FRI0155 Genes regulating bone homeostasis are differentially expressed at peripheral versus axial enthesial sites in an animal model of spondyloarthritis

Background In spondyloarthritis (SpA), mechanisms of enthesial bone formation are not well understood. Currently, no treatment can reverse this process, although TNF inhibitors may modify bone formation if initiated early. SKG mice exhibit features of SpA with both peripheral joint and axial inflammation after induction by curdlan via the Dectin-1 pathway residing upstream of TNFα, IL-23 and IL-17. Objectives The goal of this study was to identify genes and pathways that regulate enthesial bone formation. SKG mice were previously reported to develop axial bone formation within 12 weeks of curdlan injection, as well as enthesial bone formation around peripheral joints1. Methods 9 week-old SKG or control BALB/c mice were injected IP with curdlan. Histology and microCT scanning were performed at serial time points. Laser capture microscopy was performed on formalin-fixed paraffin-embedded (FFPE) tissue sections from entheseal sites around the ankle and tail vertebrae 3 weeks after curdlan injection. RNA was extracted using the Qiagen RNeasy FFPE kit and whole transcriptome analysis was performed using Affymetrix gene array MTA 1.0. Results Inflammation at axial entheses developed earliest at the tail base. Contrary to previous reports, we did not observe axial bone formation, but rather saw significant erosion of vertebral bodies, throughout the spine. The absence of axial bone formation was noted as late as 24 weeks post curdlan. In contrast, exuberant enthesial bone formation occurred around ankles at predictable sites of tendon/ligament insertions into bone. We identified distinct expression of genes involved in bone resorption and formation at ankle when compared to spine enthesial sites. IPA upstream regulator analysis identified significant gene expression of upstream regulators of TGF-beta, known to recruit early osteoblast precursors to bone (z-value 2.83), and WNT3A, a WNT signalling pathway agonist that promotes osteoblast differentiation and function (z-value 2.54), in the ankle, but not the spine. These findings support the histologic observation of bone formation at ankle, but not spine, enthesial sites. In addition, significant increased gene expression of upstream regulators of the aryl hydrocarbon receptor (AhR) (z-value 2.14) was found at spine, but not ankle, enthesial sites (z-value −1.78). AhR is a ligand-activated transcription factor that is a positive regulator of osteoclastogenesis via c-fos-mediated RANKL signalling. Conclusions Whole transcriptome analysis revealed increased expression of upstream regulators of genes in anabolic pathways, including WNT3A and TGF-beta, at ankle enthesial sites where bone formation occurs, but not at spine enthesial sites where bone erosions persist without bone formation. These pathways may thus contribute to peripheral enthesial bone formation. Significantly increased gene expression of upstream regulators of AhR, a positive regulator of osteoclastogenesis, was found at spine enthesial sites. AhR is thus a candidate regulator of continued bone resorption at spine enthesial sites in this murine model of SpA. Reference [1] Ruutu M, et al. β-Glucan Triggers Spondylarthritis and Crohn’s Disease–like Ileitis in SKG Mice. Arthritis Rheum2012Jul;64(7):2211. Acknowledgements This work was supported by a grant provided by AbbVie. Disclosure of Interest None declared