Gene Therapy to Treat Disease - Biotechnology and Your Health: Pharmaceutical Applications - page 91

UNINTENDED CONSEQUENCES OF GENE THERAPY
Immune Deficient Children
In 2000, French researchers announced the first gene therapy cure
in nine children with X-linked severe combined immune deficiency
( X-SCID ). This rare condition is caused by the inherited loss of a
protein that is part of the docking site for critical immune system
signal proteins. Because of this defect, children with X-SCID have
no mature, working lymphocytes—critical immune system cells.
As a result, they are so susceptible to viral and bacterial infections
that they rarely survive infancy. In the French scientists' research,
blood stem cells were removed from an affected child, treated with
a retroviral vector carrying a normal docking protein gene, and
returned to the child. Nine out of ten children treated in this
way developed functional, mature immune system cells, which
provided them with protection against infections. News articles
proclaimed that a cure had been found.
Stop and Consider
What would you want to know before you took part in a gene
therapy trial?
However, within a few months, two of the nine children devel-
oped a form of leukemia that had been triggered by the insertion of
the payload gene too close to a gene that controlled the division of
those cells. Uncontrolled increases in the number of white blood cells
cause leukemia. The two children were treated with chemotherapy
for the leukemia. One patient's leukemia returned and the child was
then treated with a bone marrow transplant, but subsequently died.
The same study reported that a third child showed evidence of uncon-
trolled growth of white blood cells. In addition, a U.S. researcher
reported that a monkey treated with cells changed by a similar vector
died of a white blood cell tumor. The FDA called a halt to human