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Classic Citrullinemia
ASS1 gene
Classic citrullinemia (CTLN1) is an autosomal recessive disorder of the urea cycle characterized by neonatal or intermittent onset of hyperammonemia caused by a deficiency of the enzyme argininosuccinate synthetase. An acute neonatal form is the most common. Infants are normal at birth followed by an acute illness characterized by hyperammonemia, anorexia, vomiting, lethargy, and hepatomegaly in some cases, increased intracranial pressure, seizures and coma. A later onset presentation is less frequent and may be similar to or milder than the neonatal form and present with feeding difficulties, vomiting, episodic hyperammonemia, lethargy, seizures and cerebral atrophy on CT scans or MRI. CTLN1 has been diagnosed in adult women who developed severe symptoms such as psychosis during pregnancy or postpartum.1 Another late onset form, common in Japan, has been reported in individuals as late as 20 years of age with milder presentation including slurred speech, irritability and insomnia as presenting signs. Asymptomatic individuals have also been described. In one large study of 37 patients diagnosed with CTLN1, all abnormal alleles (74/74) were detected by sequencing.
Mutation analysis of the ASS1 gene is performed on genomic DNA from the submitted specimen using bi-directional sequence analysis of coding exons (3-16), and corresponding intron/exon boundaries. If sequencing identifies a mutation on only one allele of the ASS1 gene, and if clinically indicated, reflex deletion/duplication testing (ExonArrayDx) will be performed at no additional charge to evaluate for a deletion/duplication of one or more exons of this gene. Mutations found in the first person of a family to be tested are confirmed by repeat analysis using sequencing, restriction fragment analysis, or another appropriate method.
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