Argininosuccinic aciduria (AA) is an urea cycle disorder caused by deficiency of argininosuccinic acid lyase (ASL), an enzyme catalyzing the conversion of argininosuccinate to fumarate and arginine. Consequently, argininosuccinate accumulates in blood and urine. Increased serum concentrations and urinary excretion of argininosuccinate are the biochemical hallmarks of the disease and of major importance for the diagnosis of the disease. This diagnosis must be confirmed by means of genetic testing. Treatment is mainly based on dietary restriction and arginine supplementation.
AA usually manifests early in life, but may occasionally not be noted until childhood or even adulthood. Some authors distinguish severe neonatal-onset AA (onset within the first month of life) from mild to moderate late-onset ASL deficiency (onset in patients older than one month)  .
The clinical picture is not different from that observed in other urea cycle disorders and varies according to disease severity: Parents may claim feeding problems, lethargy and irritability in their child, report vomiting, tremor, and ataxia, or present their offspring due to an acute hyperammonemic crisis. Hyperammonemia induces hypothermia, tachypnea and respiratory alkalosis, which usually precede seizures and profound loss of consciousness. In severe cases, hyperammonemic coma and even death may occur within the first days of life. Less severe forms of the disease are characterized by episodic hyperammonemia, triggered by infectious disease, fasting, stress or non-compliance with dietary restrictions and/or medication . Hyperammonemia may cause irreversible brain damage and thus, untreated AA may lead to learning difficulties, intellectual disability and cognitive impairment, or abnormal behavior. Such long-term sequelae may arise after repeated hyperammonemic crises or even in the absence of any documented episodes of metabolic decompensation .
Additionally, skin lesions, hepatomegaly and trichorrhexis nodosa, i.e., coarse and brittle hair, may be noted in AA patients . The latter is considered specific for ASL deficiency but don't suffice for a reliable diagnosis. Also, the absence of hair shaft defects does not rule out ASL deficiency .
The above-described symptoms may raise suspicion for an urea cycle disorder, but are not specific for AA. Furthermore, they may not allow for a reliable differentiation of urea cycle disorders and organic acidemias. Valuable hints at the nature of the disease can be obtained during familial and personal anamnesis. Parents of affected children should be asked for possible metabolic disorders or symptoms thereof in past generations or living relatives. It should be considered that the course of the disease in affected family members may be more or less severe than in the patient at hand. With regards to the latter, certain information concerning their medical history and eating habits may also point at an urea cycle disorder, even in the absence of documented hyperammonemic crises. For instance, patients suffering from an urea cycle disorder are prone to behavioral disorders and are known to avoid protein-rich food  .
Important measures to confirm that the patient does indeed suffer from an urea cycle disorder consist in measuring serum ammonia levels and checking for metabolic acidosis. An AA patient will suffer from hyperammonemia, but not from metabolic acidosis, but rather from respiratory alkalosis. Laboratory analyses of blood samples should also reveal citrullinemia (citrulline levels of 100 to 300 µmol/l) and increased concentrations of argininosuccinate (50 to 110 µmol/l) . Additional findings typically comprise low levels of arginine and ornithine.
Urinary argininosuccinate levels may reach 10,000 µmol/g creatinine . In fact, urine analysis is considered more sensitive with regards to the detection of mild forms of AA than blood analysis. Even if reliable conclusions can't be drawn from laboratory analyses of blood samples, urine amino acid profiles should reveal the exact type of urea cycle disorder . Urinary excretion of orotic acid may also be increased in AA patients.
Enzyme activity can be assessed in skin fibroblasts, red blood cells or - if a liver biopsy is performed - in hepatocytes. In order to do so, argininosuccinate may be provided and its breakdown observed by means of fluorometric measurements. Alternatively, the conversion of fumarate and arginine to argininosuccinate may be evaluated. Contradictory results have been published regarding the correlation of test results and disease severity, though  . Therefore, some authors recommend carrying out indirect assays (e.g., citrulline incorporation) instead of direct enzyme activity measurements: They argue indirect assays to be more sensitive and predict a better correlation between test results and the patient's phenotype . Others recommend enzyme assays only in case of inconclusive results of molecular genetic testing , as described subsequently.
Genetic analyses should be done in any case to confirm causal mutations of the ASL gene. Furthermore, mutation identification facilitates prenatal diagnosis in subsequent generations .
Dietary measures are the mainstay of AA treatment. In order to diminish protein catabolism, protein intake should be limited. It may be necessary to supplement essential amino acids to compensate for dietary restrictions and to avoid nutrient deficiencies. Arginine is usually applied in an intent to favor the formation of argininosuccinate: This seemingly apparently contradictory strategy is followed because argininosuccinate does contain two waste nitrogen atoms and has a high renal clearance. By this means, hyperammonemia can be controlled. However, doubts remain on whether arginine itself exerts neurotoxic effects . Thus, arginine should not be over-supplemented. Serum arginine levels should ideally remain below 200 µmol/l . For as of yet unknown reasons, AA patients tend to present with hypokalemia and may require potassium supplementation .
Treatment in case of acute metabolic decompensation does not differ from measures to be taken in case of other urea cycle disorders during hyperammonemic crisis. Here, the main objectives of therapy are to lower ammonia levels and to reverse protein catabolism. In order to do so, ammonia scavengers may be administered, dextrose, insulin and intra-lipids. Protein intake should be radically reduced for at least 48 hours and arginine has to be supplemented. Eventually, patients may be considered for hemodialysis or hemofiltration . AA patients have occasionally been treated by means of liver transplantation  .
If left untreated, ASL deficiency may ensue severe developmental delays and intellectual disability . However, neurocognitive deficiencies have also been described in AA patients who did not suffer from hyperammonemic crises . They may possibly be explained by argininosuccinate-mediated neurotoxicity , but other pathomechanism have been proposed, too . Additionally, argininosuccinate is known to exert toxic effects on hepatocytes, which is why AA patients are considered to be at higher risks of developing hepatic diseases and liver fibrosis . Affected individuals have also been proposed to be predisposed for hypertension . Fortunately, most of those long-term sequelae can be avoided or, at the very least, reduced by means of proper treatment. The overall mortality (neonatal plus late-onset disease) has been estimated to 6% .
AA is inherited in an autosomal recessive manner and has been associated with distinct mutations of the ASL gene, which is located on chromosome 7 and encodes for an enzyme of the same name required for catalyzing the breakdown of argininosuccinate in the late urea cycle . Data regarding the genotype–phenotype correlation have been provided, but are not necessarily consistent in different studies  . According to current knowledge, other genes are not involved in the pathogenesis of AA.
For the United States, the overall incidence of urea cycle disorders has been estimated to 1 in 35,000 inhabitants. According to Summar et al., ornithine transcarbamylase deficiency is the most common urea cycle disorder (incidence of 1 in 56,500 individuals), followed by ASL deficiency or AA, which affects 1 in 218,750 people . Even though precise data are lacking for developing regions, incidence rates are expected to be significantly higher in countries characterized by the practice of consanguineous marriage, i.e., most Arab and the Middle Eastern countries. Indeed, according to a recently published review, the incidence of AA may be as high as 1 in 70,000 individuals . Men and women are affected equally.
ASL catalyzes the conversion of argininosuccinate to fumarate and arginine in the late urea cycle. Fumarate is required for the generation of malate, whereas arginine is converted to urea and ornithine. The former reaction is catalyzed by cytosolic fumarase, the latter by arginase. In patients suffering from AA, cleavage of argininosuccinate is considerably restricted and substrates for fumarase and arginase are supplied in small amounts only. On the one hand, this leads to an accumulation of argininosuccinate in blood and urine, which, in turn, slows down earlier reactions of the urea cycle: In AA patients, citrulline-consuming formation of argininosuccinate is restricted and serum citrulline levels rise. On the other hand, levels of arginine and ornithine may decrease to below reference ranges. Indeed, lack of arginine is what causes affected individuals to develop trichorrhexis nodosa, a hair shaft anomaly that may be recognized during microscopic examination.
Early diagnosis of urea cycle disorders is crucial for avoiding hyperammonemic crises and thus, irreversible damage to the central nervous system. Although ALS deficiency is usually considered a rather mild urea cycle disorder, it is no exception to this rule.
Prevention of AA and long-term sequelae comprises the following steps:
AA results from ASL deficiency, a rare inborn error of metabolism. This enzyme is required for catalyzing the cleavage of argininosuccinate in the late urea cycle, which is why AA pertains to the group of urea cycle disorders. Accordingly, AA patients may present with symptoms of hyperammonemic encephalopathy during their first month of life or, in case of less severe disease, suffer from metabolic decompensation in times of increased demand and subsequent protein catabolism. Clinical findings rarely allow for a reliable diagnosis of AA, with hepatomegaly and hair shaft anomalies being the only symptoms that presumably distinguish this disease from other urea cycle disorders. A detailed metabolic profile is required to arrange for the enzyme assays and genetic analyses necessary to confirm the diagnosis. Treatment mainly rests on restricted protein intake and arginine supplementation. Even though hyperammonemia can usually be controlled applying these measures, AA patients remain at high risks of slowly progressive neurocognitive decline. Further research is needed to shed more light on the causes of this development and to be able to adapt treatment recommendations accordingly.
In the human body, ammonia detoxification rests on a series of biochemical reactions forming the so-called urea cycle. Distinct enzymes catalyze single reactions within that cycle, break down metabolic intermediates and supply substrates for the following reaction. Argininosuccinic acid lyase (ASL) is one of those enzymes and catalyzes the conversion of argininosuccinate to fumarate and arginine. In patients suffering from argininosuccinic aciduria (AA), mutations of the gene encoding for ASL cause a considerable reduction of the activity of this enzyme. Consequently argininosuccinate cannot be sufficiently cleaved to fumarate and arginine. It accumulates in the patient's blood and urine. This metabolic block also impairs ammonia detoxification in general and thus, affected individuals may develop hyperammonemia, i.e., increased concentrations of ammonia in their blood. Ammonia exerts toxic effects on the brain and if its concentrations increase beyond certain limits, AA patients may suffer from acute encephalopathy - presenting as seizures and profound loss of consciousness - or progressive neurodegeneration. The latter eventually results in developmental delays and intellectual disability. Additional symptoms of AA comprise loss of appetite, lethargy, irritability, hypothermia, high respiratory rate, tremor and ataxia. These symptoms often become apparent during the neonatal period, but may occasionally not be noted until well into childhood. Appropriate dietary measures can be taken as soon as the diagnosis is confirmed. In most cases, hyperammonemia and its consequences can be avoided or, at the very least, reduced.