Chapter 15: The Thalassemias

• Each of these disorders results from an inherited defect in the rate of synthesis of one or another of globin chains.

• Resultant imbalance of globin chain production may cause ineffective erythropoiesis, defective hemoglobin production, red cell hemoglobin precipitates, hemolysis, anemia of variable degree and propensity to iron overload.

Genetic Control and Synthesis of Hemoglobin

• Each hemoglobin (Hb) molecule consists of two separate pairs of identical globin chains.

• All normal human Hb molecules found in an adult have one pair of α-chains. The α-chains can combine with β-chains (α₂β₂), δ-chains (α₂δ₂), and γ-chains (α₂γ₂).

• Adult Hb is ~97% Hb A (α₂β₂), ~0.5% Hb F α2γ2, and ~2.5% Hb A₂ (α₂δ₂).

• In fetal life, Hb F (α₂γ₂) predominates. Position 136 of some γ-chains is occupied by glycine and in others by alanine. These are designated ^Gγ and ^Aγ, respectively. At birth Hb F is a mixture of α₂^Gγ₂ and α₂^Aγ₂ in a ratio of 3:1.

• In embryonic life, Hb Gower 1 is composed of dimers of zeta (ζ) and epsilon (ε) globins (ζ₂ε₂). Hb Gower 2 (α₂ε₂) and Hb Portland (ζ₂γ₂) are only present before the 8th week of intrauterine life.

• During fetal life, globin gene expression switches occur from ζ- to α- and from ∈- to γ-chain production, followed by β- and δ-chain production at perinatal life.

Globin Gene Clusters

• α-Gene cluster (chromosome 16) consists of one functional ζ gene and two α genes (α₂ and α₁).

• Exons of the two α-globin genes have identical coding sequences; however, they differ in second intron.

• Production of α₂ mRNA exceeds that of α₁, by factor of 1.5 to 3.

• β-Gene cluster (chromosome 11) consists of one functional ∈ gene, a ^Gγ gene, an ^Aγ gene, a pseudo β gene, a δ gene, and a β gene.

• Flanking regions contain conserved sequences essential for gene expression.

Regulation of Globin Gene Clusters

• Primary transcript is a large mRNA precursor, with both intron and exon sequences, which is extensively processed in the nucleus to yield the final mRNA.

• Expression of the globin genes is regulated by complex control mechanisms.

Developmental Changes in Globin Gene Expression

• β-Globin produced at low levels beginning at 8 to 10 weeks of fetal life increases considerably at about 36 weeks gestation.

• γ-Globin produced at high levels early starts to decline at ~36 weeks.

• At birth, β-globin and γ-globin production are approximately equal.

• By age 1 year, γ-globin production is less than 1% of total non–α-globin production.

• Mechanism of switches is being elucidated and involve BCL11A and leukemia-related factor encoded by the ZBTB7A gene.

• Fetal Hb synthesis may be reactivated in adults in some disease states by a yet unknown mechanism.

• A large number of mutations cause thalassemia (eg, more than 200 for β-thalassemia).

• The molecular basis of the thalassemias is discussed in detail in Chap. 48 in Williams Hematology, 9th ed.

• β-Thalassemias are of two main varieties: (Table 15–1)

  — The two types are β⁰-thalassemia, with total absence of β-chain production and β⁺-thalassemia, with partial deficiency of β-chain production.

  — The hallmark of the common forms of β-thalassemias is elevation of Hb A₂ in heterozygotes.

• δβ-Thalassemias are heterogeneous:

  — In some cases, no δ- or β-chains are produced.

  — In other cases, the non-α chains are fusion δβ-chains: N-terminal residue of δ-chain fused to C-terminal residues of the β-chain. Fusion variants are called Lepore hemoglobins.

  — Levels of Hb F, but not HbA₂, are elevated in heterozygotes.

• Hereditary persistence of fetal Hb (HPFH):

  — HPFH is heterogeneous genetically (deletion and nondeletion forms).

  — It is characterized by persistence of Hb F in adult life.

  — Although it has no clinical significance, it may have mild thalassemic changes.

• α-Thalassemias are usually caused by deletion of one or more of the four α genes (two globin genes per haploid chromosome):

  — If one of the two α-globin loci on chromosome 16 is deleted, the designation α– is used. If both are deleted, the designation αα/– – is used. Thus, a patient with two α locus deletions would be designated α–/α– or αα/– – depending on the arrangement of the deletions on the chromosomes.

  — α-Thalassemias also arise from a variety of other mechanisms, such as an elongated α-chain because of a mutated stop codon (Hb Constant Spring) or missense or nonsense mutations.

Imbalanced Globin Chain Synthesis (The Major Problem)

• Homozygous β-thalassemia (Figure 15–1):

  — β-Globin synthesis is absent or greatly reduced, resulting in hypochromic microcytic red cells.

  — Because excess α-chains are incapable of forming viable Hb tetramers, they precipitate in red cell precursors, resulting in intramedullary destruction of the abnormal erythroid cells (ineffective erythropoiesis) and hemolysis.

  — Clinical manifestations appear after neonatal switch from γ-chain to β-chain production.

• Heterozygous β-thalassemia:

  — Usually only mild hypochromic microcytic anemia with elevated Hb A₂ is apparent.

  — Some are more severe because of poor heme-binding properties and instability, with red cell inclusions containing precipitated β-chains as well as excess α-chains. These are sometimes designated as hyperunstable hemoglobins.

• α-Thalassemias:

  — There is defective α-chain production. Manifestations occur in both fetal and adult life because α-chains are present in both fetal and adult hemoglobin molecules.

  — In the newborn, excess γ-chains become soluble γ₄ homotetramers designated Hb Bart’s.

  — After infancy, as the switch from γ- to β-chains takes place, excess β-chains if sufficiently large become β₄ homotetramer (Hb H).

  — Because both γ₄ and β₄ homotetramers are soluble, they do not precipitate to any significant degree, explaining the less severe degree of ineffective erythropoiesis in α-thalassemias compared to β-thalassemias.

  — However, Hb H is unstable and precipitates readily, forming inclusion bodies.

  — Both Hb Bart’s and Hb H have high oxygen affinity.

  — Defect in Hb synthesis leads to hypochromic, microcytic cells.

Persistent Fetal Hemoglobin Production and Cellular Heterogeneity

• In β⁰-thalassemias, Hb F is the only Hb produced except for small amounts of Hb A₂.

• In thalassemias, as in normal individuals, Hb F is heterogeneously distributed among the red cells.

• Because of elevated Hb F levels in β-thalassemias, red cells have high oxygen affinity.

• The mechanism of persistent γ-chain synthesis in thalassemias is incompletely understood.

Consequence of Compensatory Mechanisms for the Anemia of Thalassemia

• Severe anemia and the high oxygen affinity of Hb F in homozygous β-thalassemia produce severe tissue hypoxia.

• High oxygen affinity of Hb Bart’s and Hb H accentuates hypoxia in severe forms of α-thalassemia.

• Erythropoietin production and consequent expansion of marrow lead to deformities of skull with frequent sinus and ear infections, porous long bones, and pathologic fractures.

• Massive erythropoiesis diverts calories and also leads to hyperuricemia, gout, and folate deficiency.

Splenomegaly; Dilutional Anemia

• Constant exposure of the spleen to red cells of precipitated globin chains leads to “work hypertrophy” of spleen, ultimately leading to splenomegaly.

• The enlarged spleen may sequester red cells and expand plasma volume, exacerbating anemia.

Abnormal Iron Metabolism

• β⁰-Thalassemia homozygotes accumulate large amounts of iron because of increased absorption and red cell transfusions.

• Iron accumulates in endocrine glands, liver and, most importantly, myocardium.

• Consequences are diabetes, hypoparathyroidism, hypogonadism, and death from heart failure.

• The role of hepcidin of erythroferrone in the abnormal regulation of iron absorption in thalassemias is discussed in Chap. 9 of this Manual.

Infection

• All forms of severe thalassemia appear to be associated with an increased susceptibility to bacterial infection; the iron overload may be one of the contributing factors. See Chap. 9 of this Manual.

Coagulation Defects

• Patients with thrombocytosis after splenectomy may develop progressive pulmonary hypertension with platelet aggregation in the pulmonary circulation.

Clinical Heterogeneity

• Most manifestations of β-thalassemia are related to excess α-chains.

• Degree of globin-chain imbalance determines severity.

• Coinheritance of α-thalassemia or of genes for enhanced γ-chain production may reduce the severity of β-thalassemias.

• β-Thalassemias: Mediterranean populations, Middle East, India and Pakistan, Southeast Asia, southern Russia, China

  — Rare in Africa, except Liberia and parts of North Africa

  — Occurs sporadically in all races

• α-Thalassemias: widespread in Africa, Mediterranean populations, Middle East, Southeast Asia

  — Loss of both functional α-globin loci on the same chromosome. This occurs in Mediterranean and Asian populations but is extremely rare in Africa and the Middle East. Thus, Hb Bart’s hydrops syndrome and Hb H disease are largely restricted to Southeast Asia and Mediterranean populations.

• Thalassemic red cells: less likely to be infected with the plasmodial organisms of malaria

β-Thalassemias

• β-Thalassemia major: clinically severe, requiring transfusions

• β-Thalassemia intermedia: milder, later onset, requiring either few or no transfusions but at risk of iron overload

• β-Thalassemia minor: heterozygous carrier, clinically asymptomatic

β-Thalassemia Major

• Homozygous or compound heterozygous state

• Infant well at birth; anemia developing in first few months of life, becoming progressively more severe, and coinciding with switch from γ to β-chains; failure to thrive

• Onset of symptoms after first year of life more typical of β-thalassemia intermedia

• Inadequately transfused child

  — Stunted growth; expanded marrow leads to bossing of skull, expanded maxilla, widened diploë, gross skeletal deformities

  — Grossly enlarged liver and spleen; secondary thrombocytopenia and leukopenia

  — Skin pigmentation; chronic leg ulceration

  — Hypermetabolic state: fever, wasting, hyperuricemia

  — Frequent infections, folate deficiency, spontaneous fractures, dental problems

  — Symptoms of iron loading by time of puberty; poor growth; endocrine problems (diabetes mellitus, adrenal insufficiency); cardiac problems, death by the third decade as a result of cardiac siderosis

• Adequately transfused child

  — Grows and develops normally until effects of iron loading appear by end of first decade

β-Thalassemia Intermedia

• Wide spectrum of disability:

  — Severe forms: later-appearing anemia than β-thalassemia major; usually requires transfusion. There is retarded growth and development, skeletal deformities, and splenomegaly.

  — Milder forms: asymptomatic, transfusion-independent, Hb levels 10 to 12 g/dL.

β-Thalassemia Minor

• Often slight anemia without functional impairment; discovered by blood cell examination

α-Thalassemias

• Interactions of α-thalassemia haplotypes result in four broad phenotypic categories:

  — Normal (αα/αα).

  — Silent carrier (α–/αα).

  — α-Thalassemia trait (α–/α–) or αα/– –); mild hematologic changes, but no clinical abnormality; low mean cell volume (MCV) and low mean cell hemoglobin (MCH). There are varying levels of Hb Bart’s (γ₄) at birth.

  — Hb H disease (– –/α–); hypochromic, severe to moderately severe hemolytic anemia often with marked splenomegaly. Red cells contain precipitates of Hb H(β₄), which is an unstable hemoglobin.

• Hb Bart’s hydrops fetalis syndrome (– –/– –) is incompatible with extrauterine life.

  — Frequent cause of stillbirth in Southeast Asia. If alive at birth, infant dies within hours.

  — Pallor, massive edema, hepatosplenomegaly. Hydrops resembles that of Rh incompatibility.

  — High incidence of maternal toxemia of pregnancy, with enlarged placenta.

  — At autopsy: massive extramedullary hematopoiesis.

β-Thalassemias

(Figure 15–2)

β-Thalassemia Major

• Severe anemia: Hb 2 to 3 g/dL; blood film: marked anisopoikilocytosis, hypochromia, target cells, basophilic stippling, large poikilocytes; nucleated red cells numerous; reticulocytes moderately increased; inclusions of Hb in hypochromic red cells (these can be supravitally stained by methyl violet)

• After splenectomy: more inclusions; large, flat macrocytes; small, deformed microcytes

• Leukocyte and platelet counts normal or slightly elevated

• Marrow: marked erythroid hyperplasia, abnormal erythroblasts with stippling, increased sideroblasts; markedly increased storage iron

• Markedly ineffective erythropoiesis; shortened red cell survival

• Hb: Hb F increased, from less than 10% to greater than 90%; Hb A absent in β⁰-thalassemia. Hb A₂ levels are low, normal, or high; always invariably elevated, however, if expressed as a proportion of Hb A

β-Thalassemia Minor

• Mild anemia: Hb 9 to 11 g/dL

• Microcytic hypochromic red cells: MCV 50 to 70 fL (MCV a valuable screen for thalassemia trait); MCH 20 to 22 pg

• Hb A₂ level: increased to 3.5% to 7%

α-Thalassemias

Hemoglobin Bart’s Hydrops Fetalis Syndrome

• Blood film: severe thalassemic changes; many nucleated red blood cells

• Hb: Hb Bart’s predominates; Hb Portland (ζ₂γ₂) 10% to 20%

Hemoglobin H Disease

• Blood film: hypochromic microcytic red blood cells, increased polychromasia

• Mild reticulocytosis (~5%)

• Hb H inclusions demonstrable in almost all red blood cells in blood incubated with brilliant cresyl blue

α⁰-Thalassemia Trait (αα/– – or α–/α–)

• Similar appearance of blood film and cell counts as in β-thalassemia trait

• 5% to 15% Hb Bart’s at birth, disappears during maturation

• Rare cells with Hb H inclusions can be demonstrated in some cases

Silent Carrier or α⁺-Thalassemia Trait (αα/α–)

• 1% to 2% Hb Bart’s at birth in some but not all cases

• Gene mapping analysis is only certain method of diagnosing α-thalassemia carrier states

• For an approach to the diagnosis of thalassemia syndromes, see Figure 15–3.

• In childhood, hereditary sideroblastic anemias may resemble thalassemia, but marrow examination should permit differentiation (see Chap. 11).

• High fetal Hb levels found in juvenile chronic myelomonocytic leukemia rarely can cause confusion, but examination of the marrow should be definitive (see Chap. 46).

• Diagnosis of the rarer forms of thalassemia is discussed in Chap. 48, of Williams Hematology, 9th ed.

β-Thalassemia Major

General Considerations

• High standard of pediatric care is required with adequate transfusions.

• Early treatment of infections is necessary.

• Folate supplements are warranted.

• Careful attention to respiratory infections and dental care must be taken because of bony deformities of skull.

• Preventive measures of iron overload are essential.

• When iron-loading is present, endocrine replacement therapy may be needed.

Allogeneic Hematopoietic Stem Cell Transplantation

• Very good results are obtained with human leukocyte antigen (HLA)–identical sibling donors if performed early.

• In the absence of risk factors (irregular chelation, hepatomegaly, portal fibrosis), approximately 90% of children have 5-year, event-free survival with a mortality risk of 4%.

• For patients with one or two risk factors, the disease-free survival rate is 82%; with all three risk factors, the disease-free survival rate is 51%.

• No case of hematologic malignancy has been observed after transplantation.

Transfusion

• In children, maintain Hb at 9.5 to 14 g/dL by transfusing red cells every 4 weeks to ensure normal growth and development. Use washed, filtered, or frozen cells to avoid transfusion reactions. Children maintained at high Hb level do not develop hypersplenism.

Iron Chelation

• Rationale: Every child on high-transfusion regimen will develop and die of myocardial siderosis.

• Subcutaneous infusion of deferoxamine, 12 hours, overnight: determine dose to achieve adequate urinary iron excretion.

• Continue nightly infusions of deferoxamine on outpatient basis and monitor by measurements of urinary iron excretion. Ascorbic acid administered orally, 50 to 100 mg/d, increases iron excretion but should be given only after deferoxamine infusion has been started.

Experimental Approaches

• Increase γ-globin synthesis in patients with β-thalassemia or sickle cell anemia using demethylating or cytotoxic agents, or arginine butyrate.

• Somatic gene therapy is discussed in Chap. 29 in Williams Hematology, 9th ed.

α-Thalassemia

Hydrops Fetalis (– –/– –)

• There is no treatment.

• Genetic counseling and prenatal diagnosis are encouraged.

Hb H Disease (α –/– –)

• Avoid “oxidant” drugs.

• Splenectomy may be needed if anemia and splenomegaly are severe.

• For prenatal diagnosis, screen mothers at first prenatal visit; if mother is a thalassemia carrier, screen father. If both are carriers of gene for severe form of thalassemia, offer prenatal diagnostic testing and termination of pregnancy.

• Chorionic villus sampling at 9 to 10 weeks and fetal DNA analysis.

β-Thalassemia

Thalassemia Major

• Cardiac complications have decreased dramatically in patients who are adequately treated by transfusion and iron chelation.

• Hematopoietic stem cell transplantation done early in life with HLA–identical sibling donors can lead to cure.

Thalassemia Intermedia

• Patients may develop iron loading and severe bone disease in 3 to 4 decades.

• There is a high incidence of diabetes mellitus due to iron loading of pancreas.