Project Summary
Fibrous dysplasia (FD) affects approximately 1 in 5000 individuals and causes focal bone lesions that result in
pain, deformity and fracture susceptibility. FD lesions are characterized by an accumulation of immature
osteoblast lineage cells as well as robust osteoclastogenesis with increased bone turnover. Receptor activator
of NFkB ligand (RANKL), the key cytokine driving osteoclast differentiation, is highly expressed by osteoblastic
cells in FD lesions. Neutralizing antibody against RANKL, aRANKL, prevents osteoclast formation and
decreases lesion size in mouse models of FD. In patients with FD, denosumab, which targets RANKL, is the
only effective treatment. However, denosumab can inhibit skeletal growth and withdrawal can cause rebound
bone resorption and fractures. Thus, alternative therapies are needed. If we could understand how aRANKL
inhibits FD bone lesions, we might be able to discover new treatment targets.
To investigate the effect of aRANKL antibody treatment in FD, we used an established mouse model
where GnasR201H expression is induced post-zygotically in skeletal stem cells; bone lesions in this model are
osteoblastic precursors which are a mosaic of wildtype and lineage traced mutant osteoblastic cells. We
demonstrated that aRANKL inhibits the fibrotic bone lesions of FD in this model. At the cellular level, we
demonstrated that aRANKL treatment altered the phenotype of Gnas mutant cells and also had a significant
impact on wildtype osteoblastic cells, inhibiting proliferation and promoting differentiation of both.
The cellular mechanism responsible for the effect of aRANKL is not known. While RANKL is best known
for its role in driving osteoclast differentiation, it has many other biologic functions. For example, RANKL is critical
for the development of lymph nodes and some specialized epithelial cells, and it can stimulate osteoblast
differentiation through reverse signaling. Thus, we hypothesize that RANKL may promote FD bone lesion
pathogenesis in an osteoclast-independent manner and that the critical target of aRANKL is RANKL itself. We
will test this hypothesis by comparing the efficacy of osteoclast inhibition with anti-CSF1R (which blocks a key
survival factor for osteoclasts) with aRANKL in inhibiting FD lesions in this model.
The molecular mechanism by which aRANKL treatment alters the cellular phenotype of Gnas mutant
cells and wild-type cells in FD lesions is unknown. To investigate the pathways downstream of aRANKL
treatment, we propose to determine the transcriptional changes induced in Gnas mutant cells and adjacent wild-
type osteoblastic cells using a spatial transcriptomic approach using the GeoMx digital spatial profiler platform.
Through a combination of determining whether aRANKL efficacy is independent of osteoclast inhibition and
molecular investigation of tissue and cell level response to aRANKL, this proposal will significantly advance our
understanding of FD pathogenesis and identify additional pathways that could be targeted for treatment.
