Phenotype and genotype characterization of adenine phosphoribosyltransferase deficiency

Abstract

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.

Figure 1.

APRT deficiency causes 2,8-DHA accumulation, leading to urolithiasis and crystalline nephropathy. (A) Metabolic pathways for the disposal of adenine in human show, in the absence of APRT activity, the alternative route of oxidation by xanthine oxydase (XO) to 2,8-DHA via the 8-hydroxy-intermediate in a manner analogous to the production of uric acid from hypoxanthine via xanthine. In human, adenine cannot be converted to adenosine as hypoxanthine to inosine by purine nucleoside phosphorylase (PNP); the only alternative pathway is oxidation. The site of inhibitory effect of allopurinol on 2,8-DHA synthesis is also indicated. ADA, adenosine deaminase; AMP, adenosine monophosphate; HGPRT, hypoxanthine-guanine phosphoribosyltransferase; IMP, inosine monophosphate; PRPP, 5-phosphoribosyl-1-pyrophosphate. (B) Morphologic features of 2,8-DHA crystals and stones. (a) Crystalluria study by polarized microscopy revealing typical 2,8-DHA crystals appearing round and reddish-brown with characteristic central Maltese cross pattern. Note the presence of few crystals of calcium oxalate dihydrate (white arrows) in addition to 2,8-DHA crystals. All crystalluria examined in our patients with APRT deficiency were positive for 2,8-DHA crystals. (b) Light microscopy study of kidney allograft biopsy, showing severe tubulointerstitial injury secondary to precipitation of crystals (arrows; Masson's Trichrome staining). (c) Kidney allograft biopsy examined by polarized microscopy showing precipitation of 2,8-DHA crystals within tubular lumen and in renal interstitium. (d) 2,8-DHA stones. Surface of stones are typically rough, humpy, soft, and friable. Color is reddish-brown turning grey when drying. Stone sections are disorganized with porosities and beige to brown color. Magnifications: ×200 in Ba; ×400 in B, b and c.

Figure 2.

Diagnosis of APRT deficiency was often made late with impaired renal function. (A) Repartition of patients depending on their age when diagnosis of APRT deficiency was made. Data are provided for the 53 patients described in Table 1. (B) Repartition of patients depending on renal function at diagnosis and last follow-up. Data are provided for the 38 patients described in Table 2. eGFR, estimated GFR by MDRD formula.

Figure 3.

The IVS4+2insT nucleotide sequence at the exon 4–intron 4 junction shows the most prevalent mutation. (A and B) T insertion between nucleotides 1831 and 1832 or 1832 and 1833 in IVS4 splice donor site results in deletion of exon 4 in mRNA (A), leading to a premature termination at amino acid 110 (Ala108GluX3; B). Adapted from reference.

Figure 4.

A diagnostic algorithm is proposed for diagnosis of complete APRT deficiency. Stone analysis (combining morphologic examination by stereomicroscope and infrared spectroscopy) allows identification of 2,8-DHA in virtually all cases. When observed by microscopy in urine samples or renal biopsy in patients with crystalline nephropathy, crystals should be studied by Fourier transformed infrared microscopy, which represents a highly specific and sensitive technique. In a second step, diagnosis of APRT deficiency must be confirmed by measure of APRT activity level in erythrocyte lysates. APRT activity assay may also be helpful in patients without analyzable stone, especially when crystalluria cannot be studied (e.g., patient with anuria; technique not available). Aprt gene analysis, although not necessary for diagnosis, may be performed to identify mutations.