Consequences of impaired purine recycling in dopaminergic neurons

Psychiatric Manifestations in Children and Adolescents with Inherited Metabolic Diseases

Background: Inherited metabolic disorders (IEMs) can be represented in children and adolescents by psychiatric disorders. The early diagnosis of IEMs is crucial for clinical outcome and treatment. The aim of this review is to analyze the most recurrent and specific psychiatric features related to IEMs in pediatrics, based on the onset type and psychiatric phenotypes. Methods: Following the PRISMA Statement, a systematic literature review was performed using a predefined algorithm to find suitable publications in scientific databases of interest. After removing duplicates and screening titles and abstracts, suitable papers were analyzed and screened for inclusion and exclusion criteria. Finally, the data of interest were retrieved from the remaining articles. Results: The results of this study are reported by type of symptoms onset (acute and chronic) and by possible psychiatric features related to IEMs. Psychiatric phenomenology has been grouped into five main clinical manifestations: mood and anxiety disorders; schizophrenia-spectrum disorders; catatonia; eating disorders; and self-injurious behaviors. Conclusions: The inclusion of a variety of psychiatric manifestations in children and adolescents with different IEMs is a key strength of this study, which allowed us to explore the facets of seemingly different disorders in depth, avoiding possible misdiagnoses, with the related delay of early and appropriate treatments.

Purine nucleoside phosphorylase enables dual metabolic checkpoints that prevent T cell immunodeficiency and TLR7-associated autoimmunity

Purine nucleoside phosphorylase (PNP) enables the breakdown and recycling of guanine nucleosides. PNP insufficiency in humans is paradoxically associated with both immunodeficiency and autoimmunity, but the mechanistic basis for these outcomes is incompletely understood. Here, we identify two immune lineage-dependent consequences of PNP inactivation dictated by distinct gene interactions. During T cell development, PNP inactivation is synthetically lethal with downregulation of the dNTP triphosphohydrolase SAMHD1. This interaction requires deoxycytidine kinase activity and is antagonized by microenvironmental deoxycytidine. In B lymphocytes and macrophages, PNP regulates Toll-like receptor 7 signaling by controlling the levels of its (deoxy)guanosine nucleoside ligands. Overriding this regulatory mechanism promotes germinal center formation in the absence of exogenous antigen and accelerates disease in a mouse model of autoimmunity. This work reveals that one purine metabolism gene protects against immunodeficiency and autoimmunity via independent mechanisms operating in distinct immune lineages and identifies PNP as a potentially novel metabolic immune checkpoint.

HGprt deficiency disrupts dopaminergic circuit development in a genetic mouse model of Lesch–Nyhan disease

In Lesch–Nyhan disease (LND), deficiency of the purine salvage enzyme hypoxanthine guanine phosphoribosyl transferase (HGprt) leads to a characteristic neurobehavioral phenotype dominated by dystonia, cognitive deficits and incapacitating self-injurious behavior. It has been known for decades that LND is associated with dysfunction of midbrain dopamine neurons, without overt structural brain abnormalities. Emerging post mortem and in vitro evidence supports the hypothesis that the dopaminergic dysfunction in LND is of developmental origin, but specific pathogenic mechanisms have not been revealed. In the current study, HGprt deficiency causes specific neurodevelopmental abnormalities in mice during embryogenesis, particularly affecting proliferation and migration of developing midbrain dopamine (mDA) neurons. In mutant embryos at E14.5, proliferation was increased, accompanied by a decrease in cell cycle exit and the distribution and orientation of dividing cells suggested a premature deviation from their migratory route. An abnormally structured radial glia-like scaffold supporting this mDA neuronal migration might lie at the basis of these abnormalities. Consequently, these abnormalities were associated with an increase in area occupied by TH⁺ cells and an abnormal mDA subpopulation organization at E18.5. Finally, dopaminergic innervation was disorganized in prefrontal and decreased in HGprt deficient primary motor and somatosensory cortices. These data provide direct in vivo evidence for a neurodevelopmental nature of the brain disorder in LND. Future studies should not only focus the specific molecular mechanisms underlying the reported neurodevelopmental abnormalities, but also on optimal timing of therapeutic interventions to rescue the DA neuron defects, which may also be relevant for other neurodevelopmental disorders.

Lesch-Nyhan disease causes impaired energy metabolism and reduced developmental potential in midbrain dopaminergic cells

Mutations in HPRT1, a gene encoding a rate-limiting enzyme for purine salvage, cause Lesch-Nyhan disease which is characterized by self-injury and motor impairments. We leveraged stem cell and genetic engineering technologies to model the disease in isogenic and patient-derived forebrain and midbrain cell types. Dopaminergic progenitor cells deficient in HPRT showed decreased intensity of all developmental cell-fate markers measured. Metabolic analyses revealed significant loss of all purine derivatives, except hypoxanthine, and impaired glycolysis and oxidative phosphorylation. real-time glucose tracing demonstrated increased shunting to the pentose phosphate pathway for de novo purine synthesis at the expense of ATP production. Purine depletion in dopaminergic progenitor cells resulted in loss of RHEB, impairing mTORC1 activation. These data demonstrate dopaminergic-specific effects of purine salvage deficiency and unexpectedly reveal that dopaminergic progenitor cells are programmed to a high-energy state prior to higher energy demands of terminally differentiated cells.

Induced pluripotent stem cells from subjects with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is an inherited disorder caused by pathogenic variants in the HPRT1 gene, which encodes the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase (HGprt). We generated 6 induced pluripotent stem cell (iPSC) lines from 3 individuals with LND, along with 6 control lines from 3 normal individuals. All 12 lines had the characteristics of pluripotent stem cells, as assessed by immunostaining for pluripotency markers, expression of pluripotency genes, and differentiation into the 3 primary germ cell layers. Gene expression profiling with RNAseq demonstrated significant heterogeneity among the lines. Despite this heterogeneity, several anticipated abnormalities were readily detectable across all LND lines, including reduced HPRT1 mRNA. Several unexpected abnormalities were also consistently detectable across the LND lines, including decreases in FAR2P1 and increases in RNF39. Shotgun proteomics also demonstrated several expected abnormalities in the LND lines, such as absence of HGprt protein. The proteomics study also revealed several unexpected abnormalities across the LND lines, including increases in GNAO1 decreases in NSE4A. There was a good but partial correlation between abnormalities revealed by the RNAseq and proteomics methods. Finally, functional studies demonstrated LND lines had no HGprt enzyme activity and resistance to the toxic pro-drug 6-thioguanine. Intracellular purines in the LND lines were normal, but they did not recycle hypoxanthine. These cells provide a novel resource to reveal insights into the relevance of heterogeneity among iPSC lines and applications for modeling LND.

Lesch-Nyhan syndrome and its variants: examining the behavioral and neurocognitive phenotype

PURPOSE OF REVIEW

Lesch-Nyhan Syndrome (LNS) is a metabolic disorder involving mutations in the HGPRT1 gene that result in hyperuricemia, intellectual disability, a dystonic movement disorder, and compulsive self-injury with self-mutilation. The aim of this review is to summarize recent research that documents the extended behavioral, neurologic, and neurocognitive phenotype in classic LNS, to describe milder variants of HGprt deficiency that do not self-injure and have less severe neurological and cognitive deficits, and to provide an update on treatment for associated psychiatric and behavioral disorders.

RECENT FINDINGS

Psychiatric management utilizes combined behavioral and pharmacological treatment in conjunction with protective equipment and dental management to avert self-injury. Pharmacological management focuses on stabilization of mood and anxiety management. S-adenosylmethionine (SAMe), a physiological intermediate in methylation and transsulfuration, has shown beneficial effects in carefully selected patients who can tolerate the drug. Deep brain stimulation is shown in several case reports and series to reduce or eliminate self-injury and aggression, and in some cases, modify dystonia.

SUMMARY

This review highlights progress in our understanding of the behavioral and neurocognitive phenotype of Lesch-Nyhan syndrome (HGprt deficiency) and its variants, describes psychiatric and behavioral management, and discusses prospects for new therapies.

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.

The Use of Perinatal 6-Hydroxydopamine to Produce a Rodent Model of Lesch-Nyhan Disease

Lesch-Nyhan disease is a neurologically, metabolically, and behaviorally devastating condition that has eluded complete characterization and adequate treatment. While it is known that the disease is intimately associated with dysfunction of the hypoxanthine phosphoribosyltransferase 1 (HPRT1) gene that codes for an enzyme of purine metabolism (hypoxanthine-guanine phosphoribosyltransferase) and is associated with neurological, behavioral, as well as metabolic dysfunction, the mechanisms of the neurobehavioral manifestations are as yet unclear. However, discoveries over the past few decades not only have created useful novel animal models (e.g., the HPRT-deficient mouse and the serendipitously discovered perinatal 6-hydroxydopamine (6-OHDA lesion model), but also have expanded into epigenetic, genomic, and proteomic approaches to better understand the mechanisms underlying this disease. The perinatal 6-OHDA model, in addition to modeling self-injury and dopamine depletion in the clinical condition, also underscores the profound importance of development in the differential course of maladaptive progression in the face of a common/single neurotoxic insult at different ages. Recent developments from clinical and basic science efforts attest to the fact that while the disease would seem to have a simple single gene defect at its core, the manifestations of this defect are profound and unexpectedly diverse. Future efforts employing the 6-OHDA model and others in the context of the novel technologies of genome editing, chemo- and opto-genetics, epigenetics, and further studies on the mechanisms of stress-induced maladaptations in brain all hold promise in taking our understanding of this disease to the next level.

Consequences of impaired purine recycling on the proteome in a cellular model of Lesch-Nyhan disease

The importance of specific pathways of purine metabolism for normal brain function is highlighted by several inherited disorders, such as Lesch-Nyhan disease (LND). In this disorder, deficiency of the purine recycling enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt), causes severe neurological and behavioral abnormalities. Despite many years of research, the mechanisms linking the defect in purine recycling to the neurobehavioral abnormalities remain unclear. In the current studies, an unbiased approach to the identification of potential mechanisms was undertaken by examining changes in protein expression in a model of HGprt deficiency based on the dopaminergic rat PC6-3 line, before and after differentiation with nerve growth factor (NGF). Protein expression profiles of 5 mutant sublines carrying different mutations affecting HGprt enzyme activity were compared to the HGprt-competent parent line using the method of stable isotopic labeling by amino acids in cell culture (SILAC) followed by denaturing gel electrophoresis with liquid chromatography and tandem mass spectrometry (LC-MS/MS) of tryptic digests, and subsequent identification of affected biochemical pathways using the Database for Annotation, Visualization and Integrated Discovery (DAVID) functional annotation chart analysis. The results demonstrate that HGprt deficiency causes broad changes in protein expression that depend on whether the cells are differentiated or not. Several of the pathways identified reflect predictable consequences of defective purine recycling. Other pathways were not anticipated, disclosing previously unknown connections with purine metabolism and novel insights into the pathogenesis of LND.

Transcriptomic approach to Lesch-Nyhan disease

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

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.

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.

Striatal neurodevelopment is dysregulated in purine metabolism deficiency and impacts DARPP-32, BDNF/TrkB expression and signaling: new insights on the molecular and cellular basis of Lesch-Nyhan Syndrome

Lesch-Nyhan Syndrome (LNS) is a neurodevelopmental disorder caused by mutations in the gene encoding the purine metabolic enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). This syndrome is characterized by an array of severe neurological impairments that in part originate from striatal dysfunctions. However, the molecular and cellular mechanisms underlying these dysfunctions remain largely unidentified. In this report, we demonstrate that HPRT-deficiency causes dysregulated expression of key genes essential for striatal patterning, most notably the striatally-enriched transcription factor B-cell leukemia 11b (Bcl11b). The data also reveal that the down-regulated expression of Bcl11b in HPRT-deficient immortalized mouse striatal (STHdh) neural stem cells is accompanied by aberrant expression of some of its transcriptional partners and other striatally-enriched genes, including the gene encoding dopamine- and cAMP-regulated phosphoprotein 32, (DARPP-32). Furthermore, we demonstrate that components of the BDNF/TrkB signaling, a known activator of DARPP-32 striatal expression and effector of Bcl11b transcriptional activation are markedly increased in HPRT-deficient cells and in the striatum of HPRT knockout mouse. Consequently, the HPRT-deficient cells display superior protection against reactive oxygen species (ROS)-mediated cell death upon exposure to hydrogen peroxide. These findings suggest that the purine metabolic defect caused by HPRT-deficiency, while it may provide neuroprotection to striatal neurons, affects key genes and signaling pathways that may underlie the neuropathogenesis of LNS.

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

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

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

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

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.

Deficiency of the purine metabolic gene HPRT dysregulates microRNA-17 family cluster and guanine-based cellular functions: a role for EPAC in Lesch-Nyhan syndrome

Lesch-Nyhan syndrome (LNS) is a neurodevelopmental disorder caused by mutations in the gene encoding the purine metabolic enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). A series of motor, cognitive and neurobehavioral anomalies characterize this disease phenotype, which is still poorly understood. The clinical manifestations of this syndrome are believed to be the consequences of deficiencies in neurodevelopmental pathways that lead to disordered brain function. We have used microRNA array and gene ontology analysis to evaluate the gene expression of differentiating HPRT-deficient human neuron-like cell lines. We set out to identify dysregulated genes implicated in purine-based cellular functions. Our approach was based on the premise that HPRT deficiency affects preeminently the expression and the function of purine-based molecular complexes, such as guanine nucleotide exchange factors (GEFs) and small GTPases. We found that several microRNAs from the miR-17 family cluster and genes encoding GEF are dysregulated in HPRT deficiency. Most notably, our data show that the expression of the exchange protein activated by cAMP (EPAC) is blunted in HPRT-deficient human neuron-like cell lines and fibroblast cells from LNS patients, and is altered in the cortex, striatum and midbrain of HPRT knockout mouse. We also show a marked impairment in the activation of small GTPase RAP1 in the HPRT-deficient cells, as well as differences in cytoskeleton dynamics that lead to increased motility for HPRT-deficient neuron-like cell lines relative to control. We propose that the alterations in EPAC/RAP1 signaling and cell migration in HPRT deficiency are crucial for neuro-developmental events that may contribute to the neurological dysfunctions in LNS.

HPRT-deficiency dysregulates cAMP-PKA signaling and phosphodiesterase 10A expression: mechanistic insight and potential target for Lesch-Nyhan Disease?

Lesch-Nyhan Disease (LND) is the result of mutations in the X-linked gene encoding the purine metabolic enzyme, hypoxanthine guanine phosphoribosyl transferase (HPRT). LND gives rise to severe neurological anomalies including mental retardation, dystonia, chorea, pyramidal signs and a compulsive and aggressive behavior to self injure. The neurological phenotype in LND has been shown to reflect aberrant dopaminergic signaling in the basal ganglia, however there are little data correlating the defect in purine metabolism to the neural-related abnormalities. In the present studies, we find that HPRT-deficient neuronal cell lines have reduced CREB (cAMP response element-binding protein) expression and intracellular cyclic AMP (cAMP), which correlates with attenuated CREB-dependent transcriptional activity and a reduced phosphorylation of protein kinase A (PKA) substrates such as synapsin (p-syn I). Of interest, we found increased expression of phosphodiesterase 10A (PDE10A) in HPRT-deficient cell lines and that the PDE10 inhibitor papaverine and PDE10A siRNA restored cAMP/PKA signaling. Furthermore, reconstitution of HPRT expression in mutant cells partly increased cAMP signaling synapsin phosphorylation. In conclusion, our data show that HPRT-deficiency alters cAMP/PKA signaling pathway, which is in part due to the increased of PDE10A expression and activity. These findings suggest a mechanistic insight into the possible causes of LND and highlight PDE10A as a possible therapeutic target for this intractable neurological disease.

Transiently transfected purine biosynthetic enzymes form stress bodies

It has been hypothesized that components of enzymatic pathways might organize into intracellular assemblies to improve their catalytic efficiency or lead to coordinate regulation. Accordingly, de novo purine biosynthesis enzymes may form a purinosome in the absence of purines, and a punctate intracellular body has been identified as the purinosome. We investigated the mechanism by which human de novo purine biosynthetic enzymes might be organized into purinosomes, especially under differing cellular conditions. Irregardless of the activity of bodies formed by endogenous enzymes, we demonstrate that intracellular bodies formed by transiently transfected, fluorescently tagged human purine biosynthesis proteins are best explained as protein aggregation.

Ageing as a primary risk factor for Parkinson's disease: evidence from studies of non-human primates

Ageing is the greatest risk factor for the development of Parkinson's disease. However, the current dogma holds that cellular mechanisms that are associated with ageing of midbrain dopamine neurons and those that are related to dopamine neuron degeneration in Parkinson's disease are unrelated. We propose, based on evidence from studies of non-human primates, that normal ageing and the degeneration of dopamine neurons in Parkinson's disease are linked by the same cellular mechanisms and, therefore, that markers of cellular risk factors accumulate with age in a pattern that mimics the pattern of degeneration observed in Parkinson's disease. We contend that ageing induces a pre-parkinsonian state, and that the cellular mechanisms of dopamine neuron demise during normal ageing are accelerated or exaggerated in Parkinson's disease through a combination of genetic and environmental factors.

HPRT deficiency coordinately dysregulates canonical Wnt and presenilin-1 signaling: a neuro-developmental regulatory role for a housekeeping gene?

We have used microarray-based methods of global gene expression together with quantitative PCR and Western blot analysis to identify dysregulation of genes and aberrant cellular processes in human fibroblasts and in SH-SY5Y neuroblastoma cells made HPRT-deficient by transduction with a retrovirus stably expressing an shRNA targeted against HPRT. Analysis of the microarray expression data by Gene ontology (GO) and Gene Set Enrichment Analysis (GSEA) as well as significant pathway analysis by GeneSpring GX10 and Panther Classification System reveal that HPRT deficiency is accompanied by aberrations in a variety of pathways known to regulate neurogenesis or to be implicated in neurodegenerative disease, including the canonical Wnt/β-catenin and the Alzheimer's disease/presenilin signaling pathways. Dysregulation of the Wnt/β-catenin pathway is confirmed by Western blot demonstration of cytosolic sequestration of β-catenin during in vitro differentiation of the SH-SY5Y cells toward the neuronal phenotype. We also demonstrate that two key transcription factor genes known to be regulated by Wnt signaling and to be vital for the generation and function of dopaminergic neurons; i.e., Lmx1a and Engrailed 1, are down-regulated in the HPRT knockdown SH-SY5Y cells. In addition to the Wnt signaling aberration, we found that expression of presenilin-1 shows severely aberrant expression in HPRT-deficient SH-SY5Y cells, reflected by marked deficiency of the 23 kDa C-terminal fragment of presenilin-1 in knockdown cells. Western blot analysis of primary fibroblast cultures from two LND patients also shows dysregulated presenilin-1 expression, including aberrant proteolytic processing of presenilin-1. These demonstrations of dysregulated Wnt signaling and presenilin-1 expression together with impaired expression of dopaminergic transcription factors reveal broad pleitropic neuro-regulatory defects played by HPRT expression and suggest new directions for investigating mechanisms of aberrant neurogenesis and neuropathology in LND and potential new targets for restoration of effective signaling in this neuro-developmental defect.

II: Effects of a dopamine receptor antagonist on fathead minnow dominance behavior and ovarian gene expression in the fathead minnow and zebrafish

Neurotransmitters such as dopamine play an important role in reproductive behaviors and signaling. Neuroendocrine-active chemicals in the environment have potential to interfere with and/or alter these processes. A companion study with the dopamine 2 receptor antagonist, haloperidol, found no evidence of a direct effect of the chemical on fish reproduction. This study considered haloperidol's potential effects on behavior and ovarian gene expression. Male fathead minnows exposed to 50 microg haloperidol/L for 96 h were found to be significantly more dominant than control males. In terms of molecular signaling, investigated using oligonucleotide microarrays, there was little similarity in the identity and functions of genes differentially expressed in the ovaries of fathead minnows (Pimephales promelas) versus zebrafish (Danio rerio) exposed under the same conditions. Results suggest that non-lethal concentrations of haloperidol do not induce ovarian molecular responses that could serve as biomarkers of exposure to D2R antagonists, but may impact behavior.

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.

Human neural stem cells: a model system for the study of Lesch-Nyhan disease neurological aspects

The study of Lesch-Nyhan-diseased (LND) human brain is crucial for understanding how mutant hypoxanthine-phosphoribosyltransferase (HPRT) might lead to neuronal dysfunction. Since LND is a rare, inherited disorder caused by a deficiency of the enzyme HPRT, human neural stem cells (hNSCs) that carry this mutation are a precious source for delineating the consequences of HPRT deficiency and for developing new treatments. In our study we have examined the effect of HPRT deficiency on the differentiation of neurons in hNSCs isolated from human LND fetal brain. We have examined the expression of a number of transcription factors essential for neuronal differentiation and marker genes involved in dopamine (DA) biosynthetic pathway. LND hNSCs demonstrate aberrant expression of several transcription factors and DA markers. HPRT-deficient dopaminergic neurons also demonstrate a striking deficit in neurite outgrowth. These results represent direct experimental evidence for aberrant neurogenesis in LND hNSCs and suggest developmental roles for other housekeeping genes in neurodevelopmental disease. Moreover, exposure of the LND hNSCs to retinoic acid medium elicited the generation of dopaminergic neurons. The lack of precise understanding of the neurological dysfunction in LND has precluded development of useful therapies. These results evidence aberrant neurogenesis in LND hNSCs and suggest a role for HPRT gene in neurodevelopment. These cells combine the peculiarity of a neurodevelopmental model and a human, neural origin to provide an important tool to investigate the pathophysiology of HPRT deficiency and more broadly demonstrate the utility of human neural stem cells for studying the disease and identifying potential therapeutics.

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

A deficiency in hypoxanthine guanine phosphoribosyltransferase (HPRT) activity leads to overproduction of uric acid. According to the degree of enzymatic deficiency, a large spectrum of neurologic features can also be observed, ranging from mild or no neurologic involvement to complete Lesch-Nyhan disease. Herein, we describe a patient with hyperuricemia, juvenile-onset gouty arthritis, nephrolithiasis, and mild neurologic symptoms, attributed to a newly identified variant of the hprt gene, c.596T>G, resulting in the amino acid change p.F199C. Residual HPRT activity (8%) protected against severe neurologic involvement in this patient. Modeling of the mutated protein was used to predict the mechanisms that led to partial enzymatic activity. Careful neurologic examination is warranted in juvenile and middle-aged patients with gout, in order to detect mild symptoms that may lead to a diagnosis of HPRT deficiency.

Lesch-Nyhan disease: from mechanism to model and back again

Lesch-Nyhan disease (LND) is a rare inherited disorder caused by mutations in the gene encoding hypoxanthine-guanine phosphoribosyltransferase (HPRT). LND is characterized by overproduction of uric acid, leading to gouty arthritis and nephrolithiasis. Affected patients also have characteristic neurological and behavioral anomalies. Multiple cell models have been developed to study the molecular and metabolic aspects of LND, and several animal models have been developed to elucidate the basis for the neurobehavioral syndrome. The models have different strengths and weaknesses rendering them suitable for studying different aspects of the disease. The extensive modeling efforts in LND have questioned the concept that an 'ideal' disease model is one that replicates all of its features because the pathogenesis of different elements of the disease involves different mechanisms. Instead, the modeling efforts have suggested a more fruitful approach that involves developing specific models, each tailored for addressing specific experimental questions.

Deficiency of the housekeeping gene hypoxanthine-guanine phosphoribosyltransferase (HPRT) dysregulates neurogenesis

Neuronal transcription factors play vital roles in the specification and development of neurons, including dopaminergic (DA) neurons. Mutations in the gene encoding the purine biosynthetic enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) cause the resulting intractable and largely untreatable neurological impairment of Lesch-Nyhan disease (LND). The disorder is associated with a defect in basal ganglia DA pathways. The mechanisms connecting the purine metabolic defect and the central nervous system (CNS) phenotype are poorly understood but have been presumed to reflect a developmental defect of DA neurons. We have examined the effect of HPRT deficiency on the differentiation of neurons in the well-established human (NT2) embryonic carcinoma neurogenesis model. We have used a retrovirus expressing a small hairpin RNA (shRNA) to knock down HPRT gene expression and have examined the expression of a number of transcription factors essential for neuronal differentiation and marker genes involved in DA biosynthetic pathway. HPRT-deficient NT2 cells demonstrate aberrant expression of several transcription factors and DA markers. Although differentiated HPRT-deficient neurons also demonstrate a striking deficit in neurite outgrowth during differentiation, resulting neurons demonstrate wild-type electrophysiological properties. These results represent direct experimental evidence for aberrant neurogenesis in HPRT deficiency and suggest developmental roles for other housekeeping genes in neurodevelopmental disease.

Hypoxanthine-guanine phosphoribosyl transferase regulates early developmental programming of dopamine neurons: implications for Lesch-Nyhan disease pathogenesis

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency results in Lesch-Nyhan disease (LND), where affected individuals exhibit a characteristic neurobehavioral disorder that has been linked with dysfunction of dopaminergic pathways of the basal ganglia. Since the functions of HPRT, a housekeeping enzyme responsible for recycling purines, have no direct relationships with the dopaminergic pathways, the mechanisms whereby HPRT deficiency affect them remain unknown. The current studies demonstrate that HPRT deficiency influences early developmental processes controlling the dopaminergic phenotype, using several different cell models for HPRT deficiency. Microarray methods and quantitative PCR were applied to 10 different HPRT-deficient (HPRT(-)) sublines derived from the MN9D cell line. Despite the variation inherent in such mutant sublines, several consistent abnormalities were evident. Most notable were increases in the mRNAs for engrailed 1 and 2, transcription factors known to play a key role in the specification and survival of dopamine neurons. The increases in mRNAs were accompanied by increases in engrailed proteins, and restoration of HPRT reverted engrailed expression towards normal levels, demonstrating a functional relationship between HPRT and engrailed. The functional relevance of the abnormal developmental molecular signature of the HPRT(-) MN9D cells was evident in impoverished neurite outgrowth when the cells were forced to differentiate chemically. To verify that these abnormalities were not idiosyncratic to the MN9D line, HPRT(-) sublines from the SK-N-BE(2) M17 human neuroblastoma line were evaluated and an increased expression of engrailed mRNAs was also seen. Over-expression of engrailed occurred even in primary fibroblasts from patients with LND in a manner that suggested a correlation with disease severity. These results provide novel evidence that HPRT deficiency may affect dopaminergic neurons by influencing early developmental mechanisms.

Is ZMP the toxic metabolite in Lesch-Nyhan disease?

The genetic deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT), located on the X chromosome, causes a severe neurological disorder in man, known as Lesch-Nyhan disease (LND). The enzyme HPRT is part of the savage pathway of purine biosynthesis and catalyzes the conversion of hypoxanthine and guanine to their respective nucleotides, IMP and GMP. HPRT deficiency is associated with a relatively selective dysfunction of brain dopamine systems. Several metabolites that accumulate in the patients (phosphoribosylpyrophosphate (PRPP), hypoxanthine, guanine, xanthine, and Z-nucleotides) have been proposed as toxic agents in LND. Some authors have pointed that Z-riboside, derived from the accumulation of ZMP, could be the toxic metabolite in LND. However, the available experimental data support a better hypothesis. I suggest that ZMP (and not Z-riboside) is the key toxic metabolite in LND. ZMP is an inhibitor of the bifunctional enzyme adenylosuccinate lyase, and a deficiency of this enzyme causes psychomotor and mental retardation in humans. Moreover, it has been reported that ZMP inhibits mitochondrial oxidative phosphorylation and induces apoptosis in certain cell types. ZMP is also an activator of the AMP-activated protein kinase (AMPK), a homeostatic regulator of energy levels in the cell. The AMPK has been implicated in the regulation of cell viability, catecholamine biosynthesis and cell structure. I propose that accumulation of ZMP will induce a pleiotropic effect in the brain by (1) a direct inhibition of mitochondrial respiration and the bifunctional enzyme adenylosuccinate lyase, and (2) a sustained activation of the AMPK which in turns would reduce cell viability, decrease dopamine synthesis, and alters cell morphology. In addition, a mechanism to explain the accumulation of ZMP in LND is presented. The knowledge of the toxic metabolite, and the way it acts, would help to design a better therapy.