A 78-year-old female presented in 1996 with bone pain, generalized muscular weakness and immobility. She had marked hypophosphataemia and osteomalacic fractures and a clinical diagnosis of oncogernic osteomalacia was made.1 No tumour could be identified at that time. Subsequent management, including partial parathyroidectomy, oral phosphate and 1,25-dihydroxycholecalciferol (1,25-D3), produced good symptomatic improvement. In 2008, elevated levels of fibroblast growth factor (FGF)-23 were found, 18F-Deoxyglucose PET scanning became available and showed a focus of emission in the right chest (Figure 1a). MRI disclosed a paraspinal tumour at the level of T9 vertebral body (Figure 1b). Further investigations showed that she had a renal Fanconi syndrome, comprising low molecular weight proteinuria, (preoperative results in Figure 2), glycosuria of 2.6–25 mmol/mmol creatinine (reference range <0.1) and aminoaciduria characterized by hyperglycinuria and borderline hyperlysinuria. Preoperative low molecular weight proteinuria was shown by very elevated urinary excretion of both retinol-binding protein (RBP) (Figure 2) and Beta-2-microglobulin (data not shown) relative to albumin.2 The ratio of the renal Transport Maximum of Phosphate/Glomerular Filtration rate, TmP/GFR, a measure of the threshold for urine phosphate reabsorption, was markedly reduced …

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.Conclusions: 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.

Correspondence to Dr. Anthony G.W. Norden, F.R.C. Path., DepartTo the Editor: We thank Dr. Greive et al for their ment of Clincial Biochemistry, Addenbrooke’s Hospital, Hills Road, comments on our estimates of human glomerular sieving Cambridge CB2 2QR. coefficients (GSCs) derived from studies of patients with E-mail: agwn2@cam.ac.uk Fanconi syndrome [1]. We did, in fact, specifically suggest in the Discussion section of our paper that any contribuREFERENCES tion of transcellular reabsorption of albumin, as pro1. Greive KA, Osicka TM, Russo LM, Comper WD: Fragmentation posed by Dr. Greive’s group, would lead to underestimaof filtered proteins and implications for glomerular protein sieving in Fanconi syndrome. Kidney Int 61:1549–1550, 2002 tion of the GSC for albumin [2]. The difficulty is that 2. Norden AGW, Lapsley M, Lee PJ, et al: Glomerular sieving and measurements of how much filtered albumin or other implications for renal failure in Fanconi syndrome. Kidney Int 60: 1885–1892, 2001 proteins might be catabolized in the way have not been

Human band 3 Walton is an AE1 mutation that results in the deletion of the 11 COOH-terminal amino acids of the protein and is associated with dominant distal renal tubular acidosis. The properties of band 3 Walton expressed with normal band 3 in the heterozygous mutant erythrocytes and the kidney isoform expressed in Xenopus oocytes and in the Madin-Darby canine kidney cell line were examined. The mutant erythrocytes have normal hematology but have reduced band 3 Walton content. Transport studies showed that erythrocyte band 3 Walton has normal sulfate transport activity, and kidney band 3 Walton has normal chloride transport activity when expressed in Xenopus oocytes. The mutant protein is clearly able to reach the cell surface of erythrocytes and oocytes. In contrast, while normal kidney band 3 was expressed at the cell surface in the kidney cell line, the Walton mutant protein was retained intracellularly within the kidney cells. The results demonstrate that band 3 Walton is targeted differently in erythrocytes and kidney cells and indicate that the COOH-terminal tail of band 3 is required to allow movement to the cell surface in kidney cells. It is proposed here that the mutant band 3 gives rise to dominant distal renal tubular acidosis by inhibiting the movement of normal band 3 to the cell surface. It is suggested that this results from the association of the normal and mutant proteins in band 3 hetero-oligomers, which causes the intracellular retention of normal band 3 with the mutant protein.

Conference Abstract| June 01 2001 Mutations in the band 3 (AE1) gene associated with distal renal tubular acidosis A. M. Toye; A. M. Toye 1Department of Biochemistry, University of Bristol, Bristol, BS8 1TD, U.K. Search for other works by this author on: This Site PubMed Google Scholar L. J. Bruce; L. J. Bruce 1Department of Biochemistry, University of Bristol, Bristol, BS8 1TD, U.K. Search for other works by this author on: This Site PubMed Google Scholar O. Wrong; O. Wrong 1Department of Biochemistry, University of Bristol, Bristol, BS8 1TD, U.K. Search for other works by this author on: This Site PubMed Google Scholar M. T. Young; M. T. Young 1Department of Biochemistry, University of Bristol, Bristol, BS8 1TD, U.K. Search for other works by this author on: This Site PubMed Google Scholar R. J. Unwin; R. J. Unwin 1Department of Biochemistry, University of Bristol, Bristol, BS8 1TD, U.K. Search for other works by this author on: This Site PubMed Google Scholar M. J. A. Tanner M. J. A. Tanner 1Department of Biochemistry, University of Bristol, Bristol, BS8 1TD, U.K. Search for other works by this author on: This Site PubMed Google Scholar Biochem Soc Trans (2001) 29 (3): A87. https://doi.org/10.1042/bst029a087a Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share MailTo Twitter LinkedIn Cite Icon Cite Get Permissions Citation A. M. Toye, L. J. Bruce, O. Wrong, M. T. Young, R. J. Unwin, M. J. A. Tanner; Mutations in the band 3 (AE1) gene associated with distal renal tubular acidosis. Biochem Soc Trans 1 June 2001; 29 (3): A87. doi: https://doi.org/10.1042/bst029a087a Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Society Transactions Search Advanced Search This content is only available as a PDF. © 2001 Biochemical Society2001 Article PDF first page preview Close Modal You do not currently have access to this content.

We describe three mutations of the red-cell anion exchangerband 3 (AE1, SLC4A1) gene associated with distalrenal tubular acidosis (dRTA) in families from Malaysia and Papua NewGuinea: Gly(701)-->Asp (G701D), Ala(858)-->Asp(A858D) and deletion of Val(850) (DeltaV850). The mutationsA858D and DeltaV850 are novel; all three mutations seem to berestricted to South-East Asian populations. South-East Asianovalocytosis (SAO), resulting from the band 3 deletion of residues400-408, occurred in many of the families but did not itselfresult in dRTA. Compound heterozygotes of each of the dRTA mutationswith SAO all had dRTA, evidence of haemolytic anaemia and abnormal red-cell properties. The A858D mutation showed dominant inheritance and therecessive DeltaV850 and G701D mutations showed a pseudo-dominantphenotype when the transport-inactive SAO allele was also present. Red-cell and Xenopus oocyte expression studies showed that theDeltaV850 and A858D mutant proteins have greatly decreased aniontransport when present as compound heterozygotes (DeltaV850/A858D,DeltaV850/SAO or A858D/SAO). Red cells with A858D/SAO had only 3% ofthe SO(4)(2-) efflux of normal cells, thelowest anion transport activity so far reported for human red cells. The results suggest dRTA might arise by a different mechanism for eachmutation. We confirm that the G701D mutant protein has an absoluterequirement for glycophorin A for movement to the cell surface. Wesuggest that the dominant A858D mutant protein is possibly mis-targetedto an inappropriate plasma membrane domain in the renal tubular cell,and that the recessive DeltaV850 mutation might give dRTA because ofits decreased anion transport activity.

UNLABELLED:Tubular proteinuria defined by a study of Dent's ( CLCN5 mutation) and other tubular diseases. BACKGROUND:The term "tubular proteinuria" is often used interchangeably with "low molecular weight proteinuria" (LMWP), although the former implies a definite etiology. A specific quantitative definition of tubular proteinuria is needed, and we address this by studying five different renal disorders. METHODS:Tubular proteinuria was assessed by measuring urinary retinol-binding protein (RBP), beta2-microglobulin (beta2M), alpha1-microglobulin (alpha1M), and albumin in 138 patients: 26 affected males and 24 female carriers of the X-linked syndrome "Dent's disease," 6 patients with other Fanconi syndromes, 17 with distal renal tubular acidosis (dRTA), 39 with glomerulonephritis (GN), and 26 with Chinese herbs nephropathy (CHN). RESULTS:RBP was better than beta2M or alpha1M in identifying the tubular proteinuria of Dent's disease. Median urinary RBP levels in mg/mmol creatinine were: affected male Dent's, 18.2, N = 26; carrier female Dent's, 0. 30, N = 24; dRTA, 0.027, N = 17; GN, 0.077, N = 39; and normal adults, 0.0079, N = 61. Elevated urinary RBP (>0.017) and albumin < (10 x RBP) + 2 identified all patients with the LMWP of Dent's disease and clearly distinguished their LMWP from that of dRTA and GN. This is a quantitative definition of tubular proteinuria. Consistent with this definition, 80% of those patients with CHN who had an elevated RBP had tubular proteinuria. Urinary RBP and albumin in carriers of Dent's disease were strikingly correlated over a 100-fold range (R = 0.933). CONCLUSION:The combination of elevated urinary RBP (>0.017) and albumin < (10 x RBP) + 2 (mg protein/mmol creatinine) is a quantitative definition of tubular proteinuria. Furthermore, our findings suggest that a shared defect in tubular RBP and albumin reuptake causes this form of proteinuria.

UNLABELLEDIsolated hypercalciuria with mutation in CLCN5: Relevance to idiopathic hypercalciuria.BACKGROUNDIdiopathic hypercalciuria (IH) is the most common risk factor for kidney stones and often has a genetic component. Dent's disease (X-linked nephrolithiasis) is associated with mutations in the CLCN5 chloride channel gene, and low molecular weight (LMW) proteinuria was universally observed in affected males. We sought to identify mutations in CLCN5 or abnormalities in LMW protein excretion in a large group of patients with IH and in a rat model of genetic hypercalciuria.METHODSOne hundred and seven patients with IH (82 adults and 25 children) and one asymptomatic hypercalciuric man with a known inactivating mutation in CLCN5 were studied. Secondary causes of hypercalciuria were excluded in all. The excretion of retinol-binding protein and beta2-microglobulin was measured by immunoassay in 101 patients with IH. Mutation analysis of the CLCN5 gene was performed in 32 patients with IH and in the genetic hypercalciuric stone-forming (GHS) rat strain.RESULTSLMW protein excretion was normal in 92 patients with IH, and only slight abnormalities were found in the other nine, none of whom had a mutation in CLCN5. One 27-year-old man who had a CLCN5 mutation was found to have isolated hypercalciuria without LMW proteinuria, renal failure, or other evidence of renal disease. Mutation analysis was normal in 32 patients with IH. The CLCN5 sequence was normal in the GHS rat.CONCLUSIONSInactivation of CLCN5 can be found in the setting of hypercalciuria without other features of X-linked nephrolithiasis. However, mutations in CLCN5 do not represent a common cause of IH.

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 tran smembrane 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.

In distal renal tubular acidosis (dRTA) the tubular secretion of hydrogen ion in the distal nephron is impaired, leading to the development of metabolic acidosis, frequently accompanied by hypokalemia, nephrocalcinosis, and metabolic bone disease. The condition can be familial, when it is usually inherited as an autosomal dominant, though there is a rarer autosomal recessive form associated with nerve deafness. It has been shown that the autosomal dominant form of dRTA is associated with a defect in the anion exchanger (AE1) of the renal collecting duct intercalated cell. This transporter is a product of the same gene (AE1) as the erythrocyte anion exchanger, band 3. In this review we will look at the evidence for this association. Studies of genomic DNA from families with this disorder have shown, both by genetic linkage studies and by DNA sequencing, that affected individuals are heterozygous for mutations in the AE1 gene whilst unaffected family members have a normal band 3 sequence. Mutations have been found in the region of proposed helices 6 and 7 of the membrane domain of band 3 and involve amino acids Arg-589 and Ser-613, and in the COOH-terminal domain of band 3. Studies of red cell band 3 from these families have provided information on the effect these mutations have on the structure and function of erythrocyte band 3. Expression studies of the erythroid and kidney isoforms of the mutant AE1 proteins, in Xenopus laevis oocytes, have shown that they retained chloride transport activity, suggesting that the disease in the dRTA families is not related simply to the anion transport activity of the mutated proteins. A possible explanation for the dominant effect of these mutant AE1 proteins in the kidney cell is that these mutations affect the targeting of AE1 from the basolateral to the apical membrane of the alpha-intercalated cell.

Primary distal renal tubular acidosis (dRTA) is characterized by reduced ability to acidify urine, variable hyperchloremic hypokalemic metabolic acidosis, nephrocalcinosis, and nephrolithiasis. Kindreds showing either autosomal dominant or recessive transmission are described. Mutations in the chloride-bicarbonate exchanger AE1 have recently been reported in four autosomal dominant dRTA kindreds, three of these altering codon Arg589. We have screened 26 kindreds with primary dRTA for mutations in AE1 . Inheritance was autosomal recessive in seventeen kindreds, autosomal dominant in one, and uncertain due to unknown parental phenotype or sporadic disease in eight kindreds. No mutations in AE1 were detected in any of the autosomal recessive kindreds, and analysis of linkage showed no evidence of linkage of recessive dRTA to AE1 . In contrast, heterozygous mutations in AE1 were identified in the one known dominant dRTA kindred, in one sporadic case, and one kindred with two affected brothers. In the dominant kindred, the mutation Arg-589/Ser cosegregated with dRTA in the extended pedigree. An Arg-589/His mutation in the sporadic case proved to be a de novo mutation. In the third kindred, affected brothers both have an intragenic 13-bp duplication resulting in deletion of the last 11 amino acids of AE1. These mutations were not detected in 80 alleles from unrelated normal individuals. These findings underscore the key role of Arg-589 and the C terminus in normal AE1 function, and indicate that while mutations in AE1 cause autosomal dominant dRTA, defects in this gene are not responsible for recessive disease.