Hepatocyte gene therapy in a large animal: A neonatal bovine model of citrullinemia

The development of gene-replacement therapy for inborn errors of metabolism has been hindered by the limited number of suitable large-animal models of these diseases and by inadequate methods of assessing the efficacy of trea tment. Such methods should provide sensitive detection of expression in viv o and should be unaffected by concurrent pharmacologic and dietary regimens . We present the results of studies in a neonatal bovine model of citrullin emia, an inborn error of urea-cycle metabolism characterized by deficiency of argininosuccinate synthetase and consequent life-threatening hyperammone mia. Measurements of the flux of nitrogen from orally administered (NH4)-N- 15 to [N-15]urea were used to determine urea-cycle activity in vivo. In con trol animals, these isotopic measurements proved to be unaffected by pharma cologic treatments. Systemic administration of a first-generation E1-delete d adenoviral vector expressing human argininosuccinate synthetase resulted in transduction of hepatocytes and partial correction of the enzyme defect. The isotopic method showed significant restoration of urea synthesis. More over, the calves showed clinical improvement and normalization of plasma gl utamine levels after treatment. The results show the clinical efficacy of t reating a large-animal model of an inborn error of hepatocyte metabolism in conjunction with a method for sensitively measuring correction in vivo. Th ese studies will be applicable to human trials of the treatment of this dis order and other related urea-cycle disorders.