Urea cycle disorder is a general term referring to a variety of hereditary diseases provoking partial or complete deficiencies of enzymes required for the breakdown of nitrogen compounds and detoxification of ammonia. Hyperammonemia is the clinical hallmark of urea cycle disorders.
The hallmark of UCD is hyperammonemia. In severe UCD, accumulation of neurotoxic ammonia provokes symptoms of hyperammonemic encephalopathy within hours or few days after birth; patients suffering from milder UCD may not experience any symptoms until adulthood, though. Also, residual activity of enzymes may delay the onset of symptoms and attenuate their severity.
Severe neonatal hyperammonemia is commonly observed in the deficiency of NAGS, CPSI, OTC, ASS, and ASL. Parents may claim feeding difficulties, may note their infants to become somnolent or lethargic. Vomiting and respiratory distress are frequently observed. Tremor and seizures may indicate cerebral edema and eventually, affected children may fall into a hyperammonemic coma. Late-onset UCD manifests in form of loss of appetite, vomiting, tremor and ataxia. Patients may experience stroke-like episodes and show psychiatric symptoms [7]. Rising levels of ammonia may provoke cerebral edema and coma in elder UCD patients, too.
Of note, patients diagnosed with and under therapy for UCD may sustain hyperammonemic crises at any time during their lives. Here, a thorough anamnesis may reveal triggers of such an episode: fasting or protein overload, infectious diseases, pregnancy or recent childbirth, surgery, and other forms of physical stress.
It is of utmost importance to recognize UCD and to initiate treatment before irreversible brain damage occurs. Thus, serum ammonia levels should be determined in all patients presenting with symptoms consistent with hyperammonemic encephalopathy. In the case of hyperammonemia, it should be evaluated whether a patient suffers from metabolic acidosis, which is not characteristic of primary UCD but may indicate organic acidemia. Further analyses of blood samples should be realized to assess the following parameters:
Urine samples should be obtained and examined with regards to their contents of orotic acid and organic acids.
Findings may be interpreted as follows:
Genetic screens may confirm the diagnosis of a particular UCD, may allow for the determination of the mutation underlying the disease, and this information may be valuable as a prognostic factor.
Neuroimaging is usually not required for the diagnosis of UCD, but may reveal brain lesions consistent with hyperammonemia.
Although there is no causative treatment for either gene defect, long-term administration of certain drugs may compensate for enzyme deficiencies:
Otherwise, general recommendations include the reduction of dietary protein intake to diminish protein catabolism. Patients should consume less than 2 g of protein per kg and day [11], but development and growth are to be monitored to prevent nutrient deficiencies. Essential amino acids are often required to this end.
Hyperammonemic encephalopathy is an emergency and requires immediate symptomatic treatment to avoid permanent brain damage. Treatment protocols depend on serum ammonia levels and may comprise the following [4]:
Finally, patients may be considered for liver transplantation.
Despite considerable improvements in survival rates of UCD patients, the neurological outcome generally remains poor [9] [1]. Cognitive and motor development may be severely affected by hyperammonemia and brain damage due to UCD. There is a negative correlation between the severity of hyperammonemia, the duration of hyperammonemic coma and the neurological outcome [6]. Therefore, any diagnostic delay that leads to the postponement of therapy is likely to exacerbate the outcome.
Recently, heterozygosity for OTC mutations has been shown to affect neurocognitive and psychological functions in women [10]. According to that study, the severity of neurological deficits correlates with the amount of residual urea synthetic capacity and the mutation type, and may indeed be predicted based on these parameters. It is tempting to speculate that this also applies to other UCD.
UCD comprise of five deficiencies of catalytic enzymes (CPS-I, OTC, ASS, ASL, and ARG), one deficiency of an enzyme providing an allosteric activator of a catalytic enzyme (NAGS), and two dysfunctional transporters (OTL, CIT) [3]. All these diseases are caused by sequence anomalies affecting those genes encoding for the respective proteins. With the exception of OTC, UCD is inherited as an autosomal recessive trait. The gene encoding for OTC is located on the X-chromosome. There is considerable heterogeneity regarding the precise mutations underlying a specific UCD. While frameshift and nonsense mutations typically provoke complete deficiencies, missense mutations may be associated with a residual activity of the respective protein. It has been proposed that such residual activity accounts for the delay of symptom onset observed in some cases [4].
In the United States, the overall incidence of UCD has been calculated to be about 1 per 35,000 inhabitants [5], while international studies yielded estimates of up to 1 per 8,000 live births [4]. Summar et al. provided detailed estimates of incidence rates of individual UCD [5]:
| NAGS deficiency | <1:2,000,000 |
| CPSI deficiency | 1:1,300,000 |
| OTC deficiency | 1:56,500 |
| ASS deficiency | 1:250,000 |
| ASL deficiency | 1:218,750 |
| ARG deficiency | 1:950,000 |
| OTL deficiency | <1:2,000,000 |
| CIT deficiency | <1:2,000,000 |
Considerable gender predilection has to be noted for OTC deficiency. Since the gene encoding for OTC is located on the X-chromosome, men are affected significantly more often. Women may compensate for a defective allele, but about 15% of carriers still develop hyperammonemia at some point in their lives [3].
The urea cycle plays a crucial role in the catabolic metabolism of nitrogen compounds since it allows for the breakdown of such molecules without an accumulation of ammonia as a by-product. Ammonia mainly arises during the conversion of glutamate to α-ketoglutarate in the liver, a reaction mediated by glutamate dehydrogenase, and deamination of adenosine monophosphate in skeletal muscles [6]; other reactions comprising oxidative deamination contribute to ammonia synthesis to a lesser extent. Detoxification of ammonia is initiated by the irreversible conversion of ammonia and bicarbonate to carbamoyl phosphate, and this reaction is catalyzed by CSPI. CSPI is dependent on the presence of N-acetyl-glutamate, which serves as an allosteric activator and is provided by NAGS. In a subsequent reaction, OTC mediates the production of citrulline from carbamoyl phosphate and ornithine. Accordingly, CSPI deficiency directly provokes the accumulation of neurotoxic ammonia. NAGS deficiency causes a reduced activity of CSPI and similarly leads to hyperammonemia. Conversion of carbamoyl phosphate and ornithine to citrulline and phosphate is impaired in OTC-deficient patients.
All reactions described so far take place in mitochondria, but at this point, citrulline needs to be transported into the cytoplasm. This transport is mediated by an antiporter, namely by OTL, which allows for the transport of citrulline into the cytoplasm and for the import of ornithine into mitochondria. Under physiological conditions, ASS catalyzes the conversion of citrulline and aspartate to argininosuccinate. Thus, besides citrulline, aspartate is required for this reaction to take place. Aspartate is transported into the cytoplasm via an aspartate-glutamate transporter termed citrin, and this protein is defective in patients suffering from CIT. Consequently, substrates needed for argininosuccinate synthetase activity are not supplied adequately in case of OTL or CIT deficiency, and the urea cycle is interrupted at this point. An inherent deficiency of ASS has similar consequences. Both ASL and ARG are required for the breakdown of argininosuccinate and recovery of ornithine. In patients suffering from ASL or ARG deficiency, the regeneration of ornithine and the synthesis of urea are impaired. Therefore, OTC activity is reduced due to lack of substrates.
The central nervous system is very sensitive to ammonia and therefore, the clinical presentation of patients suffering from UCD-related hyperammonemia is that of hyperammonemic encephalopathy. Ammonia may pass the blood-brain barrier and is subsequently converted to glutamine. This reaction takes place in astrocytes, but because glutamine is osmotically active, astrocytes start to swell [7]. Cytotoxic brain edema develops, and this condition further interferes with glia function [8]. Exacerbation of brain edema leads to increased intracranial pressure, brain herniation, and death. Histopathologically, cortical atrophy, basal ganglia lesions and white matter damage may be observed. Prolonged exposure of the brain to enhanced concentrations of ammonia results in irreversible brain damage and permanent neurological deficits.
Affected families may benefit from genetic counseling. Genetic screens are indicated to identify carriers, and this approach allows to deduce the likelihood of a child to be affected by a determined UCD. Furthermore, prenatal tests are available. Chorionic villus or amniotic fluid samples can be examined accordingly, and in the case of a positive result, parents may opt for a premature termination of pregnancy.
The urea cycle comprises several complex biochemical reactions and aims at the breakdown of proteins, amino acids, and nitrogen compounds while at the same time preventing an accumulation of ammonia. Various enzymes are required for the proper execution of each single step on the way to the production of urea. Mutations may occur in any gene encoding for these enzymes, thus giving rise to an interruption of this catabolic process. Gene defects interfering with the transport of intermediate products may ensue similar clinical features as enzyme deficiencies. In detail, the term urea cycle disorder (UCD) encompasses the following diseases:
NAGS deficiency, CPS-I deficiency, and OTC deficiency are sometimes referred to as the most severe UCD. Indeed, these UCD are all proximal UCD, i.e., enzyme deficiencies impair early steps of the urea cycle. Disturbance of late reactions is typical of distal UCD and may be associated with milder symptoms and late-onset disease. However, no significant differences have been encountered regarding the clinical outcome of proximal and distal UCD [1]. Also, entities like ARG, not usually characterized by neonatal hyperammonemia, may occasionally manifest as severe UCD [2].
Urea cycle disorder (UCD) is a general term referring to a total of eight hereditary diseases, all characterized by disturbances of protein catabolism and ammonia detoxification. These are important metabolic processes that comprise a chain of biochemical reactions which constitute the urea cycle. In simple terms, each reaction provides the substrates needed for the following reaction, with nitrogenous compounds like proteins, amino acids, and ammonia being required for the first step. These molecules are eventually converted into urea. Each reaction is catalyzed by an enzyme, and most UCDs are indeed enzyme deficiencies. Other UCDs are characterized by defective transporters that impair substrate trafficking. In any case, gene defects provoking UCD lead to an interruption of the urea cycle and to the accumulation of ammonia.
Ammonia is neurotoxic, i.e., the brain is most sensitive to enhanced levels of this compound. Affected individuals thus present with a variety of neurological deficits, ranging from loss of appetite and vomiting to tremor, seizures, and coma. Blood levels of ammonia have to be diminished as soon as possible to prevent irreversible brain damage. Unfortunately, long-term prevention of hyperammonemic crises is not always possible and the neurological outcome of UCD is often poor.