SCID – also known as severe combined immunodeficiency – is a very rare genetic disorder which only affects between 1 in 50,000 and 1 in 100,000 births. Children born with SCID do not have a effective immune system, the result of a single defective gene coding for the enzyme adenosine deaminase. Boys are more often affected than girls because at least one form of the disease is sex-linked (carried on the X chromosome).

In the past, the only way of keeping these children alive was to bring them up in a completely sterile environment, with all their food, water and air sterilised and with no direct contact with other people. Even then, affected children rarely lived into their teens as the slightest infection could kill them.

Another alternative is a bone marrow transplant if a suitable donor can be found. Although the affected child has no immune system to cause rejection, the transplanted marrow can attack the patient’s cells. What is more, the donor cells may be infected with a virus – and this can kill the recipient very quickly.

Patients can also be regularly injected with the enzyme they need, but this involves a lifetime of carefully managed therapy.

So gene therapy (inserting a healthy gene into the DNA using a specially modified virus) offers the exciting possibility of a normal life for children who otherwise have a limited life expectancy and relatively poor quality of life.

Variations on the technique were tried on children in several different countries, including Britain. The trials had considerable success – all of the children treated developed functioning immune systems which enabled them to fight off infections and to make antibodies when they were given vaccines. They could leave hospital and their sterile environments and live normal lives.

Then came disappointing news. Nine children with SCID had been treated successfully in France, using gene therapy. First one and then two of these children developed leukaemia-like symptoms. They responded well to chemotherapy, but both the French and the American governments have halted trials of gene therapy for SCID until more is known about why these boys fell ill, and whether their illness is linked to the gene therapy they underwent.

The UK government decided differently, feeling that the potential benefits outweighed the possible risks. This view was backed up both by doctors carrying out the therapy at Great Ormond Street Hospital and by the mother of little Rhys Evans, the first British boy to be given gene therapy.

Professor Nevin, who chaired the UK committee which made the decision that work should continue commented: "As with all innovative treatments, there will always be the potential for side-effects."

Dr Bobby Gaspar of Great Ormond Street Hospital said: "If we stop these studies now we will be denying extremely effective therapy to children and they may suffer as a result of not receiving this therapy. Ethically we believe it is the right thing to go on."

Marie Evans, the mother of Rhys (left) also had an opinion. "If they stop something just because one child has an adverse effect at the end of the day medicine and the world just doesn't go on," she said.
Jeans for Genes