Super-resolution microscopy reveals distinct epigenetic states regulated by estrogen receptor activity.

Changes in gene expression regulated by ligand-dependent transcription factors such as estrogen receptor-α (ERα) involves the recruitment of coactivators including p300 that acetylates histone H3 at lysine 27 (H3K27ac). While H3K27ac marks active enhancers, the detailed chromatin architecture of enhancers remains unclear. Using super-resolution microscopy, we reveal distinct structural states of H3K27ac modified chromatin in response to ERα activation. In estradiol (E2)-treated cells, H3K27ac modified chromatin adopts open, elongated structures associated with active enhancers, while ERα inhibition induces compact, spherical H3K27ac modified chromatin conformations linked to repression. A constitutively active ERα mutation linked to endocrine therapy resistance in breast cancer maintains open chromatin states independent of ligand, suggesting sustained transcriptional activity. Our findings provide the first direct visualization of H3K27ac associated chromatin structural dynamics, challenging the assumption that H3K27ac modification alone is sufficient to lead to enhancer activation. By demonstrating that H3K27ac architecture is dynamically regulated by ERα, we establish a new paradigm for understanding epigenetic regulation and highlight potential therapeutic targets for endocrine therapy resistant cancers.