urea cycle defects

1. UREA CYCLE DISORDERS
Presenter – Dr Tejaswini
Moderator – Dr Shanthi

2. Introduction
• The urea cycle, first described by Krebs and Henseleit in 1932.
• An extremely important enzymatic process in humans and
terrestrial animals
• It is the only pathway for detoxification of ammonia, the
toxic product of breakdown of protein and other nitrogen
containing molecules

3. • Mammals detoxify ammonia to urea through a series of reactions known as
the urea cycle.
• 5 enzymes: Carbamoyl phosphate synthetase 1 (CPS1)
Ornithine transcarbamylase (OTC)
Argininosuccinate synthetase (ASS)
Argininosuccinate lyase (ASL), and
Arginase 1
• A 6th enzyme, N-acetylglutamate (NAG) synthetase (NAGS), catalyzes
synthesis of NAG, which is an obligatory activator (effector) of the CPS1
enzyme.
• The urea cycle is exclusively present in the hepatocytes, distributed
between the mitochondrial matrix and the cytosol.

5. Incidence
• The worldwide incidence of urea cycle disorders (UCD) is
approximately 1 in 25,000–30,000 livebirths.
• However, this incidence is suspected to be an underestimation since
patients were evaluated based upon clinical manifestations and/or
family history rather than newborn screening.
• The mortality rate was 24 percent in neonatal-onset cases and 11
percent in late-onset cases.

6. Etiology
• Genetically determined, caused by mutations in genes coding for
enzymes, cofactor producer or transporter proteins of the urea
production pathway.
• Autosomal recessively inherited, except for OTC which is inherited in an
X-linked pattern.
• OTC deficiency may manifest in females frequently (in up to 15% cases),
with less severity than males. The symptoms in a female carrier depend
on the degree of skewed inactivation of the X chromosome.

7. Clinical presentation
• The severity of enzyme deficiency is the key to the clinical severity.
• Severe deficiency or total absence of enzyme activity leads to classical
neonatal presentation with hyperammonemia with ensuing coma and
death, if untreated.
• This presentation is classical for severe deficiency of any of the first four
enzymes of urea cycle (i.e., up to formation of arginine).

8. Clinical presentation
• Less severe deficiency may result in episodic presentation later in
life because of hyperammonemia in acute stressful conditions.
• The deficiency of the fifth enzyme, arginase 1 (ARG1), and two
transporters, ornithine and aspartate transporter leads to different
clinical presentations.

9. Typical presentation
• The neonates appear normal at birth and present within the first
week, mostly within first 48–72 hours of life with progressive
lethargy, and refusal to feed.
• This coincides with increase in oral intake of protein, as well as
with ongoing catabolism and stress of birth.
• As stage advances, the babies may have irritability, vomiting,
tachypnea (because of cerebral hyperventilation), and seizures
in 50%, as a sign of neurotoxicity.

10. • Later, there can be apnea owing to reduce respiratory drive,
metabolic acidosis (especially in argininosuccinic aciduria or
ASL deficiency), hypoglycemia, hypothermia and lactic
acidemia as encephalopathy deepens.
• Progressive coma and death ensues, if untreated, accompanied
by brain stem herniation.

11. Atypical presentation
• Some patients with partial urea cycle enzyme deficiency present with
chronic vomiting, developmental delay, a seizure disorder, sleep
disorders, or psychiatric illness.
• Others may develop symptoms following increased protein intake or
during periods of catabolic stress.
• These patients tend to prefer vegetarian diets because dietary protein
intake often is associated with headache.

12. • Particularly for ASL deficiency, manifestations that appear to be
unrelated to the severity or duration of hyperammonemic
episodes include:
(1) neurocognitive deficiencies (attention deficit hyperactivity
disorder, developmental disability, seizures, and learning
disability
(2) liver disease (hepatitis, cirrhosis)
(3) trichorrhexis nodosa (coarse brittle hair that breaks easily);
and
(4) systemic hypertension.

13. • Arginase deficiency is not typically because of
hyperammonemia.
• In infancy, there may be irritability and failure to thrive. This is
followed by development of spastic diplegia, dystonia or ataxia,
plateauing of cognitive development, and subsequent loss of
developmental milestones.
• As the symptoms advance, the children have progressive
spasticity leading to complete loss of ambulation, autonomic
dysfunction with loss of bowel/bladder control and severe
cognitive impairment.

14. Diagnosis
• Ammonia levels should be measured in patients who present
with typical clinical features of UCDs or who have a suggestive
family history or an abnormal newborn screening test.
• If the plasma ammonia concentration is greater than 100 to 150
micromol/L, further testing is performed to establish a
diagnosis.

15. • The initial laboratory evaluation for suspected UCD should
include arterial pH and carbon dioxide; serum ammonia, lactate,
glucose, electrolytes, and amino acids; and urine organic acids
and orotic acid.
• Elevated plasma ammonia concentration combined with normal
blood glucose and normal anion gap strongly suggests a UCD.

16. Plasma amino acid/urine orotic acid analyses
• Citrulline concentration is increased in argininosuccinate
synthetase (ASS) and argininosuccinate lyase (ASL)
deficiencies
• Argininosuccinic acid is absent in the former and elevated in the
latter.
• Arginine is elevated three- to fourfold above the upper limit of
normal in arginase deficiency

17. • Citrulline is absent or low in carbamyl phosphate synthetase I
(CPSI), ornithine transcarbamylase (OTC), or N-acetyl
glutamate synthetase (NAGS) deficiencies
• Arginine also is low, and glutamine is increased in these
disorders.
• If citrulline is absent urine orotic acid measurement may
differentiate OTC and CPS deficiencies.
• Orotic acid can be increased in the former and is low in the
latter.

18. Enzyme analysis — The diagnosis of a specific UCD can be
confirmed by enzyme analysis of tissue samples, as follows:
• Liver biopsy – CPSI, OTC, and NAGS deficiencies
• Fibroblasts from skin biopsy – ASS and ASL deficiencies
• Red blood cells – Arginase deficiency

19. DNA MUTATION ANALYSIS
• Increasingly, DNA sequencing (targeted or whole exome-based
approaches) in conjunction with array comparative genomic
hybridization (aCGH) is being used as noninvasive alternatives
to enzyme analysis of tissue samples.

20. Management
• Clinical outcome depends mainly on the severity and the duration of
hyperammonemia.
• Serious neurologic sequelae are likely in newborns with severe
elevations in blood ammonia (>300 μmol/L) for more than 12 hr.
• Thus, acute hyperammonemia should be treated promptly and
vigorously.

21. The initial approach to treatment consists of the following:
• Rehydrate and maintain good urine output without overhydration
• Remove nitrogen (ammonia) from the body using medications and/or
hemodialysis
• Stop protein intake and minimize catabolism
• Stimulate anabolism and uptake of nitrogen precursors by muscle

22. Fluid management
• Symptomatic patients typically are volume depleted and
reduced tissue perfusion can further increase protein
catabolism and nitrogen load and lead to increased ammonia
concentrations.
• In addition, pharmacologic treatment of UCDs requires good
kidney function.
• Thus, repletion of intravascular volume is a priority.

23. • The composition of maintenance fluid depends upon whether
pharmacologic therapy to remove ammonia is implemented.
• IV fluids should consist of 10 percent dextrose in water (D10W)

24. • A conservative approach of only initial IV fluid rehydration with
cessation of protein intake is reasonable when peak
hyperammonemia is <200 micromol/L and when the duration
of hyperammonemia is less than 24 hours.
• However, if hyperammonemia is persistent, >200 micromol/L, or
showing rapid increase, then pharmacologic therapy to remove
ammonia should be instituted immediately:

25. Pharmacotherapy
• For proximal UCDs (NAGS, CPSI, and OTC deficiencies), initiate IV arginine
hydrochloride (low maintenance dose), IV sodium phenylacetate-sodium
benzoate, and oral citrulline.
• For NAGS deficiency, initiate oral carglumic acid.
• For distal UCDs (ASS and ASL] deficiencies), initiate IV arginine
hydrochloride (high maintenance dose) and IV sodium phenylacetate-
sodium benzoate.
• For arginase deficiency, initiate IV sodium phenylacetate-sodium benzoate.
• If the diagnosis is not known initially, initiate IV sodium phenylacetate-
sodium benzoate and IV arginine hydrochloride (low maintenance dose).

26. • Acylation therapy - Exogenous organic acid that is acylated
endogenously with nonessential amino acids to form a nontoxic compound
with high renal clearance.
• The main organic acids used for this purpose are sodium salts of benzoic
acid and phenylacetic acid.
• For intravenous (IV) use, a combined formulation of benzoate and
phenylacetate (Ammonul) is commercially available.
• Sodium phenylbutyrate, metabolized to phenylacetate, is the primary
oral formulation.

27. • Benzoate forms hippurate with endogenous glycine in the
liver.
• Each mole of benzoate removes 1 mole of ammonia as glycine.
• Phenylacetate conjugates with glutamine to form
phenylacetylglutamine, which is readily excreted in the urine.
• One mole of phenylacetate removes 2 moles of ammonia as
glutamine from the body.

28. • weight ≤20 kg, loading dose of 500 mg/kg (250 mg/kg of each drug) in a
volume of 25 to 35 mL/kg of 10 percent dextrose solution infused over 90
minutes.
• weigh >20 kg, the loading dose is 11 g/m (ie, 5.5 g/m of each drug).
• Maintenance infusion of sodium phenylacetate-sodium benzoate (500
mg/kg per 24 hours for <20 kg, 11 g/m per 24 hours as a continuous
infusion for >20 kg)
• Adverse effects - Metabolic (eg, hypokalemia, hyperchloremia, acidosis)

29. • Arginine –Enzyme deficiencies in the urea cycle (with the exception of
arginase deficiency) prevent the formation of arginine, thus rendering it an
essential amino acid.
• Arginine deficiency results in a catabolic state that stimulates further
mobilization of nitrogen from protein breakdown.
• In OTC, ASS, and ASL deficiencies, arginine also is needed to generate
urea cycle intermediates, including ornithine, citrulline, and
argininosuccinic acid.

30. • In the absence of a diagnosis of the specific form of UCD and/or for patients
≤20 kg, the loading dose is 200 mg/kg.
• For patients >20 kg with a known diagnosis of a specific UCD other than
arginase deficiency, the loading dose is 4 g/m.
• IV maintenance dose is 200 mg/kg per 24 hours for patients ≤20 kg and 4 g/m
per 24 hours for patients >20 kg.
• For ASS and ASL deficiency, the maintenance dose is 600 mg/kg per 24 hours
for patients ≤20 kg and 12 g/m per 24 hours >20 kg.
• Blood pressure should be monitored since high doses of IV arginine can
decrease blood pressure. Patients should also be monitored for hyperchloremic
acidosis

31. • Citrulline – In OTC or CPSI deficiency, small oral doses of citrulline (150
to 200 mg/kg per 24 hours for patients ≤20 kg and 3 to 4 g/m per 24 hours
for patients >20 kg)
• Citrulline should not be given if the diagnosis is unknown, because
citrulline levels are elevated in ASS and ASL deficiencies.

32. • Carglumic acid – Carglumic acid (Carbaglu) - Treatment of
hyperammonemia due to NAGS deficiency.
• Carglumic acid is able to activate the first enzyme of the urea cycle (CPSI),
leading to rapid reduction of plasma ammonia to normal levels.
• The initial dose for acute hyperammonemia ranges from 100 to 250
mg/kg/day orally (prepared as a liquid and divided into two to four doses
that are given immediately before meals).

33. Hemodialysis
• Indications include an ammonia level that is rapidly increasing, acute
hyperammonemia that is resistant to initial drug therapy, and/or ammonia
that is above the range of 500 micromol/L
• Continuous arteriovenous or venovenous hemodialysis (CAVHD or
CVVHD) with flow rates >40 to 60 mL/min is optimal.
• Some use ECMO with hemodialysis. This technique rapidly reduces
ammonia levels but with greater morbidity associated with surgical
vascular access

34. • If these procedures are not available, continuous venovenous
hemofiltration (CVVH; also known as detoxification) may be used.
• Peritoneal dialysis is the least acceptable method because clearance of
ammonia is very slow and detoxification may take several days.

35. • Ammonia concentration is measured hourly during dialysis. Hemodialysis
is stopped when the ammonia concentration has dropped below 200
micromol/L.
• Hourly monitoring of ammonia levels is continued until ammonia levels
have stabilized below 200 micromol/L for at least 24 hours after stopping
dialysis.
• During this period, hemofiltration can be used to clear the newly produced
nitrogen.
• Dialysis catheters should be kept in place until ammonia levels have been
stable for at least 24 hours.

36. Inhibition of Ammonia Production
• Increase calorie supplementation to offset catabolism and protein breakdown
Provide 10–20% glucose infusion and add insulin if hyperglycemia occurs. IV lipids
infusion @ 2–3 g/kg/24 hours is also helpful.
• Little amount of protein is initiated either orally or parenterally, and titrated up to
maintain anabolism as well as to keep the ammonia levels down.

37. • Correction of Fluid and Electrolyte/Acid-Base Disturbance
• Treatment of Underlying or Triggering Factors

38. Dietary Management
• The basis of dietary management is to prevent production of ammonia.
• By keeping the protein intake to the lowest required level, which is enough to
sustain linear growth and carry out all the cellular functions appropriately

39. • oral sodium phenylbutyrate (400 to 600 mg/kg per 24 hours for patients
≤20 kg and 9 to 13 g/m per 24 hours for patients >20 kg)
• ≥2 months of age, oral glycerol phenylbutyrate (4.5 to 11.2 mL/m /day
divided in three equal doses).
• Citrulline (170 mg/kg per day orally) - OTC or CPSI deficiency
• ASS or ASL deficiency - arginine base 500 mg/kg per day orally
• carglumic acid - <100 mg/kg/day in patients with NAGS deficiency.

40. Monitoring for Growth and Development
• Regular assessment of growth and development is required to assess
appropriateness of dietary protein and energy intake of patient.
• In addition, few blood parameters, such as hemoglobin, albumin and pre-albumin
levels, vitamins and amino acid levels, should be checked regularly to assess
growth and efficacy and adequacy of maintenance therapy.
• Ammonia should be monitored more frequently both in normal state as well as
during times of acute illness.

41. Liver Transplantation as an Option for Long-term Cure
• Over the last two decades, reasonable evidence has gathered that liver
transplantation is the only modality that provides a definitive cure to patients of
UCD.
• Long-term (5-year) survivals are now close to 90% with minimum morbidity, if
offered at a time when the child has not had many neurological insults.
• Lifelong immunosuppressant therapy is required which may be a deterrent for
few patients, along with the exorbitant cost of a liver transplant.

42. OUTCOME
• Irrespective of the treatment, the outcome of acute severe neonatal
presentation of UCD remains poor.
• Outcome of children presenting later is better, provided the hyperammonemic
episodes are briskly and effectively managed and continued in the long-term.

43. Thank you