The bone microenvironment as a master regulator of disseminated tumour cells responsible for breast cancer recurrence

Advanced breast cancer is frequently associated with skeletal metastases characterised by dormancy and subsequent incurable metastatic outgrowth accompanied by skeletal related events. Disseminated tumour cells putatively assume residence in metastatic niches within the bone microenvironment, in locations that may be regularly occupied by resident cells and regulated by neighbouring cells in response to local and systemic signals. My studies explored how modification of the bone microenvironment using a dietary approach (low calcium), a surgical intervention (ovariectomy) and pharmacological inhibition of bone resorption (Zol) impacted breast cancer cell development and progression within the bone in vivo. The effect of a low calcium diet (0.1%) on the bone microenvironment of mature (12-week old) and dormant disseminated tumour cells was investigated in the absence and presence of bone-colonising tumour cells. In the absence of tumour cells, minor reductions of percent bone volume was detected after 28 days, with a reduction in PINP and increase in TRAP, without significant change to gene expression. Without existing literature on the effect of a low calcium diet on the outgrowth of dormant disseminated breast cancer cells, my research showed a low calcium diet in isolation did not sufficiently alter the bone microenvironment to trigger the outgrowth of dormant tumour cells. To investigate the effect of repeated doses of Zol on outgrowing tumour cells in bone with rapid turnover, young (6-week old) mice were treated with four once-weekly clinically-relevant doses. Zol significantly increased trabecular percent bone volume, trabecular number and reduced trabecular separation. In agreement with existing literature Zol did not prevent or significantly delay the outgrowth of tumours in the bone with rapid turnover. Profiling transcriptional changes to the mature (12-week old) bone microenvironment 14 days after ovariectomy, which has been shown to induce the outgrowth of dormant disseminated tumour cells, by RNA-seq revealed POSTN, MMP2, THBS2 and OPN as genes influencing the altered bone microenvironment. Comparison with publicly available RNA-seq data on the bone microenvironment of young (~30 years old), old (~73 years old) and old women treated with oestrogen for 3 weeks (~ 70 years old) found overlap between genes expressed by both young women and old women, suggesting caution in comparison. Comparing genes altered in the bone microenvironment of ovariectomised mice did not show overlap with genes found from in vitro dormant tumour cells. Deconvolution of bulk RNA-seq data to infer cell types did not show any significant differences. Further studies on dormancy gene signatures within the bone microenvironment are required.