SUN-368 Wnt Inhibition Decreases Trabecular Bone in a Mouse Model of Fibrous Dysplasia

Background: G protein-coupled receptor (GPCR) signaling mediates a wide spectrum of physiological functions, including bone development and remodeling. Fibrous dysplasia (FD) is a common skeletal dysplasia where normal bone and bone marrow are replaced by fibrous tissue and expansile trabecular bone lesions. The craniofacial bones are often involved, leading to pain and facial deformities. FD is a mosaic disease caused by a somatic mutation in the GNAS gene encoding the G-protein alpha subunit (Gsα) that leads to constitutive activation of the Gs signaling pathway. Unfortunately, FD has no effective medical treatments. Major challenges have hampered the development of pharmacologic strategies that specifically target GNAS or the Gsα protein. We previously developed the Col1(2.3)/Rs1 mouse model (Rs1) in which the Gs signaling pathway is activated specifically in bone by an engineered GPCR protein. These mice showed increased trabecular bone formation with loss of marrow space and cortical bone, which strongly resembles human FD (1–4). There was also a dramatic increase in the number of immature osteoblasts present in the FD lesions, suggesting that activation of Gs signaling caused an accumulation of these cells. Our prior studies showed increased Wnt signaling, which may be a major driver of this effect. Furthermore, blocking the Gs signaling could reverse the bone phenotype, providing proof-of-concept for finding drugs that could reverse the phenotype. Therefore, we administered the Wnt inhibitor LGK974, currently used in human clinical trials, to the Rs1 mice to test if the FD lesions could be pharmacologically reversed. Methods: We administered LGK974 in 4-week-old Rs1 and non-Rs1 mice. We used a low dose (5mg/kg) for 8 weeks or high dose (30mg/kg) for 4 weeks. The mice were evaluated by histology and micro computed tomography (micro-CT) for mineral density (mg/cm3), bone volume (mm3), and trabecular thickness (um). Results: LGK974 decreased β-catenin levels in bone on western blots. In the low-dose group, the histology and micro-CT showed no statistically significant differences between drug and control groups. In the high-dose group, the micro-CT showed significantly decreased trabecular bone thickness (p=0.0364, n=3) in the drug-treated group (22±2µm) compared with controls (17±2µm). Furthermore, histology showed resorption of the abnormal bone; however, the fibrocellular infiltrate in the Rs1 mice was still present. Conclusions: Wnt inhibition can lead to decreased fibrous dysplastic bone, but separates abnormal bone formation from the fibrocellular infiltrate. These results provide new insight into understanding interactions between the Wnt and Gs signaling pathways in FD pathogenesis and bone formation. References: 1. Hsiao EC et al. PNAS. 2008. 2. Hsiao EC et al. JBMR. 2010. 3. Schepers, Hsiao EC et al. Blood. 2012. 4. Cain CJ et al. Endocrinology. 2016.