight their potential, not only as non-invasive circulating biomarkers with diagnostic and prognostic significance, but also as treatment response predictors and therapeutic targets aiding BC management.