Abstract by Chelsea Hopkins

Fibrous dysplasia (FD) is a rare genetic bone disorder that commonly results in deformity, fractures, and pain. The aetiology of FD is well-characterised; it is caused by a postzygotic missense mutation in the GNAS gene encoding the Gα_s subunit of G proteins, leading to accumulation of cAMP and activation of downstream pathways in bone marrow stromal cells. FD is a mosaic disorder, where lesions can develop in one or multiple bones in a highly individualistic manner. Pain is a key problem for patients, but the underlying mechanisms of pain development are unknown and treatment is often ineffective. The overall aim of this thesis is to identify a translationally relevant mouse model of FD that develops a pain-like phenotype and investigate the underlying mechanisms that contribute to this phenotype.

Manuscript 1 is a systematic review analysing and comparing different animal models of FD. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses was utilised to identify animal models of FD. Seven unique causative animal models of FD were identified that expressed the causative genetic mutation of FD. These models were assessed for FD features and how closely they resembled observations in patients. Mechanistic models (no causative mutation) that developed FD-like lesions were also assessed. This review may assist researchers in identifying suitable models for their research on FD. It also facilitated the selection of a mouse model for the study conducted in Manuscript 2.

Manuscript 2 aims to identify mechanisms of FD pain in a translationally relevant mouse model of the disorder. A previously established site-specific, inducible model of FD was implemented for this study. Male and female mice with FD lesions displayed pain-related behaviours. This was indicated by decreased burrowing (recovery after ibuprofen treatment), grid hanging (partial recovery after morphine treatment), home cage activity, and wheel running. Tissue harvested from the mice demonstrated the presence of osteoclasts, immune cells, sensory nerve fibres, and increased vascularity within the FD lesion. Dorsal root ganglia demonstrated markers of nerve damage, suggesting the development of neuropathic-like pain. In vitro analysis demonstrated increased production of cAMP and inflammatory markers associated with painful bone diseases. In summary, the mouse model demonstrated FD development with subsequent pain-like development that suggests inflammatory and neuropathic pain components.

Manuscript 3 seeks to establish if home cage activity and wheel running in novel Digital Ventilated Cages® were affected by cancer-induced bone pain. As cancer developed, well-established pain tests – gait analysis and weight bearing – were affected, with a corresponding decline in wheel running, but not cage activity. Digital Ventilated Cages® could be a useful tool to assess spontaneous pain-like behaviour in mice, particularly wheel running. This supports the notion that the reduced wheel running described in Manuscript 2 was due to the development of pain-like behaviour.

In conclusion, a translationally relevant animal model of FD was identified and implemented. Pain-like behaviour developed, which was reversed by analgesics and underlying mechanisms corresponding to inflammatory and neuropathic pain were characterised. This model can be used to assess novel analgesic treatments for FD.