Background: Spondyloarthritis (SpA) is an inflammatory rheumatic disease characterized, in its axial form, by lesions affecting the spine and/or sacroiliac joints (SIJ). Severe forms of the disease manifest as exuberant ossification phenomena leading to complete ankylosis of the spine and SIJ, defining ankylosing spondylitis (AS), with underlying mechanisms not fully understood.

Objectives: This study aims to advance our understanding of the pathophysiology of ossification in AS.

Methods: Using whole exome sequencing analysis, we identified a mutation in the EXTL3 gene within a family exhibiting a familial form of AS. EXTL3 plays an ubiquitous role in heparan sulfate (HS) synthesis. We developed a mouse model carrying the mutation in a heterozygous state ( Extl3 ^{mut/+ )} and conducted phenotypic exploration using micro-computed tomography (microCT) and histological analyses. Cellular and molecular pathways altered by this mutation, were investigated in primary osteoblast and chondrocyte cultures at all stages of differentiation, using transcriptomic and proteomic analyses.

Results: Phenotypic analysis of 6- and 12-month-old Extl3 ^mut/+ mice compared to wild-type Extl3 ^+/+ mice revealed two major abnormalities at the SIJ and the spine: osteoarticular lesions and trabecular bone alterations. Osteoarticular lesions, including erosions of the sacral cortical bone, were shown in 6-month-old Extl3 ^mut/+ mice by microCT. In 12-month-old Extl3 ^mut/+ mice, islet formation within the iliac cortical was observed, corresponding to the inclusion of non-mineralized osteoid tissue, confirmed by histological analyses. These lesions were associated with disorganization of collagenous cartilage and chondrocyte columns, along with sacral cartilage thickening. Bridges within the SIJ space were visualized at 12 months. Trabecular bone alterations included deterioration in all parameters at 6 and 12 months, associated with an accumulation of non-mineralized osteoid tissue, defining an osteomalacia phenotype. HS accumulation was documented in both regions, primarily originating from osteoblasts. Osteoblast cultures exhibited delayed differentiation and a diminished capacity for mineralization. Furthermore, an activation of the Wnt pathway was observed, yet it appeared insufficient to rectify the observed phenotype. Conversely, chondrocyte cultures demonstrated an accelerated differentiation process toward the hypertrophic stage, associated with the activation of the Wnt pathway, suggesting a potential deregulation of endochondral ossification.

Conclusion: This study identified a rare variant of the EXTL3 gene in a family with familial aggregation of AS. The murine model carrying the mutation demonstrated an articular, including cartilage lesions, and bone phenotype. A genetic association study in a large cohort of severe AS patients is underway to investigate the aggregation of rare EXTL3 variants in this population. Thus, abnormalities in the HS synthesis pathway, already implicated in multiple exostoses disease, may also contribute to the pathophysiology of ossification phenomena in AS.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: None declared.