Hematology 425 Thalassemias

1. Hematology 425 Thalassemias Russ Morrison November 17, 2006

2. Thalassemias • Thalassemias are a diverse group of inherited disorders caused by gene mutations • These gene mutations reduce or completely eliminate the synthesis of one or more of the globin chains of the Hgb tetramer • The homozygous state for the abnormal autosomal gene for beta-globin chain synthesis (Cooley’s anemia) has become known as thalassemia major

3. Thalassemias • The heterozygous state for the abnormal gene for beta-globin chain synthesis is called thalassemia minor • The heterozygous, milder forms of thalassemia are the most frequent genetic defect in humans • The homozygous, more sever forms are capable of causing significant morbidity and mortality

4. Thalassemias • Thalassemia is a group of disorders defined as a condition in which a reduction in the rate of production of one or more of the globin chains leads to • Imbalanced globin chain production • Defective Hgb production • Damage to the RBCs or their precursors by the buildup of the globin chain that is produced in excess

5. Thalassemias • Usually it is the synthesis of either the alpha or beta chains of hemoglobin A (HbA; α2β2) that is impaired • Thalassemias are named according to the chain with reduced or absent globin synthesis

6. Thalassemias – Genetic Control of Hgb Synthesis • The normal Hgb molecule is a tetramer (double dimer) of two alpha-like chains (either α or ζ) with two beta-like chains (either β, γ, δ or ε) • Combinations of these chains produce six normal hemoglobins • Three of the normal Hgbs are embryonic • Gower-1 (ζ2ε2) • Gower-2 (α2ε2) • Portland (ζ2γ2)

7. Thalassemias – Genetic Control of Hgb Synthesis • The other three normal Hgbs are • Fetal (α2γ2) • A (α2 β2) • A2 (α2 δ 2) • By the 10th week of gestation, zeta and epsilon chain production ceases and gamma chain synthesis begins • The gamma chains combine with alpha chains to make HbF, which predominates during fetal life

8. Thalassemias – Genetic Control of Hgb Synthesis • After birth, gamma chain production decreases and beta chains are the predominant chains produced • The transition from gamma chain to beta chain globin production is called the gamma-to-beta switch • HbA is 95-97% of normal adult Hgb, HbA2 is 2-3% and HbF is 2%

9. Thalassemias – Genetic Control of Hgb Synthesis • The alpha and zeta genes are located on the short arm of chromosome 16 • The cluster of beta-like genes is distributed on the short arm of chromosome 11 • The alpha gene loci are duplicated on each chromosome 16 and named α1 and α2 • With this duplication of alpha genes a normal genotype would be αα/αα

10. Thalassemias – Genetic Control of Hgb Synthesis • An individual inherits one each of the five functional genes (β, Gγ, Aγ,δ or ε) on both chromosomes 11 • The genotype for normal beta chain synthesis would be designated as β/β

11. Categories of Thalassemia • Thalassemias are divided into β –thalassemias, which include all of the disorders of reduced globin chains affecting the cluster of genes on C11 – and • α – thalassemias, which involve the α1 and α2 loci on C16 • The β-thalassemias affect mainly the beta chain production, but may also involve delta, gamma (both types) and epsilon chains

12. Categories of Thalassemia • Included in the β-thalassemia group is β0- thalassemia, in which no beta chains are produced from the beta gene locus on one C11 • Additional designations for the main group of thalassemias are included in table 25-2 of the text

13. Thalassemia – Geographic Distribution • Thalassemias are found world-wide, but some geographic regions demonstrate higher concentrations • Beta-thalassemia is more common in Mediterranean regions (southern Italy and Greece) while alpha-thalassemia is more common in Thailand, China, the Philippines and other Asian countries

14. Thalassemia – Geographic Distribution • It has been suggested that the frequency of thalassemia may be associated with selective advantage of protection from malaria • It is theorized that malarial parasites can not acquire sufficient nutrients from digestion of Hgb in thalassemic cells • Alpha- and beta-thalassemic RBCs may bind greater levels of anti-malarial antibodies than other cells leading to greater removal of parasitized RBCs

15. Thalassemia-Pathophysiology • Pathophysiology of the thalassemias is due to the imbalance of globin chain synthesis • In B-thalassemia, imbalanced production of globin chains results in a lack of hemoglobin produced in the erythroid precursors • This, in turn, results in hypochromic, microcytic RBCs • It also results in excess unpaired globin chains, which precipitate in the developing RBCs, causing surface membrane damage in both developing and mature cells

16. Thalassemia-Pathophysiology • This causes ineffective erythropoiesis (cells being destroyed in the marrow) or premature hemolysis of peripheral RBCs through removal by macrophages • Persons are asymptomatic during fetal life and up to 4-6 months of age because they are protected by HbF (α2γ2) • They begin to demonstrate symptoms after the gamma-to-beta switch

17. Thalassemia-Pathophysiology • In α-thalassemia, non-alpha-chain production has different consequences • Because alpha chains are shared by both fetal and adult hemoglobins, all stages of life (fetus through adult) are impacted • In the fetus there is excess gamma-chain production, which produces γ4 tetramers • These tetramers do not precipitate in the BM, but do precipitate in the PB

18. Thalassemia-Pathophysiology • In the PB, the precipitates form RBC inclusion bodies followed by removal of the cells from the circulation by the spleen • A hemolytic process develops with RBCs that are microcytic and hypochromic due to decreased hemoglobin synthesis and incorporation into the RBCs

19. Thalassemia-Genetic Defects • Research has shown that there are many different types of defects at the molecular level that lead to thalassemia • Genetic defects that cause a decrease or lack of production of a particular globin chain are • Single nucleotide (or point) mutation that interferes with one of the critical steps in messenger mRNA production, causing the amount of mRNA to be decreased

20. Thalassemia-Genetic Defects • Base substitutions that alter promoter function RNA processing, or mRNA translation or modify a codon into a “nonsense codon” that leads to premature termination of translation or to the substitution of an incorrect amino acid • Insertion or deletion mutations within the coding region of the mRNA creating “frameshifts” that prevent the synthesis of a complete, normal globin polypeptide

21. Thalassemia-Genetic Defects • large deletion within the alpha- or beta-globin clusters that removes one or more genes or alters the regulation of the remaining genes in the cluster • All of these varied genetic defects or mutations cause a decrease in or lack of synthesis of one globin chain, resulting in a thalassemia syndrome

22. Clinical Syndromes of β-Thalassemia • β-thalassemia is divided into three clinical syndromes: • Β-thalassemia minor (heterozygous), a mild microcytic, hyochromic hemolytic anemia • Β-thalassemia major (homozygous), a severe transfusion-dependent anemia • Β-thalassemia intermedia, with symptoms of severity between the first two

23. Clinical Syndromes of β-Thalassemia • A fourth syndrome designated as a silent carrier has also been described • Many of the mutations cause the beta gene to not be expressed at all (β0 gene) • Others cause a variable decrease in production of beta chain (β+ gene) • β+ genes produce from 10 to 50% of normal beta-chain synthesis

24. Clinical Syndromes of β-Thalassemia • The silent carrier state results in “almost normal” beta-chain production and was recognized through family studies • If a patient is homozygous for this carrier state, serious hemolytic anemia will develop • Other thalassemias may be caused by alterations of the beta cluster genes

25. Thalassemia Major

26. Thalassemia Minor

27. β-Thalassemia - Prognosis • Individuals with thalassemia minor (thalassemia trait) usually have asymptomatic mild anemia. This state does not result in mortality or significant morbidity. • The prognosis of patients with thalassemia major is highly dependent on the patient's adherence to long-term treatment programs, namely the hypertransfusion program and life-long iron chelation. Allogeneic bone marrow transplantation may be curative.

28. α-Thalassemias • In contrast to the beta-globin cluster, in which point mutations are the most common cause of thalassemia, large deletions in the alpha-globin genes are the predominant cause of α-thalassemia • The degree of decreased production of the alpha chain depends on • The specific mutation • The number of alpha genes affected • Whether an α2 or α1 gene is affected

29. α-Thalassemias • The α2 gene is thought to produce approximately 75% of the alpha-globin chains in normal RBCs • Notation for the normal alpha gene haplotype is αα, signifying there are two normal genes (α2 and α1) on one C16 • The normal genotype is αα/αα

30. α-Thalassemias • α-thalassemias may also be divided into a+-thalassemia which have decreased production from the alpha-chain complex and a0-thalassemia in which no alpha-globin is produced • The most common deletions generate one chromosome bearing a single alpha gene and another with two alpha-globin genes

31. α-Thalassemias • Clinical syndromes of α-thalassemia are listed in table 25-5 of the text • Homozygous α-thalassemia (--/--) is incompatible with life and results in the absence of all alpha chain synthesis • The infant is born with hydrops fetalis, which is edema caused by accumulation of serous fluid in the fetal tissues as a result of severe anemia • Infants with this genotype deliver prematurely and are stillborn or die shortly after birth

32. Thalassemias • As discussed in chapter 24, a variant hemoglobin may be inherited along with a thalassemia, as seen in HbC-thalassemia • Diagnosis of thalassemia is made from the RBC morphology, Hgb electrophoresis, Heinz body test and HbA2 and HbF quantitation • Thalassemia must be differentiated from other microcytic, hypochromic anemias, especially iron deficiency anemia and iron studies are an important part of this differentiation