Adenine phosphoribosyltransferase deficiency in man. Report of a second family

Implications of disorders of purine metabolism for the kidney and the urinary tract

The spectrum of kidney and urinary tract disorders related to purines comprises acute hyperuricosuric nephropathy, chronic urate nephropathy and urolithiasis. Two factors in the development of acute hyperuricosuric nephropathy are increased uric acid concentration and low pH in the tubular fluid. Chronic urate nephropathy still possess several problems: incidence (although this seems to be decreasing, presumably owing to effective prevention), the source of interstitial urate, the cause of the interstitial deposition of urate, and the role of urate deposits in the pathogenesis of the interstitial nephropathy. The relation of the experimental nephropathy to the pathogenesis of chronic urate nephropathy in the human is not yet clear but a model is proposed according to which interstitial urate derives from two sources: hyperuricaemic plasma and hyperuricosuric tubular fluid. Urolithiasis related to purines leads to uric acid-urate stones, xanthine stones, 2,8-dihydroxyadenine stones, iatrogenic xanthine and oxipurinol stones, and possibly calcium stones. Pathogenetic factors in uric acid lithiasis are hyperuricosuria (whether due to an inborn enzyme abnormality or of unknown aetiology) and low urinary pH; oliguria is a contributory factor. There remain several open questions about uric acid lithiasis: incidence, the shift of its location from lower to upper urinary tract, the interplay of pathogenetic factors, and the role of compounds which inhibit crystallization.

Genetic disorders and urolithiasis

A recent analysis of the McKusick's On-Line Mendelian Inheritance in Man (OMIM) database revealed over 30 genetic or putatively genetic conditions in which urolithiasis contributes to the disease pathology at least to some extent. There is wide clinical, biochemical, and genetic heterogeneity in many of these conditions.

Erythrocyte adenine PRPP availability in two types of APRT deficiency using silicon oil method

Erythrocyte phosphoribosylpyrophosphate availability for adenine was measured by silicon oil method previously described. The homozygotes of Japanese type APRT deficiency (n = 6, from 4 families) showed 4.3 +/- 2.7% (mean +/- standard deviation) of adenine PRPP availability and the heterozygotes (n = 5) showed 86.0 +/- 6.0% of adenine PRPP availability. All homozygotes of Japanese type APRT deficiency from 4 unrelated families show the equally decreased adenine PRPP availability and it supports the presumption of the presence of the similar defect of APRT in all families. In a Japanese family of complete APRT deficiency, adenine PRPP availability of the homozygote was undetectable and that of the heterozygote was normal low (54.3% of normal mean activity). The adenine PRPP availability of the heterozygote of complete APRT deficiency was diagnostically different from that of the homozygotes of Japanese type APRT deficiency, despite, these two conditions showed almost the same erythrocyte APRT activity. These results prove that the silicon oil method previously written is the rapid and useful method for differential diagnosis between two types of APRT deficiency.

A coupled optical assay for adenine phosphoribosyltransferase and its extension for the spectrophotometric and radioenzymatic determination of 5-phosphoribosyl-1-pyrophosphate in mixtures and in tissue extracts

A reliable assay was developed to characterize crude cell homogenates with regard to their adenine phosphoribosyltransferase activities. The 5-phosphoribosyl-1-pyrophosphate (PRPP)-dependent formation of AMP from adenine is followed spectrophotometrically at 265 nm by coupling it with the following two-stage enzymatic conversion: AMP + H2O----adenosine + Pi (5'-nucleotidase); adenosine + H2O----inosine + NH3 (adenosine deaminase). The same principle was applied to develop a spectrophotometric and a radioenzymatic assay for PRPP. The basis of the spectrophotometric assay is the absorbance change at 265 nm associated with the enzymatic conversion of PRPP into inosine, catalyzed by the sequential action of partially purified adenine phosphoribosyltransferase, commercial 5'-nucleotidase, and commercial adenosine deaminase, in the presence of excess adenine. In the radiochemical assay PRPP is quantitatively converted into [14C]inosine via the same combined reaction. Tissue extracts are incubated with excess [14C]adenine. The radioactivity of inosine, separated by a thin-layer chromatographic system, is a measure of PRPP present in tissue extracts. The radioenzymatic assay is at least as sensitive as other methods based on the use of adenine phosphoribosyltransferase. However, it overcomes the reversibility of the reaction and the need to use transferase preparations free of any phosphatase and adenosine deaminase activities.

Selection of human cells having two different types of mutations in individual cells (genetic/artificial mutants). Application to the diagnosis of the heterozygous state for a type of adenine phosphoribosyltransferase deficiency

We have previously reported the establishment and characterization of B cell lines from patients and family members with various types of adenine phosphoribosyltransferase (APRT) deficiencies. These cell lines contain, at the APRT locus, three different alleles (APRT*1, APRT*Q0, and APRT*J) that are clearly distinguishable from each other. From five genetically heterozygous cell lines with two different genotypes (APRT*1/APRTQ0 and APRT*1/APRT*J), we have selected 48 clones resistant to 2,6-diaminopurine. Resistance to this adenine analogue is a characteristic of cells having defects in both of the APRT alleles in individual cells. The mutant clones from a cell line from a complete-type heterozygote had APRT activities close to zero (mean = 0.04 nmol/min per milligram protein) in the cell extracts, while 15 clones from four cell lines from the four Japanese-type heterozygotes had significant enzyme activities (mean = 3.88 nmol/min per milligram protein). Kinetic studies on two of the mutants from two Japanese-type heterozygous cell lines have shown that affinity to substrate 5-phosphoribosyl-1-pyrophosphate was reduced, indicating that APRT in those clones reflected the characteristics of the Japanese-type enzyme. The data presented here indicate that clones we obtained are genetic/artificial mutants, each having a genetic mutation in a single allele (APRT*J or APRT*Q0) and an artificially produced mutation in the other previously functional allele (APRT*1). The present procedure provided the only diagnostic method for Japanese-type APRT heterozygotes (APRT*1/APRT*J).

Genetic and clinical studies on 19 families with adenine phosphoribosyltransferase deficiencies

Adenine phosphoribosyltransferase (APRT) deficiency leading to 2,8-dihydroxyadenine (DHA) urolithiasis has been considered a rare cause of urolithiasis and renal insufficiency. We have examined samples from 19 Japanese families with DHA lithiasis. In 79% of the families, patients only partially lacked hemolysate APRT activities, clearly contrasting with the complete deficiency in all the patients from non-Japanese families so far reported. All patients with DHA lithiasis were homozygotes for defective APRT genes, whether the deficiency was complete or partial. In family studies we found two symptomatic and four asymptomatic homozygous family members. The segregation figures are compatible with the hypothesis of a simple autosomal recessive mode of inheritance. By analyzing the data stored by a large clinical laboratory in Japan, we estimated that 0.00368% of the general population has DHA lithiasis. These data indicate that more than 1% of the general population possess mutant alleles of the APRT gene as heterozygotes. Our present studies indicate that most of the patients with this disease are undiagnosed in Japan, and probably in other countries also.

Altered kinetic properties of a mutant adenine phosphoribosyltransferase

Three siblings in a Japanese family experienced recurrent 2,8-dihydroxyadenine urolithiasis despite the presence of adenine phosphoribosyltransferase (APRT) activities in the hemolysates (19.9% to 28.2% of normal value). However, studies on viable T cells from these patients indicated that APRT was not functional in viable cells. Further analysis of the partially purified enzymes from hemolysates disclosed that patient's APRT had a reduced affinity to 5-phosphoribosyl-1-pyrophosphate (PRPP). Seven healthy members of this family whose APRT functioned normally in viable T cells had the erythrocyte enzyme levels between the patients and normal individuals (38.2% to 65.6%), suggesting that they are carriers of the defective gene. These results indicate that the defective gene code a unique mutant APRT with a reduced affinity to PRPP, and the patients are homozygotes. The mutant enzyme was also shown to be more heat-stable than normal enzyme. However, since mutant enzyme, unlike normal enzyme, was insensitive to the stabilization effect of PRPP, the latter became more heat-stable than the former when the heat treatment was performed in the presence of PRPP. This type of defect with alterations in the kinetic and physical properties of APRT as described here is likely to be a common type of APRT deficiency in Japan.

Inhibition by adenine of in vitro immunological functions of normal and adenine phosphoribosyltransferase-deficient human lymphocytes

Exogenous adenine strongly inhibited mitogen-stimulated transformation, cytoplasmic immunoglobulin production, and natural killer activity of human mononuclear leukocytes at the high concentration of 1.0 mM. These inhibitions by adenine were not due to cytotoxicity, because the viability of cultured cells was not affected by adenine up to 1.0 mM. As the magnitude of inhibition by adenine of these in vitro immunological functions was similar in normal and adenine phosphoribosyltransferase-deficient cells, its inhibition was not mediated by corresponding nucleotides. Adenine at the concentration of 0.1 mM caused 50% inhibition of cytoplasmic immunoglobulin production without alternating cell proliferation or viability. This suggests that an appropriate concentration of adenine may inhibit the differentiation of B cells to plasma cells rather than affecting cell proliferation. Understanding the mechanisms of adenine inhibition may lead to new approaches for the regulation of immune responses.

Common characteristics of mutant adenine phosphoribosyltransferases from four separate Japanese families with 2,8-dihydroxyadenine urolithiasis associated with partial enzyme deficiencies

2,8-Dihydroxyadenine urolithiasis associated with partial deficiencies of adenine phosphoribosyltransferase (APRT) has been found only among Japanese families. All Caucasian patients with the same lithiasis are completely deficient in this enzyme. Partially purified APRT from one of the Japanese families with the lithiasis associated with a partial deficiency of APRT had a reduced affinity for 5-phosphoribosyl-1-pyrophosphate (PRPP). In the present investigations, we have shown that this characteristic is common in mutant enzymes from all the four separate Japanese urolithiasis families associated with partial APRT deficiencies so far tested. The mutant enzymes also had several other characteristics in common including increased resistance to heat in the absence of PRPP and reduced sensitivity to the stabilizing effect of PRPP. These data suggest that these families have a common mutant allele (APRT*J) at the APRT gene locus.

Rapid method for the diagnosis of partial adenine phosphoribosyltransferase deficiencies causing 2,8-dihydroxyadenine urolithiasis

More than half of the Japanese patients with 2,8-dihydroxyadenine urolithiasis only partially lack adenine phosphoribosyltransferase (APRT), while all the Caucasian patients with the same disease completely lack the enzyme. APRT activities in healthy heterozygotes for the complete APRT deficiencies were at the same levels as the Japanese patients, and simple enzyme assay does not distinguish between these two conditions. We have previously shown, using viable T-cells, that the enzyme was non-functional in the cells from the Japanese patients although they contain considerable APRT activities in the cell extracts. In the present investigations, we devised a rapid method using erythrocytes for the diagnosis of partial APRT deficiencies accompanied by severe impairment in adenine metabolism causing 2,8-dihydroxyadenine lithiasis. Thus, erythrocytes from three different families with 2,8-dihydroxyadenine urolithiasis associated with partial APRT deficiencies incorporated only minimal amounts of radioactive adenine, while normal erythrocytes incorporated significant amounts. These data indicate that severe impairment in adenine metabolism is shown not only in viable T-cells but also in viable erythrocytes. The present procedures provide a rapid method suitable for routine clinical use for the diagnosis of partial APRT deficiencies causing 2,8-dihydroxyadenine lithiasis.

Severe impairment in adenine metabolism with a partial deficiency of adenine phosphoribosyltransferase

Among three unrelated patients with recurrent 2,8-dihydroxyadenine urolithiasis, two completely lacked adenine phosphoribosyltransferase (APRT) in both erythrocytes and proliferative T cells. The third patient possessed significant enzyme activities in both hemolysates and T-cell extracts at levels comparable to heterozygotes for complete APRT deficiency. Despite significant APRT activities in cell extracts, cultured T cells from the third patient were at least 100-fold more resistant than normal T cells to an adenine analog, 6-methylpurine, whose cytotoxicity is dependent on APRT. These data indicate that APRT activity in T cells from the third patient is positive in cell extracts, but apparently not operating in viable cells. Although the cells from the patients with complete APRT deficiency were as resistant to 6-methylpurine as the cells from the third patient, the cells from the heterozygotes for complete APRT deficiency were almost as sensitive as normal T cells. Therefore, adenine metabolism in the third patient but not in the heterozygotes seems to be as severely impaired as in the patients with complete APRT deficiency, which is quite consistent with the clinical manifestations in these individuals.

Cloning the complete human adenine phosphoribosyl transferase gene

We have isolated a clone from a human genomic lambda library which cross-hybridises with the cloned hamster adenine phosphoribosyl transferase gene (aprt). After restriction mapping and further hybridisation to the hamster gene, a series of putative human aprt-containing fragments has been isolated and tested for ability to transform adenine phosphoribosyl transferase-deficient (aprt-) strains of Chinese hamster ovary (CHO) cells to APRT proficiency. Transforming activity was detected in a 48-kb lambda clone, the 17.4-kb EcoRI insert, and an 8.6-kb HincII fragment. Smaller fragments have thus far shown no transforming activity. Transformants appear to be stable for the APRT+ phenotype, and human aprt DNA sequences are present in the hamster transformants. The 8.6-kb HincII fragment has been subcloned and the insert mapped. Nonrepetitive regions of this subclone have been identified, and should prove valuable for chromosome walking studies on human chromosome 16, familial studies of a human aprt- trait, the analysis of restriction fragment length polymorphisms (RFLPs) in the area surrounding the aprt gene, and the fine structure mapping of the mutations induced by chemical carcinogens and alkylating agents.

Human adenine phosphoribosyltransferase: characterization from subjects with a deficiency of enzyme activity

Adenine phosphoribosyltransferase (APRT) was characterized with respect to specific activity and immunoreactive protein (CRM) levels in hemolysate from 18 members of an APRT-deficient kindred. In addition, lymphoblastoid cell lines were established from six of these subjects and APRT from these cells was characterized in a similar fashion. Levels of specific activity and CRM in patients homozygous for the deficiency were less than 1% of normal. Heterozygous subjects had higher levels of activity and CRM in lymphoblasts than in erythrocytes and, in all cases, the APRT present was normal in terms of isoelectric point, subunit molecular weight, and heart stability. The higher levels of activity and CRM found in lymphoblasts may be due either to expression of a mutant gene product stabilized in a normal:mutant dimer or to autologous regulation.

2,8-dihydroxyadenine urolithiasis: review of the literature and report of a case in the United States

Urolithiasis resulting from inherited metabolic derangement is rare. Only 13 cases of 2,8-dihydroxyadenine stones resulting from a deficiency of the enzyme adenine phosphoribosyl transferase have been reported since 1974. Of these cases 9 have been in children with the homozygous trait. To date, 3 homozygous and 1 heterozygous adults with urolithiasis have been reported. This disease has not been associated with any other clinical or biochemical abnormalities. Treatment includes low purine diet and allopurinol. We herein report a case of complete adenine phosphoribosyl transferase deficiency associated with 2,8-dihydroxyadenine urolithiasis in the United States, bringing the total to 14 in the literature.

Characterization of the biochemical basis of a complete deficiency of the adenine phosphoribosyl transferase (APRT)

In order to study the biochemical basis of a complete deficiency of adenine phosphoribosyl transferase (APRT) the enzyme was purified to homogeneity, its properties were characterized, and antibodies raised. The enzyme is indirectly involved in adenine uptake. Apparently, by forming AMP the internal concentration of adenine is kept low allowing it diffusion. The same APRT is present in various tissues as was revealed by antibody inactivations employing anti-erythrocyte APRT as well as by direct enzyme assays in cells from the APRT deficient patient. In vitro cultured fibroblasts derived from this patient had less than 0.02% enzyme activity. No cross-reacting material was found in erythrocytes obtained from an APRT deficient child.

Structural features of the phosphoribosyltransferases and their relationship to the human deficiency disorders of purine and pyrimidine metabolism

Similarities in the physical and chemical properties of the phosphoribosyltransferase family of enzymes suggest that they may share common structural features as observed in other functionally related proteins. The unusually high incidence of structural gene mutations of these enzymes in man are associated with several metabolic diseases of purine and pyrimidine metabolism. It is proposed that these disorders are the consequence of structural mutations to an architectural domain common to all of the phosphoribosyltransferases.

Purine reutilization and synthesis de novo in long-term human lymphocyte cell lines deficient in adenine phosphoribosyltransferase activity

Clonal lines, with either partial or total deficiency of adenine phosphoribosyltransferase (APRT) were derived from the WI-L2 long-term human lymphocyte line by selection for resistance to the adenine analogs 8-azaadenine or 2,6-diaminopurine. Resistance to 8-azaadenine also conferred resistance to 2,6 diaminopurine and vice versa. Cells with 30--40% of wild-type APRT activity were selected by resistance to 0.01 mM 2,6-diaminopurine or 1.40 mM 8-azaadenine. The APRT in the 8-azaadinine-resistant cells exhibited a four- to sevenfold increase in the apparent Km for adenine. Activities of three other purine reutilization and interconversion enzymes in the resistant cells, including hypoxanthine phosphoribosyltransferase (HPRT), adenosine kinase, and adenosine deaminase, were within the range of wild-type activities. The doubling times of the APRT-deficient cells in purine-free medium was not different from wild-type cells. The APRT in the 8-azaadenine-resistant cells did not have an altered mobility in glycerol gradients as compared to wild-type cells. The rate of purine synthesis de novo and intracellular levels of 5-phosphoribosyl-1-pyrophosphate were unchanged in the APRT-deficient cells as compared to WI-L2. The ability of the cells to reutilize exogenous adenine, however, was severely impaired.

Adenine phosphoribosyltransferase: a simple spectrophotometric assay and the incidence of mutation in the normal population

The significance of partial deficiency of erythrocyte adenine phosphoribosyltransferase (APRT), reported in a number of subjects with gout, has been investigated by studying its incidence in 700 normal blood donors. Three clearly deficient subjects were found, an incidence not significantly different from that in patients with abnormalities of urate metabolism. A new assay method for APRT is described in which an erythrocyte lysate is incubated with adenine and phosphoribosylpyrophosphate (PRPP) for a given time; both hemoglobin and adenine nucleotide (AMP) are then precipitated with lanthanum phosphate; the change in absorbance of adenine at 260 nm reflects the extent of its conversion to AMP by APRT.

Acquired increases of human erythrocyte purine enzymes

The alterations of three erythrocyte purine enzymes were studied in 12 patients with diseases associated with reticulocytosis, two patients with a partial deficiency of hypoxanthine-guanine phosphoribosyltransferase, seven patients with severe megaloblastic anemia, and 14 normal subjects. The specific activity of adenine phosphoribosyltransferase was positively correlated (r = 0.81) with the reticulocyte percentate in ten patients with a normal hypoxanthine-guanine phosphoribosyltransferase. Two apparent types of alterations of this enzyme were distinguished: (1) increased specific activity with a normal half life as in megaloblastic anemia, and (2) a prolonged half life with or without an elevation of specific activity as in the deficiency of hypoxanthine-guanine phosphoribosyltransferase. Hypoxanthine-guanine phosphoribosyltransferase and phosphoribosylpyrophosphate synthetase were increased in megaloblastic anemia, but were not correlated with the reticulocyte percentage and did not have a consistent change in the half life in the other disorders studied. The data show that acquired disorders associated with reticulocytosis may cause an elevation of the specific activity of purine enzymes in peripheral circulating erythrocytes. Therefore, these factors must be carefully considered in the interpretation of an elevated level of enzyme activity.

Erythrocyte adenosine kinase activity in gout

Erythrocyte adenosine kinase (AK) (EC 2.7.1.20) and guanosine monophosphate (GMP) reductase (EC 1.6.6.8) were measured in healthy male controls and primary gout subjects. Adenosine kinase activity in 19 controls and 26 gouty subjects was 0.717 +/- 0.176 and 0.615 +/- 0.128 nmol/mg protein/h, respectively. The difference was statistically significant (p less than 0.05). GMP reductase activity in 39 controls and 46 gouty subjects was 30.90 +/- 6.28 and 33.43 +/- 7.97 mumol/mg protein/h, respectively, without statistically significant difference.

Adenine phosphoribosyltransferase deficiency: its inheritance and occurrence in a female with gout and renal disease

A deficiency of adenine phosphoribosyltransferase (APRT) enzyme activity to approximately 40% of normal has been found in erythrocytes from a young woman aged 24 years, who had suffered from recurrent gouty arthritis since 11 years of age. She also demonstrated considerable, although asymptomatic, renal impairment with a creatinine clearance of one-third normal. Her father had suffered from gouty arthritis but had a normal APRT activity; he was obese, had a high purine intake and was a regular beer drinker. The patient's mother was asymptomatic with a normal serum urate concentration, but demonstrated a similar reduction in APRT activity to that of her daughter. Eleven other asymptomatic members of the family also demonstrated a similar reduction in APRT activity in erythrocyte lysates. The pattern of inheritance was consistent with autosomal transmission. Concentrations of phosphoribosylpyrophospate (PRPP) in erythrocytes were within normal limits both in the subjects with deficient, and in those with normal, APRT activity. Partial purification of APRT enzyme from erythrocytes of the index case did not reveal any difference from the normal enzyme as far as Michaelis constants, heat stability, or mobility in polyacrylamide gel was concerned. No primary abnormality of lipoprotein metabolism was demonstrated either in the propositus or in other members of her family. Study of urate metabolism in the propositus indicated that, although urate production was within the normal range in absolute terms, there was increased incorporation of glycine into produced urate, usually taken as one index of de novo urate production. Impaired renal excretion of urate was also shown. Although detailed study of urate metabolism has not been undertaken in other family members with APRT deficiency, no conclusive relationship has yet been demonstrated between APRT deficiency and disordered urate metabolism.