Fibrous dysplasia of bone (FD) is a crippling skeletal disease associated with post zygotic mutations (R201C, R201H) of the gene encoding the α subunit of the stimulatory G protein, Gs. By causing a characteristic structural subversion of bone and bone marrow, the disease results in deformity, hypomineralization, and fracture of the affected bones, with severe morbidity arising in childhood or adolescence. Lack of inheritance of the disease in humans is thought to reflect embryonic lethality of germline-transmitted activating Gsα mutations, which would only survive through somatic mosaicism. We have generated multiple lines of mice that express GsαR201C constitutively and develop an inherited, histopathologically exact replica of human FD. Robust transgene expression in neonatal and embryonic tissues, and embryonic stem (ES) cells was associated with normal development of skeletal tissues and differentiation of skeletal cells. As in humans, FD lesions in mice developed only in the postnatal life; a defined spatial and temporal pattern characterized the onset and progression of lesions across the skeleton. In individual bones, lesions developed through a sequence of three distinct histopathological stages: a primary modeling phase defined by endosteal/medullary excess bone formation, and normal resorption; a secondary phase, with excess, inappropriate remodeling; and a tertiary fibrous dysplastic phase, which reproduced a full-blown replica of the human bone pathology in mice of age ≥1 year. Gsα mutations are sufficient to cause FD, and are per se compatible with germline transmission and normal embryonic development in mice. Our novel murine lines constitute the first model of FD.

Fibrous dysplasia of bone/McCune‐Albright syndrome (Polyostotic FD/MAS; OMIM#174800) is a crippling skeletal disease caused by gain‐of‐function mutations of Gsα. Enhanced bone resorption is a recurrent histological feature of FD and a major cause of fragility of affected bones. Previous work suggests that increased bone resorption in FD is driven by RANKL and some studies have shown that the anti‐RANKL monoclonal antibody, denosumab, reduces bone turnover and bone pain in FD patients. However, the effect of RANKL inhibition on the histopathology of FD and its impact on the natural history of the disease remain to be assessed. In this study, we treated the EF1α‐GsαR201C mice, which develop an FD‐like phenotype, with an anti‐mouse RANKL monoclonal antibody. We found that the treatment induced marked radiographic and microscopic changes at affected skeletal sites in 2‐month‐old mice. The involved skeletal segments became sclerotic due to the deposition of new, highly mineralized bone within developing FD lesions and showed a higher mechanical resistance compared to affected segments from untreated transgenic mice. Similar changes were also detected in older mice with a full‐blown skeletal phenotype. The administration of anti‐mouse RANKL antibody arrested the growth of established lesions and, in young mice, prevented the appearance of new ones. However, after drug withdrawal, the newly formed bone was remodelled into FD tissue and the disease progression resumed in young mice. Taken together, our results show that the anti‐RANKL antibody significantly affected the bone pathology and natural history of FD in the mouse. Pending further work on the prevention and management of relapse after treatment discontinuation, our preclinical study suggests that RANKL inhibition may be an effective therapeutic option for FD patients. © 2019 American Society for Bone and Mineral Research.

We recently reported the generation and initial characterization of the first direct model of human Fibrous Dysplasia (OMIM #174800), obtained through the constitutive systemic expression of one of the disease causing mutations, GsαR201C, in the mouse. To define the specific pathogenetic role(s) of individual cell types within the stromal/osteogenic system in FD, we generated mice expressing GsαR201C selectively in mature osteoblasts using the 2.3kb Col1a1 promoter. We show here that this results in a striking high bone mass phenotype, but not in a mimicry of human FD. The high bone mass phenotype involves specifically a deforming excess of cortical bone and prolonged and ectopic cortical bone remodeling. Expression of genes characteristic of late stages of bone cell differentiation/maturation is profoundly altered as a result of expression of GsαR201C in osteoblasts, and expression of the Wnt inhibitor, Sost, is reduced. While high bone mass is in fact a feature of some types/stages of FD lesions in humans, it is marrow fibrosis, localized loss of adipocytes and hematopoietic tissue, osteomalacia and osteolytic changes that together represent the characteristic pathological profile of FD, as well as the sources of specific morbidity. None of these features are reproduced in mice with osteoblast-specific expression of GsαR201C. We further show that hematopoietic progenitor/stem cells, as well as more mature cell compartments, and adipocyte development are normal in these mice. These data demonstrate that effects of Gsα mutations underpinning FD-defining tissue changes and morbidity do not reflect the effects of the mutations on osteoblasts proper.

Identifying the molecular networks that underlie Fibrous Dysplasia (FD) is key to understand the pathogenesis of the disease, to refine current diagnostic approaches and to develop efficacious therapies. In this study, we used the NanoString nCounter Analysis System to investigate the gene signature of a series of nine Formalin Fixed Decalcified and Paraffin-Embedded (FFDPE) bone biopsies from seven FD patients. We analyzed the expression level of 770 genes. Unsupervised clustering analysis demonstrated partitioning into two clusters with distinct patterns of gene expression. Differentially expressed genes included growth factors, components of the Wnt signaling system, interleukins and some of their cognate receptors, ephrin ligands, matrix metalloproteinases, neurotrophins and genes encoding components of the cAMP-dependent protein kinase. Interestingly, two tissue samples obtained from the same skeletal site of one patient one year apart failed to segregate in the same cluster. Retrospective histological review of the samples revealed different microscopic aspects in the two groups. The results of our pilot study suggest that the genetic signature of FD is heterogeneous and varies according to the histology and, likely, to the age of the lesion. In addition, they show that the Nanostring technology is a valuable tool for molecular translational studies on archival FFDPE material in FD and other rare bone diseases.

The Gsα/cAMP signaling pathway mediates the effect of a variety of hormones and factors that regulate the homeostasis of the post-natal skeleton. Hence, the dysregulated activity of Gsα due to gain-of-function mutations (R201C/R201H) results in severe architectural and functional derangements of the entire bone/bone marrow organ. While the consequences of gain-of-function mutations of Gsα have been extensively investigated in osteoblasts and in bone marrow osteoprogenitor cells at various differentiation stages, their effect in adipogenically-committed bone marrow stromal cells has remained unaddressed. We generated a mouse model with expression of GsαR201C driven by the Adiponectin (Adq) promoter. Adq-GsαR201C mice developed a complex combination of metaphyseal, diaphyseal and cortical bone changes. In the metaphysis, GsαR201C caused an early phase of bone resorption followed by bone deposition. Metaphyseal bone formation was sustained by cells that were traced by Adq-Cre and eventually resulted in a high trabecular bone mass phenotype. In the diaphysis, GsαR201C, in combination with estrogen, triggered the osteogenic activity of Adq-Cre-targeted perivascular bone marrow stromal cells leading to intramedullary bone formation. Finally, consistent with the previously unnoticed presence of Adq-Cre-marked pericytes in intraosseous blood vessels, GsαR201C caused the development of a lytic phenotype that affected both cortical (increased porosity) and trabecular (tunneling resorption) bone. These results provide the first evidence that the Adq-cell network in the skeleton not only regulates bone resorption but also contributes to bone formation, and that the Gsα/cAMP pathway is a major modulator of both functions.