J. Inher. Metab. Dis. 18 (1995) 273-282 © SSIEM and Kluwer Academic Publishers. Printed in the Netherlands Identification of two new aberrant splicings in the ornithine carbamoyltransferase (OCT) gene in two patients with early and late onset OCT deficiency T. MATSUURA I, R. HOSHIDE 1, S. KOMAKI1, K. KIWAKI 1, F. ENDO ~, S. NAKAMURA 2, T. JITOSHO 3 and I. MATSUDA~* I Department of Pediatrics, Kumamoto University School of Medicine, Kumamoto ; 2Neonatal Medical Center, Kumamoto Municipal Hospital, Kumamoto ; 3Department of Pediatrics, Kagoshima Municipal Hospital, Kagoshima, Japan *Correspondence: Department of Pediatrics, Kumamoto University School of Medicine, 1-1-1 Honjo, Kumamoto 860, Japan MS received 29.11.94 Accepted 27.2.95 Summary: Ornithine carbamoyltransferase (OCT) is a liver-specific enzyme located in the mitochondrial matrix. OCT deficiency is an X-linked disease with a heterogeneous phenotype, even in affected males. We studied two male patients (K.M., K.G.) with early and late onset, respectively. OCT activity was zero in the autopsied liver of patient K.M. and was 6% of control in the biopsied liver of K.G. Sequencing of OCT cDNAs revealed exon 5 skipping in K.M., resulting from a T- to-C transition of the initial dinucleotide of the 5' splicing donor site of intron 5, and a G-to-T transversion at position +45 in exon 9 (L304F) in K.G., providing three OCT mRNAs of different lengths: a normally spliced transcript, 23 bp insertion of intron 8 and the first 50bp missing within exon 9. Exon 5 skipping and two other aberrant splicings produced stop codons early downstream in mature OCT mRNAs. Expression study of a missense allele, L304F, transfected to cultured Cos 1 cells revealed a 34.4% value of the control. The difference of OCT activities between the patient liver and transfected cells (6% vs. 34%) can be explained by this splicing abnormality. Ornithine carbamoyltransferase (OCT)(EC 2.1.3.3) is the second enzyme in the synthesis of urea from ammonia in the liver. OCT deficiency is X-linked and is the most common urea cycle disorder (Brusilow and Horwich 1989; Nagata et al 1991). Identified mutations of the OCT gene include gross deletion, missense mutation, nonsense mutation and splicing abnormality (Old et al 1985; Rozen et al 1985; Maddalena et al 1988a,b; Grompe et al 1989, 1991; Finkelstein et al 1990; Hata et al 1989, 1991; Carstens et al 1991). Among the missense mutations, R141Q (Lee and Nussbaum 1989), T264A, D196V and G195R (Matsuura et al 1993) abolished or reduced OCT activity in expression studies with 273