Chapter 18: Hemoglobinopathies Associated with Unstable Hemoglobin

DEFINITION

• The unstable hemoglobins discussed here result from a mutation that changes the amino acid sequence of one of the globin chains, leading to instability and precipitation of the hemoglobin molecule.

• Homotetramers of normal β chains (hemoglobin H) or γ chains (hemoglobin Bart's) are also unstable. These hemoglobins are found in α-thalassemia (see Chap. 16).

ETIOLOGY AND PATHOGENESIS

• The tetrameric hemoglobin molecule has numerous noncovalent forces that maintain the structure of each subunit and bind the subunits to each other.

• Amino acid substitutions or deletions that weaken noncovalent forces, allow hemoglobin to denature and precipitate as insoluble globins, which may attach to the cell membrane, forming Heinz bodies.

• Heinz bodies impair erythrocyte deformability, impeding the ability to negotiate the splenic sinuses; "pitting" of Heinz bodies causes loss of membrane and ultimately destruction of red cells, and a hemolytic anemia.

INHERITANCE

• An autosomal dominant disorder. The patients are heterozygotes and have a combination of hemoglobin A and unstable hemoglobin in their red cells. Homozygous and compound heterozygotes are not observed because they are thought to be lethal.

• Sometimes patients develop an unstable hemoglobin as a de novo mutation. More than 80 percent of patients have a defect in the β globin chain; α-globin defects are less likely to cause a clinical disorder because there are four α-globin genes normally and a mutation in one gene results in a minor proportion of abnormal globin in the cell.

CLINICAL FEATURES

• Hemolysis is usually compensated. Also, a patient with an unstable hemoglobin with high oxygen affinity may have a hemoglobin level in the upper normal range.

• Infection or treatment with oxidant drugs may precipitate hemolytic episodes, making the diagnosis apparent.

• In β-chain mutations, chronic hemolytic anemia becomes evident after neonatal period but during the first year of life as γ chains (fetal hemoglobin) are replaced by mutant β chains.

• Physical findings may include pallor, jaundice, splenomegaly.

• Some patients have dark urine probably from the catabolism of free heme groups or Heinz bodies.

LABORATORY FEATURES

• Hemoglobin concentration may be normal or decreased. The MCV may be decreased because of loss of hemoglobin from denaturation and pitting.

• Blood film may show hypochromia, poikilocytosis, polychromasia, anisocytosis, and basophilic stippling.

• Heinz bodies are commonly found in circulating red cells; after splenectomy they become more abundant.

• Reticulocytosis is often disproportionate to the severity of the anemia, particularly when the abnormal hemoglobin has a high oxygen affinity.

• Diagnosis is confirmed by demonstration of an unstable hemoglobin. This may be done by:

  — Isopropanol precipitation test: a simple screening test that involves the incubation of the hemolysate with a 17 percent of isopropanol; hemolysates containing unstable hemoglobin variants form a precipitate, whereas a normal hemolysate remains clear.

  — Heat denaturation test: cumbersome and usually not used.

  — Heinz body detection: ...