Idiopathic renal hypouricemia: A case report and literature review

Genotype and Phenotype of Renal Hypouricemia: A Single-Center Study from China

BACKGROUND

Renal hypouricemia (RHUC), a rare inherited disorder characterized by impaired uric acid reabsorption and subsequent profound hypouricemia, occurs mainly due to variants in SLC22A12 or SLC2A9. Only anecdotal cases and one small-scale RHUC screening study have been reported in the Chinese population.

METHODS

A total of 19 patients with RHUC from 17 unrelated families were recruited from our center. The medical history, clinical manifestations, biochemical exam, and clinical outcomes were collected. Next-generation sequencing-based targeted gene sequencing or whole exon sequencing was performed.

RESULTS

A total of 22 variants in SLC22A12 or SLC2A9 were found in 19 patients. The variant c.944G>A (p.W315X) in SLC2A9 was identified in three patients. Three variants c.165C>A (p.D55E), c.1549_1555delGAGACCC (p.E517Rfs*17), and c.1483T>C (p.W495R) in SLC22A12 and three variants c.1215+1G>A (splicing variant), c.643A>C (p.T215P), and c.227C>A (p.S76X) in SLC2A9 were novel. A proportion of 10 out of 19 patients presented with exercise-induced acute kidney injury (EIAKI). The renal outcome was favorable. Five patients had nephrolithiasis, in whom three had hypercalciuria.

CONCLUSION

The current study reported six novel variants in SLC22A12 and SLC2A9 genes of Chinese patients with RHUC. The variant c.944G>A (p.W315X) in SLC2A9 may be common in Chinese patients. EIAKI is the main clinical phenotype associated with RHUC in our cohort, with a favorable outcome. Hypercalciuria presented in some RHUC patients is a new finding.

Recurrent exercise-induced acute kidney injury associated with hypouricemia: a case report and literature review

BACKGROUND

Hereditary renal hypouricemia (RHUC) is a heterogenous disorder characterized by defective uric acid (UA) reabsorption resulting in hypouricemia and increased fractional excretion of UA. RHUC is an important cause of exercise-induced acute kidney injury (EIAKI), nephrolithiasis and posterior reversible encephalopathy syndrome (PRES). We present here an unusual case of a patient with RHUC who presented with recurrent EIAKI and had two heterozygous mutations in the SLC2A9 gene.

CASE PRESENTATION

A 43-year old man was admitted to our clinic because of bilateral loin pain, nausea and sleeplessness for 3 days after strenuous exercise. The laboratory results revealed increased levels of blood urea nitrogen (BUN) (15 mmol/l) and serum creatinine (Scr) (450 μmol/l), while the UA level was extremely low at 0.54 mg/dl, and his fractional excretion of urate (FE-UA) was 108%. The patient had an episode of acute kidney injury after playing soccer approximately 20 years ago, and on routine physical examination, his UA was less than 0.50 mg/dl. In view of the marked hypouricemia and high FE-UA, a diagnosis of RHUC was suspected, which led us to perform mutational screening of the SLC22A12 and SLC2A9 genes. DNA sequencing revealed no mutation in SLC22A12 gene, but two heterozygous mutations in the SLC2A9 gene.

CONCLUSIONS

This is a rare report of a patient with RHUC2 due to the mutation of SLC2A9. And this unique symptom of EIAKI and decreased or normal serum concentrations of UA warrant more attention as an early cue of RHUC.

Analysis of Purine Metabolism to Elucidate the Pathogenesis of Acute Kidney Injury in Renal Hypouricemia

Renal hypouricemia is a disease caused by the dysfunction of renal urate transporters. This disease is known to cause exercise-induced acute kidney injury, but its mechanism has not yet been established. To analyze the mechanism by which hypouricemia causes renal failure, we conducted a semi-ischemic forearm exercise stress test to mimic exercise conditions in five healthy subjects, six patients with renal hypouricemia, and one patient with xanthinuria and analyzed the changes in purine metabolites. The results showed that the subjects with renal hypouricemia had significantly lower blood hypoxanthine levels and increased urinary hypoxanthine excretion after exercise than healthy subjects. Oxidative stress markers did not differ between healthy subjects and hypouricemic subjects before and after exercise, and no effect of uric acid as a radical scavenger was observed. As hypoxanthine is a precursor for adenosine triphosphate (ATP) production via the salvage pathway, loss of hypoxanthine after exercise in patients with renal hypouricemia may cause ATP loss in the renal tubules and consequent tissue damage.

Mutation in the SLC2A9 Gene: A New Family with Familial Renal Hypouricemia Type 2

Familial renal hypouricemia is a rare genetic disorder characterized by a defect in renal tubular urate reabsorption. Some patients present with exercise-induced acute kidney injury and nephrolithiasis. Type II is caused by mutations in the SLC2A9 gene. Here, we report the case of a young patient who developed acute kidney injury after exercise secondary to familial renal hypouricemia type II. The same mutation was found in other asymptomatic members of his family. We review the medical literature on this condition. This case highlights the importance of considering uric acid disorders in the work-up of acute kidney injury after exercise.