Retroviral-Mediated Gene Transduction
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An obligatory part of the life cycle of retroviruses (Fig. 1 ) is a stable, chromosomally integrated form of the virus, known as the provirus. The existence of the proviral form of retroviruses has provided one of the main driving forces for their use as vectors for gene transfer because, in the instance of a replication-defective retrovirus vector, the proviral form is the end point of the transduction (infection) process of the target cell (Fig. 2 ). The use of (replication) defective retroviral vectors is therefore ideal where the goal is the stable genetic modification of the target cell. The term transduction is now used to describe the infection of a cell with such a replication-defective retrovirus. Although replication-competent retroviral vectors have also been made, these are not in general use and offer little to recommend them. In this chapter vector should be taken as referring specifically to replication-defective retroviral vectors.
Fig. 1. Retroviral life cycle. Virus particles bind to the cell via specific plasma membrane receptors resulting in endocytosis and virus disassembly. The process of conversion of the single-stranded RNA genome of the virus into double-stranded DNA (dsDNA) via reverse transcription takes place in the cytoplasm. A complex containing dsDNA and retroviral proteins then gains access to the nucleus during cell division and the dsDNA molecule integrates into the chromosome to give the proviral form. This is then transcribed to give mRNAs encoding Gag/Pol and Env proteins. After proteolytic processing, these proteins are assembled into virions along with the viral genomic RNA and virus particles released by budding.
Fig. 2. Recombinant retroviral vector transduction. The initial stages of the process, up to provirus formation, are the same as for wild-type virus (see Fig. 1 ). However, because the recombinant vector encodes no functional viral proteins, no virus can be produced. Proviral transcripts simply serve to encode (depending on the exact design of the vector) the gene of interest.
