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Oxalate Transport as Contributor to Primary Hyperoxaluria: The Jury Is Still Out

      Related Article, p. 1096
      Hyperoxaluria is a major risk factor for renal stone disease and can arise from exogenous sources, such as dietary excess and enteric hyperabsorptive states, or from endogenous overproduction. In the healthy individual, most urinary oxalate arises from endogenous production and is excreted almost entirely through the kidney. In hyperoxaluric states, urine becomes supersaturated with respect to calcium oxalate, leading to crystalluria, stone formation, and/or nephrocalcinosis. When end-stage renal failure ensues, systemic oxalate deposition follows in the heart, bone, and blood vessels, leading to significant morbidity and mortality. The accompanying paper by Monico et al
      • Monico C.G.
      • Weinstein A.
      • Jiang Z.
      • et al.
      Phenotypic and functional analysis of human SLC26A6 variants in patients with familial hyperoxaluria and calcium oxalate nephrolithiasis.
      in this issue of the American Journal of Kidney Diseases explores the hypothesis that hyperoxaluria may be caused by mutations in oxalate transport, in addition to better-known mutations in oxalate production.
      The inherited hyperoxalurias, primary hyperoxaluria type 1 and 2 (PH1 and PH2), are rare diseases which have helped to elucidate the metabolic pathways leading to endogenous hyperoxaluria (Fig 1). Both diseases are brought about by failure to remove glyoxylate: PH1 due to deficiency of peroxisomal alanine-glyoxylate aminotransferase (AGT; encoded by the AGXT gene) and PH2 as a result of cytosolic glyoxylate reductase/hydroxypyruvate reductase (GRHPR) deficiency. In both cases, the excess glyoxylate is metabolized to oxalate by lactate dehydrogenase (LDH). These diseases are at the severe end of the spectrum for hyperoxaluria, with 50% of those presenting in childhood developing end-stage renal failure by age 15.
      • Latta K.
      • Brodehl J.
      Primary hyperoxaluria I.
      Another group of patients exists in whom an inherited cause is suspected, but PH1 and PH2 have been excluded.
      • Monico C.G.
      • Persson M.
      • Ford C.H.
      • Rumsby G.
      • Milliner D.S.
      Potential mechanisms of marked hyperoxaluria not due to primary hyperoxaluria I or II.
      • Van Acker K.J.
      • Eyskens F.J.
      • Espeel M.F.
      • et al.
      Hyperoxaluria with hyperglycoluria not due to alanine:glyoxylate aminotransferase defect: A novel type of primary hyperoxaluria.
      These individuals with atypical PH (non-PH1/PH2) may represent a single entity, but equally may include several underlying causes.
      Figure thumbnail gr1
      Figure 1The metabolic pathways in primary hyperoxaluria. Abbreviations: AGT, alanine-glyoxylate aminotransferase; GO, glycolate oxidase; GRHPR, glyoxylate reductase/hydroxypyruvate reductase; LDH, lactate dehydrogenase; NAD, nicotinamide adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide.
      The genes for both AGT and GRHPR have been mapped,
      • Takada Y.
      • Kaneko N.
      • Esumi H.
      • Purdue P.E.
      • Danpure C.J.
      Human peroxisomal L-alanine:glyoxylate aminotransferase: Evolutionary loss of a mitochondrial targeting signal by point mutation of the initiation codon.
      • Cramer S.D.
      • Ferree P.M.
      • Lin K.
      • Milliner D.S.
      • Holmes R.P.
      The gene encoding hydroxypyruvate reductase (GRHPR) is mutated in patients with primary hyperoxaluria type II.
      • Rumsby G.
      • Cregeen D.
      Identification and expression of a cDNA for human hydroxypyruvate/glyoxylate reductase.
      allowing extensive mutation analysis to be carried out, and attempts have been made to look for genotype-phenotype relationships.
      • Pirulli D.
      • Marangella M.
      • Amoroso A.
      Primary hyperoxaluria: Genotype-phenotype correlation.
      • Monico C.G.
      • Olson J.B.
      • Milliner D.S.
      Implications of genotype and enzyme phenotype in pyridoxine response of patients with type 1 primary hyperoxaluria.
      • Rumsby G.
      • Williams E.
      • Coulter-Mackie M.B.
      Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias.
      In addition, the crystal structure of both proteins is now available,
      • Zhang X.
      • Roe M.
      • Hou Y.
      • et al.
      Crystal structure of alanine:glyoxylate aminotranferase and the relationship between genotype and enzymatic phenotype in primary hyperoxaluria type 1.
      • Booth M.P.S.
      • Conners R.
      • Rumsby G.
      • Brady R.L.
      Structural basis of substrate specificity in human glyoxylate reductase/hydroxypyruvate reductase.
      which has provided explanations for some of the phenotypic heterogeneity at the enzyme level. However, in spite of all this information, we are no nearer to an explanation for the wide phenotypic differences. For example, we have shown that individuals with null alleles of AGXT and GRHPR can present across a wide age range, from neonate to fifth decade,
      • Rumsby G.
      • Williams E.
      • Coulter-Mackie M.B.
      Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias.
      and that wide variation, from early onset to cryptic adult, can be seen within the same family.
      • Rumsby G.
      • Williams E.
      • Coulter-Mackie M.B.
      Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias.
      • Frishberg Y.
      • Rinat C.
      • Khatib I.
      • et al.
      Intra-familial clinical heterogeneity: absence of genotype-phenotype correlation in primary hyperoxaluria type 1 in Israel.
      One possible explanation for such variability may be the natural variation in each of the contributing enzymes of the metabolic pathway. We have shown in vitro that both AGT and GRHPR exert a protective effect in minimizing glyoxylate toxicity in cultured CHO (Chinese Hamster Ovary) cells transfected with all the relevant enzymes of the glyoxylate pathway.
      • Behnam J.T.
      • Williams E.L.
      • Brink S.
      • Rumsby G.
      • Danpure C.J.
      Reconstruction of human hepatocyte glyoxylate metabolic pathways in stably transformed Chinese-hamster ovary cells.
      These experiments illustrate a clear role for glycolate oxidase (GO; encoded by the HAO1 gene) in the production of glyoxylate, a key contributor to endogenous oxalate, and one could hypothesize a situation where variation in GO activity could lead to substantial interindividual variation in glyoxylate production. In addition, differences undoubtedly exist in the amount of urinary crystallization inhibitors present, the hydration status of an individual, and the presence or absence of infections as well as differences in intestinal oxalate uptake and renal oxalate handling.
      Oxalate uptake and renal oxalate excretion have been given a boost by the identification of the SLC26 (solute-linked carrier) family of transporters. This family of anion exchangers includes 10 protein products with multifunctional properties and a range of substrate specificities, 3 of which, SLC26A2, A3, and A4, have disease associations.
      • Everett L.A.
      • Green E.D.
      A family of mammalian anion transporters and their involvement in human genetic disease.
      Several have been shown to transport oxalate in exchange for either sulfate or chloride, with 2 of them, SLC26A6 and SLC26A3, showing high affinity for oxalate. SLC26A6 is expressed in the apical membrane of intestinal and renal epithelia, including the pancreatic duct cells
      • Lohi H.
      • Kujala M.
      • Kerkela E.
      • Saarialho-Kere U.
      • Kestila M.
      • Kere J.
      Mapping of five new putative anion transporter genes in human and characterization of SLC26A6, a candidate gene for pancreatic anion exchanger.
      and duodenum
      • Wang Z.
      • Petrovic S.
      • Mann E.
      • Soleimani M.
      Identification of an apical Cl/HCO3 exchanger in the small intestine.
      and the proximal tubule,
      • Knauf F.
      • Yang S.-L.
      • Thomson R.B.
      • Mentone S.A.
      • Giebisch G.
      • Aronson P.S.
      Identification of a chloride-formate exchanger expressed on the brush border membrane of renal proximal tubule cells.
      more specifically the distal proximal tubule, distal convoluting tubule, and intercalating cells of the collecting ducts.
      • Kujala M.
      • Tienari J.
      • Lohi H.
      • et al.
      SLC26A6 and SLC26A7 anion exchangers have a distinct distribution in human kidney.
      SLC26A3 is not found in the kidney but is present in the apical membrane of cells in the small intestine and colon.
      • Silberg D.G.
      • Wang W.
      • Moseley R.H.
      • Traber P.G.
      The Down Regulated in Adenoma (dra) gene encodes an intestine-specific membrane sulfate transport protein.
      SLC26A6 and A3 facilitate transcellular transport of anions across the apical membrane; in some cases this exchange is electrogenic and in others electroneutral.
      The Slc26a6 null mouse showed a significant increase in intestinal oxalate absorption which was insensitive to the anion exchange inhibitor DIDS (4,4′ diisothiocyanostilbene-2,2′-disulfonic acid) compared to the wildtype mouse, in which net DIDS-sensitive oxalate excretion occurred.
      • Freel R.W.
      • Hatch M.
      • Green M.
      • Soleimani M.
      Ileal oxalate absorption and urinary oxalate excretion are enhanced in Slc26a6 null mice.
      • Jiang Z.
      • Asplin J.R.
      • Evan A.P.
      • et al.
      Calcium oxalate urolithiasis in mice lacking anion transporter Slc26a6.
      These results were interpreted as unopposed oxalate uptake by an as-yet unknown apical transporter, with a reduction in the apical efflux of oxalate (in exchange for chloride) and increased export across the basal membrane. In both mouse knockout models, oxalate excretion was reduced by decreasing dietary oxalate, supporting the theory that the hyperoxaluria is due to a hyperabsorptive state. It is possible that SLC26A3, which is involved in oxalate-sensitive sulfate uptake into the intestinal cells, could be the other apical membrane transporter and that one could reduce oxalate uptake by blocking this transporter. Preliminary studies with an Slc26a3 knockout mouse do indeed show a significant decrease in mucosal to serosal flux of oxalate in both the distal ileum and distal colon.
      • Hatch M.
      • Freel R.W.
      The roles and mechanisms of intestinal oxalate transport in oxalate homeostasis.
      The interplay of these 2 transporters therefore appears to have major importance for the regulation of oxalate uptake in the gut, at least in mice.
      However, mice are not humans and mice are quite resistant to renal stone formation even in the presence of marked hyperoxaluria.
      • Freel R.W.
      • Hatch M.
      • Green M.
      • Soleimani M.
      Ileal oxalate absorption and urinary oxalate excretion are enhanced in Slc26a6 null mice.
      • Jiang Z.
      • Asplin J.R.
      • Evan A.P.
      • et al.
      Calcium oxalate urolithiasis in mice lacking anion transporter Slc26a6.
      • Hatch M.
      • Freel R.W.
      The roles and mechanisms of intestinal oxalate transport in oxalate homeostasis.
      • Salido E.C.
      • Li X.M.
      • Lu Y.
      • et al.
      Alanine-glyoxylate aminotransferase-deficient mice, a model for primary hyperoxaluria that responds to adenoviral gene transfer.
      Comparative studies of the human and mouse SLC26A6 showed similar bidirectional oxalate flux and comparable exchange rates for chloride/bicarbonate and chloride/hydroxide.
      • Chernova M.N.
      • Jiang L.
      • Friedman D.J.
      • et al.
      Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants.
      However, there were significant differences in the affinity for chloride, which appeared to reside primarily, but not exclusively, in the transmembrane domain.
      • Chernova M.N.
      • Jiang L.
      • Friedman D.J.
      • et al.
      Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants.
      • Clark J.S.
      • Vandorpe D.H.
      • Chernova M.N.
      • Heneghan J.F.
      • Stewart A.K.
      • Alper S.L.
      Species differences in Cl affinity and in electrogenicity of SLC26A6-mediated oxalate/Cl exchange correlate with the distinct human and mouse susceptibilities to nephrolithiasis.
      The significance of the lower affinity of human SLC26A6 for chloride may be that the enteric secretion of oxalate is less in humans than mouse, leading to a greater degree of oxalate absorption with a concomitant increased risk of renal stone formation.
      An adaptive mechanism has been shown in animal studies whereby in hyperoxaluric states, eg, renal failure, increasing amounts of oxalate can be secreted into the gut,
      • Hatch M.
      • Freel R.W.
      • Vaziri N.
      Intestinal excretion of oxalate in chronic renal failure.
      possibly via SLC26A6. Treatment of hyperoxaluric individuals with oxalate-degrading bacteria, specifically Oxalobacter formigenes, can potentially capitalize on this response by enhancing the enteric secretion of oxalate
      • Hoppe B.
      • Beck B.
      • Gatter N.
      • et al.
      Oxalobacter formigenes: a potential tool for the treatment of primary hyperoxaluria type 1.
      and thus has the potential to act as a generic treatment for all forms of hyperoxaluria.
      The possibility of a role of SLC26A6 in human forms of hyperoxaluria is therefore one that needs to be considered, whether as a cause of disease itself or as a disease modifier. Monico et al
      • Monico C.G.
      • Weinstein A.
      • Jiang Z.
      • et al.
      Phenotypic and functional analysis of human SLC26A6 variants in patients with familial hyperoxaluria and calcium oxalate nephrolithiasis.
      explore this hypothesis by analyzing the SLC26A6 gene in patients with PH1, PH2, and atypical PH. Five missense changes were found, including 2 known polymorphisms, p.Val206Met (a valine to methionine change at amino acid 206)
      • Waldegger S.
      • Moschen I.
      • Ramirez A.
      • et al.
      Cloning and characterization of SLC26A6, a novel member of the solute carrier 26 family.
      and p.Val185Met (the same amino acid change at position 185).
      • Clark J.S.
      • Vandorpe D.H.
      • Chernova M.N.
      • Heneghan J.F.
      • Stewart A.K.
      • Alper S.L.
      Species differences in Cl affinity and in electrogenicity of SLC26A6-mediated oxalate/Cl exchange correlate with the distinct human and mouse susceptibilities to nephrolithiasis.
      Expression studies have shown that p.Val185Met slightly reduces oxalate influx and efflux in response to extracellular chloride concentration.
      • Clark J.S.
      • Vandorpe D.H.
      • Chernova M.N.
      • Heneghan J.F.
      • Stewart A.K.
      • Alper S.L.
      Species differences in Cl affinity and in electrogenicity of SLC26A6-mediated oxalate/Cl exchange correlate with the distinct human and mouse susceptibilities to nephrolithiasis.
      The p.Val206Met polymorphism, which occurred at a relatively high frequency in controls and patients, albeit mainly in the heterozygous state, reduced oxalate influx by 30% relative to the wild type transporter,
      • Monico C.G.
      • Weinstein A.
      • Jiang Z.
      • et al.
      Phenotypic and functional analysis of human SLC26A6 variants in patients with familial hyperoxaluria and calcium oxalate nephrolithiasis.
      although the effect on oxalate efflux was not described. The authors were unable to show any effect of this variant on urine or plasma oxalate concentration, although it may be that subtle differences in oxalate excretion are overlooked in such gross hyperoxaluric states or that heterozygosity has no effect on transporter function. The results would therefore suggest that mutations in SLC26A6 do not account for atypical PH, at least in the cases examined here, but its role as a modifier of oxalate excretion cannot be ruled out at this stage.

      Acknowledgements

      Financial Disclosure: None.

      References

        • Monico C.G.
        • Weinstein A.
        • Jiang Z.
        • et al.
        Phenotypic and functional analysis of human SLC26A6 variants in patients with familial hyperoxaluria and calcium oxalate nephrolithiasis.
        Am J Kidney Dis. 2008; 52: 1096-1103
        • Latta K.
        • Brodehl J.
        Primary hyperoxaluria I.
        Eur J Pediatr. 1990; 149: 518-522
        • Monico C.G.
        • Persson M.
        • Ford C.H.
        • Rumsby G.
        • Milliner D.S.
        Potential mechanisms of marked hyperoxaluria not due to primary hyperoxaluria I or II.
        Kidney Int. 2002; 62: 392-400
        • Van Acker K.J.
        • Eyskens F.J.
        • Espeel M.F.
        • et al.
        Hyperoxaluria with hyperglycoluria not due to alanine:glyoxylate aminotransferase defect: A novel type of primary hyperoxaluria.
        Kidney Int. 1996; 50: 1747-1752
        • Takada Y.
        • Kaneko N.
        • Esumi H.
        • Purdue P.E.
        • Danpure C.J.
        Human peroxisomal L-alanine:glyoxylate aminotransferase: Evolutionary loss of a mitochondrial targeting signal by point mutation of the initiation codon.
        Biochem J. 1990; 268: 517-520
        • Cramer S.D.
        • Ferree P.M.
        • Lin K.
        • Milliner D.S.
        • Holmes R.P.
        The gene encoding hydroxypyruvate reductase (GRHPR) is mutated in patients with primary hyperoxaluria type II.
        Hum Mol Genet. 1999; 8: 2063-2069
        • Rumsby G.
        • Cregeen D.
        Identification and expression of a cDNA for human hydroxypyruvate/glyoxylate reductase.
        Biochim Biophys Acta. 1999; 1446: 383-388
        • Pirulli D.
        • Marangella M.
        • Amoroso A.
        Primary hyperoxaluria: Genotype-phenotype correlation.
        J Nephrol. 2003; 16: 297-309
        • Monico C.G.
        • Olson J.B.
        • Milliner D.S.
        Implications of genotype and enzyme phenotype in pyridoxine response of patients with type 1 primary hyperoxaluria.
        Am J Nephrol. 2005; 25: 183-188
        • Rumsby G.
        • Williams E.
        • Coulter-Mackie M.B.
        Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias.
        Kidney Int. 2004; 66: 959-963
        • Zhang X.
        • Roe M.
        • Hou Y.
        • et al.
        Crystal structure of alanine:glyoxylate aminotranferase and the relationship between genotype and enzymatic phenotype in primary hyperoxaluria type 1.
        J Mol Biol. 2003; 331: 643-652
        • Booth M.P.S.
        • Conners R.
        • Rumsby G.
        • Brady R.L.
        Structural basis of substrate specificity in human glyoxylate reductase/hydroxypyruvate reductase.
        J Mol Biol. 2006; 360: 178-189
        • Frishberg Y.
        • Rinat C.
        • Khatib I.
        • et al.
        Intra-familial clinical heterogeneity: absence of genotype-phenotype correlation in primary hyperoxaluria type 1 in Israel.
        Am J Nephrol. 2005; 25: 269-275
        • Behnam J.T.
        • Williams E.L.
        • Brink S.
        • Rumsby G.
        • Danpure C.J.
        Reconstruction of human hepatocyte glyoxylate metabolic pathways in stably transformed Chinese-hamster ovary cells.
        Biochem J. 2006; 394: 409-416
        • Everett L.A.
        • Green E.D.
        A family of mammalian anion transporters and their involvement in human genetic disease.
        Hum Mol Genet. 1999; 8: 1883-1891
        • Lohi H.
        • Kujala M.
        • Kerkela E.
        • Saarialho-Kere U.
        • Kestila M.
        • Kere J.
        Mapping of five new putative anion transporter genes in human and characterization of SLC26A6, a candidate gene for pancreatic anion exchanger.
        Genomics. 2000; 70: 102-112
        • Wang Z.
        • Petrovic S.
        • Mann E.
        • Soleimani M.
        Identification of an apical Cl/HCO3 exchanger in the small intestine.
        Am J Physiol Gastrointest Liver Physiol. 2002; 282: G573-G579
        • Knauf F.
        • Yang S.-L.
        • Thomson R.B.
        • Mentone S.A.
        • Giebisch G.
        • Aronson P.S.
        Identification of a chloride-formate exchanger expressed on the brush border membrane of renal proximal tubule cells.
        Proc Nat Acad Sci USA. 2001; 98: 9425-9430
        • Kujala M.
        • Tienari J.
        • Lohi H.
        • et al.
        SLC26A6 and SLC26A7 anion exchangers have a distinct distribution in human kidney.
        Nephron Exp Nephrol. 2005; 101: e50-e58
        • Silberg D.G.
        • Wang W.
        • Moseley R.H.
        • Traber P.G.
        The Down Regulated in Adenoma (dra) gene encodes an intestine-specific membrane sulfate transport protein.
        J Biol Chem. 1995; 270: 11897-11902
        • Freel R.W.
        • Hatch M.
        • Green M.
        • Soleimani M.
        Ileal oxalate absorption and urinary oxalate excretion are enhanced in Slc26a6 null mice.
        Am J Physiol Gastrointest Liver Physiol. 2006; 290: G719-G728
        • Jiang Z.
        • Asplin J.R.
        • Evan A.P.
        • et al.
        Calcium oxalate urolithiasis in mice lacking anion transporter Slc26a6.
        Nat Genet. 2006; 38: 474-478
        • Hatch M.
        • Freel R.W.
        The roles and mechanisms of intestinal oxalate transport in oxalate homeostasis.
        Semin Nephrol. 2008; 28: 143-151
        • Salido E.C.
        • Li X.M.
        • Lu Y.
        • et al.
        Alanine-glyoxylate aminotransferase-deficient mice, a model for primary hyperoxaluria that responds to adenoviral gene transfer.
        Proc Nat Acad Sci USA. 2006; 103: 18249-18254
        • Chernova M.N.
        • Jiang L.
        • Friedman D.J.
        • et al.
        Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants.
        J Biol Chem. 2005; 2005: 8564-8580
        • Clark J.S.
        • Vandorpe D.H.
        • Chernova M.N.
        • Heneghan J.F.
        • Stewart A.K.
        • Alper S.L.
        Species differences in Cl affinity and in electrogenicity of SLC26A6-mediated oxalate/Cl exchange correlate with the distinct human and mouse susceptibilities to nephrolithiasis.
        J Physiol. 2008; 586: 1291-1306
        • Hatch M.
        • Freel R.W.
        • Vaziri N.
        Intestinal excretion of oxalate in chronic renal failure.
        J Am Soc Nephrol. 1994; 5: 1339-1343
        • Hoppe B.
        • Beck B.
        • Gatter N.
        • et al.
        Oxalobacter formigenes: a potential tool for the treatment of primary hyperoxaluria type 1.
        Kidney Int. 2006; 70: 1305-1311
        • Waldegger S.
        • Moschen I.
        • Ramirez A.
        • et al.
        Cloning and characterization of SLC26A6, a novel member of the solute carrier 26 family.
        Genomics. 2001; 72: 43-50

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