What exactly is Type 1 Citrullinemia?
Type 1 Citrullinemia (CTLN1) is categorized as a urea cycle disorder. This means CTLN1 involves a congenital error, which results in a deficiency in one of the enzymes that function in the urea cycle. Specifically, CTLN1 is characterized as an argininosuccinate synthetase (ASS) deficiency (Woo, Park, and Lee 2014). This is problematic because the urea cycle is an essential metabolic pathway that removes excess nitrogen from the body by detoxifying ammonia into urea (Ibarra-González, Fernández-Lainez, and Vela-Amieva 2010). A deficiency in any enzyme in this cycle results in an ammonia build up in the body, which leads to various health issues seen in patients with these disorders.
Symptoms of Urea Cycle Disorders:
- Change in mental status
- Hyperactive behavior
- Mood disturbances
- Self-injurious behavior
- Avoidance of dietary protein
- Cognitive impairment
Symptoms of Type 1 Citrullinemia:
- Hyperammonemic encephalopathy
- Failure to thrive
- Loss of consciousness
- Death early in life
- Chronic intermittent hyperammonemia
- Hepatic dysfunction
- Severe neuropsychiatric symptoms
How is CTLN1 identified?
Plasma quantitative amino acid analysis
According to Quinonez and Thoene 1993, if the following are found as they are described, the patient may have CTLN1.
- Citrulline = Usually greater than 1000 µmol/L (normal: <50 µmol/L)
- Argininosuccinic acid = Absent
- Arginine and ornithine = Low to normal range
- Lysine, glutamine, and alanine = Increased; these are surrogate markers of hyperammonemia.
- Urinary organic acids = Normal, except orotic acid may be detected as part of urinary organic acid analysis by gas chromatography/mass spectrometry; however, the sensitivity depends on the extraction method.
- Argininosuccinate synthase (ASS) enzyme activity = Incorporation of radiolabeled citrulline into argininosuccinic acid is measured in cultured fibroblasts. ASS activity is also determined by a method based on the conversion of radiolabeled (14C)-aspartate to (14C)-argininosuccinate [Gao et al 2003]:
- The normal enzyme activity in fibroblasts is 0.8-3.8 nmol/min/mg protein, but this is specific to tissue, method, and laboratory.
- Cultured chorionic villus cells or cultured amniocytes from the fetus may be used for prenatal diagnosis.
All states include CTLN1 in their newborn screening programs. Elevated citrulline is detected in dried blood spots on newborn screen by tandem mass spectroscopy (MS/MS). Citrullinemia is confirmed by plasma amino acid analysis that demonstrates the findings described above. Other conditions that may result in elevated citrulline on NBS are argininosuccinic acidemia, citrullinemia II (citrin deficiency), and pyruvate carboxylase deficiency.
What is normally happening in the body without CTLN1?
Without CTLN1, the body has the functional form of ASS and therefore, the urea cycle carries on as it normally should. The Figure 2 describes what occurs in the urea cycle in a disease-free state.
Specifically, CTLN1 concerns the step where citrulline and aspartate are combined to form argininosuccinate using argininosuccinate syntheses and ATP. The chemistry of this step involves the following (Fig. 3):
Citrulline uses its amide group’s resonance to attack a phosphate group of an ATP molecule with its negatively charged oxygen. Then a nucleophilic acyl substitution will occur, kicking off a diphosphate as the leaving group and resulting in an AMP attached to the citrulline’s previously nucleophilic oxygen. Aspartate will then come in and attack the carbon that has a double bond to the positively charged nitrogen with its lone pair on the amino group. Two electrons from the double bond between the carbon and nitrogen will then move to become a lone pair on the nitrogen that was previously positively charged. This lone pair will than travel back down to reform the double bond, kicking off the AMP and oxygen as the leaving group. The resulting molecule is argininosuccinate. (McMurry and Begley 2005).
What is the history of this disease?
The first reported case of Type 1 Citrullinemia occurred in 1962 where a 9 month old baby born of two first cousins exhibited signs of mental retardation as well as severe vomiting spells (McMurray et al. 1962). The patient had very high levels of citrulline in their serum, urine, and spinal fluid. It was identified in 1967 that the enzyme argininosuccinate synthetase (ASS) was connected to this disease. Through the analysis of the enzyme’s kinetics in a patient with the disease, ASS was found to have an altered Michaelis constant and was thus found to be kinetically less efficient than the normal form (Tedesco and Mellman 1967).
Citrullinemia used to be subcategorized into three different types based on biochemical manifestations. These were called types I, II, and III (Woo, Park, and Lee 2014). However, after discovering the gene responsible for type II, the disease was subcategorized based on molecular pathogenesis, resulting in only two types of citrullinemia (Woo, Park, and Lee 2014).
More recent discoveries concerning CTLN1 are attributed to The Urea Cycle Disorders Consortium. The Urea Cycle Disorders Consortium (UCDC) is an organization funded by the NIH as a part of the Rare Diseases Clinical Research Network. The UCDC’s purpose is to investigate treatment methods and consequences of Urea Cycle Disorders (UCDs) closely in order to formulate longitudinal analyses and eventually long-term solutions (Batshaw et al. 2014). One of the diseases the UCDC studies is of course CTLN1, as it is a UCD. Patients with Type 1 Citrullinemia make up about 13.6% of those with UCDs (Quinonez and Thoene 1993).
In the 1980’s, it was found that there was a 50% five-year survival rate for infants who presented with urea cycle disruption and were treated with aggressive hemodialysis (Batshaw et al. 2014). Initial studies of these children revealed that all survivors possessed developmental disabilities that correlated with the duration, severity, and number of hyperammonemic episodes they experienced (Batshaw et al. 2014). This caused for metabolic specialists to suggest no treatment for neonatal onset disease.
Very recent studies have suggested that the mortality rate due neonatal hyperammonemic comas has decreased and that cognitive outcome is slowly progressing (Batshaw et al. 2014). Additionally, it has been recently discovered that hyperammonemia can manifest at any age and cause a 10% risk of death and large risk for developmental disabilities in patients with partial defects of the urea cycle (Batshaw et al. 2014). These recent findings suggest there is still a lot we do not know about UCDs and therefore CTLN1.