Three-dimensional oxabicycloheptene sulfonate targets the homologous recombination and repair programmes through estrogen receptor α antagonism

Selective estrogen receptor modulators (SERMs) are a class of structurally diverse compounds, which have been extensively used to treat hormone-responsive cancers due to their unique partially agonistic and antagonistic properties toward estrogen receptors. Our previous studies have identified a three-dimensional SERM, oxabicycloheptene sulfonate (OBHS), as an estrogen receptor α (ERα) ligand, which is effective for the prevention and treatment of estrogen-dependent endometriosis in vivo. Here, using genome-wide ChIP-seq and RNA-seq analysis, we report that OBHS rapidly induces genome-wide ERα occupancy and acts as a partial agonist and antagonist for ERα. Interestingly, OBHS downregulates the homologous recombination and repair (HRR) modules, resulting in increased DNA damage, apoptosis and cell cycle arrest, inducing synthetic lethality with poly (ADP-ribose) polymerase (PARP) inhibitor olaparib through ERα antagonism. Mechanistically, OBHS impairs the RNA polymerase II (Pol II) loading at the promoters of estrogen-responsive HRR genes. Furthermore, combination therapy of OBHS with olaparib significantly reduces the tumour burden and delays the progression of breast cancer in vivo. Together, our studies not only characterise a novel SERM which uniquely targets the homologous recombination and repair programmes through ERα antagonism but also propose a synthetic lethal strategy by combining OBHS with PARP inhibitor olaparib for ERα-responsive cancers.