EXTL3 dysfunction identified as a driver of aberrant bone development in severe familial ankylosing spondylitis.

OBJECTIVES: Pathological ossification leading to spinal ankylosis is a hallmark of ankylosing spondylitis (AS), yet its molecular mechanisms remain poorly understood. We investigated the genetic basis and functional consequences of severe familial AS to identify pathways driving structural progression. METHODS: Genome-wide linkage analysis and whole-exome sequencing were performed in a multiplex family with severe AS. A heterozygous knock-in mouse carrying the identified EXTL3 variant was generated and characterized using microCT, histology, transcriptomics, primary osteoblast and chondrocyte cultures, and biochemical analyses of heparan sulfate (HS) metabolism. RESULTS: A rare missense variant in EXTL3, a key enzyme involved in HS biosynthesis, was identified in affected family members. Extl3mut/+ mice developed sacroiliac structural abnormalities, osteoid accumulation and early joint bridging, together with impaired trabecular bone architecture. Mechanistically, the mutation accelerated chondrocyte hypertrophy while delaying osteoblast maturation and mineralization. These changes were associated with activation of Wnt signalling, increased HS accumulation, enhanced heparanase expression and altered glycosaminoglycan homeostasis. Collectively, these findings identify EXTL3-mediated HS dysregulation as a novel mechanism linking abnormal bone remodelling to pathological ossification. CONCLUSIONS: Our study identifies EXTL3 as a genetic modifier of structural severity in AS and reveals a previously unrecognised role for HS metabolism in pathological ossification. These findings support a model in which HLA-B27 primarily confers disease susceptibility, whereas rare variants such as EXTL3 contribute to structural progression, highlighting HS-related pathways as potential therapeutic targets.