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Normal Hemoglobins
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- Hemoglobin A. This is the designation for the normal hemoglobin that exists
after birth. Hemoglobin A is a tetramer with two alpha chains and two beta chains
(a2b2).
- Hemoglobin A2. This is a minor component of the hemoglobin found in red cells
after birth and consists of two alpha chains and two delta chains (a2d2).
Hemoglobin A2 generally comprises less than 3% of the total red cell hemoglobin.
- Hemoglobin F. Hemoglobin F is the predominant hemoglobin during fetal development.
The molecule is a tetramer of two alpha chains and two gamma chains (a2g2).
The genes for hemoglobin F and hemoglobin A are closely related, existing in the same gene cluster on chromosome 11. Hemoglobin
F production falls dramatically after birth, although some people continue
to produce small amounts of hemoglobin F for their entire lives.
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Clinically Significant Variant Hemoglobins
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- Hemoglobin S. This the predominant hemoglobin in people with sickle cell disease.
The alpha chain is normal. The disease-producing mutation exists in the beta chain,
giving the molecule the structure, a2bS2.
People who have one sickle mutant gene and one normal beta gene have sickle cell
trait which is benign.
- Hemoglobin C. Hemoglobin C results from a mutation in the beta globin gene and
is the predominant hemoglobin found in people with
hemoglobin C disease (a2bC2).
Hemoglobin C disease is relatively benign, producing a mild hemolytic anemia and
splenomegaly. Hemoglobin C trait is benign.
- Hemoglobin E. This variant results from a mutation in the hemoglobin beta chain.
People with hemoglobin E disease have a mild hemolytic anemia and mild splenomegaly.
Hemoglobin E trait is benign. Hemoglobin E is extremely common in S.E. Asia and in
some areas equals hemoglobin A in frequency.
- Hemoglobin Constant Spring.
Hemoglobin Constant Spring is a variant in which a mutation in the alpha globin gene
produces an alpha globin chain that is abnormally long.
The quantity of hemoglobin in the cells is low for
two reasons. First, the messenger RNA for hemoglobin Constant Spring is
unstable. Some is degraded prior to protein synthesis. Second, the Constant
Spring alpha chain protein is itself unstable. The result is a thalassemic
phenotype. (The designation Constant Spring derives from the isolation
of the hemoglobin variant in a family of ethnic Chinese background from
the Constant Spring district of Jamaica.)
- Hemoglobin H. Hemoglobin H is a tetramer composed of four beta globin chains. Hemoglobin H occurs only with extreme limitation of alpha chain
availability. Hemoglobin H forms in people with three-gene alpha thalassemia as well
as in people with the combination of two-gene deletion alpha thalassemia and hemoglobin Constant Spring.
- Hemoglobin Barts.
Hemoglobin Barts develops in fetuses with four-gene deletion alpha thalassemia. During
normal
embryonic development, the episilon gene of the alpha globin gene locus combines
with genes from the beta globin locus to form functional hemoglobin molecules. The episolon gene turns off at
about 12 weeks, and normally the alpha gene takes over. With four-gene deletion alpha
thalassemia no alpha chain is produced. The gamma chains produced during fetal development
combine to form gamma chain tetramers. These molecules transport oxygen poorly. Most
individuals with four-gene deletion thalassemia and consequent hemoglobin Barts die
in utero (hydrops fetalis). The abnormal hemoglobin seen during fetal development
in individuals with four-gene deletion alpha thalassemia was characterized at St.
Bartholomew's Hospital in London. The hospital has the fond sobriquet, St. Barts,
and the hemoglobin was named "hemoglobin Barts."