Update on the phenotypic spectrum of Lesch-Nyhan disease and its attenuated variants

Unleashing the Power of Induced Pluripotent stem Cells in in vitro Modelling of Lesch-Nyhan Disease

Lesch-Nyhan disease (LND) is a monogenic rare neurodevelopmental disorder caused by a deficiency in hypoxanthine-guanine phosphoribosyltransferase (HPRT), the key enzyme of the purines salvage pathway. Beyond its well-documented metabolic consequences, HPRT deficiency leads to a distinctive neurobehavioral syndrome characterized by motor disabilities, cognitive deficits, and self-injurious behavior. Although various cell and animal models have been developed to investigate LND pathology, none have adequately elucidated the underlying mechanisms of its neurological alterations. Recent advances in human pluripotent stem cell research and in vitro differentiation techniques have ushered in a new era in rare neurodevelopmental disorders research. Pluripotent stem cells, with their ability to propagate indefinitely and to differentiate into virtually any cell type, offer a valuable alternative for modeling rare diseases, allowing for the detection of pathological events from the earliest stages of neuronal network development. Furthermore, the generation of patient-derived induced pluripotent stem cells using reprogramming technology provides an opportunity to develop a disease-relevant model within the context of a patient-specific genome. In this review, we examine current stem cell-based models of LND and assess their potential as optimal models for exploring key pathological molecular events during neurogenesis and for the discovering novel treatment options. We also address the limitations, challenges, and future prospects for improving the use of iPSCs in LND research.

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

IL1A regulates the inflammation in gout through the Toll-like receptors pathway

Objective: Gout is a dangerous metabolic condition related to monosodium urate (MSU). Our aim is to study the molecular mechanisms underlying gout and to identify potential clinical biomarkers by bioinformatics analysis and experimental validation. Methods: In this study, we retrieved the overlapping genes between GSE199950-Differential Expressed Genes (DEGs) dataset and key module in Weighted Gene Co-Expression Network Analysis (WGCNA) on GSE199950. These genes were then analyzed by protein-protein interaction (PPI) network, expression and Gene Set Enrichment Analysis to identify the hub gene related to gout. Then, the gene was investigated by peripheral blood mononuclear cells (PBMCs), immunoassay and cell experiments like western blotting to uncover its underlying mechanism in gout cells. Results: From the turquoise module and 83 DEGs, we identified 62 overlapping genes, only 11 genes had mutual interactions in PPI network and these genes were highly expressed in MSU-treated samples. Then, it was found that the IL1A (interleukin 1 alpha) was the only one gene related to Toll-like receptor signaling pathway that was associated with the occurrence of gout. Thus, IL1A was determined as the hub gene in this study. In immunoassay, IL1A was significantly positively correlated with B cells and negatively correlated with macrophages. Moreover, IL1A is highly expressed in gout patients,it has a good clinical diagnostic value. Finally, the results of in vitro experiments showed that after knocking down IL1A, the expressions of pro-inflammatory cytokines and Toll-like receptor signaling pathway-related proteins (TLR2, TLR4, MyD88) were all reduced. Conclusion: It is confirmed that IL1A is a promoting gene in gout with a good diagnostic value, and specifically it affects the inflammation in gout through Toll-like receptor pathway. Our research offers fresh perspectives on the pathophysiology of gout and valuable directions for future diagnosis and treatment.

Clinical and biochemical footprints of inherited metabolic diseases. XIV. Metabolic kidney diseases

Kidney disease is a global health burden with high morbidity and mortality. Causes of kidney disease are numerous, extending from common disease groups like diabetes and arterial hypertension to rare conditions including inherited metabolic diseases (IMDs). Given its unique anatomy and function, the kidney is a target organ in about 10% of known IMDs, emphasizing the relevant contribution of IMDs to kidney disease. The pattern of injury affects all segments of the nephron including glomerular disease, proximal and distal tubular damage, kidney cyst formation, built-up of nephrocalcinosis and stones as well as severe malformations. We revised and updated the list of known metabolic etiologies associated with kidney involvement and found 190 relevant IMDs. This represents the 14th of a series of educational articles providing a comprehensive and revised list of metabolic differential diagnoses according to system involvement.

Forensically relevant challenging behaviors and the genetics domain

Impulsive and aggressive behaviors along with intellectual disabilities often manifest in the context of genetic disorders and are a persisting challenge to professionals in the forensic psychiatric and psychological setting. The following chapter comprises an overview of relevant factors in the gene-context-behavior interaction such as monoamine oxidase A activity and specific epileptic phenomena. It presents several examples of monogenetic disorders with behaviors from the aggression spectrum and summarizes emerging strategies for treatment and clinical management thereof. The final part focuses on challenges and future developments in this field with relevance for the judicial and forensic systems. It is concluded that the relationship between a genetic syndrome and forensically relevant and/or violent behaviors should typically be addressed within a multidisciplinary framework that also includes the application of modern genetic techniques.

Description of the Molecular and Phenotypic Spectrum of Lesch-Nyhan Disease in Eight Chinese Patients

Background: Lesch-Nyhan disease (LND) is a rare disorder involving pathogenic variants in the HPRT1 gene encoding the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT) that result in hyperuricemia, intellectual disability, dystonic movement disorder, and compulsive self-mutilation. The purpose of the present study was to characterize the genetic basis of LND and describe its phenotypic heterogeneity by identifying the variation in the HPRT1 gene in a cohort of Chinese LND patients. Results: The median age at diagnosis was 31 mo (interquartile range (IQR): 7-76 mo), and the initial manifestations were mainly head control weakness and motor development delay. The median age of self-mutilation behavior onset was 19 mo (IQR: 17-24 mo), and all patients were required to travel in a wheelchair and fall into the predicament of compulsive self-harm behavior. There were two patients whose blood uric acid levels were normal for their high urinary acid excretion fraction without taking uric acid-lowering drugs. Seven different pathogenic variants of the HPRT1 gene were identified among eight independent pedigrees, including four novel mutations [c.299 (exon 3) T > A; loss (exon: 6) 84 bp; c.277_281delATTGC; c.468_470delGAT]. The pathogenic variant sites were mainly concentrated in exon 3, and truncating mutations (including frameshift mutations and nonsense mutations) were the most common genetic variant types (5/7, 71.4%). Conclusion: The present study described the phenotypic and molecular spectrum of LND in eight Chinese families, including four novel mutations, which expands our understanding of LND.

Late diagnosis of Lesch-Nyhan disease complicated with end-stage renal disease and tophi burst: a case report

Background

Lesch–Nyhan disease (LND) is a rare X-linked recessive inborn error of purine metabolism. Late diagnosis of LND may cause significant morbidity. LND cases have never been reported in Indonesia.

Case report

A 15-year-old male who had been diagnosed with cerebral palsy was referred to our hospital due to renal failure requiring emergency dialysis. The patient presented with three classic manifestations of LND: increased uric acid levels, neurological disorders, and self-injurious behaviors. LND was suspected because of an abscess-like lump on the left ankle that was confirmed to be a tophus, which had burst and discharged thick masses containing blood, debris, and white crystal materials. The diagnosis of LND was confirmed by the presence of a deletion to exon 1 of the HPRT1 gene. The patient received oral allopurinol daily and treatment for end-stage renal disease (ESRD), which included regular dialysis and subcutaneous administration of erythropoietin. At a 2-month follow-up, he improved clinically with a 71% decrease in uric acid levels after regular dialysis and allopurinol treatment.

Conclusion

In developed countries, LND can be diagnosed as early as 3 days after birth. However, diagnosis in the present case was delayed due to the rarity of the disease and the limited number of facilities in Indonesia that offer genetic counseling. Late diagnosis of LND leads to ESRD and irreversible abnormalities. This is the first case of LND presenting with a unique clinical presentation of tophus burst reported in Indonesia.

Self-injurious behaviour in movement disorders: systematic review

Self-injurious behaviours (SIBs) are defined as deliberate, repetitive and persistent behaviours that are directed towards the body and lead to physical injury and are not associated with sexual arousal and without suicidal intent. In movement disorders, SIBs are typically associated with tic disorders, most commonly Tourette syndrome, and neurometabolic conditions, such as classic Lesch-Nyhan syndrome. However, beyond these well-known aetiologies, a range of other movement disorder syndromes may also present with SIBs, even though this clinical association remains less well-known. Given the scarcity of comprehensive works on this topic, here we performed a systematic review of the literature to delineate the spectrum of movement disorder aetiologies associated with SIBs. We report distinct aetiologies, which are clustered in five different categorical domains, namely, neurodevelopmental, neurometabolic and neurodegenerative disorders, as well as disorders with characteristic structural brain changes and heterogeneous aetiologies (eg, autoimmune and drug-induced). We also provide insights in the pathophysiology of SIBs in these patients and discuss neurobiological key risk factors, which may facilitate their manifestation. Finally, we provide a list of treatments, including practical measures, such as protective devices, as well as behavioural interventions and pharmacological and neurosurgical therapies.

Deep Brain Stimulation of the Internal Pallidum in Lesch-Nyhan Syndrome: Clinical Outcomes and Connectivity Analysis

BACKGROUND

Lesch-Nyhan syndrome (LNS) is a rare genetic disorder characterized by a deficiency of hypoxanthine-guanine phosphoribosyltransferase enzyme. It manifests during infancy with compulsive self-mutilation behavior associated with disabling generalized dystonia and dyskinesia. Clinical management of these patients poses an enormous challenge for medical teams and carers.

OBJECTIVES

We report our experience with bilateral deep brain stimulation (DBS) of the globus pallidus internus (GPi) in the management of this complex disorder.

MATERIALS AND METHODS

Preoperative and postoperative functional assessment data prospectively collected by a multidisciplinary pediatric complex motor disorders team, including imaging, neuropsychology, and neurophysiology evaluations were analyzed with regards to motor and behavioral control, goal achievement, and patient and caregivers' expectations.

RESULTS

Four male patients (mean age 13 years) underwent DBS implantation between 2011 and 2018. Three patients received double bilateral DBS electrodes within the posteroventral GPi and the anteromedial GPi, whereas one patient had bilateral electrodes placed in the posteroventral GPi only. Median follow-up was 47.5 months (range 22-98 months). Functional improvement was observed in all patients and discussed in relation to previous reports. Analysis of structural connectivity revealed significant correlation between the involvement of specific cortical regions and clinical outcome.

CONCLUSION

Combined bilateral stimulation of the anteromedial and posteroventral GPi may be considered as an option for managing refractory dystonia and self-harm behavior in LNS patients. A multidisciplinary team-based approach is essential for patient selection and management, to support children and families, to achieve functional improvement and alleviate the overall disease burden for patients and caregivers.

HPRT-related hyperuricemia with a novel p.V35M mutation in HPRT1 presenting familial juvenile gout

Unlike complete deficiency of hypoxanthine phosphoribosyltransferase (HPRT) (i.e., Lesch-Nyhan syndrome), partial HPRT deficiency causes HPRT-related hyperuricemia without neurological symptoms. Herein, we describe a 22-year-old man without neurological symptoms that presented gout, hyperuricemia (serum urate level, 12.2 mg/dL), multiple renal microcalculi, and a family history of juvenile gout that was exhibited by his brother and grandfather. Genetic testing revealed a novel missense mutation, c.103G>A (p.V35M), in the HPRT1 gene, and biochemical testing (conducted using the patient's erythrocytes) showed that the patient retained only 12.4% HPRT enzymatic activity compared to that exhibited by a healthy control subject. We thus diagnosed the patient with HPRT-related hyperuricemia caused by partial HPRT deficiency. After his serum urate level was controlled via treatment with febuxostat, his gout did not recur. Thus, this study emphasizes that HPRT deficiency should be considered as a potential cause of familial juvenile gout, even in the absence of neurological symptoms.

Onychotillomania: Diagnosis and Management

Onychotillomania, defined as self-induced trauma to the nail unit, either by picking or pulling at the nails, affects 0.9% of the population. It may lead to severe irreversible nail dystrophy, melanonychia, or infections. Although no large clinical trials have assessed the efficacy of treatments, cognitive-behavioral therapy, physical barrier methods, and pharmacological treatments have shown some benefits in case reports. The objective of this article is to review the prevalence, diagnostic criteria, etiology, historical and physical examination findings, pathological features, and current treatment methods. Onychotillomania remains a clinical challenge to dermatologists, pediatricians, internists, and psychiatrists in practice, as there are no evidence-based treatment methods.

Uric acid, an important screening tool to detect inborn errors of metabolism: a case series

Background

Uric acid is the metabolic end product of purine metabolism in humans. Altered serum and urine uric acid level (both above and below the reference ranges) is an indispensable marker in detecting rare inborn errors of metabolism. We describe different case scenarios of 4 Sri Lankan patients related to abnormal uric acid levels in blood and urine.

Case 1

A one-and-half-year-old boy was investigated for haematuria and a calculus in the bladder. Xanthine crystals were seen in microscopic examination of urine sediment. Low uric acid concentrations in serum and low urinary fractional excretion of uric acid associated with high urinary excretion of xanthine and hypoxanthine were compatible with xanthine oxidase deficiency.

Case 2

An 8-month-old boy presented with intractable seizures, feeding difficulties, screaming episodes, microcephaly, facial dysmorphism and severe neuro developmental delay. Low uric acid level in serum, low fractional excretion of uric acid and radiological findings were consistent with possible molybdenum cofactor deficiency. Diagnosis was confirmed by elevated levels of xanthine, hypoxanthine and sulfocysteine levels in urine.

Case 3

A 3-year-10-month-old boy presented with global developmental delay, failure to thrive, dystonia and self-destructive behaviour. High uric acid levels in serum, increased fractional excretion of uric acid and absent hypoxanthine–guanine phosphoribosyltransferase enzyme level confirmed the diagnosis of Lesch–Nyhan syndrome.

Case 4

A 9-year-old boy was investigated for lower abdominal pain, gross haematuria and right renal calculus. Low uric acid level in serum and increased fractional excretion of uric acid pointed towards hereditary renal hypouricaemia which was confirmed by genetic studies.

Conclusion

Abnormal uric acid level in blood and urine is a valuable tool in screening for clinical conditions related to derangement of the nucleic acid metabolic pathway.

Unapparent hypoxanthine-guanine phosphoribosyltransferase deficiency

Complete deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity causes Lesch Nyhan disease (LND), characterized by hyperuricemia, severe action dystonia, choreoathetosis, ballismus, cognitive and attention deficit and self-injurious behavior. Partial HPRT deficiency is present in patients with Lesch-Nyhan variant (LNV), who present with HPRT-related gout and a variable degree of neurological involvement. The diagnosis of HPRT deficiency relies on clinical, biochemical, enzymatic and molecular data. Patients with HPRT deficiency present low or undetectable HPRT activity in hemolysates, with increased adenine phosphoribosyltransferase (APRT) activity. We present a 9-year-old boy who experienced an episode of macroscopic hematuria with dysuria and left flank pain. He presented hyperuricemia and hyperuricosuria. HPRT and APRT activities were both normal in hemolysate; however, HPRT activity assayed in intact erythrocytes was 50% of control levels. A new missense point mutation c.424 A>G (T142A) was found in the HPRT1 gene. The apparent Michaelis constant (Km) for 5-phosphoribosyl-pyrophosphate assayed in patient hemolysate was 20-fold of control levels. In conclusion, we report a patient with HPRT deficiency who presented with both normal HPRT and APRT activity in hemolysate, in which the enzyme activity determined in intact erythrocytes was of diagnostic utility.

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.

Self-injurious Behavior in a Young Child with Lesch-Nyhan Syndrome

Lesch-Nyhan syndrome (LNS) is a rare inherited disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyl transferase-1. Few reports on behavioral aspects especially self-injurious behavior in LNS patients are available. We report a case of LNS in an 8-year-old male child, who presented with characteristic self-injurious behavior.

Acute renal failure unmasking Lesch-Nyhan disease in a patient with tuberous sclerosis complex

CASE REPORT

We report on a male patient with Tuberous Sclerosis Complex (TSC), which was prenatally diagnosed. At the age of 3 months the patient developed acute renal failure with excessive hyperuricemia. Kidney function improved after rehydration and application of rasburicase, however without full recovery. Due to the inappropriate high levels of uric acid compared to kidney function, screening of hypoxanthine-guanine phosphoribosyltransferase (HPRT) related diseases was initiated. Mutation analysis revealed a deletion of exon 2 and 3 of the HPRT gene confirming the diagnosis of Lesch-Nyhan Disease (LND). After initiation of allopurinol therapy renal function further improved. In the following months the patient developed clinically a typical neurological phenotype of LND and TSC with seizures, severe dystonia and developmental delay.

CONCLUSION

Acute renal failure is a rare complication of HPRT related diseases. Combination of two inherited diseases may lead to a delayed diagnosis due to a mixed and maybe misleading phenotype.

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

AIMS

Lesch-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 METHODS

Analyses were performed by high performance liquid chromatography (HPLC)-coupled quadrupole time-of-flight mass spectrometry (QTOF-MS).

KEY FINDINGS

In 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.

SIGNIFICANCE

Both 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.

Do clinical features of Lesch-Nyhan disease correlate more closely with hypoxanthine or guanine recycling?

Lesch-Nyhan disease (LND) is a rare, X-linked recessive neurodevelopmental disorder caused by deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGprt), an enzyme in the purine salvage pathway. HGprt has two functions; it recycles hypoxanthine and guanine. Which of these two functions is more relevant for pathogenesis is unclear because some evidence points to hypoxanthine recycling, but other evidence points to guanine recycling. In this study, we selectively assayed hypoxanthine (Hprt) and guanine (Gprt) recycling in skin fibroblasts from 17 persons with LND, 11 with an attenuated variant of the disease (LNV), and 19 age-, sex-, and race-matched healthy controls (HC). Activity levels of both enzymes differed across groups (p < 0.0001), but only Gprt distinguished patients with LND from those with LNV (p < 0.05). Gprt also showed slightly stronger correlations than Hprt with 13 of 14 measures of the clinical phenotype, including the severity of dystonia, cognitive impairment, and behavioral abnormalities. These findings suggest that loss of guanine recycling might be more closely linked to the LND/LNV phenotype than loss of hypoxanthine recycling.

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.

Hypoxanthine guanine phosphoribosyltransferase (HPRT) deficiencies: HPRT1 mutations in new Japanese families and PRPP concentration

Mutation of hypoxanthine guanine phosphoribosyltransferase (HPRT) gives rise to Lesch-Nyhan syndrome, which is characterized by hyperuricemia, severe motor disability, and self-injurious behavior, or HPRT-related gout with hyperuricemia. Four mutations were detected in two Lesch-Nyhan families and two families with partial deficiency since our last report. A new mutation of G to TT (c.456delGinsTT) resulting in a frameshift (p.Q152Hfs*3) in exon 3 has been identified in one Lesch-Nyhan family. In the other Lesch-Nyhan family, a new point mutation in intron 7 (c.532+5G>T) causing splicing error (exon 7 excluded, p.L163Cfs*4) was detected. In the two partial deficiency cases with hyperuricemia, two missense mutations of p.D20V (c.59A>T) and p.H60R (c.179A>G) were found. An increase of erythrocyte PRPP concentration was observed in the respective phenotypes and seems to be correlated with disease severity.

HPRT deficiency in Spain: what have we learned in the past 30 years (1984-2013)?

Since 1984, we have diagnosed at the La Paz University Hospital, Madrid, Spain, 41 patients with hypoxanthine phosphoribosyltransferase (HPRT) activity deficiency. These patients belonged to 34 families. We have also performed molecular and enzymatic diagnosis in three patients from India, one from Belgium, and three from Colombia. About 1/3 of these patients were followed up at La Paz University Hospital at least every year. This fact has allowed us to examine the complete spectrum of HPRT deficiency as well as to perform a more accurate diagnosis and treatment. In the present review, we also summarized our studies on the basis of physiopathology of the neurological manifestation of Lesch Nyhan disease (LND).

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.

Clinical severity in Lesch-Nyhan disease: the role of residual enzyme and compensatory pathways

Mutations in the HPRT1 gene, which encodes the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HGprt), cause Lesch-Nyhan disease (LND) and more mildly affected Lesch-Nyhan variants. Prior studies have suggested a strong correlation between residual hypoxanthine recycling activity and disease severity. However, the relevance of guanine recycling and compensatory changes in the de novo synthesis of purines has received little attention. In the current studies, fibroblast cultures were established for 21 healthy controls and 36 patients with a broad spectrum of disease severity related to HGprt deficiency. We assessed hypoxanthine recycling, guanine recycling, steady-state purine pools, and de novo purine synthesis. There was a strong correlation between disease severity and either hypoxanthine or guanine recycling. Intracellular purines were normal in the HGprt-deficient fibroblasts, but purine wasting was evident as increased purine metabolites excreted from the cells. The normal intracellular purines in the HGprt-deficient fibroblasts were likely due in part to a compensatory increase in purine synthesis, as demonstrated by a significant increase in purinosomes. However, the increase in purine synthesis did not appear to correlate with disease severity. These results refine our understanding of the potential sources of phenotypic heterogeneity in LND and its variants.

Mild Lesch-Nyhan Disease in a Boy with a Null Mutation in HPRT1: An Exception to the Known Genotype-Phenotype Correlation

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency results in a continuous spectrum of clinical phenotypes though all include overproduction of uric acid with hyperuricaemia, urate nephrolithiasis and gout. 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 present a boy with a LND variant phenotype due to a deletion of exon 5 of HPRT1 predicted to fully abolish HPRT activity. Metabolic analysis confirms lack of significant residual enzyme activity. The boy, currently age 10, presented with hyperuricaemia, hypotonia, developmental delay and extrapyramidal and pyramidal involvement. He has never shown any signs of self-injurious or maladaptive behaviour. 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. Key Clinical Message HPRT1 mutations that result in very low or no hypoxanthine-guanine phosphoribosyltransferase enzyme activities are generally associated with the classic Lesch-Nyhan disease. This report presents one of the rare cases with a null mutation in the HPRT1 gene that associates with a milder than expected phenotype with lack of self-injurious behaviour.

DERA is the human deoxyribose phosphate aldolase and is involved in stress response

Deoxyribose-phosphate aldolase (EC 4.1.2.4), which converts 2-deoxy-d-ribose-5-phosphate into glyceraldehyde-3-phosphate and acetaldehyde, belongs to the core metabolism of living organisms. It was previously shown that human cells harbor deoxyribose phosphate aldolase activity but the protein responsible of this activity has never been formally identified. This study provides the first experimental evidence that DERA, which is mainly expressed in lung, liver and colon, is the human deoxyribose phosphate aldolase. Among human cell lines, the highest DERA mRNA level and deoxyribose phosphate aldolase activity were observed in liver-derived Huh-7 cells. DERA was shown to interact with the known stress granule component YBX1 and to be recruited to stress granules after oxidative or mitochondrial stress. In addition, cells in which DERA expression was down-regulated using shRNA formed fewer stress granules and were more prone to apoptosis after clotrimazole stress, suggesting the importance of DERA for stress granule formation. Furthermore, the expression of DERA was shown to permit cells in which mitochondrial ATP production was abolished to make use of extracellular deoxyinosine to maintain ATP levels. This study unraveled a previously undescribed pathway which may allow cells with high deoxyribose-phosphate aldolase activity, such as liver cells, to minimize or delay stress-induced damage by producing energy through deoxynucleoside degradation.

Genotypic and phenotypic spectrum in attenuated variants of Lesch-Nyhan disease

Lesch-Nyhan disease and its attenuated variants are caused by deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). All patients exhibit excessive production of uric acid, which increases the risk for nephrolithiasis, renal failure, gouty arthritis and tophi. The mildest phenotype includes only problems related to overproduction of uric acid. The most severe clinical phenotype includes prominent neurological abnormalities and the universal feature is self-injurious behavior. In between the mildest and most severe syndromes is a broad spectrum of phenotypes with varying degrees of neurological, neurocognitive and behavioral abnormalities. The effect of HPRT1 gene mutations on residual HGprt enzyme activity is the most relevant factor contributing to disease phenotype. Attenuated clinical phenotypes are associated with residual enzyme function, whereas the most severe phenotype is usually associated with null activity. In cases of gouty arthritis with urate overproduction, a careful evaluation for motor impairments or neurocognitive abnormalities may help to identify attenuated variants of Lesch-Nyhan disease for better management.

Genetics of Huntington's disease and related disorders

Huntington's disease is the most frequent form of the hereditary choreas and has a multifaceted phenotype including cognitive and psychiatric impairment. The disorder is due to a dynamic mutation, which also influences the onset age of the disorder. Other genetic modifiers of the HD phenotypes have been suggested but often not confirmed by independent studies. Several syndromes with similar presentation have different genetic backgrounds, including the neuroacanthocytoses, mainly choreoacanthocytosis and MacLeod syndrome as a result of mutations in chorein and Kell protein, respectively, but also benign hereditary chorea, owing to mutations in NKX-2-1, and paroxysmal kinesigenic dyskinesia, as a result of recently found mutations in the proline-rich transmembrane protein 2, PRRT2. Chorea can also be a major feature in other neurogenetic disorders, including the spinocerebellar ataxias and also in neurometabolic disorders.

Molecular characterization and structure analysis of HPRT in a Chinese patient with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is caused by deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT). The aim of the present study is to characterize the molecular deficiency of a clinical diagnosed Chinese patient with attenuated variant of LND. The coding region and the intron-exon boundaries of HPRT1 gene were sequenced by standard methods, and HPRT activity was assayed by HPLC method. Structure analysis was performed to estimate the consequence of the mutant of HPRT1 gene. A new mutation c.245T>G (p.Ile82Ser) was identified in this patient, and heterozygous mutation was found in the patient's mother. The activity of HPRT in the patient was completely undetectable. Structure study indicates that the mutation of p.Ile82Ser may lead to loss of hydrophobic side chain and disrupt its normal conformation of HPRT protein. It is helpful for diagnosis of LND that sequencing analysis of HPRT1 gene is performed in male infant and juvenile with hyperuricaemia and neurologic dysfunction in Chinese.

Late diagnosis of Lesch-Nyhan disease variant

A 30-year-old man was referred for investigation and management of hyperuricaemia. History included recurrent nephrolithiasis and chronic gout with poor response to medical management. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) enzyme activity was investigated and found to be deficient confirming the diagnosis of Lesch-Nyhan disease. Hyperuricaemia was treated with allopurinol. To prevent nephrolithiasis, the patient was instructed to avoid dehydration and aim for a minimum urine output of 2 L/day. Urinary alkalinisation with potassium citrate was started. The patient was referred for genetic counselling. This case discusses the genetics, pathophysiology, clinical manifestations, diagnosis and management of HGPRT deficiency.

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.

Impairment of adenylyl cyclase 2 function and expression in hypoxanthine phosphoribosyltransferase-deficient rat B103 neuroblastoma cells as model for Lesch-Nyhan disease: BODIPY-forskolin as pharmacological tool

Hypoxanthine phosphoribosyl transferase (HPRT) deficiency results in Lesch-Nyhan disease (LND). The link between the HPRT defect and the self-injurious behavior in LND is still unknown. HPRT-deficient rat B103 neuroblastoma cells serve as a model system for LND. In B103 cell membranes, HPRT deficiency is associated with a decrease of basal and guanosine triphosphate-stimulated adenylyl cyclase (AC) activity (Pinto and Seifert, J Neurochem 96:454-459, 2006). Since recombinant AC2 possesses a high basal activity, we tested the hypothesis that AC2 function and expression is impaired in HPRT deficiency. We examined AC regulation in B103 cell membranes, cAMP accumulation in intact B103 cells, AC isoform expression, and performed morphological studies. As most important pharmacological tool, we used 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene forskolin (BODIPY-FS) that inhibits recombinant AC2 but activates ACs 1 and 5 (Erdorf et al., Biochem Pharmacol 82:1673-1681, 2011). In B103 control membranes, BODIPY-FS reduced catalysis, but in HPRT(-) membranes, BODIPY-FS was rather stimulatory. 2'(3')-O-(N-methylanthraniloyl) (MANT)-nucleoside 5'-[γ-thio]triphosphates inhibit recombinant ACs 1 and 5 more potently than AC2. In B103 control membranes, MANT-guanosine 5'-[γ-thio]triphosphate inhibited catalysis in control membranes less potently than in HPRT(-) membranes. Quantitative real-time PCR revealed that in HPRT deficiency, AC2 was virtually absent. In contrast, AC5 was up-regulated. Forskolin (FS) and BODIPY-FS induced cell clustering and rounding and neurite extension in B103 cells. The effects of FS and BODIPY-FS were much more prominent in control than in HPRT(-) cells, indicative for a differentiation defect in HPRT deficiency. Neither FS nor BODIPY-FS significantly changed cAMP concentrations in intact B103 cells. Collectively, our data show that HPRT deficiency in B103 cells is associated with impaired AC2 function and expression and reduced sensitivity to differentiation induced by FS and BODIPY-FS. We discuss the pathophysiological implications of our data for LND.