A British boy undergoing gene therapy for X-linked severe combined immunodeficiency (X-SCID) has developed leukemia. The three-year-old, who has not been named, began the gene therapy regimen two years ago at the Great Ormond Street Hospital in London.
X-SCID is an inherited disorder of the immune system, caused by the mutation in a single gene—IL2RG. It occurs primarily in boys. Patients are highly susceptible to opportunistic infections caused by bacteria, fungi and viruses, including chicken pox, meningitis and pneumonia. The condition is commonly called “bubble baby syndrome,” because patients must be confined to a sterile environment to safeguard them from infection. Without treatment, babies born with X-SCID usually die in the first year.
So far,14 British children have received gene therapy for X-SCID and a similar immune condition and none of them have developed leukemia. The gene therapy uses a genetically modifed virus to correct the DNA error that causes the disease. All the patients from this group have responded well to the therapy, with a restoration of vital immune system functions.
However, cancer has been established as a potential side effect of the treatment. Several years ago four of 11 patients in a French X-SCID gene therapy trial were also diagnosed with leukemia. One of those patients died and three are in remission. The trial was subsequently stopped in 2002. Physicians and patients’ families must carefully weigh the risks before deciding to undergo the treatment regimen.
The data “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,” said Martin Gore, chair of the Government’s Gene Therapy Advisory Committee (UK). “The more that we understand about this specific episode the better able we will be to develop safer and more effective vectors.”
Although X-SCID can be treated with a bone marrow transplant, suitable marrow donors are available for only one-third of patients. In addition, it also carries risks for these children who are already in a compromised health state. In lieu of a transplant, gene therapy has been thought to be a more promising, if somewhat risky, therapeutic option considering the extreme severity of the disorder.
Source: The Times Online (UK), December 19, 2007