MicroRNAs in HER2-Amplified Breast Cancer

Abstract: Background: Breast cancer is the most common female malignancy and the leading cause of cancer-related deaths worldwide. Targeted therapy against the main biomarkers estrogen receptor alpha (ER) and human epidermal growth factor receptor 2 (HER2/ERBB2) have greatly improved mortality rates, but ab initio or acquired therapy resistance is common. It is imperative to identify patients who will benefit from targeted therapy and develop new treatment modalities for resistant tumors. microRNAs (miRNAs) are small, non-coding RNAs that regulate expression of up to 60% of human protein-coding genes. Their frequent deregulation in cancer makes miRNAs attractive candidate biomarkers and clinical targets. We previously reported that the HER2 locus, which is amplified in 15-20% of breast cancers, also encodes an intronic miRNA, mir-4728. Aims: The aim of this thesis was to study the function of the main mature miRNA product of mir-4728, miR-4728-3p, and to investigate if this miRNA could serve as an independent diagnostic or treatment predictive biomarker in breast cancer. Results: Expression of miR-4728-3p correlated with HER2, and ER was identified as a direct target of the miRNA, connecting these two important breast cancer biomarkers. Regulation of ER occurred in a non-canonical manner, expanding our understanding of miRNA-mediated target gene regulation. During functional studies in vitro, we sometimes detected strong, batch-dependent off-target effects of commercially available miRNA mimics, stressing the need to maintain high quality standards in clinical studies using miRNA replacement therapy. Furthermore, inhibition of miR-4728-3p led to upregulation of the predicted target PAPD5, a non-canonical poly(A) polymerase, that destabilized the oncogenic microRNA miR-21. HER2 signaling is known to activate mir-21 transcription through the MAPK/ERK pathway, suggesting that the HER2 locus has a dual role in increasing levels of miR-21 through both miR-4728-3p and HER2. Blocking miR-4728-3p also strongly inhibited cellular metabolism, indicating a role for the miRNA in improved nutrient processing. Taken together, these results establish the importance of miR-4728-3p as a breast cancer oncogene.