Exploring the intrinsic and extrinsic factors that regulate breast cancer cell dormancy

Breast cancer can recur in patients months to decades after initial diagnosis and treatment. There is mounting evidence that dormant breast disseminated tumor cells (DTCs) exist in distant organs, whose reactivation results in cancer recurrence. However, the mechanisms that control tumor cell dormancy remain poorly understood, making it difficult to predict which patients will recur and develop cancer recurrence. Unfortunately, the extreme rarity of dormant DTCs has been the major obstacle to their study. To overcome this challenge, we developed an efficient system to isolate and study rare dormant tumor cells from metastatic organs. Using this system and single cell RNA-sequencing (scRNA-seq), we identified a group of genes including Cfh, Gas6, Mme and Ogn that were differentially expressed in dormant breast cancer cells present in the bone and lung. While modulation of these genes individually had no impact on the metastatic behavior of breast cancer cells, we found that as a group, these genes predicted the dormancy phenotype in murine breast cancer models. Importantly, expression of these genes in primary human breast cancer tumors correlated with disease-free survival, suggesting these genes may have predictive value in determining which patients are likely to recur.To move beyond the intrinsic factors that control breast cancer dormancy, we explored how therapy-induced microenvironmental changes contributed to breast cancer DTC dormancy. Genotoxic chemotherapy is often used in breast cancer patients as adjuvant or neoadjuvant therapy. In addition to targeting tumor cells, these therapies can induce a variety of adverse effects ranging from hair loss, fatigue, low blood counts, and bone loss to the promotion of distant metastasis. How chemotherapy affects dormant breast cancer DTCs has not be illustrated. In a well-characterized breast cancer dormancy model (D2.0R), we found that doxorubicin (Doxo), a commonly used chemotherapy in breast cancer, promoted the seeding of D2.0R cells systematically and drove dormant D2.0R cells particularly in epididymal white adipose tissue (eWAT) to grow. Moreover, a high-fat diet (HFD) further exacerbated the effect of Doxo on dormant D2.0R cells, resulting in robust outgrowth in eWAT. Interestingly, we found that this dormancy-breaking characteristic was unique to doxorubicin as other chemotherapy reagents including paclitaxel, vincristine and cisplatin failed to activate dormant D2.0R cells. Although preliminary investigations failed to elucidate the mechanism by which Doxo drove the outgrowth of dormant D2.0R cells, our data provide novel insights into an adverse effect of doxorubicin that stimulates dormant breast cancer DTC reactivation. In summary, we identified an intrinsic gene expression signature of dormant breast cancer cells that is present in both a primary and metastatic setting. Furthermore, we found an extrinsic condition that led to reactivation of dormant breast cancer DTCs, both of which have potential clinical implications for treating breast cancer patients