Genomic and epigenomic cross-talks in the regulatory landscape of miRNAs in breast cancer.

MicroRNAs (miRNA) are a class of endogenous, small noncoding RNAs found in animals, plants, and viruses that control their target gene expression posttranscriptionally. They are involved in a wide array of biological processes including cell differentiation, development, cell death and homeostasis, and fine-tune the regulation of these pathways. Their aberrant expressions have been associated with different diseases. These small RNAs are also known to function as oncogenes, oncosupressor genes, modulators of metastatic spread, and regulators of cancer stem cells. Their deregulation is a hallmark of different cancers types including breast cancer. Despite the growing evidence for their involvement in breast cancer, understanding the interplay between miRNAs and their targets leading to the disease remains largely unknown. Here, we provide a comprehensive story on miRNA signatures of breast cancer, miRNAs in breast cancer stem cells, metastamirs (i.e., metastasis regulatory miRNAs), circulating miRNAs as invasive blood-based biomarkers, and oncomiRs and oncosupressor miRNAs associated with breast cancer. Furthermore, we provide biological insights on their regulation by various mechanisms including genomic alterations and demonstration of a complicated feedback network between miRNAs and epigenetic regulators forming an epigenetics-miRNA regulatory circuit whose disruption may underlie the cause of breast cancer.