Many diseases such as cystic fibrosis, muscular dystrophy, haemophilia and cancer are caused by faulty genes. Gene therapy involves the addition of a healthy, working copy of the gene into appropriate cells in the body to replace or override the faulty copy present in the genome.
Unlike most conventional medicines, instead of treating the symptoms of a disease, gene therapy has the potential to correct the underlying cause.
Gene therapy was first proposed nearly forty years ago as a method of manipulating cells at the molecular level in order to cure rare genetic diseases like cystic fibrosis, phenylketonuria and cancers.
Following an explosion in cancer, virology and bacterial genetics research in the late 1960s, and with the biochemical characterisation of human genetic diseases, it was proposed that synthetic and viral vectors could be used to deliver corrected versions of genes into cells. It was suggested that the gene transfer would induce a corrected phenotype on the cell.
The transfer of large genetic sequences into cells presents a fundamental problem because DNA is negatively charged and will not readily cross the plasma membranes. Therefore, in addition to identifying the genetic basis of a disease and cloning a corrective copy of the sequence, much effort has gone into the production of vector systems for transporting genes into cells and expressing the genes in-vivo.
Replication deficient viruses were proposed as ideal gene transfer systems because they have the ability to infect cells, partially evade immune responses and express their genome in the host cell. A range of viral vectors including retroviruses, lentiviruses, adenoviruses and adeno associated viruses have been used in gene therapy trials.
In addition non-viral gene transfer systems using cationic liposome/DNA complexes have been developed. They allow the transfection of cells with potentially much larger pieces of DNA and none of the immune response problems associated with viral gene therapy.