Severe gouty arthritis and mild neurologic symptoms due to F199C, a newly identified variant of the hypoxanthine guanine phosphoribosyltransferase

Lesch-Nyhan Syndrome in an Indian Child

Hypoxanthine guanine phosphoribosyl transferase-1 (HGPRT-1) leading to Lesch-Nyhan syndrome (LNS) is one of the important causes of self-mutilation. Hereby, we report a case of LNS in a three and half-year-old male child, who presented with characteristic self-mutilating behavior. He had history of developmental delay, difficulty in social interaction, attention deficit and features of autism. His serum blood biochemistry was normal except for low hemoglobin levels and raised serum uric acid levels. With a diagnosis of LNS, the child was treated with allopurinol. With various modalities of physical restraint, his self-mutilating behavior came under control and currently the patient is being followed up.

2,8-Dihydroxyadenine urolithiasis: a not so rare inborn error of purine metabolism

Adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation and formation of urinary crystals and kidney stones. The disease can be present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available, including stone analysis, crystalluria, and APRT activity in red blood cells, make the diagnosis easy to confirm when APRT deficiency is suspected. However, the lack of recognition of this metabolic disorder frequently resulted in a delay in diagnosis and treatment with grave consequences. The early recognition and treatment of APRT deficiency are of crucial importance to prevent irreversible loss of renal function. This review summarizes the genetic and metabolic mechanisms underlying DHA stones formation and chronic kidney disease, along with the issues of diagnosis and management of APRT deficiency. Moreover, we report the mutations in the APRT gene responsible for APRT deficiency in 51 French patients (43 families) including 22 pediatric cases (18 families) among the 64 patients identified in the biochemistry laboratories of Necker Hospital, Paris (1978-2013).

Transcriptomic approach to Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is an X-linked metabolic disease caused by various mutations in the gene HPRT1 encoding an enzyme of purine metabolism, hypoxanthine guanine phosphoribosyltransferase (HPRT). In its most severe form, LND patients suffer from overproduction of uric acid along with neurological or behavioural difficulties including self-injurious behaviours. To gain more insight into pathogenesis, we compared the transcriptome from human LND fibroblasts to normal human fibroblasts using a microarray with 60,000 probes corresponding to the entire human genome. Using stringent criteria, we identified 25 transcripts whose expression was significantly different between LND and control cells. These genes were confirmed by quantitative RT-PCR to be dysregulated in LND cells. Moreover, bioinformatic analysis of microarray data using gene ontology (GO) highlighted clusters of genes displaying biological processes most significantly affected in LND cells. These affected genes belonged to specific processes such as cell cycle and cell-division processes, metabolic and nucleic acid processes, demonstrating the specific nature of the changes and providing new insights into LND pathogenesis.

New biomarkers for early diagnosis of Lesch-Nyhan disease revealed by metabolic analysis on a large cohort of patients

BACKGROUND

Lesch-Nyhan disease is a rare X-linked neurodevelopemental metabolic disorder caused by a wide variety of mutations in the HPRT1 gene leading to a deficiency of the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase (HGprt). The residual HGprt activity correlates with the various phenotypes of Lesch-Nyhan (LN) patients and in particular with the different degree of neurobehavioral disturbances. The prevalence of this disease is considered to be underestimated due to large heterogeneity of its clinical symptoms and the difficulty of diagnosing of the less severe forms of the disease. We therefore searched for metabolic changes that would facilitate an early diagnosis and give potential clues on the disease pathogenesis and potential therapeutic approaches.

METHODS

Lesch-Nyhan patients were diagnosed using HGprt enzymatic assay in red blood cells and identification of the causal HPRT1 gene mutations. These patients were subsequently classified into the three main phenotypic subgroups ranging from patients with only hyperuricemia to individuals presenting motor dysfunction, cognitive disability and self-injurious behavior. Metabolites from the three classes of patients were analyzed and quantified by High Performance Ionic Chromatography and biomarkers of HGprt deficiency were then validated by statistical analyses.

RESULTS

A cohort of 139 patients, from 112 families, diagnosed using HGprt enzymatic assay in red blood cells, was studied. 98 displayed LN full phenotype (86 families) and 41 (26 families) had attenuated clinical phenotypes. Genotype/phenotype correlations show that LN full phenotype was correlated to genetic alterations resulting in null enzyme function, while variant phenotypes are often associated with missense mutations allowing some residual HGprt activity. Analysis of metabolites extracted from red blood cells from 56 LN patients revealed strong variations specific to HGprt deficiency for six metabolites (AICAR mono- and tri-phosphate, nicotinamide, nicotinic acid, ATP and Succinyl-AMP) as compared to controls including hyperuricemic patients without HGprt deficiency.

CONCLUSIONS

A highly significant correlation between six metabolites and the HGprt deficiency was established, each of them providing an easily measurable marker of the disease. Their combination strongly increases the probability of an early and reliable diagnosis for HGprt deficiency.

Adenine phosphoribosyltransferase deficiency: Leave no stone unturned

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disease leading to generation of large amounts of 2,8-dihydroxyadenine (DHA). DHA is excreted in urine, where it precipitates into crystals due to its low solubility. DHA crystals can aggregate into stones or cause injury to the renal parenchyma (DHA nephropathy). Recurrent urolithiasis and DHA nephropathy are the two clinical manifestations of APRT deficiency. Diagnosis of APRT deficiency can be made during childhood as well as adulthood. Diagnosis mainly relies on the recognition of DHA in stones or urine crystals. Measurement of APRT activity and genetic testing are useful for confirmation of diagnosis, for family screening and should be considered in difficult cases of urolithiasis or crystalline nephropathy. Allopurinol therapy is the cornerstone of treatment and is highly effective in preventing recurrence of stones and kidney disease. High fluid intake and dietary modifications are also recommended. Early diagnosis and treatment are of paramount importance to prevent renal damage. Unfortunately, diagnosis of APRT deficiency is often overlooked and irreversible renal failure still occurs in a substantial proportion of patients. Clinicians must be alert to the possibility of APRT deficiency and consider the appropriate diagnostic tests in certain cases. This review discusses the genetic and biochemical mechanisms of APRT deficiency, and the issues of diagnosis and management.

Phenotypic variation among seven members of one family with deficiency of hypoxanthine-guanine phosphoribosyltransferase

We describe a family of seven boys affected by Lesch-Nyhan disease with various phenotypes. Further investigations revealed a mutation c.203T>C in the gene encoding HGprt of all members, with substitution of leucine to proline at residue 68 (p.Leu68Pro). Thus patients from this family display a wide variety of symptoms although sharing the same mutation. Mutant HGprt enzyme was prepared by site-directed mutagenesis and the kinetics of the enzyme revealed that the catalytic activity of the mutant was reduced, in association with marked reductions in the affinity towards phosphoribosylpyrophosphate (PRPP). Its Km for PRPP was increased 215-fold with hypoxanthine as substrate and 40-fold with guanine as substrate with associated reduced catalytic potential. Molecular modeling confirmed that the most prominent defect was the dramatically reduced affinity towards PRPP. Our studies suggest that the p.Leu68Pro mutation has a strong impact on PRPP binding and on stability of the active conformation. This suggests that factors other than HGprt activity per se may influence the phenotype of Lesch-Nyhan patients.

Xanthine oxido-reductase, free radicals and cardiovascular disease. A critical review

Free radical mediated pathologies occupy a special place in medical semiology and in mechanistic interpretation of diseases. Free radicals, or better reactive oxygen species (ROS) or reactive nitrogen species (RNS) play also an important role in cell signaling. This is the basis of the ambivalent (Jekyll-Hyde) situation of ROS in biology and pathology. Aging itself is attributed by a popular theory to free radicals. A number of ROS-scavenging substances and procedures were described without however reaching credibility for their therapeutic value. An interesting exception is the xanthine oxido-reductase produced ROS and their role in cardiovascular disease. Allopurinol inhibition of xanthine oxido-reductase was shown to be efficient in some cases of cardiovascular diseases. Another important aspect of xanthine oxido-reductase produced ROS is their antibacterial capacity considered to be of importance with newborns fed on milk rich in this enzyme as well as at the gastrointestinal barrier. This ambivalent role of xanthine oxido-reductase justifies this review on the basic enzymatic mechanisms involved, derived ROS production, their role in the above mentioned biological processes and especially the interest of the inhibition of this enzyme as a preventive or curative measure in some cardiovascular pathologies.

Genotype-phenotype correlations in neurogenetics: Lesch-Nyhan disease as a model disorder

Establishing meaningful relationships between genetic variations and clinical disease is a fundamental goal for all human genetic disorders. However, these genotype-phenotype correlations remain incompletely characterized and sometimes conflicting for many diseases. Lesch-Nyhan disease is an X-linked recessive disorder that is caused by a wide variety of mutations in the HPRT1 gene. The gene encodes hypoxanthine-guanine phosphoribosyl transferase, an enzyme involved in purine metabolism. The fine structure of enzyme has been established by crystallography studies, and its function can be measured with very precise biochemical assays. This rich knowledge of genetic alterations in the gene and their functional effect on its protein product provides a powerful model for exploring factors that influence genotype-phenotype correlations. The present study summarizes 615 known genetic mutations, their influence on the gene product, and their relationship to the clinical phenotype. In general, the results are compatible with the concept that the overall severity of the disease depends on how mutations ultimately influence enzyme activity. However, careful evaluation of exceptions to this concept point to several additional genetic and non-genetic factors that influence genotype-phenotype correlations. These factors are not unique to Lesch-Nyhan disease, and are relevant to most other genetic diseases. The disease therefore serves as a valuable model for understanding the challenges associated with establishing genotype-phenotype correlations for other disorders.

Hypoxanthine-guanine phosphoribosyltransferase deficiency in a patient with a Madrid II mutation

The hypoxanthine-guanine phosphoribosyltransferase deficiency is an inborn error of purine metabolism, linked to the X chromosome. The clinical phenotypes associated with HPRT deficiency varied according to the level of enzyme deficiency, with a large spectrum of neurologic features like self-injurious behaviour in patients with complete deficiency. We report a 20-year-old man who had asymmetric polyarthritis, tophi, hyperuricemia, nephrolithiasis and mild neurologic symptoms with undetectable levels of HPRT activity in lysed erythrocytes. The genetic study identified the c.143G>A mutation in exon 3, GAA CGT (CTT>GAA CAT CTT (48arg>his). The presence of gouty arthropathy and chronic hyperuricemia in a young patient with neurological symptoms, suggests HPRT deficiency for which it is necessary its enzyme and molecular determination.

Adenine phosphoribosyltransferase deficiency

Complete adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation of this compound and the formation of urinary crystals and stones. The disease can present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available-including stone analysis, crystalluria, and APRT activity measurement-make the diagnosis easy to confirm when APRT deficiency is suspected. However, the disease can present at any age, and the variability of symptoms can present a diagnostic challenge to many physicians. The early recognition and treatment of APRT deficiency are of crucial importance for preventing irreversible loss of renal function, which still occurs in a non-negligible proportion of cases. This review summarizes the genetic and metabolic mechanisms underlying stone formation and renal disease, along with the diagnosis and management of APRT deficiency.

Genotype-phenotype correlations in Lesch-Nyhan disease: moving beyond the gene

Lesch-Nyhan disease and its attenuated variants are caused by mutations in the HPRT1 gene, which encodes the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase. The mutations are heterogeneous, with more than 400 different mutations already documented. Prior efforts to correlate variations in the clinical phenotype with different mutations have suggested that milder phenotypes typically are associated with mutants that permit some residual enzyme function, whereas the most severe phenotype is associated with null mutants. However, multiple exceptions to this concept have been reported. In the current studies 44 HPRT1 mutations associated with a wide spectrum of clinical phenotypes were reconstructed by site-directed mutagenesis, the mutant enzymes were expressed in vitro and purified, and their kinetic properties were examined toward their substrates hypoxanthine, guanine, and phosphoribosylpyrophosphate. The results provide strong evidence for a correlation between disease severity and residual catalytic activity of the enzyme (k(cat)) toward each of its substrates as well as several mechanisms that result in exceptions to this correlation. There was no correlation between disease severity and the affinity of the enzyme for its substrates (K(m)). These studies provide a valuable model for understanding general principles of genotype-phenotype correlations in human disease, as the mechanisms involved are applicable to many other disorders.

The genetic basis of hyperuricaemia and gout

Gout results from elevated urate concentrations in the blood (hyperuricaemia). When super-saturation of urate is reached, monosodium urate crystals form within the joint. In some individuals, these crystals elicit a painful self-limiting inflammatory response that is characteristic of acute gouty arthritis. The most important cause of hyperuricaemia is reduced excretion of uric acid in the urine. Uric acid excretion is coordinated by a suite of urate transport molecules expressed in the renal collecting tubules, and is a key physiological checkpoint in gout. Other checkpoints in gout are hepatic production of urate, monosodium urate crystal formation, and initiation of the acute inflammatory response. Genome-wide association scans for genes regulating serum urate concentrations have identified two major regulators of hyperuricaemia- the renal urate transporters SLC2A9 and ABCG2. The risk variants at each gene approximately double the risk for gout in people of Caucasian ancestry, with SLC2A9 also resulting in higher risk for gout in people of Polynesian ancestry, a diverse population characterized by a high prevalence of gout. Ongoing genetic association studies are identifying and confirming other genes controlling serum urate concentrations; although genome-wide association studies in gout per se await recruitment of suitable case sample sets.

Pegloticase for chronic gout

BACKGROUND

Pegloticase is a potential new treatment option for patients with chronic gout intolerant to other urate-lowering therapies.

OBJECTIVES

To assess safety (adverse events, death) and efficacy (pain, function, frequency of flares, quality of life, uric acid level, radiographic damage) of pegloticase in various doses or as compared to placebo or other interventions for treatment of hyperuricemia in patients with chronic gout.

SEARCH STRATEGY

We searched six databases: The Cochrane Central Register of Controlled Trials (CENTRAL), via The Cochrane Library, OVID MEDLINE, CINAHL (via EBSCOHost), OVID SPORTdiscus, EMBASE and the Science Citation Index (Web of Science).

SELECTION CRITERIA

All published randomized controlled trials (RCTs) or controlled clinical trials that compared various doses of pegloticase alone or pegloticase alone or in combination with other urate-lowering or anti-inflammatory medications to placebo alone or placebo in combination with these medications, in patients with gout.

DATA COLLECTION AND ANALYSIS

Two review authors (AA, JS) independently extracted data from the included trials, including trial and population characteristics, primary and secondary outcomes. For dichotomous and continuous outcomes, we calculated the risk ratio and mean difference, respectively with 95% confidence interval. Major outcomes were: (a)

EFFICACY

frequency of gout flares and change in serum uric acid; and (b) safety: adverse events, serious adverse events, withdrawals and death. Minor/secondary outcomes were pain, patient/physician global assessment, tophus burden, health related quality of life, function and radiographic progression.

MAIN RESULTS

Only one open-label, phase-II RCT (n=41) met the selection criteria that compared various doses of pegloticase without comparison to placebo or another treatment. Patients were randomized to one of the four doses of pegloticase for 12 to 14 weeks - 4mg every 2 weeks, 8mg every 2 weeks, 8mg every 4 weeks and 12mg every 4 weeks. Percent responders (uric acid below 6 mg/dl 80% or more time) in the four dose groups were 56%, 88%, 52% and 62%. Percent time without hyperuricemia (uric acid below 6 mg/dl) was 78%, 92%, 76% and 76% respectively. No between dose differences were noted. Most common adverse events (10% or more patients) included nephrolithiasis, arthralgia, anemia, dyspnea, headache, muscle spasms, nausea and pyrexia. 89% reported one or more gout flares during the study. Pain, patient/physician global, function, quality of life, tophus size/regression and radiographic progression were not reported in this study.

AUTHORS' CONCLUSIONS

There are no published double-blind, placebo-controlled RCTs of pegloticase. More evidence is needed to assess risks/benefits of pegloticase in patients with chronic gout.

Attenuated variants of Lesch-Nyhan disease

Lesch–Nyhan disease is a neurogenetic disorder caused by deficiency of the enzyme hypoxanthine–guanine phosphoribosyltransferase. The classic form of the disease is described by a characteristic syndrome that includes overproduction of uric acid, severe generalized dystonia, cognitive disability and self-injurious behaviour. In addition to the classic disease, variant forms of the disease occur wherein some clinical features are absent or unusually mild. The current studies provide the results of a prospective and multi-centre international study focusing on neurological manifestations of the largest cohort of Lesch–Nyhan disease variants evaluated to date, with 46 patients from 3 to 65 years of age coming from 34 families. All had evidence for overproduction of uric acid. Motor abnormalities were evident in 42 (91%), ranging from subtle clumsiness to severely disabling generalized dystonia. Cognitive function was affected in 31 (67%) but it was never severe. Though none exhibited self-injurious behaviours, many exhibited behaviours that were maladaptive. Only three patients had no evidence of neurological dysfunction. Our results were compared with a comprehensive review of 78 prior reports describing a total of 127 Lesch–Nyhan disease variants. Together these results define the spectrum of clinical features associated with hypoxanthine–guanine phosphoribosyltransferase deficiency. At one end of the spectrum are patients with classic Lesch–Nyhan disease and the full clinical phenotype. At the other end of the spectrum are patients with overproduction of uric acid but no apparent neurological or behavioural deficits. Inbetween are patients with varying degrees of motor, cognitive, or behavioural abnormalities. Recognition of this spectrum is valuable for understanding the pathogenesis and diagnosis of all forms of hypoxanthine–guanine phosphoribosyltransferase deficiency.

Phenotype and genotype characterization of adenine phosphoribosyltransferase deficiency

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder causing 2,8-dihydroxyadenine stones and renal failure secondary to intratubular crystalline precipitation. Little is known regarding the clinical presentation of APRT deficiency, especially in the white population. We retrospectively reviewed all 53 cases of APRT deficiency (from 43 families) identified at a single institution between 1978 and 2009. The median age at diagnosis was 36.3 years (range 0.5 to 78.0 years). In many patients, a several-year delay separated the onset of symptoms and diagnosis. Of the 40 patients from 33 families with full clinical data available, 14 (35%) had decreased renal function at diagnosis. Diagnosis occurred in six (15%) patients after reaching ESRD, with five diagnoses made at the time of disease recurrence in a renal allograft. Eight (20%) patients reached ESRD during a median follow-up of 74 months. Thirty-one families underwent APRT sequencing, which identified 54 (87%) mutant alleles on the 62 chromosomes analyzed. We identified 18 distinct mutations. A single T insertion in a splice donor site in intron 4 (IVS4 + 2insT), which produces a truncated protein, accounted for 40.3% of the mutations. We detected the IVS4 + 2insT mutation in two (0.98%) of 204 chromosomes of healthy newborns. This report, which is the largest published series of APRT deficiency to date, highlights the underdiagnosis and potential severity of this disease. Early diagnosis is crucial for initiation of effective treatment with allopurinol and for prevention of renal complications.