Retrovirus gene delivery

Some 24 per cent of all gene therapy clinical trials undertaken to date have employed retroviral vectors as gene delivery systems. Retroviruses are enveloped viruses. Their genome consists of ssRNA of approximately 5-8 kb. Upon entry into sensitive cells, the viral RNA is reverse transcribed and eventually yields double-stranded DNA. This subsequently integrates into the host cell genome (Box 14.1). The basic retroviral genome contains a minimum of three structural genes ... 

Themis, M., Forbes, S.J., Chan, L., Cooper, R.G., Etheridge, C.J., Miller, A.D. et al. (1998) Enhanced in vitro and in vivo gene delivery using cationic agent complexed retrovirus vectors. Gene Ther., 5, 1180-1186. 

The initial viral-based gene delivery experiments were performed in vitro using replication defective retroviral vectors in an attempt to transduce endothelial cells (Yao et al., 1991). Preliminary viral-based in vivo cardiovascular gene delivery experiments were also performed using retroviral vectors and demonstrated this vector system s ability to successfully transduce vascular endothelial cells (Newman et al., 1991). Although attempts at direct transduction of the carotid artery were not successful, it was established that efficient endothelial transduction could be achieved through the retrovirus-mediated infection of cells ex vivo and their subsequent arterial seeding in vivo (Lynch et al., 1992). 

Viruses Viruses provide a unique and extremely efficient method of insertion of genes into mammalian cells, such as human bone marrow stem cells. SV 40, adenovirus, vaccine virus, and polyomavirus have been used as gene transfer vectors. A non-replicating adenoviral vector that efficiently infects human cells has recently been developed. The essential feature of a retroviral gene delivery system is the presence of an RNA copy of the replacement gene that is packaged in a viral particle, capable of specific and efficient entry into the cytoplasm of a cell. A retrovirus consists of... 

Figure 1 Schematic example drawing of a few gene delivery vectors. A. Retrovirus, B. adenovirus, C. adeno-associated virus, D. liposomes, E. naked DNA. Polyplex not shown. (Reprinted from Ref. 295. Courtesy of Slim Films.)...

A major drawback in the use of the traditional recombinant DNA techniques is the inability to insert foreign DNA in vivo. This problem has been addressed through the development of viral gene delivery techniques. In these approaches, viral-based vectors are used (retrovirus, adenovirus, adeno associated virus, etc.), which are able to infect targeted cells, and stably insert foreign DNA into the cells. This has tremendous advantages in that it can be used in vivo, and it is especially well suited to the correction of inborn errors of metabolism. As the safety and efficacy of these approaches are improved, it is quite likely that it will be possible to use this approach to manipulate other valuable metabolic pathways [31]. 

Many of the approved clinical trials have utilized recombinant retroviruses for gene delivery. Retroviral particles contain two copies of identical RNA genomes that are wrapped in a protein coat and further... 

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