Transcriptomic Profiling and Molecular Characterisation of Dormant and Proliferating Breast Cancer Cells in the Brain

In metastatic breast cancer, brain metastases (BrM) incidence is 15-30% and effective therapies for BrM represent an unmet clinical need. Cancer dormancy describes the extended period of time in which disseminated tumour cells stop dividing and enter a clinically undetectable, reversible state of quiescence. Under more favourable conditions, the cancer cells may be able to exit dormancy, leading to metastatic progression. It was reasoned that a deeper understanding of dormancy mechanisms may reveal novel therapeutic targets for metastatic breast cancer. I utilised a xenograft model of BrM and isolated dormant and proliferating breast adenocarcinoma (MDA-MB-231) cells from mouse brains. We performed mRNAseq and differential gene expression analysis to identify differences in gene expression profiles. GO and KEGG enrichment analysis comparing dormant and proliferating cancer cells revealed an enrichment of extracellular matrix constituents and reduced metabolic processes. This was accompanied by an enrichment of terms pertaining to HIPPO, Wnt and TGF-β signalling pathways, amongst other pathways. Inhibition of aerobic glycolysis in vitro induced reversible G2/M cell cycle arrest in MDA-MB-231 cancer cells, concurrent with phosphorylation and cytoplasmic retention of a core HIPPO transcriptional regulator, YAP. The most significantly upregulated gene identified in dormancy was the small leucine-rich proteoglycan, biglycan. In primary breast tumours, high biglycan expression was associated with increased occurrence of metastasis and relapse. In a meta-analysis of breast cancer patient gene expression data, biglycan expression was reduced in metastases compared to that of the primary tumour. Furthermore, overexpression of biglycan in vitro induced growth arrest of MDA-MB-231 cells. Immunofluorescence imaging of brain tumour xenografts revealed that biglycan expressing dormant cells as well as proliferating cancer cells in vivo localise along the brain parenchymal vasculature, implicating the perivascular niche in the induction of dormancy. The results are expected to offer a novel insight into the induction of dormancy and suggest targetable pathways for the inhibition of cancer relapse.