Virus-based vectors for gene expression in mammalian cells: Retrovirus

High-level expression of proteins in animal cells has been very informative in studies of protein and cellular function and in many cases has relied on virus-derived expression vectors. Many viruses have evolved to maximize expression of their proteins in host cells and are therefore a good starting point for construction of efficient expression vectors. Retroviruses are diploid, single-stranded, positive-sense RNA viruses [1]. As an obligate step of the retrovirus life cycle, the RNA genome is converted into DNA and then integrated into the host cell chromosome in the form of provirus. The provirus replicates as the host cell chromosome replicates and is transmitted to all progeny cells. This ability of retroviruses to stably introduce new genetic information into the target cells led to the development of retroviruses as vehicles for the stable transfer of genes [2–4]. Indeed, retroviral vectors have been used for a variety of experimental applications, including insertional mutagenesis, cell lineage studies, the creation of transgenic animals and the expression of foreign genes into mammalian cells, both in vitro and in vivo as documented by the majority of gene therapy clinical trials. An ideal vector should guarantee not only high efficient gene transfer but also appropriately regulated and stable gene expression from a safely integrated provirus. Currently, efforts are devoted to achieve these goals. This chapter focuses on recent progress on retroviral vector design and applications.