Prenatal diagnosis of ornithine transcarbamylase deficiency: Results in SPFASH mice

Abstract

Ornithine transcarbamylase (OTC) deficiency is a human X-chromosome-linked disease (McKusick 31125). The presence of OTC activity in the human placenta encouraged us to examine the possible diagnosis of the disease in an animal model (Spfash mice) by enzymatic assay on placental samples. A significant positive correlation (p < 0.02) was found between placental and hepatic activities; the pH dependence of OTC was similar in the placenta and liver when compared within normal or homozygous mutant mice. The apparent Km (ornithine) and Km (carbamoylphosphate) values of the enzyme did not show any significant differences when compared in both placentae and livers of normal fetuses. The use of OTC assay in the placenta for prenatal diagnosis of OTC deficiency in mouse fetuses obtained by the crossbreeding of Spfash/+ with +/Y has shown that our method has good diagnostic value. We made the diagnosis of OTC deficiency in male fetuses with a sensitivity and a specificity of 1.0. The + gene from the father in Spfash/+ animals is preferentially inactivated in extraembryonic tissues, explaining why very low placental OTC activity was observed in 12 of the 18 female fetuses studied. Because these 12 females have variable OTC activity in their livers, it is not possible to appreciate the true residual activity in their livers by measuring this activity in the placenta.