Circulating miRNAs in Breast Cancer Diagnosis and Prognosis

Simple Summary In the era of precision medicine, new tools for better management of breast cancer patients are still needed. From this perspective, circulating microRNA (miRNA), a small group of non-coding endogenous RNAs with a nucleotide length of 18-25, could be potentially appealing as diagnostic, prognostic and predictive biomarkers. miRNAs have been found to be deregulated in different pathological conditions including cancers. Moreover, due to their high stability in biological fluid, where they circulate freely or are associated with extracellular vesicles (EVs), they are ideal candidates to be used for non-invasive detection and monitoring. This review highlights the recent findings related to free and EV-derived miRNA applicability in breast cancer, pointing out the advantages, but also the issues, that still limit their translation in clinical practice. Great improvement has been made in the diagnosis and therapy of breast cancer patients. However, the identification of biomarkers for early diagnosis, prognosis, therapy assessment and monitoring, including drug resistance and the early detection of micro-metastases, is still lacking. Recently, circulating microRNAs (miRNAs), circulating freely in the blood stream or entrapped in extracellular vesicles (EVs), have been shown to have a potential diagnostic, prognostic or predictive power. In this review, recent findings are summarized, both at a preclinical and clinical level, related to miRNA applicability in the context of breast cancer. Different aspects, including clinical and technical challenges, are discussed, describing the potentialities of miRNA use in breast cancer. Even though more methodological standardized studies conducted in larger and selected patient cohorts are needed to support the effective clinical utility of miRNA as biomarkers, they could represent novel and accessible tools to be transferred into clinical practice.