Gene Therapy

I

Introduction

Gene Therapy, the use of specific genes to produce a therapeutic benefit in people suffering from a particular disease. Thus, if a deficiency in a specific gene has been identified as the cause of the disease, gene therapy would involve the supply of a functional copy of that gene, allowing its corresponding protein to be produced.

II

Historical Background

Once specific human genes were isolated and cloned in the 1980s, it became possible to introduce them into bacteria and use the bacteria to produce the corresponding protein. This recombinant human protein could then be used to treat individuals suffering from a genetic disease caused by a defect in the corresponding gene. Thus, a functional DNA molecule (see Nucleic Acids) is being used to make a protein which is then injected into the patient.

Evidently, a much simpler approach would be to inject the patient with a functional gene. The patient would then make functional protein, thereby overcoming the defect caused by the lack of a functional gene. Consequently, this gene therapy approach is a logical extension of previous approaches using recombinant proteins.

III

Main Approaches of Gene Therapy

A

The Direct Approach

One example of the direct approach to gene therapy involves the delivery of functional copies of the genes encoding the transmembrane protein whose inactivation results in the lung disease cystic fibrosis. Another example is applied to severe combined immunodeficiency (SCID), which affects children, making them highly prone to infection. In different forms of SCID, the disease results from the inactivation of the gene that encodes the enzyme adenosine deaminase or the gene encoding the ãc receptor which normally mediates the cells’ response to several regulatory molecules. In these cases, gene therapy using the appropriate gene involves the delivery of a functional gene encoding adenosine deaminase or the ãc receptor. Clinical trials of gene therapy using the appropriate gene in these diseases have been undertaken in the United States, the United Kingdom, and France.

B

The Indirect Approach

As well as these examples of a direct approach to gene therapy involving the supply of functional copies of a defective gene, it is also possible to envisage indirect gene therapy approaches in cases where the genetic defect is unknown as yet, or where the disease does not involve a genetic deficiency. Rather than acting by replacing a defective gene, in this case, the protein produced by the therapeutic gene will produce some indirect benefit in the patient.

Cancer is one example of this indirect approach where several trials of gene therapy have already been undertaken. In this group of diseases some means is required of killing the cancer cells before they kill the patient. One means of achieving this is to deliver a gene encoding an enzyme that converts a harmless substance, known as a pro-drug, into a lethal drug. If the gene encoding the enzyme can be delivered specifically to the cancer cells, the pro-drug will be converted to the lethal drug by the cancer cells, so killing them while the normal cells survive.

An alternative approach is based on the fact that, while the cancer cells in the patient’s body are different from normal cells, these differences are not efficiently recognized by the patient’s immune system. Therefore, it does not attack the cancer cells and destroy them, as it would, for example, invading bacteria. The immune response to the cancer cells can be stimulated by delivering to them genes encoding proteins (such as interleukin 2 or interleukin 4), which stimulate the immune system and improve its ability to recognize and destroy the cancer cells.

Evidently, novel treatments such as gene therapy are most appropriately tested in diseases that potentially have a rapid and fatal outcome, such as cancer. For this reason, 67 per cent of the first 350 clinical trials of gene therapy in human subjects in the United States were in cancer cases, and these included both immunotherapy and pro-drug approaches.

This indirect approach to gene therapy is also applicable to neurological diseases such as Alzheimer’s disease or Parkinson’s disease. These diseases involve the loss of specific neuronal cells in the brain. Hence, unlike cancer, where the therapy aims to kill the cells, the gene therapy approach to these diseases would involve the delivery of genes encoding factors that promote neuronal survival. These could include nerve growth factor, to delay or even halt the loss of neurons. Alternatively, it may be possible to deliver genes encoding proteins that may functionally replace the activity of the lost cells. Therefore, as Parkinson’s disease involves the loss of dopamine-producing neurons in the brain, it has been treated by introducing grafts of foetal cells producing dopamine. It may also prove possible to treat this disease by delivering the gene encoding an enzyme (tyrosine hydroxylase), which is one of the key steps in the pathway for the production of dopamine. This would allow dopamine production to take place by genetically modifying cells that do not normally manufacture dopamine.

The testing of gene therapy in these neurological diseases is more difficult than in cancer however. This is partly because the chronic long-term nature of these diseases renders it more difficult to test experimental treatments that may have significant side-effects. However, it is also due to difficulties in developing suitable delivery methods for safely and efficiently introducing genes into the brain. Indeed, the effectiveness of the gene therapy trials attempted so far in other systems, including cancer, has been greatly limited by the low efficiency of the gene therapy methods available. The actual and potential methods of gene delivery for human gene therapy are described below.