Single-cell Analysis Reveals Inter- and Intratumour Heterogeneity in Metastatic Breast Cancer.

Metastasis is the leading cause of cancer-related deaths of breast cancer patients. Some cancer cells in a tumour go through successive steps, referred to as the metastatic cascade, and give rise to metastases at a distant site. We know that the plasticity and heterogeneity of cancer cells play critical roles in metastasis but the precise underlying molecular mechanisms remain elusive. Here we aimed to identify molecular mechanisms of metastasis during colonization, one of the most important yet poorly understood steps of the cascade. We performed single-cell RNA-Seq (scRNA-Seq) on tumours and matched lung macrometastases of patient-derived xenografts of breast cancer. After correcting for confounding factors such as the cell cycle and the percentage of detected genes (PDG), we identified cells in three states in both tumours and metastases. Gene-set enrichment analysis revealed biological processes specific to proliferation and invasion in two states. Our findings suggest that these states are a balance between epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial transitions (MET) traits that results in so-called partial EMT phenotypes. Analysis of the top differentially expressed genes (DEGs) between these cell states revealed a common set of partial EMT transcription factors (TFs) controlling gene expression, including ZNF750, OVOL2, TP63, TFAP2C and HEY2. Our data suggest that the TFs related to EMT delineate different cell states in tumours and metastases. The results highlight the marked interpatient heterogeneity of breast cancer but identify common features of single cells from five models of metastatic breast cancer.

A high-throughput drug screen reveals means to differentiate triple-negative breast cancer.

Plasticity delineates cancer subtypes with more or less favourable outcomes. In breast cancer, the subtype triple-negative lacks expression of major differentiation markers, e.g., estrogen receptor α (ERα), and its high cellular plasticity results in greater aggressiveness and poorer prognosis than other subtypes. Whether plasticity itself represents a potential vulnerability of cancer cells is not clear. However, we show here that cancer cell plasticity can be exploited to differentiate triple-negative breast cancer (TNBC). Using a high-throughput imaging-based reporter drug screen with 9 501 compounds, we have identified three polo-like kinase 1 (PLK1) inhibitors as major inducers of ERα protein expression and downstream activity in TNBC cells. PLK1 inhibition upregulates a cell differentiation program characterized by increased DNA damage, mitotic arrest, and ultimately cell death. Furthermore, cells surviving PLK1 inhibition have decreased tumorigenic potential, and targeting PLK1 in already established tumours reduces tumour growth both in cell line- and patient-derived xenograft models. In addition, the upregulation of genes upon PLK1 inhibition correlates with their expression in normal breast tissue and with better overall survival in breast cancer patients. Our results indicate that differentiation therapy based on PLK1 inhibition is a potential alternative strategy to treat TNBC.

Glucocorticoids promote breast cancer metastasis.

Diversity within or between tumours and metastases (known as intra-patient tumour heterogeneity) that develops during disease progression is a serious hurdle for therapy1-3. Metastasis is the fatal hallmark of cancer and the mechanisms of colonization, the most complex step in the metastatic cascade4, remain poorly defined. A clearer understanding of the cellular and molecular processes that underlie both intra-patient tumour heterogeneity and metastasis is crucial for the success of personalized cancer therapy. Here, using transcriptional profiling of tumours and matched metastases in patient-derived xenograft models in mice, we show cancer-site-specific phenotypes and increased glucocorticoid receptor activity in distant metastases. The glucocorticoid receptor mediates the effects of stress hormones, and of synthetic derivatives of these hormones that are used widely in the clinic as anti-inflammatory and immunosuppressive agents. We show that the increase in stress hormones during breast cancer progression results in the activation of the glucocorticoid receptor at distant metastatic sites, increased colonization and reduced survival. Our transcriptomics, proteomics and phospho-proteomics studies implicate the glucocorticoid receptor in the activation of multiple processes in metastasis and in the increased expression of kinase ROR1, both of which correlate with reduced survival. The ablation of ROR1 reduced metastatic outgrowth and prolonged survival in preclinical models. Our results indicate that the activation of the glucocorticoid receptor increases heterogeneity and metastasis, which suggests that caution is needed when using glucocorticoids to treat patients with breast cancer who have developed cancer-related complications.

Early dissemination seeds metastasis in breast cancer.

Accumulating data suggest that metastatic dissemination often occurs early during tumour formation, but the mechanisms of early metastatic spread have not yet been addressed. Here, by studying metastasis in a HER2-driven mouse breast cancer model, we show that progesterone-induced signalling triggers migration of cancer cells from early lesions shortly after HER2 activation, but promotes proliferation in advanced primary tumour cells. The switch from migration to proliferation was regulated by increased HER2 expression and tumour-cell density involving microRNA-mediated progesterone receptor downregulation, and was reversible. Cells from early, low-density lesions displayed more stemness features, migrated more and founded more metastases than cells from dense, advanced tumours. Notably, we found that at least 80% of metastases were derived from early disseminated cancer cells. Karyotypic and phenotypic analysis of human disseminated cancer cells and primary tumours corroborated the relevance of these findings for human metastatic dissemination.