Argininosuccinate lyase deficiency

Abstract

The urea cycle consists of six consecutive enzymatic reactions that convert waste nitrogen into urea. Deficiencies of any of these enzymes of the cycle result in urea cycle disorders (UCDs), a group of inborn errors of hepatic metabolism that often result in life-threatening hyperammonemia. Argininosuccinate lyase (ASL) catalyzes the fourth reaction in this cycle, resulting in the breakdown of argininosuccinic acid to arginine and fumarate. ASL deficiency (ASLD) is the second most common UCD, with a prevalence of ~1 in 70,000 live births. ASLD can manifest as either a severe neonatal-onset form with hyperammonemia within the first few days after birth or as a late-onset form with episodic hyperammonemia and/or long-term complications that include liver dysfunction, neurocognitive deficits, and hypertension. These long-term complications can occur in the absence of hyperammonemic episodes, implying that ASL has functions outside of its role in ureagenesis and the tissue-specific lack of ASL may be responsible for these manifestations. The biochemical diagnosis of ASLD is typically established with elevation of plasma citrulline together with elevated argininosuccinic acid in the plasma or urine. Molecular genetic testing of ASL and assay of ASL enzyme activity are helpful when the biochemical findings are equivocal. However, there is no correlation between the genotype or enzyme activity and clinical outcome. Treatment of acute metabolic decompensations with hyperammonemia involves discontinuing oral protein intake, supplementing oral intake with intravenous lipids and/or glucose, and use of intravenous arginine and nitrogen-scavenging therapy. Dietary restriction of protein and dietary supplementation with arginine are the mainstays in long-term management. Orthotopic liver transplantation (OLT) is best considered only in patients with recurrent hyperammonemia or metabolic decompensations resistant to conventional medical therapy.

Figure 1. The urea cycle

consists of series of steps catalyzed by enzymes depicted in green boxes which convert nitrogen from ammonia and aspartate into urea. ASLD leads to accumulation of argininosuccinate upstream of the block as well as deficiency of arginine downstream of the block. CPS1, carbamoyl phosphate synthase 1; OTC, ornithine transcarbamylase; ASS, argininosuccinate synthase; ASL, argininosuccinate lyase; ARG, arginase 1; ORNT1, ornithine transporter,

Figure 2. Metabolic fates of arginine

Arginine is derived from dietary sources, protein catabolism, or endogenous synthesis. The citrulline-NO cycle (shaded green) is responsible for regeneration of arginine in various tissues. Arginine serves as the precursor for many biologically important molecules; a decrease in arginine may result in decreased production of compounds for which it serves as a precursor. ADC - arginine decarboxylase, Arg1 - arginase 1, ASA - argininosuccinic acid, ASL - argininosuccinate lyase, ASS - argininosuccinate synthase, GABA - γ-amino butyric acid, GATM - glycine amidinotransferase, NO - nitric oxide, NOS - nitric oxide synthase, OTC - ornithine transcarbamylase.