Gene therapy

Genes are coded messages that tell cells how to behave. Our genes decide what we look like and control how our body works. They are in the nucleus of every cell in our body. The nucleus is the control centre of the cell. Genes are made of DNA. Scientists estimate that we have 25,000 separate genes.

Genes are grouped together to make chromosomes. We inherit half our chromosomes from our mother and half from our father.

Genes carry instructions on how to make proteins. The cell needs these proteins to work properly. For example they need them to repair damage in the cell.

Cancer cells are different from normal cells. Cancer cells have changes (mutations) in several of their genes which make them behave differently. The genes that are damaged may be

• Genes that encourage the cell to multiply (known as oncogenes)
• Genes that stop the cell multiplying (tumour suppressor genes)
• Genes that repair other damaged genes

There is more about these types of genes in our page about how cancers start.

Most gene changes that may make a cell become cancerous are caused by environmental or lifestyle factors (for example, smoking). But some people have inherited faulty genes that increase their risk of particular types of cancer. Inherited damaged genes cause between 2 and 3 out of every 100 cancers (2% to 3%). There is more information about how cancer starts in our about cancer section.

Some types of gene therapy treat cancer by blocking abnormal genes in cancer cells or repairing or replacing abnormal genes. So the cancer cells go back to normal.

Other types of gene therapy encourage more genes to become abnormal in cancer cells. So the cells die or become sensitive to other types of cancer treatment. Some types of gene therapy don’t change the cancer cell genes. But they make a change in a virus gene so that the virus kills cancer cells, but not normal cells.

The ideas for these new treatments have come about because we now know in detail how cancer cells are different from normal cells. There is information about this in our section about the cancer cell. It’s very early days for this type of treatment. A number of new types of gene therapy treatment are being researched.

Some clinical trials have looked at using gene therapy for cancer. But progress has been slow in this very complicated area. One major hurdle that researchers continue to work on is how to get new genes inside cancer cells.

Researchers are looking at different ways to use gene therapy

• Gene therapy and TS genes
• Gene therapy and repair genes
• Using altered viruses

Gene therapy and TS genes

A tumour suppressor gene (TS gene) is a gene that acts to stop cells growing and dividing into new cells. p53 is a TS gene that is damaged in most human cancers. Scientists have been working on a way to replace a damaged p53 gene with a normal one. They have used viruses in the lab to carry the new gene into the cancer cells. If used in people, these carrier viruses are weakened so they can't multiply and cause disease. But weakened viruses might be killed by the immune system before they have a chance to carry the new gene to the cancer cells. Researchers are trying to find a balance between

• Stopping the carrier virus causing disease
• Making sure the carrier virus is strong enough to get past the immune system without being killed

We are still investigating the technical side of developing the treatment – for example, using the carrier virus to get the genes into cells. These are early days and it is a long process.

Gene therapy and repair genes

In theory, gene therapy could be used to replace damaged DNA repairing genes in cancer cells. Or we could try damaging even more of these genes. so that the cancer cell dies. Or the cell is so damaged that it is more easily killed by other cancer treatments (such as radiotherapy or chemotherapy). Laboratory work is going on in this area, but clinical trials are still a little way off.

Using altered viruses

Some viruses are lethal to cells. They infect cells and then kill them. Some research has used lethal viruses, but changed a gene. So the virus no longer damages normal cells but it still kills cancer cells.

This isn’t cancer gene therapy as such – it isn’t trying to affect or change the genes inside cancer cells. It’s just trying to kill the cancer cells. But it comes under the general heading of gene therapy, because a gene in the virus has been deliberately removed.

One example of this type of research uses the cold sore virus (herpes simplex). The changed virus is called Oncovex. It has been tested in early clinical trials for advanced melanoma, pancreatic cancer and head and neck cancers.