Chapter 198

Abstract

Abstract

1. Hereditary renal hypouricemia (MIM 242050 and 307830) is an inborn error of membrane transport, presumably in urate reabsorption in the proximal tubule. It is inherited in an autosomal recessive manner. In homozygotes, it manifests as hypouricemia and increased renal urate clearance. Heterozygosity may be detected by moderately decreased serum urate levels and moderately but significantly increased renal urate clearance.

2. Homozygosity is associated with moderate or excessive uricosuria, reflecting the diversion of intestinal urate elimination to urinary urate excretion consequent to the hypouricemia. There is no evidence for purine overproduction. Hypercalciuria, probably of the hyperabsorptive type, is associated with renal hypouricemia in about 20 percent of the propositi. The mechanism for this abnormality is not yet clarified. The hyperuricosuria and/or hypercalciuria are etiologic factors in uric acid or calcium oxalate urolithiasis, occurring in about 25 percent of the propositi.

3. Transport of urate through the intestinal wall and through the erythrocyte membrane appears to be normal.

4. The differential diagnosis of hereditary renal hypouricemia includes familial conditions, in which the defective renal urate transport is one component in a generalized transport abnormality, such as Hartnup syndrome or the group of diseases with Fanconi renal tubulopathy (Wilson disease, cystinosis, galactosemia, and hereditary fructose intolerance).

5. The model of the renal handling of urate in humans includes four components: free glomerular filtration, net early proximal tubular reabsorption (segment S1), net tubular secretion (segment S2), and net postsecretory tubular reabsorption (segment S3). It is assumed that reabsorption and secretion of urate in the proximal tubule occur simultaneously and that the manifestation of net reabsorption or secretion at the various segments reflects different intensities of the two processes. The residual urinary urate in the S1 region derives originally from filtration, whereas that in the S3 segment arises from secretion in the S2 segment.

6. Several types of renal hypouricemia may be distinguished, according to the nature and site of the transport defect. The classification is based on present knowledge concerning the renal handling of urate in humans and on the mechanism of action of pyrazinamide (assumed to inhibit specifically urate secretion) and probenecid (assumed to inhibit specifically urate reabsorption at the postsecretory site). In hereditary renal hypouricemia, the most common type appears to be that of a presecretory reabsorption defect. Some of patients may have a total transport defect (no reabsorption, no secretion) or total reabsorption defect. A postsecretory reabsorption defect has not been documented in hereditary renal hypouricemia but was found to characterize acquired renal hypouricemia and familial conditions in which renal hypouricemia is a part of a generalized tubular reabsorption defect (Fanconi syndrome). A hypersecretion defect was suggested in one family with hereditary renal hypouricemia and in three patients with isolated renal hypouricemia without evidence for an hereditary basis.

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