Mutation Analysis of the Ornithine Transcarbamylase (OTC) Gene in Five Japanese OTC Deficiency Patients Revealed Two Known and Three Novel Mutations Including a Deep Intronic Mutation

Key word: cryptic splice site, deep intronic mutation, ornithine transcarbamylase Ornithine transcarbamylase (OTC) deficiency is the most common inborn error of the urea cycle. Although a combination of molecular methods have been used including DNA sequencing of all 10 exons and exon-intron boundaries of OTC gene, only ~80% of patients with OTC deficiency are found to have mutations. We report two known and three novel mutations of the OTC gene in five Japanese patients including two neonatal-onset, one late-onset, and two symptomatic female patients. Known nonsense mutations (c.578G>A and c.421C>T) were detected in a neonatal-onset male and a symptomatic female patient, respectively. Mutation analysis revealed two novel mutations including one splice site mutation (c.386+1G>C) in a symptomatic female patient and one missense mutation (c.515T>A) in a late-onset male patient. In the remaining case, which was a neonatal-onset male patient, no mutation was disclosed by direct sequencing of all 10 exons and their flanking intron sequences. Therefore, OTC mRNA in the liver was analyzed by RT-PCR, and remarkably, a 135-nt insertion was detected between exons 5 and 6. Genomic DNA analysis of intron sequences revealed a single nucleotide change at 265 bp downstream from the 3’ end of exon 5, which created the novel splice acceptor site. Thereby, a 135-nt exon was created from the central part of an intron sequence. This is the first report of mutation deep in the intronic sequence in the OTC gene. Molecular analysis using genomic DNA and mRNA will increase the mutation detection ratio in the OTC gene.