2 Gene therapy
Gene therapy is often reported to be one of the most hopeful lines of experimentation to grow up alongside the Human Genome Project. If we know so much about all these genes, surely we can use this knowledge to try to treat some diseases in which genes are involved at a fundamental level? But what exactly is gene therapy, and where is the work heading?
Just looking at the two words coupled in the phrase ‘gene therapy’ raises an important ambiguity.
From what you have learnt so far, what might gene therapy mean?
It could mean treatment that involves correcting genes that are involved in causing an illness in an individual. Or, it could mean using genes to treat an illness caused in some other way.
It turns out that it means both. Most of the public debate has been about the former meaning, i.e. correcting or repairing genes, but early applications have focused on the latter meaning. These applications involve using ‘designer’ DNA to tackle diseases that are not inherited – by using altered viruses designed specifically to attack cancer cells, say. Here, the DNA is working more or less like a drug. In fact, many ‘gene therapy’ trials approved so far have been attempts to treat a variety of cancers.
Here, though, we will focus on the first of the two meanings of gene therapy, correcting genes. Some genetic diseases can be tackled by modifying the phenotype, without worrying about how DNA might be involved. Most simply, a multifactorial problem like a cleft palate can be corrected surgically. But for many diseases, such as cystic fibrosis (CF), non-genetic treatments can help to alleviate some symptoms, but so far that is all. So in a single-gene disorder such as CF, the theoretical focus is now on the gene and its product. We begin by focusing on gene products, i.e. proteins, and then consider genes.
If the product, that is a particular protein, is abnormal, can it be corrected? If it is missing altogether, can it be supplied? Sometimes, the answer involves genetic manipulation (or genetic modification) of other organisms. This technique is described in Study Note 1.
Study Note 1: The technique of genetic manipulation of organisms
The technique of genetic manipulation, or genetic modification, of organisms relies on restriction enzymes to cut large molecules of DNA in order to isolate the gene or genes of interest from human DNA, which has been extracted from cells. After the gene has been isolated, it is inserted into bacterial cells and cloned. This process enables large amounts of identical copies of the human DNA to be extracted for further experiments. Once inside the bacterial cells, if the human gene is active or ‘switched on’ then the bacteria behave like ‘living factories’, manufacturing large amounts of the human protein encoded by the gene (Figure 1). This can be extracted and purified from the bacterial cultures, ready for use by humans.
Genetic manipulation has enabled unlimited quantities of certain human proteins to be produced more easily and less expensively than was previously possible. Problems exist with this approach, however, as proteins must fold themselves up into very specific structures to have a biological effect. Often this doesn't happen very effectively in bacteria. In order to overcome this problem, the cloned human DNA has been introduced into sheep. In this case, the human protein is secreted into the milk, allowing for a continuous process of production (Figure 1). Alternatively, the cloned human DNA can be used for gene therapy by direct intervention in the individual's DNA (Figure 1 and Study Note 2.
Human clotting factor, the protein used to treat haemophilia, can be made by splicing the human gene into bacteria (Figure 1). Insulin, which is used to treat diabetes, can be produced by sheep in their milk. Then you can supply the missing gene product to the patient like any other medicine.
The methods of using either bacteria or sheep, like others involving production of genetically modified organisms for food, have been controversial in their way. Much of the technology involved in making genetically manipulated or modified organisms, involves doing the same sort of thing – inserting DNA into cells – as involved in gene therapy (as shown in Figure 1).
However, this technology is not gene therapy proper. Why not?
All that happens is that the protein is extracted and given to patients, usually by injection on a regular basis, so the gene is not being corrected or repaired.
Suppose you could inject the gene instead – to provide direct intervention in the individual's DNA.
Even if you could get a new gene into a human, what else would be needed to make sure it worked as a therapy?
The gene would need to be active – get transcribed into mRNA and translated into protein. And it would have to enter the right cells.
There is no point making haemoglobin in skin cells, or in producing in blood cells the protein that CF patients need in the lining of their lungs. And ideally it would have to go on working, perhaps in the cells’ descendants, otherwise repeat treatments would be needed.
The idea of, in effect, treating the genotype, has been around from the 1960s. Since the new technologies of genetic manipulation came into widespread use in the 1980s, many trials of gene therapy have been carried out on humans. Despite high hopes, few have yet shown clear benefits to patients. At the turn of the millennium, it looked as though gene therapy would be more complicated, and take longer to deliver, than was thought 10 or 20 years previously. On the other hand, it may turn out that the techniques now being tried are superseded by more successful ones as our knowledge increases. The remarkable pace of technical developments suggests that it would be unwise to discount changes to human genes in the medium term. The rest of this chapter considers some of the possibilities and problems to look out for.