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There are over 4,000 genetic disorders recognized in humans and most cannot be completely alleviated by conventional treatment. The idea behind gene therapy is to introduce healthy genes into patients who carry faulty or undesirable genes; thus the root cause of the disease can be treated by genetic engineering rather than by simply attempting to alleviate symptoms with drugs. There are two possibilities for gene therapy, though one, germ line therapy (in which the genome of the gamete-producing cells is altered) has far-reaching ethical implications and is therefore unlikely to be countenanced by regulatory authorities, at least in the near future. Many scientists consider that germ line therapy can never be permitted as the long-term implications to the human genome cannot be calculated.
Gene therapy of somatic (non-gamete-producing) cells is a much more realistic prospect as it affects only the individual patient. At present it is only feasible in diseases where a gene is absent or faulty, in that it underproduces its protein product; in this case the replacement gene can alleviate the disease by augmenting the production of protein. In practice, the large number of new cells to which such a gene must be introduced, in order to have a significant effect, means that it is necessary to use a virus vector to insert the gene into the genome of each cell by a process known as transduction. Such viruses can be produced in the laboratory and modified so that they do not complete their life cycle. This is essential to avoid the spread of the virus, and with it the gene, to other individuals. Cells that received the new gene would then multiply it by dividing to produce new cells. The number of cells which would need to receive the gene, therefore, depends upon the rate at which the cell type multiplies. If the absence of the gene reduces the life span of the cell, then the proportion of genetically altered cells will steadily increase. In diseases such as cystic fibrosis, where one important gene is absent, the gene therapy could have a life-saving effect even if only a small minority of cells were genetically altered. At present it is necessary to remove the affected cells for gene therapy; for this reason most success has been seen with tissues such as bone marrow, the cells of which can be readily removed and replaced. RB
See also eugenics; genetic engineering; transformation.Further reading Zsolt Harsanyi, Genetic Prophesy Beyond the Double Helix. |
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