Gene therapy is used to describe any medical precedure that uses genetic material to try to correct a disease. This ranges from simply inserting plain DNA or RNA into a cell to attempt to temporarily express the gene, to sophisticated packages of genes and promoters in a complex viral vector, in an effort to completely replace a defective gene. gene therapy has great promise for curing many genetic disorders, and could also be used to reduce the incidence of inherited cancer or coronary heart disease, but cannot currently be used because of the difficulties in getting the body to express foreign DNA, many of which are not fully understood, and the stringent clinical trials that any gene therapy product would have to undergo.


The DNAor RNA used in gene therapy is composed of at least two components, a correct, therapeutic gene, and a promoter, a sequence that tells the cell to express the therapeutic gene. Ideally the promoter should be specific to the area of the body affected by the disease, but at the moment generic promoters are used. Other sequences to increase gene expression, integration or specificity can be included, but this is limited by the size of the vector.


In order to insert material into a complex organism like a human, a vector has to be used, which will perform a number of functions:
1. The vector must be capable of avoiding the immune system, which would otherwise destroy the foreign DNA/RNA as it entered the body.
2. The vector must be capable of entering cells and passing through membranes. The main reason naked DNA/RNA cannot be used is that DNA/RNA has a negatively charged sugar-phosphate backbone, which is repelled by phospolipid membranes which are also negatively charged.
3. The vector must allow the DNA/RNA to interact with transcription/translation mechanisms inside the cell, but must also resist digestion by nuclease enzymes inside the cell.

Five main classes of vector have been used, four of them viral, one a completely artificial, non-viral vector. These are:
Adenoviruses are viruses that normally cause respiratory infections in humans. This makes them especially good for treatment of cystic fibrosis, one of the main targets of gene therapy research. WIth the genes that can stimulate an immune response removed, adenoviruses can deliver a therapeutic gene into a cell, however, the gene expression is low and temporary as the gene is no integrated into the genome.
Adeno-Associated Virus (AAV)
AAV is a good candidate for a vector as it naturally inserts itself into an apparently unused patch of the human chromosome 19, one of the best types of integration that can occur, as not only is there the possibility of permenant gene expression, but the integration is very unlikely to do any damage to the existing genome. Unfortunately this specificity is lost as some of the viral genes are removed to insert the gene. Only very small genes can be used as the original AAV genome is very small.
Herpes Simplex Virus
The latent Herpes Simplex Virus is nonpathogenic and is particularly attracted to nerve cells making it important for targeting nerves, however issues about its safety and difficulties in making sure the virus remains latent, and does not start killing cells make it an unlikely candidate for any immediate gene therapy products.
Retroviruses are viruses that can stably integrate into a genome without causing any serious immune response, however, they are limited by the inability to infect nondividing cells and the difficulties in purification. A more promising subclass of retroviruses, called lentiviruses, are being investigated, that have the abliity to infect nondividing cells.

All these viruses are usually manufactured by using packaging cell lines, where a plasmid containing the DNA for both the therapeutic gene and the viral DNA required is assembled as a plasmid, and inserted into a bacteria (almost always E.coli )to produce the virus.

Nonviral vectors
A number of nonviral gene therapy vectors have been created, most using cationic lipid complexes. These are groups of positively charged lipids that bind to the negatively charged sugar-phosphate backbone of the DNA. These complexes have a net positive charge, so they are attracted to and pass through negatively charged cell membranes. They are not capable of integrating the DNA into the genome, although the idea of using them to introduce an entire artificial chromosome to a cell.

The Future

Gene therapy treatments for both cystic fibrosis and heamophilia, using the CFTR and clotting factor IX genes repectively, are now being heavily researched by several organisations, and it seems likely that a commercial product will be available withing the next 5-10 years, depending on the clinical trials required. The possibilities for gene therapy are huge once the technology becomes reliable (gene therapy is still not a precise science). Many genetic disorders will be permenantly eliminated by gene therapy in utero before they even appear, while more advanced human genetic engineering will become possible.