Aquaporin 9 induction in human iPSC‐derived hepatocytes facilitates modeling of ornithine transcarbamylase deficiency

BACKGROUND AND AIMS: Patient‐derived human‐induced pluripotent stem cells (hiPSCs) differentiated into hepatocytes (hiPSC‐Heps) have facilitated the study of rare genetic liver diseases. Here, we aimed to establish an in vitro liver disease model of the urea cycle disorder ornithine transcarbamylase deficiency (OTCD) using patient‐derived hiPSC‐Heps. APPROACH AND RESULTS: Before modeling OTCD, we addressed the question of why hiPSC‐Heps generally secrete less urea than adult primary human hepatocytes (PHHs). Because hiPSC‐Heps are not completely differentiated and maintain some characteristics of fetal PHHs, we compared gene‐expression levels in human fetal and adult liver tissue to identify genes responsible for reduced urea secretion in hiPSC‐Heps. We found lack of aquaporin 9 (AQP9) expression in fetal liver tissue as well as in hiPSC‐Heps, and showed that forced expression of AQP9 in hiPSC‐Heps restores urea secretion and normalizes the response to ammonia challenge by increasing ureagenesis. Furthermore, we proved functional ureagenesis by challenging AQP9‐expressing hiPSC‐Heps with ammonium chloride labeled with the stable isotope [(15)N] ((15)NH(4)Cl) and by assessing enrichment of [(15)N]‐labeled urea. Finally, using hiPSC‐Heps derived from patients with OTCD, we generated a liver disease model that recapitulates the hepatic manifestation of the human disease. Restoring OTC expression—together with AQP9—was effective in fully correcting OTC activity and normalizing ureagenesis as assessed by (15)NH(4)Cl stable‐isotope challenge. CONCLUSION: Our results identify a critical role for AQP9 in functional urea metabolism and establish the feasibility of in vitro modeling of OTCD with hiPSC‐Heps. By facilitating studies of OTCD genotype/phenotype correlation and drug screens, our model has potential for improving the therapy of OTCD.