Modelling urea cycle disorders using iPSCs

Abstract

The urea cycle is a liver-based pathway enabling disposal of nitrogen waste. Urea cycle disorders (UCDs) are inherited metabolic diseases caused by deficiency of enzymes or transporters involved in the urea cycle and have a prevalence of 1:35,000 live births. Patients present recurrent acute hyperammonaemia, which causes high rate of death and neurological sequelae. Long-term therapy relies on a protein-restricted diet and ammonia scavenger drugs. Currently, liver transplantation is the only cure. Hence, high unmet needs require the identification of effective methods to model these diseases to generate innovative therapeutics. Advances in both induced pluripotent stem cells (iPSCs) and genome editing technologies have provided an invaluable opportunity to model patient-specific phenotypes in vitro by creating patients' avatar models, to investigate the pathophysiology, uncover novel therapeutic targets and provide a platform for drug discovery. This review summarises the progress made thus far in generating 2- and 3-dimensional iPSCs models for UCDs, the challenges encountered and how iPSCs offer future avenues for innovation in developing the next-generation of therapies for UCDs.

Fig. 1. The urea and citrulline-nitric oxide cycles.

Ammonia is converted to urea via five consecutive enzymatic reactions within the liver. The urea cycle begins in the mitochondria (blue box), where ammonia and bicarbonate are converted into carbamoyl phosphate. Carbamoyl phosphate requires allosteric activation by N-acetylglucosamine (NAG). Ornithine transcarbamylase (OTC) then catalyses formation of citrulline from carbamoyl phosphate and ornithine. Citrulline is channelled out of the mitochondria via the ornithine transporter (ORNT1), whilst citrin transports aspartate from the mitochondria to the cytoplasm. Argininosuccinate synthetase then enables the synthesis of argininosuccinate from citrulline and aspartate. Argininosuccinate is subsequently broken down into arginine and fumarate by argininosuccinate lyase, with fumarate directed to the Krebs cycle. Arginine is broken down into urea and ornithine by arginase. Ornithine is transported into the mitochondria by the ORNT1 transporter. CA5A carbonic anhydrase VA, CPS1 carbamoyl phosphate synthetase 1, NAG N-acetyl glutamate, NAGS N-acetyl glutamate synthase, OTC ornithine transcarbamylase, OTC1 ornithine transporter, ASS argininosuccinate synthetase, ASL argininosuccinate lyase, ARG arginase, NOS nitric oxide synthase.