Cooperative role of ESR1 mutations and midasin in endocrine resistance and the therapeutic potential of dual targeting

While most cases of estrogen receptor-positive (ER+) breast cancer initially respond to endocrine therapy, inevitably many women acquire resistance. A major hurdle hampering the response of metastatic breast cancer patients to endocrine therapy is constitutively active point mutations in the ER ligand binding domain (LBD). We previously identified midasin (MDN1), a novel ribosomal protein, which was significantly overexpressed in MCF-7 endocrine-resistant breast cancer stem cells. Additional studies also indicated that midasin was required for growth, and stemness, but there was a mutation-dependent impact of midasin inhibition on mammosphere morphology and protein expression. However, the consequences of increased MDN1 expression and ESR1 mutations remain unclear. We hypothesize that ESR1 mutations along with dysregulated MDN1 cooperate to alter the microenvironment and position breast cancer cells for a survival advantage. The objective of this study was to elucidate the mechanism by which midasin inhibition alters the biology of breast cancer cells with ESR1 mutations. To address this, breast cancer patient tumor samples were queried using CPTAC, and MDN1 protein expression was significantly elevated in luminal, HER2+, and triple-negative tumors compared with normal breast tissue. In a metastatic breast cancer cohort (n = 379), 10% of patients exhibited alterations in MDN1, whereas 25% exhibited ESR1 alterations, primarily amplifications and missense mutations. A significant co-occurrence (log₂ OR = 1.60; q = 0.027) between these alterations suggested functional interplay between midasin and ER signaling. Molecular docking identified Rbin-2 as a compound interacting with midasin (docking score: -5.70). Proliferation assays using MCF-7 wild-type, MCF-7 D538G , and MCF-7 Y537S cells showed that 12 µM Rbin-2 produced ~50% growth inhibition. In 3D spheroid assays, Rbin-2 significantly decreased invasion, disrupted spheroid integrity, and promoted cell death, particularly in ER-mutant cells. Given these results and the clinical data, we examined co-targeting ER point mutations and midasin. Cells treated with 12 µM Rbin-2 plus 2.5-10 µM elacestrant exhibited ~79% growth inhibition. Combination Index analysis showed the strongest synergy in MCF-7 Y537S and MCF-7 D538G cells compared with the wildtype cells. Immunoblots demonstrated that elacestrant reduced ER and midasin expression, while the combination produced a synergistic decrease in MDN1, supporting a cooperative mechanism between ER signaling and midasin. Overall, our results indicate that aberrant ESR1 activity and elevated MDN1 cooperate to drive aggressive tumor phenotypes, and that simultaneous inhibition of ER and midasin may offer a novel therapeutic avenue for patients with ESR1-mutant, endocrine-resistant breast cancer.