Vitamin C transporter SVCT1 serves a physiological role as a urate importer: functional analyses and in vivo investigations

Uric acid, the end product of purine metabolism in humans, is crucial because of its anti-oxidant activity and a causal relationship with hyperuricemia and gout. Several physiologically important urate transporters regulate this water-soluble metabolite in the human body; however, the existence of latent transporters has been suggested in the literature. We focused on the Escherichia coli urate transporter YgfU, a nucleobase-ascorbate transporter (NAT) family member, to address this issue. Only SLC23A proteins are members of the NAT family in humans. Based on the amino acid sequence similarity to YgfU, we hypothesized that SLC23A1, also known as sodium-dependent vitamin C transporter 1 (SVCT1), might be a urate transporter. First, we identified human SVCT1 and mouse Svct1 as sodium-dependent low-affinity/high-capacity urate transporters using mammalian cell-based transport assays. Next, using the CRISPR-Cas9 system followed by the crossing of mice, we generated Svct1 knockout mice lacking both urate transporter 1 and uricase. In the hyperuricemic mice model, serum urate levels were lower than controls, suggesting that Svct1 disruption could reduce serum urate. Given that Svct1 physiologically functions as a renal vitamin C re-absorber, it could also be involved in urate re-uptake from urine, though additional studies are required to obtain deeper insights into the underlying mechanisms. Our findings regarding the dual-substrate specificity of SVCT1 expand the understanding of urate handling systems and functional evolutionary changes in NAT family proteins.

Influence of urate-lowering therapies on renal handling of uric acid

The purpose of this study is to investigate the effect of urate-lowering therapies (ULTs) on renal uric acid excretion in gout patients. This prospective observational study involved 106 primary gout patients and 51 healthy controls. Gout patients received ULT with either xanthine oxidase inhibitors or the uricosuric agent benzbromarone. Parameters such as 24-h urinary uric acid, creatinine clearance, uric acid clearance, glomerular filtration load of uric acid, fractional excretion of uric acid, excretion of uric acid per volume of glomerular filtration, and urinary uric acid to urinary creatinine ratio were used to evaluate the pre- and post-treatment renal capacity for uric acid clearance in gout patients and were compared with the values in the healthy controls. Compared to healthy controls, gout patients had higher glomerular filtration load of uric acid and lower uric acid clearance, creatinine clearance, and fractional uric acid excretion. After ULT, both the xanthine oxidase inhibitor group and benzbromarone group patients showed reduction in glomerular filtration load of uric acid. Creatinine clearance was significantly improved in the xanthine oxidase inhibitor group. Excretion function was remarkably enhanced in patients who reached the treatment target (serum uric acid <6 mg/dl). Changes in glomerular uric acid filtration load were significantly correlated with changes in serum urate levels. Gout patients have impaired renal uric acid excretion. ULTs reduce renal urate load and enhance the renal capacity of uric acid clearance. Xanthine oxidase inhibitors showed superiority over benzbromarone in improving renal function.

Frequency of CYP2C9 polymorphisms in Polynesian people and potential relevance to management of gout with benzbromarone

OBJECTIVES

Gout is a major health problem in Polynesians and allopurinol, the drug of choice for the management gout, appears to be less effective in Polynesian patients. The uricosuric drug benzbromarone is an alternative treatment but CYP2C9 poor metabolisers (PMs) may be at a heightened risk of benzbromarone-induced hepatotoxicity. The objectives of this study were to determine the frequency of the PM alleles CYP2C9*2 and CYP2C9*3 in New Zealand (NZ) Caucasian and Polynesian gout cohorts; and then to test for novel CYP2C9 polymorphisms in Polynesians.

METHODS

Eight hundred and fifty-two Caucasians (537 controls, 315 gout patients) and 1072 Māori and Pacific Island (Polynesian) people (620 controls, 452 gout patients) were genotyped for CYP2C9*2 and CYP2C9*3. Forty Polynesians were screened for novel CYP2C9 polymorphisms using whole genome sequencing.

RESULTS

Frequency of CYP2C9 PM alleles was significantly higher in Caucasians compared to Polynesians (CYP2C9*2: 13.5% versus 3.1%; CYP2C9*3: 5.5% versus 1.6%, P<1.2E-11). Within Polynesians, CYP2C9 PM alleles were rarer in Western Polynesians (Samoa, Tonga) than Eastern Polynesians (NZ and Cook Island Maori; CYP2C9*2: 0.6% versus 2.5%; CYP2C9*3: 0.4% versus 2.0%; P<0.03). A total of 152 SNPs were found by sequencing. None of these variants were predicted by in silico analysis to significantly impact on CYP2C9 expression or activity.

CONCLUSION

Prospective CYP2C9 genotyping of Caucasian gout patients may be warranted for benzbromarone, whereas the low frequencies of CYP2C9 PM alleles in Polynesians suggests that the CYP2C9 polymorphism may be of little or no relevance to benzbromarone prescribing in this population.