Normal HPRT coding region in a male with gout due to HPRT deficiency

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.

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.

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.

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.

Lesch-Nyhan syndrome: mRNA expression of HPRT in patients with enzyme proven deficiency of HPRT and normal HPRT coding region of the DNA

Inherited mutation of the purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT) gives rise to Lesch-Nyhan syndrome (LNS) or Lesch-Nyhan variants (LNV). We report a case of two LNS affected members of a family with deficiency of activity of HPRT in intact cultured fibroblasts in whom mutation could not be found in the HPRT coding sequence but there was markedly decreased HPRT expression of mRNA.

Novel mutations in the human HPRT gene

Inherited mutation of a purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT), gives rise to Lesch-Nyhan Syndrome (LNS) or HPRT-related gout. Here, we report five novel independent mutations in the coding region of the HPRT gene from five unrelated male patients manifesting different clinical phenotypes associated with LNS: exon 2: c.133A > G, p.45R > G; c.35A > C, p.12D > A; c.88delG; exon 7: c.530A > T, p.177D > V; and c.318 + 1G > C: IVS3 + 1G > C splice site mutation.

Methylation status of HPRT1 promoter in HPRT deficiency with normal coding region

Lesch-Nyhan syndrome is an X-linked recessive inborn error of metabolism due to a complete deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity (OMIM 300322). Partial deficiency of HPRT (OMIM 300323) is characterized by the effects of excess uric acid synthesis and a continuum spectrum of neurological manifestations, without the manifestations of full-blown Lesch-Nyhan syndrome. Both diseases have been associated with mutations in the HPRT1 gene. We have described one Lesch-Nyhan patient and four partial HPRT deficient patients with a normal HPRT1 coding region. These patients showed markedly decreased HPRT mRNA expression, but no mutation in their genomic regulatory sequences from HPRT1 gene. In this study, we analyzed the promoter region methylation status of the HPRT1 gene in these five HPRT deficient patients.

METHODS

DNA was bisulphite modified and a 620 bp fragment including 320 bp 5' to start codon was amplified and sequenced. The methylation status of 35 CpG island 5' to start codon and 28 CpG island 3' to start codon were investigated in male controls, female controls, patients, and the patient's mothers. Primer pairs were designed for methylated-specific and unmethylated-specific amplification and PCR was performed employing DNA bisulphite treated as template.

RESULTS

No alterations in the methylation pattern of the HPRT1 promoter were found in the five HPRT deficient patients.

CONCLUSIONS

The promoter region methylation status of these five HPRT deficient patients was similar to that of normal subjects. Thus, some other genetic alteration must explain a reduced enzyme activity with a normal gene coding region.

Plasmodium falciparum: new molecular targets with potential for antimalarial drug development

Malaria remains one of the world's most devastating infectious diseases. Drug resistance to all classes of antimalarial agents has now been observed, highlighting the need for new agents that act against novel parasite targets. The complete sequencing of the Plasmodium falciparum genome has allowed the identification of new molecular targets within the parasite that may be amenable to chemotherapeutic intervention. In this review, we investigate four possible targets for the future development of new classes of antimalarial agents. These targets include histone deacetylase, the aspartic proteases or plasmepsins, aminopeptidases and the purine salvage enzyme hypoxanthine-xanthine-guanine phosphoribosyltransferase.

Attenuated variants of Lesch-Nyhan disease

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

Inhibition of hypoxanthine-guanine phosphoribosyltransferase by acyclic nucleoside phosphonates: a new class of antimalarial therapeutics

The purine salvage enzyme hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) is essential for purine nucleotide and hence nucleic acid synthesis in the malaria parasite, Plasmodium falciparum. Acyclic nucleoside phosphonates (ANPs) are analogues of the nucleotide product of the reaction, comprising a purine base joined by a linker to a phosphonate moiety. K(i) values for 19 ANPs were determined for Pf HGXPRT and the corresponding human enzyme, HGPRT. Values for Pf HGXPRT were as low as 100 nM, with selectivity for the parasite enzyme of up to 58. Structures of human HGPRT in complex with three ANPs are reported. On binding, a large mobile loop in the free enzyme moves to partly cover the active site. For three ANPs, the IC(50) values for Pf grown in cell culture were 1, 14, and 46 microM, while the cytotoxic concentration for the first compound was 489 microM. These results provide a basis for the design of potent and selective ANP inhibitors of Pf HGXPRT as antimalarial drug leads.

Inborn errors of purine and pyrimidine metabolism

Genetic disorders of purine and pyrimidine (PP) metabolism are under-reported and infrequently mentioned in the general literature, as well as in reviews dedicated to other inborn errors of metabolism. Owing to limited awareness, relatively recent recognition, as well as considerable phenotypic variation, these disorders may often be misdiagnosed or remain undiagnosed. Disorders that arise as a result of dysfunction in PP metabolism represent some of the most challenging diagnostic problems in medicine. In addition to their low prevalence rates, they also present with extremely variable signs and symptoms. They may affect any system in a variety of manners, and often mimic other, more recognizable disorders. The diagnostic problem is compounded by the fact that some biochemically affected patients are symptom-free. Rapidly evolving laboratory techniques such as high-performance liquid chromatography coupled to tandem mass spectrometry are now well established as the preferred method for detection for these defects, but currently the most important step in diagnosis consists of suspecting the disorder. Diagnosis is vital because genetic counselling can be provided and, in some cases, specific treatment can be offered that may slow or even reverse clinical symptoms. If undiagnosed, these disorders can be devastating to patients and their families, resulting in early death or institutionalization for the rest of patient's life. This article describes the current state of knowledge about inborn errors of purine and pyrimidine metabolism, focusing on the varying clinical presentations, the laboratory findings and discusses indications for selective screening for these disorders.

Hypoxanthine-guanine phosphoribosylotransferase deficiency--the spectrum of Polish mutations

Hypoxanthine-guanine phosphoribosyltransferase (HPRT; EC 2.4.2.8) deficiency (OMIM 308000) is an inborn error of purine metabolism. The defect causes three overlapping clinical syndromes: Lesch-Nyhan disease (LND; OMIM 300322), HPRT-related hyperuricaemia with neurologic dysfunction (HRND) and hyperuricaemia alone (HRH; OMIM 300322). During the period 1977-2007, 18 patients belonging to 12 Polish families and one Latvian family with HPRT deficiency have been identified. The majority of patients had a typical LND phenotype, three patients were classified as HRH and one patient as an intermediate phenotype (HRND). Genetic analysis revealed 12 different HPRT1 mutations, five of them being unique. In two typical Lesch-Nyhan families a novel single-base substitution, c.220T>G (p.Phe74Val), and a deletion of seven nucleotides, c.395_401del7 (p.Ile132LysfsX3), were found. Another novel single-base substitution, c.295T>G (p.Phe99Val), was identified in a patient with severe partial deficiency of HPRT with neurological dysfunction. In patients belonging to the HRH group, two transitions were detected: c.481G>A (p.Ala161Thr) and c.526C>T (p.Pro176Ser). Other mutations identified in Polish patients, c.131A>G (p.Asp44Gly), c.222C>A (p.Phe74Leu), c.385-1G>A (p.Asn129_Glu134del), c.482C>A (p.Ala161Glu), c.508C>T (p.Arg170Ter) and c.569G>A (p.Gly190Glu), have been reported previously in unrelated patients and are located within one of the clusters of hot spots of the HPRT1 gene (exons 3, 7 and 8). Patients with partial phenotypes presented mutations predicted to permit some degree of residual enzyme function (single-base substitutions). All mutations, except c.508C>T (p.Arg170Ter), were found in single families only, indicating the lack of any common mutation causing HPRT deficiency in Poland.

The diagnosis of HPRT deficiency in the 21st century

We have studied 36 patients with HPRT deficiency, 25 with Lesch-Nyhan syndrome and 11 with partial HPRT deficiency (grades 1 to 3). Patients diagnosed with HPRT deficiency have increased 50% since 2000. The most relevant recent advances have been made in molecular diagnosis. Nevertheless, enzyme determinations are still essential for the diagnosis of HPRT deficiency. Therapy for the neurological manifestations of HPRT deficiency has not advanced. Allopurinol remains the drug of choice to diminish uric acid overproduction, but the optimal allopurinol dose must be established in each patient to prevent xanthine or uric acid urolithiasis, a process aided by sequential determination of urinary oxypurines and uric acid.

Normal HPRT coding region in complete and partial HPRT deficiency

Lesch-Nyhan syndrome is an X-linked recessive inborn error of metabolism due to a virtually complete lack of hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity (OMIM 300322). Partial deficiency of HPRT (OMIM 300323) is characterized by the effects of excess uric acid synthesis and a continuum spectrum of neurological manifestations, without the manifestations of full-blown Lesch-Nyhan syndrome. Both diseases have been associated with mutations in the HPRT gene. These mutations are heterogeneous and disperse throughout the entire HPRT gene. In 2005 Dawson et al. described, for the first time, an individual with gout in whom HPRT deficiency appeared to be due to a defect in gene regulation. In the present study we present four patients with partial HPRT deficiency and one patient with Lesch-Nyhan syndrome who showed a normal HPRT coding sequence and markedly decreased HPRT mRNA expression. This is the first report of a patient with Lesch-Nyhan syndrome due to a defect in HPRT gene expression regulation.