Our laboratory has been pioneer in the use of cell therapy to treat patients with inborn errors of disease. However, for preclinical studies, many small animal models of monogenetic liver disease do not faithfully recreate the phenotype observed in human patients. Using special FAH and immune deficient mice (FRGN), we have created a chimeric model where the mouse liver can be highly repopulated with human hepatocytes. Hepatocytes were isolated from normal donors or from a patient who received a liver transplant for a severe urea cycle defect, carbamoyl phosphate synthase (CPS1) deficiency, an autosomal recessive urea cycle defect which causes hyperammonemia and central nervous system symptoms because of a greatly reduced capacity to convert the excess nitrogen from protein intake into urea for excretion. Isolated human hepatocytes were transplanted into the liver of FRGN mice, replacing 85–95% of the mouse hepatocytes with human hepatocytes as quantified by plasma human albumin levels.
Mice repopulated with normal hepatocytes (CPS1-proficient) displayed normal ammonia levels (<90 micromolar), while mice highly repopulated with CPS1-deficient hepatocytes displayed exhibited characteristic symptoms of human CPS1-deficiency including increased basal ammonia (150–300 micromolar), 80% reduction in CPS1 metabolic activity, delayed clearance of an ammonium chloride infusion, elevated glutamine and glutamate levels and impaired metabolism of [15N]ammonium chloride into urea, with no other obvious phenotypic differences.
Conclusions. Since most metabolic liver diseases result from mutations that alter critical pathways in hepatocytes, a model that incorporates actual disease-affected, mutant human hepatocytes is useful for the investigation of the molecular, biochemical and phenotypic differences induced by that mutation. The model is also expected to be useful for investigations of modified RNA, gene, cellular and small molecule therapies for CPS1-deficiency. Liver-humanized models for this and other monogenic liver diseases afford the ability to assess the therapy on actual disease-affected human hepatocytes, in vivo, for long periods of time and will provide data that is highly relevant for investigations of the safety and efficacy of gene editing technologies directed to human hepatocytes and the translation of gene editing technology to the clinic.