Carbamoyl phosphate synthetase deficiency is a urea cycle disorder inherited as an autosomal recessive trait. Hyperammonemia is the clinical hallmark of the disease and results from the reduced synthesis of carbamoyl phosphate from ammonia and bicarbonate.
Neonatal-onset CPSI deficiency is the most common form of the disease. Prenatal period and birth are usually uneventful, and affected neonates may develop normally during the first few days of life. Most commonly, parents present their children aged less than a week, claiming feeding difficulties and vomiting. Additionally, the child may be noted to suffer from intermittent hypothermia; hyperventilation and hypotonia may be observed. These symptoms result from an accumulation of neurotoxic ammonia and consequent hyperammonemic encephalopathy. Thus, they are frequently accompanied by reduced consciousness, which may range from lethargy and somnolence to coma, and seizures. Although these symptoms are not specific for CPSI deficiency, they should prompt a strong suspicion of UCD.
Less commonly, symptom onset occurs in patients who experienced an unremarkable postnatal period. Complaints reported by individuals suffering from late-onset CPSI deficiency generally comprise chronic headaches, nausea, and vomiting. Similar to neonates suffering from hyperammonemia, elder patients may experience seizures and present with a reduced level of consciousness. Their mental state may be altered, and motor functions may be compromised [10]. Symptoms typically exacerbate during hyperammonemic crises, and those may be triggered by protein overload, fasting, infectious diseases or other stressors that stimulate protein catabolism. Non-compliance with therapeutic regimens may also lead to an acute aggravation of symptoms.
Because there is a direct correlation between the severity of hyperammonemia, the duration of hyperammonemic coma and permanent brain damage [11], it is important to recognize symptoms of hyperammonemic encephalopathy and to identify the cause of neurological deterioration. In this context, serum ammonia levels should be determined in every patient suspicious of UCD or sepsis. In healthy individuals, blood concentrations of ammonia are maintained between 10 and 40 μmol/l [6]. Blood samples should also be analyzed with regards to glucose levels, concentrations of amino acids and acylcarnitines as well as hepatic and renal parameters. Hyperammonemia is the clinical hallmark of CPSI deficiency and other UCD, but the former entity is also associated with increased levels of glutamine and alanine and low concentrations of citrulline and arginine. Urine samples should be tested for excess excretion of orotic acid, and in a case of CPSI deficiency, the corresponding tests typically yield negative results. This diagnostic measure is of importance to distinguish CPSI deficiency from other, distal UCD. Eventually, molecular biological techniques may be employed to prove sequence anomalies within the gene encoding for CPSI.
Of note, magnetic resonance imaging of the brain is not required to confirm a tentative diagnosis of CPSI deficiency. It may be of prognostic value, though. Typical findings include cortical atrophy, basal ganglia lesions and white matter damage.
Causative treatment is not available. Affected individuals are recommended to adhere to a diet low in protein but should avoid fasting and insufficient supply of nutrients. In this context, it may be necessary to supplement essential amino acids, arginine and citrulline. Minor shares of patients may benefit from the administration of N-carbamoyl glutamate, a compound generally prescribed to individuals diagnosed with hyperammonemia type 3 [12]. N-carbamoyl glutamate serves as an allosteric activator of CPSI and thus complements N-acetyl glutamate provided by NAGS.
Patients presenting with hyperammonemic encephalopathy require urgent medical attention to prevent permanent brain damage. In order to decrease ammonia production and to revert the catabolic state of the patient, protein intake should be stopped completely for up to 48 hours and affected individuals should receive glucose infusions. Additional measures should be undertaken depending on the severity of hyperammonemia [13]:
Liver transplantation may be an option for patients repeatedly suffering from hyperammonemic crises.
Unrecognized and untreated CPSI deficiency is generally fatal, and this particularly applies to the more severe, neonatal-onset form of the disease. Even if medical care is ensured, about one-fourth of neonates carrying CPSI gene defects succumb to the disease. According to a retrospective study conducted in Japan, late-onset CPSI deficiency is not associated with significant mortality [9]. Nevertheless, hyperammonemic crises may induce irreversible brain damage and result in mental retardation and disability of survivors. Any diagnostic or therapeutic delay increases the likelihood of a poor neurological outcome.
CPSI deficiency is caused by mutations in the gene encoding for CPSI. This gene is located on the short arm of chromosome 2 and is physiologically expressed by hepatocytes and epithelial cells of the intestinal mucosa [2]. It spans more than 120 kb corresponding to a total of 38 exons and 37 introns that are subsequently translated into a 1,500-amino-acid protein [3]. To date, more than 200 mutations of the CPSI gene have been described, and they may result in an absent or reduced activity of the enzyme [2] [4]. Indeed, the level of residual enzymatic activity has been related to the delay of symptom onset, i.e., absent CPSI activity is associated with neonatal-onset disease while partial enzymatic activity is detectable in those individuals presenting with late-onset CPSI deficiency. The majority of gene defects related to CPSI deficiency are missense mutations, which are most likely to provoke an incomplete functional impairment of the enzyme [4]. Insertion, deletion, and duplication of sequence segments have also been detected. All forms of CPSI deficiency known to date are inherited as an autosomal recessive trait.
In the United States, the annual incidence of CPSI deficiency has been calculated to be 1 in 300,000 live births [5]. Based on estimates of an overall incidence of UCD of 1 in 35,000 neonates, CPSI deficiency accounts for about one in nine cases of UCD. Neither racial nor gender predilection have been reported to date. Most commonly, CPSI deficiency is diagnosed in neonates. Otherwise, symptom onset may occur at any age and indeed, case reports on hyperammonemic encephalopathy due to previously undiagnosed CPSI deficiency in adults aged 40 years and older are available [6].
In the human body, ammonia mainly arises from the conversion of glutamate to α-ketoglutarate, and the hepatic urea cycle is the single most important pathway of ammonia detoxification. Incorporation of ammonia into the urea cycle requires bicarbonate and active CPSI; subsequent steps of the urea cycle cannot take place if CPSI does not supply carbamoyl phosphate. Consequently, CPSI deficiency is associated with an accumulation of ammonia. Excess ammonia is distributed throughout the whole body and is able to cross the blood-brain barrier. Here, it provokes cytotoxic and possibly vasogenic brain edema, which may manifest in form of seizures, tremor, asterixis, delirium and a reduced level of awareness [7]. Increases in intracranial pressure may be detrimental and cause hyperammonemic coma and death, but even less severe hyperammonemic crises may provoke irreversible neuronal loss.
To a lesser extent, CPSI is expressed by epithelial cells lining the intestinal tract. Similar to its function in hepatocytes, the intestinal enzyme catalyzes the formation of carbamoyl phosphate, which is subsequently converted to citrulline. Citrulline, in turn, is utilized for the synthesis of arginine, a semi-essential amino acid. Thus, patients suffering from CPSI deficiency present with low serum levels of citrulline and arginine. Little is known about the clinical consequences of those amino acid deficiencies although it has been suggested that endothelial function and cytokine release by immune cells may be altered [8].
Families affected by CPSI deficiency should be offered genetic counseling and prenatal screenings. Fetal DNA may be extracted from chorionic villus or amniotic fluid samples and may be examined accordingly [12].
Although analyses of serum ammonia levels in few-days-old neonates may allow for the detection of UCD before hyperammonemic encephalopathy occurs, they are not yet routinely carried out.
Carbamoyl phosphate synthetase deficiency is a proximal urea cycle disorder (UCD) that may also be referred to as carbamoyl phosphate synthetase I (CPSI) deficiency or hyperammonemia type 1. The former designation specifies the disease to result from the reduced activity of the mitochondrial enzyme CPSI. Of note, carbamoyl phosphate synthetase II (CPSII) is a cytosolic enzyme that catalyzes the conversion of glutamine, bicarbonate, and ATP to glutamate, ADP, phosphate, and carbamoyl phosphate. CPSII plays a crucial role in pyrimidine biosynthesis but is not involved in ammonia detoxification.
CPSI, in turn, is required for the synthesis of carbamoyl phosphate from ammonia and bicarbonate, and this reaction is the rate-limiting step of the urea cycle [1]. Consequently, CPSI deficiency is associated with an accumulation of neurotoxic ammonia. CPSI deficiency is a hereditary disorder and thus, affected individuals often develop severe hyperammonemia within few days after birth. Indeed, neonatal-onset CPSI deficiency is the most common form of the disease. Nevertheless, symptom onset may be delayed until years after birth and even until well into adulthood. Late onset of symptoms may be related to a greater residual activity of the enzyme.
Of note, CPSI depends on the supply of N-acetyl-glutamate, which acts as an allosteric activator of the former. N-acetyl glutamate is provided by N-acetyl glutamate synthase (NAGS), an enzyme that catalyzes the conversion of glutamate and acetyl-CoA to N-acetyl glutamate. Hyperammonemia type 3 is a pathogenetically different but clinically similar UCD provoked by NAGS deficiency.
Carbamoyl phosphate synthetase deficiency is a rare urea cycle disorder. It is caused by mutations in the gene encoding for carbamoyl phosphate synthetase I (CPSI), an enzyme required for ammonia detoxification.
Ammonia is a by-product arising during the breakdown of proteins, amino acids and other molecules containing nitrogen. CPSI catalyzes the conversion of ammonia and bicarbonate to carbamoyl phosphate, which is subsequently processed to urea. Several biochemical reactions take place in between, and this complex pathway is referred to as the urea cycle. It allows for the conversion of toxic ammonia into a nitrogen compound that can easily be excreted. In a case of CPSI deficiency, carbamoyl phosphate is not or is only insufficiently synthesized. Consequently, ammonia accumulates.
The central nervous system is most sensitive to increased levels of ammonia and patients suffering from CPSI deficiency thus present with hyperammonemic encephalopathy. Due to the disease being caused by a gene defect, symptom onset generally occurs early in life. In fact, parents of healthy born neonates may note feeding difficulties and vomiting after the first few days of life of their child. Additionally, they may note hypothermia, hyperventilation, and a decreased muscle tone. Children may become lethargic, somnolent and fall into coma. Seizures are common. Less frequently, residual activity of CPSI delays symptom onset until later periods of life. Here, chronic headaches, an altered mental status, and motor dysfunction are characteristic of CPSI deficiency.
It is of utmost importance to recognize hyperammonemic crises and to initiate treatment as early as possible. This way, permanent brain damage, and death may possibly be avoided. In such cases, immediate measures are undertaken to lower serum concentrations of ammonia, but patients have to adhere to a life-long therapeutic regimen including a diet low in protein and amino acid supplementation. Unfortunately, causative treatment is not available and CPSI deficiency continues to be a life-threatening disease, particularly in neonates.