NHLBI Communications Office
Scientists Use Gene Therapy to Correct Sickle Cell Disease
"Scientists have been working to accomplish this since the creation
of an animal model for sickle cell disease several years ago. Although much
more research is needed before human application, this is a significant achievement
that brings us closer to human gene therapy for what is a very serious genetic
blood disorder," said NHLBI Director Claude Lenfant, M.D.
Sickle cell disease affects about 1 in 500 African Americans and 1 in 1,000
Hispanic Americans. The disease is caused by a mutation in one of the two
genes that determines the structure of hemoglobin, a critical molecule found
in red blood cells. Hemoglobin transports oxygen from the lungs to other parts
of the body. In patients with sickle cell disease, abnormal hemoglobin molecules
stick to one another and form long, rod-like structures. These structures
cause the red blood cells to become stiff assuming a sickle shape.
The sickled red cells pile up, causing blockages and damaging vital organs
In the study led by scientists at Harvard Medical School and the Massachusetts
Institute of Technology, mice were bioengineered to contain a human gene that
produces defective hemoglobin, causing sickle cell disease. The defect is
an amino acid substitution on the so-called "beta" chain of amino
acids that makes up part of the hemoglobin molecule. Since no single mouse
model perfectly mimics human sickle cell disease, the scientists performed
the experiment using two different mouse models. One mouse model contained
only defective human hemoglobin and the other model contained a mixture of
defective human hemoglobin and normal mouse hemoglobin.
Bone marrow containing the defective human beta-hemoglobin gene was removed
from the bioengineered mice and genetically "corrected" by the addition
of an anti-sickling human beta-hemoglobin gene. The new gene produces a beta
chain of amino acids that when incorporated into the hemoglobin molecule gives
rise to a modified normal hemoglobin molecule that prevents the sickling process.
After adding the anti-sickling gene, the corrected marrow was then transplanted
into other mice with sickle cell disease whose bone marrow had been removed
by radiation. Three months later, blood samples from the transplanted mice
showed a high level of expression of the anti-sickling beta-hemoglobin gene,
verified by identifying high levels of anti-sickling hemoglobin protein in
the blood cells.
"Gene expression continued for at least 10 months in all mice in up
to 99 percent of their circulating red blood cells. Up to 52 percent of the
total hemoglobin incorporated the anti-sickling globin protein," said
Dr. Philippe Leboulch, principal investigator of the study and assistant professor
of medicine at Harvard Medical School and the Massachusetts Institute of Technology.
Leboulch noted that gene expression above 15 percent is likely to have some
therapeutic benefit in human patients.
Further analysis of the structure of the transplanted mice's red blood cells
showed a dramatic reduction in the number of irreversibly sickled cells. For
one of the mouse models transplanted, no irreversibly sickled cells could
be detected. These mice also had changes in the density of the transplanted
red blood cells that "showed a clear shift towards normal," according
to the scientists.
Two signs of sickle cell disease enlarged spleen and a characteristic
defect in urine concentration were also corrected following the gene
The "lentiviral" vector used to deliver the therapeutic gene is
based on human immunodeficiency virus (HIV). However, unlike the HIV virus,
the vector is not capable of replicating or causing disease.
"The next step is to see how effective this vector is in larger animals
more similar to humans. It will also be important to assess the safety of
the vector when it is produced in large quantities in particular with
respect to its ability to replicate," said Greg Evans, Ph.D., a scientist
with the Sickle Cell Disease Scientific Research Group of the Blood Diseases
Program within NHLBI.
In addition to vector safety, another scientific issue to be addressed before
human application is the toxicity of the regimen used to partially destroy
the bone marrow of the transplant recipient before he or she receives the
genetically corrected bone marrow.
"A number of research studies are underway to develop less toxic regimens
which would still allow the new bone marrow to produce normal red blood cells
for the long term," added Evans.
The first human application of gene therapy for sickle cell disease would
be done with autologous transplantation. In this procedure, some of the patient's
own bone marrow cells would be removed and genetically corrected. The remaining
original marrow would be partially destroyed to "make room" for
the genetically altered cells, which would then be returned to the patient.
Currently, the only cure available for sickle cell disease is bone marrow
transplantation. In this procedure, a sick patient (the recipient) is transplanted
with bone marrow from a healthy, genetically compatible ("matched")
sibling donor. However, only about 18 percent of children with sickle cell
disease have a healthy, matched sibling donor.
For other children and adults with the disease, treatment includes transfusions,
which correct anemia and prevent strokes, and pain-killing drugs. In addition,
a drug called hydroxyurea is used in adults to reduce the frequency of painful
crises and acute chest syndrome. The drug's use in children is still being
To interview Greg Evans of NHLBI, call the NHLBI Communications Office at
Other collaborating scientists on this study were from the Albert Einstein
College of Medicine, INSERM in France, and Genetix Pharmaceuticals.
NHLBI press releases and information on sickle cell disease can be found
online at : www.nhlbi.nih.gov