Cystinosis-associated metabolic bone disease across ages and CKD stages 1-5D/T.

Context: The pathophysiology of cystinosis-associated metabolic bone disease is complex.
Objective: We hypothesized a disturbed interaction between osteoblasts and osteoclasts.
Design: Binational cross-sectional multicenter study.
Setting: Hospital clinics.
Patients: One hundred and three patients with cystinosis (61% children) with chronic kidney disease (CKD) stages 1-5D/T.
Main Outcome Measures: Ten key bone markers.
Results: Skeletal complications occurred in two-thirds of the patients, with adults having a five-fold increased risk compared to children. Patients with CKD stages 1-3 showed reduced z-scores for serum phosphate and calcium, 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.
Conclusions: Bone health in cystinosis deteriorates with age, which is associated with increased osteoclast activity despite counterregulation of osteoblasts via OPG/RANKL, which in conjunction with elevated sclerostin levels and persistent rickets/osteomalacia may promote progressive bone loss.
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