Molecular mechanisms and tumour heterogeneity in breast cancer

MicroRNAs play a critical role in regulation of gene expression and are known to be dysregulated in breast cancer. The aim of this study was investigate four microRNAs miR-875-5p, miR-339-5p, miR-10a and miR-379 which were selected based on their predicted binding site on target mRNAs Sodium Iodide Symporter (NIS), Retinoic acid receptor alpha (RARA), RARA, oestrogen receptor alpha (ERA) and Thyroid hormone receptor alpha (THRA) in breast cancer. The secondary aim was to investigate tumour heterogeneity through detection of Mesenchymal Stem Cells (MSCs) within the stroma of primary breast tumours. The expression of both microRNAs and target mRNAs was investigated in patient breast tissues. Gene expression analysis revealed significant loss of both RARA and THRA expression in breast cancer patients compared to healthy controls, highlighting their relevance in disease. While miR-875-5p and miR-339-5p were not found to be dysregulated in breast cancer or involved in regulation of the target genes analysed, miR-10a and miR-379 expression was found to be significantly decreased in breast cancer patients compared to healthy controls, suggesting a tumour suppressor role for these miRNAs. Further, miR-379 was linked with patient clinicopathological details and with increasing tumour stage the level of miR-379 decreased significantly. Subsequently Cyclin B1 was identified as a target gene for this miRNA. Decreased Cyclin B1 protein levels and associated inhibition of cellular proliferation was observed in the presence of a miR-379 mimic in vitro. In vivo studies revealed that cells engineered to stably express miR-379 had reduced tumour forming potential and reduced growth rate, confirming a tumour suppressor role for this miRNA in breast cancer. Analysis and culture of primary stromal cells highlighted the heterogeneity of the tumour microenvironment. Along with activated myofibroblasts, a subpopulation of MSCs was identified within the tumour microenvironment. This subset of stromal cells had characteristic MSC morphology, cell surface antigens and the proven ability to differentiate into osteoblasts and chondrocytes. This promising data revealed loss of miR-379 in the tumour setting and supports the potential for replacement strategies in the clinical setting