Disparate enzyme activity in erythocytes and leukocytes. A variant of hypoxanthine phosphoribosyl-transferase deficiency with an unstable enzyme

Lesch-Nyhan variant syndrome: real-time rt-PCR for mRNA quantification in variable presentation in three affected family members

Inherited mutations of hypoxanthine guanine phosphoribosyltransferase (HPRT) give rise to Lesch-Nyhan syndrome (LNS) or variants (LNV). We report molecular insights from real-time RT-PCR for HPRT mRNA quantification into the mechanism by which a single mutation located in exon 7 of the HPRT gene: c.500G>T, p.R167M, led to different clinical phenotypes from three male LNV-affected patients in the same family manifesting parallel differences in enzymatic activities. This approach can be applied for understanding genotype-phenotype correlations for other human genetic diseases.

Mechanisms for phenotypic variation in Lesch-Nyhan disease and its variants

Lesch-Nyhan disease is a neurogenetic disorder caused by mutation of the HPRT1 gene on the X chromosome. There is significant variation in the clinical phenotype, with more than 300 different known mutations. There are few studies that have addressed whether similar mutations result in similar phenotypes across different patients because hypoxanthine-guanine phosphoribosyltransferase (HGprt) deficiency is rare, and most mutations are unique or limited to individual families. However, recent studies have revealed multiple unrelated patients with similar mutations, providing an opportunity to examine genotype-phenotype correlations. We found significant variation among the clinical features of 10 patients from 8 unrelated families all carrying a mutation replacing guanine with adenine at base position 143 (c.143G>A) in the HPRT1 gene. This mutation results in replacement of arginine by histidine at amino acid position 48 (p.arg48his) in the HGprt enzyme. Biochemically, the enzyme exhibits reduced thermal integrity, a mechanism that may explain clinical variation. The literature reveals similar clinical variation among other patients with similar mutations, although the variation is relatively minor across the whole population of patients. Identifiable sources of clinical variation include known limitations of clinical ascertainment and mechanisms that affect residual enzyme activity and stability. These results are helpful for understanding genotype-phenotype correlations and discordance and likely are applicable to other neurogenetic disorders where similar variation occurs.

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.

The spectrum of mutations causing HPRT deficiency: an update

Mutations in the gene encoding hypoxanthine-guanine phosphoribosyltransferase (HPRT) cause Lesch-Nyhan disease, which is characterized by hyperuricemia, severe motor disability, and self-injurious behavior. Mutations in the same gene also cause less severe clinical phenotypes with only some portions of the full syndrome. A large database of 271 mutations associated with both full and partial clinical phenotypes was recently compiled. Since the original database was assembled, 31 additional mutations have been identified, bringing the new total to 302. The results demonstrate a very heterogeneous collection of mutations for both LND and its partial syndromes. The differences between LND and the partial phenotypes cannot be explained by differences in the locations of mutations, but the partial phenotypes are more likely to have mutations predicted to allow some residual enzyme function. The reasons for some apparent exceptions to this proposal are addressed.

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

In humans, mutations in the gene encoding the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) are associated with a spectrum of disease that ranges from hyperuricemia alone to hyperuricemia with profound neurological and behavioral dysfunction. Previous attempts to correlate different types or locations of mutations with different elements of the disease phenotype have been limited by the relatively small numbers of available cases. The current article describes the molecular genetic basis for 75 new cases of HPRT deficiency, reviews 196 previously reported cases, and summarizes four main conclusions that may be derived from the entire database of 271 mutations. First, the mutations associated with human disease appear dispersed throughout the hprt gene, with some sites appearing to represent relative mutational hot spots. Second, genotype-phenotype correlations provide no indication that specific disease features associate with specific mutation locations. Third, cases with less severe clinical manifestations typically have mutations that are predicted to permit some degree of residual enzyme function. Fourth, the nature of the mutation provides only a rough guide for predicting phenotypic severity. Though mutation analysis does not provide precise information for predicting disease severity, it continues to provide a valuable tool for genetic counseling in terms of confirmation of diagnoses, for identifying potential carriers, and for prenatal diagnosis.

Synergistic interaction between acivicin (AT-125) and 6-thioguanine in the murine leukemia L1210. Biochemical and cytokinetic considerations

The effect of the purine antagonist acivicin (AT-125) on the metabolism and cytotoxicity of 6-thioguanine was examined in the murine leukemia L1210. Cells exposed to 5 X 10(-6) M acivicin for 18 hr followed by 10(-5) M [14C]-6-thioguanine for 2 hr accumulated 2.90 +/- 0.17 nmoles 6-thioguanine/10(6) cells compared to 0.69 +/- 0.07 nmoles 6-thioguanine/10(6) cells in untreated controls. Intracellular accumulation of 6-thioguanine monophosphate, a lethal 6-thioguanine metabolite, increased from 0.27 +/- 0.05 to 1.08 +/- 0.13 nmoles 6-thioguanine monophosphate/10(6) cells following the same acivicin exposure. A similar increment was observed for the formation of 6-thioguanine triphosphate. These alterations in 6-thioguanine metabolism were associated with an increase in the intracellular level of 5-phosphoribosyl-1-pyrophosphate, an obligatory substrate in 6-thioguanine activation (57.9 +/- 7.6 vs 13.4 +/- 2.3 ng 5-phosphoribosyl-1-pyrophosphate/10(6) cells). In contrast, there was a 50% reduction in the amount of 6-thioguanine incorporated into RNA and DNA following acivicin pretreatment. Cytofluorometric analysis revealed that an 18-hr exposure to 5 X 10(6) M acivicin increased the population 5-phase cells, which are more sensitive to the actions of 6-thioguanine, by 50% relative to untreated controls. In both suspension culture growth and soft agar studies, the sequential administration of acivicin followed by 6-thioguanine resulted in substantial growth inhibitory activity; in contrast, the effects of the reverse sequence were subadditive. Pretreatment of L1210 cells with acivicin potentiates the action of subsequently administered 6-thioguanine, and the mechanism may involve both biochemical as well as cytokinetic factors. In vivo studies involving the sequential administration of these agents appear warranted.

Hypoxanthine guanine phosphoribosyl transferase deficiency presenting with gout and renal failure in infancy

A month old infant had gout and renal failure caused by hypoxanthine guanine phosphoribosyl transferase (HGPRTase) deficiency. Investigations showed a high uric acid value, crystal nephropathy on ultrasound, and uric acid deposition on renal biopsy. The HGPRTase value was low in red cells and fibroblasts.

Regulation of expression of genes for enzymes of the mammalian urea cycle in permanent cell-culture lines of hepatic and non-hepatic origin

We present here the results of investigations conducted by ourselves and others on the regulation of the expression of genes encoding the enzymes of the mammalian urea cycle as manifest in cultured cells of both hepatic and extrahepatic origin. Upon consideration of the recently discovered discrete non-hepatic arginase genetic locus in man and our consequent hypothesis that the form of arginase thus transcribed in such extrahepatic cells functions principally in providing ornithine for protein anabolism and polyamine biosynthesis, rather than in detoxifying ammonia through urea formation, we have chosen instead to study permanent cell lines that are derived from liver and continue to perform a variety of hepatic functions in culture as experimental models for probing the molecular mechanisms underlying the control of ureagenesis within the mature liver cell. Of two such arginase-positive rat-hepatoma lines, we have characterized extensively in one (H4-II-E-C3) the mode of action of glucocorticoids in augmenting the cellular levels of this enzyme as well as of argininosuccinate synthetase. To this end, we have recently demonstrated that these stimulations are both mediated by binding of the hormones to classical cytoplasmic steroid receptors in a specific and saturable fashion and have thus concluded that the H4-II-E-C3 line will provide a suitable cell culture system for subsequent more detailed experiments from which the information garnered will continue to be relevant to the ureagenic pathway as modulated in the differentiated hepatocyte in vivo.

Effect of L-glutamine antagonists on 5-phosphoribosyl 1-pyrophosphate levels in P388 leukemia and in murine colon adenocarcinomas in vivo

The intratumoral content of 5-phosphoribosyl 1-pyrophosphate (PRPP) and the activity of the enzymes anabolizing and catabolizing the sugar phosphate were determined following i.p. administration of an LD10 dose of an L-glutamine antagonist or saline to tumor-bearing animals. Elevation of PRPP pool size following administration of L-[alpha S,5S]-alpha-amino-3-chloro-4,5-dihydro-5-isopazoleacetic acid (NSC-163501) (AT-125) was maximal at 8 hr and returned to pretreatment levels by 24 hr. In P388 leukemia, dose for dose, at 4 hr, 6-diazo-5-oxo-L-norleucine (NSC-7365) (DON) was the most potent of the L-glutamine antagonists in elevating basal PRPP pool size (50% above control) followed by AT-125 and azaserine, 300 and 100% above control respectively. Moreover, such augmentation in PRPP pool size preferentially affected P388 tumor rather than the small intestine. Following i.p. administration of LD10 doses of AT-125, DON and azaserine, the specific activities of PRPP anabolizing and catabolizing enzymes were determined. A significant inhibition of PRPP amidotransferase was demonstrated with DON and AT-125 (P less than 0.05), and no inhibition with azaserine. A similar modulation of PRPP pool size demonstrated in vivo following administration of 250 mg/kg of ART-125 in mice bearing colonic adenocarcinoma lines. It was suggested that a significant increase of PRPP pool size might cause the possible synergism of a selected L-glutamine antagonist and 5-fluorouracil as reported after the appropriately scheduled administration of methotrexate and 5-fluorouracil.

Properties of arginase from liver of Macaca fascicularis; comparison of normals with red blood cell arginase deficient monkeys

Deficiency of arginase (E.C. 3.5.3.1), the fifth enzyme of the urea cycle, was found in the red blood cells (RBCs) of Macaca fascicularis monkeys (less than 0.2 micromol arginine cleaved/g Hb/min; normal equals 49.2). Liver biopsies were obtained from two of these monkeys and from one monkey with normal levels of RBC arginase activity. Arginase from both groups of animals required Mn2+ for maximal enzyme activity and demonstrated a pH optimum of 10.2 in vitro. The activity of arginase in the livers of all three monkeys was 1.1 millimol arginine cleaved per g protein per min. The apparent Km for arginine of arginase in the livers of the RBC-deficient monkeys was 7.4 and 5.9 mM and in the normal monkey was 6.9 mM. Similar patterns of heat denaturation was seen at 69 C without Mn2+ present and 79 C in the presence of 20mM Mn2+. No difference in mobility on either RBC-deficient or normal monkeys was found. In addition, liver arginase from all three monkeys reacted similarly with anti-human liver arginase antibody. Liver arginases in RBC-deficient and normal monkeys were identical by ten criteria. These studies do not distinguish among several hypotheses for the genetic determination of arginase in different organs of this species and of man.

Partial deficiency of hypoxanthine-phosphoribosyltransferase:evidence for a structural mutation in a patient with gout

A mutant hypoxanthine-phosphoribosyltransferase (EC 2.4.2.8.) from a patient with gout is examined. The activity of the erythrocyte enzyme is about 5% of normal in this case. Immunoprecipitation studies using antiserum against highly purified human hypoxanthine-phosphoribosyltransferase reveal that the patient's erythrocytes contain a normal amount of cross-reacting material. The mutant enzyme has an altered net charge as shown by preparative isoelectric focusing (pI values of 5.75 and 4.55). The influence of chemical modification on enzymic activity was studied using a number of different reagents directed against sulfhydryl-, amino-, and guanidino-groups. Compared with normal hypoxanthine-phosphoribosyltransferase the mutant enzyme shows a generally lowered susceptibility to active site-directed inhibition. It is concluded that the patient's enzyme is the product of a structural mutation.

Molecular and tissue-specific heterogeneity in HPRT deficiency

In several patients with different degrees of HPRT deficiencies, residual activities have been determined in both lysed and intact erythrocytes. No close correlation could be found between the degree of HPRT deficiency and the severity of the clinical expression. Unless HPRT activity in both intact and lysed erythrocytes was below detection level, the residual activity in intact red blood cells was higher than in lysates. Tissue-specific heterogeneity was illustrated with a patient suffering from X-linked gout. Lysates from erythrocytes, leukocytes, and cultured fibroblasts showed 1%, 8%, and 100% of normal HPRT activity, respectively. Characterization of the erythrocyte and fibroblast HPRT from this patient showed no kinetic abnormalities. However, there was a decreased heat stability. It is concluded that for a better understanding of the pathophysiology in HPRT deficiency studies on nucleated cells from the different tissues are needed.

The incorporation of homologous and heterologous hypoxanthine-guanine phosphoriboxyltransferase into mutant cells

Experiments are described leading to partial compensation of a deficiency in the enzyme hypoxanthine-guanine phosphoribosyltransferase in mutant cells by supplying the cells with exogenous purified enzymes. DEAE-dextran is an effective helper agent, whereas poly (L-lysine), lysolecithin and amphotericin B seem to inhibit the entry of the enzymes of their activity. Enzyme preparation from Chinese hamster was found to have different effects in different mutant cell lines. In mutant Chinese hamster cells, the electrophoretic activity pattern remains unchanged for the Chinese hamster enzyme, but changes progressively to faster-moving activity peaks for the human enzyme after several hours. The metabolic effect of the incorporated enzyme is in the range between 3 and 4% of the normal cellular enzyme activity which corresponds to a 10--20 fold increase of hypoxanthine-guanine phosphoribosyltransferase activity in the mutant cells.

Hypoxanthine phosphoribosyltransferase activity in intact fibroblasts from patients with X-linked hyperuricemia

Discordance between clinical phenotype and the level of a mutant enzyme activity may reflect differences between enzyme function in vivo and that measured by the customary enzyme assays on cell extracts. In the present study, the conversion of hypoxanthine to phosphorylated products was measured in intact skin fibroblasts and in cell extracts from seven patients with mutant hypoxanthine-guanine phosphoribosyltransferase (HPRT) and six control subjects. The patient's phenotypes ranged from asymptomatic hyperuricemia to the Lesch-Nyhan syndrome. Although there was a general correlation between the HPRT activity in cell extracts assayed by the usual methods and the function of the purine salvage pathway in patients, as reflected by urinary oxypurine excretion, there were notable exceptions. A more accurate appraisal of the functioning of the pathway at the cellular level is achieved by measuring the conversion of substrate to product in the intact cell at physiological concentrations of substrates, activators, and product and metabolite inhibitors, and in a physiological ionic environment. In one of the seven patients, the standard enzyme assay indicated normal function, whereas measurements in the intact cell exposed severe dysfunction of the salvage system. In another, the standard assay suggested a severe deficiency not evident in the intact cell or in the patient.

Purine and phosphoribosylpyrophosphate synthesis in differentiating murine virus-induced erythroleukemic cells in vitro

Phosphoribosylpyrophosphate synthetase activity was determined in Friend virus-inducted erythroleukemic cells in culture, stimulated to differentiate in the presence of dimethylsulfoxide. The activity of phosphoribosylpyrophosphate synthetase did not decrease in cells which had acquired the specialized function of hemoglobin synthesis, nor was the phosphoribosylpyrophosphate content of untreated erythroleukemic cells significantly different from that of cultures exposed to dimethylsulfoxide for 96 hours. However, the rate of the early steps of de novo purine biosynthesis as measured by the incorporation of [1-14C] glycine and [1-14C] formate into formyglycinamide ribonucleotide, was significantly lower in differentiating cell cultures. The addition of glutamine or ammonia increased glycine incorporation of control cultures, but failed to do so in treated cultures. In the course of the normal development of erythrocytes in vivo, phosphoribosylpyrophosphate synthetase activity is preserved, while the capacity to synthesize purines de novo is lost, as is the activity of the phosphoribosyl-l-amine synthesizing enzymes. Our present study suggests that the rate of de novo purine biosynthesis in this erythroleukemic cell line is not limited by the availability of phosphoribosylphrophosphate, but rather by a decrease in the phosphoribosyl-l-amine synthesizing enzymes. These findings provide further evidence that during dimethylsulfoxide-stimulated erythroid maturation, the same regulatory mechanisms are operative as in normal cellular development, and that ammonia-dependent purine biosynthesis is subject to the same regulatory mechanisms as is glutamine-dependent biosynthesis.

Hypoxanthine-guanine phosphoribosyl transferase deficiency

In man congential lack of enzyme of the purine salvage system, hypoxanthineguanine phosphoribosyl transferase (HG-PRT E.C. 2.4.2.8), is mostly accompanied by a picture known as the Lesch-Nyhan snydrome. The degree of deficiency may vary from zero to a few percent of normal activity but a correlation between the severity of HG-PRT deficiency and the clinical picture has not been observed, no more than a correlation HG-PRT deficiency and neurological dysfunction. But individuals with undetectable HG-PRT activity but without the Lesch-Nyhan syndrome have been described. Patients with partial HG-PRT defiency have clinically distinctive findings. Sometimes mild neurological abnormalities are observed. Because of marked overproduction of ric acid severe gouty arthritis and renal dysfunction are often encountered in both complete and partial deficiency. There is considerable molecular heterogeneity in HG-PRT deficiency in man. Mutant ebnzymes may exhibit different kinetic and electrophoretic properties, indicating that hterwe might be a mutation on the structural gene coding for HG-PRT. Lack of HG-PRT disturbs purine interconversions profoundly. In addition to an important function of HG-PRT in the uptake of the purine hypoxantine and guanine into the cell, the effective uptake of inosine, guanosine and adenosine also seems to be dependent on HG-PRT...

Hypoxanthine-guanine phosphoribosyltransferase. Characterization of a mutant in a patient with gout

The mutation in a young gouty male with a partial deficiency of hypoxanthine-guanine phosphoribosyltransferase has been evaluated. The serum uric acid was 11.8 mg/100 ml, and the urinary uric acid excretion was 1,279 mg/24 h. Erythrocyte hypoxanthine-guanine phosphoribosyltransferase was 34.2 nmol/h/mg, adenine phosphoribosyltransferase was 36.5 nmol/h/mg and phosphoribosylpyrophosphate was 2.6 muM. Hypoxanthine-guanine phosphoribosyltransferase from peripheral leukocytes and cultured diploid skin fibroblasts was within the normal range, but enzyme activity in rectal mucosa was below the normal range. Initial velocity studies of the normal enzyme and the mutant enzyme from erythrocytes with the substrates hypoxanthine, guanine, or phosphoribosylpyrophosphate showed that the Michaelis constants were similar. Product inhibition studies distinguished the mutant enzyme from the normal enzyme. Hyperbolic kinetics with increasing phosphoribosylpyrophosphate were converted to sigmoid kinetics by 0.2 mM GMP with the mutant enzyme but not with the normal enzyme. The mutant erythrocyte hypoxanthine-guanine phosphoribosyltransferase was inactivated normally at 80 degrees C and had a normal half-life in the peripheral circulation. The mol wt of 48,000 was similar to the normal enzyme mol wt of 47,000. With isoelectric focusing, the mutant erythrocyte enzyme had two major peaks with isoelectric pH's of 5.50 and 5.70, in contrast to the isoelectric pH's of 5.76, 5.82, and 6.02 of the normal isozymes. Isoelectric focusing of leukocyte extracts from the patient revealed the presence of the mutant enzyme. Cultured diploid fibroblasts from the propositus appeared to function normally, as shown by the inability to grow in 50-100 muM azaguanine and by the normal incorporation of [14C]hypoxanthine into nucleic acid. In contrast, erythrocytes from the patient displayed abnormal properties, including the increased synthesis of phosphoribosylphyrophosphate and elevated functional activity of orotate phosphoribosyltransferase and orotidylic decarboxylase. These unique kinetic, physical, and functional properties provide support for heterogeneous structural gene mutations in partial deficiencies of hypoxanthine-guanine phosphoribosyltransferase.

Hysteretic characteristic of adenine phosphoribosyltransferase

Preassay-incubation of the highly purified human erythrocyte adenine phosphoribosyltransferase (EC 2.4.2.7) (AMP pyrophosphorylase) with one of its substrates, 5-phosphoribosyl 1-pyrophosphate (PRibPP), changes the apparent V max value of the enzyme reaction. The extent of inhibition by preassay-incubation with an inhibitor, fructose 1,6-diphosphate (FDP), or a destabilizer, hypoxanthine (Hx), is found not to be proportional to the amount of the inhibitor present. The maximum inhibition achieved by preassay-incubation was about 40%. The PRibPP, FDP, and Hx induced changes in AMP pyrophosphorylase do not require the presence of divalent ions. The inhibtion of AMP pyrophosphorylase produced by preincubation with Hx was prevented when PRibPP was added to the preassay-incubation system. However, the preassay-incubation effect of FDP was only partially diminished under the same conditions. Contrary to the PRibPP-bound AMP pyrophosphorylase, the adenine-bound enzyme was found to be more heat labile than the unbound enzyme. Similar thermal instability was also observed with FDP- and Hx-bound enzyme. Our experimental results indicate that a conformational change of AMP pyrophosphorylase induced by the binding of metabolites is a slow process as compared to the overall catalytic reaction. This hysteretic characteristic of AMP pyrophosphorylase may be one of the regulatory mechanisms in purine intermediary metabolism.