The relationship between higher serum sex steroid

The relationship between higher serum sex steroid levels and increased breast cancer risk is well established in postmenopausal women (Fourkala et al., 2012; Key et al., 2011), but documenting a premenopausal link is challenging due to hormonal fluctuations associated with menstrual cycle progression (Key et al., 2013; Kaaks et al., 2014). The temporal dynamics of oestradiol (E2), P, and other hormones during the menstrual cycle may be more important for breast cancer risk than absolute hormonal levels at a single time point (Brisken, 2013). Moreover, hormonal context, life stage, and reproductive status may significantly impact hormone-associated risk: transient exposure to high levels of P in women with multiple pregnancies has not been associated with increased breast cancer risk, whereas persistent exposure to relatively low progestin levels postmenopausally has been associated with breast cancer in epidemiologic (Beral, 2003) and clinical (Chlebowski et al., 2003) studies. BRCA mutations may lead to cell-autonomous defects including defects in chromosome duplication and cytokinesis (Venkitaraman, 2014). Although cell nonautonomous alterations such as hormonal alterations leading to aberrant growth of hormone-sensitive target endothelin receptor antagonist may be particularly relevant to sporadic cancers (Veronesi et al., 2005), recent evidence linking BRCA-associated risk and hormonal factors (Domchek et al., 2010; Dubeau, 2015) suggests an interplay between cell-autonomous and cell nonautonomous factors in BRCA-mutation carriers. Preclinical studies (Chodankar et al., 2005; Hong et al., 2010; Yen et al., 2012) show mice carrying a BRCA1 mutation in the steroid-hormone-producing granulosa cells have a longer pro-oestrous phase, corresponding with the oestrogen-dominant follicular phase of the human menstrual cycle, as well as elevated basal E2 levels and evidence of increased oestrogen exposure in target organs such as bones. Recently, we demonstrated altered endometrial thickness and higher E2 and P levels in well-defined parts of the luteal phase in BRCA-mutation carriers vs wild-type controls (Widschwendter et al., 2013). Targeting cell nonautonomous hormonal targets may thus be an effective, noninvasive strategy for breast cancer prevention in BRCA-mutation carriers. A majority of breast cancers among BRCA1-mutation carriers are oestrogen-receptor negative (Foulkes et al., 2004), and removal of ovaries in BRCA1-mutation carriers with an oestrogen-receptor negative breast cancer dramatically reduces breast cancer-specific mortality (hazard ratio 0.07, p=0.009) (Metcalfe et al., 2015), suggesting that breast cancer biology in BRCA1-mutation carriers is determined by ovarian hormones other than oestrogens. Hence, strategies targeting cell nonautonomous oestrogen pathways (i.e. selective oestrogen receptor modulators like tamoxifen or aromatase inhibitors), although successful for primary prevention and adjuvant treatment in oestrogen receptor-positive cancers (Cuzick et al., 2015; Cuzick et al., 2014; Forbes et al., 2008), are unlikely to be successful in BRCA1-mutation carriers. Therefore, we aimed to explore alternative pathways as targets for breast cancer prevention in BRCA carriers. Receptor activator of nuclear factor kappa-B ligand (RANKL), a member of the tumour necrosis factor (TNF) superfamily, plays a key role in bone remodelling and immune function. RANKL is an important mediator of sex hormone-driven mammary gland development, proliferation, and carcinogenesis (Gonzalez-Suarez et al., 2010; Schramek et al., 2010; Wood et al., 2013). Blocking RANKL (Gonzalez-Suarez et al., 2010; Schramek et al., 2010; Joshi et al., 2010) or progesterone-receptor (PgR) pathways (Poole et al., 2006) substantially reduces mammary cancers in mice. In bone, one of the other main sources of RANKL and its physiological antagonist osteoprotegerin (OPG), evidence suggests a direct and inverse tissue/serum relation for OPG and RANKL (Findlay et al., 2008).