Hemoglobinopathies (PDF)

HB STRUCTURE & H E M O G LO B I N O PAT H I E S Prof. Dr. Maha El Gammal Hemoglobin structure Hemoglobin is the oxygen-carrying protein within RBCs. Hemoglobin is a tetramer consisting of two pairs of globin chains. Heme, a complex of ferrous iron and protoporphyrin, is linked covalent...

HB STRUCTURE & H E M O G LO B I N O PAT H I E S Prof. Dr. Maha El Gammal Hemoglobin structure Hemoglobin is the oxygen-carrying protein within RBCs. Hemoglobin is a tetramer consisting of two pairs of globin chains. Heme, a complex of ferrous iron and protoporphyrin, is linked covalently to each globin monomer and can reversibly bind one oxygen molecule. Hemoglobin production The two main types of globins are the α-globins and the β-globins, which are made in equivalent amount in precursors of RBCs. The β-globin gene cluster is on chromosome 11 and includes an embryonic ε-globin gene, the two fetal γ-globin genes (Aγ and Gγ), and the two adult δ- and β-globin genes. The α-globin gene cluster is on chromosome 16 and includes the embryonic ζ-globin gene and the duplicated α-globin genes (α1 and α2) which are expressed in both fetal and adult life. Both clusters also contain nonfunctional genes (pseudogenes) designated by the prefix ψ.  Normal adult hemoglobin (Hb A) has two α-globins and two β-globins (α2β2), while HbA2 has two α-globins and two δ-globins (α2δ2). There are also distinct embryonic ( Hb Gower-1 (ζ2ɛ2), Hb Gower-2 (α2ɛ2) and Hb Portland-1 (ζ2γ2)), fetal=HbF (α2γ2), and minor adult analogs of the α- globins and β-globins encoded by separate genes. Human haemoglobin production is characterized by two‘switches’ The switch from embryonic to fetal haemoglobin production begins as early as week 5 of gestation and is completed by week 10. At birth, HbF (α2γ2) comprises 60–80% of the total haemoglobin, falling to about 5% at 6months of age and eventually reaching the adult level of 0.5–1.0% at 2 years Disorders of hemoglobin Disorders of hemoglobin can be classified as qualitative or quantitative disorders. 1. Qualitative abnormalities of hemoglobin arise from mutations that change the amino acid sequence of the globin, thereby producing structural and functional changes in hemoglobin. Ex, Sickle cell disease 2. Quantitative hemoglobin disorders result from the decreased and imbalanced production of generally structurally normal globins. Ex. Thalassemias.  B-thalassemia Pathophysiology The defect in β-thalassemia is a reduced or absent production of β-globin chains with a relative excess of α-chains. The decreased β-chain synthesis leads to:-  Impaired production of the α2β2 tetramer of Hb A,  Decreased hemoglobin production, and an  Imbalance in globin chain synthesis. The reduction in Hb A in each of the circulating RBCs results in hypochromic, microcytic RBCs with target cells, a characteristic finding in all forms of β-thalassemia. Aggregates of excess α-chains precipitate and form inclusion bodies, leading to premature destruction of erythroid precursors in the bone marrow (ineffective erythropoiesis)  In more severe forms, circulating RBCs also may contain inclusions, leading to early clearance by the spleen. The precipitated α-globin chains and products of degradation may also induce changes in RBC metabolism and membrane structure, leading to shortened RBC survival. The response to anemia and ineffective erythropoiesis is increased production of erythropoietin leading to erythroid hyperplasia which can produce skeletal abnormalities, splenomegaly, extramedullary masses and osteoporosis.  α-THALASSEMIA Normal individuals have four α - globin genes arranged as linked pairs, α2 and α1, at the tip of each chromosome 16, the normal α genotype being represented as αα/αα. The α thalassaemias can be classified as α0 thalassaemia, in which no α- chains are produced from the linked pair, and α+ thalassaemia, in which production of α - chain from the affected chromosome is reduced. Pathophysiology The pathophysiology of α thalassaemia is different to that of β thalassaemia. A deficiency of α - chains leads to the production of excess γ or β chains, which form Hb Bart’s (γ4) and HbH (β4), respectively. These soluble tetramers do not precipitate extensively in the bone marrow and hence erythropoiesis is more effective than in β thalassaemia. HbH is unstable and precipitates in red cells as they age. The inclusion bodies cause red cell membrane damage and obstruction in the spleen leading to shortened red cell survival.  Hb Bart is soluble and does not precipitate; however, it has a very high oxygen affinity and is unable to deliver oxygen to the tissues. This leads to severe fetal tissue hypoxia, resulting in edema, congestive heart failure, and death. Genotype – phenotype relationship Loss of one functioning α gene (αα/−α) is almost completely silent, with normal or only slightly hypochromic red cells. Loss of two α genes (−−/αα or −α/−α) produces a mild hypochromic microcytic anaemia, the α thalassaemia trait. Homozygotes for α0 thalassaemia ( − − /− −) have a lethal condition with intrauterine haemolytic anaemia called Hb Bart’s hydrops fetalis syndrome.  Clinical features of thalassemias 1- Anemia Individuals with β-thalassemia have presented with variable but often  severe degrees of anemia  expansion of the bone marrow spaces secondary to erythroid hyperplasia eg Frontal bossing  hepatosplenomegaly.  pallor and slight jaundice, Usually, these manifestations are absent or minimally present in patients with thalassemia major if transfusion therapy is initiated early during the first year of life provided that the hemoglobin levels are maintained at 9– 2- Osteoporosis 3- Gallstones 4- Thromboembolic complications 5- Iron Overload Cardiac dysfunction is the main clinical problem that may lead to early death. Endocrine abnormalities, particularly hypogonadism, low growth hormone, hypothyroidism, and diabetes mellitus. 6- chronic skin ulceration 7- Secondary Gout  CLINICAL Types 1- β-Thalassemia minor (trait) is asymptomatic and is characterized by mild microcytic anemia -physicians often mistake the small red blood cells of the person with beta thalassemia minor as a sign of iron- deficiency anemia. 2- β-Thalassemia major (Cooley’s anemia) is characterized by absence of or severe deficiency in β-chain synthesis. Symptoms are usually evident within the first 6-12 months of life. In the absence of adequate RBC transfusions, the infant will experience failure to thrive and a variety of clinical findings. thalassaemic facies ’, and osteopenia, predisposing to fractures. Growth retardation, progressive hepatosplenomegaly, gallstone formation, thromboembolic complications and cardiac disease are common. 3- β-thalassemia intermedia is often grouped under the term non-transfusion dependent thalassemia (NTDT), which is used to describe patients with moderate anemia who do not need life-long regular transfusions for survival, but need occasional or frequent transfusions in certain clinical settings for short periods of time. These patients exhibit a wide spectrum of clinical findings including hepatosplenomegaly, extramedullary hematopoietic, bone deformities, thrombotic events and gallstones.  Investigations 1- CBC: shows a microcytic, hypochromic anaemia (β- thalassaemia carrier status is often confused with iron deficiency due to reduced MCV and MCH). In the severe forms of thalassaemia, the haemoglobin level ranges from 2-8 g/dL. White blood cell (WBC) count is usually elevated from the haemolytic process. Unless hypersplenism where Platelet count and WBC may be depressed. RBCs show fragments, tear drop cells, together with large and pale target cells. nucleated red cells are present. 2- The reticulocyte count is elevated but less than expected for the degree of anaemia due to ineffective erythropoiesis. 3- Brilliant Cresyl Blue Stain  Incubation with brilliant cresyl blue stain causes Hemoglobin H to precipitate. Results in characteristic appearance of multiple discrete inclusions - golf ball appearance of RBCs. Inclus. 4- Iron profile: Serum iron level is elevated, with saturation as high as 80%. Ferritin is also raised. 5- Erythropoietin levels will be high. 6- A bone marrow aspirate is not essential to make the diagnosis, but if performed shows very marked erythroid hyperplasia 7- liver function tests show elevation of bilirubin, AST and LDH, with a normal ALT. 8- Hb electrophoresis: In thalassemia Electrophoresis usually show an increased amount of HbA2 &HBf -Small amounts of HbA may be present 9- Molecular studies: Determine specific defect at molecular DNA level. 10- Imaging Skeletal surveys show classical changes to the bones but only in patients who are not regularly transfused. Plain skull X-ray shows the classical 'hair on end' appearance.  Management Asymptomatic carriers: require no specific treatment but should be protected from detrimental iron supplementation, which should only be given after confirmation of iron deficiency. Thalassaemia intermedia : – Occasional blood transfusion may be required during periods of rapid growth, infection-associated aplastic and in pregnancy. Thalassaemia major: – Regular transfusion to maintain a haemoglobin level higher than 9.5 g/dL. – Iron chelation to prevent overload syndrome. – If hypersplenism develops, splenectomy may be considered. All families should be offered genetic counselling.  Bone marrow transplantation Bone marrow transplantation is generally seen as the treatment of choice if there is an HLA - identical sibling and it is clear that the child is transfusion dependent. The prognosis for HSCT from an HLA-matched sibling donor is excellent. The success of stem cell transplantation is generally reduced as children get older, iron overload increases and iron-related organ damage increases. 6- Gene therapy: is a future prospect for treatment.