Hemoglobinopathies

HEMOGLOBINOPATHIES

Hemoglobinopathies are defined as a group of genetic disorders caused by production of a structurally abnormal hemoglobin molecule; synthesis of insufficient quantities of normal hemoglobin; or, rarely, both. Sickle cell anemia (HbS), hemoglobin C disease (HbC), hemoglobin SC disease (HbS + HbC = HbSC), and the thalassemias are representative hemoglobinopathies that can have severe clinical consequences. The first three conditions result from production of hemoglobin with an altered amino acid sequence (qualitative hemoglobinopathy), whereas the thalassemias are caused by decreased production of normal hemoglobin (quantitative hemoglobinopathy).

A. Sickle cell anemia (hemoglobin S disease)

Sickle cell anemia, the most common of the RBC sickling diseases, is a genetic disorder of the blood caused by a single nucleotide substitution in the gene for β-globin. It is the most common inherited blood disorder in the United States, affecting 50,000 Americans. It occurs primarily in the African American population, affecting one of 500 newborn African American infants in the United States. Sickle cell anemia is an autosomal recessive disorder. It occurs in individuals who have inherited two mutant genes (one from each parent) that code for synthesis of the β chains of the globin molecules. [Note: The mutant β-globin chain is designated βS, and the resulting hemoglobin, α₂βS₂, is referred to as HbS.] An infant does not begin showing symptoms of the disease until sufficient HbF has been replaced by HbS so that sickling can occur (see below). Sickle cell anemia is characterized by lifelong episodes of pain (“crises”); chronic hemolytic anemia with associated hyperbilirubinemia; and increased susceptibility to infections, usually beginning in infancy. [Note: The lifetime of a RBC in sickle cell anemia is less than 20 days, compared with 120 days for normal RBC, hence, the anemia.] Other symptoms include acute chest syndrome, stroke, splenic and renal dysfunction, and bone changes due to marrow hyperplasia. Heterozygotes, representing 1 in 12 African Americans, have one normal and one sickle cell gene. The blood cells of such heterozygotes contain both HbS and HbA. These individuals have sickle cell trait. They usually do not show clinical symptoms (but may under conditions of extreme physical exertion with dehydration) and can have a normal life span.

Figure 3.18 Amino acid substitutions in hemoglobin S (HbS) and hemoglobin C (HbC).

1. Amino acid substitution in HbS β chains: A molecule of HbS contains two normal α-globin chains and two mutant β-globin chains (βS), in which glutamate at position six has been replaced with valine (Figure 3.18). Therefore, during electrophoresis at alkaline pH, HbS migrates more slowly toward the anode (positive electrode) than does HbA (Figure 3.19). This altered mobility of HbS is a result of the absence of the negatively charged glutamate residues in the two β chains, thereby rendering HbS less negative than HbA. [Note: Electrophoresis of hemoglobin obtained from lysed RBC is routinely used in the diagnosis of sickle cell trait and sickle cell disease. DNA analysis also is used.]

Figure 3.19 Diagram of hemoglobins (HbA), (HbS), and (HbC) after electrophoresis.

Figure 3.20 Molecular and cellular events leading to sickle cell crisis. HbS = hemoglobin S.

2. Sickling and tissue anoxia: The replacement of the charged glutamate with the nonpolar valine forms a protrusion on the β chain that fits into a complementary site on the β chain of another hemoglobin molecule in the cell (Figure 3.20). At low oxygen tension, deoxyhemoglobin S polymerizes inside the RBC, forming a network of insoluble fibrous polymers that stiffen and distort the cell, producing rigid, misshapen RBC. Such sickled cells frequently block the flow of blood in the narrow capillaries. This interruption in the supply of oxygen leads to localized anoxia (oxygen deprivation) in the tissue, causing pain and eventually death (infarction) of cells in the vicinity of the blockage. The anoxia also leads to an increase in deoxygenated HbS. [Note: The mean diameter of RBC is 7.5 µm, whereas that of the microvasculature is 3–4 µm. Compared to normal RBC, sickled cells have a decreased ability to deform and an increased tendency to adhere to vessel walls and so have difficulty moving through small vessels, thereby causing microvascular occlusion.]

3. Variables that increase sickling: The extent of sickling and, therefore, the severity of disease is enhanced by any variable that increases the proportion of HbS in the deoxy state (that is, reduces the affinity of HbS for O₂). These variables include decreased pO₂, increased pCO₂, decreased pH, dehydration, and an increased concentration of 2,3-BPG in RBC.

4. Treatment: Therapy involves adequate hydration, analgesics, aggressive antibiotic therapy if infection is present, and transfusions in patients at high risk for fatal occlusion of blood vessels. Intermittent transfusions with packed RBC reduce the risk of stroke, but the benefits must be weighed against the complications of transfusion, which include iron overload (hemosiderosis), bloodborne infections, and immunologic complications. Hydroxyurea (hydroxycarbamide), an antitumor drug, is therapeutically useful because it increases circulating levels of HbF, which decreases RBC sickling. This leads to decreased frequency of painful crises and reduces mortality. [Note: The morbidity and mortality associated with sickle cell anemia has led to its inclusion in newborn screening panels to allow prophylactic antibiotic therapy to begin soon after the birth of an affected child.]

5. Possible selective advantage of the heterozygous state: The high frequency of the bS mutation among black Africans, despite its damaging effects in the homozygous state, suggests that a selective advantage exists for heterozygous individuals. For example, heterozygotes for the sickle cell gene are less susceptible to the severe malaria caused by the parasite Plasmodium falciparum. This organism spends an obligatory part of its life cycle in the RBC. One theory is that because these cells in individuals heterozygous for HbS, like those in homozygotes, have a shorter life span than normal, the parasite cannot complete the intracellular stage of its development. This fact may provide a selective advantage to heterozygotes living in regions where malaria is a major cause of death. Figure 3.21 illustrates that in Africa, the geographic distribution of sickle cell anemia is similar to that of malaria.

Figure 3.21 A. Distribution of sickle cell in Africa expressed as a percentage of the population with disease. B. Distribution of malaria in Africa.

B. Hemoglobin C disease

Like HbS, HbC is a hemoglobin variant that has a single amino acid substitution in the sixth position of the β-globin chain (see Figure 3.18). In HbC, however, a lysine is substituted for the glutamate (as compared with a valine substitution in HbS). [Note: This substitution causes HbC to move more slowly toward the anode than HbA or HbS does (see Figure 3.19).] Rare patients homozygous for HbC generally have a relatively mild, chronic hemolytic anemia. These patients do not suffer from infarctive crises, and no specific therapy is required.

C. Hemoglobin SC disease

HbSC disease is another of the RBC sickling diseases. In this disease, some β-globin chains have the sickle cell mutation, whereas other β-globin chains carry the mutation found in HbC disease. [Note: Patients with HbSC disease are doubly heterozygous. They are called compound heterozygotes because both of their β-globin genes are abnormal, although different from each other.] Hemoglobin levels tend to be higher in HbSC disease than in sickle cell anemia and may even be at the low end of the normal range. The clinical course of adults with HbSC anemia differs from that of sickle cell anemia in that symptoms such as painful crises are less frequent and less severe. However, there is significant clinical variability.

D. Methemoglobinemias

Oxidation of the heme iron in hemoglobin to the ferric (Fe³⁺) state forms methemoglobin, which cannot bind O₂. This oxidation may be caused by the action of certain drugs, such as nitrates, or endogenous products such as reactive oxygen species. The oxidation may also result from inherited defects, for example, certain mutations in the α- or β-globin chain promote the formation of methemoglobin (HbM). Additionally, a deficiency of NADH-cytochrome b5 reductase (also called NADH-methemoglobin reductase), the enzyme responsible for the conversion of methemoglobin (Fe³⁺) to hemoglobin (Fe²⁺), leads to the accumulation of HbM. [Note: The RBC of newborns have approximately half the capacity of those of adults to reduce HbM. They are, therefore, particularly susceptible to the effects of HbM-producing compounds.] The methemoglobinemias are characterized by “chocolate cyanosis” (a brownish blue coloration of the skin and mucous membranes and brown-colored blood) as a result of the dark-colored HbM. Symptoms are related to the degree of tissue hypoxia and include anxiety, headache, and dyspnea. In rare cases, coma and death can occur. Treatment is with methylene blue, which is oxidized as Fe+3 is reduced.

E. Thalassemias

The thalassemias are hereditary hemolytic diseases in which an imbalance occurs in the synthesis of globin chains. As a group, they are the most common single gene disorders in humans. Normally, synthesis of the α- and β-globin chains is coordinated, so that each α-globin chain has a β-globin chain partner. This leads to the formation of α₂β₂ (HbA). In the thalassemias, the synthesis of either the α- or the β-globin chain is defective. A thalassemia can be caused by a variety of mutations, including entire gene deletions, or substitutions or deletions of one to many nucleotides in the DNA. [Note: Each thalassemia can be classified as either a disorder in which no globin chains are produced (α^o- or β^o-thalassemia), or one in which some chains are synthesized but at a reduced level (α+- or β+-thalassemia).]

Figure 3.22 A. β-Globin gene mutations in the β-thalassemias. B. Hemoglobin (Hb) tetramers formed in β-thalassemias.

1. β-Thalassemias: In these disorders, synthesis of β-globin chains is decreased or absent, typically as a result of point mutations that affect the production of functional mRNA. However, α-globin chain synthesis is normal. Excess α-globin chains cannot form stable tetramers and so precipitate, causing the premature death of cells initially destined to become mature RBC. Increase in α₂δ₂ (HbA₂) and α₂γ₂ (HbF) also occurs. There are only two copies of the β-globin gene in each cell (one on each chromosome 11). Therefore, individuals with β-globin gene defects have either β-thalassemia trait (β-thalassemia minor) if they have only one defective β-globin gene or β-thalassemia major (Cooley anemia) if both genes are defective (Figure 3.22). Because the β-globin gene is not expressed until late in fetal gestation, the physical manifestations of β-thalassemias appear only several months after birth. Those individuals with β-thalassemia minor make some β chains, and usually do not require specific treatment. However, those infants born with β-thalassemia major are seemingly healthy at birth but become severely anemic, usually during the first or second year of life due to ineffective erythropoiesis. Skeletal changes as a result of extramedullary hematopoiesis also are seen. These patients require regular transfusions of blood. [Note: Although this treatment is lifesaving, the cumulative effect of the transfusions is iron overload (a syndrome known as hemosiderosis). Use of iron chelation therapy has improved morbidity and mortality.] The only curative option available is hematopoietic stem cell transplantation.

2. α-Thalassemias: In these disorders, synthesis of α-globin chains is decreased or absent, typically as a result of deletional mutations. Because each individual’s genome contains four copies of the α-globin gene (two on each chromosome 16), there are several levels of α-globin chain deficiencies (Figure 3.23). If one of the four genes is defective, the individual is termed a silent carrier of α-thalassemia, because no physical manifestations of the disease occur. If two α-globin genes are defective, the individual is designated as having α-thalassemia trait. If three α-globin genes are defective, the individual has hemoglobin H (β₄) disease, a hemolytic anemia of variable severity. If all four α-globin genes are defective, hemoglobin Bart (γ₄) disease with hydrops fetalis and fetal death results, because α-globin chains are required for the synthesis of HbF.

Figure 3.23 A. α-Globin gene deletions in the α-thalassemias. B. Hemoglobin (Hb) tetramers formed in α-thalassemias.