Sickle Cell Anemia
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Sickle cell disease is the most common inherited blood disorder in the United
States. Approximately 80,000 Americans have the disease.
In the United States, sickle cell disease is most prevalent among African
Americans. About one in 12 African Americans and about one in 100 Hispanic
Americans carry the sickle cell trait, which means they are carriers of the
disease.
Sickle cell disease is caused by a mutation in the hemoglobin-Beta gene
found on chromosome 11. Hemoglobin transports oxygen from the lungs to other
parts of the body. Red blood cells with normal hemoglobin (hemoglobin-A)
are smooth and round and glide through blood vessels.
In people with sickle cell disease, abnormal hemoglobin molecules - hemoglobin
S - stick to one another and form long, rod-like structures. These structures
cause red blood cells to become stiff, assuming a sickle shape. Their shape
causes these red blood cells to pile up, causing blockages and damaging vital
organs and tissue.
Sickle cells are destroyed rapidly in the bodies of people with the disease,
causing anemia. This anemia is what gives the disease its commonly known
name - sickle cell anemia.
The sickle cells also block the flow of blood through vessels, resulting
in lung tissue damage that causes acute chest syndrome, pain episodes, stroke
and priapism (painful, prolonged erection). It also causes damage to the
spleen, kidneys and liver. The damage to the spleen makes patients - especially
young children - easily overwhelmed by bacterial infections.
A baby born with sickle cell disease inherits a gene for the disorder from
both parents. When both parents have the genetic defect, there's a 25 percent
chance that each child will be born with sickle cell disease.
If a child inherits only one copy of the defective gene (from either parent),
there is a 50 percent chance that the child will carry the sickle cell trait.
People who only carry the sickle cell trait typically don't get the disease,
but can pass the defective gene on to their children.
Until recently, people with sickle cell disease were not expected to survive
childhood. But today, due to preventive drug treatment, improved medical
care and aggressive research, half of sickle cell patients live beyond 50
years.
Treatments for sickle cell include antibiotics, pain management and blood
transfusions. A new drug treatment, hydroxyurea, which is an anti-tumor drug,
appears to stimulate the production of fetal hemoglobin, a type of hemoglobin
usually found only in newborns. Fetal hemoglobin helps prevent the "sickling" of
red blood cells. Patients treated with hydroxyurea also have fewer attacks
of acute chest syndrome and need fewer blood transfusions.
Currently the only cure for sickle cell disease is bone marrow transplantation.
In this procedure a sick patient is transplanted with bone marrow from healthy,
genetically compatible sibling donors. However only about 18 percent of children
with sickle cell disease have a healthy, matched sibling donor. Bone marrow
transplantation is a risky procedure with many complications.
Researchers are experimenting with attempts to cure sickle cell disease
by correcting the defective gene and inserting it into the bone marrow of
those with sickle cell to stimulate production of normal hemoglobin. Recent
experiments show promise. In December 2001, scientists at Harvard Medical
School and MIT, supported by the National Institutes of Health (NIH), announced
that they had corrected sickle cell disease in mice using gene therapy.
Researchers used bioengineering to create mice with a human gene that produces
the defective hemoglobin causing sickle cell disease. Bone marrow containing
the defective hemoglobin gene was removed from the mice and genetically "corrected" by
the addition of the anti-sickling human beta-hemoglobin gene. The corrected
marrow was then transplanted into other mice with sickle cell disease. The
genetically corrected mice began producing high levels of normal red blood
cells and showed a dramatic reduction in sickled cells. Scientists are hopeful
that the techniques can be applied to human gene transplantation using autologous
transplantation, in which some of the patient's own bone marrow cells would
be removed and genetically corrected.
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Doctors diagnosis sickle cell through a blood test that checks for hemoglobin
S - the defective form of hemoglobin. To confirm the diagnosis, a sample
of blood is examined under a microscope to check for large numbers of sickled
red blood cells - the hallmark trait of the disease.
In more than 40 states, testing for the defective sickle cell gene is routinely
performed on newborns.
Sickle cell disease can also be detected in an unborn baby. Amniocentesis,
a procedure in which a needle is used to take fluid from around the baby
for testing, can show whether the fetus has sickle cell disease or carries
the sickle cell gene. If the test shows that the child will have sickle cell
disease, some parents may choose not to continue the pregnancy. Genetic counselors
can help parents make these difficult decisions.
A new technique used in conjunction with in vitro fertilization,
called pre-implantation genetic diagnosis (PGD), enables parents who carry
the sickle cell trait to test embryos for the defective gene before implantation,
and to choose to implant only those embryos free of the sickle cell gene.
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NHGRI Clinical Research on Sickle Cell Disease
- Current NHGRI Clinical Studies
- Search ClinicalTrials.gov
- Clinical Research FAQ
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Genes and Disease:
Sickle Cell Anemia
Sickle Cell Anemia
Sickle
Cell Anemia: Your Genes, Your Health
Sickle
Cell Disease
Sickle Cell
Newborn Screening: FAQ's
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