A few years ago, a clinical trial began in France in the hope of curing children with a type of genetic immune deficiency called SCID-X1. Children with this disease have a defective gene, called gamma-c, which prevents a subset of the cells of the immune system from forming, and predisposes the children to life-threatening infections. In an attempt to cure the childrenwho would otherwise die at a young agephysicians used gene therapy to provide them with normal gamma-c genes.
This particular trial has had striking success as well as tragedy. Eight of the eleven children are currently thriving. However, in two cases the therapy successfully introduced gamma-c genes, but these children have since developed leukemia. In both children, a gamma-c gene inserted next to another gene, called LMO2. The LMO2 gene has previously been linked to leukemia, and scientists speculate that the insertion of the gamma-c gene next to LMO2 may have overstimulated the gene, causing T cells to proliferate in excess. An LMO2 effect, in combination with the proliferation-inducing effects of the gamma-c gene itself, may be the cause of the leukemia in these two patients. Scientists are still investigating other possible causes.
From this single trial, it is clear that gene therapy holds significant promise, yet it is also clear that it poses significant risks. To learn more about the application of gene therapy in SCID, view the accompanying animation.