BACKGROUND: Breast cancer is a leading cause of cancer-related death in women. Most cases are invasive ductal carcinomas of no special type (NST breast carcinomas). METHODS AND RESULTS: In this prospective, multicentric biomarker discovery study, we analyzed the expression of small non-coding RNAs (mainly microRNAs) in plasma by qPCR and evaluated their association with NST breast cancer. Large-scale expression profiling and subsequent validations have been performed in patient and control groups and compared with clinicopathological data. Small nuclear U6 snRNA, miR-548b-5p and miR-451a have been identified as candidate biomarkers. U6 snRNA was remarkably overexpressed in all the validations, miR-548b-5p levels were generally elevated and miR-451a expression was mostly downregulated in breast cancer groups. Combined U6 snRNA/miR-548b-5p signature demonstrated the best diagnostic performance based on the ROC curve analysis with AUC of 0.813, sensitivity 73.1% and specificity 82.6%. There was a trend towards increased expression of both miR-548b-5p and U6 snRNA in more advanced stages. Further, increased miR-548b-5p levels have been partially associated with higher grades, multifocality, Ki-67 positivity, and luminal B rather than luminal A samples. On the other hand, an association has been observed between high miR-451a expression and progesterone receptor positivity, lower grade, unifocal samples, Ki-67-negativity, luminal A rather than luminal B samples as well as improved progression-free survival and overall survival. CONCLUSIONS: Our results indicated that U6 snRNA and miR-548b-5p may have pro-oncogenic functions, while miR-451a may act as tumor suppressor in breast cancer.

3rd Department of Medicine Department of Endocrinology and Metabolism 1st Faculty of Medicine Charles University and General University Hospital Prague Prague 2 Czech Republic

Department of Obstetrics and Gynecology Masaryk University and University Hospital Brno Obilní trh 11 602 00 Brno Czech Republic

Department of Pathology Faculty of Medicine Masaryk University and University Hospital Brno Obilní trh 11 602 00 Brno Czech Republic

Faculty Transfusion Center General University Hospital Prague U Nemocnice 2 128 08 Prague Czech Republic

Institute for Mother and Child Care Podolské nábřeží 157 36 147 00 Prague Podolí Czech Republic

Institute of Biology and Medical Genetics 1st Faculty of Medicine Charles University Prague and General University Hospital Prague Albertov 4 128 00 Prague Czech Republic

Institute of Pharmacology 1st Faculty of Medicine Charles University Prague and General University Hospital Prague Albertov 4 128 00 Prague Czech Republic

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