Fibrous dysplasia of bone - transgenic models of diseases, and models of therapy

Fibrous Dysplasia (OMIM174800) is a crippling skeletal disease caused by activating mutations in the Gsa gene. Animal models of FD have been woefully needed for understanding disease mechanisms, and for developing and testing mechanistic therapeutic approaches. We generated and extensively analyzed multiple lines of transgenic mice that express the disease causing mutation, GsaR201C, either constitutively or as targeted to specific cell types. We have now obtained a detailed, comprehensive, and entirely novel appraisal of the disease biology,and specific prospects for therapy. First,in the mouse, constitutive expression of GsaR201C is compatible with germline transmission,resulting in a replica of the human disease,precise but inherited, and independent of mosaicism; it does not affect development, skeletogenesis, skeletal cell differentiation or prenatal bone formation. Second,targeting the mutation to osteoblasts generates a marked high bone mass phenotype (mediated by downregulation of SOST), which phenocopies human disorders caused by loss-of-function mutations in the SOST gene (Van Buchem’sdisease, Sclerosteosis), but not FD. All of the defining tissue changes and direct morbidity factors in FD bone (osteolysis, fracture, osteomalacia, marrow fibrosis)reflect the effects of the mutationon compartments of the stromal/osteogenic lineage other than osteoblasts proper. Third, GsaR201C expression in ADRB2-enriched perivascular osteoprogenitors, but not in PTH1R-enriched mature bone cells, causes overexpression of RANKL in osteoprogenitors,but not in mature bone cells; mutated osteoprogenitors cue excess osteoclastogenesis, which causes osteolysis and fracture in FD bone. In WT osteoprogenitors, this effect of the mutation is mimicked by stimulation of ADRB2 (ISO), but not of PTH1R (PTH). Fourth, bone marrow fat cells are the unsuspected culprits of abnormal bone formation in FD. GsaR201C causes upregulation of UCP-1 and thermogenic lipolysis in marrow fat, WAT, and BAT, mimicking the effects of b-adrenergic stimulation in ‘beige’ fat. In bone, conversion of mutated fat cells into dysplastic,fat-derived, osteoblasts ensues. Ectopic expression of the mineralization inhibitor, MGP, is the hallmark of such unique osteoblasts, and the cause of osteomalacia and deformity in FD bone. MGP is not expressed in osteoblasts with targeted GsaR201C expression; MGP-producing osteoblasts only arise from mutated precursors. Fifth, distinct changes and mechanisms mark distinct temporal phases in the natural history of the disease, conceivably responsive to distinct treatment modalities. Sixth, the Gsa-dependent, key mechanisms in FD (RANKL, MGP, beige marrow fat) do operate in human FD, and can be reproduced in human osteoprogenitors (transduced with the same constructs used to generate FD mice). FD is a dysregulation of the neural/b -adrenergic control of theosteogeniclineageas a system. All the novel disease mechanisms we identified are druggable.