A Caenorhabditis elegans model of orotic aciduria reveals enlarged lysosome‐related organelles in embryos lacking umps‐1 function

Gut granules are cell type‐specific lysosome‐related organelles found within the intestinal cells of Caenorhabditis elegans . To investigate the regulation of lysosome‐related organelle size, we screened for C. elegans mutants with substantially enlarged gut granules, identifying alleles of the vacuolar‐type H + ‐ATPase and uridine‐5′‐monophosphate synthase (UMPS)‐1. UMPS‐1 catalyzes the conversion of orotic acid to UMP; this comprises the two terminal steps in de novo pyrimidine biosynthesis. Mutations in the orthologous human gene UMPS result in the rare genetic disease orotic aciduria. The umps‐1(−) mutation promoted the enlargement of gut granules to 250 times their normal size, whereas other endolysosomal organelles were not similarly affected. UMPS‐1::green fluorescent protein was expressed in embryonic and adult intestinal cells, where it was cytoplasmically localized and not obviously associated with gut granules. Whereas the umps‐1(−) mutant is viable, combination of umps‐1(−) with mutations disrupting gut granule biogenesis resulted in synthetic lethality. The effects of mutations in pyr‐1 , which encodes the enzyme catalyzing the first three steps of de novo pyrimidine biosynthesis, did not phenotypically resemble those of umps‐1(−) ; instead, the synthetic lethality and enlargement of gut granules exhibited by the umps‐1(−) mutant was suppressed by pyr‐1(−) . In a search for factors that mediate the enlargement of gut granules in the umps‐1(−) mutant, we identified WHT‐2, an ABCG transporter previously implicated in gut granule function. Our data suggest that umps‐1(−) leads to enlargement of gut granules through a build‐up of orotic acid. WHT‐2 possibly facilitates the increase in gut granule size of the umps‐1(−) mutant by transporting orotic acid into the gut granule and promoting osmotically induced swelling of the compartment.