The spectrum of inherited mutations causing HPRT deficiency: 75 new cases and a review of 196 previously reported cases

Detailed genetic and clinical analysis of a novel de novo variant in HPRT1: Case report of a female patient from Saudi Arabia with Lesch-Nyhan syndrome

Background: Hypoxanthine-guanine phosphoribosyltransferase (HPRT1) deficiency is an inborn error of purine metabolism responsible for Lesch-Nyhan syndrome (LNS). The disease is inherited in an X-linked recessive manner and predominantly affects male individuals. Female individuals can carry a mutation as heterozygotes, but typically, they are asymptomatic because of the random inactivation of the affected allele. Nevertheless, although rare, heterozygote female individuals may manifest LNS with full characteristics. Herein, we describe a female patient from Saudi Arabia with LNS. Results: The patient (a 4-year-old girl) presented with typical characteristics of the disease, which include global developmental delay, self-mutilation, hyperuricemia, hypotonia, speech delay, spasticity, and seizures. Her general biochemical laboratory results were normal except for high levels of uric acid. The abdominal MRI\MRS, mostly unremarkable, showed bilateral echogenic foci within the renal collecting system. Genetic testing (whole-exome sequencing, iterative variant filtering, segregation analysis, and Sanger sequencing) pointed a novel de novo frameshift variant in HPRT1. X-inactivation assay using HpaII showed the presence of a 100% skewed X chromosome carrying the affected allele. RT-PCR of the cDNA indicated complete loss of the expression of the normal allele. Conclusion: Our study presents a female patient who has a severe case of LNSand found to be the 15th female patient with the disease in the world. The study emphasizethe need for a streamlined protocol that will help an early and accurate diagnosis of female LNS patients to avoid unnecessary interventions that lead to costly patient care.

Deep brain stimulation in Lesch-Nyhan syndrome: a systematic review

Given the good results of deep brain stimulation (DBS) in the treatment of movement disorders, DBS was initially tried to treat Lesch-Nyhan syndrome (LNS) with the aim to alleviate LNS-related dystonia. Some cases have reported clinical results of DBS in LNS thus far. This systematic review was conducted to comprehensively summarize cases of LNS treated with DBS and evaluate the efficacy and safety of DBS in LNS. Eight publications covering 12 LNS patients were included in this review. DBS improved dystonia of the LNS to varying degrees. All the included cases achieved partial or complete control of self-injurious behavior (SIB). Overall, DBS is a promising treatment for both motor and behavior disorders of LNS patients, but the results reported thus far have varied widely, especially for motor outcomes. The ultimate clinical benefits in LNS patients were still unpredictable. DBS-related complications were rather common, which raised questions about the safety of the procedure in LNS. More research is needed to further clarify the safety and effectiveness of this treatment.

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.

PUS7 deficiency in human patients causes profound neurodevelopmental phenotype by dysregulating protein translation

Protein translation is a highly regulated process involving the interaction of numerous genes on every component of the protein translation machinery. Upregulated protein translation is a hallmark of cancer and is implicated in autism spectrum disorder, but the risks of developing each disease do not appear to be correlated with one another. In this study we identified two siblings from the NIH Undiagnosed Diseases Program with loss of function variants in PUS7, a gene previously implicated in the regulation of total protein translation. These patients exhibited a neurodevelopmental phenotype including autism spectrum disorder in the proband. Both patients also had features of Lesch-Nyhan syndrome, including hyperuricemia and self-injurious behavior, but without pathogenic variants in HPRT1. Patient fibroblasts demonstrated upregulation of protein synthesis, including elevated MYC protein, but did not exhibit increased rates of cell proliferation. Interestingly, the dysregulation of protein translation also resulted in mildly decreased levels of HPRT1 protein suggesting an association between dysregulated protein translation and the LNS-like phenotypic findings. These findings strengthen the correlation between neurodevelopmental disease, particularly autism spectrum disorders, and the rate of protein translation.

Case report: Early-onset renal failure as presenting sign of Lesch-Nyhan disease in infancy

Lesch-Nyhan disease (LND) is a rare X-linked recessive disease caused by pathogenic mutations of the HPRT1 gene. The typical clinical manifestations include cerebral palsy, intellectual disability, dysarthria, self-injurious behavior, and gouty arthritis in children. This report describes a Chinese boy aged 2 months and 7 days with a significantly elevated uric acid concentration accompanied by renal dysfunction and, notably, brain imaging changes. Whole-exome sequencing revealed a hemizygous mutation of HPRT1 in nucleotide 508 from cytosine to thymine (c.508C > T), resulting in a nonsense mutation (p.R170X). The incidence of LND is extremely low in China, and hyperuricemia is a common clinical manifestation. Therefore, the possibility of LND should be considered in children with increased uric acid in infancy accompanied by brain imaging changes or neurological dysfunction. Moreover, genetic testing is needed to provide adequate genetic counseling to the family, and should be conducted as early as possible in such children to avoid misdiagnosis or delayed diagnosis.

Contribution of Human Pluripotent Stem Cell-Based Models to Drug Discovery for Neurological Disorders

One of the major obstacles to the identification of therapeutic interventions for central nervous system disorders has been the difficulty in studying the step-by-step progression of diseases in neuronal networks that are amenable to drug screening. Recent advances in the field of human pluripotent stem cell (PSC) biology offers the capability to create patient-specific human neurons with defined clinical profiles using reprogramming technology, which provides unprecedented opportunities for both the investigation of pathogenic mechanisms of brain disorders and the discovery of novel therapeutic strategies via drug screening. Many examples not only of the creation of human pluripotent stem cells as models of monogenic neurological disorders, but also of more challenging cases of complex multifactorial disorders now exist. Here, we review the state-of-the art brain cell types obtainable from PSCs and amenable to compound-screening formats. We then provide examples illustrating how these models contribute to the definition of new molecular or functional targets for drug discovery and to the design of novel pharmacological approaches for rare genetic disorders, as well as frequent neurodegenerative diseases and psychiatric disorders.

Involvement of APOBEC3B in mutation induction by irradiation

To better understand the cancer risk posed by radiation and the development of radiation therapy resistant cancer cells, we investigated the involvement of the cancer risk factor, APOBEC3B, in the generation of radiation-induced mutations. Expression of APOBEC3B in response to irradiation was determined in three human cancer cell lines by real-time quantitative PCR. Using the hypoxanthine-guanine phosphoribosyl transferase (HPRT) mutation assay, mutations in the HPRT gene caused by irradiation were compared between APOBEC3B-deficient human hepatocellular carcinoma (HepG2) cells [APOBEC3B knocked out (KO) using CRISPR-Cas9 genome editing] and the parent cell line. Then, HPRT-mutated cells were individually cultured to perform PCR and DNA sequencing of HPRT exons. X-Irradiation induced APOBEC3B expression in HepG2, human cervical cancer epithelial carcinoma (HeLa) and human oral squamous cell carcinoma (SAS) cells. Forced expression of APOBEC3B increased spontaneous mutations. By contrast, APOBEC3B KO not only decreased the spontaneous mutation rate, but also strongly suppressed the increase in mutation frequency after irradiation in the parent cell line. Although forced expression of APOBEC3B in the nucleus caused DNA damage, higher levels of APOBEC3B tended to reduce APOBEC3B-induced γ-H2AX foci formation (a measure of DNA damage repair). Further, the number of γ-H2AX foci in cells stably expressing APOBEC3B was not much higher than that in controls before and after irradiation, suggesting that a DNA repair pathway may be activated. This study demonstrates that irradiation induces sustained expression of APOBEC3B in HepG2, HeLa and SAS cells, and that APOBEC3B enhances radiation-induced partial deletions.

Molecular Analysis of Mutations in the Human HPRT Gene

The HPRT assay uses incorporation of toxic nucleotide analogues to select for cells lacking the purine scavenger enzyme hypoxanthine-guanine phosphoribosyl transferase. A major advantage of this assay is the ability to isolate mutant cells and determine the molecular basis for their functional deficiency. Many types of analyses have been performed at this locus: the current protocol involves generation of a cDNA and multiplex PCR of each exon, including the intron/exon junctions, followed by direct sequencing of the products. This analysis detects point mutations, small deletions and insertions within the gene, mutations affecting RNA splicing, and the products of illegitimate V(D)J recombination within the gene. Establishment of and comparisons with mutational spectra hold the promise of identifying exposures to mutation-inducing genotoxicants from their distinctive pattern of gene-specific DNA damage at this easily analyzed reporter gene.

Animal Model Contributions to Congenital Metabolic Disease

Genetic model systems allow researchers to probe and decipher aspects of human disease, and animal models of disease are frequently specifically engineered and have been identified serendipitously as well. Animal models are useful for probing the etiology and pathophysiology of disease and are critical for effective discovery and development of novel therapeutics for rare diseases. Here we review the impact of animal model organism research in three examples of congenital metabolic disorders to highlight distinct advantages of model system research. First, we discuss phenylketonuria research where a wide variety of research fields and models came together to make impressive progress and where a nearly ideal mouse model has been central to therapeutic advancements. Second, we review advancements in Lesch-Nyhan syndrome research to illustrate the role of models that do not perfectly recapitulate human disease as well as the need for multiple models of the same disease to fully investigate human disease aspects. Finally, we highlight research on the GM2 gangliosidoses Tay-Sachs and Sandhoff disease to illustrate the important role of both engineered traditional laboratory animal models and serendipitously identified atypical models in congenital metabolic disorder research. We close with perspectives for the future for animal model research in congenital metabolic disorders.

Phenotypic and molecular spectrum of Korean patients with Lesch-Nyhan syndrome and attenuated clinical variants

Lesch-Nyhan syndrome (LNS) is an X-linked recessive disorder caused by mutations in the HPRT1 gene. The clinical features and mutation spectrum of 26 Korean LNS patients from 23 unrelated families were retrospectively reviewed. The HPRT1 gene was analyzed by direct sequencing of genomic DNA. The median age at diagnosis was 2.3 years (range, 4 months-22.6 years) and the initial presenting features included developmental delay, orange colored urine, and self-injurious behaviors. Most patients were wheelchair-bound and suffered from urinary complications and neurologic problems such as self-mutilation and developmental delay. Twenty different mutations in HPRT1 were identified among 23 independent pedigrees, including six novel mutations. The most common mutation type was truncating mutations including nonsense and frameshift mutations (45%). Large deletions in the HPRT1 gene were identified in exon 1, exons 5-6, exons 1-9, and at chr X:134,459,540-134,467,241 (7702 bp) including the 5'-untranslated region, exon 1, and a portion of intron 1. In conclusion, this study describes the phenotypic spectrum of LNS and has identified 20 mutations from 23 Korean families, including six novel mutations in Korean patients with LNS.

Understanding the structure-function relationship of HPRT1 missense mutations in association with Lesch-Nyhan disease and HPRT1-related gout by in silico mutational analysis

The nucleotide salvage pathway is used to recycle degraded nucleotides (purines and pyrimidines); one of the enzymes that helps to recycle purines is hypoxanthine guanine phosphoribosyl transferase 1 (HGPRT1). Therefore, defects in this enzyme lead to the accumulation of DNA and nucleotide lesions and hence replication errors and genetic disorders. Missense mutations in hypoxanthine phosphoribosyl transferase 1 (HPRT1) are associated with deficiencies such as Lesch-Nyhan disease and chronic gout, which have manifestations such as arthritis, neurodegeneration, and cognitive disorders. In the present study, we collected 88 non-synonymous single nucleotide polymorphisms (nsSNPs) from the UniProt, dbSNP, ExAC, and ClinVar databases. We used a series of sequence-based and structure-based in silico tools to prioritize and characterize the most pathogenic and stabilizing or destabilizing nsSNPs. Moreover, to obtain the structural impact of the pathogenic mutations, we mapped the mutations to the crystal structure of the HPRT protein. We further subjected these mutant proteins to a 50 ns molecular dynamics simulation (MDS). The MDS trajectory showed that all mutant proteins altered the structural conformation and dynamic behavior of the HPRT protein and corroborated its association with LND and gout. This study provides essential information regarding the use of HPRT protein mutants as potential targets for therapeutic development.

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

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

Background

Lesch-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 presentation

We 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.

Conclusions

Pediatricians treating these children should be aware of these rare conditions and promptly manage the potential complications that may require medical or surgical intervention.

Altered gastrointestinal motility in an animal model of Lesch-Nyhan disease

Mutations in the HGPRT1 gene, which encodes hypoxanthine-guanine phosphoribosyltransferase (HGprt), housekeeping enzyme responsible for recycling purines, lead to Lesch-Nyhan disease (LND). Clinical expression of LND indicates that HGprt deficiency has adverse effects on gastrointestinal motility. Therefore, we aimed to evaluate intestinal motility in HGprt knockout mice (HGprt¯). Spontaneous and neurally evoked mechanical activity was recorded in vitro as changes in isometric tension in circular muscle strips of distal colon. HGprt¯ tissues showed a lower in amplitude spontaneous activity and atropine-sensitivity neural contraction compared to control mice. The responses to carbachol and to high KCl were reduced, demonstrating a widespread impairment of contractility. L-NAME was not able in the HGprt¯ tissues to restore the large amplitude contractile activity typical of control. In HGprt¯ colon, a reduced expression of dopaminergic D1 receptor was observed together with the loss of its tonic inhibitory activity present in control-mice. The analysis of inflammatory and oxidative stress in colonic tissue of HGprt¯ mice revealed a significant increase of lipid peroxidation associated with over production of oxygen free radicals. In conclusion, HGprt deficiency in mice is associated with a decrease in colon contractility, not dependent upon reduction of acetylcholine release from the myenteric plexus or hyperactivity of inhibitory signalling. By contrast the increased levels of oxidative stress could partially explain the reduced colon motility in HGprt¯ mice. Colonic dysmotility observed in HGprt¯ mice may mimic the gastrointestinal dysfunctions symptoms of human syndrome, providing a useful animal model to elucidate the pathophysiology of this problem in the LND.

HPRT1 activity loss is associated with resistance to thiopurine in ALL

Acute lymphoblastic leukemia (ALL) is an aggressive hematological tumor resulting from the malignant transformation of lymphoid progenitors. Thiopurine is a widely used drug in the maintaining treatment of ALL. After a period of chemotherapy, 20% of pediatric patients and over 50% of adult patients will relapse. To investigate the mechanisms of drug resistance in vitro, we established the thiopurine resistant cell lines Reh-6MPR (6-MP Resistant cell) and Reh-6TGR (6-TG Resistant cell) by stepwise selection of the ALL cell line Reh. Cell viability assay revealed that 6MPR and 6TGR cells were almost 1000-fold more resistant to thiopurine comparing with the control Reh cells, and thiopurine conversion was significantly impaired in the resistant cells. Mechanistically, a same novel hypoxanthine phosphoribosyl transferase 1 (HPRT1) mutation c.495_496insA (p.V165fs) was found by whole exome sequencing in both resistant cells. The HPRT1 mutation dramaticly decreased the production of [¹³C₅,¹⁵N₄]-IMP from [¹³C₅,¹⁵N₄]-hypoxanthine (HX), showed a loss-of-funciton mechanism. Notably, re-expression the wildtype HPRT1 in Reh-6MPR cell can reverse the drug resistance and thiopurine conversion in Reh-6MPR cells. These results highlight the importance of HPRT1's activity in thiopurine resistance.

Kinetic Isotope Effects and Transition State Structure for Hypoxanthine-Guanine-Xanthine Phosphoribosyltransferase from Plasmodium falciparum

Plasmodium falciparum parasites are purine auxotrophs that rely exclusively on the salvage of preformed purines from their human hosts to supply the requirement for purine nucleotides. Hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) catalyzes the freely reversible Mg²⁺-dependent conversion of 6-oxopurine bases to their respective nucleotides and inorganic pyrophosphate. The phosphoribosyl group is derived from 5-phospho-α-d-ribosyl 1-pyrophosphate (PRPP). The enzyme from malaria parasites (PfHGXPRT) is essential as hypoxanthine is the major precursor in purine metabolism. We used specific heavy atom labels in PRPP and hypoxanthine to measure primary (1-¹⁴C and 9-¹⁵N) and secondary (1-³H and 7-¹⁵N) intrinsic kinetic isotope effect (KIE) values for PfHGXPRT. Intrinsic isotope effects contain information for understanding enzymatic transition state properties. The transition state of PfHGXPRT was explored by matching KIE values predicted from quantum mechanical calculations to the intrinsic values determined experimentally. This approach provides information about PfHGXPRT transition state bond lengths, geometry, and atomic charge distribution. The transition state structure of PfHGXPRT was determined in the physiological direction of addition of ribose 5-phosphate to hypoxanthine by overcoming the chemical instability of PRPP. The transition state for PfHGXPRT forms nucleotides through a well-developed and near-symmetrical D_N*A_N, S_N1-like transition state.

Hypoxanthine Induces Neuroenergetic Impairment and Cell Death in Striatum of Young Adult Wistar Rats

Hypoxanthine is the major purine involved in the salvage pathway of purines in the brain. High levels of hypoxanthine are characteristic of Lesch-Nyhan Disease. Since hypoxanthine is a purine closely related to ATP formation, the aim of this study was to investigate the effect of intrastriatal hypoxanthine administration on neuroenergetic parameters (pyruvate kinase, succinate dehydrogenase, complex II, cytochrome c oxidase, and ATP levels) and mitochondrial function (mitochondrial mass and membrane potential) in striatum of rats. We also evaluated the effect of cell death parameters (necrosis and apoptosis). Wistar rats of 60 days of life underwent stereotactic surgery and were divided into two groups: control (infusion of saline 0.9%) and hypoxanthine (10 μM). Intrastriatal hypoxanthine administration did not alter pyruvate kinase activity, but increased succinate dehydrogenase and complex II activities and diminished cytochrome c oxidase activity and immunocontent. Hypoxanthine injection decreased the percentage of cells with mitochondrial membrane label and increased mitochondrial membrane potential labeling. There was a decrease in the number of live cells and an increase in the number of apoptotic cells by caused hypoxanthine. Our findings show that intrastriatal hypoxanthine administration altered neuroenergetic parameters, and caused mitochondrial dysfunction and cell death by apoptosis, suggesting that these processes may be associated, at least in part, with neurological symptoms found in patients with Lesch-Nyhan Disease.

Lesch-Nyhan disease in two families from Chiloé Island with mutations in the HPRT1 gene

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. The authors report two independent point mutations leading to splicing errors: IVS 2 +1G>A, c.134 +1G>A, and IVS 3 +1G>A, c.318 +1G>A in the hypoxanthine-phosphoribosyltransferase1 (HPRT1) gene which result in exclusion of exon 2 and exon 3 respectively, in the HGprt enzyme protein from different members of two Chiloé Island families. Molecular analysis has revealed the heterogeneity of genetic mutation of the HPRT1 gene responsible for the HGprt deficiency. It allows fast, accurate carrier detection and genetic counseling.

Genome-wide genetic screening with chemically mutagenized haploid embryonic stem cells

In model organisms, classical genetic screening via random mutagenesis provides key insights into the molecular bases of genetic interactions, helping to define synthetic lethality, synthetic viability and drug-resistance mechanisms. The limited genetic tractability of diploid mammalian cells, however, precludes this approach. Here, we demonstrate the feasibility of classical genetic screening in mammalian systems by using haploid cells, chemical mutagenesis and next-generation sequencing, providing a new tool to explore mammalian genetic interactions.

Lesch-Nyhan Syndrome: Models, Theories, and Therapies

Lesch-Nyhan syndrome (LNS) is a rare X-linked disorder caused by mutations in HPRT1, an important enzyme in the purine salvage pathway. Symptoms of LNS include dystonia, gout, intellectual disability, and self-mutilation. Despite having been characterized over 50 years ago, it remains unclear precisely how deficits in hypoxanthine and guanine recycling can lead to such a profound neurological phenotype. Several studies have proposed different hypotheses regarding the etiology of this disease, and several treatments have been tried in patients, though none have led to a satisfactory explanation of the disease. New technologies such as next-generation sequencing, optogenetics, genome editing, and induced pluripotent stem cells provide a unique opportunity to map the precise sequential pathways leading from genotype to phenotype.

Novel mutation in HPRT1 causing a splicing error with multiple variations

Lesch-Nyhan disease (LND) is a rare X-linked recessive disorder caused by deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT), encoded by the HPRT1. To date, nearly all types of mutations have been reported in the whole gene; however, duplication mutations are rare. We here report the case of a 9-month-old boy with LND. He showed developmental delay, athetosis, and dystonic posture from early infancy, but no self-injurious behaviors. Hyperuricemia was detected, and his HPRT enzyme activity in erythrocytes was completely deficient. A novel duplication mutation (c.372dupT, c.372_374 TTT > c.372_375 TTTT) was identified in exon 4 of the HPRT1, which causes aberrant splicing. This is the third case of a duplication mutation in the HPRT1 that causes splicing error.

The Effect of S-Adenosylmethionine on Self-Mutilation in a Patient with Lesch-Nyhan Disease

BACKGROUND

Lesch-Nyhan disease (LND) is an X-chromosomal disorder of purine metabolism characterized by hyperuricemia, dystonia, and self-mutilation, leading to an extremely high burden of disease in affected patients and families. Although allopurinol therapy can control hyperuricemia, it has no effect on self-mutilation and neurological symptoms. Single reports describe a beneficial effect of S-adenosylmethionine (SAM) on the neurological symptoms, which motivated us to evaluate this alternative treatment.

METHODS

We performed a double-blind placebo-controlled trial to analyze the effects of SAM on self-mutilation attempts in a male patient affected by LND. The trial lasted for 282 days and comprised three alternating verum and placebo periods of 50 days each. The mother of the patient recorded attempts of self-mutilation during the entire trial.

RESULTS

While verum and placebo were both well tolerated, a total of 1,762 events of self-mutilation were recorded, of which 1,281 events were in the placebo period and 481 in the verum period. The daily mean of events was 8.6 with placebo and 4.5 with SAM corresponding to a 50 % decrease in self-mutilation events under SAM treatment (p < 0.05).

CONCLUSION

The results of this double-blind placebo-controlled single-case trial suggest that SAM can have a beneficial effect on self-mutilation in patients with LND, possibly by replenishing the purine pool in affected brain cells.

Genetic background of uric acid metabolism in a patient with severe chronic tophaceous gout

Hyperuricemia depends on the balance of endogenous production and renal excretion of uric acid. Transporters for urate are located in the proximal tubule where uric acid is secreted and extensively reabsorbed: secretion is principally ensured by the highly variable ABCG2 gene. Enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) plays a central role in purine metabolism and its deficiency is an X-linked inherited metabolic disorder associated with clinical manifestations of purine overproduction. Here we report the case of a middle-aged man with severe chronic tophaceous gout with a poor response to allopurinol and requiring repeated surgical intervention. We identified the causal mutations in the HPRT1 gene, variant c.481G>T (p.A161S), and in the crucial urate transporter ABCG2, a heterozygous variant c.421C>A (p.Q141K). This case shows the value of an analysis of the genetic background of serum uric acid.

The eye and the skin in nonendocrine metabolic disorders

As metabolism is controlled by the input of genes and the environment, metabolic disorders result from some disturbance in the interaction between genes and environmental factors. Many metabolic disorders consist in congenital enzyme deficiencies, also known as "inborn errors of metabolism," that may be disabling or cause severe illness and death and are predominantly inherited in an autosomal recessive fashion. The deposit in cells and tissues of storage substances from errors in metabolic processes may produce a wide variety of disorders affecting different organs and functions, with different degrees of severity, and often present around the time of birth or early childhood. Distinctive ocular and skin manifestations accompany many metabolic diseases and may provide clues for their diagnosis and evolution.

Use of the HPRT gene to study nuclease-induced DNA double-strand break repair

Understanding the mechanisms of chromosomal double-strand break repair (DSBR) provides insight into genome instability, oncogenesis and genome engineering, including disease gene correction. Research into DSBR exploits rare-cutting endonucleases to cleave exogenous reporter constructs integrated into the genome. Multiple reporter constructs have been developed to detect various DSBR pathways. Here, using a single endogenous reporter gene, the X-chromosomal disease gene encoding hypoxanthine phosphoribosyltransferase (HPRT), we monitor the relative utilization of three DSBR pathways following cleavage by I-SceI or CRISPR/Cas9 nucleases. For I-SceI, our estimated frequencies of accurate or mutagenic non-homologous end-joining and gene correction by homologous recombination are 4.1, 1.5 and 0.16%, respectively. Unexpectedly, I-SceI and Cas9 induced markedly different DSBR profiles. Also, using an I-SceI-sensitive HPRT minigene, we show that gene correction is more efficient when using long double-stranded DNA than single- or double-stranded oligonucleotides. Finally, using both endogenous HPRT and exogenous reporters, we validate novel cell cycle phase-specific I-SceI derivatives for investigating cell cycle variations in DSBR. The results obtained using these novel approaches provide new insights into template design for gene correction and the relationships between multiple DSBR pathways at a single endogenous disease gene.

Nucleotide salvage deficiencies, DNA damage and neurodegeneration

Nucleotide balance is critically important not only in replicating cells but also in quiescent cells. This is especially true in the nervous system, where there is a high demand for adenosine triphosphate (ATP) produced from mitochondria. Mitochondria are particularly prone to oxidative stress-associated DNA damage because nucleotide imbalance can lead to mitochondrial depletion due to low replication fidelity. Failure to maintain nucleotide balance due to genetic defects can result in infantile death; however there is great variability in clinical presentation for particular diseases. This review compares genetic diseases that result from defects in specific nucleotide salvage enzymes and a signaling kinase that activates nucleotide salvage after DNA damage exposure. These diseases include Lesch-Nyhan syndrome, mitochondrial depletion syndromes, and ataxia telangiectasia. Although treatment options are available to palliate symptoms of these diseases, there is no cure. The conclusions drawn from this review include the critical role of guanine nucleotides in preventing neurodegeneration, the limitations of animals as disease models, and the need to further understand nucleotide imbalances in treatment regimens. Such knowledge will hopefully guide future studies into clinical therapies for genetic diseases.

Purinosome formation as a function of the cell cycle

The de novo purine biosynthetic pathway relies on six enzymes to catalyze the conversion of phosphoribosylpyrophosphate to inosine 5'-monophosphate. Under purine-depleted conditions, these enzymes form a multienzyme complex known as the purinosome. Previous studies have revealed the spatial organization and importance of the purinosome within mammalian cancer cells. In this study, time-lapse fluorescence microscopy was used to investigate the cell cycle dependency on purinosome formation in two cell models. Results in HeLa cells under purine-depleted conditions demonstrated a significantly higher number of cells with purinosomes in the G1 phase, which was further confirmed by cell synchronization. HGPRT-deficient fibroblast cells also exhibited the greatest purinosome formation in the G1 phase; however, elevated levels of purinosomes were also observed in the S and G2/M phases. The observed variation in cell cycle-dependent purinosome formation between the two cell models tested can be attributed to differences in purine biosynthetic mechanisms. Our results demonstrate that purinosome formation is closely related to the cell cycle.

Partial HPRT Deficiency with a Novel Mutation of the HPRT Gene in Combination with Four Previously Reported Variants Associated with Hyperuricemia

A 15-year-old boy was referred to our department due to gout. The laboratory findings showed hyperuricemia with a decreased erythrocyte hypoxanthine phosphoribosyl transferase (HPRT) activity. The HPRT cDNA sequence was revealed to be 206A>T, which has not been previously reported. In addition, direct sequencing of genomic DNA showed the patient to possess four variants reported to be associated with hyperuricemia. This is the first case report of partial HPRT deficiency due to a novel HPRT mutation accompanied by variants associated with hyperuricemia. Combination treatment consisting of benzbromarone and febuxostat had a significant effect in reducing the urate level in our patient.

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.

Hyperuricemia and gout due to deficiency of hypoxanthine-guanine phosphoribosyltransferase in female carriers: New insight to differential diagnosis

BACKGROUND

X-linked hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency in an inherited disorder of purine metabolism is usually associated with the clinical manifestations of hyperuricemia. A variable spectrum of neurological involvement occurs predominantly in males. Females are usually asymptomatic. Carrier status cannot be confirmed by biochemical and enzymatic methods reliably.

METHODS

We studied clinical, biochemical and molecular genetic characteristics of Czech families with hyperuricemia and HPRT deficiency. We analyzed age at diagnosis, clinical symptoms, uricemia, urinary hypoxanthine and xanthine, HPRT activity in erythrocytes, mutation in the HPRT1 gene, X-inactivation, and major urate transporters.

RESULTS

A mutation in the HPRT1 gene in family A was confirmed in one boy and four females. Three females with hyperuricemia had normal excretion of purine. One female was normouricemic. An 8-month-old boy with neurological symptoms showed hyperuricemia, increased excretion of urinary hypoxanthine and xanthine and a very low HPRT activity in erythrocytes. We have found three other unrelated female carriers with hyperuricemia and normal excretion of hypoxanthine and xanthine among other families with HPRT deficiency.

CONCLUSIONS

HPRT deficiency needs to be considered in females with hyperuricemia with normal excretion of purine metabolites. Familiar hyperuricemia and/or nonfamiliar gout should always be further investigated, especially in children.

Lesch Nyhan syndrome: a novel complex mutation in a Tunisian child

Lesch Nyhan syndrome (LNS) is an X-linked recessive disorder due to complete deficiency of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) enzyme. Defect of the enzymatic activity is related to mutations of the HPRT1 gene. The disorder severity is due to neurological features and renal complications. Up to now, more than 300 mutations have been reported. We report on a Tunisian child with a severe phenotype due to a novel identified complex mutation.

Novel hypoxanthine guanine phosphoribosyltransferase gene mutations in Saudi Arabian hyperuricemia patients

Over the past decade, a steady increase in the incidence of HPRT-related hyperuricemia (HRH) has been observed in Saudi Arabia. We examined all the nine exons of HPRT gene for mutations in ten biochemically confirmed hyperuricemia patients, including one female and three normal controls. In all, we identified 13 novel mutations in Saudi Arabian HPRT-related hyperuricemia patients manifesting different levels of uric acid. The Lys103Met alteration was highly recurrent and was observed in 50% of the cases, while Ala160Thr and Lys158Asn substitutions were found in two patients. Moreover, in 70% of the patients ≥2 mutations were detected concurrently in the HPRT gene. Interestingly, one of the patients that harbored Lys103Met substitution along with two frameshift mutations at codons 85 and 160 resulting in shortened protein demonstrated unusually high serum uric acid level of 738 μmol/L. Two of the seven point mutations that resulted in amino acid change (Lys103Met and Val160Gly) were predicted to be damaging by SIFT and Polyphen and were further analyzed for their protein stability and function by molecular dynamics simulation. The identified novel mutations in the HPRT gene may prove useful in the prenatal diagnosis and genetic counseling.

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.

Treatment of Lesch-Nyhan disease with S-adenosylmethionine: experience with five young Malaysians, including a girl

BACKGROUND

Lesch-Nyhan disease (LND) is a rare X-linked recessive neurogenetic disorder caused by deficiency of the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT, EC 2.4.2.8) which is responsible for recycling purine bases into purine nucleotides. Affected individuals have hyperuricemia leading to gout and urolithiasis, accompanied by a characteristic severe neurobehavioural phenotype with compulsive self-mutilation, extrapyramidal motor disturbances and cognitive impairment.

AIM

For its theoretical therapeutic potential to replenish the brain purine nucleotide pool, oral supplementation with S-adenosylmethionine (SAMe) was trialed in 5 Malaysian children with LND, comprising 4 related Malay children from 2 families, including an LND girl, and a Chinese Malaysian boy.

RESULTS

Dramatic reductions of self-injury and aggressive behaviour, as well as a milder reduction of dystonia, were observed in all 5 patients. Other LND neurological symptoms did not improve during SAMe therapy.

DISCUSSION

Molecular mechanisms proposed for LND neuropathology include GTP depletion in the brain leading to impaired dopamine synthesis, dysfunction of G-protein-mediated signal transduction, and defective developmental programming of dopamine neurons. The improvement of our LND patients on SAMe, particularly the hallmark self-injurious behaviour, echoed clinical progress reported with another purine nucleotide depletion disorder, Arts Syndrome, but contrasted lack of benefit with the purine disorder adenylosuccinate lyase deficiency. This first report of a trial of SAMe therapy in LND children showed remarkably encouraging results that warrant larger studies.

Loss of dopamine phenotype among midbrain neurons in Lesch-Nyhan disease

OBJECTIVE

Lesch-Nyhan disease (LND) is caused by congenital deficiency of the purine recycling enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). Affected patients have a peculiar neurobehavioral syndrome linked with reductions of dopamine in the basal ganglia. The purpose of the current studies was to determine the anatomical basis for the reduced dopamine in human brain specimens collected at autopsy.

METHODS

Histopathological studies were conducted using autopsy tissue from 5 LND cases and 6 controls. Specific findings were replicated in brain tissue from an HGprt-deficient knockout mouse using immunoblots, and in a cell model of HGprt deficiency by flow-activated cell sorting (FACS).

RESULTS

Extensive histological studies of the LND brains revealed no signs suggestive of a degenerative process or other consistent abnormalities in any brain region. However, neurons of the substantia nigra from the LND cases showed reduced melanization and reduced immunoreactivity for tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis. In the HGprt-deficient mouse model, immunohistochemical stains for TH revealed no obvious loss of midbrain dopamine neurons, but quantitative immunoblots revealed reduced TH expression in the striatum. Finally, 10 independent HGprt-deficient mouse MN9D neuroblastoma lines showed no signs of impaired viability, but FACS revealed significantly reduced TH immunoreactivity compared to the control parent line.

INTERPRETATION

These results reveal an unusual phenomenon in which the neurochemical phenotype of dopaminergic neurons is not linked with a degenerative process. They suggest an important relationship between purine recycling pathways and the neurochemical integrity of the dopaminergic phenotype.

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.

Transition-state inhibitors of purine salvage and other prospective enzyme targets in malaria

Malaria is a leading cause of human death within the tropics. The gradual generation of drug resistance imposes an urgent need for the development of new and selective antimalarial agents. Kinetic isotope effects coupled to computational chemistry have provided the relevant details on geometry and charge of enzymatic transition states to facilitate the design of transition-state analogs. These features have been reproduced into chemically stable mimics through synthetic chemistry, generating inhibitors with dissociation constants in the pico- to femto-molar range. Transition-state analogs are expected to contribute to the control of malaria.

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.

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.

Purine metabolism during neuronal differentiation: the relevance of purine synthesis and recycling

Purines are a class of small organic molecules that are essential for all cells. They play critical roles in neuronal differentiation and function. Their importance is highlighted by several inherited disorders of purine metabolism, such as Lesch-Nyhan disease, which is caused by a deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). Despite the known importance of purines in the nervous system, knowledge regarding their metabolism in neurons is limited. In the current studies, purine pools and their metabolism were examined in rat PC6-3 cells, a PC12 pheochromocytoma subclone that undergoes robust differentiation with nerve growth factor. The results were compared with five new independent PC6-3 subclones with defective purine recycling because of different mutations affecting HGprt enzyme activity. The results demonstrate an increase in most purines and in energy state following neuronal differentiation, as well as specific abnormalities when purine recycling is lost. The loss of HGprt-mediated purine recycling also is associated with significant loss of dopamine and related metabolites in the mutant PC6-3 lines, suggesting an important connection between purine and dopamine pathways. These results provide insights into how purine pools and metabolism change with neuronal differentiation, and how specific enzyme defects may cause neuronal dysfunction. Differentiation of dopaminergic PC6-3 cells is accompanied by increased purine pools and energy state. The lack of a functional purine recycling pathway causes purine limitation in both undifferentiated and differentiated cells, as well as profound loss of dopamine content. The results imply an unknown mechanism by which intracellular purine levels regulate dopamine levels.

Role of human hypoxanthine guanine phosphoribosyltransferase in activation of the antiviral agent T-705 (favipiravir)

6-Fluoro-3-hydroxy-2-pyrazinecarboxamide (T-705) is a novel antiviral compound with broad activity against influenza virus and diverse RNA viruses. Its active metabolite, T-705-ribose-5'-triphosphate (T-705-RTP), is recognized by influenza virus RNA polymerase as a substrate competing with GTP, giving inhibition of viral RNA synthesis and lethal virus mutagenesis. Which enzymes perform the activation of T-705 is unknown. We here demonstrate that human hypoxanthine guanine phosphoribosyltransferase (HGPRT) converts T-705 into its ribose-5'-monophosphate (RMP) prior to formation of T-705-RTP. The anti-influenza virus activity of T-705 and T-1105 (3-hydroxy-2-pyrazinecarboxamide; the analog lacking the 6-fluoro atom) was lost in HGPRT-deficient Madin-Darby canine kidney cells. This HGPRT dependency was confirmed in human embryonic kidney 293T cells undergoing HGPRT-specific gene knockdown followed by influenza virus ribonucleoprotein reconstitution. Knockdown for adenine phosphoribosyltransferase (APRT) or nicotinamide phosphoribosyltransferase did not change the antiviral activity of T-705 and T-1105. Enzymatic assays showed that T-705 and T-1105 are poor substrates for human HGPRT having Km(app) values of 6.4 and 4.1 mM, respectively. Formation of the RMP metabolites by APRT was negligible, and so was the formation of the ribosylated metabolites by human purine nucleoside phosphorylase. Phosphoribosylation and antiviral activity of the 2-pyrazinecarboxamide derivatives was shown to require the presence of the 3-hydroxyl but not the 6-fluoro substituent. The crystal structure of T-705-RMP in complex with human HGPRT showed how this compound binds in the active site. Since conversion of T-705 by HGPRT appears to be inefficient, T-705-RMP prodrugs may be designed to increase the antiviral potency of this new antiviral agent.

The cellular and molecular carcinogenic effects of radon exposure: a review

Radon-222 is a naturally occurring radioactive gas that is responsible for approximately half of the human annual background radiation exposure globally. Chronic exposure to radon and its decay products is estimated to be the second leading cause of lung cancer behind smoking, and links to other forms of neoplasms have been postulated. Ionizing radiation emitted during the radioactive decay of radon and its progeny can induce a variety of cytogenetic effects that can be biologically damaging and result in an increased risk of carcinogenesis. Suggested effects produced as a result of alpha particle exposure from radon include mutations, chromosome aberrations, generation of reactive oxygen species, modification of the cell cycle, up or down regulation of cytokines and the increased production of proteins associated with cell-cycle regulation and carcinogenesis. A number of potential biomarkers of exposure, including translocations at codon 249 of TP53 in addition to HPRT mutations, have been suggested although, in conclusion, the evidence for such hotspots is insufficient. There is also substantial evidence of bystander effects, which may provide complications when calculating risk estimates as a result of exposure, particularly at low doses where cellular responses often appear to deviate from the linear, no-threshold hypothesis. At low doses, effects may also be dependent on cellular conditions as opposed to dose. The cellular and molecular carcinogenic effects of radon exposure have been observed to be both numerous and complex and the elevated chronic exposure of man may therefore pose a significant public health risk that may extend beyond the association with lung carcinogenesis.

Metabolic disorders of purine metabolism affecting the nervous system

The purines are a group of molecules used by all cells for many vital biochemical processes including energy-requiring enzymatic reactions, cofactor-requiring reactions, synthesis of DNA or RNA, signaling pathways within and between cells, and other processes. Defects in some of the enzymes of purine metabolism are known to be associated with specific clinical disorders, and neurological problems may be a presenting sign or the predominant clinical problem for several of them. This chapter describes three disorders for which the clinical features and metabolic basis are well characterized. Deficiency of adenylosuccinate-lyase (ADSL) causes psychomotor retardation, epilepsy, and autistic features. Lesch-Nyhan disease is caused by deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT) and is characterized by hyperuricemia, motor and cognitive disability, and self-injurious behavior. Deficiency of myoadenylate deaminase (mAMPD) is associated with myopathic features. In addition to these disorders, several other disorders are briefly summarized. These include defects of phosphoribosylpyrophosphate synthase, adenosine deaminase (ADA), purine nucleoside phosphorylase (PND), deoxyguanosine kinase (dGK), or IMP dehydrogenase (IMPDH). Each of these disorders provides an unusual window on the unique importance of purine metabolism for function of different parts of the nervous system.