Ornithine Transcarbamylase Deficiency – Definition
Ornithine transcarbamylase deficiency (OTC deficiency,) is an X-chromosome linked, hereditary metabolic disease caused by mutations – or genetic changes – in the OTC gene. This defect leads to the incorrect breakdown of ammonia, which can cause comas and result in severe brain damage in male newborns due to very elevated levels of ammonia in the blood. However, a later onset of the disease and a milder progression is also possible, and it may also affect females.
Synonyms:
OTC deficiency, ornithine carbamoyltransferase deficiency
Gene:
OTC
Gene product:
OTC
Function:
Part of the urea cycle
Pattern of inheritance:
X-chromosomal recessive or X-chromosomal dominant
Prevalence:
Varies from 1:14.000 to 1:77.000 depending on the study
Genotype-phenotype correlation
Missense mutations resulting in neonatal occurrence in hemizygous boys.
Both nonsense mutations, insertions, and deletions that cause a frameshift as well as single-nucleotide variants lead to a complete loss of OTC function and result in neonatal occurrence.
Affected heterozygous females can also develop symptoms later on.
Penetrance:
Complete in affected hemizygous males
Ornithine Transcarbamylase Deficiency – Diagnosis
Suspected Diagnosis
OTC deficiency is suspected with the following findings:
Clinical Symptoms – Newborn Boys:
- Normal at birth
- Development of reduced oral food intake with decreased suction
- Acute neonatal encephalopathy (lethargy, somnolence) with hyperventilation and low body temperature
Clinical Symptoms – Children, Adolescents, Adults (Male or Female):
- Encephalopathic or psychotic episodes
- Recurring vomiting
- Migraines
- Reye-like syndrome
- Cramping seizures
- Unexplained cerebral paresis
Family History:
Death of a male newborn in the family: due to sepsis within the first week after birth, unexplained somnolence, refusal to eat, tachypnea, or severe illness
Laboratory Findings:
Please note: OTC deficiency is not included in newborn screening in Germany!
- Plasma ammonia concentration: In acute encephalopathy, the plasma ammonia levels are typically above 200 µmol/L and often above 500-1000 µmol/L.
- Concurrent amino acid levels in plasma: High glutamine and deficient citrulline concentrations are characteristic of proximal urea cycle defects such as OTC deficiency.
- Urine: Elevated concentration of orotic acid in the urine
Diagnostic Criteria
Male Individuals:
The diagnosis of “OTC deficiency” is confirmed by the clinical and laboratory-chemical findings listed above and/or with at least one of the following findings:
- A proven hemizygous mutation in the OTC gene (see below)
- Abnormally large increase in the excretion of orotic acid after an allopurinol loading test with or without a concomitant family history of OTC deficiency
- Reduced OTC enzyme activity in the liver
Female Individuals:
The diagnosis of “OTC deficiency” is confirmed by the clinical and laboratory-chemical findings listed above and/or with at least one of the following findings:
- A proven heterozygous mutation in the OTC gene (see below)
- Abnormally large increase in the excretion of orotic acid after an allopurinol loading test with or without a concomitant family history of OTC deficiency
A liver biopsy to diagnose OTC deficiency is not recommended for females.
Genetic Diagnostics
Sequence analysis and, if necessary, a subsequent targeted deletion/duplication analysis can provide genetic evidence of a mutation in the OTC gene. Panel examinations consisting of multiple genes may also be helpful.
Differential Diagnoses
- N-acetylglutamate synthetase deficiency
- Severe carbamoyl phosphate synthetase 1 deficiency
- Citrullinemia (argininosuccinate synthetase deficiency)
- Argininosuccinic acid disease (argininosuccinic aciduria)
Clinical Presentation
OTC deficiency can occur as a severe disease in male neonates (very rarely in female neonates) or as a post-neonatal disease (partial deficiency) in male and female individuals.
Neonatal Occurrence
Typically, affected children appear to be healthy at birth; when they are two to three days old, however, they increasingly lose their sucking behavior, resulting in a reduced food intake and hypotension, followed by somnolence and even a hyperammonemic coma. Other symptoms are hyperventilation leading to respiratory alkalosis, cramping seizures, decreased body temperature, and severe encephalopathy.
Following a hyperammonemic coma, there continues to be an increased risk of an elevated level of ammonia. Generally, a liver transplant at around 6 months (sometimes earlier) is necessary to prevent further brain damage and increase the quality of life.
Post-Neonatal Occurrence
In affected hemizygous males and heterozygous females with partial OTC deficiency, the disease can manifest for the first time anywhere from infancy to adulthood. It is often the case that initial symptoms, such as episodic vomiting, lethargy, irritability, failure to thrive, or developmental delay, occur when a change is made from nursing to baby formula or cow’s milk.
In adults with very mild forms of the disease, initial symptoms often manifest after a trigger, such as major injuries, following surgery, after giving birth, during cancer therapy, after prolonged fasting, a protein-rich diet, high-dosage steroid therapy, or a feverish illness.
Affected Heterozygous Females
Affected heterozygous females may either be without phenotypic symptoms or suffer from significant symptoms. The degree of expression depends on the inactivation of the X-chromosome. Approximately 15% of affected females develop symptoms over the course of their lives.
Hepatocellular Carcinoma (HCC)
The risk of developing HCC is probably the highest in patients exhibiting long-term liver damage who have not undergone a transplant.
Special Features of Treatment
The treatment should be interdisciplinary and involve the corresponding specialist disciplines.
The level of ammonia should be lowered as quickly as possible during the acute phase. Hemodialysis is the treatment of choice for this in both newborns as well as older patients. In addition, ammonia-binding medications can be administered as well.
Long-term treatments include restricting protein consumption and therapy with ammonia-binding medications.
A liver transplant may be necessary following hyperammonemic crises or a hyperammonemic coma.
Diagnosis of Ornithine Transcarbamylase Deficiency- What's Next?
Once diagnosed, it is recommended that a cancer predisposition specialist manage the patient. The following section explains whether cancer screening tests or other measures are necessary and how they should be performed. Some additional information, including links to support groups, is also included at the end of this page.
Diagnosis of Ornithine Transcarbamylase Deficiency- What's Next?
Once diagnosed, it is recommended that a cancer predisposition specialist manage the patient. The following section explains whether cancer screening tests or other measures are necessary and how they should be performed. Some additional information, including links to support groups, is also included at the end of this page.
Recommendations for Early Detection in Your Patients
Plasma ammonia levels should be measured on a regular basis when starting treatment: at first every two weeks, then less frequently as time goes on, and ultimately every four months.
A plasma amino acid analysis should be conducted when starting treatment: at first every two weeks, then less frequently as time goes on, and ultimately every four months.
Depending on the symptoms, liver function tests every 3-6 months or more regularly if symptoms increase
Neuropsychological examinations at times when developmental milestones should have been reached (e.g. at 6-9 months, 18 months, 3 years)
Fasting, stress, and drug therapy with valproate, haloperidol, or the systematic administering of corticosteroids should be avoided.
Awareness of the potential for an increased risk of HCC should be raised in patients. Specific cancer surveillance is not recommended.
