Familial hypouricaemia associated with renal tubular uricosuria and uric acid calculi: case report

Clinical and functional characterization of URAT1 variants

Idiopathic renal hypouricaemia is an inherited form of hypouricaemia, associated with abnormal renal handling of uric acid. There is excessive urinary wasting of uric acid resulting in hypouricaemia. Patients may be asymptomatic, but the persistent urinary abnormalities may manifest as renal stone disease, and hypouricaemia may manifest as exercise induced acute kidney injury. Here we have identified Macedonian and British patients with hypouricaemia, who presented with a variety of renal symptoms and signs including renal stone disease, hematuria, pyelonephritis and nephrocalcinosis. We have identified heterozygous missense mutations in SLC22A12 encoding the urate transporter protein URAT1 and correlate these genetic findings with functional characterization. Urate handling was determined using uptake experiments in HEK293 cells. This data highlights the importance of the URAT1 renal urate transporter in determining serum urate concentrations and the clinical phenotypes, including nephrolithiasis, that should prompt the clinician to suspect an inherited form of renal hypouricaemia.

Two male siblings with hereditary renal hypouricemia and exercise-induced ARF

Familial renal hypouricemia with exercise-induced acute renal failure (ARF) is rare. A 45-year-old man presented with abdominal pain, vomiting, and oliguria after severe exercise. The diagnosis was ARF based on high serum creatinine (SCr) level (5.1 mg/dL [451 micromol/L]). Renal function recovered completely within 2 weeks of conservative treatment (creatinine clearance [Ccr], 100.4 mL/min [1.67 mL/s]). After remission, laboratory results showed serum urate (SUA) of 0.8 mg/dL (48 micromol/L), and fractional excretion of uric acid (FE(UA)) of 46%. The final diagnosis was ARF associated with idiopathic renal hypouricemia. Other diseases that could increase the excretion of urate were excluded. Because only mild responses were observed both in pyradinamide and benzbromarone loading tests, he was considered to be a presecretory reabsorption disorder type. The younger brother (42 years old) also had episodes of low and middle back pain after severe exercise and experienced similar attacks at least 5 times since the age of 29. SCr level was elevated in every attack. Hypouricemia (SUA, 1.0 mg/dL [59 micromol/L]) and high urinary urate excretion (FE(UA), 65.7%) also were detected. Renal function recovered almost completely without any specific treatment. Radiologic examination of the 2 cases showed bilateral urolithiasis probably caused by the high urinary urate excretion. Sequence analysis of a urate anion exchanger known to regulate blood urate level (URAT1 gene) in both brothers showed homozygous mutation in exon 4 (W258Stop), resulting in a premature truncated URAT1 protein. Both their parents and their children showed heterozygous mutation of the URAT1 gene. This is the first report of the 2 male siblings of familial renal hypouricemia complicated with exercise-induced ARF, with definite demonstration of genetic abnormality in the responsible gene (URAT1).

Renal hypouricemia: prevention of exercise-induced acute renal failure and a review of the literature

Isolated renal hypouricemia from defective uric acid reabsorption and/or secretion is a well-described entity, with a prevalence of 0.12% to 0.20% in Japan. It is rarely associated with exercise-induced acute renal failure (ARF). The etiology of ARF is debated. Prevention of ARF in renal hypouricemia has not been previously addressed. A 29-year-old Pakistani man had recurrent exercise-induced ARF. He was found to have isolated renal hypouricemia; serum uric acid 0.5 mg/dL, 24-hour urine uric acid 472 +/- 25 mg (+/- SD), and fractional excretion of uric acid 55.2% to 69.4%. Both pyrazinamide and probenecid decreased fractional excretion of uric acid and uric acid excretion rate (UV(Urate)) in our patient, suggesting either a partial presecretory and postsecretory reabsorption defect or increased secretion. We investigated renal uric acid excretion during exercise in our patient and four control subjects. All five subjects underwent a physical fitness test (PFT). Our patient developed ARF. Uric acid excretion rate increased in our patient, from 0.48 mg/min at baseline to 1.49 mg/min 4 hours after the PFT, as did the urine uric acid to urine creatinine ratio (UUa)/UCr) (0.29 to 1.49). In the controls, UV(Urate) and UUA/UCr were unchanged after the PFT: UV(Urate) was 0.46 +/- 0.10 mg/min at baseline and 0.59 +/- 0.04 mg/min 4 hours after the PFT, while UUA/UCr was 0.30 +/- 0.04 at baseline and 0.36 +/- 0.04 at 4 hours. All five subjects took allopurinol 300 mg daily for 5 days and repeated the PFT. In our patient, allopurinol prevented the ARF as well as the exercise-induced increases in UV(Urate) (0.28 mg/min to 0.22 mg/min) and UUA/UCr (0.25 to 0.17). In the controls, the UV(Urate) and UUA/UCr responses to exercise were not altered. We conclude that increased renal excretion of uric acid during exercise was responsible for the ARF in our patient with renal hypouricemia and that successful prophylaxis with allopurinol is possible.

Acute renal failure after exercise in a child with renal hypouricaemia

We describe a 12-year-old boy with renal hypouricaemia who presented with acute renal failure after a 6-h sports training session. Back and loin pain were noted at presentation. Although serum creatinine and blood urea nitrogen were elevated, the serum uric acid concentration was normal (2.6 mg/dl). This decreased to 0.3 mg/dl after recovery. The pyrazinamide and probenecid tests revealed that the cause of renal hypouricaemia in this patient was totally defective uric acid reabsorption. Acute renal failure after exercise is a rare but serious complication in paediatric patients with renal hypouricaemia.

Bilateral obstructing ureteral uric acid stones in an infant with hereditary renal hypouricemia

We report on a 15-month-old boy with renal hypouricemia who presented with acute renal failure, anuria and sepsis due to bilateral obstructing ureteral uric acid stones. He was treated successfully with extracorporeal shock wave lithotripsy. Metabolic survey of 10 relatives revealed a rare hereditary disorder in 4 siblings: isolated renal hypouricemia and hyperuricosuria. To our knowledge this is the youngest reported case of hereditary renal hypouricemia and 1 of the youngest patients to be treated with extracorporeal shock wave lithotripsy.

Familial hypouricaemia due to an isolated tubular defect of urate reabsorption

Hypouricaemia due to an isolated renal tubular defect is a rare condition. Several members from an affected family are described. The propositus is a 12-year-old girl with hypouricaemia (0.7-1.1 mg/dl) and increased fractional excretion of uric acid (50%). Pyrazinamide and sulphinpyrazone tests revealed an attenuated response in this subject to both drugs. The mother and one of the propositus' two brothers have the same defect. The other brother has uric acid levels at the lower limit of normal and increased fractional excretion of uric acid. These results suggest familial hypouricaemia due to a pre-secretory reabsorptive tubular defect, transmitted by autosomal dominant inheritance.

Hereditary renal hypouricemia in children

The renal handling of urate was investigated in four children with hereditary renal hypouricemia and in their parents. The urate/creatinine clearance ratios in the four patients were 1.02 +/- 0.28, 0.93 +/- 0.11, 1.03 +/- 0.24, and 1.46 +/- 0.26, markedly higher than those in control subjects. Except for a partial response to pyrazinamide (change in clearance ratio from 1.46 to 1.07) in one patient, pyrazinamide and benzbromarone did not affect the clearance ratios in our patients. In the parents, the urate/creatinine clearance ratios were intermediate between those of the patients and control subjects, but responses to pyrazinamide and benzbromarone were normal. These data indicate that our patients have a combined defect in renal urate reabsorption, and that one of them might be subclassified as having the hypersecretion of defect. Results also show that heterozygotes can be identified by testing their urate/creatinine clearance ratio.

Familial hyper- and hypouricemias in random and hyperlipidemic recall cohorts: the Princeton School District Family Study

Using the Princeton School Family Study, our specific aim was to estimate the prevalence of familial hyper- and hypouricemia, to estimate the proportion of probands' first-degree relatives who were similarly affected, and to evaluate the contribution of diseases, drugs, and alcohol intake (if any) to uric acid levels. We studied 379 probands and a total of 1928 subjects, 125 and 52 black probands from a randomly recalled group, 147 white and 55 black probands from a hyperlipidemic recall (top decile cholesterol and/or triglyceride) group. Familial hyper- and hypouricemias were arbitrarily identified in those kindreds having at least two first-degree relatives in the same decile as the proband, top or bottom respectively, for serum uric acid. No probands had symptomatic gout. Diseases, drugs, and alcohol intake were not consistently associated with aggregations of high and/or low uric acid levels in families, and had little relationship to uric acid levels in individuals. Of the 177 randomly recalled probands, and of the 55 black probands in hyperlipidemic recall familial hyperuricemia, with concurrent primary hyperlipoproteinemia and hypertension. Familial hypouricemia was present in 1 of 125 white and in 1 of 52 randomly recalled black kindreds, and in 3 of 147 white and 3 of 55 hyperlipidemic recall black kindreds. While familial clustering of hyperuricemia was limited, clustering of hypouricemia was much more marked. Seventy-four and 84% respectively of first-degree relatives of hypouricemic white and black probands had uric acid less than the 50th percentile. In randomly recalled probands and their first-degree relatives there were significant inverse partial correlations between uric acid and high density lipoprotein cholesterol. Inverse associations of uric acid with high density lipoprotein cholesterol and the concurrence of hyperlipoproteinemia and hypertension in hyperuricemic families points to the importance of lipoprotein and blood pressure screening in families with asymptomatic hyperuricemia. The potential ramifications of within-family clustering of hypouricemia need to be further assessed in populations, particularly in regards to uric acid nephrolithiasis.