The urea cycle is the major pathway for detoxification of ammonia in mammals. Arginase I deficiency is thought to be the least common of the urea cycle disorders and results in hyperargininemia. In humans, deficiency of this enzyme is characterized clinically by progressive mental impairment, spasticity, growth retardation, and periodic episodes of hyperammonemia. This proposal is two-fold: 1) to develop gene-based correction of arginase deficiency with viral vectors and to closely examine corrected animals behaviorally and biochemically; and 2) to evaluate the role elevated arginine and related metabolites (beyond just ammonia) have on the developing brain and development of mental retardation in arginase deficiency. Preliminary data: Our research group has: 1) constructed and characterized the arginase I knockout mouse; 2) demonstrated at least short-term (if not longer) correction and rescue with recombinant helper-dependent adenoviral vectors and adeno-associated viral vectors; 3) demonstrated that knockout mouse neurons have a higher percentage of cells in the S phase of the cell cycle; 4) shown that differentiated neurons from knockout mice have a more mature morphology; and 5) shown that knockout neurons demonstrate induction of genes related to protection from oxidative damage. In Aim 1, recombinant viral vectors will be used to rescue mice from lethality and to obtain a better understanding of the challenges facing neonatal gene therapy that involve rapid cellular proliferation and potential loss with episomal vs. integrated vectors. In Aim 2, the effect of hyperargininemia on the nervous system will be examined in vitro along with GABA and glutamine synthesis/release, and extensive microarray analysis. In Aim 3, the effect of hyperargininemia on the nervous system will be examined in vivo along with GABA and glutamine synthesis/release, guanidino compound determination, and the role that nitric oxide and its metabolites may have on nervous system injury. Successful completion of the proposed studies will provide a molecular understanding of the mechanism of injury to the brain in arginase I deficiency and related disorders. In addition, it is expected that a gene replacement strategy will have been developed for arginase deficiency through these studies and that the information obtained in treating neonates that have developing tissues undergoing rapid proliferation will be applicable to other inborn errors of metabolism.