Gene therapy is the deliberate 'repair' or replacement of damaged genes. Success has been limited to somatic (body) cells rather than sex cells (gametes). This means that any changes are not passed on to subsequent generations (inherited). The modification of germ line cells (germ line therapy) is likely to be discouraged on moral grounds.
Using mice with artificially induced Cystic fibrosis (CF) copies of a gene CFTR enclosed in liposomes (tiny lipid droplets) were squirted into their lungs.
The liposomes fused with the lung cell membranes, and this allowed the DNA in the CFTR genes to pass through into the cells.
Not all cells were successfully changed, but those that had the gene added were then corrected for CF, temporarily.
Later attempts were made using viruses as vectors (carriers) to introduce the gene as the virus enters (infects) the cells.
Cystic fibrosis in humans.
Cause: The inheritance of two recessive alleles for cystic fibrosis.
Symptoms: The transport of chloride ions and water by the cells in the airways, lungs and gut is disrupted. This causes thick mucus to line the lungs and ducts in the gut. Because the thick mucus is not shifted easily, it is more likely that a sufferer will pick up a bacterial infection.
Gene therapy treatment: Copies of the DNA for the normal allele were inserted into other loops of DNA, which were then attached to liposomes. The liposome complexes were then sprayed as an aerosol of fine droplets into the patients' noses. The DNA is then taken up by some of the cells lining the airways.
Fortunately only about 10% of these cells need to take up the DNA for symptoms to be relieved.
In an example, called severe combined immunodeficiency disease (SCID) the gene for making an enzyme (adenosine deaminase, ADA) is defective and this prevents the immune system from defending the body against infections.
The missing ADA gene is extracted from healthy bone marrow tissue. The gene is then inserted into a virus, which is then rendered harmless (non invasive) by removing its reproductive genes.
Treatment of affected babies involves taking samples of its defective bone marrow cells and subjecting them to this modified virus. The cells are cultures for several days, and then reintroduced, by injection, into the baby's bone marrow.
Once in the bone marrow, these modified cells then produce healthy blood cells, complete with the ADA gene. Although not all the cells are 'repaired' in this way, sufficient numbers exist in the body to produce an improvement in the health of the baby.