microRNAS in Therapy Resistance of Breast Cancer

Breast cancer is one of the deadliest cancer types worldwide and consists of several subtypes differing in their molecular characteristics; each subtype requires various effective treatment strategies. Development of resistance to radiation or therapeutic agents is one of the main factors leading to the death of about 450,000 breast cancer patients each year. Since microRNAs (miRNAs) have been shown to be key players in health and disease, it is not surprising that they influence the development of resistance to treatment and thereby affect the fate of patients suffering from different types of cancer. miRNAs typically modulate the expression of hundreds of targets, forming a complex regulatory layer which we have only begun to understand. This review summarises miRNAs that confer resistance to different treatment options or sensitise breast cancer cells to a particular treatment. Moreover, this review addresses the high clinical value of miRNAs as biomarkers that allow prediction or monitoring therapy response. The focus of the review is to illustrate how much we know already but also to emphasise that a vast part of the miRNome and its implications for breast cancer therapy resistance remains in the dark and requires further investigation.