Purine metabolism in heterozygous carriers of hypoxanthine-guanine phosphoribosyltransferase deficiency

The urate pool and daily turnover of urate, together with the rate of incorporation of glycine into urate, were measured in three asymptomatic mothers who had sons with various degrees of deficiency of hypoxanthine-guanine phosphoribosyltransferase activity. Two of these mothers had abnormally increased values for the urate pool, urate turnover, and 24-hour urinary excretion of uric acid. These two mothers also had reduced hypoxanthine-guanine phosphoribosyltransferase activity and increased adenine phosphoribosyltransferase activity in erythrocyte lysates. All three mothers showed an abnormal increase in urate production, as judged by the rate of incorporation of glycinie into urate.

Role of neuronal nitric oxide in the dopamine deficit of HPRT-deficient mice

Lesch-Nyhan disease is a debilitating disorder caused by a lack of purine salvage activity. Basal ganglia dopamine deficits manifest in both patients and hypoxanthine phosphoribosyltransferase (HPRT) mutant mice. We previously reported decreased activity in an oxidant sensitive enzyme in the brain of HPRT-deficient mice. In the present study, we have investigated whether one source of free radicals, neuronal nitric oxide synthase (NOS1), contributes to the dopamine deficit associated with HPRT deficiency. HPRT knockout and wild-type mice were bred, either to lack, or to have the full complement of NOS1 alleles. Double mutant mice had striatal dopamine and dopamine metabolite levels indistinguishable from the HPRT single mutant counterparts. These results indicate that NOS1 produced nitric oxide does not contribute to the dopamine deficit seen in HPRT deficiency.

Intrastriatal injection of hypoxanthine reduces striatal serotonin content and impairs spatial memory performance in rats

The aim of this study was to investigate the effects of intrastriatal injection of hypoxanthine, a metabolite accumulated in Lesch-Nyhan disease, on rats' performance in the Morris water maze tasks, along with the monoamine content in striatum of rats. Male adult Wistar rats were divided in two groups: (1) saline-injected and (2) hypoxanthine-injected group. Seven days after solutions infusion, animals were trained in the Morris Water Maze or were sacrificed for evaluation of the striatal monoamine content. Results show that hypoxanthine administration caused impairment on spatial navigation in the acquisition phase in reference memory task in the Morris Water Maze, as well as in the latency to cross over the platform location in probe trial, when compared to the saline group (control). Hypoxanthine also altered rat performance in the working memory. Although striatal dopamine metabolites content did not change, treated animals showed a reduction of tissue levels of serotonin (5-HT) and 5- hydroxyl-indoleacetic acid (5-HIAA). These results show that intra-striatal hypoxanthine administration provoked impairment of spatial learning/memory in rats without affecting striatal dopaminergic system, although serotonergic pathways seem to have been affected.

Intrastriatal hypoxanthine reduces Na(+),K (+)-ATPase activity and induces oxidative stress in the rats

The main objective of this study was to investigate the effects of a single intrastriatal injection of hypoxanthine, a metabolite accumulated in Lesch Nyhan disease and possibly involved in its neuropathology, on Na(+),K(+)-ATPase activity, as well as on some parameters of oxidative stress, namely chemiluminescence (an index of lipid peroxidation), total radical-trapping antioxidant parameter--TRAP (an index of total antioxidant capacity of the tissue) and total thiol protein membrane content, in striatum, cerebral cortex and hippocampus of rats. Results show that hypoxanthine significantly decreased Na(+),K(+)-ATPase activity and TRAP while increased chemiluminescence in all ipsislateral structures tested. However, no effect on total thiol protein membrane content was detected. We suggest that hypoxanthine induces oxidative stress in all cerebral structures studied (striatum, hippocampus and cerebral cortex) and that the reduction of Na(+),K(+)-ATPase activity was probably mediated by reactive oxygen species.

Study of the adenosinergic system in the brain of HPRT knockout mouse (Lesch-Nyhan disease)

BACKGROUND

Lesch-Nyhan disease (LND), an X-linked genetic disease caused by complete deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT), is characterized by hyperuricemia and psychiatric disturbance, mainly self-aggressiveness. Literature dates support the hypothesis that dopaminergic deficit and serotonergic excess in the circuit of basal ganglia are responsible for the aggressive behavior. Altered adenosine transport across the membrane of HPRT-deficient lymphocytes has been reported, suggesting adenosine involvement in LND.

METHODS

The expression of several genes related to the adenosinergic system (ADORA1A, ADORA2A, ADORA2B) were studied in the brain of the murine model of LND by real time PCR. Nucleotide levels and enzyme activities possibly involved in adenosine release were also measured.

RESULTS

Studies performed by real time PCR showed 95% increase in ADORA1A expression, 15% decrease in ADORA2A expression, and no change in ADORA2B expression in knockout mice compared to controls. No significant differences were found in the level of nucleotides or enzyme activities between control and mutant mice.

CONCLUSIONS

Our results suggest that adenosine neurotransmission might be involved in the specific neurobehavioral features of LND by increased expression of adenosine A1 receptors.

Molecular, biochemical, and genetic characterization of a female patient with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is a rare X-linked recessive disorder caused by virtually complete deficiency of activity of the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8). Human HPRT is encoded by a single structural gene located on the long arm of the X-chromosome (Xq26). The classical LND phenotype occurs almost exclusively in males, manifested in excessive purine production and characteristic neurological manifestations, including compulsive self-mutilation, choreoathetosis, spasticity, and occasionally developmental delay. Heterozygous females are usually phenotypically normal, due to the random inactivation of the X chromosome (Lyonization mechanism). However, six females were reported to be affected with the full biochemical and clinical manifestations of LND. All these cases were heterozygous for an HPRT mutation. Absence of transcription of the normal HPRT allele was attributed in all of them to non-random inactivation of the X chromosome carrying the normal allele. Here we describe an additional LND female, who presented with acute renal failure at the age of two months, in whom absence of transcription of the two HPRT alleles occurred due to as yet undescribed mechanism in LND females: the transcription of one HPRT allele was blocked due to a de novo X chromosome-autosome translocation 46,XX,t(X:2)(q26:p25), with a breaking point encompassing the HPRT gene locus, whereas the transcription of the normal allele was inhibited due to non-random inactivation of the second X-chromosome. Cultured fibroblasts from this patient exhibited the biochemical alterations in purine nucleotide metabolism characteristic of male LND fibroblasts.

A Golgi study of neuronal architecture in a genetic mouse model for Lesch–Nyhan disease

Lesch-Nyhan disease (LND) is an inherited disorder associated with deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT), an enzyme essential for purine recycling. The clinical manifestations of the disorder and several neurochemical studies have pointed towards a defect in the striatum, but histological studies of autopsied brain specimens have not revealed any consistent abnormalities. An HPRT-deficient (HPRT-) mouse that has been produced as a model for the disease also exhibits neurochemical abnormalities of the striatum without obvious histological correlates. In the current studies, Golgi-Cox histochemistry was used to evaluate the fine structure of medium spiny I neurons from the striatum in the HPRT- mice. To determine if any abnormalities might be restricted to striatal neurons, the pyramidal projection neurons of layer 5 of the cerebral cortex were also evaluated. Neurons from both regions demonstrated a normal distribution, orientation, and gross morphology. There was no evidence for an abnormal developmental process or degeneration. However, both regions demonstrated a paucity of neurons with very long dendrites and a reduction in dendritic spines that depended upon the distance from the cell body. These findings demonstrate that HPRT deficiency is associated with changes in neuronal architecture in the HPRT- mice. Similar abnormalities in the LND brain could underlie some of the clinical manifestations.

Altered membrane NTPase activity in Lesch-Nyhan disease fibroblasts: comparison with HPRT knockout mice and HPRT-deficient cell lines

Lesch-Nyhan disease (LND) is a rare disorder caused by a defect of an enzyme in the purine salvage pathway, hypoxanthine phosphoribosyl transferase (HPRT). It is still unknown how the metabolic defect translates into the complex neuropsychiatric phenotype characterized by self-injurious behavior, dystonia and mental retardation. There are abnormalities in purine and pyrimidine nucleotide content in HPRT-deficient cells. We hypothesized that altered nucleotide concentrations in HPRT deficiency change G-protein-mediated signal transduction. Therefore, our original study aim was to examine the high-affinity GTPase activity of G-proteins in membranes from primary human skin and immortalized mouse skin fibroblasts, rat B103 neuroblastoma cells and mouse Neuro-2a neuroblastoma cells. Unexpectedly, in membranes from human fibroblasts, B103- and Neuro-2a cells, V(max) of low-affinity nucleoside 5'-triphosphatase (NTPase) activities was decreased up to 7-fold in HPRT deficiency. In contrast, in membranes from mouse fibroblasts, HPRT deficiency increased NTPase activity up to 4-fold. The various systems analyzed differed from each other in terms of K(m) values for NTPs, absolute V(max) values and K(i) values for nucleoside 5'-[beta,gamma-imido]triphosphates. Our data show that altered membrane NTPase activity is a biochemical hallmark of HPRT deficiency, but species and cell-type differences have to be considered. Thus, future studies on biochemical changes in LND should be conducted in parallel in several HPRT-deficient systems.

Hypothesized deficiency of guanine-based purines may contribute to abnormalities of neurodevelopment, neuromodulation, and neurotransmission in Lesch-Nyhan syndrome

The Lesch-Nyhan syndrome is a devastating sex-linked recessive disorder resulting from almost complete deficiency of the activity of hypoxanthine phosphoribosyltransferase (HPRT). The enzyme deficiency results in an inability to synthesize the nucleotides guanosine monophosphate and inosine monophosphate from the purine bases guanine and hypoxanthine, respectively, via the "salvage" pathway and an accelerated biosynthesis of these purines via the de novo pathway. The syndrome is characterized by neurologic manifestations, including the very dramatic symptom of compulsive self-mutilation. The neurologic manifestations may result, at least in part, from a mixture of neurodevelopmental (eg, a failure to "arborize" dopaminergic synaptic terminals) and neurotransmitter (eg, disruption of GABA and glutamate receptor-mediated neurotransmission) consequences. HPRT deficiency results in elevated extracellular levels of hypoxanthine, which can bind to the benzodiazepine agonist recognition site on the GABA(A) receptor complex, and the possibility of diminished levels of guanine-based purines in discrete "pools" involved in synaptic transmission. In addition to their critical roles in metabolism, gene replication and expression, and signal transduction, guanine-based purines may be important regulators of the synaptic availability of L-glutamate. Guanine-based purines may also have important trophic functions in the CNS. The investigation of the Lesch-Nyhan syndrome may serve to clarify these and other important neurotransmitter, neuromodulatory, and neurotrophic roles that guanine-based purines play in the central nervous system, especially the developing brain. A widespread and general deficiency of guanine-based purines would lead to impaired transduction of a variety of signals that depend on GTP-protein-coupled second messenger systems. This is less likely in view of a prominent localized pathologic effect of HPRT deficiency on presynaptic dopaminergic projections to the striatum. A possible more circumscribed effect of a deficiency of guanine-based purines could be interference with modulation of glutamatergic neurotransmission. Guanosine has been shown to be an important modulator of glutamatergic neurotransmission, promoting glial reuptake of L-glutamate. A deficiency of guanosine could lead to dysregulated glutamatergic neurotransmission, including possible excitotoxic damage. Unfortunately, although the biochemical lesion has been known for quite some time (ie, HPRT deficiency), therapeutically beneficial interventions for these affected children and adults have not yet emerged based on this elucidation. Conceivably, guanosine or its analogues and excitatory amino acid receptor antagonists could participate in the pharmacotherapy of this devastating disorder.

Effect of hypoxanthine on Na+,K+-ATPase activity and some parameters of oxidative stress in rat striatum

The main objective of this study was to investigate the effects of preincubation of rat striatum homogenate in the presence of hypoxanthine, a metabolite accumulated in Lesch-Nyhan disease, on Na+,K+-ATPase activity and on some parameters of oxidative stress namely thiobarbituric acid-reactive substances (TBA-RS), total radical-trapping antioxidant parameter (TRAP) and membrane protein thiol content. Results showed that hypoxanthine significantly increased TBA-RS and reduced Na+,K+-ATPase activity, TRAP and membrane protein thiol content. In addition, we also evaluated the effect of glutathione, trolox, allopurinol and Nvarpi-nitro-L-arginine methyl ester (L-NAME) on the inhibitory effect of hypoxanthine on Na+,K+-ATPase activity in the same rat cerebral structure. All tested compounds per se did not alter Na+,K+-ATPase activity, but only glutathione and trolox prevented the effect of hypoxanthine on the enzyme activity. The effect of glutathione and trolox on hypoxanthine-induced increase of TBA-RS levels was also investigated. These antioxidants alone or combined with hypoxanthine reduced TBA-RS levels. Our present findings show that hypoxanthine induces oxidative stress in rat striatum and that the inhibition of Na+,K+-ATPase activity caused by this oxypurine was probably mediated by reactive oxygen species. It is presumed that these results might be associated with the neuronal dysfunction of patients affected by Lesch-Nyhan disease.

Effects of hypoxanthine on adenosine transport in human lymphocytes. Implications in the pathogenesis of Lesch-Nyhan syndrome

We have examined the effect of hypoxanthine on adenosine transport and [3H] NBTI binding in peripheral blood lymphocytes (PBL) cultures. Pre-incubation with hypoxanthine originates a dose dependent decrease of adenosine transport and [3H] NBTI binding sites in PBL.

Adenosine transport in HPRT deficient lymphocytes from Lesch-Nyhan disease patients

We have analysed adenosine transport and [3H] NBTI binding in peripheral blood lymphocytes obtained from Lesch-Nyhan patients, in basal conditions and following 24 h incubation with hypoxanthine. We found that adenosine transport and [3H] NBTI binding were significantly decreased in PBL-LN with respect to PBL-C in basal conditions. Following 25 microM hypoxanthine incubation, adenosine transport is decreased in PBL-LN with respect to basal transport, however, [3H] NBTI binding in PBL-LN was not decreased following hypoxanthine incubation.

Inherited hyperuricemic disorders

Inherited hyperuricemic disorders fall into two major classes, metabolic overproduction of purines and renal tubular undersecretion. The aim was to explore both. Methodology was a combination of personal experience and review of relevant literature. The overproduction of hyperuricemias result from deficiency of hypoxanthine-guanine phosphoribosyl transferase, overactivity of phosphoribosylpyrophosphate synthetase and deficiency of glucose-6-phosphatase. The undersecretion disorders are autosomal dominantly inherited and are heterogeneous. A major number of these patients result from mutations in the gene that codes for uromodulin. Treatment is with allopurinol.

Guanine nucleotide depletion induces differentiation and aberrant neurite outgrowth in human dopaminergic neuroblastoma lines: a model for basal ganglia dysfunction in Lesch-Nyhan disease

Lesch-Nyhan disease (LND), caused by complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT), is characterized by a neurological deficit, the etiology of which is unknown. Evidence has accumulated indicating that it might be related to dysfunction of the basal ganglia with a prominent loss of striatal dopamine fibers. Guanine nucleotide depletion has been shown to occur in cells from Lesch-Nyhan patients. In this study we demonstrate that chronic guanine nucleotide depletion induced by inhibition of inosine monophosphate dehydrogenase with low levels (50 nM) of mycophenolic acid (MPA) lead human neuroblastoma cell lines to differentiate toward the neuronal phenotype. The MPA-induced morphological changes were more evident in the dopaminergic line LAN5, than in the cholinergic line IMR32. MPA-induced differentiation, unlike that induced by retinoic acid, caused a less extensive neurite outgrowth and branching (similar to that observed in cultured HPRT-deficient dopaminergic neurons) and involved up-regulation of p53, p21 and bax, and bcl-2 down-regulation without p27 protein accumulation. These results suggest that guanine nucleotide depletion following HPRT deficiency, might lead to earlier and abnormal brain development mainly affecting the basal ganglia, displaying the highest HPRT activity, and could be responsible for the specific neurobehavioral features of LND.

Inhibition of Na+, K+-ATPase activity in rat striatum by the metabolites accumulated in Lesch-Nyhan disease

In the present study, we investigated the in vitro effect of hypoxanthine, xanthine and uric acid, metabolites accumulating in tissue of patients with Lesch-Nyhan disease, on Na(+), K(+)-ATPase activity in striatum of neonate rats. Results showed that all compounds significantly inhibited Na(+), K(+)-ATPase activity. We also studied the kinetics of the inhibition of Na(+), K(+)-ATPase activity caused by hypoxanthine. The apparent K(m) and V(max) of Na(+), K(+)-ATPase activity for ATP as the substrate and hypoxanthine as the inhibitor were 0.97 mM and 0.69 nmol inorganic phosphate (Pi) released per min per mg of protein, respectively. K(i)-value was 1.9 microM, and the inhibition was of the non-competitive type. We also observed that the inhibitory effects of hypoxanthine, xanthine and uric acid probably occur through the same mechanism, suggesting a common binding site for these oxypurines on Na(+), K(+)-ATPase. Therefore, it is conceivable that inhibition of brain Na(+), K(+)-ATPase activity may be involved at least in part in the neuronal dysfunction characteristic of patients with Lesch-Nyhan disease.

Adenosine transport in peripheral blood lymphocytes from Lesch-Nyhan patients

We postulated that adenosine function could be related to some of the neurological features of Lesch-Nyhan syndrome and therefore characterized adenosine transport in PBLs (peripheral blood lymphocytes) obtained from Lesch-Nyhan patients (PBL(LN)) and from controls (PBL(C)). Adenosine transport was significantly lower in PBL(LN) when compared with that in PBL(C) and a significantly lower number of high affinity sites for [(3)H]nitrobenzylthioinosine binding were quantified per cell ( B (max)) in PBL(LN) when compared with that in PBL(C). After incubation with 25 microM hypoxanthine, adenosine transport was significantly decreased in PBL(LN) with respect to PBL(C). Hypoxanthine incubation lowers [(3)H]nitrobenzylthioinosine binding in PBL(C), with respect to basal conditions, but does not affect it in PBL(LN). This indicates that hypoxanthine affects adenosine transport in control and hypoxanthine-guanine phosphoribosyltransferase-deficient cells by different mechanisms.

Tetrahydrobiopterin deficiency and dopamine loss in a genetic mouse model of Lesch-Nyhan disease

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) is an enzyme that catalyses the conversion of hypoxanthine and guanine into their respective nucleotides. Inherited deficiency of the enzyme is associated with a loss of striatal dopamine in both mouse and man. Although HPRT is not directly involved in the metabolism of dopamine, it contributes to the supply of GTP, which is used in the first and rate-limiting step in the synthesis of tetrahydrobiopterin (BH4). Since BH4 is required as a cofactor for tyrosine hydroxylase in the synthesis of dopamine, any limitation in the supply of GTP could interfere with the synthesis of dopamine. The current studies were designed to address the hypothesis that the reduced striatal dopamine in mice with HPRT deficiency results from reduced availability of BH4. The mutant mice had small reductions in striatal BH4, with normal BH4 levels in other brain regions. Liver BH4 was normal in HPRT-deficient mutant mice, and a phenylalanine challenge test failed to reveal any evidence for impaired hepatic phenylalanine hydroxylase, another BH4-dependent enzyme. Although striatal BH4 content is not normal, supplementation with BH4 or L-dopa failed to correct the striatal dopamine deficiency of the mutant mice, suggesting that BH4 limitation is not responsible for the dopamine loss.

The biochemical basis of the neurobehavioral abnormalities in the Lesch–Nyhan syndrome: a hypothesis

Lesch-Nyhan syndrome (LNS) is a rare X-recessive disorder that leads to virtually complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Partial HPRT deficiency results in uric acid overproduction with subsequent hyperuricemia, nephrolithiasis, renal failure and gouty arthritis. In contrast, at complete HPRT deficiency, besides overproduction of uric acid neurological problems appear including spasticity, choreoathetosis, mental retardation, and compulsive self-mutilation. The cause for the uric acid overproduction has been clarified, but the connection between the enzyme deficiency and the neurological manifestations in LNS remains unclear. A hypothesis, which explains this relation, is proposed in the paper. The hypothesis has several important points most substantial of which is the accelerated biosynthesis of semiessential amino acid histidine that against the background of accelerated purine de novo biosynthesis results in 5-aminoimidazole-4-carboxamideribotide (AICAR) and histamine accumulation. The histamine and AICAR were determined to be the compounds that cause the neurobehavioral symptoms of LNS for several reasons. First, in the basal ganglia a balance between the direct (activating) and the indirect (inhibiting) pathways arising on the basis of the antagonistic and reciprocal dopamine-adenosine interactions normally exists. This balance can tonically regulate smooth voluntary movements and the activity of the thalamus, which, in turn, processes the afferent sensorimotor signals from the whole body to the all areas of the cerebral cortex and is concerned to modulate mental development and bring sensory information into awareness. Second, histamine is known to induce a selective damage in dopaminergic neurons inhibiting the direct dopaminergic pathway, which could lead to muscular rigidity, and slowness in initiating movements as well as tremor that are characteristic of Parkinsonism in LNS. Third, AICAribosid (AICAR breakdown product) is a potent adenosine A2a receptor antagonist inhibiting the indirect dopamine-adenosinergic pathway and, therefore, could be responsible for the choreoathetosis, dystonia and ballismus found in LNS. The excitatory-inhibitory disbalance in the basal ganglia could result in inadequate modification of the thalamus activity with subsequent mental retardation and symptoms that include the patients not being aware for their own bodies that could give rise to self-mutilation. Finally, a possibility for the creation of a new animal model that could exactly match the human LNS is proposed in the paper.

5'-aminoimidazole-4-carboxamide riboside induces apoptosis in human neuroblastoma cells

5'-Aminoimidazole-4-carboxamide riboside (AICA riboside) has been previously shown to be toxic to two neuronal cell models [Neuroreport 11 (2000) 1827]. In this paper we demonstrate that AICA riboside promotes apoptosis in undifferentiated human neuroblastoma cells (SH-SY5Y), inducing a raise in caspase-3 activity. In order to exert its effect on viability, AICA riboside must enter the cells and be phosphorylated to the ribotide, since both a nucleoside transport inhibitor, and an inhibitor of adenosine kinase produce an enhancement of the viability of AICA riboside-treated cells. Short-term incubations (2 h) with AICA riboside result in five-fold increase in the activity of AMP-dependent protein kinase (AMPK). However, the activity of AMPK is not significantly affected at prolonged incubations (48 h), when the apoptotic effect of AICA riboside is evident. The results demonstrate that when the cell line is induced to differentiate both toward a cholinergic phenotype (with retinoic acid) or a noradrenergic phenotype (with phorbol esters), the toxic effect is significantly reduced, and in the case of the noradrenergic phenotype differentiation, the riboside is completely ineffective in promoting apoptosis. This reduction of effect correlates with an overexpression of Bcl-2 during differentiation. AICA riboside, derived from the hydrolysis of the ribotide, an intermediate of purine de novo synthesis, is absent in normal healthy cells; however it may accumulate in those individuals in which an inborn error of purine metabolism causes an increase in the rate of de novo synthesis and/or an overexpression of cytosolic 5'-nucleotidase, that appears to be the enzyme responsible for AICA ribotide hydrolysis. In fact, 5'-nucleotidase activity has been shown to increase in patients affected by Lesch-Nyhan syndrome in which both acceleration of de novo synthesis and accumulation of AICA ribotide has been described, and also in other neurological disorders of unknown etiology. Our results raise the intriguing clue that the neurotoxic effect of AICA riboside on the developing brain might contribute to the neurological manifestations of syndromes related to purine dismetabolisms.

Severe pyridine nucleotide depletion in fibroblasts from Lesch-Nyhan patients

The relationship between a complete deficiency of the purine enzyme hypoxanthine-guanine phosphoribosyltransferase and the neurobehavioural abnormalities in Lesch-Nyhan disease remains an enigma. In vitro studies using lymphoblasts or fibroblasts have evaluated purine and pyrimidine metabolism with conflicting results. This study focused on pyridine nucleotide metabolism in control and Lesch-Nyhan fibroblasts using radiolabelled salvage precursors to couple the extent of uptake with endocellular nucleotide concentrations. The novel finding, highlighted by specific culture conditions, was a marked NAD depletion in Lesch-Nyhan fibroblasts. ATP and GTP were also 50% of the control, as reported in lymphoblasts. A 6-fold greater incorporation of [(14)C]nicotinic acid into nicotinic acid- adenine dinucleotide by Lesch-Nyhan fibroblasts, with no unmetabolized substrate (20% in controls), supported disturbed pyridine metabolism, NAD depletion being related to utilization by poly(ADP-ribose) polymerase in DNA repair. Although pyrimidine nucleotide concentrations were similar to controls, Lesch-Nyhan cells showed reduced [(14)C]cytidine/uridine salvage into UDP sugars. Incorporation of [(14)C]uridine into CTP by both was minimal, with more than 50% [(14)C]cytidine metabolized to UTP, indicating that fibroblasts, unlike lymphoblasts, lack active CTP synthetase, but possess cytidine deaminase. Restricted culture conditions may be neccesary to mimic the situation in human brain cells at an early developmental stage. Cell type may be equally important. NAD plus ATP depletion in developing brain could restrict DNA repair, leading to neuronal damage/loss by apoptosis, and, with GTP depletion, affect neurotransmitter synthesis and basal ganglia dopaminergic neuronal systems. Thus aberrant pyridine nucleotide metabolism could play a vital role in the pathophysiology of Lesch-Nyhan disease.

GTP cyclohydrolase I feedback regulatory protein-dependent and -independent inhibitors of GTP cyclohydrolase I

GTP cyclohydrolase I feedback regulatory protein (GFRP) mediates the feedback inhibition of GTP cyclohydrolase I activity by (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) through protein complex formation. Since guanine and BH4 have a common pyrimidine ring structure, we examined the inhibitory effect of guanine and its analogs on the enzyme activity. Guanine, 8-hydroxyguanine, 8-methylguanine, and 8-bromoguanine inhibited the enzyme activity in a GFRP-dependent and pH-dependent manner and induced complex formation between GTP cyclohydrolase I and GFRP. The type of inhibition by this group is a mixed type. All these properties were shared with BH4. In striking contrast, inhibition by 8-azaguanine and 8-mercaptoguanine was GFRP-independent and pH-independent. The type of inhibition by 8-azaguanine and 8-mercaptoguanine was a competitive type. The two compounds did not induce complex formation between the enzyme and GFRP. These results demonstrate that guanine compounds of the first group bind to the BH4-binding site of the GTP cyclohydrolase I/GFRP complex, whereas 8-azaguanine and 8-mercaptoguanine bind to the active site of the enzyme. Finally, the possible implications in Lesch-Nyhan syndrome and Parkinson diseases of the inhibition of GTP cyclohydrolase I by guanine and 8-hydroxyguanine are discussed.

Hypoxanthine impairs morphogenesis and enhances proliferation of a neuroblastoma model of Lesch Nyhan syndrome

Extracellular purines have essential roles in neuronal development; hence, disruptions in their metabolism as reported in Lesch Nyhan syndrome (LNS) could result in developmental abnormalities. The deficiency of hypoxanthine-guanine phosphoribosyl transferase (HGPRT) in LNS leads to increased hypoxanthine and uric acid production. We have reported that HGPRT-deficient B103-4C neuroblastoma, a neuronal model of LNS, proliferated less and differentiated more than their HGPRT-positive B103 counterparts. Here, we sought to determine whether differences in proliferation and differentiation would occur when these cells were cultured in the presence of hypoxanthine or in a hypoxanthine-/serum-free chemically defined media (NBMN2). In media with 1% serum, hypoxanthine (50 microM) significantly increased the proliferation of both cell lines with a greater effect on B103-4C cells. In 1% serum media, hypoxanthine increased differentiation of B103 but decreased B103-4C differentiation. In NBMN2, B103 proliferated far more than B103-4C, but both cell types differentiated to the same extent. These results are interpreted to suggest that elevated levels of central nervous system (CNS) hypoxanthine as reported in LNS may affect neuronal development, and to implicate hypoxanthine and abnormal neuronal development as causative factors in the etiology of LNS.

Abnormalities in cellular adhesion of neuroblastoma and fibroblast models of Lesch Nyhan syndrome

Lesch Nyhan syndrome is a neurological paediatric condition characterized by mental retardation, choreathotosis and self-mutilation. Biochemically, this condition has been attributed to a deficiency in the purine enzyme, hypoxanthine guanine phosphoribosyltransferase, however, the way this affects the development of the nervous system is still unknown. Ma et al.(15) and Stacey et al.(25) found that hypoxanthine guanine phosphoribosyltransferase-deficient neuroblastoma, differentiated significantly more than cells with this enzyme. Here, we report that adhesion of hypoxanthine guanine phosphoribosyltransferase-deficient neuroblastoma as well as fibroblasts from patients with Lesch Nyhan syndrome, exhibited dramatically enhanced adhesion compared to control cells. This increase in adhesion was dependent upon the cell type, density of the cells and upon the substrate used. Development of the nervous system is dependent on adhesion, in particular in the processes of migration, nucleation, differentiation and fasciculation. Our results suggest that the increased adhesion of hypoxanthine guanine phosphoribosyltransferase-deficient neuroblastoma and fibroblasts in vitro underpins the neuropathology of Lesch Nyhan syndrome.

Dopamine function in Lesch-Nyhan disease

Lesch-Nyhan disease is a disorder of purine metabolism resulting from mutations in the gene for hypoxanthine guanine phosphoribosyl transferase on the X chromosome. It is characterized by hyperuricemia and all of its consequences, as in gout; but in addition, patients have impressive disease of the central nervous system. This includes spasticity, involuntary movements, and retardation of motor development. The behavioral phenotype is best remembered by self-injurious biting behavior with attendant destruction of tissue. The connection between aberrant metabolism of purines and these neurologic and behavioral features of the disease is not clear. Increasing evidence points to imbalance of neurotransmitters. There is increased excretion of the serotonin metabolite 5-hydroxyindoleacetic acid in the urine. There are decreased quantities and activities of a number of dopaminergic functions. Positron emission tomography scanning has indicated deficiency in the dopamine transporter.