XDH-1 inactivation causes xanthine stone formation in Caenorhabditis elegans which is inhibited by SULP-4-mediated anion exchange in the excretory cell

Xanthine dehydrogenase (XDH) is a molybdenum cofactor (Moco) requiring enzyme that catabolizes hypoxanthine into xanthine and xanthine into uric acid, the final steps in purine catabolism. Human patients with mutations in XDH develop xanthinuria which can lead to xanthine stones in the kidney, recurrent urinary tract infections, and renal failure. Currently, there are no therapies for treating human XDH deficiency. Thus, understanding mechanisms that maintain purine homeostasis is an important goal of human health. Here, we used the nematode Caenorhabditis elegans to model human XDH deficiency using two clinically relevant paradigms: Moco deficiency or loss-of-function mutations in xdh-1, the C. elegans ortholog of XDH. Both Moco deficiency and xdh-1 loss of function caused the formation of autofluorescent xanthine stones in C. elegans. Surprisingly, only 2% of xdh-1 null mutant C. elegans developed a xanthine stone, suggesting additional pathways may regulate this process. To uncover such pathways, we performed a forward genetic screen for mutations that enhance the penetrance of xanthine stone formation in xdh-1 null mutant C. elegans. We isolated multiple loss-of-function mutations in the gene sulp-4 which encodes a sulfate permease homologous to human SLC26 anion exchange proteins. We demonstrated that SULP-4 acts cell-nonautonomously in the excretory cell to limit xanthine stone accumulation. Interestingly, sulp-4 mutant phenotypes were suppressed by mutations in genes that encode for cystathionase (cth-2) or cysteine dioxygenase (cdo-1), members of the sulfur amino acid catabolism pathway required for production of sulfate, a substrate of SULP-4. We propose that sulfate accumulation caused by sulp-4 loss of function promotes xanthine stone accumulation. We speculate that sulfate accumulation causes osmotic imbalance, creating conditions in the intestinal lumen that favor xanthine stone accumulation. Supporting this model, a mutation in osm-8 that constitutively activates the osmotic stress response also promoted xanthine stone accumulation in an xdh-1 mutant background. Thus, our work establishes a C. elegans model for human XDH deficiency and identifies the sulfate permease sulp-4 as a critical player controlling xanthine stone accumulation.