Cross-Roads to Drug Resistance and Metastasis in Breast Cancer: miRNAs Regulatory Function and Biomarker Capability.

Breast cancer and specifically metastatic breast cancer (mBC) constitutes a major health burden worldwide with the highest number of cancer-related mortality among women across the globe. Despite having similar subtypes, breast cancer patients present with a spectrum of aggressiveness and responsiveness to therapy due to cancer heterogeneity. Drug resistance and metastasis contribute to therapy failure and cancer recurrence. Research in the past two decades has focused on microRNAs (miRNAs), small endogenous non-coding RNAs, as active players in tumorigenesis, therapy resistance and metastasis and as novel non-invasive cancer biomarkers. This is due to their unique dysregulated signatures throughout tumor progression and their tumor suppressive/oncogenic roles. Identifying miRNAs signatures capable of predicting therapy response and metastatic onset in breast cancer patients might improve prognosis and offer prolonged median and relapse-free survival rate. Despite the growing reports on miRNAs as novel non-invasive biomarkers in breast cancer and as regulators of breast cancer drug resistance or metastasis, the quest on whether some miRNAs are capable of regulating both simultaneously is inevitable, yet understudied. This chapter will review the role of miRNAs as biomarkers and as active players in inducing/reversing anti-cancer drug resistance, driving/blocking metastasis or regulating both simultaneously in breast cancer.