Hypoxanthine-guanine Phosphoribosyltransferase Deficiency Research Articles

Red Blood Cells from Individuals with Lesch-Nyhan Syndrome: Multi-Omics Insights into a Novel S162N Mutation Causing Hypoxanthine-Guanine Phosphoribosyltransferase Deficiency.

Lesch-Nyhan syndrome (LN) is an is an X-linked recessive inborn error of metabolism that arises from a deficiency of purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). The disease manifests severely, causing intellectual deficits and other neural abnormalities, hypercoagulability, uncontrolled self-injury, and gout. While allopurinol is used to alleviate gout, other symptoms are less understood, impeding treatment. Herein, we present a high-throughput multi-omics analysis of red blood cells (RBCs) from three pediatric siblings carrying a novel S162N HPRT1 mutation. RBCs from both parents-the mother, a heterozygous carrier, and the father, a clinically healthy control-were also analyzed. Global metabolite analysis of LN RBCs shows accumulation of glycolytic intermediates upstream of pyruvate kinase, unsaturated fatty acids, and long chain acylcarnitines. Similarly, highly unsaturated phosphatidylcholines are also elevated in LN RBCs, while free choline is decreased. Intracellular iron, zinc, selenium, and potassium are also decreased in LN RBCs. Global proteomics documented changes in RBC membrane proteins, hemoglobin, redox homeostasis proteins, and the enrichment of coagulation proteins. These changes were accompanied by elevation in protein glutamine deamidation and methylation in the LN children and carrier mother. Treatment with allopurinol incompletely reversed the observed phenotypes in the two older siblings currently on this treatment. This unique data set provides novel opportunities for investigations aimed at potential therapies for LN-associated sequelae.

СИНДРОМ КЕЛЛИ–СИГМИЛЛЕРА: ОПИСАНИЕ КЛИНИЧЕСКОГО НАБЛЮДЕНИЯ И ОБЗОР ЛИТЕРАТУРЫ

Kelley-Seegmiller syndrome (KSS) is a hereditary recessive X-linked disease based on a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase that leads to impaired purine metabolism, which - in its turn - causes the development of gout in early childhood and the formation of neurological symptoms. The scientific literature represents an insufficient number of clinical cases of KSS described to date, most of which are characterized by the delayed diagnosis verification. The main reason for this may be the lack of information about this orphan disease. The article presents a rare clinical case observation of the development of KSS with mental retardation and the onset of gout and arthritis in early childhood and discusses the characteristic symptoms of the disease as well as the methods for its diagnosis and therapy.

Lesch-Nyhan syndrome due to a splice-site mutation in a 14-month-old boy presenting as acute renal failure

Lesch-Nyhan disease (LND) is a rare X-linked recessive inherited disorder caused by mutations in HPRT1 gene resulting in deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT). LND is characterized by hyperuricemia and a spectrum of neurological and behavioral manifestations. We describe a rare case of a 14-month-old boy presenting with acute renal failure and hyperuricemia. The patient exhibited all features of LNS apart from self-injurious behavior. The enzymatic analysis demonstrated total inactivity of the HPRT, and the molecular analysis revealed a splice-site mutation in intron 3 leading to exon 4 exclusion. This splice-site mutation has been previously reported only twice.

Lesch-Nyhan Disease and Its Variants: Phenotypic and Mutation Spectrum of Hypoxanthine-Guanine Phosphoribosyltransferase Deficiency in Argentine Patients

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency is a disorder of purine metabolism responsible for Lesch-Nyhan Disease (LND) and its variants, HPRT-related hyperuricemia with neurologic dysfunction (HND) and HPRT-related hyperuricemia (HRH). The objective of this study was to characterize a cohort of Argentine patients with HPRT deficiency diagnosed in a single center. Results: Twenty nine patients were studied, including 12 LND, 15 HND and 2 HRH. The average onset age was 0.64 years for LND with motor delay as the main manifestation, 8.84 years for HND and 2.5 years for HRH; nephrological manifestations predominated as presenting features in these variants. The average diagnosis age was 3.58 years for LND, 17.21 years for HND and 2.5 years for HRH. Clinical heterogeneity was more evident in HND, even in members of the same family. All patients presented hyperuricemia and no detectable HPRT activity in erythrocyte lysate. The molecular study allowed to identify 9 different mutations in HPRT1 gene from 24 patients (11 independent pedigrees) and to establish genotype-phenotype correlation. In conclusion, this study describes the genotypic/phenotypic spectrum of HPRT deficiency in Argentine patients and highlights the need to increase awareness about the suspicion of these diseases, especially the LND variants with high clinical heterogeneity.

LESCH-NYHAN SYNDROME – late diagnosis of rare disease: clinical case

Background. Lesch-Nyhan syndrome is inherent X-linked recessive genetic disorder with decreased activity of hypoxanthine-guanine phosphoribosyltransferase (HGPRT). The disease is characterized by presence of the classical triad: hyperuricemia, neurological and behavioral changes. In the article we present a clinical case of Lesch-Nyhan syndrome first diagnosed only at 16 years old despite the fact that the clinical clues were already found at the patient`s early age. Case presentation. An 18-year-old Caucasian man was admitted to the rheumatology department because of gouty arthritis. In neonatal period he was diagnosed with mild intrauterine growth restriction by hypoplastic type. Uric acid crystals were found in our patient`s urine at 6-month-old. In the first year of life, delayed motor development was noted together with permanent neurological changes which were referred to rickets. During school years, severe dysgraphia, dyslexia, dysarthria, logoneurosis warranted observation by a speech therapist. At his 12 he had been diagnosed with nephrocalcinosis, at 14 – with chronic kidney disease and symptomatic arterial hypertension. The family history was remarkable for gout in grandmother and great-grandmother, chronic pyelonephritis – in mother, urate nephropathy – in both brothers. In physical examination hyperemia and edema of the left first metatarsophalangeal joint, left ankle defiguration, funnel chest, gynecomastia, tophi on the ears were noted. On examination, some neurological disorders and mild cognitive impairment were found. In investigations hyperuricemia, arthritis of the first metatarsophalangeal joint, diffuse changes in the renal parenchyma with impaired renal excretory function were detected. Despite the clues in patient`s anamnesis, objective examination and additional investigation, as well as the presence of a family anamnesis suggesting the hereditary nature of hyperuricemia, the diagnosis of HGPRT deficiency was not made until the age of 16 years. Conclusion. The presence of Lesch-Nyhan syndrome can be assumed with the progression of muscle tone impairment and movement disorders in a child after the first six months of life in combination with high plasma uric acid concentration and its increased urinary excretion. Difficulties in the syndrome diagnosis are associated not only with a rare occurrence, but with a slight or moderate degree of central nervous system impairment that is often related by doctors to rickets or delivery trauma, as well as low accessibility of molecular genetic testing.

Lesch-Nyhan Syndrome: Evaluation of a Modified Bite Device to Prevent Bite Injuries

Lesch-Nyhan syndrome (LNS) is a serious form of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency, a hereditary purine metabolism disorder. The prevalence reported in the literature is 1/380,000 to 235,000 births. Males are affected and females are heterozygous carriers. LNS patients present a combination of hypotonia, spasticity, and neurological and behavioral disorders. They also show an obsessive-compulsive self-injurious behavior with bites and injuries to the lips, tongue, cheeks and fingers. The literature offers little scientific contribution related to the management of this problem. The authors describe their experience with a 4-year-old LNS patient and present a viable solution to control and avoid bite injuries, namely a specifically modified bite. The patient was treated at the Pediatric Dentistry Department of “Sapienza” University of Rome with a modified bite with the internal surface, in contact with the teeth, realized in 2-mm-thick soft silicone, and the exterior part consisting of a transparent resin shell with front and rear shields to separate lips and cheeks from the dental arches. At a 12-month follow-up visit, compliance was excellent: the child wore the device with regularity and without discomfort, even during soft-food feeding. No intraoral bite injuries were found, with a general improvement of the young patient’s quality of life.

Rescuing compounds for Lesch-Nyhan disease identified using stem cell–based phenotypic screening

Lesch-Nyhan disease (LND) is a rare monogenic disease caused by deficiency of the salvage pathway enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). LND is characterized by severe neuropsychiatric symptoms that currently cannot be treated. Predictive in vivo models are lacking for screening and evaluating candidate drugs because LND-associated neurological symptoms are not recapitulated in HGPRT-deficient animals. Here, we used human neural stem cells and neurons derived from induced pluripotent stem cells (iPSCs) of children affected with LND to identify neural phenotypes of interest associated with HGPRT deficiency to develop a target-agnostic-based drug screening system. We screened more than 3000 molecules and identified 6 pharmacological compounds, all possessing an adenosine moiety, that corrected HGPRT deficiency-associated neuronal phenotypes by promoting metabolism compensations in an HGPRT-independent manner. This included S-adenosylmethionine, a compound that had already been used as a compassionate approach to ease the neuropsychiatric symptoms in LND. Interestingly, these compounds compensate abnormal metabolism in a manner complementary to the gold standard allopurinol and can be provided to patients with LND via simple food supplementation. This experimental paradigm can be easily adapted to other metabolic disorders affecting normal brain development and functioning in the absence of a relevant animal model.

HPRT and Purine Salvaging Are Critical for Hematopoietic Stem Cell Function.

Adult hematopoietic stem cells (HSCs) maintain tissue homeostasis and regenerative capacity of the hematopoietic system through self-renewal and differentiation. Metabolism is recognized as an important regulatory entity controlling stem cells. As purine nucleotides are essential for metabolic functions, we analyzed the role of hypoxanthine guanine phosphoribosyl transferase (HPRT)-associated purine salvaging in HSCs. Here, we demonstrate that hematopoietic stem and progenitor cells (HSPCs) show a strong dependence on HPRT-associated purine salvaging. HSPCs with lower HPRT activity had a severely reduced competitive repopulation ability upon transplantation. Strikingly, HPRT deficiency resulted in altered cell-cycle progression, proliferation kinetics and mitochondrial membrane potential primarily in the HSC compartment, whereas more committed progenitors were less affected. Our data thus imply a unique and important role of HPRT and the purine salvage pathway for HSC function. Stem Cells 2019;37:1606-1614.

Lesch Nyhan Syndrome: A Rare Occurrence

Human populations display a marked variation in their patterns of health and disease. This variation, may have multiple underlying causes, which may not be completely in the hands of an individual. One such condition is “Lesch Nyhan Syndrome”, a rare inherited disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT), produced by mutations in the HPRT gene located on the X chromosome, where persons affected are cognitively impaired and have behavioral disturbances and an uncontrollable self-injury causing habit. This paper is a case report of this extreme condition.

Inhibiting PNP for the therapy of hyperuricemia in Lesch-Nyhan disease: Preliminary in vitro studies with analogues of immucillin-G.

Lesch-Nyhan disease (LND) is a rare X-linked genetic disorder, with complete hypoxanthine-guanine phosphoribosyltransferase (HGPRT) deficiency, uric acid (UA), hypoxanthine and xanthine accumulation, and a devastating neurologic syndrome. UA excess, causing renal failure, is commonly decreased by xanthine oxidoreductase (XOR) inhibitors, such as allopurinol, yielding a xanthine and hypoxanthine increase. Xanthine accumulation may result in renal stones, while hypoxanthine excess seems involved in the neurological disorder. Inhibition of purine nucleoside phosphorylase (PNP) represents a different strategy for lowering urate. PNP catalyzes the cleavage of purine ribo- and d-ribo-nucleosides into ribose/deoxyribose phosphate and free bases, starting catabolism to uric acid. Clinical trials demonstrated that PNP inhibitors, initially developed as anticancer drugs, lowered UA in some gouty patients, in association or not with allopurinol. The present study tested the reliability of an analogue of immucillin-G (C1a), a PNP inhibitor, as a therapy for urate, hypoxanthine, and xanthine excess in LND patients by blocking hypoxanthine production upstream. The therapeutic aim is to limit the administration of XOR inhibitors to LND patients by supplying the PNP inhibitor in low doses, avoiding d-nucleoside toxicity. We report studies conducted in primary cultures of skin fibroblasts from controls and LND patients grown in the presence of the PNP inhibitor. Cell viability, oxypurine release in culture medium, and endocellular nucleotide pattern have been monitored in different growth conditions (inhibitor concentration, time, added inosine). Our results demonstrate effective PNP inhibition by low inhibitor concentration, with reduced hypoxanthine release, and no appreciable toxicity in control or patient cells, suggesting a new therapeutic strategy for LND hyperuricemia.

Lesch-Nyhan disease: A rare disorder with many unresolved aspects

Abstract Lesch-Nyhan Disease (LND) is a rare X-linked recessive metabolic and neurological syndrome due to the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT). Besides its well known “housekeeping” function this purine salvage enzyme has revealed an unexpected role in neurodevelopment, unveiled by the peculiar neurological symptoms flanking hyperuricemia in LND: dystonia, choreoathetosis, compulsive self-injurious behaviour. Several lines of research have tried to find the molecular basis for the neurological phenotype after the disease was first described in 1964. Dopaminergic deficit was then found to underlie the neurologic symptoms but the aetiology for such alteration seemed inexplicable. A number of detailed studies in the last 50 years addressed the genetic, metabolic, cognitive, behavioral and anatomical features of this disease. Initial investigations seeked for accumulation of toxic metabolites or depletion of essential molecules to disclose potential connections between purine recycling and neuronal dysfunction. In the last two decades sophisticated biotechnological methods were used for a deeper insight in the genetic and molecular aspects, unveiling a network of combined gene dysregulations in neuronal development and differentiation producing neurotransmission defects. These studies, conducted with several different approaches, allowed consistent steps forward, demonstrating transcriptional aberrations affecting different metabolic pathways in HPRT deficiency, yet leaving many questions still unsolved.

Xanthine calculi in a patient with Lesch-Nyhan syndrome and factor V Leiden treated with allopurinol: case report

BackgroundLesch-Nyhan syndrome is a rare inborn error of purine metabolism marked by a complete deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Inherited as an X-linked recessive genetic disorder that primarily affects males, patients with Lesch-Nyhan syndrome exhibit severe neurological impairments, including choreoathetosis, ballismus, cognitive dysfunction, and self-injurious behavior. Uric acid levels are usually abnormally high, leading to kidney and bladder stones which often necessitate urological intervention. Factor V Leiden is an autosomal dominant disorder of blood clotting associated with hypercoagulability, thrombophilia, and renal disease.Case presentationWe present the first reported case of xanthine calculi in a patient with Lesch-Nyhan syndrome and Factor V Leiden who was treated with allopurinol. A renal ultrasound and CT scan demonstrated bilateral staghorn calculi in the kidneys as well as nephrocalcinosis. Two years earlier the patient underwent cystoscopy with bilateral ureteroscopy and laser lithotripsy, and he was stone free afterwards. The patient subsequently underwent bilateral percutaneous nephrolithotomy (PCNL) and was stone free following the procedure. Patients with endogenous overproduction of uric acid who are being treated with allopurinol have a higher chance of developing xanthine stones.ConclusionsPediatricians treating these children should be aware of these rare conditions and promptly manage the potential complications that may require medical or surgical intervention.

GLUT9 influences uric acid concentration in patients with Lesch-Nyhan disease.

Patients with deficient hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity present hyperuricemia and/or hyperuricosuria, with a variable degree of neurological manifestations. Hyperuricemia in HPRT deficiency is due to uric acid overproduction and is frequently treated with allopurinol. Renal uric acid excretion is sharply increased in these patients. In recent years, several renal tubular urate transporter single nucleotide polymorphisms (SNPs), including those of the GLUT9, ABCG2 and URAT1 genes, have been described that influence the renal handling of uric acid and modulate serum urate levels. In the present study, we analyzed whether GLUT9, ABCG2 and URAT1 gene SNPs are able to influence uric acid levels and allopurinol response in patients with HPRT deficiency. Three SNPs, URAT1 rs11231825, GLUT9 rs16890979 and ABCG2 rs2231142, previously associated in our population with hyperuricemia and gout, were analyzed in 27 patients with HPRT deficiency treated with allopurinol for at least 5years. Patients with HPRT deficiency having allele A of rs16890979 in the GLUT9 gene present with a lower serum urate concentration at diagnosis, before allopurinol treatment is instituted, and need lower allopurinol doses to maintain serum urate levels between 268 and 446μmol/L (4.5 and 7.5mg/dL). No relationship between rs2231142 in the ABCG2 gene or rs11231825 in the URAT1 gene and serum urate levels or allopurinol response was found in our patients with HPRT deficiency. GLUT9 SNPs influence the renal handling of uric acid and modulate serum urate levels and the response to treatment in patients with uric acid overproduction due to HPRT deficiency.

Aicar effect in early neuronal development

ABSTRACTThe neurological manifestations of Lesch-Nyhan disease (LND) have been attributed to the effect of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency on nervous system development. An increase has been reported in the levels of 5-aminoimidazole-4-carboxamide-1-β-D-ribotide (AICAR) and its triphosphate form ZTP in the red blood cells of patients with LND. AICAR accumulation in the brain has been hypothesized as the cause of some of the neurological symptoms of patients with LND. In this study, we examined the effect of AICAR on the differentiation of neurons in the well-established human NTERA-2 cl.D1 (NT2/D1) embryonic carcinoma neurogenesis model. NT2/D1 cells were differentiated along neuroectodermal lineages after exposure to 10-µM retinoic acid (RA), with or without the addition of 25-µM AICAR to the culture medium. The effect of AICAR on RA differentiation were examined through changes in the expression of genes essential to neuronal differentiation, as well as genes from the Wnt/β-catenin, transforming growth factor beta (TGFβ) and sonic hedgehog (SHH) pathways.Results: RA-induced differentiation in the NT2/D1 cells significantly increased the expression of MAP2, NRG1, NRP1, NRP2, NEUROG1 and EN1 genes (genes linked to neural differentiation) compared with undifferentiated NT2/D1 cells. We found that AICAR increased the expression of the SHH gene and the WNT2 and WNT7B genes but did not influence the expression of genes whose overexpression characterize early neurodevelopmental processes.Conclusion: The relevance of the AICAR related changes in the SHH and Wnt/β-catenin pathway genes expression in the physiopathology of LND warrants further exploration.

Skewed X inactivation in Lesch-Nyhan disease carrier females.

X chromosome inactivation (XCI) ratios of normal females can range from a highly skewed ratio of 0:100 to a 50:50 ratio. In several X-linked disorders, female carriers present skewed X inactivation. Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency is an X-linked disorder. Males are affected and present with the complete Lesch-Nyhan disease (LND) or with a partial phenotype (Lesch-Nyhan variant, LNV). Female carriers are usually asymptomatic. The aim of the present study was to analyze the XCI pattern of HPRT-deficiency carrier females. As a group, 75% of HPRT-deficiency carrier females presented skewed XCI. Moreover, skewed XCI is significantly more frequent in LND carriers (83%) than in LNV (0-50%, depending on the phenotype severity). The ratios of the preferentially inactivated allele of carrier females were significantly higher than the ratios of the preferentially inactivated allele of noncarrier females (89.4±15, n=52 vs 65.2±12, n=52; P<0.0001). For carrier diagnosis, the presence of skewed XCI presents a sensitivity of 75% with a specificity of 85%. In LND families, the presence of skewed XCI is more sensitive for carrier diagnosis than in LNV families; however, we believe that this test is not accurate for carrier diagnostic purposes.

A Trivalent Enzymatic System for Uricolytic Therapy of HPRT Deficiency and Lesch-Nyhan Disease

PurposeBecause of the evolutionary loss of the uricolytic pathway, humans accumulate poorly soluble urate as the final product of purine catabolism. Restoration of uricolysis through enzyme therapy is a promising treatment for severe hyperuricemia caused by deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT). To this end, we studied the effect of PEG conjugation on the activity and stability of the enzymatic complement required for conversion of urate into the more soluble (S)-allantoin.MethodsWe produced in recombinant form three zebrafish enzymes required in the uricolytic pathway. We carried out a systematic study of the effect of PEGylation on the function and stability of the three enzymes by varying PEG length, chemistry and degree of conjugation. We assayed in vitro the uricolytic activity of the PEGylated enzymatic triad.ResultsWe defined conditions that allow PEGylated enzymes to retain native-like enzymatic activity even after lyophilization or prolonged storage. A combination of the three enzymes in an appropriate ratio allowed efficient conversion of urate to (S)-allantoin with no accumulation of intermediate metabolites.ConclusionsPharmaceutical restoration of the uricolytic pathway is a viable approach for the treatment of severe hyperuricemia.

The renal phenotype of allopurinol-treated HPRT-deficient mouse.

Excess of uric acid is mainly treated with xanthine oxidase (XO) inhibitors, also called uricostatics because they block the conversion of hypoxanthine and xanthine into urate. Normally, accumulation of upstream metabolites is prevented by the hypoxanthine-guanine phosphoribosyltransferase (HPRT) enzyme. The recycling pathway, however, is impaired in the presence of HPRT deficiency, as observed in Lesch-Nyhan disease. To gain insights into the consequences of purine accumulation with HPRT deficiency, we investigated the effects of the XO inhibitor allopurinol in Hprt-lacking (HPRT-/-) mice. Allopurinol was administered in the drinking water of E12-E14 pregnant mothers at dosages of 150 or 75 μg/ml, and mice sacrificed after weaning. The drug was well tolerated by wild-type animals and heterozygous HPRT+/- mice. Instead, a profound alteration of the renal function was observed in the HPRT-/- model. Increased hypoxanthine and xanthine concentrations were found in the blood. The kidneys showed a yellowish appearance, diffuse interstitial nephritis, with dilated tubules, inflammatory and fibrotic changes of the interstitium. There were numerous xanthine tubular crystals, as determined by HPLC analysis. Oil red O staining demonstrated lipid accumulation in the same location of xanthine deposits. mRNA analysis showed increased expression of adipogenesis-related molecules as well as profibrotic and proinflammatory pathways. Immunostaining showed numerous monocyte-macrophages and overexpression of alpha-smooth muscle actin in the tubulointerstitium. In vitro, addition of xanthine to tubular cells caused diffuse oil red O positivity and modification of the cell phenotype, with loss of epithelial features and appearance of mesenchymal characteristics, similarly to what was observed in vivo. Our results indicate that in the absence of HPRT, blockade of XO by allopurinol causes rapidly developing renal failure due to xanthine deposition within the mouse kidney. Xanthine seems to be directly involved in promoting lipid accumulation and subsequent phenotype changes of tubular cells, with activation of inflammation and fibrosis.

Non-targeted metabolomics by high resolution mass spectrometry in HPRT knockout mice

AimsLesch-Nyhan disease (LND) is characterized by hyperuricemia as well as neurological and neuropsychiatric symptoms including repetitive self-injurious behavior. Symptoms are caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) as a result of a mutation on the X chromosome. To elucidate the pathophysiology of LND, we performed a metabolite screening for brain and serum extracts from HPRT knockout mice as an animal model for LND. Main methodsAnalyses were performed by high performance liquid chromatography (HPLC)-coupled quadrupole time-of-flight mass spectrometry (QTOF-MS). Key findingsIn brain extracts, we found six metabolites with significantly different contents in wild-type and HPRT-deficient mice. Two compounds we could identify as 5-aminoimidazole-4-carboxamide ribotide (AICAR) and 1-methylimidazole-4-acetic acid (1-MI4AA). Whereas AICAR was accumulated in brains of HPRT knockout mice, 1-MI4AA was decreased in these mice. SignificanceBoth metabolites play a role in histidine metabolism and, as a consequence, histamine metabolism. AICAR, in addition, is part of the purine metabolism. Our findings may help to better understand the mechanisms leading to the behavioral phenotype of LND.

Mild Lesch-Nyhan disease in a boy with a null-mutation in HPRT1: an exception to the known genotype-phenotype correlation: three-year follow up

Lesch-Nyhan disease and its attenuated variants are caused by deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HPRT). HPRT deficiency results in a continuous spectrum of clinical phenotypes though all include overproduction of uric acid with nephrolithiasis, renal failure, gouty arthritis and tophi.HPRT1 mutations that result in very low or no HPRT enzyme activities are generally associated with the classic Lesch-Nyhan disease (LND) phenotype with intellectual disability, motor handicap and self-injurious behaviour. Mutations that permit a higher residual HPRT activity are seen in some patients with the milder LND variant phenotypes with varying degrees of cognitive, motor handicap and maladaptive behaviour without recurrent self-injury. We have previously presented a 10-year-old boy with a LND variant phenotype due to a deletion of exon 5 of HPRT1 predicted to fully abolish HPRT activity. Metabolic analysis confirmed lack of significant residual enzyme activity. He presented with hyperuricemia, hypotonia, developmental delay, extrapyramidal and pyramidal involvement. This boy is one of the rare cases with a suspected null-mutation in HPRT1, that associates with a milder than expected phenotype with lack of self-injurious behaviour. He is currently 13-years-old and has yet to show any signs of self-injurious or maladaptive behaviour. We furthermore present our experience with oral baclofen in this patient.

Altered histamine neurotransmission in HPRT-deficient mice

Lesch–Nyhan syndrome (LNS) is an X-chromosomal disorder with congenital deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) as underlying defect. We determined the concentrations of dopamine, histamine and their metabolites in brains of HPRT knockout mice, which serve as an animal model for LNS, and compared the results to those obtained from wild-type controls. Analyses were performed by high performance liquid chromatography (HPLC)-coupled tandem mass spectrometry (MS/MS). Besides a decrease of dopamine and 3-methoxytyramine (3-MT) concentrations in the cerebral hemisphere, HPRT-deficient mice also exhibited significantly reduced 1-methylhistamine (1-MH) and 1-methylimidazole-4-acetic acid (1-MI4AA) concentrations in the brain hemisphere and medulla. Moreover, the amount of 1-MI4AA was significantly decreased in the cerebellum. Our findings show that neuronal perturbations caused by HPRT deficiency are not restricted to the dopamine system but also affect histaminergic neurotransmission. These new insights into the brain metabolism of an LNS mouse model may help to find new therapeutic strategies to improve the quality of life of LNS patients.