One of the first children in Britain to receive gene therapy for an immune system disorder has developed leukaemia as a result of his treatment.

The boy, aged 3, is the first gene therapy patient in Britain to fall ill with leukaemia, a known risk of the procedure. A similar programme in France has caused four cases of the blood cancer, and one death.

The child, who has not been named, was born with X-linked severe combined immunodeficiency (X-SCID), a genetic condition in which the immune system fails to develop. It is often called “bubble baby syndrome”, because sufferers are shielded in a sterile pouch. Without treatment, they die in their first year, from infections such as pneumonia or chicken pox.

Two years ago the boy became the eighth patient to be treated in London on Great Ormond Street Hospital’s gene therapy programme, which uses a genetically modified virus to correct the faulty DNA that causes X-SCID.

While his immune system responded “extremely well” to the procedure, leukaemia was diagnosed last month, the hospital said. This is an acknowledged risk of gene therapy, as inserting the replacement DNA can activate another gene that promotes cancer.

None of the other 14 British children who have had gene therapy for X-SCID and a similar condition, known as ada-SCID, has developed leukaemia so far, while all have had immune function restored.

The cancer, however, has been diagnosed in 4 of the 11 patients in a French trial, one of whom died while three are in remission.

Bobby Gaspar and Adrian Thrasher, who treated the boy, said in a statement: “Our first thoughts are to secure the best treatment for this child and to support his family.

“The development of leukaemia is now a recognised side-effect in this study, though the risks are balanced by the severity of the condition and the lack of good alternative treatments for X-SCID.”

While 80 per cent of children with leukaemia make a full recovery, untreated X-SCID is always fatal. The doctors said that gene therapy was usually a better option than the alternative, a bone marrow transplant.

A suitable bone marrow donor is available for only a third of X-SCID children, and even when a transplant is possible it carries a risk of complications, including death, because the children are so ill.

Dr Gaspar said that while a donor had been found for the boy, his family had chosen gene therapy after independent counselling about the risks. “His parents are obviously very distressed, but they are as sanguine about it as can be expected.” There is every chance that the boy will survive leukaemia and be cured of X-SCID.

The trial has now been closed to new patients, but Dr Gaspar and Professor Thrasher plan to start another programme using a slightly different technique next year.

They have already completed successful animal trials of a viral vector that can carry new DNA into cells without the risk of inadvertently switching on a cancer gene.

Martin Gore, chairman of the Government’s Gene Therapy Advisory Committee, said the data that his committee had seen “suggests that the risk of leukaemia after gene therapy may be confined to this patient group and the particular vectors used in these two trials.

“The more that we understand about this specific episode the better able we will be to develop safer and more effective vectors.”

The X factor

— X-linked severe combined immunodeficiency (X-SCID) is caused by a missing gene on the X chromosome. It affects only boys

— Sufferers have no functioning immune system, leaving them highly susceptible to infection

— The traditional treatment is a bone marrow transplant, but a suitable donor can be found for only one in three patients

— Gene therapy was developed as an alternative by teams at Great Ormond Street and in Paris

— There are also gene trials on arthritis, eye disease and cancer

Sources: Great Ormond Street Hospital, Times database

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