Background: Common variants within the urate transporter genes SLC22A11 (OAT4) and SLC22A12 (URAT1) have been associated with hyperuricaemia and gout in multiple populations, but these associations have been with variants with no obvious role in protein function. This research aimed to characterise the exonic sequences of SLC22A11 and SLC22A12 in European and Polynesian individuals and assess whether rare non-synonymous variants in these genes affect protein function and cause hyperuricaemia.
Methods: The exonic regions of SLC22A11 and SLC22A12 were sequenced in 422 individuals with hyperuricaemia or gout (Polynesian = 227, European = 195) and 386 individuals without hyperuricaemia or gout (Polynesian = 213, European = 173). All non-synonymous variants were identified and analysed using burden testing methods. The difference in uptake of [14C]-labelled urate between each modified protein (non-synonymous variant construct) and the wild-type protein was assessed in vitro.
Results: Sixteen missense (SLC22A11 = 9, SLC22A12 = 7) and two nonsense variants (SLC22A11 = 1, SLC22A12 = 1) were identified in the SLC22A11 (OAT4) and SLC22A12 (URAT1) exonic sequences. Burden analyses of the SLC22A12 variants produced a significant protective effect (P = 0.01) in the European cohort, but not in the Polynesian cohort (P = 0.73). This effect in European was reflected in significantly reduced [14C]-labelled urate uptake in all but one of the modified URAT1 proteins (~2 to 40% of wild-type activity). In contrast, burden analysis of SLC22A11 non-synonymous variants identified a significant risk of hyperuricaemia in Europeans (P = 0.02), but not Polynesians (P = 0.29). Wild-type OAT4 transporter activity was substantially less than that of wild-type URAT1, and only four of the seven OAT4 protein variants exhibited some reduction in activity (~60 to 70% of wild-type activity).
Conclusions: Rare non-synonymous variants in both SLC22A12 and SLC22A11 significantly associate with hyperuricaemia in European individuals. This is the first time rare genetic variants in SLC22A11 have been implicated in causing hyperuricaemia, and our findings challenge the current understanding of OAT4 (SLC22A11) as a transporter that mediates renal uric acid reabsorption; modest loss-of-function variants are associated with hyperuricemia, suggesting that OAT4 may function as a secretory urate transporter in vivo.