Gene therapy using retroviruses

Several methods of gene therapy have been developed to insert foreign genes into cells with genetic defects. Microinjection of a gene into a target cell has been successful in some cases, but is very time consuming and requires a high degree of expertise. Another approach is electroporation, in which DNA is stimulated to enter cells by exposure to an electric shock. However, this procedure is traumatic to the cells. To date, the most effective method of introducing foreign genes into cells is through a viral vector. With this method, foreign genes enter the cell via a normal viral infection mechanism.

The genome of a virus may consist of DNA or RNA, and may be either single- or double-stranded. When certain DNA viruses infect a cell, their DNA is inserted into the host’s genome. Once integrated, viral genes can be transcribed into mRNA, which is then translated into protein. By contrast, the genomes of simple RNA viruses are translated directly into mRNA by the enzyme RNA replicase. In retroviruses, the RNA genome is transcribed by the enzyme reverse transcriptase into DNA, which is then inserted into the host’s genome. The viral genes can then be expressed, directing the synthesis of viral RNA and proteins. Retroviruses consist of an outer protein envelope surrounding a protein core that contains viral RNA and reverse transcriptase. The retroviral RNA contains the three coding regions gag, pol, and env, which code for the viral core proteins, reverse transcriptase, and the coat protein, respectively.

So far, retroviruses seem a more promising tool for gene therapy than either DNA viruses or simple RNA viruses. A retrovirus containing a gene of interest enters a cell via receptor-mediated endocytosis, and its RNA is then transcribed into DNA. This DNA randomly integrates into the cellular DNA, forming a provirus that is copied along with the chromosomal DNA during cell division. Retroviral vectors are constructed such that the therapeutic gene takes the place of gag, pol, or env. Problems associated with this form of gene therapy include the possibility that random integration could lead to activation of an oncogene, the fact that integration can occur only in cells that can divide, and the limitation that gene expression cannot be precisely controlled due to the randomness of integration.