show Abstracthide AbstractWe hypothesized that circulating miRNAs could work as biomarkers for early detection of breast cancer brain metastasis (BCBM), and their targets could constitute new targets for modulation. We used a mouse model of BCBM and Next-Generation Sequencing to establish the circulating miRNAs alterations along brain metastasis development and performed bioinformatics analysis to identify their targets with relevance in the metastatic process. We additionally analyzed human resected brain metastasis of breast cancer patients for target's expression validation. In our mouse model, we observed a deregulation of circulating miRNAs profile during BCBM progression, with a downregulation of miR-802-5p and miR-194-5p in plasma prior to brain metastases detection. The transcription factor myocyte enhancer factor 2C (MEF2C), was identified as a target for both miRNAs, and its expression was increasingly observed in malignant cells along brain metastasis development. Its upregulation was also observed in peritumoral astrocytes and in human BCBM. Collectively, downregulation of circulating miR-802-5p and miR-194-5p appear as precocious biomarkers for BCBM and MEF2C emerges as a new player and a potential target for modulation. Overall design: We used a mouse model of breast cancer brain metastasis (BCBM), relying on the innoculation of breast cancer cells (or vehicle; control) in the carotid artery of Balb/c mouse, as a reliable model of brain metastasization of breast cancer, and collected brain and plasma samples at different time points (3, 7 and 10 days). Brain metastasis development was profiled by histologic analysis of brain sections and alterations in the circulating miRNA signature were determined by Next-Generation Sequencing miRNA analysis of plasma samples. MiRNAs with an aberrant expression prior to brain metastasis detection were selected for further study. This included the validation of their altered expression by qPCR and the identification of their potential targets with relevance for the metastatic process by bioinformatics analayis. Finnally, the expression of the most promising target was determined by immunofluorescence analysis of brain sections along metastasis development and the data obtained in the mouse model was further validated by analysis of ressected brain metastasis from breast cancer patients.