More than 230 disease-causing OTC mutations have been identified, many of which are unique to the families in which they occur [2
]. In general, mutations that affect amino acids in the interior of the enzyme near the active site—such as proline-225—cause neonatal disease, whereas mutations affecting residues on the surface of the protein are associated with milder phenotypes or later onset of disease [1
]. However, in some cases, mutations that affect the catalytic function of the mutant enzyme are associated with late-onset disease. In particular, this report demonstrates that OTC deficiency resulting from the P225T mutation can cause such late-onset disease.
The proline-225 residue affected in this patient is conserved among many species and may be involved in the processing of the OTC protein [3
]. Different amino acid substitutions at proline-225 can have substantially different disease presentations. For example, Pro-225-Arg [3
] and Pro-225-Leu [4
] have been found in males with severe neonatal OTC deficiency. The P225T mutation has been documented in one other patient, a previously healthy boy who manifested late-onset disease at 8 years of age [5
]. In a stable isotope study of ureagenesis, this particular mutation resulted in a 42% reduction in 15
N incorporation into urea, indicating that P225T allows for significant residual OTC activity [6
A variety of genetic and environmental factors exist that can lead a previously healthy hemizygote to become acutely ill later in life. Some late-onset mutations, such as R40H, have been shown to affect posttranslational processing events such as targeting to the mitochondrion [1
]. Several environmental factors may also contribute to the timing of disease onset and disease manifestations. These factors include high-protein diets, valproic acid treatment, and exposure to the insect repellant N
]. Electron microscopic studies of the liver of a late-onset OTC patient revealed mitochondrial abnormalities beyond those expected with OTC deficiency per se, suggesting primary mitochondrial injury by exogenous agents [10
]. Organophosphates, such as the pesticides to which this patient was exposed to, have been reported to induce apoptosis by disrupting mitochondrial membranes and activating caspases [11
]. Our patient was in good health until 62 years of age, when he became acutely ill and died. In the days preceding his illness, he worked with several home-gardening products, including pesticides, fertilizers, and sealants. We speculate that exposure to organic chemicals may have contributed to the onset of symptoms in this patient; that is, chemicals may have entered his system through his lungs or skin, accumulated in his liver, and compromised his OTC function.
The differential diagnosis of hyperammonemia in an older child or adult can be addressed by routine laboratory testing. Causes of hyperammonemia include both nongenetic (acquired) and genetic etiologies, and include postviral aspirin-induced Reye syndrome; acute or chronic hepatitis; Wilson disease; α1-antitrypsin deficiency; alcoholic cirrhosis; drug-induced liver failure (e.g., anticonvulsants); chemotherapy; infection; and late-onset urea cycle defects. In a patient with hyperammonemia, a complete blood cell (CBC) count, serum electrolytes (including measurement of the anion gap), blood pH, blood glucose, liver function chemistries, and urinalysis can be helpful. In this case, a respiratory alkalosis was the only abnormality found initially on laboratory analysis (Table ). Specifically, a normal anion gap existed, liver functions and CBC were normal, and blood glucose (145 mg/dl) and urinalysis (trace ketones) were near normal values. As a first approximation, hyperammonemia in the presence of normal liver function and a respiratory alkalosis is highly suggestive of a urea cycle defect. Additional tests are needed to determine which urea cycle defect is present, and this additional testing includes measurement of quantitative plasma amino acids and urine orotic acid.
Serial laboratory values of patient
Treatment guidelines for hyperammonemia include the following:
- Intravenous hydration with glucose and lipids to minimize protein catabolism and endogenous ammonia production.
- Cessation of protein intake from all sources to restrict nitrogen intake.
- Administration of arginine HCl or arginine free base for all urea cycle defects except arginase deficiency. This allows the urea cycle to partially proceed with removal of some fraction of accumulated nitrogen.
- Administration of compounds that facilitate the removal of ammonia through alternate pathways, including oral sodium phenylbutyrate, which conjugates glutamine, leading to the removal of two nitrogens as phenylacetylglutamine, and oral sodium benzoate, which conjugates glycine, leading to removal of 1 nitrogen as hippurate. These can be given orally or, in more severe presentations, combined in an intravenous formulation.
- Hemodialysis can be performed in severe cases marked by refractory hyperammonemia or encephalopathy, or when there is clinical deterioration.
- Treatment of shock, sepsis, seizures, and increased intracranial pressure.
The family members of the patient presented here raise an important clinical issue: when and whether to treat asymptomatic individuals with OTC mutations, given an index patient with late-onset disease [12
]. In this case, we elected close clinical monitoring of the affected relatives, although a more aggressive approach may be warranted. Female carriers should be followed in addition to males.
Asymptomatic family members can opt for diet modification and prophylactic urea cycle defect therapy, but it is unclear whether these measures are necessarily more effective than close clinical monitoring in the prevention of hyperammonemia and metabolic decompensation.
Treatment possibilities for asymptomatic family members include:
- Carrying an emergency letter indicating that they are at risk of hyperammonemia. Included in the letter should be the laboratory testing to be obtained, the contact number of a metabolic specialist, and instructions for the dose and administration of arginine, sodium benzoate, and sodium phenylacetate.
- Monitoring of blood ammonia levels during intercurrent illnesses to determine whether ammonia levels are elevated during catabolic stress. If elevations occur, an emergency interval diet plan can be provided for fluid and nutritional requirements, including cessation of protein intake and provision of adequate calories.
- Ongoing dietary restriction of protein and supplementation with an essential amino acid formula.
In conclusion, it is important to recognize that acute neurologic decline and hyperammonemia in a previously healthy adult may be due to a urea cycle defect (Table ). Vigilance by clinicians can lead to rapid treatment and appropriate involvement of metabolic specialists. Finally, molecular studies in patients such as the one in this report will help identify genotype–phenotype correlations and contribute to our understanding of disease mechanisms in this heterogenous disorder.
Neurologic status of patient