Epigenome remodelling is an important mechanism of gene deregulation in cancer.

However, its potential as a target to counteract therapy resistance in breast cancer remains largely unaddressed. In this study, we reveal for the first time that epigenetic therapy with FDA and TGA approved Decitabine (5-Aza-mC), a DNA methyltransferase inhibitor, suppresses tumour growth in xenograft models of pre-clinical metastatic estrogen receptor positive (ER+) breast cancer. We found that Decitabine-induced genome-wide DNA hypomethylation specifically reprograms ER binding at enhancers and alters three-dimensional (3D) chromatin interactions, leading to activation of tumour suppressive transcriptional profiles. Importantly, long-term withdrawal of epigenetic therapy partially restores methylation at ER-enhancer elements, resulting in a loss of ectopic 3D chromatin interactions and associated gene repression. Our study illustrates the potential of epigenetic therapy to target ER+ endocrine-resistant breast cancer by DNA methylation-dependent rewiring of 3D chromatin interactions, which are associated with the suppression of tumour growth.