An international team of researchers led by Professors Mark Caulfield and Patricia Munroe, from the William Harvey Research Institute at Barts and The London School of Medicine and Dentistry with Chris Cheeseman at the University of Alberta in Canada and Kelle Moley at the University of Washington in USA, have shown that the SLC2A9 gene, which encodes a glucose transporter, is also a high-capacity urate transporter, and thus possibly a new drug target for gout. Their findings are published in this week's PLoS Medicine (7 October 2008).
Several urate transporters have already been identified but recently, using an approach called genome-wide association scanning, Caulfield and others found that some genetic variants of a human gene called SLC2A9 are more common in people with high serum urate levels than in people with normal levels. SLC2A9 encodes a glucose transporter (a protein that helps to move the sugar glucose through cell membranes) and is highly expressed in the kidney's main urate handling site. Professor Caulfield and his team investigated the possibility that the protein made by the SLC2A9 gene might be a urate transporter and tested whether genetic variations in SLC2A9 might be responsible for the association between serum urate levels and high blood pressure.
The team first expressed SLC2A9 in frog eggs, a type of cell that does not have its own urate transporter. They found that SLC2A9 transported urate about 50 times faster than glucose, and that glucose facilitated SLC2A9-mediated urate transport. Similarly, over expression of SLC2A9 in human embryonic kidney cells more than doubled their urate uptake. Conversely, when the researchers used a technique called RNA interference to reduce the expression of mouse SLC2A9 in mouse cells that normally makes this protein, urate transport was reduced. Researchers then looked at two genetic variations within SLC2A9 that vary between individuals (so-called single polynucleotide polymorphisms) in nearly 900 men who had had their serum urate levels and urinary urate excretion rates measured. They found that certain genetic variations at these two sites were associated with increased serum urate levels and decreased urinary urate excretion. Finally, the researchers used a statistical technique called meta-analysis to look for an association between one of the SLC2A9 gene variants and blood pressure. In two separate meta-analyses that together involved more than 20,000 participants in several studies, there was no association between this gene variant and blood pressure.
Overall, these findings indicate that SLCA9 is a high capacity urate transporter, and suggest that this protein plays an important part in controlling serum urate levels. They provide confirmation that common genetic variants in SLC2A9 affect serum urate levels to a marked degree, although they do not show exactly which genetic variant is responsible for increasing serum urate levels. They also provide important new insights into how the kidneys normally handle urate and suggest ways in which this essential process may sometimes go wrong. The findings could eventually lead to new treatments for gout and possibly for other diseases that are associated with increased serum urate levels.
Professor Mark Caulfield said: "This MRC funded study shows how a team of international researchers can find a completely unsuspected mechanism for urate handling in the kidney. Such discoveries could pave the way for new medicines."
Citation: Caulfield MJ, Munroe PB, O'Neill D, Witkowska K, Charchar FJ, et al. (2008) SLC2A9 is a high-capacity urate transporter in humans. PLoS Med 5(9): e197. doi:10.1371/journal.pmed.0050197
Journalists may access the paper at http://www.plos.org/press/plme-05-10-caulfield.pdf
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Barts and The London School of Medicine and Dentistry
Barts and The London School of Medicine and Dentistry – at Queen Mary, University of London - offers international levels of excellence in research and teaching while serving a population of unrivalled diversity amongst which cases of diabetes, hypertension, heart disease, TB, oral disease and cancers are prevalent, within east London and the wider Thames Gateway. Through partnership with our linked trusts, notably Barts and The London NHS Trust, and our associated University Hospital trusts – Homerton, Newham, Whipps Cross and Queen's – the School's research and teaching is informed by an exceptionally wide ranging and stimulating clinical environment.
At the heart of the School's mission lies world class research, the result of a focused programme of recruitment of leading research groups from the UK and abroad and a £100 million investment in state-of-the-art facilities. Research is focused on translational research, cancer, cardiology, clinical pharmacology, inflammation, infectious diseases, stem cells, dermatology, gastroenterology, haematology, diabetes, neuroscience, surgery and dentistry.
The School is nationally and internationally recognised for research in these areas, reflected in the £40 million it attracts annually in research income. Its fundamental mission, with its partner NHS Trusts, and other partner organisations such as CRUK, is to ensure that that the best possible clinical service is underpinned by the very latest developments in scientific and clinical teaching, training and research.
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