Ornithine transcarbamylase (OTC) deficiency is a rare X-linked genetic metabolic disorder that prevents the body from properly breaking down nitrogen, a waste product of protein.
Ornithine transcarbamylase (OTC) deficiency is the most common urea cycle disorder, caused by mutations in the X-linked OTC gene, leading to impaired conversion of ammonia to urea and resulting in life-threatening hyperammonemia.
The OTC enzyme is almost exclusively expressed in the liver and catalyzes the synthesis of citrulline from carbamoyl phosphate and ornithine.
OTC is the most common of the urea cycle disorders.
Without the functional OTC enzyme, nitrogen builds up in the blood as toxic ammonia (hyperammonemia), which can lead to severe neurological damage, coma, or death if untreated.
OTC deficiency follows an X-linked inheritance pattern, distinguishing it from other urea cycle disorders which are autosomal recessive.
Symptoms of OTC vary based on the amount of residual enzyme activity and the age of onset:
Neonatal-Onset (Severe): Usually occurs in males within the first few days of life. Infants appear normal at birth but quickly develop poor feeding, vomiting, lethargy, seizures, and hypothermia.
Late-Onset (Partial): Can appear from infancy to late adulthood in both males and females.
Symptoms may be triggered by metabolic stressors like high-protein diets, illness, surgery, or pregnancy.
Common signs: Chronic headaches or migraines, unexplained vomiting, protein avoidance, and psychiatric episodes like delirium, agitation, or confusion.
As an X-linked recessive condition, it primarily affects males who have only one X chromosome.
Males with pathogenic variants typically present more severely, while heterozygous females show variable manifestations depending on X-inactivation patterns (lyonization).
Females have two X chromosomes and are often carriers and they may be asymptomatic or show varying degrees of symptoms depending on a biological process called X-inactivation.
Diagnosis is typically established through clinical findings and laboratory tests showing high ammonia and low citrulline levels, confirmed by molecular genetic testing of the OTC gene.
Treatment:
The acute management of hyperammonemic crisis requires:
Rapid ammonia reduction to ≤200 μmol/L using renal replacement therapy with continuous kidney replacement therapy or hemodialysis for severe cases.
Intravenous ammonia scavengers-sodium phenylacetate and sodium benzoate.
Reversal of catabolism with IV dextrose and intralipids
Protein restriction
Supplements: Arginine or citrulline to support the remaining urea cycle function.
Management of cerebral edema if present
Long-term management includes:
Protein-restricted diet with essential amino acid supplementation
Oral nitrogen-scavenging drugs (sodium phenylbutyrate or sodium benzoate)
Avoidance of triggers (fasting, valproate, corticosteroids) Dietary Control: Lifelong restriction of dietary protein to limit nitrogen intake.
Hemodialysis may be required to rapidly lower ammonia during an acute crisis.
Liver Transplant: Often the only curative option, especially for severe neonatal cases, as it replaces the defective enzyme system.
Neonatal-onset disease, primarily in males, presents within the first week of life, typically on days 2-3, with poor feeding, lethargy, vomiting, and tachypnea that rapidly progresses to hyperammonemic coma and death if untreated.
These infants appear normal at birth but become catastrophically ill once symptomatic.
Late-onset (partial) disease can occur in males and heterozygous females from infancy through adulthood.
Hyperammonemic crises can be precipitated by catabolic stressors, protein overload, fasting, or certain medications (valproate, haloperidol, corticosteroids) at any age.
OTC deficiency can also present as acute liver failure, particularly in females.
Diagnosis:
Diagnosis is based on elevated plasma ammonia levels, characteristic plasma amino acid patterns with elevated glutamine, low citrulline, elevated urinary orotic acid, and molecular genetic testing of the OTC gene.
Note that orotic aciduria may be absent with limited oral intake, potentially delaying diagnosis.
Liver transplantation is typically performed by age 6 months in severe neonatal-onset cases to prevent recurrent hyperammonemic crises and neurodevelopmental deterioration.
It may also be considered in partial deficiency with frequent hyperammonemic episodes.
Neuropsychological complications include developmental delay, learning disabilities, intellectual disability, ADHD, and executive function deficits.
Treatment with ammonia scavengers has shown >90% five-year survival in symptomatic females and reduced frequency of hyperammonemic episodes.
Early aggressive management of hyperammonemic crises, including therapeutic hypothermia when indicated, can result in favorable neurologic outcomes.