Retroviruses as Vectors

The motive behind gene delivery may be simply to mark the target cell population (elsewhere in this volume). Alternatively, it may lead to the generation of novel mutations that can be recognized by virtue of their linkage to the retroviral genome. More usually a gene is delivered to a specific cell we, where iti expression is driven from sequence elements included in the
vector and the effects of expression can be monitored. If the expressed gene encodes a correct version of a defective gene, its transfer into diseased cells may even allow the correcting of the genetic deficiency, either on a cellular level or in the animal as a whole. Such approaches may eventually lead to gene therapy for specific monogenic disorders (for review, see ref. 4).

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Advantages of retroviral vectors over other mechanisms of delivery of DNA to cells include the potential of infecting nearly every cell in the target population (5). Physical methods can be used to induce cells to take up DNA, such as coprecipitation with calcium phosphate, forming complexes of DNA with lipids, electroporation, direct microinjection ofDNA, or the encapsidation of DNA into liposomes. However, infection by high-titer retroviruses can usually exceed the efficiency (about 1% transfer) of these methods. In addition, infection leads to integration of a single copy of the vector genome in a predictable, stable fashion, which is rarely the case using nonviral transfer techniques.