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

Correction of the aprt Gene Using Repair-Polypurine Reverse Hoogsteen Hairpins in Mammalian Cells

In this study, we describe the correction of single-point mutations in mammalian cells by repair-polypurine reverse Hoogsteen hairpins (repair-PPRHs). These molecules consist of (1) a PPRH hairpin core that binds to a polypyrimidine target sequence in the double-stranded DNA (dsDNA), producing a triplex structure, and (2) an extension sequence homologous to the DNA sequence to be repaired but containing the wild-type nucleotide instead of the mutation and acting as a donor DNA to correct the mutation. We repaired different point mutations in the adenosyl phosphoribosyl transferase (aprt) gene contained in different aprt-deficient Chinese hamster ovary (CHO) cell lines. Because we had previously corrected mutations in the dihydrofolate reductase (dhfr) gene, in this study, we demonstrate the generality of action of the repair-PPRHs. Repaired cells were analyzed by DNA sequencing, mRNA expression, and enzymatic activity to confirm the correction of the mutation. Moreover, whole-genome sequencing analyses did not detect any off-target effect in the repaired genome. We also performed gel-shift assays to show the binding of the repair-PPRH to the target sequence and the formation of a displacement-loop (D-loop) structure that can trigger a homologous recombination event. Overall, we demonstrate that repair-PPRHs achieve the permanent correction of point mutations in the dsDNA at the endogenous level in mammalian cells without off-target activity.

Adenine phosphoribosyltransferase deficiency in the United Kingdom: two novel mutations and a cross-sectional survey

Background

Adenine phosphoribosyltransferase deficiency is an inborn error of metabolism that can cause kidney disease from crystalline nephropathy or kidney stones.

Methods

We present three cases from a single centre with varied presentations to illustrate how increasing awareness led to better patient identification. We then undertook a cross-sectional survey of all the patients identified from the Purine Research Laboratory in the UK since 1974.

Results

Our index case presented with recurrent nephrolithiasis and was diagnosed on stone analysis, the second case presented with acute kidney injury and the third case was identified from a biopsy undertaken for acute on chronic kidney injury. Genetic studies identified two novel mutations. Twenty patients were retrospectively identified. The mean age at diagnosis was 25 years (range 2–70); eight were <20 years, seven were 20–40 years and five were >40 years. Five of the 20 patients were deceased, 3 after end-stage renal disease (ESRD). Twelve have normal renal function, one had CKD stage 3, one had severe kidney disease and one was on dialysis.

Conclusions

Adenine phosphoribosyltransferase deficiency presents in a wide spectrum in all age groups. Patients can be completely asymptomatic and kidney disease may be incorrectly attributed to other conditions. Outcome is poor in late diagnosis and there is a high prevalence of ESRD. Patients with unexplained renal stone disease or deterioration in kidney function should be considered for screening. Identification and surveillance of patients in the UK can improve. There is now a rare disease registry with meetings organized that include patients, families and health care providers to improve awareness.

Recurrence of crystalline nephropathy after kidney transplantation in APRT deficiency and primary hyperoxaluria

Purpose of review

To provide transplant physicians with a summary of the pathogenesis and diagnosis of adenine phosphoribosyl transferase (APRT) deficiency and primary hyperoxaluria and, focussed on kidney transplantation, and to discuss interventions aimed at preventing and treating the recurrence of crystalline nephropathy in renal transplant recipients.

Source of information

Pubmed literature search.

Setting

Primary hyperoxaluria and APRT deficiency are rare inborn errors of human metabolism. The hallmark of these diseases is the overproduction and urinary excretion of compounds (2,8 dihydroxyadenine in APRT deficiency, oxalate in primary hyperoxaluria) that form urinary crystals. Although recurrent urolithiasis represents the main clinical feature of these diseases, kidney injury can occur as a result of crystal precipitation within the tubules and interstitium, a condition referred to as crystalline nephropathy. Some patients develop end-stage renal disease (ESRD) and may become candidates for kidney transplantation. Since kidney transplantation does not correct the underlying metabolic defect, transplant recipients have a high risk of recurrence of crystalline nephropathy, which can lead to graft loss. In some instances, the disease remains undiagnosed until after the occurrence of ESRD or even after kidney transplantation.

Key messages

Patients with APRT deficiency or primary hyperoxaluria may develop ESRD as a result of crystalline nephropathy. In the absence of diagnosis and adequate management, the disease is likely to recur after kidney transplantation, which often leads to rapid loss of renal allograft function. Primary hyperoxaluria, but not APRT deficiency, becomes a systemic disease at low GFR with oxalate deposition leading to malfunction in non-renal organs (systemic oxalosis). We suggest that these diagnoses should be considered in patients with low glomerular filtration rate (GFR) and a history of kidney stones. In APRT deficiency, stones may be confused with uric acid stones, unless specialized techniques are used (infrared spectroscopy or X-ray crystallography for urinary crystals or stone analysis; Fourier transform infrared microscopy for crystals in kidney biopsy). Where these are unavailable, and for confirmation, the diagnosis can be made by measurement of enzyme activity in red blood cell lysates or by genetic testing. In patients with primary hyperoxaluria, levels of urinary and plasma oxalate; and the presence of nearly pure calcium oxalate monohydrate in stones, which often also have an unusually pale colour and unorganized structure, increase diagnostic suspicion. Molecular genetic testing is the criterion measure. Lifelong allopurinol therapy, with high fluid intake if appropriate, may stabilize kidney function in APRT deficiency; if ESRD has occurred or is near, results with kidney transplantation after initiation of allopurinol are excellent. In primary hyperoxaluria recognized before ESRD, pyridoxine treatment and high fluid intake may lead to a substantial decrease in urinary calcium oxalate supersaturation and prevent renal failure. In non-responsive patients or those recognized later in their disease, liver transplantation cures the underlying defect and should be considered when the GFR falls below 30 ml/min/1.73 m²; in those which or near ESRD, liver transplantation and intensive dialysis before kidney transplantation may be considered to reduce the total body oxalate burden before kidney transplantation.

Limitations

The availability of diagnostic tests varies between countries and centres. Data on long term outcomes after kidney transplantation are limited, especially for APRT deficiency patients.

Implications

Increasing transplant physicians knowledge of APRT deficiency and primary hyperoxaluria should enable them to implement adequate diagnostic and therapeutic interventions, thereby achieving good outcomes after kidney transplantation.

2,8-Dihydroxyadenine urolithiasis: a not so rare inborn error of purine metabolism

Adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation and formation of urinary crystals and kidney stones. The disease can be present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available, including stone analysis, crystalluria, and APRT activity in red blood cells, make the diagnosis easy to confirm when APRT deficiency is suspected. However, the lack of recognition of this metabolic disorder frequently resulted in a delay in diagnosis and treatment with grave consequences. The early recognition and treatment of APRT deficiency are of crucial importance to prevent irreversible loss of renal function. This review summarizes the genetic and metabolic mechanisms underlying DHA stones formation and chronic kidney disease, along with the issues of diagnosis and management of APRT deficiency. Moreover, we report the mutations in the APRT gene responsible for APRT deficiency in 51 French patients (43 families) including 22 pediatric cases (18 families) among the 64 patients identified in the biochemistry laboratories of Necker Hospital, Paris (1978-2013).

Adenine phosphoribosyltransferase deficiency: Leave no stone unturned

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disease leading to generation of large amounts of 2,8-dihydroxyadenine (DHA). DHA is excreted in urine, where it precipitates into crystals due to its low solubility. DHA crystals can aggregate into stones or cause injury to the renal parenchyma (DHA nephropathy). Recurrent urolithiasis and DHA nephropathy are the two clinical manifestations of APRT deficiency. Diagnosis of APRT deficiency can be made during childhood as well as adulthood. Diagnosis mainly relies on the recognition of DHA in stones or urine crystals. Measurement of APRT activity and genetic testing are useful for confirmation of diagnosis, for family screening and should be considered in difficult cases of urolithiasis or crystalline nephropathy. Allopurinol therapy is the cornerstone of treatment and is highly effective in preventing recurrence of stones and kidney disease. High fluid intake and dietary modifications are also recommended. Early diagnosis and treatment are of paramount importance to prevent renal damage. Unfortunately, diagnosis of APRT deficiency is often overlooked and irreversible renal failure still occurs in a substantial proportion of patients. Clinicians must be alert to the possibility of APRT deficiency and consider the appropriate diagnostic tests in certain cases. This review discusses the genetic and biochemical mechanisms of APRT deficiency, and the issues of diagnosis and management.

Adenine phosphoribosyl transferase 1 is a key enzyme catalyzing cytokinin conversion from nucleobases to nucleotides in Arabidopsis

In plants, the cytokinin metabolic processes, including cytokinin biosynthesis, interconversion, inactivation, and degradation, play critical roles in the regulation of cytokinin homeostasis and plant development. Purine metabolic enzymes have been implied to catalyze the cytokinin interconversion in previous works. In this study, we report that Adenine Phosphoribosyl Transferase 1 (APT1) is the causal gene of the high-dose cytokinin-resistant mutants. APT1 catalyzes the cytokinin conversion from free bases to nucleotides, and is functionally predominant among the five members of the Arabidopsis Adenine Phosphoribosyl Transferase family. Loss of APT1 activity in plants leads to excess accumulation of cytokinin bases, thus evoking myriad cytokinin-regulated responses, such as delayed leaf senescence, anthocyanin accumulation, and downstream gene expression. Thus, our study defines APT1 as a key metabolic enzyme participating in the cytokinin inactivation by phosphoribosylation.

2,8-Dihydroxyadenine uroliths in a dog

CASE DESCRIPTION

A 43-kg (95-lb) 4-year-old neutered male mixed-breed dog was evaluated because of a 2-day history of dysuria.

CLINICAL FINDINGS

Radiography and ultrasonography revealed hydronephrosis, hydroureter, and radiolucent, hyperechoic uroliths in the right kidney and ureter and the urinary bladder. Serum bile acids concentration was within the reference interval.

TREATMENT AND OUTCOME

The uroliths in the bladder and right ureter were surgically removed and submitted for analysis. They were initially identified as urate uroliths; however, results of further analysis indicated uroliths were composed of 2,8-dihydroxyadenine (2,8-DHA), and 2,8-DHA was identified in a urine sample of the dog. Allopurinol was prescribed for the dog, and a purine-restricted diet was recommended.

CLINICAL RELEVANCE

2,8-DHA uroliths are extremely rare in humans and dogs. Such uroliths may be underdiagnosed in humans because of variability of clinical signs and difficulty in differentiating 2,8-DHA and urate uroliths and crystalluria. Uroliths composed of 2,8-DHA may be misdiagnosed as urate uroliths in dogs.

Adenine phosphoribosyltransferase deficiency

Complete adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation of this compound and the formation of urinary crystals and stones. The disease can present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available-including stone analysis, crystalluria, and APRT activity measurement-make the diagnosis easy to confirm when APRT deficiency is suspected. However, the disease can present at any age, and the variability of symptoms can present a diagnostic challenge to many physicians. The early recognition and treatment of APRT deficiency are of crucial importance for preventing irreversible loss of renal function, which still occurs in a non-negligible proportion of cases. This review summarizes the genetic and metabolic mechanisms underlying stone formation and renal disease, along with the diagnosis and management of APRT deficiency.

Genetic determinants of urolithiasis

Urolithiasis affects approximately 10% of individuals in Western societies by the seventh decade of life. The most common form, idiopathic calcium oxalate urolithiasis, results from the interaction of multiple genes and their interplay with dietary and environmental factors. To date, considerable progress has been made in identifying the metabolic risk factors that predispose to this complex trait, among which hypercalciuria predominates. The specific genetic and epigenetic factors involved in urolithiasis have remained less clear, partly owing to the candidate gene and linkage methods that have been available until now, being inherently low in their power of resolution and in assessing modest effects in complex traits. However, together with investigations of rare, Mendelian forms of urolithiasis associated with various metabolic risk factors, these methods have afforded insights into biological pathways that seem to underlie the development of stones in the urinary tract. Monogenic diseases account for a greater proportion of stone formers in children and adolescents than in adults. Early diagnosis of monogenic forms of urolithiasis is of importance owing to associated renal injury and other potentially treatable disease manifestations, but diagnosis is often delayed because of a lack of familiarity with these rare disorders. In this Review, we will discuss advances in the understanding of the genetics underlying polygenic and monogenic forms of urolithiasis.

A Japanese boy with adenine phosphoribosyltransferase (APRT) deficiency caused by compound heterozygosity including a novel missense mutation in APRT gene

We describe a 2-year-old Japanese boy with radiolucent urolithiasis and recurrent urinary tract infection. Urinalysis showed typical 2,8-dihydroxyadenine (2,8-DHA) crystals, leading to a diagnosis as adenine phosphoribosyltransferase (APRT) deficiency. The sensitivity of proliferating T cells to an adenine analogue, whose cytotoxicity is dependent on APRT, showed that he was homozygous or compound heterozygous for the APRT gene mutation. A genetic analysis revealed a compound heterozygous state for M136T and a novel missense mutation L33P, not previously reported in patients with APRT deficiency.

CONCLUSION

Adenine phosphoribosyltransferase deficiency should be suspected in all patients with radiolucent kidney stones, urinary 2,8-DHA crystals were an important finding for an early diagnosis of APRT deficiency. Appropriate treatment should be initiated to prevent the development of urolithiasis or renal failure in APRT-deficient children. The T cell method was useful to detect a homozygote or a compound heterozygote of the pathogenic allelic gene in APRT deficiency, and a genetic analysis revealed a novel mutation L33P.

Phenotype and genotype characterization of adenine phosphoribosyltransferase deficiency

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder causing 2,8-dihydroxyadenine stones and renal failure secondary to intratubular crystalline precipitation. Little is known regarding the clinical presentation of APRT deficiency, especially in the white population. We retrospectively reviewed all 53 cases of APRT deficiency (from 43 families) identified at a single institution between 1978 and 2009. The median age at diagnosis was 36.3 years (range 0.5 to 78.0 years). In many patients, a several-year delay separated the onset of symptoms and diagnosis. Of the 40 patients from 33 families with full clinical data available, 14 (35%) had decreased renal function at diagnosis. Diagnosis occurred in six (15%) patients after reaching ESRD, with five diagnoses made at the time of disease recurrence in a renal allograft. Eight (20%) patients reached ESRD during a median follow-up of 74 months. Thirty-one families underwent APRT sequencing, which identified 54 (87%) mutant alleles on the 62 chromosomes analyzed. We identified 18 distinct mutations. A single T insertion in a splice donor site in intron 4 (IVS4 + 2insT), which produces a truncated protein, accounted for 40.3% of the mutations. We detected the IVS4 + 2insT mutation in two (0.98%) of 204 chromosomes of healthy newborns. This report, which is the largest published series of APRT deficiency to date, highlights the underdiagnosis and potential severity of this disease. Early diagnosis is crucial for initiation of effective treatment with allopurinol and for prevention of renal complications.

Is adenine phophorybosiltransferase deficiency a still underdiagnosed cause of urolithiasis and chronic renal failure? A report of two cases in a family with an uncommon novel mutation

We describe two patients that had a history of recurrent renal stones and chronic renal insufficiency. The first case was a 51-year-old man with an adenine phophoribosyltransferase (APRT) deficiency who was diagnosed only after he had been referred for severe renal failure requiring hemodialysis. This led to a screening of the entire family, which identified six carriers and an additional affected relative (a 41-year-old man and the second case reported herein). Genetic analysis of the APRT gene revealed an atypical mutation previously described only once in a compound heterozygote.

Clinical, biochemical and molecular diagnosis of a compound homozygote for the 254 bp deletion-8 bp insertion of the APRT gene suffering from severe renal failure

OBJECTIVE

To determine the type of mutation in a patient with clinical diagnosis of suspected APRT deficiency.

DESIGN AND METHODS

A 51-year-old male patient, with a clinical history of two prior episodes of renal colic with urinary stone excretion (reported as uric acid stones in the first episode and as calcium oxalate stones in the second), was admitted to the hospital with severe non-oliguric renal failure (1.06 mmol/L serum creatinine), severe hyponatremia (114 mmol/L Na(+)), metabolic acidosis (14 mmol/L HCO(3)(-)) and uricemia in the normal range. Abnormalities at renal scan and persistency of severe renal failure required to start haemodialysis. Results of renal biopsy prompted us to undertake a biochemical and molecular biological evaluation of the patient for suspected adenine phosphoribosyltransferase (APRT) deficiency.

RESULTS

HPLC analysis of serum and urine, for determining purine derivative profile, showed the pathological presence of adenine in both biological fluids (3.57 micromol/L and 7.11 micromol/mmol creatinine in serum and urine, respectively; not detectable in both fluids in healthy controls). APRT assay in a sample of patient hemolysate showed no detectable activity of the enzyme (25.56+/-9.55 U/L red blood cells in control healthy subjects). Molecular biological analysis of the amplified APRT gene revealed that the patient harboured in exon 3 a homozygous 254 bp deletion-8 bp insertion, previously described only once in a compound heterozygote. Analysis of the patient family showed that heterozygotes for this APRT gene mutation, in spite of a 69% lower APRT enzymatic activity than that of healthy subjects, had no detectable adenine concentrations in both serum and urine.

CONCLUSIONS

Results of the first patient harbouring the homozygous 254 bp deletion-8 bp insertion of the APRT gene strongly indicated that definitive diagnosis of APRT deficiency (often under or misdiagnosed) would require a combined clinical, biochemical and molecular biological evaluation.

Automated screening system for purine and pyrimidine metabolism disorders using high-performance liquid chromatography

An automated screening system for purine and pyrimidine metabolism disorders using high-performance liquid chromatography (HPLC) with column switching is described. The system consists of a reversed-phase column, a cation-exchange column, a column switch, four sets of ultraviolet absorbance detectors, a microcomputer and other conventional equipment. As this system permits the simultaneous determination of urinary orotic acid, uracil, dihydrouracil, pseudouridine, xanthine, 2,8-dihydroxyadenine and succinyladenosine, it offers a useful method for the detection of orotic aciduria, dihydropyrimidine dehydrogenase deficiency, dihydropyrimidinuria, xanthinuria, adenine phosphoribosyltransferase deficiency and adenylosuccinase deficiency.

Chronic renal failure secondary to 2,8-dihydroxyadenine deposition: the first report of recurrence in a kidney transplant

We report a case of adenine phosphoribosyltransferase deficiency in which the initial presentation was chronic renal failure. Diagnosis was made after infrared microscopy analysis of microcrystalline deposits on a kidney allograft biopsy specimen. This type of presentation is rarely seen, the most frequent manifestation of this disease being urolithiasis. This is the first report of recurrence of the microcrystalline nephritis in a kidney transplant with subsequent loss of allograft function.

Accumulation of 2,8 dihydroxyadenine in bovine liver, kidneys, and lymph nodes

A variety of tissues from 20 cattle slaughtered at federally inspected facilities contained abundant light green to greenish-yellow material. Gross lesions were most common in the liver and hepatic lymph nodes. Less frequent lesions were present in the mediastinal, renal, intercostal, and gastric lymph nodes. The material was most prominent in the portal triads, and in the medullary sinuses of the lymph nodes, at times occupying up to one half of the nodal mass. Renal calculi were present in one animal. Histologically, the condition was characterized by the intracytoplasmic accumulation of innumerable brown, acicular crystals in hepatocytes, macrophages, and renal tubular epithelial cells. Less frequent large aggregates of extracellular crystals were found in the lumens of renal tubules and in portal triads. Crystals were highly birefringent when examined using polarized light. The crystals were identified as 2,8 dihydroxyadenine using X-ray diffraction, electron diffraction, infrared spectroscopy, and mass spectrometry. In mammals, adenine is normally converted to adenylate by the enzyme adenine phosphoribosyltransferase. When adenine phosphoribosyltransferase is absent, deficient, or inhibited, adenine is oxidized to 2,8 dihydroxyadenine, which is extremely insoluble at physiological pH. In human beings, an autosomal recessive disease known as 2,8 dihydroxyadeninuria is caused by a deficiency of adenine phosphoribosyltransferase.

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.

Adenine phosphoribosyltransferase deficiency: 2,8-dihydroxyadenine urolithiasis in a 48-year-old woman

We report the first patient in Finland and Scandinavia with a deficiency of adenine phosphoribosyltransferase (APRT). About 30 clinically affected patients have been reported in the literature. APRT deficiency is an enzyme disorder which is inherited autosomally in a recessive manner. The use of adenine in purine metabolism is disturbed and it accumulates in the body, where it is oxidised to poorly insoluble 2,8-dihydroxyadenine by xanthine oxidase. The dihydroxyadenine forms stones which can be mistaken for uric acid stones. Our patient had had frequent episodes of urolithiasis and the diagnosis was finally made after pyelolithotomy and stone analysis. The total APRT deficiency was detected in the haemolysate of erythrocytes. Partial deficiency of APRT in the patient's relatives showed heterozygosity of the enzyme defect. The only clinical manifestation of the defect is the formation of urinary stones. This can be prevented by diet and allopurinol.

Distribution of patients with 2,8-dihydroxyadenine urolithiasis and adenine phosphoribosyltransferase deficiency in Japan

2,8-Dihydroxyadenine urolithiasis is caused by genetic deficiencies of adenine phosphoribosyl-transferase. This disease has occurred in a large number of Japanese patients and more than half of all families with this disease are only partially deficient in enzyme activities (Japanese type adenine phosphoribosyltransferase deficiency). To clarify the reasons for the preponderance of Japanese cases we sent questionnaires to 948 Japanese urological departments. The data thus obtained indicated that 76 families had 2,8-dihydroxyadenine lithiasis and of 51 families in which adenine phosphoribosyltransferase activities were assayed 76 per cent were only partially deficient in adenine phosphoribosyltransferase activities. The distribution of the 2,8-dihydroxyadenine families was roughly similar to that of the population in Japan and the rates of the Japanese type adenine phosphoribosyltransferase deficiency families were not significantly different among the various parts of Japan. These data indicate that the wide distribution of the unique mutant gene, APRT*J, that was created many years ago in a Japanese ancestor, explains at least in part the large number of 2,8-dihydroxyadenine lithiasis and adenine phosphoribosyltransferase deficiency families among the Japanese.

Human adenine phosphoribosyltransferase deficiency. Demonstration of a single mutant allele common to the Japanese

Complete adenine phosphoribosyltransferase (APRT) deficiency causes 2,8-dihydroxyadenine urolithiasis. In previous reports, analysis of the kinetic properties of APRT from APRT-deficient Japanese subjects revealed strikingly similar abnormalities suggesting a distinct "Japanese-type" mutation. In this paper, we report studies of 11 APRT-deficient lymphoblast cell lines. Nucleotide sequence analysis of APRT genomic DNA from WR2, a Japanese-type homozygote, identified a T to C substitution in exon 5, giving rise to the substitution of threonine for methionine at position 136. RNase mapping analysis confirmed this mutation in WR2 and revealed that six other Japanese-type homozygotes carry the same mutation on at least one allele. The remaining Japanese subject, who does not express the Japanese-type phenotype, did not demonstrate this mutation. Southern blot analysis showed that all seven Japanese-type subjects were confined to one TaqI restriction fragment length polymorphism (RFLP) haplotype. These studies provide direct evidence for the nature of the mutation in the Japanese-type APRT deficiency.

Screening for adenine and hypoxanthine phosphoribosyltransferase deficiencies in human erythrocytes by high-performance liquid chromatography

A screening method using high-performance liquid chromatography (HPLC) for the simultaneous detection of deficiencies of adenine phosphoribosyltransferase (APRT) and hypoxanthine phosphoribosyltransferase (HPRT) activities in human erythrocytes is described. Both enzyme reactions of APRT and HPRT in lysates treated with a charcoal-dextran were simultaneously carried out in the same reaction tube and the enzyme activities were determined by measuring the increases in absorbance at 260 nm of adenosine and inosine converted from adenosine-5'-monophosphate and inosine-5'-monophosphate with alkaline phosphatase. Adenosine and inosine were separated from adenine and hypoxanthine by a reversed-phase column. The method could detect 1% of normal APRT activity and 0.3% of normal HPRT activity. The within-run coefficients of variation for APRT and HPRT activities were 3.2 and 3.4%, respectively.

Human adenine phosphoribosyltransferase. Identification of allelic mutations at the nucleotide level as a cause of complete deficiency of the enzyme

This study reports the first demonstration of specific mutations leading to human adenine phosphoribosyltransferase (APRT) deficiency. The molecular basis of the deficiency was investigated by determining the sequence of both alleles of a patient with a complete deficiency in APRT activity. A trinucleotide deletion, corresponding to phenylalanine on the deduced amino acid sequence, was confirmed on one allele. A single nucleotide insertion, immediately adjacent to the splice site at the 5' end of the fourth intervening sequence, was confirmed on the other allele. This insertion lead to aberrant splicing, as was demonstrated by the absence of exon 4 in the complementary DNA sequence and by altered RNase mapping analysis of the abnormal messenger RNA.

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.

2,8-Dihydroxyadenine urolithiasis: report of an adult case in the United States

2,8-Dihydroxyadeninuria is a rare purine metabolic disorder that has been reported to have caused urolithiasis in 14 cases, mostly children. Excretion of the hydroxylated metabolites of adenine results from a deficiency of adenine phosphoribosyltransferase. The insoluble calculi have a similar chemical structure to uric acid and frequently are misdiagnosed as uric acid calculi. Management differs from that of uric acid urolithiasis. We report on an adult with 2,8-dihydroxyadenine urolithiasis in the United States.

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.

Simple screening methods for hypoxanthine-guanine phosphoribosyltransferase and adenine phosphoribosyltransferase deficiencies using dried blood spots on filter paper

Simple methods for the detection of hypoxanthine-guanine phosphoribosyltransferase and/or adenine phosphoribosyltransferase deficiencies using dried filter paper blood spots were studied. Enzyme activities in the eluate from dried filter paper blood spots stored for 4 weeks at room conditions were shown to be quite stable. Autoradiographs prepared from dried filter paper blood spots and DE-81 papers soaked with enzyme reaction mixtures containing 14C-hypoxanthine and/or 14C-adenine showed sharp radioactive spots in normal subjects. No activity was evident in the cases of the Lesch-Nyhan syndrome and/or adenine phosphoribosyltransferase deficiency. The methods seem to be suitable for screening.

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.

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.

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.

Spectrum of 2,8-dihydroxyadenine urolithiasis in complete APRT deficiency

APRT deficiency may be totally benign or life threatening. The importance of early recognition/diagnosis is thus stressed. Urolithiasis (2,8-DHA stones: the precipitating factor in all cases) is treatable. With early recognition and treatment allopurinol without alkali and a diet low in purine homozygotes have remained clinically and biochemically normal to date. 'Uric acid' stones in children must always be suspect and subjected to sophisticated analysis. Diagnosis from red cell APRT activity may also have its pitfalls.

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.

Purification of adenine phosphoribosyltransferase by affinity chromatography

The purine salvage pathway enzyme adenine phosphoribosyltransferase (AMP: pyrophosphate phosphoribosyltransferase EC 2.42.7) has been purified to greater than 85% homogeneity from crude rat liver 100,000 x g supernatant in one step by affinity chromatography. The enzyme binds to an AMP-agarose column and is eluted off the column by 1 mM 5-phosphoribosyl pyrophosphate with a 50 to 80% recovery. Enzyme kinetics indicate that the mechanism of the specific elution is due to competition of the product AMP and substrate 5-phosphoribosyl pyrophosphate for the same site on the enzyme.

Hypoxanthine-guanine phosphoribosyltransferase: a simple spectrophotometric assay

A simple spectrophotometric assay is described based on the conversion of hypoxanthine to inosine monophosphate and precipitation of both the reaction product and protein with lanthanum phosphate. The extent of conversion is determined by the fall in absorbance of hypoxanthine at 249 nm. The assay is suitable for screening red cell lysates for hypoxanthine-guanine phosphoribosyltransferase deficiency.