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Wednesday, 3 April, 2002, 09:57 GMT 10:57 UK
Q&A: Bubble baby 'cure'

Rhys Evans has been given a second chance of life by a pioneering gene therapy treatment at London's Great Ormond Street Hospital.

BBC News Online examines the significance of the successful treatment for those with diseases caused by defective genes.

What illness has been successfully treated?

The toddler suffered from severe combined immunodeficiency (SCID) - a condition which robbed him of the immune cells needed to fight infection.

As a result he had to live permanently in a sterile environment as a precaution against the transmission of bacteria and viruses which could kill him.

It is caused by a single defective gene - a tiny biological instruction, found in every cell.

What did the scientists do to help him?

Normally, the immune system is generated from cells produced in the bone marrow.

In Rhys' case, this was not happening because "master cells" in the marrow were not working properly owing to the genetic fault.

One solution is to give a bone marrow transplant, entirely replacing his marrow with a matching sample from someone else.

However, no match was found either inside or outside Rhys' family.

Instead, they took some of his own marrow, and extracted the "master cells", or stem cells, from it.

In the laboratory, they created a new, correct, version of his faulty gene.

This new genetic code was inserted into the genes of a particular type of viruses called retroviruses, which had been altered so they could not cause disease.

When the viruses and the stem cells were put together, the viruses "infected" the stem cells.

Retroviruses reproduce by incorporating their own genetic code into the code of the cell they have infected, and waiting for the cell to reproduce - reproducing their genes with it.

So, the new version of the gene spliced into the retrovirus was inserted into the make-up of the stem cell.

The result was a sample of bone marrow cells with a fully-functional gene.

These cells were injected back into Rhys - and started working properly, dividing into the correct immune system cells, and helping him fight disease.

Has this been done before?

Yes. A team of experts at a Paris hospital has carried out the procedure on a few babies, starting in 2000.

Their results have been similarly impressive.

This is the first treatment of its kind in the UK.

Why is this exciting for scientists researching other gene therapies?

Faulty genes are responsible for, or greatly contribute to, many other serious illnesses.

Hundreds of scientists worldwide are trying to find ways to use the same principles to tackle them.

This treatment for SCID is one of the first successful examples of gene therapy, proving that, in principle, if you can get to the cells you want to change, you may be able to treat previously intractable illnesses.

Can this advance be applied to other illnesses?

The Great Ormond Street team is hoping to apply it to other conditions similar to SCID - faulty genes within bone marrow cells that affect the development of the immune system.

Elsewhere, the potential of targeted gene therapy - both to correct single gene disorders, and to attack unwanted cells, such as cancer cells, is said to be immense.

However, other studies have illustrated the difficulties faced by doctors trying to develop gene therapy.

In this case, doctors were able to remove exactly the cells they wanted to change, and then alter them in a laboratory test tube before putting them back.

Other teams face the challenge of getting their viruses to cells in organs deep within the body, or to cells spread throughout the body.

This means finding a carrier virus which is effective at targeting the right cells in a living, breathing, human, and at doses which will not harm the patient.

Often the obstacle faced by doctors has been that, in order to have any effect, it is necessary to give very high doses of their gene therapy - which can cause problems.

It may be many years before effective gene therapies even for single-gene disorders can be developed for older patients.

See also:

27 Apr 01 | Health
Gene therapy restores dogs' sight
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