MicroRNAs in Molecular Classification and Pathogenesis of Breast Tumors

SIMPLE SUMMARY: Breast cancer is an ideal model of a heterogeneous disease that is triggered by genetic changes in the normal mammary epithelial cells and manifest as variants of breast tumor subtypes in individuals. Advancement in molecular and genomic profiling techniques, in particular the microRNA profiling have improved the ambiguity related to the presence of multiple breast tumor subtypes. This review discusses in detail, the efficient categorization of breast tumor subtypes based on expression of microRNAs and also highlights the significant role of microRNAs in regulating both the tumor cells and the host microenvironment in driving tumor initiation, progression, chemoresistance and eventual spread of the disease. MicroRNAs may be rightfully deemed as excellent biomolecules deserving a detailed investigation. ABSTRACT: The current clinical practice of breast tumor classification relies on the routine immunohistochemistry-based expression analysis of hormone receptors, which is inadequate in addressing breast tumor heterogeneity and drug resistance. MicroRNA expression profiling in tumor tissue and in the circulation is an efficient alternative to intrinsic molecular subtyping that enables precise molecular classification of breast tumor variants, the prediction of tumor progression, risk stratification and also identifies critical regulators of the tumor microenvironment. This review integrates data from protein, gene and miRNA expression studies to elaborate on a unique miRNA-based 10-subtype taxonomy, which we propose as the current gold standard to allow appropriate classification and separation of breast cancer into a targetable strategy for therapy.