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Gene therapy for X-SCID; additional briefing

18 December 2007

Why was gene therapy developed for X-SCID?

X-SCID is fatal unless treated. The conventional treatment is bone marrow transplantation, which has excellent results with a full matched donor, but less satisfactory results with a poorer match. Only one third of X-SCID patients will have a fully matched donor. Bone marrow transplantation involves powerful chemotherapy and regrettably, up to 20 per cent of children transplanted with a less than full match die. Gene therapy has therefore been a highly valuable treatment option for this group of children.

In gene therapy, a working copy of the defective gene is placed in the child’s own bone marrow cells and these are then returned to the child. The gene is placed in the cells using a vector, a modified virus. Modified cells grow rapidly and ensure that the child’s immune function returns, sometimes completely.

X-SCID gene therapy and leukaemia

A similar X-SCID trial in Paris led to a number of children developing leukaemia (four cases from 11 treated). Three were treated and recovered, one, unfortunately, died. There is no doubt in the Paris cases that the leukaemia was caused by the gene therapy, where the introduced gene was implanted next to, and switched on, an oncogene (a cancer causing gene).

The three other children are in remission from leukaemia, their immune systems are now working again, and they are doing very well.

We have now conducted a number of molecular tests and can demonstrate that our case of leukaemia was also caused by the gene therapy. The actual mechanism is similar to the cases in Paris and further work is ongoing.

Families are carefully counselled about the possible risks, and since the first Paris case, this has included discussion of the possibility of leukaemia. We are extremely thorough in seeking fully informed consent and all have been aware of this risk before entering the study. In every case, we have required the approval of the GTAC regulator before treating a patient and a number of children including the present case received independent counselling from a specialist outside of GOSH before treatment.

Prospects for the children who get leukaemia

Leukaemia is a very serious illness but can be treated and success rates now exceed 80 per cent. Taking our results and Paris’ together, present indications are that gene therapy is still safer and less intrusive than the conventional treatment, for those children without a good bone marrow donor match. Obviously as trials continue we will see how we can minimise risks to patients through protocols and vector design.

The future

The current gene therapy treatment for X-SCID is being replaced by a new vector which has been designed to reduce the risk. There are no plans to use our old vector to treat future children.

No problems have been seen so far on the ada-SCID trial and patients are monitored closely.  

Contact information:

GOSH-ICH Press Office: 020 7239 3125
Email: Coxs@gosh.nhs.uk
For genuine and urgent out of hours call speak to switchboard on 020 7405 9200

Notes to editors

Great Ormond Street Hospital for Children NHS Trust is the country’s leading centre for treating sick children, with the widest range of specialists under one roof.

With the UCL Institute of Child Health, we are the largest centre for paediatric research outside the US and play a key role in training children’s health specialists for the future.

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