The heterogeneous nature of mammary tumours may arise from different initiating genetic lesions occurring in distinct cells of origin. Here, we generated mice in which Brca2, Pten and p53 were depleted in either basal mammary epithelial cells or luminal oestrogen receptor (ER)-negative cells. Basal cell-origin tumours displayed similar histological phenotypes, regardless of the depleted gene. In contrast, luminal ER-negative cells gave rise to diverse phenotypes, depending on the initiating lesions, including both ER-negative and, strikingly, ER-positive invasive ductal carcinomas. Molecular profiling demonstrated that luminal ER-negative cell-origin tumours resembled a range of the molecular subtypes of human breast cancer, including basal-like, luminal B and 'normal-like'. Furthermore, a subset of these tumours resembled the 'claudin-low' tumour subtype. These findings demonstrate that not only do mammary tumour phenotypes depend on the interactions between cell of origin and driver genetic aberrations, but also multiple mammary tumour subtypes, including both ER-positive and -negative disease, can originate from a single epithelial cell type. This is a fundamental advance in our understanding of tumour aetiology.

Vestigial-like 1 (VGLL1) is a poorly characterized gene encoding a transcriptional co-activator structurally homologous to TAZ and YAP that modulates the Hippo pathway in Drosophila. In this study, we examined the expression of VGLL1 and its intronic miRNA, miR-934, in breast cancer. VGLL1 and miR-934 expression miRNA profiling was carried out on frozen samples of grade 3 invasive ductal carcinomas. VGLL1 protein was also examined in 433 sporadic and BRCA1-associated breast carcinomas on tissue microarrays. RNA-seq data from The Cancer Genome Atlas (TCGA) was used to confirm differences in VGLL1 and miR-934 expression in different breast cancer subtypes, and to correlate their expression with that of other genes and miRNAs. Of 28 miRNAs differentially expressed in estrogen receptor (ER)-positive and ER-negative grade 3 breast carcinomas, miR-934 was most strongly upregulated in ER-negative carcinomas, and its expression was correlated with that of VGLL1. Nuclear VGLL1 expression was observed in 13% of sporadic breast carcinomas, and while VGLL1 was only occasionally found in luminal A (0.70%) and B (5.60%) carcinomas, it was often expressed in HER2-positive (17%), triple-negative (TN) breast carcinomas (O40%) and BRCA1-associated TN carcinomas (O50%). These findings were confirmed in the TCGA dataset, which revealed positive associations with luminal progenitor genes (GABRP, SLC6A14, FOXC1, PROM1, and BBOX1) and strong negative correlations with ER-associated genes (ESR1, C6ORF211, GATA3, and FOXA1). Moreover, VGLL1 expression was associated with reduced overall survival. In conclusion, VGLL1 and miR-934 are mainly expressed in sporadic and BRCA1-associated TN basal-like breast carcinomas, and their coordinated expression, at least partially mediated by the direct modulation of ESR1, might be involved in the maintenance of a luminal progenitor phenotype.

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