Induced Gnas ^R201H expression from the endogenous Gnas locus causes fibrous dysplasia by up-regulating Wnt/β-catenin signaling

Abstract

Fibrous dysplasia (FD; Online Mendelian Inheritance in Man no. 174800) is a crippling skeletal disease caused by activating mutations of the gene, which encodes the stimulatory G protein Gα FD can lead to severe adverse conditions such as bone deformity, fracture, and severe pain, leading to functional impairment and wheelchair confinement. So far there is no cure, as the underlying molecular and cellular mechanisms remain largely unknown and the lack of appropriate animal models has severely hampered FD research. Here we have investigated the cellular and molecular mechanisms underlying FD and tested its potential treatment by establishing a mouse model in which the human FD mutation (R201H) has been conditionally knocked into the corresponding mouse locus. We found that the germ-line FD mutant was embryonic lethal, and Cre-induced FD mutant expression in early osteochondral progenitors, osteoblast cells, or bone marrow stromal cells (BMSCs) recapitulated FD features. In addition, mosaic expression of FD mutant Gα in BMSCs induced bone marrow fibrosis both cell autonomously and non-cell autonomously. Furthermore, Wnt/β-catenin signaling was up-regulated in FD mutant mouse bone and BMSCs undergoing osteogenic differentiation, as we have found in FD human tissue previously. Reduction of Wnt/β-catenin signaling by removing one copy in an FD mutant line significantly rescued the phenotypes. We demonstrate that induced expression of the FD Gα mutant from the mouse endogenous locus exhibits human FD phenotypes in vivo, and that inhibitors of Wnt/β-catenin signaling may be repurposed for treating FD and other bone diseases caused by Gα activation.

RANKL Inhibition in Fibrous Dysplasia of Bone: A Preclinical Study in a Mouse Model of the Human Disease

“…Recently, a role for receptor activator of nuclear factor kappa‐B ligand (RANKL; a member of the tumour necrosis factor superfamily and a potent stimulator of osteoclastogenesis), in the disease has emerged from studies performed on human skeletal progenitors transduced with G s α R201C , transgenic mice expressing G s α R201C in skeletal cells, and human FD tissue and cells . In addition, serum levels of RANKL have been reported to be increased in mouse models that replicate human FD, as well as in FD patients, in which they strongly correlated with the burden of the disease . Accordingly, a limited number of studies have already tested the effect of the humanized anti‐RANKL antibody, denosumab, in FD patients and have reported a positive effect on bone turnover, on the growth rate of lesions, as assessed by CT analysis, and on bone pain .…”

An Inverse Agonist Ligand of the PTH Receptor Partially Rescues Skeletal Defects in a Mouse Model of Jansen's Metaphyseal Chondrodysplasia

“…Potential effects on bone length are of interest because impaired limb development and reduced overall height are prominent clinical features of JMC . An inverse agonist could conceivably induce changes in bone length in C1HR mice via actions in the growth plates, because expression of the PTHR1‐H223R transgene is detected at low levels in growth plate chondrocytes of the C1HR mice, and PTHR1 signaling is well known to play a major role in growth plate chondrocyte differentiation and hence bone growth . We administered the inverse agonist starting at 7 days of age, and so the analog was present in the mice during a period of active bone growth.…”

“…(6) An inverse agonist could conceivably induce changes in bone length in C1HR mice via actions in the growth plates, because expression of the PTHR1-H223R transgene is detected at low levels in growth plate chondrocytes of the C1HR mice, (30) and PTHR1 signaling is well known to play a major role in growth plate chondrocyte differentiation and hence bone growth. (39,(43)(44)(45) We administered the inverse agonist starting at 7 days of age, and so the analog was present in the mice during a period of active bone growth. Elucidating how processes of growth plate maturation and bone growth might be altered by constitutive PTHR1 signaling and potentially modulated with an inverse agonist ligand are important research objectives, given the impact that such altered signaling has on skeletal development in children with JMC.…”

Clinical and translational pharmacological aspects of the management of fibrous dysplasia of bone

“…Several models have been developed; however, none have been able to fully capture all the characteristics of the human disease. Specific models mimicking the skeletal disease or the extraskeletal manifestations came to development allowing the possibility to not only evaluate the anti‐RANKL antibody denosumab, but also the speculation on new therapies. Zhao and colleagues [2] recently developed a conditional tetracycline‐inducible animal model expressing the Gsα mutation (R201C) in the skeletal stem cells.…”