Viral gene delivery

For millions of years, viruses have been transferring genes into all types of cells, including plant, animal and human cells. The experimental technique of viral gene delivery was developed from this natural ability, which offers many intrinsic advantages to scientists and clinicians: . specific cell-binding and entry properties, efficient targeting of the transgene to the nucleus of the cell and .the ability to avoid intracellular degradation. The general principle involved in the development of most viral vector systems is that an intact wild-type virus is modified for safe use and effective gene transfer; for example, the specific genes that are involved in viral replication can be modified or deleted, thus rendering the new recombinant virus ‘replication defective’ and safer for use in gene therapy protocols . Usually, the transgene that is to be delivered by the virus must be inserted into the viral genome, using molecular biological techniques; transgenes are often inserted into the space created by the removal of viral replication genes. In general, the more severely attenuated the viral vector is from its wild-type state (i.e. the greater the number of virulence-associated genes that have been removed), the safer the virus is for use in gene therapy protocols. The size of the transgene has to be matched to the potential space in the viral genome; if the new viral genome is too large, it cannot be packaged into an infectious particle. Because many of the viruses that are used as vectors lack replication genes and therefore cannot replicate in normal cells, the recombinant virus with its transgene must be grown up to higher titres in a packaging cell line. This is a cell line that contains all of the complementary genes that the virus requires to replicate (i.e. those that were previously removed). The recombinant viral particles can then be purified as live infectious virus from the packaging cell line and used to infect (transduce) cells or tissues in vivo or ex vivo.

Retroviral vectors