Repair of urea cycle defects in mice by rAAV-mediated gene transfer: Towards gene therapy for genetic liver disease

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Gene therapy has the potential to cure many genetic metabolic liver diseases. The key challenge is the development of gene transfer technologies-strategies with the necessary efficacy and safety. Vectors based on adeno-associated virus (AAV) show special promise for gene transfer to the liver, having been extensively evaluated in small and large animal models. The ongoing challenge is to achieve the higher levels of gene transfer required for human therapy. A recent quantum advance has been the development of improved AAV vectors with dramatically higher gene transfer efficiencies (up to two orders of magnitude in the liver). This places successful liver-directed gene therapy within reach. Initial human studies will only be possible in the context of severe diseases where existing therapies are high risk or inadequate. Accordingly, we have chosen the most common urea cycle defect, OTC deficiency, as a disease model. In its severe form neonatal hyperammonaemia is associated with a high risk of death and significant disability in those who survive the newborn period. Using the spf(ash) mouse model of OTC deficiency we propose to develop gene therapy strategies capable of achieving life-long disease cure. Preliminary data has confirmed feasibility, and suggests that the greater number of cells in the human liver requiring genetic repair will not prove insurmountable. The proposal focuses on issues critical to success in humans. These include strategies to minimise the number of repaired liver cells required for clinical benefit, overcoming the effects of liver growth, investigating the potential impact of OTC mutations on gene therapy, and establishing the likely efficiency of gene transfer in human liver cells and large animal livers equivalent in size to the human neonate. These studies are part of a long-term commitment to progress through to human clinical trials of gene therapy for urea cycle defects. The potential health and economic benefits are immense.