Clinical and Functional Characterization of URAT1 Variants

Citation: Tasic V, Hynes AM, Kitamura K, Cheong HI, Lozanovski VJ, et al. ( Clinical and Functional Characterization of URAT1 Variants Velibor Tasic 0 Ann Marie Hynes 0 Kenichiro Kitamura 0 Hae Il Cheong 0 Vladimir J. Lozanovski 0 Zoran Gucev 0 Promsuk Jutabha 0 Naohiko Anzai 0 John A. Sayer 0 Shree Ram Singh, National Cancer Institute, United States of America 0 1 Medical School, University Children's Hospital , Skopje , Macedonia , 2 Institute of Genetic Medicine, Newcastle University , Central Parkway, Newcastle upon Tyne , United Kingdom , 3 Department of Nephrology, Kumamoto University Graduate School of Life Sciences , Kumamoto , Japan , 4 Department of Pediatrics, Seoul National University Children's Hospital , Seoul , Korea , 5 Department of Pharmacology and Toxicology, Dokkyo Medical University School of Medicine , Mibu, Tochigi , Japan 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. - Funding: This work was supported in part by grants from the Japan Society for the Promotion of Science (JSPS KAKENHI 21390073, 21659216, 21890245), the Nakatomi Foundation, and Gout Research Foundation of Japan (NA, PJ). This study was partly supported by a grant of the Korea Healthcare technology R&D Project, Ministry for Health, Welfare and Family Affairs, Republic of Korea (A080588)(HIC). AMH is supported by Newcastle Healthcare Charities and the Northern Counties Kidney Research Fund, UK. JAS is a GlaxoSmithKline clinician scientist. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. Competing Interests: The authors have read the journals policy and have the following conflicts: JAS is a GlaxoSmithKline clinician scientist. This does not alter the authors adherence to all the PLoS ONE policies on sharing data and materials. In man, the level of serum uric acid is determined primarily by the production of urate, as an end product of purine metabolism (for which the liver enzyme xanthine oxidase is necessary) versus biliary and urinary tract elimination. In the majority of other mammals, uric acid is metabolized by uricase (urate oxidase) to allantoin, before urinary excretion. Thus man (and other species lacking uricase, such as great apes), has comparably higher serum uric acid levels than most mammals. The renal handling of uric acid is a complex and incompletely understood process [1,2]. Uric acid is freely filtered at the glomerulus, the majority undergoes reabsorption via proximal tubular urate transporter proteins and a proportion (,10%) is secreted back into the filtrate in the late proximal tubule. Molecular genetic and genome wide association studies have recently allowed the identification of several proximal tubule urate transporters including URAT1 (alias SLC22A12) [3] and GLUT9 (alias SLC2A9) [4,5,6]. Proposed models of urate transport in the proximal tubule [7] suggest an initial uptake of uric acid from the filtrate by URAT1, coupled to organic acid transporters. GLUT9, in two different isoforms, allows for basolateral exit of urate from the proximal tubule (isoform I) and regulation of urate entry/exit at the apical membrane (GLUT9DN isoform). Finally, in the late proximal tubule there are transporter proteins mediating uric acid secretion (including ABCG2, NPT1 and NPT4) [7]. As uric acid excretion is mediated through molecular transporters, certain drugs such as fenofibrate, valproic acid, trimethoprim and losartan may be used to manipulate these processes [8,9,10], thus allowing manipulation of serum uric acid levels. In humans, genetic defects in the activity of xanthine oxidase or an acquired defect in liver enzyme function or renal uric acid handling may result in hypouricaemia. Acquired hypouricaemia may be seen in a number of clinical disorders, including Fanconi syndrome [11], type 1 and type 2 diabetes mellitus [12,13], thyrotoxicosis [14], pseudohypoparathyroidism type 1b [15], pseudoaldosteronism due to licorice ingestion [16], distal renal tubular acidosis [17,18], obstructive jaundice [19] and severe acute respiratory syndrome [20]. Idiopathic renal hypouricaemia is an inherited form of hypouricaemia that is characterized by excessive urinary wasting of uric acid leading to an increased clearance (and increased fractional excretion) of uric acid. The majority of patients are asymptomatic, but some may present with uric acid nephrolithiasis or acute kidney injury following severe exercise [21]. In 2002, Enomoto et al. reported that mutations in gene SLC22A12 encoding the URAT1 transporter were responsible for most cases of idiopathic renal hypouricaemia [3]. Recently Anzai et al. found mutations in SLC2A9, encoding GLUT9, in patients with severe renal hypouricaemia [22]. It is noteworthy that reports of idiopathic renal hypouricaemia secondary to mutations in uric acid transporters URAT1 and GLUT9 were initially reported from Japan, Korea and China [23]. More recently, three Jewish Israeli families of Iraqi origin have been reported to have renal hypouricaemia, with a common mutation in SLC22A12 [24]. Inactivating mutations in SLC22A12 have not yet, to our knowledge, been reported in a Caucasian population. The typical presentation of idiopathic renal hypouricaemia is that of exercise induced acute kidney injury with a preceding history of loin pain with nausea and vomiting for several hours after physical exercise. The exact mechanism of renal damage is unclear, but may relate to damage from oxygen free radicals [21]. In contrast to this dramatic presentation, most patients are well with no overt clinical symptoms, although renal stones and hematuria may be presenting symptoms and signs. Here we present data from Skopje (Macedonia) and Newcastle upon Tyne (UK) where we have investigated the underlying genetic cause of hypouricaemia in patients of European descent. We present mutations in SLC22A12 encoding URAT1 alongside thei (...truncated)