MicroRNAs as Biomarkers

Biomarkers are molecules affected by biochemical, molecular, and cellular processes, which can be measured in biological samples, like human cells, fluids and tissues. This definition has been expanded to cover any biological process that can be measured and assessed objectively, as an indicator of the presence of a disease, its state of progression, or its response to treatment [1, 2]. According to the National Cancer Institute, biomarkers are defined as biological molecules in blood, fluids, or tissues of the body that may be affected by a normal or abnormal process, or the presence of a disease like cancer [3]. Moreover, biomarkers are capable of distinguishing diseased individuals from healthy individuals [3]. There are many types of biomarkers, including proteins (receptors or enzymes) [4, 5], peptides, or antibodies [6], as well as nucleic acids (micro-RNAs or other non-coding RNAs) [7, 8]. In addition, biomarkers are affected by alterations in gene expression profiles, and proteomic or metabolomic signatures. Furthermore, many biomarkers can be detected in the circulation (plasma, whole blood, or serum), or in excretions or secretions (stool, urine, sputum, or nipple discharge). The evaluation of biomarkers is relatively easy and can be conducted in a noninvasive serial manner. However, it is only possible to detect some biomarkers in intact tissue samples, which therefore requires specific biopsies or advanced imaging methods [3, 9].