In the studies leading up to the human clinical studies, mice were studied post-transplant for less than 6 months, which is a traditional research protocol. The Salk research team, however, allowed the mice to live through their natural life span, which is about one-and-a-half years. Mice that developed lymphoma did so at an average of 10 months of age.
In the human gene therapy trials, leukemia did not appear until 2-3 years after treatment, Woods says.
"This indicates that preclinical experimental treatments involving transgenes should include long-term follow-up before entering a clinical trial," says Woods.
But, more fundamentally, the Salk study suggests that replacement of a gene that serves multiple functions in the body may be much more problematic than therapy to replace a gene that serves a single function, says Verma.
"The bottom line here is that if you replace a gene that has multiple effects, you have to know more about its regulation and its ability to affect other genes, and that requires extensive preclinical work and a much more careful analysis," he says.
X-SCID is caused by mutations in IL2RG, which provides instructions for making the common gamma chain protein. This powerful protein, found on the surface of immature blood cells in the bone marrow, works with other proteins to direct the growth and maturation of a number of different immune system cells, including T cells, B cells, and natural killer cells. These immune system cells that kill invading viruses and bacteria, produce antibodies as well as help regulate the entire immune system. Without the common gamma chain, these cells cannot develop normally, and are unable to protect the body.
Researchers who also contributed to this paper include Virginie Bottero, Ph.D., in the Laboratory of Genetics at the Salk Institute, as well as Manfred Schmidt, Ph.D., and Christof von Kalle, Ph.D., bo