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SCID - a case study

What is SCID?

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, and it is all 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 contamination 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.

Treatment of SCID with gene therapy

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 – the children treated all 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 the news that first one and then two of the nine children with SCID treated successfully using gene therapy in France had developed leukaemia-like symptoms. They responded well to chemotherapy, but both the French and the American governments halted trials of gene therapy for SCID until they had more information on why these boys fell ill and whether it was linked to the gene therapy.

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 Rhys Evans, the first British boy to be given gene therapy. Rhys was born in 2000, received gene therapy in 2002, and is currently well and happy, enjoying normal life with a functioning immune system. In total to date (2011) Great Ormond Street Hospital has treated 16 children with SCID. One of them also developed leukaemia but 14 are able to live normal lives. The oldest is now starting secondary school.

Rhys Evans and his family in 2010

Professor Adrian Thrasher of Great Ormond Street Hospital explains that they now know why some SCID patients developed leukaemia after gene therapy. The implanted gene was positioned next to a gene that causes cancer and switched it on. Scientists have now changed the vector which carries the healthy gene into the cells of the patient with SCID so the risk of leukaemia is now much lower. Leukaemia can also be treated successfully in 80% of cases. This means that the risks of SCID are now much greater than the risks of treatment.

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."

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