Human Molecular Genetics Advance Access published online on July 5, 2007
Human Molecular Genetics, doi:10.1093/hmg/ddm172
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In vitro demonstration of intra-locus compensation using the Ornithine transcarbamylase protein as model
1 Institute of Molecular Pathology and Immunology of the University of Porto (IPATIMUP), Portugal 2 Faculty of Medicine, University of Porto, Portugal 3 Dept. of Inorganic, Physical and Material Chemistry, University of Torino, Italy
* Author to whom correspondence should be addressed: Gianpaolo Suriano, IPATIMUP rua Dr. Roberto Frias, s/n 4200-465 Porto, PORTUGAL. Tel.: +351 4225570730 Fax: +351225570799 Email: gsuriano{at}ipatimup.pt
Received May 16, 2007; Revised July 2, 2007; Accepted July 2, 2007
Ornithine transcarbamylase deficiency (OTCD) is an X-linked inborn defect of metabolism of the urea cycle, which causes hyperammonia. Mutations of the OTC gene have been recognised as the genetic cause underlying the OTC deficiency. The severity of the disease is associated to the type of mutation, leading either to neonatal onset of hyperammonemia or to a later appearance of the disease. The mutation Thr125Met is associated to neonatal hyperammonemia. Recently, the disease-causing Thr125Met mutation in humans was reported as wild-type neutral allele in chimpanzees. Further analysis confirmed the presence of Met125 fixed in chimpanzees together with Thr135, representing the only two divergent positions between human and chimpanzee OTCs. Thr125 and Thr135 was identified as ancestral mammalian combination, so that the Thr135Ala substitution occurred as human specific event, whereas the substitution of Thr125Met was characteristic of the chimpanzee linage. Only when Met125 emerges in a background with the human-specific Ala135 a highly deleterious effect is observed, suggesting among other hypotheses the existence of a compensatory effect in chimpanzee. To explore this hypothesis, we built an in vitro cell model system to study the effect of the three distinct genetic backgrounds (Ala135-Thr125; Ala135-Met125 and Thr135-Met125) on the OTC protein function. We observed that the human Thr125Met mutant is inactive, whereas the chimp OTC shows an enzymatic activity comparable to the wild-type human OTC. We concluded that the presence of a threonine at position 135 in chimps rescues the deleterious effect of the methionine at position 125, in a mechanism of intra-locus compensation.