Hemoglobinopathy

• Haemoglobinopathies are disorders where the production of normal adult hemoglobin is either partly or completely suppressed or replaced by a variant hemoglobin.

Classification of Haemoglobin Disorders

• Qualitative disorders involve differences in the sequence of amino acids composing the globin chain, resulting in a structural defect and is called a haemoglobinopathy.
• Quantitative disorders are characterized by a decreased rate of hemoglobin production, with no alteration in the amino acid sequence and is called thalassemia

Thalassemia and Haemoglobinopathy

• Inherited haemoglobin (Hb) disorders are classified into haemoglobinopathy caused by structural defects in Hb and thalassemia, caused by the impaired synthesis of globin .
• Both are typically inherited in an autosomal recessive manner.

Qualitative Disorders

• Qualitative abnormalities involve changes in globin structure, usually involving beta-chain.
• The heme portion remains normal.
• These disorders arise from single amino acid substitutions or deletions.
• Multiple substitutions are rarely observed.
• Qualitative disorders may or may not cause abnormal laboratory test results.

Sickle Cell Anaemia

• Haemoglobin S (HbS) forms due to a point mutation at position 6 of the beta-globulin chain, where valine is substituted for glutamic acid.
• It is the most common hemoglobinopathy
• Sickle Cell Anaemia provides increased resistance to Malaria infection by killing the parasite and causing infected RBCs to be sequestered in the spleen and destroyed.
• This is a world-wide disease with high incidence in Africans.
• Sickle cell disease occurs in 1 in 375 live African American births.
• Sickle cell trait occurs in 20-40% of Africans.
• It is autosomal co-dominant.
• Hemoglobin A and Hemoglobin S are produced.
• AS indicates sickle cell trait.
• SS indicates sickle cell disease.
• Patients with SS are homozygous for HbS.
• SS results in very severe anemia

Pathophysiology of Sickle Cell Anemia

• HbS cells are normal and soluble when fully oxygenated.
• Sickling occurs when O₂ decreases and HbS polymerizes into insoluble aggregates.
• Sickling can also be caused by decreased pH and dehydration.
• Cells become rigid, increasing blood viscosity and impeding blood flow to tissues.
• Blockage of the small microvasculature by sickled cells leads to hypoxia and tissue damage, organ infarction, and sickle cell crisis.
• Sickling is reversible up to a point and, upon re-oxygenation, the RBC may return to its original shape.
• Repeated sickling can damage the permeability of the RBC membrane leading to premature death.
• Repeated sickling may cause the RBCs to become irreversibly sickled and destroyed by the spleen
• Both extravascular and intravascular hemolysis occurs.

Clinical Findings of Sickle Cell Anemia

• Clinical signs typically appear at 6 months of age, when HbF is replaced with HbS.
• Patients have physical symptoms of anemia, along with slower growth and sexual maturation.
• A crisis is very painful and anything that deoxygenates blood, such as exercise, illness, and airplane flights etc., acts as a trigger where Sickle cells get stuck in capillaries.
• Strokes
• Organs such as the liver, heart, spleen, skin, kidney, lungs, brain, blood, bones are affected.
• Chronic haemolytic anaemia results from extravascular haemolysis.

Three Types of Sickle Cell Crises

• Aplastic crisis, is associated with infections which cause temporary suppression of erythropoiesis.
• Hemolytic crisis results in exaggerated anemia.
• Vaso-occlusive crisis is associated with severe pain and is the hallmark symptom of sickle cell anemia.

Laboratory Findings of Sickle Cell Anemia

• Hb is 6-10 g/dL
• Normocytic, normochromic anemia is present
• There is marked anisocytosis and poikilocytosis, and sickle cells and target cells can be observed.
• Nucleated RBCs with Polychromasia
• Increased Retics (10-20%)
• Basophilic Stippling
• Howell Jolly Bodies and Pappenheimer Bodies
• Leukocytosis with left shift
• Thromocytosis
• Hemoglobin Electrophoresis uses either alkaline/acid mediums to separate individual hemoglobins.
• Detects differences in the charge of proteins
• HbS/HbS is the most severe pattern.
• Hemoglobin Solubility Test is positive
• Osmotic Fragility is decreased
• ESR is decreased

Sickle Cell Treatment

• There is no known effective long term therapy
• Blood transfusions, bone marrow transplants and gene therapy are options
• Prevention of infection is important
• Reduce organ damage.
• Hydroxyurea is believed to induce production of Hgb F, and uses chemotherapeutic agents.
• Avoid situations that could cause a crisis.
• Median life expectancy is 50 years.

Sickle Cell Trait

• Heterozygous AS with more HbA than HbS, condition is compensated for.
• Patients often have a normal life span
• Usually asymptomatic with occasional episodes of hematuria.
• Sickling can occur with drastic reduction of oxygen tension, severe respiratory infection, air travel in unpressurized aircraft, anesthesia, or congestive heart failure.
• Exercise that causes a buildup of lactic acid can cause sickling due to lowered pH.
• Normal CBC – Few target cells
• Hemoglobin solubility test is positive
• Electrophoresis will show Both A and S

Hemoglobin C

• Amino acid substitution of lysine for glutamic acid occurs at the sixth position of the Beta chain.(α2β26Glu-Lys)
• It is homozygous CC
• Patients have chronic hemolytic anemia with associated splenomegaly and abdominal discomfort.
• Mild to moderate anemia (8-12 g/dL)
• Normochromic/normocytic
• Retic count is 4-8% (slightly increased)
• Electrophoresis
• Most hemoglobin is HbC
• no HbA present
• May or may not have increase in Hb F.
• Numerous target cells, few microspherocytes, schistocytes, and folded cells
• Hexagonal or rod-shaped crystals may be seen ("bar of gold").
• Usually intracellular.
• Elongated with blunt ends and parallel sides.

Haemoglobin C Trait (AC)

• No symptoms – no anemia.
• Target cells are a frequent finding.
• A splenectomy may be beneficial for symptomatic CC homozygous persons.
• AC heterozygous persons are usually asymptomatic, so no treatment is required.

Hemoglobin E Disease

• Lysine is substituted for glutamic acid in the 26th position of the beta chain (α2β226Glu-Lys)
• Heterozygous and homozygous forms exist
• Homozygous individuals have mild microcytic anaemia with decreased RBC survival, target cells, and increased osmotic fragility.
• Heterozygous individuals have no symptoms
• Frequently occurs with Beta thalassemia
• No clinical symptoms
• HbE has a decreased affinity for oxygen
• Mild, microcytic, hypochromic hemolytic anemia
• Many target cells
• Electrophoresis shows an E band with normal Hb F and no Hb A
• May protect against malaria.

Unstable Hemoglobin Variants

• Unstable hemoglobins are hemoglobin variants where weaken the binding forces that maintain the structure of the molecule which leads to decrease stability.
• Instability may cause Hb to denature and precipitate in the red cells as Heinz bodies, leading to increased cell rigidity, membrane damage, and RBC hemolysis.
• Results in hemolytic anemia.
• Most cases inherited as autosomal dominant disorders.
• When anemia is present, the degree of hemolysis varies considerably. Most patients have mild compensated anemia with mild reticulocytosis.
• Some have severe, chronic hemolysis with splenomegaly and jaundice
• Hb electrophoresis is usually not very helpful in diagnosis.

Methemoglobinemia (Hb M)

• HbM contains ferric iron (Fe3+), and it cannot carry oxygen, leading to cyanosis.
• There exist five variants of Hb M which result from single amino acid substitution in the globin chain that stabilizes iron in the ferric form.
• There are three causes of methemoglobinemia:
• Methemoglobin reductase system overwhelmed.
• Methemoglobin reductase system deficient.
• Molecule is resistant to methemoglobin reductase.