8. TARGETING AND GENE EXPRESSION

One of the most important aspects of gene therapy is the ability to target a specific gene to a specific cell type and also regulating its expression in quantities needed for the therapy. This could be achieved by either making use of cell properties such as its rate of cell division. So it involves altering the gene transfer vector, so that it only delivers its gene to the dividing cells.  Retroviral vectors are examples for this type of targeting, where they can only infect the rapidly dividing cells. This strategy is suitable in targeting tumour cells.  (Salmon and Guinzburg, 1993,and Dachs. et al 1997),

Other strategy depends on controlling the target gene expression, at the level of transcription.  It focuses on restricting the transgene expression in target cells through the regulatory mechanism of the genetic elements such as cell specific promoter and or/ enhancer elements. Retroviral vectors that contain the tetracycline inducible system have been developed by Paulus et al (1996). In this system, the gene can be switched on and off by administering the tetracycline to the cell. (Figure 14).

Another example for cell targeting is by activating the expression of certain gene using steroid receptors.  In the presence of a certain steroid, it is bounded to a cognate ligands attaches itself to the specific target cell, enters and moves towards the nucleus. They can then selectively induce the transcription of the specific gene. For example, a drug known as antiprogestin activates a recombinant steroid receptor that in turn binds specifically to the promoter region of the therapeutic gene. This gene switch does not affect other cellular pathways.  In vivo experiment showed that when antiprogestin is administered orally, it activates the gene to be switched on. (Kasahar, et al 1994), increased the efficiency of transfection by incorporating adenoviruses or other endosome-lysing agents into molecular conjugate vector composition (Figure 14). Similarly, adenovirus was coupled with the transferrin-polylysine/DNA complexes that enhance the receptor-mediated gene delivery and expression of transfected genes (Wagner, et al 1992).

Figure 14 : Regulation of gene expression by the action of drugs on the promoter

Other system of targeting is the use of liposomes that were prepared to incorporate specific proteins onto their surface and combined with DNA. This type of liposomes has the ability to recognize a specific cell population to deliver genes to these cells. Molecules such as monoclonal antibodies, carbohydrate ligands and protein ligands are used to be incorporated with the lysosomes to target the DNA to specific cell types.