Only a handful of somatic alterations have been linked to endocrine therapy resistance in hormone-dependent breast cancer (HDBC), potentially explaining ~40% of relapses. If other mechanisms underlie the evolution of HDBC under adjuvant therapy is currently unknown. In this work, we employ functional genomics to dissect the contribution of cis-regulatory elements (CREs) to cancer evolution by focusing on 12 megabases of non-coding DNA, including clonal enhancers, gene promoters, and boundaries of topologically associating domains. Parallel epigenetic perturbation (CRISPRi) in vitro reveals context-dependent roles for many of these CREs, with a specific impact on dormancy entrance and endocrine therapy resistance. Profiling of CRE somatic alterations in a unique, longitudinal cohort of patients treated with endocrine therapies identifies a limited set of non-coding changes potentially involved in therapy resistance. Overall, our data uncover how endocrine therapies triggers the emergence of transient features which could ultimately be exploited to hinder the adaptive process.

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