[Uric Acid Metabolism, Uric Acid Transporters and Dysuricemia].

Serum urate levels are determined by the balance between uric acid production and uric acid excretion capacity from the kidneys and intestinal tract. Dysuricemia, including hyperuricemia and hypouricemia, develops when the balance shifts towards an increase or a decrease in the uric acid pool. Hyperuricemia is mostly a multifactorial genetic disorder involving several disease susceptibility genes and environmental factors. Hypouricemia, on the other hand, is caused by genetic abnormalities. The main genes involved in dysuricemia are xanthine oxidoreductase, an enzyme that produces uric acid, and the urate transporters urate transporter 1/solute carrier family 22 member 12 (URAT1/SLC22A12), glucose transporter 9/solute carrier family 2 member 9 (GLUT9/SLC2A9) and ATP binding cassette subfamily G member 2 (ABCG2). Deficiency of xanthine oxidoreductase results in xanthinuria, a rare disease with marked hypouricemia. Xanthinuria can be due to a single deficiency of xanthine oxidoreductase or in combination with aldehyde oxidase deficiency as well. The latter is caused by a deficiency in molybdenum cofactor sulfurase, which is responsible for adding sulphur atoms to the molybdenum cofactor required for xanthine oxidoreductase and aldehyde oxidase to exert their action. URAT1/SLC22A12 and GLUT9/SLC2A9 are involved in urate reabsorption and their deficiency leads to renal hypouricemia, a condition that is common in Japanese due to URAT1/SLC22A12 deficiency. On the other hand, ABCG2 is involved in the secretion of urate, and many Japanese have single nucleotide polymorphisms that result in its reduced function, leading to hyperuricemia. In particular, severe dysfunction of ABCG2 leads to hyperuricemia with reduced extrarenal excretion.

Xanthinuria Type 1 with a Novel Mutation in Xanthine Dehydrogenase and a Normal Endothelial Function.

Whether or not extremely low levels of serum uric acid (SUA) in xanthinuria are associated with impairment of the endothelial function and exercise-induced acute kidney injury (EIAKI) is unclear. A 59-year-old woman without EIAKI or urolithiasis had undetectable levels of UA in serum and urine and elevated levels of hypoxanthine and xanthine in urine. A genetic analysis revealed homozygous mutations in the XDH gene [c.1585 C>T (p. Gln529*)]. Flow-mediated dilation was within the normal range. This is the first report of a case with extremely low levels of SUA, xanthinuria with novel mutations of xanthine dehydrogenase (XDH) and a normal endothelial function.

Xanthine oxidoreductase knockout mice with high HPRT activity were not rescued by NAD+ replenishment.

Although xanthinuria is nonfatal in human, xanthine oxidoreductase knockout (Xor-KO) mice have only a short lifespan. Hypoxanthine phosphoribosyltransferase activity (HPRT) in human and wild mice is higher than in laboratory mice. The aim of this study was to investigate the underlying mechanisms that give rise to the longer lifespan of high-HPRT/Xor-KO mice. Before Xor-KO mice die, urinary excretion of hypoxanthine increased with a corresponding decrease in excretion of xanthine. The switch of excretion from xanthine to hypoxanthine might be a cause of death for Xor-KO mice, suggesting inhibition of NAD+-dependent IMP dehydrogenase. Because hypoxanthine inhibits the synthesis of nicotinamide mononucleotide (NMN), a precursor of NAD+, the accumulation of hypoxanthine in Xor-KO mice may cause a depletion in the levels of NAD+. Moreover, urinary excretion of urate in high-HPRT/Uox-KO/Xor-KO mice means urate derived from gut microbiota is absorbed by the intestine. Likewise, over excretion of oxypurine in mice may be caused by intestinal absorption of oxypurine. For NAD+ replenishment, oral supplementation with 1% L-tryptophan, an alternative precursor of NAD+, resulted in a recovery of body weight gain in high-HPRT/Uox-KO/Xor-KO mice. In conclusion, the death of Xor-KO mice by renal failure seems to be caused by a depletion in NAD+ levels due to the intracellular accumulation of hypoxanthine. NAD+ replenishment by oral supplementation of NMN or tryptophan was complicated by the effect of gut microbiota and failed to rescue high-HPRT/Xor-KO mice. The attenuation of intestinal absorption of oxypurines seems to be necessary to avoid hypoxanthine accumulation and over excretion of oxypurine.

Xanthine urolithiasis: Inhibitors of xanthine crystallization.

OBJECTIVE: To identify in vitro inhibitors of xanthine crystallization that have potential for inhibiting the formation of xanthine crystals in urine and preventing the development of the renal calculi in patients with xanthinuria. METHODS: The formation of xanthine crystals in synthetic urine and the effects of 10 potential crystallization inhibitors were assessed using a kinetic turbidimetric system with a photometer. The maximum concentration tested for each compound was: 20 mg/L for 3-methylxanthine (3-MX); 40 mg/L for 7-methylxanthine (7-MX), 1-methylxanthine (1-MX), theobromine (TB), theophylline, paraxanthine, and caffeine; 45 mg/L for 1-methyluric acid; 80 mg/L for 1,3-dimethyluric acid; and 200 mg/L for hypoxanthine. Scanning electron microscopy was used to examine the morphology of the crystals formed when inhibitory effects were observed. RESULTS: Only 7-MX, 3-MX, and 1-MX significantly inhibited xanthine crystallization at the tested concentrations. Mixtures of inhibitors had an additive effect rather than a synergistic effect on crystallization. CONCLUSION: Two of the inhibitors identified here-7-MX and 3-MX-are major metabolites of TB. In particular, after TB consumption, 20% is excreted in the urine as TB, 21.5% as 3-MX, and 36% as 7-MX. Thus, consumption of theobromine could protect patients with xanthinuria from the development of renal xanthine calculi. Clinical trials are necessary to demonstrate these effects in vivo.

Congenital and acquired diseases related to stone formation.

PURPOSE OF REVIEW: To summarize the latest findings of congenital and acquired diseases related to stone formation and help understanding the multitude of cofactors related to urolithiasis. RECENT FINDINGS: Urolithiasis is related to a broad spectrum of congenital and acquired diseases and its management varies according to the stone type, underlying disease or recurrence rate, but it also changes according to recent findings and developments. As prevalence of urolithiasis is constantly increasing, identification of high-risk stone formers and early treatment is essential. Therefore, genetic evaluation like whole exome sequencing becomes a pertinent part of further diagnostics. SUMMARY: Stone formation is a very heterogeneous pathomechanism. This prompt us to look at every patient individually particularly in high-risk patients, including stone and 24-h-urine analysis and additional diagnostic work-up based on stone type or underlying disease.

Thiopurine-induced toxicity is associated with dysfunction variant of the human molybdenum cofactor sulfurase gene (xanthinuria type II).

BACKGROUND: The aim of our study was to identify the genetic background of thiopurine-induced toxicity in a patient with a wild-type thiopurine methyltransferase genotype and activity. A 38-year-old Caucasian woman presented with cutaneous necrotizing vasculitis pancytopenia one month after starting azathioprine therapy. METHODS: During a routine biochemical follow-up of the patient, undetectable serum uric acid (<10 µl) was observed. A high performance liquid chromatography analysis of urinary purines revealed increased levels of xanthine (137 mmol/mol creatinine). The suspected diagnosis of hereditary xanthinuria, a rare autosomal recessive disorder of the last two steps of purine metabolism, was confirmed by sequence analysis. RESULTS: An analysis of XDH/XO and AOX1 revealed common polymorphisms, while analysis of the MOCOS gene identified a rare homozygous variant c.362C > T. Dysfunction of this variant was confirmed by significantly decreased xanthine dehydrogenase/oxidase activity in the patient's plasma (<2% of control mean activity). CONCLUSIONS: We present a biochemical, enzymatic, and molecular genetic case study suggesting an important association between a hitherto undescribed dysfunction variant in the MOCOS gene and thiopurine-induced toxicity. The identified variant c.362C > T results in slower thiopurine metabolism caused by inhibition of 6-mercaptopurine oxidation (catabolism) to 6-thioxanthine and 6-thiouric acid, which increases the formation of the nucleotide 6-thioguanine, which is toxic. This is the first clinical case to identify the crucial role of the MOCOS gene in thiopurine intolerance and confirm the impact of genetic variability of purine enzymes on different therapeutic outcomes in patients undergoing thiopurine treatment.

Hereditary xanthinuria is not so rare disorder of purine metabolism.

Hereditary xanthinuria (type I) is caused by an inherited deficiency of the xanthine oxidorectase (XDH/XO), and is characterized by very low concentration of uric acid in blood and urine and high concentration of urinary xanthine, leading to urolithiasis. Type II results from a combined deficiency of XDH/XO and aldehyde oxidase. Patients present with hematuria, renal colic, urolithiasis or even acute renal failure. Clinical symptoms are the same for both types. In a third type, clinically distinct, sulfite oxidase activity is missing as well as XDH/XO and aldehyde oxidase. The prevalence is not known, but about 150 cases have been described so far. Hypouricemia is sometimes overlooked, that´s why we have set up the diagnostic flowchart. This consists of a) evaluation of uric acid concentrations in serum and urine with exclusion of primary renal hypouricemia, b) estimation of urinary xanthine, c) allopurinol loading test, which enables to distinguish type I and II; and finally assay of xanthine oxidoreductase activity in plasma with molecular genetic analysis. Following this diagnostic procedure we were able to find first patients with hereditary xanthinuria in our Czech population. We have detected nine cases, which is one of the largest group worldwide. Four patients were asymptomatic. All had profound hypouricemia, which was the first sign and led to referral to our department. Urinary concentrations of xanthine were in the range of 170-598 mmol/mol creatinine (normal < 30 mmol/mol creatinine). Hereditary xanthinuria is still unrecognized disorder and subjects with unexplained hypouricemia need detailed purine metabolic investigation.

[Xanthinuria type 1 in a woman with arthralgias: a combined clinical and molecular genetic investigation]. / Xanthinurie Typ 1 bei einer Patientin mit Gelenkschmerzen: Eine kombinierte konventionelle und molekulargenetische Ursachensuche.

HISTORY AND CLINICAL PRESENTATION: A 53-year old woman with recurrent polyarthralgias, negative test results in a recent rheumatologic work-up and an unmeasurably low uric acid serum concentration presented for suspected IgM paraproteinemia. INVESTIGATIONS: Physical examination, abdominal ultrasound and routine laboratory test results were unremarkable. Repeat determination confirmed a markedly decreased uric acid (UA) serum concentration. Urinary xanthine and hypoxanthine concentrations were increased by 14-fold and 7.5-fold, respectively. Fractional urinary UA excretion was not increased and the allopurinol loading test was normal. Sequencing of the xanthine dehydrogenase (XDH) gene revealed the pathogenic deletion c.641delC in the homozygous state. Segregation analysis showed that the patient's mother and her two adult sons were carriers of the mutation but not a half-sister and a half-brother of her deceased father. There was no evidence of parental consanguinity. These results established xanthinuria type 1 as the cause of the patient's recurrent polyarthralgias due to a previously unreported homozygosity for the known mutation c.641delC of the XDH gene. TREATMENT AND COURSE: The patient was advised to adhere to a low-purine diet and to ensure an increased daily fluid-intake of at least 2.5 l. She has since remained symptom free. CONCLUSION: Markedly lowered serum uric acid concentrations are a hallmark of xanthinuria and of hereditary renal hypouricemia, and in the absence of severe hepatic failure or evidence of an untoward drug effect should raise suspicion of these diseases. A targeted diagnostic work-up should then be initiated and factitious hypouricemia due to IgM paraproteinemia considered only in the case of equivocal test results. Molecular-genetic characterization and segregation analysis will ultimately establish the underlying genotype.

Molybdenum cofactor deficiency.

Molybdenum cofactor deficiency (MoCD) is a severe autosomal recessive inborn error of metabolism first described in 1978. It is characterized by a neonatal presentation of intractable seizures, feeding difficulties, severe developmental delay, microcephaly with brain atrophy and coarse facial features. MoCD results in deficiency of the molybdenum cofactor dependent enzymes sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase and mitochondrial amidoxime reducing component. The resultant accumulation of sulfite, taurine, S-sulfocysteine and thiosulfate contributes to the severe neurological impairment. Recently, initial evidence has demonstrated early treatment with cyclic PMP can turn MoCD type A from a previously neonatal lethal condition with only palliative options, to near normal neurological outcomes in affected patients. We review MoCD and focus on describing the currently published evidence of this exciting new therapeutic option for MoCD type A caused by pathogenic variants in MOCD1.

[Type 1 xanthinuria: Report on three cases]. / Xanthinurie héréditaire de type 1 : à propos de trois cas.

Type 1 xanthinuria is a rare cause of urolithiasis due to xanthine dehydrogenase deficiency. Pediatric cases are exceptional. Through the genetic analysis of two cases, we discovered three mutations responsible for a loss of enzyme activity. The first one had a C.3536T>C missense mutation in the XDH gene and the other one was heterozygous for two mutations c.700+1G>T and c.31778_82delTCAT. We review the diagnostic methods, possible complications, and preventive measures for stone formation.

Using Next-Generation Sequencing to Identify a Mutation in Human MCSU that is Responsible for Type II Xanthinuria.

BACKGROUND: Hypouricemia is caused by various diseases and disorders, such as hepatic failure, Fanconi renotubular syndrome, nutritional deficiencies and genetic defects. Genetic defects of the molybdoflavoprotein enzymes induce hypouricemia and xanthinuria. Here, we identified a patient whose plasma and urine uric acid levels were both extremely low and aimed to identify the pathogenic gene and verify its mechanism. METHODS: Using next-generation sequencing (NGS), we detected a mutation in the human molybdenum cofactor sulfurase (MCSU) gene that may cause hypouricemia. We cultured L02 cells, knocked down MCSU with RNAi, and then detected the uric acid and MCSU concentrations, xanthine oxidase (XOD) and xanthine dehydrogenase (XDH) activity levels, and xanthine/hypoxanthine concentrations in cell lysates and culture supernatants. RESULTS: The NGS results showed that the patient had a mutation in the human MCSU gene. The in vitro study showed that RNAi of MCSU caused the uric acid, human MCSU concentrations, the XOD and XDH activity levels among cellular proteins and culture supernatants to be extremely low relative to those of the control. However, the xanthine/hypoxanthine concentrations were much higher than those of the control. CONCLUSIONS: We strongly confirmed the pathogenicity of the human MCSU gene.

Modern diagnostic approach to hereditary xanthinuria.

Hereditary xanthinuria (HX) is a rare inherited disorder caused by a deficiency of xanthine dehydrogenase/oxidase (XDH/XO). Missing XDH/XO activity leads to undetectable levels of uric acid excessively replaced by xanthine in serum/urine. The allopurinol loading test has been traditionally used to differentiate between HX types I and II. Final confirmation of HX has been based on the biopsy finding of the absent XDH/XO activity in the small intestine or liver. We present the clinical, biochemical, ultrasound and molecular genetics findings in three new patients with HX and suggest a simple three-step approach to be used for diagnosis, typing and confirmation of HX. In the first step, the diagnosis of HX is determined by extremely low serum/urinary uric acid excessively replaced by xanthine. Second, HX is typed using urinary metabolomics. Finally, the results are confirmed by molecular genetics. We advocate for this safe and non-invasive diagnostic algorithm instead of the traditional allopurinol loading test and intestinal or liver biopsy used in the past.

Mice heterozygous for the xanthine oxidoreductase gene facilitate lipid accumulation in adipocytes.

OBJECTIVE: Xanthine oxidoreductase (XOR) catalyzes the production of uric acid with concomitant generation of reactive oxygen species. XOR has been shown to regulate adipogenesis through the control of peroxisome proliferator-activated receptor γ, but its role in adipose tissue remains unclear. The aim of this study was to examine the role of XOR in adipose tissue using XOR genetically modified mice. APPROACH AND RESULTS: Experiments were performed using 2-, 4-, and 18-month-old XOR heterozygous mice (XOR(+/-)) and their wild-type littermates to evaluate the physiological role of XOR as the mice aged. Stromal vascular fraction cells were prepared from epididymal white adipose tissue in 2-month-old XOR mice to assess adipogenesis. At 18 months, XOR(+/)- mice had significantly higher body weight, higher systolic blood pressure, and higher incidence of insulin resistance compared with wild-type mice. At 4 months, blood glucose and the expressions of CCAAT enhancer-binding protein ß, peroxisome proliferator-activated receptor γ, monocyte chemoattractant protein-1, and tumor necrosis factor α mRNA in epididymal white adipose tissue were significantly higher in XOR(+/-) than in wild-type mice. Furthermore, histological analysis of epididymal white adipose tissue in XOR(+/-) mice revealed that adipocyte size and the F4/80-positive macrophage count were increased. Experiments with a high-fat diet exhibited that body weight gain was also significantly higher in XOR(+/-) than in wild-type mice. In stromal vascular fraction cells derived from XOR(+/-) mice, the levels of peroxisome proliferator-activated receptor γ, fatty acid-binding protein 4, and CCAAT enhancer-binding protein α mRNA were upregulated, and oxidative stress levels were elevated during differentiation into adipocytes. CONCLUSIONS: These results suggest that the reduction in XOR gene expression in mice augments lipid accumulation in adipocytes, accompanied by an increase in oxidative stress, and induces obesity with insulin resistance in older age.

Multi-exon deletion in the XDH gene as a cause of classical xanthinuria.

Xanthinuria Type I is caused by mutations in the xanthine dehydrogenase gene (XDH). We report on a patient suffering from xanthinuria. Genomic DNA was screened for point mutations and imbalances in the XDH gene by sequencing and microarray typing. We could identify homozygosity of a multiexon deletion in the XDH gene; large genomic imbalances have not yet been reported in this disease. As our case and other studies on genetic alterations in kidney diseases show, large deletions (and duplications) significantly contribute to the etiology of these entities, specific assays to discover these imbalances should therefore be included in genetic testing approaches.

Molybdenum in human health and disease.

Molybdenum is an essential trace element and crucial for the survival of animals. Four mammalian Mo-dependent enzymes are known, all of them harboring a pterin-based molybdenum cofactor (Moco) in their active site. In these enzymes, molybdenum catalyzes oxygen transfer reactions from or to substrates using water as oxygen donor or acceptor. Molybdenum shuttles between two oxidation states, Mo(IV) and Mo(VI). Following substrate reduction or oxidation, electrons are subsequently shuttled by either inter- or intra-molecular electron transfer chains involving prosthetic groups such as heme or iron-sulfur clusters. In all organisms studied so far, Moco is synthesized by a highly conserved multi-step biosynthetic pathway. A deficiency in the biosynthesis of Moco results in a pleitropic loss of all four human Mo-enzyme activities and in most cases in early childhood death. In this review we first introduce general aspects of molybdenum biochemistry before we focus on the functions and deficiencies of two Mo-enzymes, xanthine dehydrogenase and sulfite oxidase, caused either by deficiency of the apo-protein or a pleiotropic loss of Moco due to a genetic defect in its biosynthesis. The underlying molecular basis of Moco deficiency, possible treatment options and links to other diseases, such as neuropsychiatric disorders, will be discussed.