Benefits outweigh gene therapy risks

The risks of developing cancer as a side-effect of gene therapy were outweighed by the potential benefit to the patients, one of the scientists involved in gene therapy clinical trials said yesterday at the XIX International Congress of Genetics.

Dr Adrian Thrasher, from the Institute of Child Health in London, said that risks needed to be individualised. Patients with X-linked severe combined immunodeficiency (X-SCID) who were unable to find a matched donor for bone marrow transplantation had a 15 per cent mortality rate, he said, and the long-term effects of chemotherapy used to ablate the immune system prior to transplantation seriously affected growth and development.

Thrasher has been involved in one of two clinical trials for X-SCID, using a gibbon ape leukaemia virus pseudotyped retroviral vector to transfer a functional common cytokine receptor gamma chain to CD34 positive bone marrow cells ex vivo, before infusing the cells into the patient.

The four children in the trial have responded well to the treatment, Thrasher said, showing signs of immunological recovery without severe side-effects. In fact, patient 1, who was treated with the cells almost two years ago, responded to a chicken pox (varicella zoster virus) infection with an "entirely normal" immunological response.

While the trial is ongoing, Thrasher said that recent problems with the development of leukaemia-like disease in two patients from a similar French trial meant that it would continue on a case-by-case basis.

"Our feeling for this therapy is that gene therapy is highly effective for X-SCID, and is promising for other haematological and immunological disorders," he said.