Long-term multi-modal recording reveals epigenetic adaptation routes in dormant breast cancer cells [scRNA-Seq]

Patients diagnosed with hormone dependent breast cancer (HDBC) receive five or more years of adjuvant endocrine therapies (ETs). Adjuvant ETs delay relapse by targeting clinically undetectable micro-metastatic deposits, yet up to 50% of patients receiving ET relapse at a constant rate over the course of decades after surgery. The mechanisms driving these clinical dynamics are largely unknown but likely involve dormancy. We developed two approaches to study the fate of dormant cells in long-term experiments. Firstly, we profiled samples from late relapse (10-35 yrs.) and patients treated with 16-44 months of neoadjuvant ETs until progression to dissect the contribution of genetic and transcriptional changes to tumour awakening. Next, we developed a first of its kind in vitro evolutionary study to systematically record adaptive strategies of individual lineages in unperturbed parallel experiments through several months. Collectively our data demonstrate that ETs induce a heritable cell state transition into dormancy via epigenetic reprogramming. Single lineages with divergent phenotypes awaken unpredictably via epigenetic erosion. Targeting the dormant epigenome shows promising activity against adapting cancer cells. Overall, this study uncovers the contribution of epigenetic adaptation to the evolution of resistance to ETs with profound implications for breast cancer.