Pathology > Basic Hematology > Red Cell Disorders > Structural Abnormalities of Hgb (Part 1)

Structural Abnormalities of Hgb (Part 1)

Disorders of abnormal hemoglobin synthesis are called hemoglobinopathies. Hemoglobinopathies may result in hemolysis (ª25%) because of changes in hemoglobin solubility or because of instabilities in the hemoglobin molecule. Such changes are caused by either structural or quantitative defects.

Structural abnormalities are due to alterations in the polypeptide sequence changing the molecular structure and, often, the function of the globin chains.Can you name at least two examples of structural abnormalities?

 

Quantitative abnormalities are secondary to decreased rates of globin chain synthesis. Can you give an example of a quantitative abnormality?

The hemoglobinopathies are inherited disorders, some of which are autosomal dominant (unstable hemoglobins) and others autosomal recessive (Hgb S).

The term "hemoglobin disease" is the occurence of a disease causing gene in the homozygous state or the heterozygous occurence of a dominant hemolytic disease causing gene.

The term "hemoglobin trait" is the usually asymptomatic heterozygous occurence of a disease causing gene.

The most common hemoglobinopathies are thalassemia and sickle cell disease/trait.

Thalassemia, a production abnormality with decreased rates of globin chain synthesis is discussed in the 'thalassemia' section of Decreased RBC production (See Navigational Outline).

Sickle cell disease and trait will be discussed in the following cards where the discussion focuses on structural defects.

Hemoglobinopathies

Sickle cell disease
Sickle cell trait
Hemoglobin C disease
Hemoglobin C trait
Hemoglobin D disease
Hemoglobin E disease
Hemoglobin S-b thalassemia
Hemoglobin C-b thalassemia
Hemoglobin E-b thalassemia
Hemoglobin S-a thalassemia

The most common of the hemoglobinopathies is Hgb S.

In sickle cell anemia a point mutation (GAG Æ GTG) in the ß-chain at codon position 6 results in encoding of a valine instead of the normal glutamine. The resulting abnormal ßs chains combine with normal a-chains to form the abnormal hemoglobin 'S'.

The abnormal position of the ß-6 valine* on the outside of the ß tetramer allows it to stick to the ß-85 phenylalanine and the ß-88 leucine on adjacent b tetramers.

The resulting abnormal ßs chains combine with normal a-chains to form the abnormal hemoglobin 'S'.

Hb S is poorly soluble in low oxygen tension situations, forming a gel and polymerizing into fibrilary structures or tactoids. This distorts the red cells causing them to become rigid and sickled.

* The normal glutamate is tucked inside the tetramer.

The PBS chacteristically shows sickle cells or drepanocytes. These cresent shaped RBCs are said to be "irrevesibly" sickled. Polychromasia, spherocytes, target cells, and Howell-Jolly bodies may also be seen. NRBCs may be present in sickle cell crisis.

The sickle cell solubility test is a widely used screening method for sickle cell anemia. The sickle cell solubility test relies on the relative insolubility of Hgb S in concentrated phosphate buffers compared to Hgb A and other Hgb variants. Hgb S precipitates causing a cloudy solution.

Hgb CHarlem also will also cause a positive cloudy solution.

Hemoglobin electrophoresis is the most important diagnostic test for Hgb S.

In children diagnosis of Hgb S is best done after age 6 months when Hgb F levels have declined to "mature" quantities.


 

  Hgb Electrophoresis Review

RBCs are lysed and electrophoresed on cellulose acetate at pH 8.4 separating the hemoglobin proteins into bands by charge. After staining hemoglobins can be identified by their position and quantitated.

A citrate gel electrophoresis at pH 6.2 in which hemoglobins migrate to different positions.

Note that certain Hgbs migrate together on one gel but differently on the other. Compare the controls on each gel. The different migration patterns can be used to identify Hgb types.

For instance, how would you determine if the band in the C position at pH 8.6, is really C or is it A2 , or E?

 

In sickle cell disease (Hgb SS)100% of the hemoglobin is Hgb S.

Most people with sickle cell disease have chronic severe hemolytic anemia Hgb levels of 60-80 g/L; yet, the symptoms of anemia are frequently milder than expected as Hb S gives up oxygen (O2) to tissue with ease compared to Hb A. The O2-dissociation curve of Hgb S is right shifted.

 

 

Although some patients live a near normal life, most develop "sickle crises". Three types of "crises" are described:

hemolytic
aplastic
vaso-occlusive (painful)

Hemolytic crises are characterized by increased hemolysis resulting in an acute drop of the Hct and Hgb. This is accompanied by extraordinary reticulocytosis.

Aplastic crises in which an acute drop in Hct and Hgb is accompanied by a fall in reticulocytes. Aplastic crises are most often associated with infections (>90% due to parvovirus B19).

Vascular-occlusive crises are the "classic" sickle cell crises. These may be caused by infection, acidosis, dehydration or hyoxia (altitude, anesthesia, circulatory stasis, pregnancy, strenuous exercise, etc). Sickled RBCs block small vessels causing infarcts, splenic sequestration (RBC trapping), and priapism .

Infarcts may occur in any organ, but are most notable in bones (childhood dactylitis of small bones may cause variable length digits), lungs, kidney, skin (leg ulcers), spleen, eye, and CNS.

The acute chest syndrome (respiratory distress; roentgenographic infiltrates of chest), caused by vascular occlusion commonly results in death.

The clinical manifestations of Hb SS are variable. While splenomegaly may be present in childhood, recurrent infarcts lead to autosplenectomy in most adults. Although some patients live a near normal life, most develop recurrent illnesses, severe crises, and organ failure.

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