By Sally Roberson

Gene therapy is a strategy used to treat disease by correcting defective genes or modifying  how genes they are expressed. The techniques used involve administrating a specific DNA or RNA sequence. Researchers hope that in the future, gene therapy will enable patients to be treated by inserting genes into their cells rather than administering drugs or subjecting them to surgery.

This therapy offers a promising new approach to treating a range of diseases including various forms of cancer, inherited disease and certain viral infections. However, further studies are still required to ensure the safety and effectiveness of these techniques. Currently, the therapy is only used to treat diseases where other therapies are already known to be ineffective.

There are two basic types of gene therapy: germline therapy and somatic gene therapy. These are described in more detail below.

Germline therapy

This therapy involves the modification of the genes inside germ cells (sperm or ova). During reproduction, these gamete cells fuse to form a zygote, which would divide and pass on the modified gene into all other cells of the body during the development of offspring. In this way, the therapy alters the genome of future generations to come.

Although theoretically this could counteract hereditary disease, jurisdictions in various countries such as Switzerland, Australia and Germany prohibit the use of germline therapy due to fears over unknown risks and long-term effects in future generations. In addition, the therapy is very costly.

Somatic gene therapy

Unlike germline therapy, somatic gene therapy only involves the insertion of therapeutic DNA into body cells and not the germ cells or gametes. This means any effects of the therapy are confined to the individual being treated and are not inherited by future offspring.

The field of somatic gene therapy is surrounded by fewer ethical issues compared with germline gene therapy, although the therapeutic approach is also still in the early stages of design and prone to obstacles.

The first hurdle is successful incorporation into the genome; integrating the modified gene into the wrong part of the DNA could induce rather than prevent disease. Secondly, the desired gene needs to be expressed. Thirdly, the gene expression needs regulating to prevent over expression triggering any disease.

Inserting genes into cancer cells

One of the most challenging aspects of gene therapy is inserting genes into cancer cells and experts are striving to find new and improved techniques for achieving this. One of the main ways this is carried out is through the use of a vector, which carries a gene into a cancer cell. Usually, the vector is a virus because viruses are built to target and enter cells so they can deliver the their genetic material once inside them. Scientists have found ways to alter these viruses so that they only deliver genes to cancer cells rather than healthy cells. Other vectors are also being tested such as inactivated bacteria.

Gene therapy techniques in cancer treatment

Some of the techniques that may be used in gene therapy to treat cancer are described below.

Harnessing the immune response

Some forms of gene therapy are designed to strengthen the body’s existing ability to target and kill cancer cells. The role of certain cells of the immune system is to recognise and kill these cells. Adding certain genes to a patient’s immune cells can improve their ability to find or kill certain forms of cancer. These techniques are currently being tested in a few trials across the UK.

Gene therapy to enhance cancer treatment

Some techniques insert genes into cancer cells that can make the cancer cells more vulnerable to radiotherapy or chemotherapy, therefore improving the effectiveness of these treatments.

Blocking the protection of cancer cells

Certain processes cancer cells use to survive can be blocked using gene therapy. For example, one process called apoptosis refers to the programmed cell death a cell undergoes if it contains DNA that is damaged and beyond repair. In cancer cells, apoptosis is stopped and the cells divide to form new cells that also contain the damage DNA. Some gene therapy techniques are designed to prevent this inhibition of apoptosis to ensure that the cancer cells do in fact die rather than survive.

Pro-drug gene therapy

Certain gene therapy techniques insert genes into cancer cells that allow conversion of an inactive drug called a pro-drug into the active form. The converting gene is given in the form of a tablet or capsule and the pro-drug is then administered. The pro-drug does not harm normal cells and only reaches cancer cells, where it is activated by the gene to become destructive.

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