Objective: To carry out rapid genetic diagnosis for a child affected with citrullinemia type Ⅰ.
Methods: Peripheral venous blood samples were obtained from the two-day-old child and his parents as well as 100 healthy controls. Serum ammonia and citrulline was determined by biochemical test and tandem mass spectrometry. Sixteen pairs of primers were designed for high-resolution melting (HRM) analysis of all exons and adjacent intronic sequences of the ASS1 gene in the proband, parents and healthy controls. Suspected mutations were confirmed by DNA sequencing, while the mRNA transcripts of the ASS1 gene were determined by reverse transcription (RT)-PCR. Functional impact of the mutation sites was predicted with PolyPhen-2 and SIFT Blink software.
Results: Blood ammonia and citrulline of the proband have respectively reached 286 μmol/L and 487.69 μmol/L, which far superseded the normal values. HRM analysis and DNA sequencing have identified in the child a homozygous c.380G>A (p.R127Q) mutation in exon 6 of the ASS1 gene, in addition with a homozygous IVS8+60G>A substitution in intron 8, while his parents were heterozygous carriers for both mutations. RT-PCR assay indicated that the IVS8+60G>A mutation did not result in abnormal splicing of the ASS1 gene transcripts. Bioinformatic analysis suggested that the site for p.R127Q was conserved among 45 species of vertebrates and may play a crucial role in citrulline metabolism.
Conclusion: The severe urea cycle disorder in the proband was probably due to the compound homozygous R127Q and IVS8+60G>A mutations of the ASS1 gene.