The lung as a metabolic factory for gene therapy.

Over the past decade, the development of effective gene delivery vehicles to the lung has created numerous innovative strategies for treating inherited and acquired lung disorders. Currently, one of the most promising vectors for gene delivery to the lung is recombinant adenoassociated virus (rAAV). To date, gene therapy to the lung has focused on correcting genetic defects for diseases with intrinsic lung pathophysiology. One largely unexplored application of gene delivery to the lung is its use as a genetic metabolic factory for the production and systemic delivery of proteins into the blood for treatment of genetic diseases not directly associated with lung pathology. The study by Auricchio and coworkers described in this issue of the JCI focuses on this novel application (1). A conceptual extension of this work also includes the use of the lung as a catabolic factory for degradation of toxic, blood-borne molecules associated with genetic abnormalities in distant organs. Because the lung has one of the most expansive epithelial–blood capillary networks and is easily accessible for non-invasive gene delivery, it is uniquely suited for such applications. Genetic metabolic factories to treat genetic diseases The concept of metabolic factories for the ectopic production and delivery of genetically engineered protein products into the circulation is not new. Early reports of protein factories called organoids demonstrated therapeutic potential as self-contained genetic neo-organs for the delivery of secreted proteins into the blood (2). More recently, clinical trials in hemophilia B patients first attempted therapeutic delivery of factor IX to muscle (as an ectopic site for factor IX production) with encouraging results (3). Hemophilia B is an ideal disease for testing such approaches, since as little as 1% of the normal circulating factor IX protein level is likely to be therapeutic in hemophilia B patients (4). However, several concerns linger regarding the functional processing of factor IX in ectopic organs and are the impetus for new clinical trials directing expression of factor IX to the liver, its normal site of production. rAAV-mediated delivery of transgenes to the lung, as employed by Auricchio et al., is advantageous in several respects. Ectopic expression of a blood-borne therapeutic protein can be limited by the immunogenicity of the vector or the transgene product. Immunogenicity to the vector may result in acute toxicity or may reduce transgene expression following repeated vector administration. In the context of rAAV, the lung appears to be a far less immunogenic target than the muscle or liver. The lung is also uniquely suited for delivery of therapeutic proteins into the circulation because it offers a large number of target cells with good access to the blood for expressed proteins but not for the applied vector — the latter being a significant safety concern because of the possibility of inadvertent germ line delivery. Finally, the target cells in the lung appear competent to produce at least some therapeutic proteins with normal post-translational modifications and secretory properties, allowing them to be released into the bloodstream in a functional form. The study described by Auricchio and colleagues demonstrates the therapeutic potential of using the lung as a genetic metabolic factory to produce and deliver proteins into the circulation. In this study they compared the efficiency of pseudotyped recombinant AAV1, 2, and 5 to deliver erythropoietin (Epo) and factor IX into the blood following lung gene transfer. Tests using alkaline phosphatase as a histochemical marker showed that rAAV2/5 was the most efficient in transduction of both the conducting airway epithelia, and alveolar regions of the lung and substantiated earlier work with this serotype in mouse lung (5). The efficiency of rAAV2/1 transduction was less than that for rAAV2/5, but greater than that for rAAV2. This was the first report evaluating AAV Type 1 capsid for in vivo lung delivery and it appears to be similar in efficiency to its close relative AAV6 (6). The most interesting finding from this study, however, was the high efficiency with which Epo and factor IX were secreted from the airway epithelium into the circulation. Levels of factor IX in the blood were within the minimal range thought to be therapeutic for hemophilia B (1–2% endogenous levels) and Epo levels also significantly increased hematocrit. Given the fact that the airway epithelium is generally thought to secrete proteins predominantly into COMMENTARY