Design of Retroviral Expression Vectors

Several characteristics of amphotropic murine retroviruses have made them useful as vectors for gene transfer with some distinct advantages over other methods of transduction. First, the normal replication cycle of retroviruses includes integration of the viral genome into the host’s chromosomal DNA, and these viruses have evolved a very efficient mechanism of stable gene transfer for this purpose. Second, because the integration machinery uses the termini of the viral DNA as substrate, insertion of viral DNA takes place in a predictable manner with the long terminal repeat (LTR) sequences flanking the genes they carry. Third, retroviruses have a broad host range, which allows gene transfer and expression of foreign genes in many cell types, even including some cell types that are refractory to gene transfer by other means. Fourth, a cytopathic effect on infected cells is lacking, which is especially important for gene expression studies and the generation of stable cell lines. Fifth, all retrovial proteins required for the assembly of infectious virions can be supplied in trans , thereby enabling the expression of exogenous genes up to approx 8 kbp. Finally, retroviral vectors can be conveniently manipulated in plasmid form and, as discussed below, are somewhat flexible in terms of vector design.