Citrullinemia, Type I via ASS1 Gene

Urea cycle defects are characterized by hyperammonemia, encephalopathy and respiratory alkalosis (Brusilow and Horwich 2014). Eight clinical disorders have been described involving defective urea cycle enzymes or transporter proteins: N-acetylglutamate synthase deficiency, carbamoyl phosphate synthetase I deficiency, ornithine transcarbamolase deficiency, argininosuccinate synthetase deficiency (also known as citrullinemia type I), argininosuccinate lyase deficiency, arginase deficiency, citrin deficiency (also known as citrullinemia type II) and hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome (Ah Mew et al. 2015).

Type I citrullinemia presents in the neonate following protein intake with massively elevated serum citrulline levels resulting in hyperammonemia. Patients exhibit lethargy, refusal to feed, vomiting, and tachypnea or stroke (Häberle et al. 2003; Ah Mew et al. 2015; Quinonez and Thoene 2016). Untreated, argininosuccinate synthetase deficiency can lead to increased intracranial pressure, increased neuromuscular tone, seizures, loss of consciousness, and death (Quinonez and Thoene 2016). A milder adult-onset form of the disease has also been described (Häberle et al. 2003; Martín-Hernández et al. 2014). Clinical signs of the milder form include recurrent lethargy, somnolence, mental retardation, and chronic or recurrent hyperammonemia. Treatment includes a protein restricted diet and sodium benzoate to scavenge ammonia (Quinonez and Thoene 2016).

Type I or classic citrullinemia is an autosomal recessive disorder. Pathogenic variants in the ASS1 gene cause both the severe neonatal and milder adult onset forms of citrullinemia type I (Häberle et al. 2003; Martín-Hernández et al. 2014). The ASS1 gene encodes argininosuccinate synthetase, a urea cycle enzyme that converts citrulline and aspartate to argininosuccinate. Missense variants are the predominant type of disease-causing variants in ASS1, though nonsense, splicing, small deletions and insertions and gross deletions have been reported as well (Gao et al. 2003; Martín-Hernández et al. 2014; Human Gene Mutation Database). Pathogenic variants are spread throughout the gene and most reported variants appear to be private. However, a few variants have been more commonly reported, such as c.421-2A>G (common in Japanese and Korean patients), c.535T>C (p.Trp179Arg), c.910C>T (p.Arg304Trp), c.970G>A (p.Gly324Ser) and c.1168G>A (p.Gly390Arg) (Engel et al. 2009; Woo et al. 2014).

Type II Citrullinemia, a multisystem disorder, is caused by pathogenic variants in the SLC25A13 gene and has clinically and biochemically distinct features (Kobayashi et al. 2014). Testing for both types of citrullinemia is available individually as well as via our Urea Cycle Disorders NextGen Sequencing (NGS) panel.

Test sensitivity should be very high in the newborn with clinical symptoms of ammonium intoxication and a plasma amino acid profile suggestive of argininosuccinate synthetase deficiency. Gao et al. (2003) reported 2 alleles in 100% of the 38 patients studied, and Häberle et al. (2003) reported 2 alleles in 100% of the 21 patients (from 17 families) they studied. Quinonez and Thoene (2016) report a clinical sensitivity of ~96%.

Although the clinical sensitivity of deletion/duplication testing is expected to be relatively low, at least four gross deletions have been reported in the ASS1 gene (Human Gene Mutation Database).

This test provides full coverage of all coding exons of the ASS1 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

This test also includes coverage for the intronic variant designated as c.174-1119G>A.

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).