Elevated serum levels of soluble receptor activator of nuclear factors-κB ligand (sRANKL) and reduced bone mineral density in patients with ankylosing spondylitis (AS)

Objective. To assess bone mineral density (BMD), serum levels of soluble receptor activator of nuclear factors-kappa B ligand (sRANKL) and osteoprotegerin (OPG) in patients with ankylosing spondylitis (AS), and to determine their relationships with disease activities. Methods. Serum levels of sRANKL and OPG in AS were measured by sandwich enzyme-linked immunosorbent assay. The disease activities were determined using Bath Ankylosing Spondylitis Disease Activity Score Index, Bath Ankylosing Spondylitis Functional Index, Bath Ankylosing Spondylitis Metrology Index and Bath Ankylosing Spondylitis Patient Global Score. BMD of femur and lumbar spine was measured by dual energy X-ray absorptiometry. Radiological grading was determined by New York criteria for sacroiliitis and modified Stoke Ankylosing Spondylitis Spine Score. Results. Osteoporosis and osteopaenia of femoral neck were found in 33 and 41% of patients, respectively. BMD of femoral neck showed negative correlation with disease activity indexes, erythrocyte sedimentation rate and C-reactive protein. The serum sRANKL levels and the ratio of sRANKL to OPG were significantly higher in patients with AS than those of controls. The sRANKL/OPG ratio tended to increase in patients with reduced BMD and radiological findings of active inflammation. Conclusion. About 74% of AS patients have reduced BMD and this change reflects disease activity. Serum sRANKL levels and sRANKL/OPG ratios are up-regulated in patients with AS and have relationship with BMD and radiological changes. These results suggest that the imbalance between RANKL and OPG might be involved in the pathogenesis and clinical courses of osteoporosis in AS.