A gene therapy trial for an inherited immune deficiency disorder has been suspended again, following the appearance of complications in a third child. Eleven patients affected by X-linked severe combined immunodeficiency disorder (X-SCID) have so far been treated by the team, based at the Necker Hospital in Paris. While most have responded extremely well to the therapy, the trial was suspended in late 2002, after two patients developed symptoms of leukaemia. One of these boys is now in remission, but the other has since died. The AFSSAPS (French Agency for Health Product Safety) gave permission for the trial to restart in May 2004, but has now suspended the experimental treatment again.
Children affected by SCID have a faulty gene that means they have no working immune system, so their bodies cannot fight infections. This life-threatening condition is sometimes called 'bubble boy' disease, as unless they can be successfully treated with a matched bone marrow transplant, patients must spend their lives in a sterile environment. To carry out the gene therapy treatment, the French researchers harvested bone marrow from the patients, from which they isolated blood stem cells. They then infected these cells with a retrovirus (a virus that inserts its genetic material into the host cell's DNA) carrying a working gene, before returning the modified cells back to the patients.
Scientists think that the leukaemia in the two patients reported in 2002 was caused by the gene therapy, although other factors may also have contributed. In both cases, researchers found that the retrovirus had inserted its genetic material close to the 'on-switch' of a cancer-causing gene called LMO2. It is thought that this event caused the unregulated growth of the bone marrow cells, which in turn triggered the leukaemia. Now, a patient who was treated in April 2002, at the age of nine months, is also showing signs of 'lymphoproliferation' - overgrowth of white blood cells. The AFSSAPS has suspended the trial while the causes of this latest complication are investigated, French newspapers reported last week.
Sources and References
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Gene therapy: new complication for 'bubble boy'
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Survenue d'une troisieme complication dans l'essai clinique  de therapie genique DICS X
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