Gene therapy for ornithine transcarbamylase deficiency

Multiple advantages of alpha-fetoprotein as a marker for in vivo gene transfer

The development of improved gene transfer vectors has been hampered by the lack of a nonimmunogenic reporter gene that can be serially quantified in the serum or from other sites. In response to the need to develop a new reporter protein, we have evaluated alpha-fetoprotein (AFP) as a potential candidate. A first-generation E1/E3-deleted adenoviral vector expressing human AFP (hAFP) was generated as a preliminary tool to evaluate AFP as a reporter. Using both mouse and baboon models, hAFP expression was evaluated in serum after intravenous delivery and in serum and bronchioalveolar lavage (BAL) fluid after delivery to the lung. In immunocompetent animals, intravenous delivery of the hAFP adenoviral vector resulted in hAFP expression in the serum early after injection, which declined rapidly over time. Disappearance of hAFP from the serum was complete by 3-4 weeks after administration and was accompanied by robust antibody responses to hAFP and loss of infected cells. After lung delivery, hAFP could be detected in both serum and BAL. This allowed the analysis of the kinetics of gene expression in the lung without sacrificing the animals. In both liver and lung, immunohistochemical analysis correlated well with hAFP levels as detected in serum or BAL, indicating that serum levels were a reliable marker of tissue expression. Preliminary results with a mouse AFP expressed in a helper-dependent adenoviral vector indicate that use of a species-specific version of AFP will eliminate the complication of antibody development. These initial evaluations suggest that AFP is useful as a reporter gene to evaluate gene expression of therapeutic cassettes in multiple tissues, and it should be considered for use in human subjects.