Hypoxanthine-guanine phosphoribosyl transferase deficiency

Chemo-Enzymatic Generation of Highly Fluorescent Nucleoside Analogs Using Purine-Nucleoside Phosphorylase

Chemo-enzymatic syntheses of strongly fluorescent nucleoside analogs, potentially applicable in analytical biochemistry and cell biology are reviewed. The syntheses and properties of fluorescent ribofuranosides of several purine, 8-azapurine, and etheno-purine derivatives, obtained using various types of purine nucleoside phosphorylase (PNP) as catalysts, as well as α-ribose-1-phosphate (r1P) as a second substrate, are described. In several instances, the ribosylation sites are different to the canonical purine N9. Some of the obtained ribosides show fluorescence yields close to 100%. Possible applications of the new analogs include assays of PNP, nucleoside hydrolases, and other enzyme activities both in vitro and within living cells using fluorescence microscopy.

Microcystin-LR-induced nuclear translocation of cGAS promotes mutagenesis in human hepatocytes by impeding homologous recombination repair

Microcystin-LR (MC-LR) has been recognized as a typical hepatotoxic cyclic peptides produced by cyanobacteria. Nowadays, due to the frequent occurrence of cyanobacterial blooms, the underlying hepatotoxic mechanism of MC-LR has become the focus of attention. In our present work, the mutagenic effect of MC-LR on human normal hepatic (HL-7702) cells regulated by cGAS was mainly studied. Here, we showed that exposure to MC-LR for 1-4 days could activate the cGAS-STING signaling pathway and then trigger immune response in HL-7702 cells. Notably, relative to the treatment with 1 μM MC-LR for 1-3 days, it was observed that when HL-7702 cells were exposed to 1 μM MC-LR for 4 days, the mutation frequency at the Hprt locus was remarkably increased. In addition, cGAS in HL-7702 cells was also found to complete the nuclear translocation after 4-day exposure. Moreover, co-immunoprecipitation and homologous recombination (HR)-directed DSB repair assay were applied to show that homologous recombination repair was inhibited after 4-day exposure. However, the intervention of the nuclear translocation of cGAS by transfecting BLK overexpression plasmid restored homologous recombination repair and reduced the mutation frequency at the Hprt locus in HL-7702 cells exposed to MC-LR. Our study unveiled the distinct roles of cGAS in the cytoplasm and nucleus of human hepatocytes as well as potential mutagenic mechanism under the early and late stage of exposure to MC-LR, and provided a novel insight into the prevention and control measures about the hazards of cGAS-targeted MC-LR.

The spectrum of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency. Clinical experience based on 22 patients from 18 Spanish families

The enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) catalyzes the reutilization of hypoxanthine and guanine to the purine nucleotides IMP and GMP, respectively. HPRT deficiency is an X-linked disorder characterized by uric acid overproduction and variable neurologic impairment. The complete deficiency of HPRT is diagnostic of Lesch-Nyhan syndrome manifested by choreoathetosis, spasticity, mental retardation, and self-injurious behavior. In some HPRT-deficient patients the enzyme defect appeared to be "partial" and the neurologic symptoms mild to severe (Kelley-Seegmiller syndrome). This has prompted the classification of HPRT deficiency in 2 distinct groups: Lesch-Nyhan syndrome and Kelley-Seegmiller syndrome, which has created much confusion. A spectrum of clinical consequences of HPRT deficiency has been recognized in small series of patients, but the complete spectrum of the neurologic disorder has not been described in a single series of patients examined by the same observers. We analyzed our experience with 22 patients belonging to 18 different families with HPRT deficiency diagnosed at "La Paz" University Hospital in Madrid over the past 16 years. The clinical spectrum of these HPRT-deficient Spanish patients was similar to the different phenotypes occasionally reported in the literature, in some cases diagnosed as Lesch-Nyhan "variants." The clinical, biochemical, enzymatic, and molecular genetic studies on these 22 patients allowed us to delineate a new classification of HPRT deficiency. Based on the neurologic symptoms, dependency for personal care, HPRT activity in hemolysate and in intact erythrocytes, and predicted protein size, patients were classified into 4 groups: Group 1 (2 patients), normal development with no neurologic symptoms, HPRT activity was detectable in hemolysates and in intact erythrocytes, and the mutation did not affect the predicted protein size. Group 2 (3 patients) mild neurologic symptoms that did not prevent independent lives, HPRT activity was detectable in intact erythrocytes, and the protein size was normal. Group 3 (2 patients), severe neurologic impairment that precluded an independent life, no residual HPRT activity, and normal protein size. Group 4 (15 patients), clinical characteristics of Lesch-Nyhan syndrome (some may not show self-injurious behavior), no residual HPRT activity, and in most (7 of 8 patients in whom the mutation could be detected) the mutation affected the predicted protein size. This classification of HPRT deficiency into 4 groups may be more useful in terms of accuracy, reproducibility, assessment for treatment trials and prognosis. The study of this Spanish series allows us to conclude that HPRT deficiency may be manifested by a wide spectrum of neurologic symptoms; the overall severity of the disease is associated with mutations permitting some degree of residual enzyme activity; and mutation analysis provides a valuable tool for prognosis, carrier identification, and prenatal diagnosis.

European surveillance on Creutzfeldt-Jakob disease: a case-control study for medical risk factors

Medical risk factors for Creutzfeldt-Jakob disease (CJD) were analyzed in a prospective ongoing case-control study based on European CJD surveillance. Detailed data on past and recent medical history were analyzed in 405 cases and controls matched by sex, age, and hospital. Data were correlated with polymorphism at codon 129 of the prion protein gene. Our analysis did not support a number of previously reported associations and failed to identify any common medical risk factor for CJD. Although not statistically significant, brain surgery was associated with an increased risk of CJD. A detailed medical history should be obtained in every suspected CJD case in order to identify iatrogenic sources of CJD.

Clinical and biochemical aspects of uric acid overproduction

Purine nucleotides are synthesized and degraded through a regulated series of reactions which end in the formation of uric acid. Increased uric acid synthesis may be the result of two major pathophysiological disorders: increased de novo purine synthesis and enhanced purine nucleotide degradation, both of which may be the result of an increased or decreased enzyme activity. In addition, some conditions and disorders associated with uric acid overproduction have been recognized as the result of increased ATP degradation or decreased synthesis of ATP. The clinical manifestations of the diseases leading to excess uric acid synthesis are heterogenous, but symptoms related to uric acid overproduction are always secondary to the precipitation of crystals in soft tissues, joints, and the kidney excretory system. In clinical practice, serum urate concentration and urinary uric acid excretion are used to assess uric acid synthesis, taking into account that a purine-rich diet can be a confounding variable. Quantification of uric acid precursors, such as adenosine, inosine, guanosine, hypoxanthine, and xanthine, in biological fluids and intracellular nucleotides has provided further insight into the metabolic disturbances underlying disorders associated with uric acid overproduction. Additional studies are necessary to define precisely the metabolic derangement in idiopathic uric acid overproduction and to assess fully the consequences of increased purine nucleotide degradation, such as free-radical formation, increased adenosine synthesis, and reduced synthesis of signal transducers.

Use of cultured human cells for biochemical analysis

The use of cultured human cells for biochemical analysis is increasing. This reflects the many advantages of such cells over tissue samples: independence of the measurements from the biological milieu of the donor, better control of experimental variables and the possibility of immortalizing the patient via his cells. Concomitant with these advantages, however, are certain new experimental variables which, if not properly controlled, can yield spurious results. This paper describes the types of cultured human cells that are generally available and the basic steps involved in their culture, as well as a discussion of the sources of variability in biochemical measurements which can be attributed to cell culture.

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.

Enzymological aspects of disorders in purine metabolism

Congenital enzyme defects of purine synthesis de novo and the salvage pathway are responsible for excessive uric acid production and are often associated with hyperuricemia and gout. On the other hand, defects of enzymes essential for the purine nucleotide cycles are the biochemical basis of dysfunction of the immune system. The influence of several congenital enzyme deficiencies on the regulation of biosynthesis de novo, on the regulation of purine nucleotide concentrations, and on adenosine concentration, as well as the effect on purine transport through cell membranes are discussed. The determination of enzymes involved in purine metabolism in noncongenital diseases seems to be of diagnostic importance. As examples, enzyme activities in lymphocytes of leukemic patients, and the determination of serum guanase activity in patients with liver dysfunction are described.

Inherited purine enzyme deficiencies: prenatal diagnosis and heterozygote detection

A number of genetically determined enzyme defects leading to disturbances of purine metabolism can prenatally be monitored, and heterozygote detection is possible in several cases. In severe hypoxanthine-guanine phosphoribosyl transferase (HGPRT) deficiency, associated with a neurological disease known as the Lesch-Nyhan Syndrome, rapid prenatal diagnosis can be performed by means of a simple quantitative radiochemical enzyme assay at the single cell level. In this X-linked recessive disease, the female heterozygotes can be detected by using cultured skin fibroblasts, but alternatively single hair root enzymes can directly be assayed. Two other genetic purine enzyme defects lead to deranged immune function: in adenosine deaminase (ADA) deficiency both T- and B-lymphocyte function are severely impaired. In purine nucleoside phosphorylase (PNP) deficiency, only certain T-cell abnormalities have been observed, with apparently normal B-cell function. Both diseases are transmitted as autosomal recessive traits. Prenatal diagnosis is possible, e.g. by means of the above mentioned microtechniques. Heterozygote detection can be done using blood cells or cultured fibroblasts. Microchemical methods offer the possibility to perform enzyme characterisation even when a very limited amount of material is available. In the case of HGPRT, ADA, and PNP, for a substrate affinity curve and pH optimum curve a few hundred cells are sufficient. These and other possibilities illustrate the general usefulness of simple microtechniques in clinical enzymology.

Human lymphocytes resistant to 6-thioguanine: restrictions in the use of a test for somatic mutations arising in vivo studied by flow-cytometric enrichment of resistant cell nuclei

A method is presented that facilitates the enumeration of 6-thioguanine(TG)-resistant human lymphocytes from peripheral blood. By the use of flow cytometry, nuclei from cells stimulated to divide in vitro and having reached their late S and G2 phases of growth, were sorted out from cultures incubated for 48 h, the last 6 h in medium containing [3H]thymidine. The out-sorted nuclei were subjected to autoradiography, and the labelling index was determined. The frequency of TG-resistant cells was calculated from the quotient (labelling index in the presence of TG/labelling index in the absence of TG). Over a period of several months, two blood donors serving as internal standards showed a variation of 20% in their frequencies of resistant lymphocytes. In a referent group of 48 blood donors, inter-individual differences in the frequency of resistant cells (range 10(-5)-10(-4) ) were established. There was a positive correlation between an increased frequency of resistant cells and the age of the blood donors. Patients treated with cytostatics or psoralen and ultraviolet radiation (PUVA) showed elevated frequencies of resistant cells (about 10(-3)-10(-2) ). We suggest that at least a part of the resistant cells scored are phenocopies and thus have not originated from somatic mutation. The high number of phenocopies after treatments with cytostatics or PUVA may depend on a temporary depletion from the cells of functioning HGPRT enzyme owing to a transcriptional block caused by the agents combined with a rather fast turnover of the enzyme. We conclude that there are restrictions in the use of this system for determining genotoxic effects in vivo.

High-performance liquid chromatographic determination of purine and pyrimidine bases, ribonucleosides, deoxyribonucleosides and cyclic ribonucleotides in biological fluids

A method is presented for the separation and quantitative determination of compounds normally related to purine and pyrimidine metabolism in biological material. The retention behaviour of nucleobases, ribonucleosides, deoxyribonucleosides and cyclic ribonucleotides has been systematically investigated by reversed-phase high-performance liquid chromatography using a non-linear gradient. Ultimately a separation of the purine and pyrimidine compounds was achieved in a 35-min run with an average detection limit of 5-10 pmol per injection. Recoveries of standards added to urine, plasma or serum were 96 +/- 5%.

Induction of 8-azaguanine resistance and sister chromatid exchange in human lymphocytes exposed to mitomycin C and X rays in vitro

Indirect evidence implies that 8-azaguanine-resistant (AGr) lymphocytes in human peripheral blood are mutants associated with the loss of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus on the active X chromosome, the mutation frequency increasing linearly with age. AGr variants are readily induced in lymphocytes exposed to mitomycin C in vitro, their incidence correlating with induced sister chromatid exchanges (SCEs). Although SCE events and the development of an AGr phenotype may reflect a common type of DNA damage, mitomycin C-induced Agr variants are not mutants but are suggested to be cells having a transcriptional block at the HPRT locus. AGr variants are also readily induced by X rays in vitro, their incidence correlates closely with the incidence of aberrations induced in the X chromosome and they are considered to have a mutational origin.

6-Thioguanine resistant peripheral blood lymphocytes in humans following psoralen, long-wave ultraviolet light (PUVA) therapy

A recently described method that enumerates variant 6-thioguanine resistant peripheral blood lymphocytes present in vivo in man as a potential marker of somatic cell mutations occurring in vivo was used to study 18 psoriatic patients receiving PUVA therapy, 16 conventinally treated psoriatic patients, 10 vitiligo patients receiving PUVA therapy and 7 untreated individuals with vitiligo. Variant lymphocyte frequencies determined for these individuals were compared with those determined for groups of 10 concurrent and 63 cumulative healthy control individuals. Variant frequencies were elevated in psoriatic and vitiligo patients receiving PUVA therapy and in conventionally-treated psoriatic patients. They were not elevated over control values in untreated vitiligo patients.

Enumeration of 6-thioguanine-resistant peripheral blood lymphocytes in man as a potential test for somatic cell mutations arising in vivo

An autoradiographic method to enumerate variant 6-thiogunanine-resistant (TGr) peripheral blood lymphocytes (PBLs) that occur in vivo in man is described. Variant cells are detected in PBL cultures stimulated to tritiated thymidine (3HTdr) incorporation in vitro with phytohemagglutinin (PHA) in the presence of TG. Cells with the naturally-occurring Lesch--Nyhan (LN) mutation served as prototype-variant cells. PBLs from a LN hemizygous male were found to be resistant to TG inhibition of PHA-stimulated 3HTdr in corporation in vitro while a LN heterozygous female was found to be a mosaic with 2/1000 PBLs resistant to 2 X 10(-4) M TG. Experiments with artificial mixtures of LN and normal PBLs showed that the LN cells were virtually all detectable even when present in low frequency (10(-5)). TGr PBLs were found in healthy non-LN individuals at median frequencies of 1.0 X 10(-4) and 1.1 X 10(-4) when determined at 2 X 10(-3) M TG and 2 X 10(-4) M TG respectively. Their frequencies were not age-related. TGr PBL-variant frequencies (Vf's) were determined in 47 cancer patients who were being treated with cytotoxic agents that are known to be mutagens. The median TGr PBL Vf determined at 2 X 10(-3) M TG in cancer patients was 2.2 X 10(-4) while, when determined at 2 X 10(-4) M TG, it was 8.5 X 10(-4). The distribution of Vf's for the treated cancer-patient group differed from that for the normal control group in that more than half of the treated cancer patients had TGr PBL Vf's greater than the highest seen for controls. Unlike those of the normal controls, the TGr PBL Vf's of treated cancer patients differed if determined at 2 X 10(-3) M TG and 2 X 10(-4) M TG, a behavior that suggested partial resistance and mimicked that seen with LN TGr PBLs. PBLs resistant to 2,6-diaminopurine (DAPr) were not found in two individuals, although the TGr PBL Vf was elevated in one. TGr PBL Vf's were greatly elevated under conditions of in vivo selection in patients receiving purine-analogue immunosuppressive therapy. The TGr PBL enumerative assay system is presented as one of potential value to detect somatic cell mutations occurring in vivo in man.

Glucose-6-phosphate dehydrogenase deficiency: biochemical and histochemical studies on hair roots for carrier detection

Kinetic properties of human hair root glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were studied in order to optimize the assay of these enzymes in lysates from single hair roots. In contrast to previously reported methods, an excess of purified 6-phosphogluconate dehydrogenase was added to the glucose-6-phosphate dehydrogenase reaction mixtures, thus allowing a more exact quantification of glucose-6-phosphate dehydrogenase activity. Although enzyme histochemical techniques suggest a similar distribution of hair root glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, enzyme assays on hair root segments after microdissection nevertheless indicate differences in the distribution of these enzymes. Upon storage a gradual drop in the activity of both hair root enzymes was found, but the rate of decrease in enzyme activity was about equal: the enzyme activity ratio was, therefore, not affected. This opens interesting possibilities for mailing hair roots for screening purposes without any special precautions.

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.

Rapid prenatal diagnosis of HG-PRT deficiency using ultra-microchemical methods

A previously developed simple ultramicromethod has been used for the rapid prenatal diagnosis of hypoxanthine-guanine phosphoribosyl transferase (HG-PRT) deficiency. The method is based on the incubation of small numbers of visually selected, lyophilized fibroblasts (in the present study five cells per incubation) with radioactive substrate in an end volume of 0.3 microliter. Fibroblasts derived from the amniotic fluid of a 15-week male fetus in a heterozygote for the X-linked Lesch-Nyhan syndrome showed a severe degree of HG-PRT deficiency. In total 50 fibroblasts were used. The diagnosis was confirmed upon termination of the pregnancy by the demonstration of HG-PRT deficiency in fetal erythrocytes and cultured skin fibroblasts.

Purine mutants of mammalian cell lines: III. Control of purine biosynthesis in adenine phosphoribosyl transferase mutants of CHO cells

Spontaneous and mutagen-induced 2,6-diaminopurine-resistant mutants of Chinese hamster ovary (CHO-K1) cells were isolated. Such mutants fell into two classes: spontaneous and ethylmethane-sulfonate-induced mutants had approximately 5% wild-type adenine phosphoribosyl transferase (APRT) activity, whereas ICR-170G-induced mutants had barely detectable APRT activity. Since it has been reported that human hypoxanthine-guanine phosphoribosyl transferase (HGPRT) (Lesch-Nyhan syndrome) and APRT mutants over-produce purines, we examined the control and rate of purine biosynthesis in the Chinese hamster mutants. End product inhibition by adenine could not be demonstrated in such mutants, indicating that the active feedback inhibitor is a nucleotide rather than the free purine base, HGPRT activity was normal in all mutants examined except in one isolate. Purine biosynthesis as measured by the accumulation of the purine biosynthetic intermediate phosphoribosyl formylglycineamide was not elevated in the mutants as might have been predicted from work with Lesch-Nyhan cells. The data also suggest that our strain of CHO-K1 is physically or functionally haploid for the APRT locus.

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.