Cystinosis-Associated Metabolic Bone Disease Across Ages and CKD Stages 1 to 5D/T.

Context: The pathophysiology of cystinosis-associated metabolic bone disease is complex.

Objective: We hypothesized a disturbed interaction between osteoblasts and osteoclasts.

Methods: This binational cross-sectional multicenter study included 103 patients with cystinosis (61% children) with chronic kidney disease (CKD) stages 1 to 5D/T at hospital clinics. Ten key bone markers were evaluated.

Results: Skeletal complications occurred in two-thirds of the patients, with adults having a 5-fold increased risk compared with children. Patients with CKD stages 1 to 3 showed reduced z-scores for serum phosphate and calcium and suppressed fibroblast growth factor 23 (FGF23) and parathyroid hormone levels, in conjunction with elevated bone-specific alkaline phosphatase levels. Serum phosphate was associated with estimated glomerular filtration rate, combined phosphate and active vitamin D treatment, and native vitamin D supplementation, while serum calcium was associated with age and dosage of active vitamin D. Sclerostin was generally elevated in children, and associated with age, FGF23 levels, and treatment with active vitamin D and growth hormone. The osteoclast marker tartrate-resistant acid phosphatase 5b was increased, and associated with age and treatment with active vitamin D. The ratio of soluble ligand of receptor activator of nuclear factor-κB (sRANKL) and osteoprotegerin (OPG), a surrogate for the regulation of osteoclastogenesis by osteoblasts, was decreased and associated with phosphate and 1,25(OH)2D3 levels. These changes were only partly corrected after transplantation.

Conclusion: Bone health in cystinosis deteriorates with age, which is associated with increased osteoclast activity despite counter-regulation of osteoblasts via OPG/RANKL, which in conjunction with elevated sclerostin levels and persistent rickets/osteomalacia, may promote progressive bone loss.