A Potential New Method to Estimate Tissue Cystine Content in Nephropathic Cystinosis

ObjectivesTo evaluate intestinal mucosal cystine crystal (CC) load as a way to estimate tissue cystine content in children with cystinosis.Study designIntestinal mucosal biopsies were obtained endoscopically from children (ages 2-18 years) with cystinosis. Using a special processing technique, CC within histiocytes were easily visible and enumerable in the mucosal tissue. Mean CC counts, calculated from stomach and duodenum combined (CC-GD), were correlated with duration of cysteamine treatment, estimated glomerular filtration rate (eGFR), and mean white blood cells (WBC) cystine levels.ResultsSeventeen subjects (6 male) were enrolled in 2 studies from 2001 and 2003. The CC-GD count (mean 12.5 ± 1.41 crystals/histiocyte) was lower than the colonic crystal count (mean 23.6 ± 3.38, P = .0031). Nine of 17 subjects underwent repeated endoscopy 2 years later and the trend for CC-GD was to decrease over time (P = .065). Biopsies, however, were never completely depleted of CC. In subjects who were diagnosed before age 18 months, the percent change from baseline of both eGFR and CC-GD were inversely correlated (P = .026). Mean WBC cystine levels were positively correlated with CC-GD (P = .023).ConclusionsCC are easily visible in the intestinal mucosa. CC-GD counts appear to correlate with eGFR and may help monitor response to treatment. Even when mean WBC cystine levels are low, the mucosal CC are not depleted suggesting that tissue cysteamine levels may not achieve therapeutic efficacy.