Future treatments for hyperoxaluria.

PURPOSE OF REVIEW: The review of potential therapies in the treatment of hyperoxaluria is timely, given the current excitement with clinical trials and the mounting evidence of the importance of oxalate in both kidney stone and chronic kidney disease. RECENT FINDINGS: Given the significant contribution of both endogenous and dietary oxalate to urinary oxalate excretions, it is not surprising therapeutic targets are being studied in both pathways. This article covers the existing data on endogenous and dietary oxalate and the current targets in these pathways. SUMMARY: In the near future, there will likely be therapies targeting both endogenous and dietary oxalate, especially in subsets of kidney stone formers.

Hydroxyproline Metabolism and Oxalate Synthesis in Primary Hyperoxaluria.

Background Endogenous oxalate synthesis contributes to calcium oxalate stone disease and is markedly increased in the inherited primary hyperoxaluria (PH) disorders. The incomplete knowledge regarding oxalate synthesis complicates discovery of new treatments. Hydroxyproline (Hyp) metabolism results in the formation of oxalate and glycolate. However, the relative contribution of Hyp metabolism to endogenous oxalate and glycolate synthesis is not known.Methods To define this contribution, we performed primed, continuous, intravenous infusions of the stable isotope [15N,13C5]-Hyp in nine healthy subjects and 19 individuals with PH and quantified the levels of urinary 13C2-oxalate and 13C2-glycolate formed using ion chromatography coupled to mass detection.Results The total urinary oxalate-to-creatinine ratio during the infusion was 73.1, 70.8, 47.0, and 10.6 mg oxalate/g creatinine in subjects with PH1, PH2, and PH3 and controls, respectively. Hyp metabolism accounted for 12.8, 32.9, and 14.8 mg oxalate/g creatinine in subjects with PH1, PH2, and PH3, respectively, compared with 1.6 mg oxalate/g creatinine in controls. The contribution of Hyp to urinary oxalate was 15% in controls and 18%, 47%, and 33% in subjects with PH1, PH2, and PH3, respectively. The contribution of Hyp to urinary glycolate was 57% in controls, 30% in subjects with PH1, and <13% in subjects with PH2 or PH3.Conclusions Hyp metabolism differs among PH types and is a major source of oxalate synthesis in individuals with PH2 and PH3. In patients with PH1, who have the highest urinary excretion of oxalate, the major sources of oxalate remain to be identified.

Effects of alanine:glyoxylate aminotransferase variants and pyridoxine sensitivity on oxalate metabolism in a cell-based cytotoxicity assay.

The hereditary kidney stone disease primary hyperoxaluria type 1 (PH1) is caused by a functional deficiency of the liver-specific, peroxisomal, pyridoxal-phosphate-dependent enzyme, alanine:glyoxylate aminotransferase (AGT). One third of PH1 patients, particularly those expressing the p.[(Pro11Leu; Gly170Arg; Ile340Met)] mutant allele, respond clinically to pharmacological doses of pyridoxine. To gain further insight into the metabolic effects of AGT dysfunction in PH1 and the effect of pyridoxine, we established an "indirect" glycolate cytotoxicity assay using CHO cells expressing glycolate oxidase (GO) and various normal and mutant forms of AGT. In cells expressing GO the great majority of glycolate was converted to oxalate and glyoxylate, with the latter causing the greater decrease in cell survival. Co-expression of normal AGTs and some, but not all, mutant AGT variants partially counteracted this cytotoxicity and led to decreased synthesis of oxalate and glyoxylate. Increasing the extracellular pyridoxine up to 0.3µM led to an increased metabolic effectiveness of normal AGTs and the AGT-Gly170Arg variant. The increased survival seen with AGT-Gly170Arg was paralleled by a 40% decrease in oxalate and glyoxylate levels. These data support the suggestion that the effectiveness of pharmacological doses of pyridoxine results from an improved metabolic effectiveness of AGT; that is the increased rate of transamination of glyoxylate to glycine. The indirect glycolate toxicity assay used in the present study has potential to be used in cell-based drug screening protocols to identify chemotherapeutics that might enhance or decrease the activity and metabolic effectiveness of AGT and GO, respectively, and be useful in the treatment of PH1.

Metabolism of (13)C5-hydroxyproline in mouse models of Primary Hyperoxaluria and its inhibition by RNAi therapeutics targeting liver glycolate oxidase and hydroxyproline dehydrogenase.

Excessive endogenous oxalate synthesis can result in calcium oxalate kidney stone formation and renal failure. Hydroxyproline catabolism in the liver and kidney contributes to endogenous oxalate production in mammals. To quantify this contribution we have infused Wt mice, Agxt KO mice deficient in liver alanine:glyoxylate aminotransferase, and Grhpr KO mice deficient in glyoxylate reductase, with (13)C5-hydroxyproline. The contribution of hydroxyproline metabolism to urinary oxalate excretion in Wt mice was 22±2%, 42±8% in Agxt KO mice, and 36%±9% in Grhpr KO mice. To determine if blocking steps in hydroxyproline and glycolate metabolism would decrease urinary oxalate excretion, mice were injected with siRNA targeting the liver enzymes glycolate oxidase and hydroxyproline dehydrogenase. These siRNAs decreased the expression of both enzymes and reduced urinary oxalate excretion in Agxt KO mice, when compared to mice infused with a luciferase control preparation. These results suggest that siRNA approaches could be useful for decreasing the oxalate burden on the kidney in individuals with Primary Hyperoxaluria.

Pyridoxamine and pyridoxal are more effective than pyridoxine in rescuing folding-defective variants of human alanine:glyoxylate aminotransferase causing primary hyperoxaluria type I.

Vitamin B6 in the form of pyridoxine (PN) is one of the most widespread pharmacological therapies for inherited diseases involving pyridoxal phosphate (PLP)-dependent enzymes, including primary hyperoxaluria type I (PH1). PH1 is caused by a deficiency of liver-peroxisomal alanine: glyoxylate aminotransferase (AGT), which allows glyoxylate oxidation to oxalate leading to the deposition of insoluble calcium oxalate in the kidney. Only a minority of PH1 patients, mostly bearing the F152I and G170R mutations, respond to PN, the only pharmacological treatment currently available. Moreover, excessive doses of PN reduce the specific activity of AGT in a PH1 cellular model. Nevertheless, the possible effect(s) of other B6 vitamers has not been investigated previously. Here, we compared the ability of PN in rescuing the effects of the F152I and G170R mutations with that of pyridoxamine (PM) and PL. We found that supplementation with PN raises the intracellular concentration of PN phosphate (PNP), which competes with PLP for apoenzyme binding leading to the formation of an inactive AGT-PNP complex. In contrast, PNP does not accumulate in the cell upon PM or PL supplementation, but higher levels of PLP and PM phosphate (PMP), the two active forms of the AGT coenzyme, are found. This leads to an increased ability of PM and PL to rescue the effects of the F152I and G170R mutations compared with PN. A similar effect was also observed for other folding-defective AGT variants. Thus, PM and PL should be investigated as matter of importance as therapeutics for PH1 patients bearing folding mutations.

Pharmacologic rescue of an enzyme-trafficking defect in primary hyperoxaluria 1.

Primary hyperoxaluria 1 (PH1; Online Mendelian Inheritance in Man no. 259900), a typically lethal biochemical disorder, may be caused by the AGT(P11LG170R) allele in which the alanine:glyoxylate aminotransferase (AGT) enzyme is mistargeted from peroxisomes to mitochondria. AGT contains a C-terminal peroxisomal targeting sequence, but mutations generate an N-terminal mitochondrial targeting sequence that directs AGT from peroxisomes to mitochondria. Although AGT(P11LG170R) is functional, the enzyme must be in the peroxisome to detoxify glyoxylate by conversion to alanine; in disease, amassed glyoxylate in the peroxisome is transported to the cytosol and converted to oxalate by lactate dehydrogenase, leading to kidney failure. From a chemical genetic screen, we have identified small molecules that inhibit mitochondrial protein import. We tested whether one promising candidate, Food and Drug Administration (FDA)-approved dequalinium chloride (DECA), could restore proper peroxisomal trafficking of AGT(P11LG170R). Indeed, treatment with DECA inhibited AGT(P11LG170R) translocation into mitochondria and subsequently restored trafficking to peroxisomes. Previous studies have suggested that a mitochondrial uncoupler might work in a similar manner. Although the uncoupler carbonyl cyanide m-chlorophenyl hydrazone inhibited AGT(P11LG170R) import into mitochondria, AGT(P11LG170R) aggregated in the cytosol, and cells subsequently died. In a cellular model system that recapitulated oxalate accumulation, exposure to DECA reduced oxalate accumulation, similar to pyridoxine treatment that works in a small subset of PH1 patients. Moreover, treatment with both DECA and pyridoxine was additive in reducing oxalate levels. Thus, repurposing the FDA-approved DECA may be a pharmacologic strategy to treat PH1 patients with mutations in AGT because an additional 75 missense mutations in AGT may also result in mistrafficking.

Four of the most common mutations in primary hyperoxaluria type 1 unmask the cryptic mitochondrial targeting sequence of alanine:glyoxylate aminotransferase encoded by the polymorphic minor allele.

The gene encoding the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT, EC. 2.6.1.44) exists as two common polymorphic variants termed the "major" and "minor" alleles. The P11L amino acid replacement encoded by the minor allele creates a hidden N-terminal mitochondrial targeting sequence, the unmasking of which occurs in the hereditary calcium oxalate kidney stone disease primary hyperoxaluria type 1 (PH1). This unmasking is due to the additional presence of a common disease-specific G170R mutation, which is encoded by about one third of PH1 alleles. The P11L and G170R replacements interact synergistically to reroute AGT to the mitochondria where it cannot fulfill its metabolic role (i.e. glyoxylate detoxification) effectively. In the present study, we have reinvestigated the consequences of the interaction between P11L and G170R in stably transformed CHO cells and have studied for the first time whether a similar synergism exists between P11L and three other mutations that segregate with the minor allele (i.e. I244T, F152I, and G41R). Our investigations show that the latter three mutants are all able to unmask the cryptic P11L-generated mitochondrial targeting sequence and, as a result, all are mistargeted to the mitochondria. However, whereas the G170R, I244T, and F152I mutants are able to form dimers and are catalytically active, the G41R mutant aggregates and is inactive. These studies open up the possibility that all PH1 mutations, which segregate with the minor allele, might also lead to the peroxisome-to-mitochondrion mistargeting of AGT, a suggestion that has important implications for the development of treatment strategies for PH1.

Multiple mechanisms of action of pyridoxine in primary hyperoxaluria type 1.

Primary hyperoxaluria type 1 (PH1) is a rare hereditary calcium oxalate kidney stone disease caused by a deficiency of the liver-specific pyridoxal-phosphate-dependent peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). About one third of patients are responsive to pharmacological doses of pyridoxine (vitamin B6), but its mechanism of action is unknown. Using stably transformed Chinese Hamster Ovary (CHO) cells expressing various normal and mutant forms of AGT, we have shown that pyridoxine increases the net expression, catalytic activity and peroxisomal import of the most common mistargeted mutant form of AGT (i.e. Gly170Arg on the background of the polymorphic minor allele). These multiple effects explain for the first time the action of pyridoxine in the most common group of responsive patients. Partial effects of pyridoxine were also observed for two other common AGT mutants on the minor allele (i.e. Phe152Ile and Ile244Thr) but not for the minor allele mutant AGT containing a Gly41Arg replacement. These findings demonstrate that pyridoxine, which is metabolised to pyridoxal phosphate, the essential cofactor of AGT, achieves its effects both as a prosthetic group (increasing enzyme catalytic activity) and a chemical chaperone (increasing peroxisome targeting and net expression). This new understanding should aid the development of pharmacological treatments that attempt to enhance efficacy of pyridoxine in PH1, as well as encouraging a re-evaluation of the extent of pyridoxine responsiveness in PH1, as more patients than previously thought might benefit from such treatment.

Factors influencing clinical outcome in patients with primary hyperoxaluria type 1.

The renal outcome in patients with primary hyperoxaluria type 1 is partly determined by AGXT mutations, including but not limited to the p.Gly170Arg mutation. The study by Mandrile et al. reports on the largest cohort of patients genotyped yet, with long-term renal survival and medical treatment by pyridoxine. In addition to the common p.Gly170Arg mutation, three other mutations were shown to be potentially associated with slower evolution.

Primary hyperoxaluria Type 1: indications for screening and guidance for diagnosis and treatment.

Primary hyperoxaluria Type 1 is a rare autosomal recessive inborn error of glyoxylate metabolism, caused by a deficiency of the liver-specific enzyme alanine:glyoxylate aminotransferase. The disorder results in overproduction and excessive urinary excretion of oxalate, causing recurrent urolithiasis and nephrocalcinosis. As glomerular filtration rate declines due to progressive renal involvement, oxalate accumulates leading to systemic oxalosis. The diagnosis is based on clinical and sonographic findings, urine oxalate assessment, enzymology and/or DNA analysis. Early initiation of conservative treatment (high fluid intake, pyridoxine, inhibitors of calcium oxalate crystallization) aims at maintaining renal function. In chronic kidney disease Stages 4 and 5, the best outcomes to date were achieved with combined liver-kidney transplantation.

Primary hyperoxaluria type 1: pathophysiology and genetics.

Primary hyperoxaluria type 1 (PH1) is a rare genetic form of calcium oxalate kidney stone disease. It is caused by a deficiency in the liver-specific enzyme, alanine:glyoxylate aminotransferase (AGT), a pyridoxal-5'-phosphate (PLP)-dependent enzyme involved in the metabolism of glyoxylate. The excessive endogenous synthesis of oxalate that ensues leads to hyperoxaluria, and the crystallization of the poorly soluble calcium salt of oxalate is responsible for a severe kidney stone disease, which can progress to end-stage renal disease, systemic deposition of oxalate and death. Knowledge about metabolic precursors of glyoxylate and oxalate, molecular pathology of AGT and analytical methods for diagnosis and clinical assessment have allowed a better understanding of the mechanisms underlying PH1 and opened the door to new therapeutic strategies.

Generation of a GLO-2 deficient mouse reveals its effects on liver carbonyl and glutathione levels.

OBJECTIVE: Hydroxyacylglutathione hydrolase (aka as GLO-2) is a component of the glyoxalase pathway involved in the detoxification of the reactive oxoaldehydes, glyoxal and methylglyoxal. These reactive metabolites have been linked to a variety of pathological conditions, including diabetes, cancer and heart disease and may be involved in the aging process. The objective of this study was to generate a mouse model deficient in GLO-2 to provide insight into the function of GLO-2 and to determine if it is potentially linked to endogenous oxalate synthesis which could influence urinary oxalate excretion. METHODS: A GLO-2 knock out mouse was generated using CRISPR/Cas 9 techniques. Tissue and 24-h urine samples were collected under baseline conditions from adult male and female animals for biochemical analyses, including chromatographic measurement of glycolate, oxalate, glyoxal, methylglyoxal, D-lactate, ascorbic acid and glutathione levels. RESULTS: The GLO-2 KO animals developed normally and there were no changes in 24-h urinary oxalate excretion, liver levels of methylglyoxal, glyoxal, ascorbic acid and glutathione, or plasma d-lactate levels. GLO-2 deficient males had lower plasma glycolate levels than wild type males while this relationship was not observed in females. CONCLUSIONS: The lack of a unique phenotype in a GLO-2 KO mouse model under baseline conditions is consistent with recent evidence, suggesting a functional glyoxalase pathway is not required for optimal health. A lower plasma glycolate in male GLO-2 KO animals suggests glyoxal production may be a significant contributor to circulating glycolate levels, but not to endogenous oxalate synthesis.

Effect of alanine supplementation on oxalate synthesis.

The Primary Hyperoxalurias (PH) are rare disorders of metabolism leading to excessive endogenous synthesis of oxalate and recurring calcium oxalate kidney stones. Alanine glyoxylate aminotransferase (AGT), deficient in PH type 1, is a key enzyme in limiting glyoxylate oxidation to oxalate. The affinity of AGT for its co-substrate, alanine, is low suggesting that its metabolic activity could be sub-optimal in vivo. To test this hypothesis, we examined the effect of L-alanine supplementation on oxalate synthesis in cell culture and in mouse models of Primary Hyperoxaluria Type 1 (Agxt KO), Type 2 (Grhpr KO) and in wild-type mice. Our results demonstrated that increasing L-alanine in cells decreased synthesis of oxalate and increased viability of cells expressing GO and AGT when incubated with glycolate. In both wild type and Grhpr KO male and female mice, supplementation with 10% dietary L-alanine significantly decreased urinary oxalate excretion ~30% compared to baseline levels. This study demonstrates that increasing the availability of L-alanine can increase the metabolic efficiency of AGT and reduce oxalate synthesis.

Genotype-phenotype correlation in primary hyperoxaluria type 1: the p.Gly170Arg AGXT mutation is associated with a better outcome.

We sought to ascertain the long-term outcome and genotype-phenotype correlations available for primary hyperoxaluria type 1 in a large retrospective cohort study. We examined the clinical history of 155 patients (129 families primarily from Western Europe, North Africa, or the Middle East) as well as the enzymatic or genetic diagnosis. The median age at first symptom was 4 years, and at diagnosis 7.7 years, at which time 43% had reached end-stage renal disease. Presentations included: (1) early nephrocalcinosis and infantile renal failure, (2) recurrent urolithiasis and progressive renal failure diagnosed during childhood, (3) late onset with occasional stone passage diagnosed in adulthood, (4) diagnosis occurring on post-transplantation recurrence, and (5) family screening. The cumulative patient survival was 95, 86, and 74% at ages 10, 30, and 50 years, respectively, with the cumulative renal survival of 81, 59, 41, and 10% at ages 10, 20, 30, and 50 years, respectively; 72 patients had undergone a total of 97 transplantations. Among the 136 patients with DNA analysis, the most common mutation was p.Gly170Arg (allelic frequency 21.5%), with a median age at end-stage renal disease of 47 years for homozygotes, 35 years for heterozygotes, and 21 years for other mutations. Our results underscore the severe prognosis of primary hyperoxaluria type 1 and the necessity for early diagnosis and treatment, as well as confirm a better prognosis of the p.Gly170Arg mutation.

Tacrolimus nephrotoxicity: beware of the association of diarrhea, drug interaction and pharmacogenetics.

Tacrolimus is known to potentially lead to adverse events in recipients with diarrhoea and/or calcium channel blocker (CCB) co-administration. We report a renal transplant recipient who suffered from severe nephrotoxicity related to a toxic tacrolimus trough concentration in both conditions, diarrhoea and CCB co-administration, and with genotyped CYP3A system and P-glycoprotein (P-gp) polymorphisms. To our knowledge, this is the first case to be investigated for such polymorphisms. Clinicians should be reminded of the possibility of highly increased levels of tacrolimus in situations of diarrhoea and/or co-administration of CCBs. It also highlights the key role in tacrolimus pharmacokinetics of the CYP3A system and P-gp polymorphisms, and their influence in high-risk situations when enzyme activity is already affected by enterocyte damage due to diarrhoea and CCB competition.

Bone metabolism in oxalosis: a single-center study using new imaging techniques and biomarkers.

The deposition of calcium oxalate crystals in the kidney and bone is a hallmark of primary hyperoxaluria type 1 (PH1). We report here an evaluation of the bone status of 12 PH1 children based on bone biomarkers [parathyroid hormone, vitamin D, fibroblast growth factor 23 (FGF23)] and radiological assessments (skeletal age, three-dimensional high-resolution peripheral quantitative computed tomography, HR-pQCT) carried out within the framework of a cross-sectional single-center study. The controls consisted of healthy and children with chronic kidney disease already enrolled in local bone and mineral metabolism studies. The mean age (+ or - standard deviation) age of the patients was 99 (+ or - 63) months. Six children suffered from fracture. Bone maturation was accelerated in five patients, four of whom were <5 years. The combination of new imaging techniques and biomarkers highlighted new and unexplained features of PH1: advanced skeletal age in young PH1 patients, increased FGF23 levels and decreased total volumetric bone mineral density with bone microarchitecture alteration.

Primary hyperoxaluria type 1: strategy for organ transplantation.

PURPOSE OF REVIEW: Primary hyperoxaluria type 1, the most common form of primary hyperoxaluria, is an autosomal recessive disorder caused by a deficiency of the liver-specific enzyme alanine:glyoxylate aminotransferase. This results in increased synthesis and subsequent urinary excretion of the metabolic end-product oxalate and the deposition of insoluble calcium oxalate in the kidney and urinary tract. As glomerular filtration rate decreases due to progressive renal involvement, oxalate accumulates and results in systemic oxalosis. RECENT FINDINGS: Diagnosis is still often delayed. It is mainly established on the basis of clinical and sonographic findings, urinary oxalate ± glycolate assessment, and DNA analysis. SUMMARY: Following specific conservative measures, the ultimate management is based on organ transplantation. Correction of the enzyme defect by liver transplantation should be planned before systemic oxalosis develops to optimize outcomes and may be either simultaneous (immunological benefit) or sequential (biochemical benefit) liver-kidney transplantation depending on disease staging, facilities, and access to deceased or living donors. Allograft and patient survival currently approaches that of transplant patients with kidney transplantation alone and with other diseases requiring combined liver-kidney transplantation. In addition, this strategy has also provided significant improvement in both quality of life and statural growth.

Contribution of Dietary Oxalate and Oxalate Precursors to Urinary Oxalate Excretion.

Kidney stone disease is increasing in prevalence, and the most common stone composition is calcium oxalate. Dietary oxalate intake and endogenous production of oxalate are important in the pathophysiology of calcium oxalate stone disease. The impact of dietary oxalate intake on urinary oxalate excretion and kidney stone disease risk has been assessed through large cohort studies as well as smaller studies with dietary control. Net gastrointestinal oxalate absorption influences urinary oxalate excretion. Oxalate-degrading bacteria in the gut microbiome, especially Oxalobacter formigenes, may mitigate stone risk through reducing net oxalate absorption. Ascorbic acid (vitamin C) is the main dietary precursor for endogenous production of oxalate with several other compounds playing a lesser role. Renal handling of oxalate and, potentially, renal synthesis of oxalate may contribute to stone formation. In this review, we discuss dietary oxalate and precursors of oxalate, their pertinent physiology in humans, and what is known about their role in kidney stone disease.

The effects of the inactivation of Hydroxyproline dehydrogenase on urinary oxalate and glycolate excretion in mouse models of primary hyperoxaluria.

The major clinical manifestation of the Primary Hyperoxalurias (PH) is increased production of oxalate, as a consequence of genetic mutations that lead to aberrant glyoxylate and hydroxyproline metabolism. Hyperoxaluria can lead to the formation of calcium-oxalate kidney stones, nephrocalcinosis and renal failure. Current therapeutic approaches rely on organ transplants and more recently modifying the pathway of oxalate synthesis using siRNA therapy. We have recently reported that the metabolism of trans-4-hydroxy-L-proline (Hyp), an amino acid derived predominantly from collagen metabolism, is a significant source of oxalate production in individuals with PH2 and PH3. Thus, the first enzyme in the Hyp degradation pathway, hydroxyproline dehydrogenase (HYPDH), represents a promising therapeutic target for reducing endogenous oxalate production in these individuals. This is supported by the observation that individuals with inherited mutations in HYPDH (PRODH2 gene) have no pathological consequences. The creation of mouse models that do not express HYPDH will facilitate research evaluating HYPDH as a target. We describe the phenotype of the Prodh2 knock out mouse model and show that the lack of HYPDH in PH mouse models results in lower levels of urinary oxalate excretion, consistent with our previous metabolic tracer and siRNA-based knockdown studies. The double knockout mouse, Grhpr KO (PH2 model) and Prodh2 KO, prevented calcium-oxalate crystal deposition in the kidney, when placed on a 1% Hyp diet. These observations support the use of the Grhpr KO mice to screen HYPDH inhibitors in vivo. Altogether these data support HYPDH as an attractive therapeutic target for PH2 and PH3 patients.