Renal failure due to 2,8-dihydroxyadenine urolithiasis

Crystalline Nephropathy due to APRT Deficiency: A Preventable Cause of Renal and Renal Allograft Failure

Adenine phosphororibosyl transferase (APRT) deficiency, a rare inborn error of metabolism is inherited as an autosomal recessive trait. It presents with 2,8-dihydroxyadenine (2,8-DHA) crystal nephropathy and recurrent nephrolithiasis and often progresses to end stage renal disease (ESRD). After transplant, it can recur in the allograft. If APRT deficiency is recognized early, renal failure can be prevented, arrested or reversed in native kidney and in allograft by treatment with allopurinol, which inhibits xanthine oxidase and reduces 2,8-DHA formation. We report two cases of APRT deficiency from our center. DNA sequencing of APRT gene performed in one of the cases revealed a pathogenic variant in Exon1 of APRT gene (c.3G>C; p.Met1). This variant affects the translation initiation codon and results in a start loss. The variant has previously been reported in two cases with APRT deficiency.

Long-term renal outcomes of APRT deficiency presenting in childhood

BACKGROUND

Adenine phosphoribosyltransferase (APRT) deficiency is a hereditary purine metabolism disorder that causes kidney stones and chronic kidney disease (CKD). The purpose of this study was to examine the course of APRT deficiency in patients who presented in childhood.

METHODS

The disease course of 21 (35%) patients in the APRT Deficiency Registry of the Rare Kidney Stone Consortium, who presented with manifestations of APRT deficiency and/or were diagnosed with the disorder before the age of 18 years, was studied. The effect of pharmacotherapy on renal manifestations and outcomes was thoroughly assessed.

RESULTS

Fourteen children were placed on allopurinol, 100 (25-200) mg/day, at the age of 2.6 (0.6-16.5) years. Six of these patients had experienced kidney stone events and three had developed acute kidney injury (AKI) prior to allopurinol treatment. During 18.9 (1.7-31.5) years of pharmacotherapy, stones occurred in two patients and AKI in three. Six adult patients started allopurinol treatment, 200 (100-300) mg/day, at age 29.8 (20.5-42.4) years. Five of these patients had experienced 28 stone episodes and AKI had occurred in two. Stone recurrence occurred in four patients and AKI in two during 11.2 (4.2-19.6) years of allopurinol therapy. Lack of adherence and insufficient dosing contributed to stone recurrence and AKI during pharmacotherapy. At latest follow-up, estimated glomerular filtration rate (eGFR) was 114 (70-163) and 62 (10-103) mL/min/1.73 m² in those who initiated treatment as children and adults, respectively. All three patients with CKD stages 3-5 at the last follow-up were adults when pharmacotherapy was initiated.

CONCLUSION

Timely diagnosis and treatment of APRT deficiency decreases renal complications and preserves kidney function.

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.

Recurrent 2,8-dihydroxyadenine nephropathy: a rare but preventable cause of renal allograft failure

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive enzyme defect of purine metabolism that usually manifests as 2,8-dihydroxyadenine (2,8-DHA) nephrolithiasis and more rarely chronic kidney disease. The disease is most often misdiagnosed and can recur in the renal allograft. We analyzed nine patients with recurrent 2,8-DHA crystalline nephropathy, in all of whom the diagnosis had been missed prior to renal transplantation. The diagnosis was established at a median of 5 (range 1.5-312) weeks following the transplant procedure. Patients had delayed graft function (n=2), acute-on-chronic (n=5) or acute (n=1) allograft dysfunction, whereas one patient had normal graft function at the time of diagnosis. Analysis of allograft biopsies showed birefringent 2,8-DHA crystals in renal tubular lumens, within tubular epithelial cells and interstitium. Fourier transformed infrared microscopy confirmed the diagnosis in all cases, which was further supported by 2,8-DHA crystalluria, undetectable erythrocyte APRT enzyme activity, and genetic testing. With allopurinol therapy, the allograft function improved (n=7), remained stable (n=1) or worsened (n=1). At last follow-up, two patients had experienced allograft loss and five had persistent chronic allograft dysfunction. 2,8-DHA nephropathy is a rare but underdiagnosed and preventable disorder that can recur in the renal allograft and may lead to allograft loss.

Hereditary causes of kidney stones and chronic kidney disease

Adenine phosphoribosyltransferase (APRT) deficiency, cystinuria, Dent disease, familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), and primary hyperoxaluria (PH) are rare but important causes of severe kidney stone disease and/or chronic kidney disease in children. Recurrent kidney stone disease and nephrocalcinosis, particularly in pre-pubertal children, should alert the physician to the possibility of an inborn error of metabolism as the underlying cause. Unfortunately, the lack of recognition and knowledge of the five disorders has frequently resulted in an unacceptable delay in diagnosis and treatment, sometimes with grave consequences. A high index of suspicion coupled with early diagnosis may reduce or even prevent the serious long-term complications of these diseases. In this paper, we review the epidemiology, clinical features, diagnosis, treatment, and outcome of patients with APRT deficiency, cystinuria, Dent disease, FHHNC, and PH, with an emphasis on childhood manifestations.

Adenine phosphoribosyltransferase deficiency in children

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder characterized by 2,8-dihydroxyadenine (2,8-DHA) crystalluria that can cause nephrolithiasis and chronic kidney disease. The aim of our study was to assess the clinical presentation, diagnosis, and outcome of APRT deficiency in a large pediatric cohort. All pediatric cases of APRT deficiency confirmed at the same French reference laboratories between 1978 and 2010 were retrospectively reviewed. Twenty-one patients from 18 families were identified. The median age at diagnosis was 3 years. Diagnosis was made after one or more episodes of nephrolithiasis (17 patients), after urinary tract infection (1 patient), and by family screening (3 patients). The diagnosis was based on stone analysis and microscopic examination of urine and/or enzymatic determination of APRT on red blood cells. All children had null APRT enzyme activity in erythrocytes. APRT gene sequencing was performed on 18 patients, revealing six homozygous and 12 compound heterozygous mutations. At diagnosis, half of the patients had decreased kidney function, and two children presented with acute renal failure. Allopurinol treatment was given to all patients at a median dose of 9 mg/kg/day. After a median follow-up of 5 years, all patients showed stabilization or improvement of kidney function, normal growth and development, and six patients had recurrence of nephrolithiasis. Based on these results, we conclude that an excellent outcome can be achieved in children with APRT deficiency who receive the proper treatment.

Two families with compound heterozygosity for adenine phosphoribosyltransferase deficiency

Adenine phosphoribosyltransferase deficiency is a disorder in which 2,8-dihydroxyadenine (2,8-DHA) crystalluria is caused by a congenital deficiency in the enzyme adenine phosphoribosyltransferase (APRT). In most cases, APRT deficiency is caused by autosomal recessive inheritance of a homozygote of the mutant gene APRT*Q0 or APRT*J, but there are also some cases in which the disorder is caused by the compound heterozygote APRT*Q0 and APRT*J. In the patients described here, brown round crystals were found in their urinary sediment. Crystalluria was the first sign of APRT deficiency, thereafter confirmed by genetic screening for APRT*/Q0 and APRT*. We performed genetic screening for APRT*Q0 and APRT*J in two families and diagnosed three cases of APRT*Q0 /APRT*J compound heterozygote-type APRT deficiency. Genetic screening for APRT*Q0 and APRT*J of family members is effective for early diagnosis and early treatment for family members.

Crystalline nephropathy due to 2,8-dihydroxyadeninuria: an under-recognized cause of irreversible renal failure

BACKGROUND

2,8-dihydroxyadeninuria (DHA) disease (also called 2,8 dihydroxyadeninuria) is a rare autosomal recessive disorder caused by complete adenine phosphoribosyltransferase deficiency and typically manifests as recurrent nephrolithiasis. Only rare cases of DHA nephrolithiasis have been reported from the USA. Herein, we report three American patients who developed DHA crystalline nephropathy leading to end-stage renal disease (ESRD) with recurrence in the allograft.

METHODS

Three cases of DHA crystalline nephropathy were identified from the Renal Pathology Laboratory of Mayo Clinic. Detailed clinical and pathologic descriptions are provided.

RESULTS

All three patients were Caucasian adults with no history of obstructive nephropathy. Two patients had no history of nephrolithiasis and one had a single episode of stones 36 years prior to presentation. All patients presented with severe renal failure with a mean serum creatinine of 7.5 mg/dl. Renal biopsies revealed numerous tubular and interstitial brown DHA crystals, tubular degenerative changes and moderate to marked tubulointerstitial scarring. Two patients were initially misdiagnosed, one as primary hyperoxaluria and the other as chronic interstitial nephritis. All three patients progressed to ESRD, within 1 month following renal biopsy in two and after 9 months in one. All three patients underwent renal transplantation with early disease recurrence in three allografts in two patients.

CONCLUSIONS

DHA disease is an under-recognized condition that can lead to irreversible renal failure and frequently recurs in the transplant. It should be included in the differential diagnosis of crystalline nephropathy, even in the absence of history of nephrolithiasis.

Adenine phosphoribosyltransferase deficiency in Iceland

Two children and two adults of four unrelated families were on regular light microscopic examination found to exhibit identical, spherical urine crystals. Their characteristic appearance led to the diagnosis of 2,8-dihydroxyadenine crystalluria by spectrophotometric or gas-chromatographic/mass-spectrometric analysis. Total deficiency of adenine phosphoribosyltransferase was confirmed by direct measurements of the enzyme activity in lysed red blood cells. Close family members were also examined for the enzyme defect, revealing no additional homozygotes, but 13 heterozygotes among 14 relatives. We suggest that round, brownish urine crystals, even without radiolucent kidney stones, should alert the physician to search for the existence of 2,8-dihydroxyadenine. Proper treatment could then be instituted without delay, preventing eventual kidney damage.

D-ribose therapy in four Polish patients with adenylosuccinate lyase deficiency: absence of positive effect

Deficiency of adenylosuccinate lyase (ADSL) (OMIM 103050) is an autosomal recessive disorder of the purine de novo synthesis pathway and purine nucleotide cycle, diagnosed so far in approximately 50 patients. The clinical presentation is characterized by severe neurological involvement including hypotonia, seizures, developmental delay and autistic features. Epilepsy in ADSL deficiency is frequent and occurs in approximately two-thirds of patients, beginning either early in the neonatal period or after the first year of life. At present there is no treatment of proven clinical efficacy. Despite of the increasing number of ADSL-deficient patients reported, there are only a few communications of therapeutic considerations or efforts. Among them only two showed some beneficial effects in ADSL-deficient patients. D-ribose, a simple and relatively cheap therapy, has been associated with improvement of behaviour and progressive reduction of the seizure frequency in one 13-year-old patient with ADSL deficiency. In this study we have re-examined D-ribose treatment in four ADSL-deficient patients. Assessments consisted of biochemical markers and neurological outcome. The 12-month trial of D-ribose failed to show any clinical benefit in ADSL patients with both milder and severe phenotype. D-ribose administration was accompanied by neither reduction in seizure frequency nor growth enhancement. Additionally, patients with milder type II presented the first seizure after 4 and 8 months of the D-ribose treatment. Therefore, we could not confirm a positive effect of D-ribose as previously reported.

Structural complexes of human adenine phosphoribosyltransferase reveal novel features of the APRT catalytic mechanism

Adenine phosphoribosyltransferase (APRT) is an important enzyme component of the purine recycling pathway. Parasitic protozoa of the order Kinetoplastida are unable to synthesize purines de novo and use the salvage pathway for the synthesis of purine bases rendering this biosynthetic pathway an attractive target for antiparasitic drug design. The recombinant human adenine phosphoribosyltransferase (hAPRT) structure was resolved in the presence of AMP in the active site to 1.76 A resolution and with the substrates PRPP and adenine simultaneously bound to the catalytic site to 1.83 A resolution. An additional structure was solved containing one subunit of the dimer in the apo-form to 2.10 A resolution. Comparisons of these three hAPRT structures with other 'type I' PRTases revealed several important features of this class of enzymes. Our data indicate that the flexible loop structure adopts an open conformation before and after binding of both substrates adenine and PRPP. Comparative analyses presented here provide structural evidence to propose the role of Glu104 as the residue that abstracts the proton of adenine N9 atom before its nucleophilic attack on the PRPP anomeric carbon. This work leads to new insights to the understanding of the APRT catalytic mechanism.

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.

Simple non-radiochemical HPLC-linked method for screening for purine metabolism disorders using dried blood spot

BACKGROUND

Pathologies associated with rare inherited disorders affecting purine metabolic pathways range from renal failure to neurological dysfunction and immunodeficiency. The disorders are usually diagnosed by measuring enzyme activities in hemolysates. A non-radiochemical HPLC-linked method is described for simultaneous determination of the activities of hypoxanthine-guanine phosphoribosyltransferase (HPRT: E.2.4.2.8.), adenine phosphoribosyltransferase (APRT: E.2.4.2.7.), adenosine deaminase (ADA: E.3.5.4.4.) and purine nucleoside phosphorylase (PNP: E.2.4.2.1.) in dried blood spots.

METHOD

7-mm-diameter blood spots stored at 4 degrees C or room temperature were transferred to an Eppendorf tube and eluted with 500-microl 0.1 mol/l Tris-HCl buffer, pH 7.4. The eluate was added to substrate solutions and incubated at 37 degrees C. Reaction products were analysed by HPLC.

RESULTS AND CONCLUSIONS

The enzyme activities tested in spot eluates were similar to those in erythrocyte lysates from the same subjects. None of the enzymatic activities tested were significantly affected by different storage temperatures. The main advantages of the proposed method are small blood volume required, easy sample collection and transfer, and accurate results. The method is therefore suitable for screening inborn errors of purine metabolism even in newborns.

Effect of D-ribose on purine synthesis and neurological symptoms in a patient with adenylosuccinase deficiency

Oral supplementation of 10 mmol/kg/day of D-ribose to a patient with an inherited deficit of adenylosuccinase, severe psychomotor retardation, and epilepsy caused a marked increase in plasma concentration and urinary excretion of urate, while minor changes in succinylpurine levels were observed. D-Ribose administration was accompanied by a slight improvement of behaviour and a progressive reduction of seizure frequency, which increased dramatically upon two attempts to withdraw the drug. Substitution of D-ribose with an equivalent amount of D-glucose did not result in an increase of seizure frequency.

Chronic renal failure in a mouse model of human adenine phosphoribosyltransferase deficiency

In humans, adenine phosphoribosyltransferase (APRT, EC 2.4.2.7) deficiency can manifest as nephrolithiasis, interstitial nephritis, and chronic renal failure. APRT catalyzes synthesis of AMP from adenine and 5-phosphoribosyl-1-pyrophosphate. In the absence of APRT, 2,8-dihydroxyadenine (DHA) is produced from adenine by xanthine dehydrogenase (XDH) and can precipitate in the renal interstitium, resulting in kidney disease. Treatment with allopurinol controls formation of DHA stones by inhibiting XDH activity. Kidney disease in APRT-deficient mice resembles that seen in humans. By age 12 wk, APRT-deficient male mice are, on average, mildly anemic and smaller than normal males. They have extensive renal interstitial damage (assessed by image analysis) and elevated blood urea nitrogen (BUN), and their creatinine clearance rates, which measure excretion of infused creatinine as an estimate of glomerular filtration rate (GFR), are about half that of wild-type males. APRT-deficient males treated with allopurinol in the drinking water had normal BUN and less extensive visible renal damage, but creatinine clearance remained low. Throughout their lifespans, homozygous null female mice manifested significantly less renal damage than homozygous null males of the same age. APRT-deficient females showed no significant impairment of GFR at age 12 wk. Consequences of APRT deficiency in male mice are more pronounced than in females, possibly due to differences in rates of adenine or DHA synthesis or to sex-determined responses of the kidneys.

Enhanced neoplastic lesion development with adenine-induced experimental multicystic nephropathy by adenine--a model system for the analysis of renal tumor generation in long-term hemodialysis patients

To cast light on the high incidence of renal cell tumors (RCT) in long-term hemodialysis patients, the role of background multicystic nephropathy was studied in a rat model. Group 1 animals were initially given N-ethyl-N-hydroryethylnitrosamine (EHEN) then subjected to adenine feeding until killing during weeks 20-27. Groups 2 and 3 received EHEN and adenine, respectively. All rats receiving adenine developed multicystic nephropathy. The incidence of renal cell hyperplasias (RCH) and multiplicities of both RCH and RCT in Group 1 were significantly increased as compared with Group 2, suggesting multicystic nephropathy provides favorable environment for tumor development.

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.

Gout, uric acid and purine metabolism in paediatric nephrology

Although gout and hyperuricaemia are usually thought of as conditions of indulgent male middle age, in addition to the well-known uricosuria of the newborn, there is much of importance for the paediatric nephrologist in this field. Children and infants may present chronically with stones or acutely with renal failure from crystal nephropathy, as a result of inherited deficiencies of the purine salvage enzymes hypoxanthine-guanine phosphoribosyltransferase (HPRT) and adenine phosphoribosyltransferase (APRT) or of the catabolic enzyme xanthine dehydrogenase (XDH). Genetic purine overproduction in phosphoribosylpyrophosphate synthetase superactivity, or secondary to glycogen storage disease, can also present in infancy with renal complications. Children with APRT deficiency may be difficult to distinguish from those with HPRT deficiency because the insoluble product excreted, 2,8-dihydroxyadenine (2,8-DHA), is chemically very similar to uric acid. Moreover, because of the high uric acid clearance prior to puberty, hyperuricosuria rather than hyperuricaemia may provide the only clue to purine overproduction in childhood. Hyperuricaemic renal failure may be seen also in treated childhood leukaemia and lymphoma, and iatrogenic xanthine nephropathy is a potential complication of allopurinol therapy in these conditions. The latter is also an under-recognised complication of treatment in the Lesch-Nyhan syndrome or partial HPRT deficiency. The possibility of renal complications in these three situations is enhanced by infection, the use of uricosuric antibiotics and dehydration consequent upon fever, vomiting or diarrhoea. Disorders of urate transport in the renal tubule may also present in childhood. A kindred with X-linked hereditary nephrolithiasis, renal urate wasting and renal failure has been identified, but in general, the various rare types of net tubular wasting of urate into the urine are recessive and relatively benign, being found incidentally or presenting as colic from crystalluria. However, the opposite condition of a dominantly inherited increase in net urate reabsorption is far from benign, presenting as familial renal failure, with hyperuricaemia either preceding renal dysfunction or disproportionate to it. Paediatricians need to be aware of the lower plasma urate concentrations in children compared with adults when assessing plasma urate concentrations in childhood and infancy, so that early hyperuricosuria is not missed. This is of importance because most of the conditions mentioned above can be treated successfully using carefully controlled doses of allopurinol or means to render urate more soluble in the urine. Xanthine and 2,8-DHA are extremely insoluble at any pH. Whilst 2,8-DHA formation can also be controlled by allopurinol, alkali is contraindicated. A high fluid, low purine intake is the only possible therapy for XDH deficiency.

Renal insufficiency secondary to 2,8-dihydroxyadenine urolithiasis

A 48-year-old man with a history of recurrent urolithiasis and chronic renal failure underwent a nephrectomy for a renal mass. At surgery the mass proved to be a calculus impacted in a dilated calyx. Gross examination of the kidney revealed chalky white deposits in the deep medulla and papillary tips. Histologic examination revealed chronic interstitial nephritis with brown spicules within some tubular epithelial cells and larger deposits of brown crystals within tubular lumina, the interstitium of the medulla, and papillary tips. Polarization microscopy revealed individual crystals scattered throughout the renal parenchyma. Although the arrangement of the crystals was reminiscent of uric acid, and, in fact, a clinical diagnosis of gouty nephropathy was made, x-ray diffraction analysis demonstrated crystals of 2,8-dihydroxyadenine. Enzymatic studies confirmed the complete absence of adenine phosphoribosyltransferase activity in erythrocyte lysates.

Purine enzyme defects as a cause of acute renal failure in childhood

Acute renal failure (ARF) is not listed as a usual form of presentation in hypoxanthine-guanine phosphoribosyltransferase deficiency, despite the gross uric acid overproduction in the defect. We found that a third of such patients may present in ARF when the urinary uric acid/creatinine ratio may be normal, not raised, and the defect may be suspected from the disproportionate increase in plasma uric acid. This is important in view of the potential confusion of uric acid with 2,8-dihydroxyadenine, the even more insoluble purine excreted in the other salvage enzyme disorder, adenine phosphoribosyltransferase deficiency. In that disorder, presentation in ARF is well recognized, the uric acid/creatinine ratio is also normal, but plasma urate is not raised. Our combined experience in these two disorders underlines the importance of early recognition and treatment with carefully adjusted doses of allopurinol, which may reverse or postpone renal failure.

Urolithiasis in children: current medical management

The mechanism of stone formation in the urinary tract is reviewed. Diet, urinary tract infection and metabolic disorders account for the different epidemiological patterns of stone formation. The diagnosis and management of renal tract calculi are discussed. Calcium stones are associated with hypercalciuria, urine acidification defects, the use of furosemide in premature babies, hypercalcaemia, hyperoxaluria, hyperuricosuria, an alkaline urine and hypocitraturia. Uric acid stones occur in acid urine, from increased purine synthesis with lympho- or myeloproliferative disorders or from several inborn errors of purine metabolism which can also cause xanthine or dihydroxyadenine stones. Cystinuria, inherited as an autosomal recessive disorder is best treated with a low sodium diet, a fluid intake exceeding 40 ml/kg per day maintaining urine pH between 7.5 and 8 and, if necessary, with oral penicillamine. Oxalate stones occur in relation to diet, bowel disease and primary inherited defects in oxalate metabolism. Urinary tract infection causing struvite and carbonate apatite formation is the commonest cause of stones in Europe.

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.

Urolithiasis in childhood

Urolithiasis occurs less frequently in children than it does in adults living in contemporary industrialized nations. However, renal calculi continue to be identified with greater frequency in certain children: those who live in some areas of North America (e.g., the Southeastern United States), in those with relatively common metabolic disorders such as idiopathic hypercalciuria or with congenital urinary tract malformations, and in patients who have remained immobilized for long periods. Evaluation of children with suspected urolithiasis should include a careful history and physical examination to identify associated symptoms and signs and factors known to predispose to calculus formation, appropriate radiographic and blood studies, and timed urine collections. Appropriate management varies with etiology but should include maintaining adequate fluid intake, and long-term monitoring of the activity of the stone disease.

Adenine phosphoribosyltransferase deficiency: a case diagnosed by GC-MS identification of 2,8-dihydroxyadenine in urinary crystals

Light microscopy of the urinary sediment from a child suffering from urinary tract disease showed massive crystalluria. Most of the sediment consisted of characteristic round and brownish crystals. 2,8-Dihydroxyadenine was identified in the crystals by means of gas chromatography-mass spectrometry. The diagnosis of adenine phosphoribosyltransferase deficiency was established by the finding of a very low activity of this enzyme in erythrocytes from the patient, and of half the normal activity in the patients. The patient was first treated with a diet low in purine and with a high liquid intake. She stayed symptomless on this regimen, but the crystalluria persisted. On low doses of allopurinol the crystalluria disappeared.

2,8-Dihydroxyadenine lithiasis

2,8-Dihydroxyadenine (2,8-DHA) lithiasis is a form of kidney stone previously mistaken for uric acid because of identical reactivity in non-specific tests used routinely in stone analysis. Unlike uric acid, the stones crush easily and do not react with uricase. The biochemical basis for the defect is a deficiency of the enzyme adenine phosphoribosyltransferase (APRT). A complete deficiency has been reported in 29 patients from 11 countries. The number of stone formers reported in Japan (10 homozygotes, 16 heterozygotes) Austria (3), and Switzerland (2) suggests more efficient diagnosis in those countries. The defective enzyme in heterozygotes in Japan is a kinetic mutant demonstrable in intact not lysed cells. 20% of APRT-deficient subjects have been asymptomatic. An equal number have presented in acute renal failure, three of whom are now on dialysis. Formation of the nephrotoxic 2,8-DHA can be prevented by allopurinol. This underlines the importance of early diagnosis, since such severe renal damage should be avoidable.

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.