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Idiopathic renal hypouricaemia (IRH), an autosomal recessive disorder, is an uncommon disease that presents with an increase in uric acid excretion. Most patients with IRH have homozygous deleterious mutations in the SLC22A12 gene that encodes for URAT1 protein. Gout is a multifactorial metabolic disease caused by the deposition of urate crystals. This study describes a patient with high levels of uric acid, primary gout and a novel SLC22A12 gene mutation, which is associated to IRH. The patient showed a 1 bp homozygous insertion (680insG) that resulted in substitution of threonine instead of alanine and in a premature stop codon. This finding provides information about the influence of environmental and/or epigenetic factors in Mendelian inheritance.
Idiopathic renal hypouricaemia (IRH) is an autosomal recessive disorder characterised by increased uric acid excretion.1 The main biochemical defect is abnormal uric acid transport at the proximal tubule. URAT protein seems to be the major mechanism regulating blood urate levels.2,3 Most patients with IRH (Online Mendelian Inheritance in Man 607096) harbour homozygous deleterious mutations in the SLC22A12 gene, which encodes for URAT1 protein.2 The major clinical manifestation of IRH is the presence of uric acid stones in the urinary tract.4 Around 50% of patients with IRH are Japanese.
Gout is a multifactorial metabolic disease caused by the deposition of urate crystals that produce characteristic joint inflammation. Hyperuricaemia is the most important risk factor for gout and, in 90% of patients, is secondary to renal uric acid under‐excretion. SLC22A12 has been studied in patients with gout, the mutation W258X is observed in a healthy population but in none of the patients with gout; the mutation W258X seems to be a suppressing factor for the development of gout.5SLC22A12 polymorphisms have been associated with reduced uric acid excretion and hyperuricaemia in a German population.6 In a previous study,7 authorised by the patients and the ethics committee, we did molecular analysis of the SLC22A12 gene in patients with primary gout, in first‐grade relatives and in healthy controls. We found mutations in 16 patients with gout, 3 in first‐grade relatives and none in healthy controls. Most of the mutations were heterozygous with one exception, the subject of this report. This finding provides information about the influence of environmental and/or epigenetic factors in the Mendelian inheritance.
A 39‐year‐old man was referred to our rheumatology department with a history of arthritis in the lower limbs. His parents were healthy and non‐consanguineous. The other members of his family (wife and two daughters 8 and 10 years old) were healthy. He had no familial history of gout or hyperuricaemia.
The patient had a 6 year history of podagra, acute attacks affecting the knees and ankles, and tophi. At physical examination his height was 1.70 m, weight 73 kg and waist circumference 83 cm; he had no hypertension, history of lithiasis, or chronic renal failure nor previous treatment with diuretics or salycilates.
Laboratory evaluation reported serum uric acid 11.8 mg/dl (702 μmol/l), urinary uric acid 403 mg/24 h (2.39 mmol/d), and uric acid clearance 3.7 ml/min/1.73 m2, serum creatinine 0.89 mg/dl (78.8 μmol/l), serum urea 36.4 mg/dl (6.07 mmol/l), glucose 102 mg/dl (5.67 mmol/l), triglycerides 413 mg/dl (4.66 mmol/l), cholesterol 203 mg/dl (5.25 mmol/l) and creatinine clearance 82.8 ml/min/1.73 m2. The patient had hyperuricaemia and hypertriglyceridemia, but no other criteria for metabolic syndrome according to ATP III criteria.
Gout diagnosis was made according to the American College of Rheumatology criteria. The patient received treatment with allopurinol 300 mg/day, colchicine 1 mg/day and fenofibrate 200 mg/day. Laboratory test of his daughters were within normal limits, but unfortunately it was not possible to determine blood and urinary laboratory values in his siblings because they lived in another city.
As part of our previously mentioned study,7SLC22A12 gene was analysed in this patient. DNA extraction from peripheral blood was performed as described elsewhere.8 All exons of the SLC22A12 gene were initially analysed through PCR. Conditions and primers to amplify exons 1–2 and 7–10 are described elsewhere,9 and conditions to amplify exons 3–6 are shown in table 11.. In general, PCR consisted of 30 cycles of denaturation at 94°C for 60 s, annealing at 62°C and extension at 72°C for 60 s. PCR products were purified with a PCR purification kit (Qiagen, Valencia, California, USA). DNA sequence analysis was performed using ABI PRISM 310 genetic analyser (Applied Biosystems, Foster City, California, USA) according to manufacturter's conditions. All procedures were performed twice in patients and healthy controls.
Hyperuricaemia is defined as serum urate concentration >7 mg/dl in men. Genetic and/or environmental factors such as diuretics, obesity and high alcohol intake are necessary for gout expression.10 In this study, we describe a patient with primary gout, hyperuricaemia and a novel SLC22A12 gene mutation. SLC22A12 mutations were first associated with hyporuricaemia and IRH.
Our patient showed a 1 bp homozygous insertion (680insG) within exon 4 (fig 11)) that resulted in substitution of threonine instead of alanine and in frameshift of the open reading frame; this 1 bp insertion generated a TGA premature stop codon 91 amino acids downstream from the last original amino acid (Ala 227). This mutation resulted in the subsequent generation of a non‐functional short polypeptide (318 amino acids) that lacks its intracellular C‐terminal region or well in a null allele. In this region, URAT1 directly interacts with PDZK1, a PDZ domain‐containing protein that interacts with several membrane proteins through its PDZ motif.11 PDZK1 regulates the functional activity of URAT1‐mediated urate transport in the apical membrane of renal proximal tubule.12 The association of URAT1 with PDZK1 enhances urate transport activities in HEK293 cells (1.4‐fold), and the deletion of the patient lacks its PDZ domain. The 680Gins was not present in 240 chromosomes of normal individuals, so it is likely that his abnormal allele represents rare polymorphisms. On the other hand, primary gout phenotype of our patient is not due to this kind of mutation. Our finding that the N‐terminus region of the SLC22A12 gene seems to be associated with reduced renal uric acid is worth mentioning.6
Interestingly, the mutation in our patient is similar to that reported for the classic IRH phenotype (W258X); however, our patient had unexpectedly high blood levels of uric acid, primary gout and decreased uric acid clearance. We have no explanation for this finding. We consider that normal levels of uric acid (serum and urinary) and no clinical features in the rest of the members of the family exclude the possibility that the patient has other diseases. We additionally support this because the patient has no other clinical features. The non‐penetrant IRH genotype could be attributed to single/multiple Mendelian modifier loci or environmental/epigenetic factors as in other entities.13,14,15,16 It would be interesting to know whether his young daughters will develop gout and hyperuricaemia in the third or fourth decade of life. In conclusion, we report a novel SLC22A12 deleterious mutation in a patient with high levels of serum uric acid and primary gout. This case illustrates the complexity of the pathogenesis of primary gout and IRH, and possibly of other genetic entities.
Competing interests: None.