Clinical features of HAT3 are those of hyperammonemic encephalopathy. Parents of neonates born with NAGS gene defects often claim feeding difficulties; retrospectively, poor feeding can generally be traced back to the first day of the child's life, but affected neonates are usually not presented until several days later. Meanwhile, about one-third of patients becomes somnolent or lethargic, two out of three suffer from vomiting [10]. Fluctuations of body temperature and hyperventilation may also be noted. Seizures are common, but subclinical seizures may not be readily observable. Eventually, the patient's level of consciousness decreases and they may fall into a hyperammonemic coma.

Late-onset HAT3 may manifest at any age. Chronic headaches and nausea are the most common cause of presentation in adolescents and adults developing hyperammonemia due to NAGS deficiency. Ataxia, tremor, visual impairment, confusion and psychiatric symptoms may also be experienced.

Patients who are currently under treatment for HAT3 are still at risk of hyperammonemia crises, independent of age. Such crises are stroke-like episodes, possibly triggered by recent infections and fever, fasting and weight loss as well as protein overload or intense physical exercise. Drugs that may provoke a hyperammonemia crisis are valproate, asparaginase and other chemotherapeutics, glucocorticoids and much more [12].

In neonates, hyperammonemia encephalopathy is frequently misdiagnosed as sepsis. Thus, blood levels of ammonia should be assessed in all neonates presenting with clinical symptoms consistent with sepsis. This also applies to elder patients presenting with otherwise not explainable neurological deficits, reduced consciousness, or suspected intoxication. If hyperammonemia is detected, hepatic failure and urea cycle disorders move up the list of differential diagnoses. Immediately following, plasma acylcarnitines, amino acids, liver enzymes, urine organic acids and orotic acid should be determined. Findings may be interpreted as follows:

• Accumulation of specific acylcarnitines may imply branched-chain organic acidemias, e.g., enhanced propionyl carnitine concentrations may indicate methylmalonic acidemia or propionic acidemia
• Glutamine and alanine levels are increased in case of HAT3, CPSI deficiency, and ornithine transcarbamylase deficiency, with the latter urea cycle disorders being much more common than NAGS deficiency
• Contrary to HAT3, CPSI deficiency is usually associated with reduced concentrations of citrulline
• Highly increased urine orotic acid is characteristic of ornithine transcarbamylase deficiency, but not of HAT3 or CPSI deficiency

Furthermore, routine blood biochemistry and blood gas analyses should be carried out to assess the overall condition of the patient.

In order to confirm HAT3, the enzymatic activity of NAGS in a liver biopsy specimen may be evaluated. Furthermore, molecular biological techniques may be employed to determine the precise gene defect underlying HAT3. Molecular diagnostic tests are most sensitive [4].

Therapy of HAT3 comprises the immediate treatment of hyperammonemia and long-term prevention of renewed increases in blood ammonia levels by daily administration of carbamoyl glutamate. The latter is a structural analogue of N-acetylglutamate, may activate CSPI and thus compensates for NAGS deficiency. Although the affinity of N-acetylglutamate to CPSI is higher, this compound has poor pharmacokinetic properties [1]. Recommended dosages vary and range from 15 mg/kg/d to 200 mg/kg/d [13]. Initially, higher doses are preferred; an adjustment of the total daily dose is then performed according to the patient's response to therapy and normalization of ammonia levels [14]. Furthermore, a restriction of dietary protein intake to less than 3 g/kg/day may be necessary, especially during metabolic crises.

Emergency treatment of hyperammonemia is not specific for HAT3 and may comprise the following measures [12]:

• Prevent any further intake of proteins for up to 36 hours
• Fluid therapy, intravenous application of dextrose (possibly plus insulin) and Intra lipids
• Provide L-arginine, L-citrulline and nitrogen scavenger medication
• In case of an unsatisfactory response to therapy or severe hyperammonemia (>250 μmol/l), prepare patient for hemodialysis or hemofiltration
• In any case, monitor blood ammonia levels every three hours

If at all possible, patients known to suffer from HAT3 should not be treated with drugs that possibly induce a hyperammonemia crisis.

If left untreated, HAT3 has a poor outcome. Hyperammonemia may provoke hyperammonemia coma, irreversible brain damage, and death. In fact, acute mortality of patients suffering from severe neonatal hyperammonemia encephalopathy open link due to complete enzymatic deficiencies interfering with the urea cycle may amount to almost 50% [11]. Survivors frequently suffer from developmental delays and their median survival time is less than four years. Thus, an early diagnosis and initiation of treatment are of utmost importance. Affected individuals who receive carbamyl glutamate before brain damage occurs are generally able to lead a normal life. Neither motor nor cognitive development is affected in these cases.

In older literature, HAT3 is defined as a severely reduced activity of NAGS as assessed in hepatic tissue [3]. Detoxification of ammonia takes place in the liver, and NAGS deficiency is indeed the trigger of HAT3. Later, NAGS deficiency could be related to sequence anomalies affecting the gene encoding for NAGS. This gene is located on the long arm of chromosome 17. More than twenty mutations of this gene have been described to date and an excellent review on this topic has been published a few years ago [4]. Still, HAT3-related gene defects unknown by then have been reported afterwards [5] [6]. In sum, both nonsense and frame-shift mutations, as well as missense mutations, have been described. While the former are generally associated with a complete absence of NAGS activity, missense mutations may be related to the residual enzymatic activity. The latter has been proposed to account for late-onset HAT3 [4]. All mutations known to date show recessive behavior; thus, heterozygous individuals don't develop HAT3. In contrast, NAGS activity is severely diminished or undetectable in homozygous patients.

Of note, secondary NAGS deficiency is a clinically indistinguishable entity of different etiology. Here, NAGS activity is diminished by toxic metabolites or provoked by lack of substrates [7]. This may be the case in isovaleric acidemia, methylmalonic acidemia, propionic acidemia and other branched-chain organic acidemias as well as valproate-induced hyperammonemia.

The overall incidence of urea cycle disorders has been estimated to be about 1 per 35,000 inhabitants. HAT3 is the least common urea cycle disorder, accounting for less than 1% of those cases. Accordingly, less than 1 per 3,500,000 people is expected to suffer from HAT3. Analyses based on newborn screens yield similar values, with an estimation of a maximum incidence of 1 in 2,000,000 [8]. Neither racial nor gender predilection have been reported to date. HAT3 has initially been described as a congenital disease with symptom onset within days after birth, but case reports of adult-onset HAT3 have been published later [5] [9]. While symptom onset may occur at any age, neonatal HAT3 is still considered the most common form of the disease.

The urea cycle comprises complex biochemical reactions that allow for the breakdown of proteins, amino acids and other nitrogen compounds while at the same time preventing the accumulation of neurotoxic ammonia: The main source of ammonia in the human body is the conversion of glutamate to α-ketoglutarate, catalyzed by glutamate dehydrogenase and yielding ammonia as a by-product. Considerable shares of ammonia are used for the production of carbamoyl phosphate, and this reaction is catalyzed by CPSI. The latter is dependent on N-acetylglutamate that, in turn, is synthesized from glutamate and acetyl-CoA. Because this reaction is mediated by NAGS, NAGS deficiency triggers a chain of pathophysiological events including reduced levels of N-acetylglutamate, diminished activity of CPSI and impaired detoxification of ammonia. In fact, carbamoyl phosphate synthase deficiency entails similar clinical consequences as HAT3 and is an important differential diagnosis of NAGS deficiency.

HAT3 patients are constantly at risk of sustaining hyperammonemia crises, which is usually defined as clinical symptoms due to ammonia levels >100 μmol/l [10]. They may be triggered by infectious diseases, lack of nutrients and protein overload, drug intake and a variety of other factors. As has been indicated above, the central nervous system is most sensitive to increased concentrations of ammonia. Patients who survive a hyperammonemia crisis are thus at risk of irreversible brain damage, persistent neurological deficits and developmental delays.

Most neonates suffering from HAT3 and other urea cycle disorders develop hyperammonemia shortly after birth, more than half of them within four days, two out of three within a week [15]. Consequently, analyses of blood samples within their first week of life may yield increased concentrations of ammonia and may prompt a corresponding workup. Moreover, prenatal diagnosis of HAT3 is feasible but requires a target-oriented, more arduous approach [16]. Despite such tests being readily available in many countries, neither pre- nor postnatal screens for urea cycle disorders are routinely conducted at present. In any case, parents-to-be with a familial history of HAT3 should be offered the corresponding analyses. Affected families that wish to have children may benefit from genetic counseling.

Hyperammonemia type 3 (HAT3) is a genetic disease associated with disturbances of urea metabolism caused by the deficiency of the enzyme N-acetyl glutamate synthase (NAGS) [1]. This enzyme catalyzes the conversion of glutamate and acetyl-CoA to N-acetyl glutamate, and the latter serves as an allosteric activator of carbamoyl phosphate synthetase I (CPSI). This enzyme catalyzes the production of carbamoyl phosphate from ammonia and bicarbonate. Consequently, NAGS deficiency results in a reduced activity of CPSI and an accumulation of ammonia in the body of the affected individual.

Due to the genetic etiology of the disease, HAT3 is a congenital disorder. However, symptoms don't necessarily manifest shortly after birth. Despite neonatal HAT3 being the most common form of the disease, symptom onset may be delayed until well into adulthood. Symptoms are provoked by a deficient detoxification of ammonia, which is a neurotoxic substance. Patients frequently develop seizures and may suffer from lethargy, vomiting, diarrhea and breathing difficulties. Severe hyperammonemia is often associated with a reduced level of consciousness, and patients may fall into a hyperammonemic coma.

Molecular testing is indicated to confirm a tentative diagnosis of HAT3. Treatment mainly consists in compensating for NAGS deficiency by administration of carbamoyl glutamate. This compound assumes the function of N-acetyl-glutamate and activates CPSI. Therapy largely improves the outcome of this potentially life-threatening disease: Hyperammonemic crises are associated with mortality rates of up to 10% [2]. Adequate treatment, however, generally allows for a good quality of life.

Hyperammonemia type 3 (HAT3) is a metabolic disorder associated with disturbances of urea metabolism. It is caused by a deficiency of the enzyme N-acetyl-glutamate synthase (NAGS), which, in turn, is provoked by mutations of the gene encoding for NAGS. For a better understanding of the clinical consequences of NAGS deficiency, biochemical reactions that play a crucial role in the urea cycle shall be illustrated briefly:

• The breakdown of proteins, amino acids, and other nitrogen compounds may yield ammonia as a by-product.
• The enzyme carbamoyl phosphate synthetase I (CPSI) catalyzes the conversion of ammonia and bicarbonate to carbamoyl phosphate.
• The activity of CPSI depends on the presence of an allosteric activator, namely N-acetyl glutamate.
• N-acetyl-glutamate is synthesized from glutamate and acetyl-CoA, and this reaction is mediated by NAGS.

Consequently, NAGS deficiency results in a reduced activity of CPSI and an accumulation of ammonia. Ammonia is neurotoxic and patients suffering from HAT3 develop hyperammonemia. This condition may cause brain damage, coma, and death. Because HAT3 is a genetic disorder, most patients develop symptoms shortly after birth. In the case of residual NAGS activity, late-onset HAT3 may occur.

Daily administration of carbamoyl glutamate is the mainstay of HAT3 therapy. This compound may activate CPSI and thus compensates for NAGS deficiency. In order to avoid permanent brain damage, treatment should be initiated as early as possible. If parents note lethargy, poor feeding, vomiting and diarrhea, hyperventilation or seizures in their babies, a physician should be advised immediately. Patients who receive carbamoyl glutamate from the start may not sustain brain damage, develop normally and lead a normal life.

1. Ah Mew N, Caldovic L. N-acetylglutamate synthase deficiency: an insight into the genetics, epidemiology, pathophysiology, and treatment. Appl Clin Genet. 2011; 4:127-135.
2. Batshaw ML, Msall M, Beaudet AL, Trojak J. Risk of serious illness in heterozygotes for ornithine transcarbamylase deficiency. J Pediatr. 1986; 108(2):236-241.
3. Bachmann C. Long-term outcome of patients with urea cycle disorders and the question of neonatal screening. Eur J Pediatr. 2003; 162 Suppl 1:S29-33.
4. Caldovic L, Morizono H, Tuchman M. Mutations and polymorphisms in the human N-acetylglutamate synthase (NAGS) gene. Hum Mutat. 2007; 28(8):754-759.
5. van de Logt AE, Kluijtmans LA, Huigen MC, Janssen MC. Hyperammonemia due to Adult-Onset N-Acetylglutamate Synthase Deficiency. JIMD Rep. 2016.
6. Al Kaabi EH, El-Hattab AW. N-acetylglutamate synthase deficiency: Novel mutation associated with neonatal presentation and literature review of molecular and phenotypic spectra. Mol Genet Metab Rep. 2016; 8:94-98.
7. Häberle J. Role of carglumic acid in the treatment of acute hyperammonemia due to N-acetylglutamate synthase deficiency. Ther Clin Risk Manag. 2011; 7:327-332.
8. Summar ML, Koelker S, Freedenberg D, et al. The incidence of urea cycle disorders. Mol Genet Metab. 2013; 110(1-2):179-180.
9. Cartagena A, Prasad AN, Rupar CA, et al. Recurrent encephalopathy: NAGS (N-acetylglutamate synthase) deficiency in adults. Can J Neurol Sci. 2013; 40(1):3-9.
10. Lee B, Diaz GA, Rhead W, et al. Blood ammonia and glutamine as predictors of hyperammonemic crises in patients with urea cycle disorder. Genet Med. 2015; 17(7):561-568.
11. Maestri NE, Clissold D, Brusilow SW. Neonatal onset ornithine transcarbamylase deficiency: A retrospective analysis. J Pediatr. 1999; 134(3):268-272.
12. Häberle J, Boddaert N, Burlina A, et al. Suggested guidelines for the diagnosis and management of urea cycle disorders. Orphanet J Rare Dis. 2012; 7:32.
13. Gessler P, Buchal P, Schwenk HU, Wermuth B. Favourable long-term outcome after immediate treatment of neonatal hyperammonemia due to N-acetylglutamate synthase deficiency. Eur J Pediatr. 2010; 169(2):197-199.
14. Guffon N, Schiff M, Cheillan D, Wermuth B, Haberle J, Vianey-Saban C. Neonatal hyperammonemia: the N-carbamoyl-L-glutamic acid test. J Pediatr. 2005; 147(2):260-262.
15. Bachmann C. Long-term outcome of patients with urea cycle disorders and the question of neonatal screening. Eur J Pediatr. 2003; 162(Suppl 1):S29-33.
16. Häberle J, Koch HG. Genetic approach to prenatal diagnosis in urea cycle defects. Prenat Diagn. 2004; 24(5):378-383.