Thalassemia

• Haemoglobinopathy refers to disorders where the production of normal adult hemoglobin is partly or completely suppressed or replaced by a variant hemoglobin.

Thalassemia Distribution

• Thalassemia is present in areas around the Mediterranean, Africa, the Middle East, India and South East Asia.
• Sickle cell anemia has a broadly similar range, present in areas across Africa, India and the Middle East

Thalassemia

• Individual α-like and β-like globin chains are encoded by two distinct gene clusters.
• The α-globin gene cluster is on the short arm of chromosome 16.
• The β-globin gene cluster is on the short arm of chromosome 11.
• Thalassemia causes an excess or absence of alpha (α) or beta (β) chains resulting in an overall decrease in hemoglobin production.
• Thalassemia can cause unusual hemoglobins to form and may induce hemolysis.
• Two major types of thalassemia: alpha (α) and beta (β).
• Alpha (α) thalassemia is caused by a defect in the rate of synthesis of alpha chains, and beta (β) thalassemia is caused by a defect in the rate of synthesis of beta chains.
• The α-globin gene cluster comprises three functional globin genes, the embryonic ζ gene (HBZ), and two fetal/adult α (α1 and α2) genes.
• The β-globin gene cluster contains five functional genes, the embryonic ε gene, two fetal Gγ and Aγ genes, and adult δ and β genes.
• The embryonic Hb includes Hb Portland (ζ2γ2), Hb Gower-1 (ζ2ε2), and Hb Gower-2 (α2ε2).
• Fetal Hb consists of α2γ2.
• Adult Hb A (α2β2) accounts for 95% of the total Hb.
• Hb A2 (α2δ2) constitutes the remaining 5%.
• Normal alpha chain genotype: 2 alpha genes on each chromosome 16, totaling 4 alpha genes.
• Normal beta chain genotype: 1 beta gene on each chromosome 11, totaling 2 beta genes.

Lab Studies for Thalassemia

• Complete Blood Count (CBC) results: HGB, HCT, MCV, MCH, MCHC are decreased.
• Blood Smear shows varying degrees of microcytosis, hypochromia, target cells, anisocytosis, poikilocytosis, and nRBCs.
• Retic Count is increased.
• Chemistry: Ferritin and serum iron are normal to increased, and bilirubin is increased.
• Hemoglobin Electrophoresis plays an important role in diagnosing and differentiating various forms of thalassemias.
• It can differentiate among Hb A, Hb A₂, and Hb F.
• Hemoglobin Electrophoresis can detect presence of abnormal hemoglobins like Hemoglobin Lepore, Hemoglobin Bart's, or Hemoglobin Constant Spring.
• It also aids in detecting combinations of thalassemia and hemoglobinopathies.

Beta Thalassemia

• Beta-thalassemia results from decreased production of beta-polypeptide chains due to mutations or deletions in the beta globin gene, leading to impaired hemoglobin (Hb) production.
• Beta- chain reduction (mainly β- gene mutation).
• Alpha-chain in excess.
• Compensatory increase in γ and δ chain synthesis results in increased levels of Hb F and A2.
• Alpha-chain in excess is unstable and precipitates within the cell, causing damage.
• Macrophages destroy the damaged RBCs in the bone marrow, leading to ineffective erythropoiesis.
• The spleen also removes damaged RBCs, leading to chronic extravascular hemolysis.
• Beta-thalassemia alleles are classified into three categories based on the degree of quantitative reduction in β-globin production: absence of β-globin (β0), reduced β-globin production (β+), and minimally reduced β-globin production (β++, also known as silent).

Types of Beta Thalassemia

• Beta thalassemia minima/ Silent carrier state is the mildest form.
• Beta thalassemia minor is a heterozygous disorder resulting in mild hypochromic, microcytic hemolytic anemia.
• Beta thalassemia intermedia: Severity lies between minor and major.
• Beta thalassemia major is a homozygous disorder resulting in severe transfusion-dependent hemolytic anemia.

Beta Thalassemia Minor

• Also called carrier or trait
• A heterozygous disorder of βº or β+ with one normal beta gene and one mutated beta gene.
• Hb A is still the major haemoglobin.
• Leads to mild hypochromic, microcytic anemia.
• Rarely see hepatomegaly or splenomegaly and normally require no treatment.

Beta Thalassemia Intermedia

• Heterozygous disorder of βº or β+ or homozygous disorder of β+.
• Severity lies between the minor and major, distinguished clinically by transfusion dependence.

Beta Thalassemia Major

• Homozygous disorder of βº or heterozygous of β° or β+.
• Results in severe transfusion-dependent hemolytic anemia caused by ineffective erythropoiesis, which also causes marked expansion of marrow space and skeletal changes.
• Typically detected early in childhood (6 months- 2 years).
• Hb A production is reduced.
• HbA₂ and Hb F production is increased.
• Significant physical findings are present.
• Leads to hypochromic microcytic anaemia.
• Anisocytosis and poikilocytosis, with schistocytes, ovalocytes, and target cells.
• Basophilic stippling from alpha chain precipitation.
• Increased reticulocytes and presence of nucleated red blood cells (nRBC).
• Hb can be as low as 2-4 g/dL.
• Microcytic hypochromic with MCV < 67 fL, and decreased MCH and MCHC.
• Retic count- mild-moderate increase.
• Peripheral blood smear show Anisocytosis and poikilocytosis, basophilic stippling, polychromasia, NRBCs and ↑ RDW.
• Iron and Indirect bilirubin is increased.
• Treatment involves regular transfusions to minimize anemia and suppress ineffective erythropoiesis, and iron-chelating agents to reduce excess iron absorption, as well as splenectomy.
• Untreated individuals die during the 1st or 2nd decade, but hypertransfusion with iron chelation can extend life for ≥ 1 decade.

Alpha Thalassemia

• The α-globin gene cluster comprises three functional globin genes: the embryonic ζ gene (HBZ) and two fetal/adult α genes (α1 and α2).
• There are normally four alpha globulin genes (two from each parent)
• At birth, excess γ-chains are present, and later, β-chains.
• Alpha thalassemia is usually due to deletion of the α-gene.
• Conditions include Bart's Hydrops Fetalis Syndrome.
• Alpha-thalassemia is classified into α0-thalassemia in which both α-globin genes are deleted (--/), and α+-thalassemia, in which one of two α-globin genes is deleted (-α/).

Types of Alpha Thalassemia

• Silent carrier state.
• Alpha+ -thalassemia minor.
• Haemoglobin H Disease.
• Bart's Hydrops Fetalis Syndrome.
• A normal haploid genotype is α / α.
• If one gene is deleted, the haploid phenotype is α thal 2.
• If both genes are deleted, the haploid phenotype is α thal 1.
• Since one gets two genes from each parent, there are four types of α thalassemia.
• Alpha / α thal 2 = silent carrier
• Alpha / α thal 1 or α thal 2/ α thal 2 = α thal trait with mild anemia.
• The combination of α thal 1/ α thal 2 results in hemoglobin H disease (β4= Hb H).
• Hb H has a higher affinity for O2 and precipitates in older cells. Anemia may be chronic to moderate to severe.
• Alpha thal 1/ alpha thal 2 = hydrops fetalis, which is fatal with stillbirth or death within hours of birth.
• Haemoglobin Barts has high affinity for O2, resulting in no O2 delivery to the tissues.

HbH Disease

• Only one alpha gene out of four is functional.
• Results in accumulation of excess unpaired gamma or beta chains.
• The excess chains pair up to form tetrads: Beta forms hemoglobin H (adults) while Gamma forms hemoglobin Bart's (infants).
• Hgb H precipitates within RBCs triggers hemolysis and a high affinity for oxygen reduces oxygen delivery to the tissues.
• RBCs are microcytic, hypochromic with marked poikilocytosis and numerous target cells.
• Incubation with brilliant cresyl blue stain causes Hemoglobin H to precipitate.
• Results in the characteristic appearance of multiple discrete inclusions (golf ball appearance) of RBCs which are smaller than Heinz bodies and are evenly distributed throughout the cell.

Bart's Hydrops Fetalis

• There are no functioning alpha chain genes.
• The predominant hemoglobin is Hemoglobin Bart, along with Hemoglobin Portland and traces of Hemoglobin H which have high oxygen affinity so they cannot carry oxygen to tissues.
• The baby is born with Hydrops Fetalis, which is edema and ascites caused by accumulation of serous fluid in fetal tissues as a result of severe anemia.
• Hepatosplenomegaly and cardiomegaly are present.
• In α0-thalassemia, the two most common deletion forms are (/--SEA, South Asia) and (/--MED, Mediterranean).
• Unequal recombination between two homologous segments that are 3.7 kb apart results in the formation of a chromosome with one a-globin gene (/-α3.7).
• The other mispaired homologous segment, 4.2 kb apart, produces the /-a4.2 allele.
• Delta/beta (δβ) thalassemia results from a deletion in both δ and β genes, increasing γ-chain synthesis and the amount of HbF.
• δβ-thalassemia heterozygotes show characteristics of thalassemia minor clinically.
• δβ-thalassemia homozygotes show characteristics of thalassemia intermedia with mild hypochromic, microcytic anaemia clinically.
• Hereditary persistence of HbF (HPHF) is a benign condition in which significant fetal hemoglobin production continues into adulthood, disregarding the normal shutoff.
• The incorrect expression might range from 10-15% to as high as 100% of the total hemoglobin, and is usually higher in homozygotes than in heterozygotes.
• HPFH is usually caused by mutations in the β- or α-globin gene cluster or the γ promoter gene region.