Altered purine and pyrimidine metabolism in erythrocytes with purine nucleoside phosphorylase deficiency

Purine and pyrimidine metabolism was compared in erythrocytes from three patients from two families with purine nucleoside phosphorylase deficiency and T-cell immunodeficiency, one heterozygote subject for this enzyme deficiency, one patient with a complete deficiency of hypoxanthine-guanine phosphoribosyltransferase, and two normal subjects. The erythrocytes from the heterozygote subject were indistinguishable from the normal erythrocytes. The purine nucleoside phosphorylase deficient erythrocytes had a block in the conversion of inosine to hypoxanthine. The erythrocytes with 0.07% of normal purine nucleoside phosphorylase activity resembled erythrocytes with hypoxanthine-guanine phosphoribosyltransferase deficiency by having an elevated intracellular concentration of PP-ribose-P, increased synthesis of PP-ribose-P, and an elevated rate of carbon dioxide release from orotic acid during its conversion to UMP. Two hypotheses to account for the associated immunodeficiency--that the enzyme deficiency leads to a block of PP-ribose-P synthesis or inhibition of pyrimidine synthesis--could not be supported by observations in erythrocytes from both enzyme-deficient families.

Hypoxanthine salvage in man: its importance in urate overproduction in the Lesch-Nyhan syndrome

1. A daily urinary excretion of 0.8 percent of the administered radioactivity results from the turnover of the labeled adenine nucleotide pool. 2. A 4-fold increase of urinary radioactivity excretion occurs in patients with the Lesch-Nyhan syndrome and support the role of impaired hypoxanthine salvage in the purine overexcretion associated with HGPRT deficiency. 3. Our data do not support the possibility that the increased radioactivity excretion in the HGPRT deficient subjects results from an elevated rate of adenine nucleotide degradation.

Overproduction of uric acid in hypoxanthine-guanine phosphoribosyltransferase deficiency. Contribution by impaired purine salvage

The contribution of reduced purine salvage to the hyperuricemia associated with hypoxanthine-guanine phosphoribosyltransferase deficiency was measured by the intravenous administration of tracer doses of [8-(14)C]adenine to nine patients with normal enzyme activity, three patients with a partial deficiency of hypoxanthine-guanine phosphoribosyltransferase, and six patients with the Lesch-Nyhan syndrome. The mean cumulative excretion of radioactivity 7 d after the adenine administration is 5.6+/-2.4, 12.9+/-0.9, and 22.3+/-4.7% of infused radioactivity for control subjects, partial hypoxanthine-guanine phosphoribosyltransferase-deficient subjects, and Lesch-Nyhan patients, respectively. To assess relative rates of nucleotide degradation in control and hypoxanthine-guanine phosphoribosyltransferase-deficient patients two separate studies were employed. With [8-(14)C]inosine administration, three control subjects excreted 3.7-8.5% and two enzyme-deficient patients excreted 26.5-48.0% of the injected radioactivity in 18 h. The capacity of the nucleotide catabolic pathway to accelerate in response to d-fructose was evaluated in control and enzyme-deficient patients. The normal metabolic response to intravenous fructose is a 7.5+/-4.2-mmol/g creatinine increase in total urinary purines during the 3-h after the infusion. The partial hypoxanthine-guanine phosphoribosyltransferase-deficient subjects and Lesch-Nyhan patients show increases of 18.6+/-10.8 and 17.3+/-11.8 mmol/g creatinine, respectively. Of the observed rise in purine exretion in control subjects, 40% occurs from inosine excretion and 32% occurs from oxypurine excretion. The rise in total purine excretion with Lesch-Nyhan syndrome is almost entirely accounted for by an elevated uric acid excretion. Increases in urine radioactivity after fructose infusion are distributed in those purines that are excreted in elevated quantities.The observations suggest that purine salvage is a major contributor to increased purine excretion and that the purine catabolic pathway responds differently to an increased substrate load in hypoxanthine-guanine phosphoribosyltransferase deficiency. The purine salvage pathway is normally an important mechanism for the reutilization of hypoxanthine in man.

Effects of L-5-hydroxytryptophan on monoamine and amino acids turnover in the Lesch-Nyhan syndrome

In a patient with the Lesch-Nyhan syndrome we found decreased spinal fluid 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of serotonin, and decreased homovanillic acid (HVA), the major metabolite of dopamine, indicating a decrease in monoamine metabolism. Administration of 5-hydroxytryptophan and carbidopa produced an increase in spinal fluid 5-HIAA, indicating that it might be possible to correct the serotonin deficiency in this syndrome, but there were no changes in the marked mental retardation and neurological deficits. Self-mutilation appeared to be suppressed by therapy but the effectiveness of the drugs decreased with time. There were also changes in the spinal fluid concentration of amino acids that might affect brain protein synthesis. These changes were corrected during administration of 5-hydroxytryptophan and carbidopa.

Lack of enhanced purine biosynthesis in HGPRT- and Lesch-Nyhan cells

The rate of de novo purine biosynthesis was measured in a series of hypoxanthine guanine phosphoribosyl transferase deficient (HGPRT-) cells from a variety of sources, including human Lesch-Nyhan cells. Under optimum growth conditions, no enhanced purine biosynthesis was detected (in contrast to previous reports). An 'elevated' level of de novo purine biosynthesis could be detected in mutants following starvation for glutamine. However, this was the result of depression of purine biosynthesis in normal cells, with a resulting artifactual overproduction in mutants.

Increased de novo purine synthesis in cultured skin fibroblasts from heterozygotes for the Lesch-Nyhan syndrome. A sensitive marker for carrier detection

The metabolic consequence of hypoxanthine-guanine phosphoribosyltransferase deficiency, the accelerated rate of purine synthesis de novo, was utilized as a marker for the detection in cultured fibroblasts of heterozygosity for the Lesch-Nyhan syndrome. This marker was found to be very sensitive allowing the detection of mutant cells in nonselected mixed mutant: normal cell cultures even at low proportion of 1 to 10. Exposure of the mixed cultures to selection for the mutant cell with azaguanine increased the sensitivity of the test. Cultures from different biopsies, obtained from heterozygote females, were found to contain different proportions of the mutant cell, ranging from 10 to 84%.

Gilles de la Tourette syndrome: further studies and thoughts

A possible association between the Gilles de la Tourette and Lesch-Nyhan syndromes has recently been postulated. Fourteen patients with Tourette syndrome demonstrated no similarity to Lesch-Nyhan based upon patterns of inheritance, behavioral changes, or alterations of purine metabolism. Despite a strong male predominance, a sex-linked pattern of inheritance could not be confirmed. Self-mutilating behavior was found in 4 male patients but was readily differentiated from that characteristic of the Lesch-Nyhan syndrome. Quantitation of hypoxanthine-guanine phosphoribosyltransferase and isoelectric focusing of its isoenzymes produced results that were indistinguishable from those in controls. We speculate that, pathophysiologically, Tourette syndrome represents an imbalance between the central neurotransmitters dopamine and serotonin rather than an alteration in purine metabolism.

Quantification of chemical mutagenesis in diploid human fibroblasts: induction of azaguanine-resistant mutants by N-methyl-N'-nitro-N-nitrosoguanidine

The toxic and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on cultured diploid human fibroblasts were studied. When 10(5) cells per 60 mm dish were exposed to MNNG for 4 h in Ham's medium F10 containing 0.02 M HEPES buffer at pH 6.8, MNNG concentrations of less than 1 X 10(-6) M resulted in cell survivals near 100%, while the average survival was less than one percent at concentrations greater than 5 X 10(-6) M. After treatment with MNNG, cells were subjected to selection using optimal conditions for the detection of diploid human fibroblasts that are resistant to the guanine-analogs AG and TG because they contain altered or deficient HPRT. The induced mutant frequency was maximized by allowing a 5 to 7 day post-treatment interval for the expression of the mutant phenotype and by replating the cells at the beginning of selection at a population density of less than 450 cells per cm2. Careful attention was given to counting statistically adequate numbers of mutants and to accurately determining cell survival and replating cloning efficiencies. Independent dose-response experiments gave induced mutant frequencies as high as 7.0 X 10(-4) to 8.8 X 10(-4) mutants per viable cell at about 5% survival, compared to a spontaneous mutation rate of 3.7 X 10(-6) to 7.2 X 10(-6) mutants per cell generation. The AGr mutants observed after treatment with MNNG were phenotypically stable and closely resembled prototype AGr cultures derived from humans who have inherited mutant X-chromosomal alleles for HPRT.

Regulation of de novo purine biosynthesis in normal and 8-azaguanine-resistant Chinese hamster cells

Azaguanine-resistant mutants of Chinese hamster ovary cells were isolated following mutagenesis with ICR-17OG. Of the eight mutant isolates examined, only one, Ag-5 had detectable hypoxanthine(guanine)phosphoribosyltransferase activity. Under normal conditions of growth, de novo purine biosynthesis in the mutants was not significantly different from wild type. However, when the cultures were starved for glutamine over a 2 h period before measuring 5'phosphoribosyl formylglycinamide (a relative measure of de novo purine biosynthesis), elevated levels of 5'-phosphoribosyl formylglycinamide accumulated in some of the mutants, and decreased levels in wild type and Ag-5. The level of purine biosynthesis could be related to the levels of glutamine in the pregrowth medium. The rate of purine biosynthesis correlated with 5-phosphoribosyl pyrophosphate levels, which were enhanced in the mutant (Ag-C) following the starvation period. No alterations were found in levels of 5-phosphoribosyl pyrophosphate synthetase or glutamine synthetase. The extent of feedback inhibition was normal in both mutant and wild type cells. These data suggest that the hypoxanthine (guanine) phosphoribosyltransferase locus is a regulatory gene.

Phosphoribosylpyrophosphate synthesis in cultured human cells

Phosphoribosylpyrophosphate (PRPP) concentrations and PRPP synthetase activity were studied in cultured human fibroblasts with control and mutant hypoxanthine-guanine phosphoribosylpyrophosphate (HPRT) activity. The PRPP concentrations increased 20- to 50-fold and PRPP synthetase activity 3-fold in cells from patients with the Lesch-Nyham syndrome when aminopterin, an inhibitor of de novo purine synthesis, was added to the medium. Concentrations of PRPP and PRPP synthetase activity did not increase in control cells in medium containing aminopterin unless hypoxanthine was removed from the medium. Exposure of cells to cycloheximide, a protein synthesis inhibitor, prevented the induction of PRPP synthetase and the formation of high PRPP concentrations. Cells from a patient with a mutant HPRT with a high Michaelis constant for PRPP increased PRPP levels and PRPP synthetase activity to a new steady state when they were grown in medium containing aminopterin. Inhibitors of de novo purine synthesis, the presence of hypoxanthine in the medium, and mutant HPRT activity affect the regulation of PRPP levels and PRPP synthetase activity.

Evaluation of metabolic pathway activity in cultured skin fibroblasts and blood leukocytes

Screening for a variety of blocked catabolic mutants can be achieved in cultured skin fibroblasts and in peripheral blood leukocytes with a simple radioisotope method that is reliable and convenient. The method measures the incorporation into trichloroacetic acid (TCA) insoluble macromolecules of a 14C-labelled intermediate in the pathway under question; incorporation of a 3H-labelled metabolite, not in the same pathway, is used as an internal control of metabolism in the cell population. The 14C/3H incorporation ratio is decreased in blocked pathways; use of the ratio method eliminates the delay required by radioautography (used in earlier adaptations of this method), the problems involved in CO2 collection, and the need to standardize cultures for cell number. This method has been used to identify cells with biochemical lesions in the oxidation of propionate, galactose, hypoxanthine and pyruvate; it has allowed us to identify a new variant of methylmalonicaciduria; we believe it can be extended to include other metabolites and pathways.

Lesch-Nyhan syndrome: the synthesis of inosine 5'-phosphate in the hypoxanthine-guanine phosphoribosyltransferase-deficient erythrocyte by alternate biochemical pathways

Erythrocytes, obtained from a normal adult male and from a patient with Lesch-Nyhan syndrome, were incubated with [8-14C]adenine and [8-14C]hypoxanthine (Table 1). The labeled adenine was utilized to about the same extent for the synthesis of AMP by the normal subject's and the patient's erythrocytes. Deamination of AMP to IMP occurred to about the same extent in both samples. In contrast, hypoxanthine was utilized extensively for IMP synthesis in the normal erythrocyte only. The amount of total label in the IMP was about 100 times that of the Lesch-Nyhan erythrocyte, a consequence of the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity in the syndrome. No significant labeling of the AMP occurred. When aliquots of erythrocytes from both sources were incubated with 4-amino-5-imidazolecarboxamide (AICA) and sodium [14C]formate, extensive labeling of the IMP occurred in normal and in Lesch-Nyhan erythrocytes. The data suggest that AICA serves as a substrate for the adenine phosphoribosyltransferase (APRT) of the Lesch-Nyhan erythrocyte and that the ribotide of AICA, 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide (AICAR), undergoes formylation by labeled N10-formyl tetrahydrofolic acid formed from the reaction of sodium [14C]formate with the tetrahydrofolic acid of the cell. The formyl-AICAR undergoes ring closure to IMP by a series of reactions comparable to those described for the normal erythrocyte. When 5-amino-1-ribosyl-4-imidazolecarboxamide (rAICA) and sodium [14C]formate were incubated with erythrocyte suspensions, extensive utilization for IMP synthesis was also observed in normal erythrocytes and in erythrocytes from Lesch-Nyhan patients (Table 2). The reaction sequence is somewhat different from that of AICA. AICA is not a substrate for the purine nucleoside phosphorylase of rabbit or human erythrocytes. The mechanism of rAICA utilization is visualized as a direct phosphorylation of the ribosyl compound, possibly by the adenosine kinase of the human cell. The ribotide, AICAR, formed by this mechanism, undergoes formylation and ring closure, yielding IMP. The glutamine antagonist, diazooxonorleucine (DON), was added to aliquots of patients' cells incubated with rAICA and sodium [14C]formate. DON is an effective inhibitor of the conversion of IMP to GMP and its presence in an incubation suspension resulted in a somewhat greater radioactivity of the total cellular IMP. The extension of the current studies to Lesch-Nyhan cells in culture may serve to assist in the direct evaluation of the regulatory role of IMP in the de novo pathway of purine nucleotide biosynthesis. Because of the substrate requirements of the reactions, the metabolism of AICA and rAICA may also serve to differentiate the roles of purine nucleotides and of phosphoribosylpyrophosphate (PRPP) in the pathway regulation. The findings presented also offer a possible therapeutic approach to the early treatment of the disease in the afflicted neonate...

Genetic heterogeneity of hypoxanthine-phosphoribosyl transferase in human fibroblasts of 3 families

Incorporation of hypoxanthine, resistance to 8-azaguanine and activation by lyophilisation have been studied in cultured human fibroblasts. Cells from one family where there was a boy with Lesch-Nyham syndrome, from two families with variant H-PRT mutations and three cell strains from patients with the Lesch-Nyham syndrome were investigated. Cells from patients with the Lesch-Nyham syndrome showed almost no hypoxanthine incorporation and resistance to concentrations of 8-azaguanine up to 10(-3) M, whereas cells of patients with partial H-PRT deficiency demonstrated variant patterns of hypoxanthine uptake and partial resistance to 8-azaguanine. Lyophilisation of fibroblast sediment from patients with the Lesch-Nyhan syndrome and patients with variant H-PRT mutations showed activation of the deficient or partially deficient H-PRT enzyme. No such activation was observed in healthy controls. Activation of lyophilised fibroblast extract from patients and controls was not obtained. These results suggest that H-PRT could be associated with the cell membranes.

Adenosine metabolism in permanent lymphocyte lines and in erythrocytes of patients with the Lesch-Nyhan syndrome

Adenosine metabolism was studied in permanent lymphocyte cell lines and in erythrocytes of L.N. patients and compared with that of normal subjects. It was found that L.N. lymphocytes incorporated adenosine into purine ribonucleotides in spite of the fact that they cannot incorporate inosine or hypoxanthine, the major metabolites of adenosine. These findings suggest that adenosine may be a potentially important salvage pathway for purines and a source of IMP and guanine nucleotides in L.N. cells.