Background: BRCA1 mutation carriers commonly undergo prophylactic mastectomy to reduce their risk of breast cancer. The identification of an effective and acceptable prevention therapy remains a ‘holy grail’ for the field. Precancerous BRCA1mut/+ tissue contains an aberrant population of progenitor cells and deregulated progesterone signalling has been implicated in BRCA1-associated oncogenesis. Since Receptor Activator of Nuclear Factor-kappa B ligand (RANKL) is a key paracrine effector of progesterone signalling, and RANKL and its receptor RANK contribute to mammary tumorigenesis, we investigated a role for this pathway in preneoplastic breast tissue from BRCA1 mutation carriers.
Methods: Freshly isolated, histologically normal patient specimens from BRCA1 mutation carriers were studied using a variety of assays. RANK expression was evaluated in formalin fixed paraffin embedded archival sections from the kConFab and the Amgen Tissue Banks with HREC approval. The MMTV-cre/Brca1fl/fl/p53+/– mouse model was used to investigate RANKL inhibition as a chemoprevention strategy.
Results: We identified two subsets of luminal progenitors (RANK+ and RANK–) in histologically normal tissue of BRCA1 mutation carriers. RANK+ cells were highly proliferative, exhibited grossly aberrant DNA repair and bore a molecular signature similar to that of basal-like breast cancer. Moreover, high levels of RANK expression prevailed in established BRCA1-associated tumours. These data suggest that RANK+ and not RANK– progenitors are a key target population in these women. Notably, inhibition of RANKL signalling by denosumab in 3D breast organoids derived from pre-neoplastic BRCA1mut/+ tissue attenuated progesterone-induced proliferation. Furthermore, inhibition of RANKL with either the RANKL inhibitor OPG-Fc or a RANKL monoclonal antibody in a Brca1-deficient mouse model significantly curtailed mammary tumorigenesis, when compared to controls (p<0.001).
Conclusions: Together these findings identify a targetable pathway in a putative cell of origin population in BRCA1 mutation carriers and implicate RANKL blockade as a promising breast cancer prevention strategy.