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This article is an overview of gene therapy vectors, their problems, and some new gene therapy vectors that are being developed.
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Gene therapy is not yet a routine procedure; the technology is tricky and beset with hurdles to overcome. There have only been a handful of successful trials (which are a cause for great optimism), and so the technology is still being tweaked and advanced in laboratories all over the world.
Viruses are the most common vectors as they infect cells easily and dump their genetic load. Scientists hijack viral cells to neutralise their virulence factors and insert the "good" gene that's to be carried into a body. Target cells such as a patient's heart cells or bone marrow cells are infected by the virus vector. It drops its genetic payload containing the therapeutic gene which is taken up by the cell's genome which goes on to produce the correct protein.
Adenovirus - a double-stranded DNA virus that usually causes respiratory and eye infections.
Adeno-associated virus - single-stranded DNA virus that inserts genetic material into chromosome 19.
Retroviruses - create double-stranded DNA copies of their RNA genomes which can be incorporated in host cell genomes.
Herpes simplex virus - double-stranded DNA virus that infects neurons.
Non-viral vectors include, injecting naked DNA into target tissue and inserting genes into lipid complexes.
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The Problem with Vectors
Currently there are several problems which are preventing gene therapy from becoming a common way of treating genetic diseases.
First of all injecting any foreign material into a person's body runs the risk of firing up the immune system. Viral vectors pose many potential risks such as an inflammatory response, increased toxicity in the body and there's another danger lurking. Some people fear that once inside a cell a virus may recover its ability to cause disease.
Then there's just the sheer complexity of trying to get a desired gene to just where you want it in a cell. It's not easy, and at the moment we're just talking about single gene disorders. Multi-gene disorders add a whole new layer of complexity.
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So the challenges are immense, but perhaps not insurmountable. Technology is moving forward all the time.
One possibility that scientists are tinkering with is the idea of creating an artificial chromosome that would house beneficial genes. This would sit alongside the 46 chromosomes in a cell without interfering with them. However, its large size will make it difficult to get inside a target cell without destroying it.
DNA scissors are showing a lot of promise - these are proteins that can bind to specific places on chromosomes, for example where the bad gene is. They make an incision (hence the 'scissors' nickname) which causes the chromosome to heal itself along with a good gene that's been supplied by researchers.
Another non-viral approach has been created by scientists at Katholieke Universiteit Leuven (Belgium) with researchers from the Max Delbruck Center in Berlin (Germany). It uses transposons which are small mobile pieces of DNA that integrate into DNA sequences by "cutting and pasting." Scientists have modified transposons to contain a desired gene that can be safely incorporated into a host cell genome. This approach does away with some of the disadvantages of viral vectors, and has been used successfully in stem cells. It is now being looked at to treat some cancers.
Vectors hold the key to the success of gene therapy. They must be able to safely and effectively deliver genes to target cells if the technology is ever to get off the ground. Perhaps these latest developments will bring about the breakthroughs that are so desperately needed.