A more practical approach has been to permit the in-troduced genes to integrate into the genome in a site-nonspecific manner. The newly added gene could then function to provide a missing or mutated gene product (Fig. 58.5A). This is the approach of most current gene therapy protocols and is exemplified by the development of clinical trials for adenosine deaminase (ADA) defi-ciency, which is an example of inherited SCID.
ADA is a reasonable target for these reasons: (1) It is an auto-somal recessive disorder in which a defect in a single gene produces absence of or diminished ADA activity with fatal combined immunodeficiency. (2) ADA ex-pression is characteristic of a normal maintenance gene with considerable variation in the normal ADA levels, suggesting that stringent regulation of expression is un-necessary. (3) A significant level of expression is not re-quired to correct the phenotype. (4) Ex vivo gene trans-fer studies can be conducted. (5) Replacement of ADA may reduce the production of toxic DNA metabolites and thus provide a growth advantage for transfected cells.
For ethical reasons, children enrolled in these clini-cal trials have also received standard therapy of enzyme infusions, so the results of these studies have been diffi-cult to interpret and are controversial. Nevertheless, there is some evidence that the ex vivo gene transfer ap-proach may evoke a biological response relevant to the treatment of ADA deficiency. Such interpretations have stimulated efforts to use the ex vivo strategy for other monogenic disorders, such as familial hypercholes-terolemia, hemophilia B, and Gaucher’s disease.
Alternatively, the introduced gene could generate a protein that acts to block or suppress the function of an-other undesirable protein in a dominant-negative man-ner (Fig. 58.5B). Last, the introduced gene could result in the production of an entirely new and unique protein that provides the recipient cell with a desirable pheno-type (Fig. 58.5C). In theory, an enzyme required for the metabolic activation of a prodrug could be expressed, leading to the desired pharmacological activity near the genetically altered cell. This approach is used in cancer gene therapy in which tumor cells are transfected with a gene encoding for an enzyme such as thymidine kinase in the presence of systemic administration of a nontoxic prodrug.
The transfected enzyme in the tumor cells con-verts the prodrug, such as ganciclovir, to an active cyto-toxic compound. Theoretically, such an approach selec-tively kills tumor cells and is nontoxic to untransfected cells. Clinical trials to assess the safety and efficacy of enzyme–prodrug cancer therapy are under way.
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