PMID 21290785


Deficiency of argininosuccinate lyase (ASL), the enzyme that cleaves argininosuccinic acid to produce arginine and fumarate in the fourth step of the urea cycle, is characterized by a severe neonatal onset form and a late onset form. The severe neonatal onset form, which is indistinguishable from that of other urea cycle disorders, is characterized by hyperammonemia within the first few days after birth accompanied by vomiting, lethargy, hypothermia, and poor feeding. In the absence of treatment, lethargy, seizures, and coma worsen, resulting in death. In contrast, the late onset form ranges from episodic hyperammonemia triggered by acute infection or stress to cognitive impairment, behavioral abnormalities, and/or learning disabilities in the absence of any documented episodes of hyperammonemia. Manifestations of ASL deficiency that appear to be unrelated to the severity or duration of hyperammonemic episodes include: (1) neurocognitive deficiencies (attention deficit hyperactivity disorder [ADHD], developmental disability, seizures, and learning disability); (2) liver disease (hepatitis, cirrhosis); (3) trichorrhexis nodosa (coarse brittle hair that breaks easily); and (4) systemic hypertension. Elevated plasma ammonia concentration (>150 µmol/L, sometimes up to ≥2000-3000 µmol/L), elevated plasma citrulline concentration (usually 200-300 µmol/L), and elevated argininosuccinic acid in the plasma or urine establish the diagnosis of ASL deficiency. Molecular genetic testing of ASL (the only gene in which mutation is known to be causative) and assay of ASL enzyme activity may be helpful when the biochemical findings are equivocal. Note: All 50 states in the US include ASL deficiency in their newborn screening programs. Treatment of manifestations: Treatment of acute metabolic decompensation with hyperammonemia involves rapid control of hyperammonemia by discontinuing oral protein intake, supplementing oral intake with intravenous lipids and/or glucose, and use of intravenous arginine and nitrogen scavenging therapy. If ammonia levels do not normalize, hemodialysis is the next step. Prevention of primary manifestations: Dietary restriction of protein and dietary supplementation with arginine are the mainstays in long-term management; for those not responsive to these measures, oral nitrogen scavenging therapy can be considered. Orthotopic liver transplantation (OLT) is considered only in patients with recurrent hyperammonemia or metabolic decompensations resistant to conventional medical therapy. Surveillance: Monitoring the concentration of plasma amino acids to identify deficiency of essential amino acids and impending hyperammonemia at intervals depending on the clinical scenario. Agents/circumstances to avoid: Excess protein intake; less than recommended intake of protein; prolonged fasting or starvation; obvious exposure to communicable diseases; valproic acid; intravenous steroids; hepatotoxic drugs (in those with hepatic involvement). Evaluation of relatives at risk: Testing of at-risk sibs (either by molecular genetic testing if the family-specific pathogenic variants are known or by biochemical testing) shortly after birth can reduce morbidity by permitting early diagnosis and treatment of those who are affected. ASL deficiency is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal testing for pregnancies at increased risk are possible if the pathogenic variants in the family have been identified.