Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Gene therapy

Therapy means treatment for a disease or condition. But whereas a drug like aspirin can be used to treat the symptoms of a headache, or an antibiotic like penicillin can be used to kill off infecting microorganisms, it is much more difficult to deal with genetic conditions.

Someone with a condition like cystic fibrosis has a combination of faulty alleles in every cell of their body. It started at fertilisation so the fertilised egg or zygote contained in this case 2 copies of the recessive allele, one from each parent. The zygote divided to make all the cells in the embryo, foetus and eventually the adult's body. By the process of nuclear division (mitosis) each of these cells were given 2 copies of the faulty allele.

Each of these cells contains non-functioning DNA so that their cell membranes have a different makeup from ordinary cells. This is most troublesome in the lungs, where sticky mucus is produced, but there can be problems with the ducts leading out of the pancreas, and the sperm ducts in men.

To deal with the problem in the lungs, it has been proposed that normal working copies of the allele (i.e. sections of DNA) could be introduced into the lungs, either in a spray of liposome droplets, or in a modified virus. There are several viruses that infect the respiratory system and it should be possible to take out (some of) the virus's infective material and get it to insert the required gene instead.

This is far from easy and there is much work to be done. It is uncertain how many cells would have to actually take up the altered gene to have a good result, and how long any improvement would last. It could be that altered lung cells will gradually be replaced by cells with the original condition. Alternatively, stem cells which have had the gene added to them might be implanted, and these could divide to replace any cells as required.

It must be said that the best candidates for gene therapy are organs where the affected cells are in a thin layer, like the lungs, or the eyes.

A very recent example concerns the successful introduction of a gene into the retina cells of someone with an inherited condition causing degeneration of these cells. The replacement DNA was packaged into a virus which normally infects eyes and this was injected into patches of the retina. This was partly successful in one case, although others who received the same treatment have not had the same results.

On the other hand conditions like (the inherited susceptibility to) breast cancer are more difficult to deal with because the individual cells are less easily accessible.

Some questions

How would a medical scientist know what section of DNA was faulty in a person who seems to have an inherited genetic condition?
>Compare (a targeted section of) the patient’s DNA with results from “normal” DNA e.g. DNA from the Human genome project
Would someone who has had successful gene therapy pass on faulty genes to their offspring?
>Unfortunately they still would – the replacement genes would not have got into the reproductive cells.
Why must gene therapy be repeated?
>Because cells in the body die and are regularly replaced, and so unmodified cells might take over.
Why are there some concerns about the use of viruses to insert missing genes?
>The virus must be inactivated, or it might cause an unwanted infection.
Some people talk about the CF gene, whereas in the account above it is referred to as a faulty allele.
What is the difference between a gene and an allele?
>An allele is an alternative form (faulty, in this case) of a gene.
Why has it not been called the CF gene?
>It is only a form of the gene, and the usual (non-faulty) version of it simply makes a working protein in the cell membrane.
Why do you think there is so much emphasis on cystic fibrosis and breast cancer?
>These are both relatively common conditions.

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