Thalassemia The thalassemias are a heterogeneous group of inherited disorders caused by mutations that decrease the rate of synthesis of α- or β-globin chains. Quantitative abnormality of globin chain synthesis A diverse collection of molecular defects Inherited as autosomal codominant conditions. As a consequence there is a deficiency of hemoglobin, with additional secondary red cell abnormalities caused by the relative excess of the other unaffected globin chain. Found most frequently in the Mediterranean, Africa, Western and Southeast Asia, India and Burma Distribution parallels that of Plasmodium falciparum

Types  Thalassemia Symbol Greek letter used to designate globin chain:  β-Thal is common in blacks, Greeks, and Italians. β chains are encoded by a single β-globin gene located on chromosome 16. /  (Normal) + (diminished, but some production of globin chain by gene) 0 (No production of globin chain ) Alpha Thalassemia Symbols Greek letter used to designate globin chain:  α-Thal is common in Southeast Asians and in blacks. The α chains are encoded by two α-globin genes, which lie in tandem on chromosome 11 Each chromosome carries 2 genes. Therefore the total complement of  genes in an individual is 4 / - Normal - ( Indicates a gene deletion) -/ -/- --/ --/- --/--

α-Thalassemias The α-thalassemia disorders are characterized by reduced or absent synthesis of α-globin chains. There are normally four α-globin genes. The severity of α-thalassemia varies greatly depending on the number of α-globin genes affected. The anemia stems both from lack of adequate hemoglobin and the effects of excess unpaired non-α chains (β, γ, δ). Thus, in the newborn with α-thalassemia, excess unpaired γ-globin forms γ4-tetramers known as hemoglobin Barts, In adults excess β-globin chains form β4 tetramers known as HbH. Since free β and γ chains are more soluble than free α chains and form fairly stable homotetramers, hemolysis and ineffective erythropoiesis are less severe than in β-thalassemias.

α-Thalassemias - DELETIONS Clinical Nomenclature Genotype Disease Molecular Genetics Hydrops fetalis Electrophoresis shows Hb Barts --/-- Fatal in utero HbH disease Severe hemolytic anemia Hb electrophoresis detects HbH. --/-α Moderately severe anemia Gene deletions spanning one or both α-globin loci α- thalassemia trait Decreased MCV, Hb, and Hct, Increased RBC Normal RDW, serum ferritin,Hb electrophoresis.Tear drop RBCs in PBS --/αα(Asian) or -α/-α(black African Mild anemia with an increased RBC count Silent carrier -α/αα Asymptomatic, normal red cells

Clinical and Genetic Classification of Thalassemia β-Thalassemias - β-globin Mutations Clinical Nomenclature Genotype Disease Molecular Genetics Thalassemia major Homozygous or compound heterozygous β0/β0, β0/ β+, or β+/β+) Severe, requires blood transfusions regularly MUTATIONS Defects in transcription, processing, or translation of mRNA, resulting in (β0) or (β+) β -thalassemia trait β/ β+ or β/β0 Asymptomatic, with mild microcytic anemia, or microcytosis without anemia SAME AS ABOVE β0, in which no β-globin chains are produced; and β+, in which there is reduced (but detectable) β-globin synthesis.

Molecular Pathogenesis : The β-globin gene and some sites at which point mutations giving rise to β-thalassemia have been localized. Mutations that lead to aberrant mRNA processing are the most common cause of β-thalassemia. Mutations in promoter region lead to reduced globin gene transcription. Because some β-globin is synthesized, such alleles are designated β+. Mutations in the coding sequences are associated with serious consequences. Eg: A single-nucleotide change in one of the exons leads to the formation of a termination, or "stop" codon, which interrupts translation of β-globin mRNA & completely prevents the synthesis of β-globin. Such alleles are designated β0 If the mutation alters the normal splice junctions splicing does not occur, and all of the mRNA formed is abnormal. Unspliced mRNA is degraded within the nucleus no β-globin is made. β0 mutations at locations away from the normal intron-exon splice junction create new sites that are substrates for the action of splicing enzymes at abnormal locations-within an intron, Because normal splice sites remain intact, both normal and abnormal splicing occur, and normal β-globin mRNA is decreased but not absent. β+ globin

Pathogenesis of β-thalassemia major 2 Major effects Decreased β globin Red cell hemolysis Inadequate HbA Decreased MCHC Hypochromic microcytic anemia Unpaired α chain forms insoluble aggregates Precipitates within the RBC Membrane damage- Extravascular hemolysis – (splenomegaly) erythroblasts are also damaged – ineffective erythropoeisis Increased iron absorption Iron overload

Lab diagnosis MCV, MCH are reduced Bilirubin level is usually slightly raised Haptoglobin depleted Peripheral blood smears Thalassemia minor Red cells appear small (microcytic), pale (hypochromic), and regular in shape. Presence of extraordinarily hypochromic, often wrinkled and folded cells (leptocytes) containing irregular inclusion bodies of precipitated α-globin chains. Target cells are often seen, a feature that results from the relatively large surface area-to-volume ratio, which leads Hb to collect in a central, dark-red "puddle.“ Thalassemia major microcytosis and hypochromia are much more pronounced, Reticulocytosis. Nucleated red cells (normoblasts) are also seen Bone marrow Striking hyperplasia of erythroid progenitors .The expanded erythropoietic marrow completely fill the intramedullary space of the skeleton, invade the bony cortex, impair bone growth, and produce skeletal deformities. Hemoglobin electrophoresis : Reveals absence or almost complete absence of HbA with almost all the circulating Hb being HbF

Clinical features β-Thal minor (β/β+) (1) Mild anemia is most often due to DNA splicing defects. (2) Severe anemia is due to a nonsense mutation with formation of a stop codon. β-Thal minor (β/β+) (1) Mild microcytic anemia (2) Mild protective effect against falciparum malaria (3) Decreased MCV, Hb, and Hct (4) Increased RBC count (5) Normal RDW, serum ferritin (6) Hb electrophoresis (a) Decreased HbA (2α/2β) β-Thal minor: (b) Increased HbA2 (2α/2δ) and HbF (2α/2γ) (7) There is no treatment. Do not treat with iron; danger of iron overload.

(1) Mild microcytic anemia Thal minor (β/β+) (1) Mild microcytic anemia (2) Mild protective effect against falciparum malaria RBC life span is shorter than normal. (3) Decreased MCV, Hb, and Hct (4) Increased RBC count (5) Normal RDW, serum ferritin (6) Hb electrophoresis (a) Decreased HbA (2α/2β) (b) Increased HbA2 (2α/2δ) and HbF (2α/2γ) (7) There is no treatment. Do not treat with iron; danger of iron overload.

β-Thal major (Cooley's anemia; β0/β0) Severe hemolytic anemia β-Thal major: β0/β0 a .RBCs with α-chain inclusions are removed by macrophages in the spleen - increase in unconjugated bilirubin (jaundice) b. The ineffective erythropoietic precursors consume nutrients and produce growth retardation and a degree of cachexia (2) Extramedullary hematopoiesis : The extramedullary hematopoiesis and the hyperplasia of the mononuclear phagocytes result in prominent splenomegaly, hepatomegaly, and lymphadenopathy (3) Increased RDW and reticulocytes (4) Hb electrophoresis (a) No synthesis of HbA (b) Increase in HbA2 and HbF β-Thal major: no HbA; ↑ HbA2, HbF (5) Long-term transfusion requirement Danger of iron overload (called hemosiderosis)

Microcytic Hypochromic Target cells