The Dysregulated Pharmacology of Clinically Relevant <i>ESR1</i> Mutants is Normalized by Ligand-activated WT Receptor

The estrogen receptor (ER/<i>ESR1</i>) is expressed in a majority of breast cancers and drugs that inhibit ER signaling are the cornerstone of breast cancer pharmacotherapy. Currently, aromatase inhibitors are the frontline endocrine interventions of choice although their durability in metastatic disease is limited by activating point mutations within the ligand-binding domain of <i>ESR1</i> that permit ligand-independent activation of the receptor. It has been suggested that the most commonly occurring <i>ESR1</i> mutations would likely compromise the clinical activity of selective estrogen receptor downregulators and selective estrogen receptor modulators (SERMs) when used as second-line therapies. It was unclear, however, how these mutations, which are likely coexpressed in cells with ER^WT, may impact response to ER ligands in a clinically meaningful manner. To address this issue, we dissected the molecular mechanism(s) underlying <i>ESR1</i>-mutant pharmacology in models relevant to metastatic disease. These studies revealed that the response of <i>ESR1</i> mutations to ligands was dictated primarily by the relative coexpression of ER^WT in cells. Specifically, dysregulated pharmacology was only evident in cells in which the mutants were overexpressed relative to ligand-activated ER^WT; a finding that highlights the role of allelism in determining ER-mutant pharmacology. Importantly, we demonstrated that the antagonist activity of the SERM, lasofoxifene, was not impacted by mutant status; a finding that has led to its clinical evaluation as a treatment for patients with advanced ER-positive breast cancer whose tumors harbor <i>ESR1</i> mutations.