Gene therapy is a novel approach to treat, cure, and
ultimately prevent disease by correcting for defective genes in the body.
The premise of gene therapy is based on correcting a disease at the level
of DNA and thus compensating for these abnormal genes.
There are essentially two forms of gene therapy
- Somatic gene therapy. This involves the manipulation of gene
*G*expression in the patient's cells; however, the modification is not inherited
by the next generation.
- Germ-line gene therapy, where genetic modification of *G*germ cells
will pass the selected change on to the next generation.
Delivery
The fundamental task for gene therapy is to develop
approaches for delivering genetic material to the appropriate cells of the
patient in a way that is specific, efficient and safe. Only the cancer cells
must be targeted and the healthy cells must remain unaffected. The problem
of "drug delivery," where the gene is the drug, is particularly challenging
for genes that are large and complex and require targeting to the nuclei
of cells.
Gene delivery vehicles called "vectors" are being employed for therapeutic
gene delivery to the cells. Viruses are currently employed as vectors.The
dangerous genetic part of the virus is removed and is replaced with therapeutic
*G*recombinant genes. However, the immune and inflammatory responses of the
patient often interfere with this method of delivery.
Gene therapy is in its infancy with most human clinical trials only in
the research stages. The approaches that are in trial are discussed below:
- A new class of genes called suicide genes stimulate the cancer cells
to die without affecting the healthy cells. These genes attach themselves
to cancer cells and lead to self-destruction. They act by converting a harmless
chemical into a powerful anti-cancer drug.Example of suicide genes are
- HSV-tk (Herpes Simplex-thymidine kinase) gene
- FHIT gene
The phenomenon by which suicide of the cancerous cell is induced
is referred to as the "bystander effect".
[58], [59]
- Senescent cells are usually sent a chemical message to stop dividing
and die (apoptosis). Cancer cells usually ignore these messages and continue
to multiply. Telomerase is one of the enzymes that protect cancer cells.
Scientists at Brunel University have succeded in "switching off" this
enzyme. [60]
- Canadian scientists have discovered a new virus called reo virus,
which kills only the cancerous cells leaving the normal cells. Normally
cells are protected against the reo virus. In cancerous cells, this protection
is lost by mutations in the RAS oncogene. Hence this virus targets and infects
cancer cells leading to their destruction.
[61]
- Scientists in Northern Ireland have found that nitric oxide, when
used in conjunction with radiotherapy, destroys cancer cells without affecting
normal cells. Injecting the nitric oxide gene in cancerous tumours sensitizes
the tumour. Subsseuqntly, when the cell is targeted with radiotherapy,
it dies without harming other healthy cells around it. The same group were
also successful in delivering cancer drugs using red blood cells (RBC's)
where a quick blast of ulrasound turns the RBC's impregnated with toxic
drugs into cancer killers. [62]
- The Imperial Cancer Research Fund has designed a new vaccine for breast
cancer which 'marks' the cancer cells for response by the immune system.
Normal cells have a protective coating of sugars on their membranes. In
breast cancer cells, this coating is lost and sugars close to the core**what
does this mean?** are exposed. The vaccine targets one of these sugars,
known as STn. Antibodies in the immune system can then home in on the cancer
cells and destroy them. [63]