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IntroductionThe female mammary gland is a very dynamic organ that undergoes continuous tissue remodelling during adulthood. Although it is well established that the number of menstrual cycles and pregnancy increase the risk of breast cancer, the reasons are unclear. Clinical and experimental evidence indicates that improper involution plays a role in the development of this malignancy. Recently, we described the miR-424(322)/503 cluster as an important regulator of mammary epithelial involution after pregnancy and that miR-424(322)/503 is commonly lost in a subset of aggressive breast cancers. Through the use of a knockout mouse model, we demonstrated for the first time that loss of miR-424(322)/503 promotes breast tumorigenesis in vivo. Remarkably, we found that loss of miR-424(322)/503 promotes chemoresistance due to the up-regulation of two of its targets: BCL-2 and insulin-like growth factor-1 receptor (IGF1R). Importantly, targeted therapies blocking the aberrant activity of these targets restore sensitivity to chemotherapy.Material and methodsPatient data were assessed from METABRIC and TCGA corresponding to breast cancer samples with available whole-genome DNA CNAs, mRNA expression data, and clinicopathological data.Targets of the miR-424(322)/503 were validated by western blot, 3’UTR reporter assays and PAR-CLIP. Wnt localization and activity was assessed by cell fractionation and qRT-PCR.Results and discussionsTumours generated in the miR-424(322)/503 knock out mouse model present morphological and molecular characteristics of metaplasic squamous cell carcinoma (SSC) of the breast. These include the presence of a non-glandular component, spindle cells and expression of cytokeratin 6. Primary SSC of the breast is a very aggressive tumour with high metastatic activity and refractory to treatment. It has been reported that nearly all the primary metaplasic carcinomas present activation of Wnt pathway, and about 40% of them carry mutations in genes of the Wnt pathway. By analysing the expression data of over 3000 breast tumours, we have determined that the loss of mir-424(322)/503 contributes to the activation of Wnt/b-catenin pathway in SSC. We also show that the miR-424(322)/503 directly targets the Wnt receptor LRP-6, preventing the translocation of b-catenin to the nucleous and inhibiting the transcription of b-catenin-regulated genes.ConclusionMiR-424(322)/503 is a tumour suppressor in breast cancer and provide a link between mammary epithelial involution, tumorigenesis, and chemoresistance.