Characterization of Urate Metabolism and Complications of Patients with Renal Hypouricemia.

Objective Both renal hypouricemia (RHU) and gout are associated with renal dysfunction and urolithiasis. The difference in renal complications associated with RHU and gout, however, has not been studied. We characterized the urate metabolism and complications of patients with RHU and compared them with patients with gout. Methods Eighteen patients with RHU who had a serum uric acid (SUA) level <2 mg/dL (10 men and 8 women), 44 patients with gout (44 men) and 16 normouricemic patients (4 men and 12 women) were included. The blood and urinary biochemical data were evaluated. A genetic analysis of uric acid transporter 1 (URAT1) was also conducted in 15 cases with RHU. Results The SUA level of RHU was 0.9±0.5/mg/dL, and the Uur/Ucr and Cur/Ccr were 0.56±0.14% and 45.7±18.0%, respectively. A genetic analysis of URAT1 in 15 RHU patients showed that 13 harbored a URAT1 gene mutation, whereas 2 harbored the wild-type gene. The SUA level was significantly lower in RHU patients (n=11) than in either gout patients (n=44) or normouricemic patients (n=16). This reduction was accompanied by the elevation of Cua/Ccr. Urinary beta 2-microglobulin levels were higher in RHU patients than in gout or normouricemia patients. Cua/Ccr correlated with normalized urinary beta 2-microglobulin levels. The prevalence of urolithiasis was 18.2% in RHU cases and 6.8% in gout cases. A homozygous URAT1 mutation was associated with urolithiasis. Conclusion Besides urolithiasis, RHU can be associated with tubular dysfunction, such as elevated urinary beta 2-microglobulin levels.