{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nCOL9A3 encodes the α3 chain of type IX collagen, a unique component of the cartilage extracellular matrix that assembles with the products of COL9A1 and COL9A2 to form a heterotrimer. This complex plays a crucial structural role by interacting with fibrillar collagens (notably type II and type XI), thereby ensuring matrix stability and proper biomechanical function in articular cartilage and intervertebral discs. Experimental investigations into collagen assembly have demonstrated that specific domains within COL9A3 are essential for determining the chain register and efficient cross‐linking required for maintaining tissue integrity."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "7"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nGenetic polymorphisms and mutations in COL9A3 have been widely implicated in musculoskeletal disorders. Specific allelic variants—including those introducing a tryptophan residue (the so‐called Trp3 allele)—are associated with an increased susceptibility to lumbar intervertebral disc degeneration, herniation, and osteoarthritis. Large-scale population studies, meta-analyses, and genotype–phenotype evaluations have underscored the complex interplay of COL9A3 mutations with age, inflammatory mediators, and other genetic factors; similar variants have also been linked to skeletal dysplasias such as multiple epiphyseal dysplasia and pseudoachondroplasia. Intriguingly, further studies employing proteomic approaches and detailed radiologic analyses have refined our understanding of COL9A3’s impact on disc morphology and overall skeletal development."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "8", "end_ref": "23"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nBeyond its structural role, COL9A3 is expressed in sensory tissues such as the inner ear, where its high expression levels suggest a function in auditory physiology. Indeed, mutations in COL9A3 have been associated with sensorineural hearing loss and ocular abnormalities observed in conditions like Stickler syndrome—findings that have expanded its recognized clinical spectrum. These observations support the inclusion of COL9A3 in genetic screening panels for patients affected by congenital hearing impairment and related connective tissue disorders, emphasizing its multifaceted contributions to both skeletal and sensory system integrity."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "24", "end_ref": "26"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Toshiyuki Ikeda, Akihiko Mabuchi, Akira Fukuda, et al. 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