Argininosuccinic aciduria is a rare genetic disorder characterized by the deficiency or lack of the enzyme argininosuccinate lyase (ASL). This enzyme is one of six enzymes that play a role in the breakdown and removal of nitrogen from the body, a process known as the urea cycle. The lack of this argininosuccinate lyase results in excessive accumulation of nitrogen, in the form of ammonia (hyperammonemia), in the blood. Ammonia is a neurotoxin, which means that it damages or inhibits the function of neurons, the cells of the central nervous system. Excess ammonia travels to the central nervous system through the blood, resulting in the symptoms and physical findings associated with the disorder. Affected infants may experience vomiting, refusal to eat, progressive lethargy, and coma. Argininosuccinic aciduria is inherited as an autosomal recessive trait.
The urea cycle disorders are a group of rare disorders affecting the urea cycle, a series of biochemical processes in which nitrogen is converted into urea and removed from the body through the urine. Nitrogen is a waste product of protein metabolism. Failure to break down nitrogen results in the abnormal accumulation of nitrogen, in the form of ammonia, in the blood.
Symptoms
The severity and specific symptoms of argininosuccinic aciduria vary from one person to another. A severe form of the disorder, which is characterized by a complete or near-complete lack of the ASL enzyme, occurs shortly after birth (neonatal period). A milder form of the disorder, which is characterized by partial lack of the ASL enzyme, affects some individuals later during infancy or childhood, or even adulthood (late-onset form).
Symptoms are caused by the accumulation of ammonia in the blood. The severe form occurs within 24-72 hours after birth, usually following a protein feeding. This form is initially characterized by a refusal to eat, lethargy, lack of appetite, vomiting, and irritability. Affected infants may also experience seizures, breathing (respiratory) abnormalities, the accumulation of fluid in the brain (cerebral edema), and an abnormally large liver (hepatomegaly). Less commonly, some individuals develop progressive liver disease and dysfunction such as the buildup of scar tissue (fibrosis) and cirrhosis. In rare instances, chronic kidney (renal) disease has been reported. Abnormally rapid breathing (tachypnea) may be detected and sometimes is the first sign recognized of elevated ammonia in the blood. As affected individuals grow older, they may have coarse and brittle (friable) hair that breaks off easily and can leave patches of hair loss, a condition known as trichorrhexis nodosa.
In some instances, due to high levels of ammonia in the blood (hyperammonemia coma), the disorder may progress to coma. In such instances, argininosuccinic aciduria may potentially result in neurological abnormalities including delays in reaching developmental milestones (developmental delays) and intellectual disability. The severity of such neurological abnormalities is more severe in infants who are in hyperammonemia coma for more than three days. If left untreated, the disorder will result in life-threatening complications. However, even individuals without significant hyperammonemia may develop neurological abnormalities suggesting alternative causes of injury.
In infants with partial enzyme deficiency, the onset of the disorder may not occur until later during infancy or childhood (late-onset form). Symptoms may include failure to grow and gain weight at the expected rate (failure to thrive), avoidance of protein from the diet, inability to coordinate voluntary movements (ataxia), lethargy, and vomiting. Affected infants and children may also have dry, brittle hair. Some individuals with the late-onset form may not develop any symptoms (asymptomatic).
Infants with the mild form may alternate between periods of wellness and hyperammonemia. Episodes of hyperammonemia are usually triggered by acute infection, stress, certain medications, or non-compliance with the recommended dietary restrictions (e.g. high protein intake). Other individuals with the mild form may not have any documented episodes of hyperammonemia, but can still develop behavioral abnormalities such as attention-deficit/hyperactivity disorder, cognitive impairment, and learning disabilities.
Both the severe and late-onset forms of argininosuccinic aciduria can be associated with long-term complications including liver dysfunction, neurocognitive deficits such as cognitive impairment, seizures, brittle hair, and high blood pressure (hypertension). These long-term complications appear to be unrelated to the frequency, length, or severity of episodes of hyperammonemia. Increasingly, high blood pressure has been diagnosed in children and adults with this condition. This may be due to an inability of the body to generate a chemical called nitric oxide.
Causes
Argininosuccinic aciduria is caused by alterations (mutations) in the ASL gene. Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a mutation of a gene occurs, the protein product may be faulty, inefficient, or absent. Depending upon the functions of the particular protein, this can affect many organ systems of the body.
Symptoms of argininosuccinic aciduria develop due to the near-complete or partial lack of the enzyme argininosuccinate lyase. The ASL gene is responsible for regulating the production of this enzyme. Alterations in the ASL gene lead to low levels of functional argininosuccinate lyase, which is needed to break down nitrogen in the body. Failure to properly break down nitrogen leads to the abnormal accumulation of nitrogen, in the form of ammonia, in the blood (hyperammonemia).
Researchers have determined that argininosuccinic aciduria is a more complex metabolic disorder than originally suspected. Affected individuals have developed some of the long-term complications described above (e.g. liver disease, hypertension, neurocognitive issues) despite not having any episodes of hyperammonemia and having an overall good metabolic profile. Researchers theorize that the deficient enzyme, argininosuccinate lyase, may have more roles in the body other than breaking down nitrogen (i.e. its role in the urea cycle) including the production of nitric oxide. More research is necessary to fully understand the complex, underlying mechanisms of argininosuccinic aciduria.
The alteration in the ASL gene is inherited in an autosomal recessive manner. Most genetic diseases are determined by the status of the two copies of a gene, one received from the father and one from the mother. Recessive genetic disorders occur when an individual inherits two copies of an altered gene for the same trait, one from each parent. If an individual inherits one normal gene and one gene for the disease, the person will be a carrier of the disease but usually will not show symptoms. The risk for two carrier parents to both pass the altered gene and have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents is 25%. The risk is the same for males and females.
Mutations in the ASL gene cause argininosuccinic aciduria. This condition belongs to a class of genetic diseases called urea cycle disorders because they are caused by problems with a process in the body called the urea cycle. The urea cycle is a sequence
of reactions that occurs in liver cells. This cycle breaks down excess nitrogen, which is made when protein is used by the body, to make a compound called urea. Urea is removed from the body in urine. Breaking down excess nitrogen and excreting it as urea
prevents it from accumulating in the body as ammonia. The ASL gene provides instructions for making an enzyme called argininosuccinate
lyase, which is needed for the fourth step of the urea cycle. The specific role of the argininosuccinate lyase enzyme is to start the reaction in which the amino acid arginine,
Diagnosis
A diagnosis of a urea cycle disorder, such as argininosuccinic aciduria, should be considered in any newborn that has an undiagnosed illness characterized by vomiting, progressive lethargy, and irritability. All 50 states in the U.S. include argininosuccinic aciduria in newborn screening programs.
- Family history. A three-generation family history with attention to other relatives (particularly children) with neurologic signs and symptoms suggestive of UCD should be obtained. Documentation of relevant findings in relatives can be accomplished either through direct examination of those individuals or a review of their medical records including the results of biochemical testing, molecular genetic testing, and autopsy examination. A family history consistent with X-linked inheritance suggests OTC deficiency.
- Physical examination. No findings on physical examination distinguish among the eight types of urea cycle defect; however, trichorrhexis nodosa can be suggestive of ASL deficiency, and progressive spasticity of the lower extremities suggestive of arginase deficiency.
- Blood tests – may reveal excessive amounts of ammonia in the blood, which is the main criterion for a diagnosis of urea cycle disorders including argininosuccinic aciduria. Blood tests may also reveal high levels of an amino acid called citrulline. However, high levels of ammonia or citrulline in the blood may characterize other disorders such as the organic acidemias, congenital lactic acidosis, and fatty acid oxidation disorders and are also present in other urea cycle disorders.
- Plasma ammonia concentration – elevation is usually the first identified laboratory abnormality in most urea cycle disorders. A plasma ammonia concentration of 150 μmol/L or higher is associated with a normal anion gap and a normal plasma glucose concentration is a strong indication of a UCD [Summar & Tuchman 2001].
Quantitative plasma amino acid analysis can be used to arrive at a tentative diagnosis. (As the liver is not fully mature at birth, affected newborns often have plasma amino acid concentrations that are quite different from those in older children and adults.)
- Plasma concentration of citrulline helps discriminate between the proximal and distal urea cycle defects, as citrulline is the product of the proximal enzymes (CPS1, OTC, and NAGS) and a substrate for the distal enzymes (ASS1, ASL, ARG1).
- Plasma citrulline is either absent or present only in trace amounts in neonatal-onset CPS1 deficiency, NAGS deficiency, and OTC deficiency and present in low to low-normal concentrations in late-onset disease. Plasma citrulline is also reduced in ORNT1 deficiency.
- Marked elevation in plasma citrulline concentration is seen in ASS1 deficiency.
- Moderate elevation in plasma citrulline may be observed in citrin deficiency, along with an elevated threonine/serine ratio.
- A more moderate (~2- to 5-fold) increase in plasma citrulline concentration is seen in ASL deficiency, which is also associated with high levels of argininosuccinic acid (ASA) in plasma and urine. Note: ASA is absent in unaffected individuals[Summar 2001, Summar & Tuchman 2001].
- Plasma citrulline concentration is usually normal in ARG1 deficiency.
- Plasma concentration of arginine is markedly elevated in ARG1 deficiency. It may be reduced in all other urea cycle disorders; however, in partial UCD enzyme defects, it may be normal.
- Plasma concentration of ornithine is elevated in ORNT1 deficiency, in which urine homocitrulline is also elevated. Ornithine is not elevated in OTC deficiency.
Note: Plasma concentrations of glutamine, alanine, and asparagine, which serve as storage forms of waste nitrogen, are frequently elevated.
Urinary orotic acid is measured to distinguish CPS1 deficiency or NAGS deficiency from OTC deficiency. It is normal or low in CPS1 deficiency and NAGS deficiency and significantly elevated in OTC deficiency. Note: Urinary orotic acid excretion can also be increased in arginine mix (ARG1 deficiency) and citrullinemia type I (ASS1 deficiency).
Urine amino acid analysis may be used to identify the presence of urine homocitrulline, observed in ORNT1 deficiency. Additionally, ASA concentrations are higher in urine than in plasma, and therefore urine amino acid profile may be helpful when small peaks of ASA or its anhydrides are difficult to resolve on plasma amino acid analysis.
Molecular Genetic Testing
Molecular genetic testing is the primary method of diagnostic confirmation for all eight CDs. Molecular testing has supplanted the measurement of enzyme activity as the definitive diagnostic test. However, enzymatic testing remains available for most disorders and may be helpful if DNA-based investigations are not informative.
- Serial single-gene testing can be considered if the biochemical findings indicate that mutation of a particular gene is most likely.Sequence analysis of the gene of interest is performed first, followed by gene-targeted deletion/duplication analysis if a hemizygous pathogenic variant in the case of OTC deficiency or only one or no pathogenic variant is found in the case of deficiencies of NAGS, CPS1, ASS1, ASL, ARG1, ORNT1, or citrin.
- A multigene panel that includes the eight genes discussed in this GeneReview may be considered. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview; thus, clinicians need to determine which multigene panel is most likely to identify the genetic cause of the condition at the most reasonable cost while limiting the identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype.
A diagnosis can be confirmed by identifying elevated levels of argininosuccinic acid in blood or urine samples. A diagnosis can also be confirmed by molecular genetic testing, which detects the gene alteration that causes the disorder.
Treatment
Treatment may require the coordinated efforts of a team of specialists. Pediatricians, neurologists, geneticists, dieticians, and physicians who are familiar with metabolic disorders may need to work together to ensure a comprehensive approach to treatment. Occupational, speech-language, and physical therapists may be needed to treat children with developmental disabilities. Genetic counseling is recommended for affected individuals and their families.
The treatment of argininosuccinic aciduria is aimed at preventing excessive ammonia from being formed or from removing excessive ammonia during a hyperammonemia episode. Long-term therapy combines dietary restrictions and the stimulation of alternative methods of converting and excreting nitrogen from the body (alternative pathways therapy).
Dietary restrictions in individuals with argininosuccinic aciduria are aimed at limiting the amount of protein intake to avoid the development of excess ammonia. However, enough protein must be taken in by an affected infant to ensure proper growth. Infants with argininosuccinic aciduria are placed on a low protein, a high-calorie diet supplemented by essential amino acids. A combination of a high biological value natural protein such as breast milk or cow’s milk formulate, an essential amino acid formula (e.g., UCD-1 Ross, or Cyclinex, Mead Johnson), and a calorie supplement without protein is often used (e.g., MJ80056, Mead Johnson).
Individuals with argininosuccinic aciduria benefit from treatment with arginine, which helps to promote the excretion of nitrogen. Arginine supplementation has shown benefits in improving or reversing changes to the hair, but its impact on the long-term, chronic complications of the disorder is not fully understood. The dose of arginine is often higher than is used in other forms of urea cycle disorder and it is effective in decreasing ammonia in emergent situations of elevated ammonia. However, chronic treatment with high doses of arginine may contribute to liver disease as it produces higher levels of argininosuccinic acid. Therefore, in individuals with liver disease, lower doses should be considered for long-term treatment. In this situation, other medications like alternative pathway therapies may be needed. Multiple vitamins and calcium supplements may also be used in the treatment of argininosuccinic aciduria. Finally, because of decreased production of nitric oxide in patients with argininosuccinic aciduria, the addition of low protein foods rich in nitrite may be helpful.
Prompt treatment is necessary when individuals have extremely high ammonia levels (severe hyperammonemia episode). Prompt treatment can avoid hyperammonemia coma and associated neurological symptoms. However, in some individuals, especially those with complete enzyme deficiency, prompt treatment will not prevent recurrent episodes of hyperammonemia and the potential development of serious complications.
In some instances, despite early treatment and good metabolic control, affected individuals may develop certain symptoms such as neurocognitive deficiencies, behavior issues such as ADHD, developmental disability, and seizures.
In addition to dietary restrictions and supplements, individuals with argininosuccinic aciduria are treated with medications that stimulate the removal of nitrogen from the body. These medications provide an alternative method to the urea cycle in converting and removing nitrogen waste. This is known as alternative pathway therapy or nitrogen scavenging therapy. This includes sodium benzoate, sodium phenylbutyrate, and glycerol triphenylbutyrate.
In 2013, the U.S. Food and Drug Administration (FDA) approved Ravicti (glycerol phenylbutyrate) for the chronic management of urea cycle disorders including argininosuccinic aciduria in affected individuals aged 2 years and older. Ravicti is a liquid therapy that helps to remove ammonia from the body. Ravicti is used in individuals who cannot manage the disorder through a low-protein diet and dietary supplements alone.
In 1996, the FDA approved Buphenyl (sodium phenylbutyrate) for chronic management of urea cycle disorders including argininosuccinic aciduria. Biphenyl is a powder therapy that helps to remove ammonia from the body. A generic form of Buphenyl is also now available.
Sodium benzoate is a powder that is not FDA approved for the treatment of urea cycle disorders, but it has been used in the chronic treatment of urea cycle disorders. It is not believed to be as effective as Buphenyl or Ravicti based on theoretical considerations, though this has never been tested in patients.
In 2005, the FDA approved the use of Ammonul (sodium benzoate and sodium phenylacetate) as an intravenous, rescue therapy for the prevention and treatment of hyperammonemia and associated disease of the brain (encephalopathy) in individuals with urea cycle disorders.
Aggressive treatment is needed in hyperammonemia episodes that have progressed to vomiting and increased lethargy. Affected individuals may be hospitalized and protein may be eliminated from the diet for 24 hours. Affected individuals may also receive treatment with intravenous administration of arginine and a combination of sodium benzoate and sodium phenylacetate. Non-protein calories may be also provided as glucose.
In individuals where there is no improvement or where hyperammonemic coma develops, the removal of wastes by filtering an affected individual’s blood through a machine (hemodialysis) may be necessary. Hemodialysis is also used to treat infants, children, and adults who are first diagnosed with argininosuccinic aciduria during hyperammonemia coma.
In some individuals, a liver transplant may be recommended. This is an option of last resort for specific individuals who have progressive liver disease, experience recurrent medical crises and hospitalizations despite therapy, or who have a poor quality of life.
Rapidly return plasma ammonia concentrations to normal physiologic levels
This is necessary even without a definitive diagnosis, given the toxic effect of elevated plasma ammonia concentration. The best way to reduce plasma ammonia concentration quickly is by dialysis. The faster the flow rate, the faster the clearance. The method employed depends on the affected individual’s circumstances and available resources. In general, the best choice for an individual patient is whatever method the local treating team is most comfortable with and can implement most quickly. The various renal replacement modalities are reviewed by Gupta et al [2016].
- The fastest method is the use of pump-driven dialysis, in which an extracorporeal membrane oxygenation (ECMO) pump is used to drive a hemodialysis machine.
- Intermittent hemofiltration (both arteriovenous and venovenous) and hemodialysis are more likely to be available than ECMO-driven dialysis.
- Continuous renal replacement therapies have lower clearance than intermittent dialysis, but are less prone to interruption and may be better tolerated in sick neonates.
- Clearance with peritoneal dialysis is substantially lower than with hemodialysis; therefore, hemodialysis (if available) is typically preferred. However, published outcome data are limited.
- Intermittent dialysis can usually be discontinued when plasma ammonia concentration falls below 150 µmol/L but may vary based on clinical evaluation by a clinician experienced in the treatment of metabolic disease. Affected individuals often experience a “rebound” hyperammonemia that may require further dialysis. This may be attenuated with the use of continuous renal replacement therapies following intermittent HD [Gupta et al 2016].
Perform pharmacologic interventions to allow alternative pathway excretion of excessive nitrogen.
- Nitrogen scavenger therapy (sodium phenylacetate and sodium benzoate) is available as an intravenous infusion for acute management and an oral preparation for long-term maintenance.
- Deficient urea cycle intermediates need to be replaced depending on the diagnosis; these can include arginine (IV infusion) and/or citrulline (oral preparation).Note: Continuous arginine hydrochloride (HCl) infusion requires central access, as extravasation from a peripheral IV has on multiple occasions resulted in severe cutaneous necrosis.
- Sodium phenylacetate and sodium benzoate can be infused through a peripheral IV; however, central access is preferred.
- In persons with NAGS deficiency and some with CPS1 deficiency, replacement of n-acetyl glutamate with the analog molecule carbamyl glutamate (Carbaglu®) can improve the clinical symptoms or in NAGS deficiency can be almost curative. This compound is available in the US and should be added to the treatment regimen in a patient without a clear diagnosis at initial presentation. Dosing in adults and children is 100 mg/kg/day to 250 mg/kg/day divided into two to four doses. The only form currently available is oral preparation; thus, administration of the medication by nasogastric/jejunal tube is necessary for the treatment of acute manifestations.
Preventive Care
After a diagnosis of argininosuccinic aciduria, steps can be taken to anticipate the onset of a hyperammonemia episode. Affected individuals should receive periodic blood tests to determine the levels of ammonia in the blood. Detection of elevated levels of ammonia may allow treatment before clinical symptoms appear. Monitoring for complications such as high blood pressure, liver inflammation, and fibrosis, and developmental delay should be closely monitored from the time of diagnosis.
Investigational Therapies
Researchers have studied nitric oxide supplementation for the treatment of certain long-term complications associated with argininosuccinic aciduria. Affected individuals have a deficiency of nitric oxide, a naturally-occurring compound in humans. Nitric oxide has several roles in the body. Initial reports of nitric oxide supplementation for individuals with argininosuccinic aciduria have led to a resolution in hypertension and improvement in other neurocognitive issues. So far, only a small number of individuals have been treated with nitric oxide supplementation. More research is necessary including appropriate clinical trials to determine the long-term safety and effectiveness of this potential therapy for individuals with argininosuccinic aciduria.
Enzyme replacement therapy and gene therapy have shown potential promise for the treatment of metabolic disorders such as urea cycle disorders. Similarly, liver-directed gene therapy and mRNA replacement therapy are also being studied. Research on these types of therapy is being explored for urea cycle disorders including argininosuccinic aciduria. More research is necessary to determine the long-term safety and effectiveness of these treatments.
References
