Familial distal renal tubular acidosis is associated with mutations in the red cell anion exchanger (Band 3, AE1) gene.

All affected patients in four families with autosomal dominant familial renal tubular acidosis (dRTA) were heterozygous for mutations in their red cell HCO3-/Cl- exchanger, band 3 (AE1, SLC4A1) genes, and these mutations were not found in any of the nine normal family members studied. The mutation Arg589--> His was present in two families, while Arg589--> Cys and Ser613--> Phe changes were found in the other families. Linkage studies confirmed the co-segregation of the disease with a genetic marker close to AE1. The affected individuals with the Arg589 mutations had reduced red cell sulfate transport and altered glycosylation of the red cell band 3 N-glycan chain. The red cells of individuals with the Ser613--> Phe mutation had markedly increased red cell sulfate transport but almost normal red cell iodide transport. The erythroid and kidney isoforms of the mutant band 3 proteins were expressed in Xenopus oocytes and all showed significant chloride transport activity. We conclude that dominantly inherited dRTA is associated with mutations in band 3; but both the disease and its autosomal dominant inheritance are not related simply to the anion transport activity of the mutant proteins.

Renal chloride channel, CLCN5, mutations in Dent's disease.

Dent's disease is an X-linked renal tubular disorder characterized by low-molecular-weight proteinuria, hypercalciuria, nephrocalcinosis, nephrolithiasis, and renal failure. Patients with Dent's disease may also suffer from rickets and other features of the renal Fanconi Syndrome. Patients may have mutations in the X-linked renal chloride channel gene, CLCN5, which encodes a 746-amino-acid protein with 12-13 transmembrane domains. We have investigated the 11 coding exons of CLCN5 for mutations in eight unrelated patients with Dent's disease. Leukocyte DNA was used for the polymerase chain reaction amplification of CLCN5 and the products analyzed for single-stranded conformational polymorphisms (SSCPs). Abnormal SSCPs were sequenced and revealed eight mutations. These consisted of three nonsense mutations (Arg34Stop, Arg648Stop, Arg704Stop), four deletions involving codons 40, 86, 157, and 241, and one acceptor splice consensus sequence mutation tgcag --> tgaag. The mutations were confirmed either by restriction endonuclease or sequence-specific oligonucleotide hybridization analysis. In addition, an analysis of 110 alleles from 74 unrelated normal individuals demonstrated that the DNA sequence changes were not common polymorphisms. All of the mutations predict truncated chloride channels that are likely to result in a functional loss. Thus, our findings expand the spectrum of CLCN5 mutations associated with Dent's disease and the results will help to elucidate further the functional domains of this novel chloride channel.

Nitrogen metabolism in the gut.

All three of the major human nitrogenous waste products--urea,, creatinine, and uric acid--are significantly degraded by intestinal bacteria. The breakdown products of creatinine and uric acid are not fully known, but metabolites of these complex heterocyclic nitrogen compounds may well turn out to play a role in uremic toxicity. Urea degradation is almost certainly by way of ammonia, but the exact site of urea hydrolysis in the alimentary tract is not known, and it is uncertain whether urea is the major source of intestinal ammonia. Ammonia is absorbed from the colon predominantly in unionized form, and the bicarbonate ion secreted by the colonic mucosa plays an important role in facilitating this absorption.

Dent's disease; a familial proximal renal tubular syndrome with low-molecular-weight proteinuria, hypercalciuria, nephrocalcinosis, metabolic bone disease, progressive renal failure and a marked male predominance.

We describe a familial form of renal Fanconi syndrome characterized by hypercalciuria, low-molecular-weight proteinuria, nephrocalcinosis and slowly progressive renal failure. Males are much more severely affected than females. The patients studied included 15 males and 10 females, and five families with up to three generations involved. Studies of the two largest families described here have already shown that their disease is inherited on the X-chromosome. The series contains the two unrelated patients originally described by Dent and Friedman in 1964 as 'hypercalcuric rickets'.

A study of immune responses to Tamm-Horsfall glycoprotein in the sera of patients with renal tubular acidosis.

Antibody to Tamm-Horsfall glycoprotein in the sera of patients with distal renal tubular acidosis (dRTA) was measured by radioimmunoassay, as well as in samples of normal human serum. Normal human serum contains small amounts of IgG capable of interacting with Tamm-Horsfall glycoprotein. Appropriate assays were carried out on antiserum raised in rabbits against human Tamm-Horsfall glycoprotein serially diluted with normal human serum. Corrections were applied for the presence of interfering substances in serum. The amounts of antibody found in samples of normal and patient sera were not significantly different, although some of the patients were diagnosed as having immune as opposed to familial dRTA. Studies of cell-mediated immunity to Tamm-Horsfall glycoprotein was found not to differentiate between the normal and patient samples. dRTA does not appear to be associated with immune responses to Tamm-Horsfall glycoprotein.

The incorporation of ammonia nitrogen into albumin in man: the effects of diet, uremia and growth hormone.

15 N-ammonium chloride was given orally to 17 individuals in 20 studies. Retention of 15N by those with normal renal function was inversely proportional to the protein intake; enrichment of albumin with 15N increased during protein restriction. Protein restriction appeared to be a more potent stimulus to the synthetic use of ammonia than a large pool of urea nitrogen. Both retention of isotope and enrichment of albumin decreased when a high protein diet was given. Enrichment of albumin with 15N in response to 10 or 20 mg of growth hormone daily was similar to that in healthy individuals on a 20 g protein diet for 3 weeks. The relevance of these findings to the nutritional potential of recycled urea nitrogen is discussed.

Treatment of chronic renal failure with dietary fiber.

We have tested the hypothesis that dietary fiber, by inhibiting colonic bacterial ammonia generation and increasing fecal nitrogen excretion, might decrease hepatic urea synthesis and thereby reduce plasma urea in patients with chronic renal failure. Six and 8 week courses of two different hemicelluloses, arabinogalactan and ispaghula, reduced mean plasma urea in uremic subjects by 11% and 19% respectively. Ispaghula also reduced the rate of rise of plasma creatinine to zero and, in one formal balance study, increased fecal nitrogen excretion by 39%. Experiments in vitro showed that ispaghula depressed anaerobic fecal bacterial net ammonia generation by 30%, and adsorbed neither urea nor ammonia. The reduction in plasma urea caused by dietary fiber is likely to be due to inhibition of colonic bacterial production of ammonia; such therapy could conceivably alleviate some of the symptoms of uremia and postpone dialysis in patients with endstage renal disease.

Tropical distal renal tubular acidosis: clinical and epidemiological studies in 78 patients.

BACKGROUND: Distal renal tubular acidosis (dRTA) caused by mutations of the SLC4A1 gene encoding the erythroid and kidney isoforms of anion exchanger 1 (AE1 or band 3) has a high prevalence in some tropical countries, particularly Thailand, Malaysia, the Philippines and Papua New Guinea (PNG). Here the disease is almost invariably recessive and can result from either homozygous or compound heterozygous SLC4A1 mutations. METHODS: We have collected and reviewed our own and published data on tropical dRTA to provide a comprehensive series of clinical and epidemiological studies in 78 patients. RESULTS: Eight responsible SLC4A1 mutations have been described so far, four of them affecting multiple unrelated families. With the exception of the mutation causing South-East Asian ovalocytosis (SAO), none of these mutations has been reported outside the tropics, where dRTA caused by SLC4A1 mutations is much rarer and almost always dominant, resulting from mutations that are quite different from those found in the tropics. SLC4A1 mutations, including those causing dRTA, may cause morphological red cell changes, often with excess haemolysis. In dRTA, these red cell changes are usually clinically recessive and not present in heterozygotes. The high tropical prevalence of dRTA caused by SLC4A1 mutations is currently unexplained. CONCLUSION: A hypothesis suggesting that changes in red cell metabolism caused by these mutations might protect against malaria is put forward to explain the phenomenon, and a possible mechanism for this effect is proposed.