N-Acetylglutamate Synthetase Deficiency

Normal enzyme function of N-acetylglutamate synthetase (NAGS) deficiency is confined to the hepatic mitochondria and mediates the reaction acetyl-coenzyme A (CoA) + glutamate → N-acetylglutamate + CoA. As a mitochondrial reaction, each of the substrates is normally omnipresent. Acetyl-CoA is a cofactor in many mitochondrial reactions, and glutamate is the transamination product of α-ketoglutarate and alanine; α-ketoglutarate is produced by the Krebs cycle.

The normal function of N-acetylglutamate (NAG), the reaction product, is to act as an activator of carbamyl phosphate synthetase (CPS), which is also a mitochondrial enzyme. See the image below.

The activation process requires physical binding of NAG to the CPS enzyme, in turn, causing the inactive form of CPS to convert to an active state. Thus, CPS activity is regulated by the relationship of available NAG to inactive CPS enzyme protein.

The biochemical effect of NAGS deficiency is an inability to form adequate NAG; this results in failure to activate the enzyme responsible for the reaction NH4+ + CO2 + ATP → H2 N-CO-PO32- + ADP, which is the entry step into the urea cycle (see Carbamyl Phosphate Synthetase Deficiency).[1]

Clinical signs and symptoms of NAGS deficiency occur when ammonia fails to fix into carbamoyl phosphate (CP) effectively, thus disabling the urea cycle. This leads to accumulation of alanine and glutamine (transamination products of pyruvate and glutamate, respectively) and, finally, of ammonia. The condition is progressive without intervention.

Overall, the hepatic urea cycle is the major route for waste nitrogen disposal, generation of which is chiefly from protein and amino acid metabolism.[2] Low-level synthesis of certain cycle intermediates in extrahepatic tissues makes a small contribution to waste nitrogen disposal as well. A portion of the cycle is mitochondrial in nature; mitochondrial dysfunction may impair urea production and result in hyperammonemia. Overall, activity of the cycle is regulated by the rate of synthesis of NAG, the enzyme activator that initiates incorporation of ammonia into the cycle.

Frequency

United States

Too few cases have been reported to cite any incidence figures. However, recognition of affected patients is increasing. Because the clinical presentation is indistinguishable from that of CPS deficiency and because the diagnosis is difficult, the true incidence may be underestimated. This is further emphasized by the fact that genetically affected individuals may remain asymptomatic for many years.

A newly formed Urea Cycle Disorders Consortium in the United States reported on a cross-section of patients throughout the country; remarkably, not a single case of NAGS deficiency was identified.[3] However, because NAG is the requisite activator for CPS, occasional mistaken diagnoses of CPS deficiency may have obscured cases of NAGS deficiency.

A series of adult cases of NAGS deficiency has been reported, suggesting that some mutations may result in milder clinical variants while complicating accurate diagnosis in children and teens.[4]

International

Only a handful of cases have been reported worldwide.

Mortality/Morbidity

NAGS deficiency is associated with significant morbidity and mortality. Patients who present with hyperammonemia are at risk for cerebral edema and death if treatment is not immediately begun. Survivors of hyperammonemic coma are likely to suffer brain damage and resulting developmental delays, learning disabilities, and/or mental retardation.

Sex

Case reports of NAGS deficiency have shown the condition to occur in both sexes; because the mutation is inherited as an autosomal recessive trait, this is to be expected.

Age

NAGS deficiency can present at any age. As with many inherited metabolic diseases, the most likely time of presentation is in the newborn period.

Long-term prognosis is unclear; most likely, the future intelligence quotient score depends on the severity of the initial presentation and the subsequent hyperammonemic episodes suffered.

Inform parents of their obligate heterozygote status given the likelihood that this is an autosomal recessive trait.

Parents must understand that the chance of recurrence is 1:4 (25%) with each subsequent pregnancy.

Advise parents to seek early medical attention for the patient in the event of intercurrent illness.

The multiple primary causes of hyperammonemia, specifically those due to urea cycle enzyme deficiencies, somewhat vary in presentation, diagnostic features, and treatment. For these reasons, the family of urea cycle defects is considered individually in this article; however, the common denominator, hyperammonemia, can be manifested clinically by some or all of the following:

• Anorexia
• Irritability
• Heavy or rapid breathing
• Lethargy
• Vomiting
• Disorientation
• Somnolence
• Asterixis (rare)
• Combativeness
• Obtundation
• Coma
• Cerebral edema
• Death, if treatment is not forthcoming or effective

As a consequence, the most striking clinical findings of each urea cycle disorder relate to this constellation of symptoms and rough temporal sequence of events.

See the list below:

• General

  • Signs of severe hyperammonemia may be present.

  • Poor growth may be evident.

• Head, ears, eyes, nose, and throat (HEENT): Papilledema may be present if cerebral edema and increased intracranial pressure have ensued.

• Pulmonary

  • Tachypnea or hyperpnea may be present.

  • Apnea and respiratory failure may occur in later stages.

• Abdominal: Hepatomegaly may be present and is usually mild.

• Neurologic

  • Poor coordination

  • Dysdiadochokinesia

  • Hypotonia or hypertonia

  • Ataxia

  • Tremor

  • Seizures and hypothermia

  • Lethargy progressing to combativeness to obtundation to coma

  • Decorticate or decerebrate posturing

The mode of inheritance is an autosomal recessive inheritance pattern.

The NAGS gene was the last one of the urea cycle to be cloned. The gene locus is 17q21.31, spans 4.5 kb, and contains 6 introns and 7 exons. The 534 amino acid residues contained in the ribosomal protein are reduced to 486 by cleavage at the N -terminus upon import to the mitochondrion. At least 22 pathogenic mutations have been reported, 10 of which were associated with acute neonatal presentation.[5] Interestingly, no mutations were found in exon 1, which is believed to code for the 50 amino acid mitochondrial-targeting segment that is cleaved.

Urea cycle defects with resulting hyperammonemia are due to deficiencies of the enzymes involved in the metabolism of waste nitrogen. The enzyme deficiencies lead to disorders with nearly identical clinical presentations. The exception is arginase, the last enzyme of the cycle; arginase deficiency causes a somewhat different set of signs and symptoms.

Possible complications include cerebral edema with resulting brain damage or death.

Affected newborns may experience fulminant hyperammonemia, which remains undetected unless index of suspicion is high.

No routine laboratory tests provide definitive clues. The BUN level may be low, but this is an unreliable index of high blood ammonia. A respiratory alkalosis may be present. Urine orotic acid levels are within reference ranges.

Plasma alanine and glutamine levels are elevated.

Urine amino acids are nondiagnostic in N- acetylglutamate synthetase (NAGS) deficiency but are important in order to help rule out hyperammonemia-hyperornithinemia-homocitrullinuria (HHH) or lysinuric protein intolerance (LPI) (see Differentials).

Urine organic acids are within reference ranges in NAGS deficiency. Ruling out organic acid disorders, which can present with similar signs and symptoms and hyperammonemia, is important.

Definitive diagnosis rests with DNA sequencing; to date, there is no newborn metabolic screen which can detect the defect.

Imaging studies generally are not helpful, with the exception of brain imaging when cerebral edema is suspected. Documenting a finding of cerebral edema is important.

DNA sequencing is available in selected laboratories; mutation analysis provides a definitive diagnosis

Immediate cessation of protein intake is mandatory in the face of high blood ammonia levels with provision of supplementary nonprotein energy to close the caloric gap. In most cases of severe hyperammonemia, the patient is given nothing enterally until the hyperammonemia is well controlled.

Treatment of severe hyperammonemia is a true emergency. Reduction of blood ammonia can usually be achieved with intravenous sodium benzoate and phenylacetate. Intravenous sodium benzoate and phenylacetate (Ammonul)[6] was approved in the United States in February, 2005. Alternatively, hemodialysis is usually effective in bringing down the ammonia level, especially with the initial presentation. Exchange transfusion is ineffective and is not generally recommended. Intravenous fluids with glucose and sometimes arginine hydrochloride (HCl) added may be indicated.[7] Maintaining as high of an energy intake as possible is important.

Specific therapy of N- acetylglutamate synthetase (NAGS) deficiency following diagnosis depends on dietary protein restriction and provision of arginine to enhance availability of ornithine and administration of carbamylglutamate (which is not widely available), a functional analogue of NAG. Some patients have done well using this regimen. Whether or not oral sodium phenylbutyrate is helpful in this condition is unclear.

NAGS deficiency is an extremely rare disorder with complex treatment.

Consultation with a metabolic disease/medical genetic specialist is usually necessary for assistance with laboratory diagnosis and clinical care. Contact these consultants by telephone if they are not locally available.

A low-protein diet is generally recommended with dietary supervision under the direction of a dietitian experienced in the care of patients with metabolic disease.

The patient should be under the care of a biochemical geneticist (metabolic disease specialist) who is an expert in the care of patients with urea cycle defects.

Make medication adjustments based on continued growth and frequently measured plasma amino acids; include the input of a highly trained nutritionist.

Consider transferring the patient to a facility equipped for emergent hemodialysis (if the patient is a neonate) and where the appropriate consultants (see Consultations) are immediately available.

The specific treatment of N -acetylglutamate synthetase (NAGS) deficiency following diagnosis depends on dietary protein restriction and provision of arginine to enhance availability of ornithine and administration of carbamylglutamate (which is not widely available), a functional analogue of NAG. Some patients have done well using this regimen.

Intravenous sodium benzoate and phenylacetate can usually reduce blood ammonia levels. The addition of intravenous fluids with glucose and sometimes arginine hydrochloride (HCl) may also be indicated.

Class Summary

In the absence of any ability to fix nitrogen generated from endogenous catabolism of protein, the urea cycle is of no use whatsoever to the homeostasis of nitrogen metabolism. In order to stimulate urea cycle action, investigators have used N -carbamoyl-L-glutamate as an analogue of N -acetyl-L-glutamate to activate CPS.[7, 8]

Safety and efficacy of carglumic acid was studied in 23 patients with NAGS who received the treatment for times ranging from 6 months to 21 years. In these patients, carglumic acid reduced blood ammonia levels within 24 h and normalized ammonia levels within 3 days. Majority of those in the study appeared to maintain normal plasma ammonia levels with long-term treatment.

Carglumic acid (Carbaglu)

Also called N -carbamoyl-L-glutamate, carbamylglutamic acid, or carglutamic acid. Structural analogue of N -acetylglutamate, which enters cells and enables activation of CPS I (first enzyme in urea cycle) in vivo. Decreases hyperammonemia by converting ammonia into urea. More resistant to enzymatic degradation by hydrolysis compared with N -acetylglutamate. Indicated for NAGS deficiency, a rare genetic disorder that results in hyperammonemia.

Available as a 200-mg dispersible tab. Tab is scored and can be split to provide accurate dose.

Class Summary

These agents assist in the excretion of nitrogen and serve as an alternative to urea to reduce waste nitrogen levels. Administer only in a large medical facility with close laboratory monitoring available.

Arginine (R-Gene 10)

Enhances production of ornithine, which facilitates incorporation of waste nitrogen into the formation of citrulline and argininosuccinate. Provides 1 mol of urea plus 1 mol ornithine per mol arginine when cleaved by arginase. Pituitary stimulant for the release of human growth hormone (HGH). Often induces pronounced HGH levels in patients with intact pituitary function.

Sodium phenylacetate and sodium benzoate (Ammonul)

Benzoate combines with glycine to form hippurate, which is excreted in urine. 1 mol of benzoate removes 1 mol of nitrogen. Phenylacetate conjugates (via acetylation) glutamine in the liver and kidneys to form phenylacetylglutamine, which is excreted by the kidneys. The nitrogen content of phenylacetylglutamine per mol is identical to that of urea (2 mol of nitrogen). Ammonul must be administered with arginine for CPS, ornithine transcarbamylase (OTC), argininosuccinate synthetase, or argininosuccinate lyase (ASL) deficiencies. Indicated as adjunctive treatment of acute hyperammonemia associated with encephalopathy caused by urea cycle enzyme deficiencies. Serves as an alternative to urea to reduce waste nitrogen levels.