Thalassemias

Thalassemias: Basics & Globin Genes - Chain Gang Genetics

• Quantitative genetic disorders: Reduced/absent synthesis of α or β globin chains, components of hemoglobin (e.g., $HbA (\alpha_2\beta_2)$).
• Causes globin chain imbalance → ineffective erythropoiesis, hemolysis, anemia.
• Globin Gene Details:
  • α-globin cluster: Chromosome 16 (HBA1, HBA2) - 4 genes (αα/αα).
  • β-globin cluster: Chromosome 11 (HBB) - 2 genes (β/β).

⭐ Normal adult Hb (HbA) is two α-chains & two β-chains. Humans: 4 α-globin genes (Chr 16, 2/chromosome) & 2 β-globin genes (Chr 11, 1/chromosome).

Alpha Thalassemias - Alpha‑Numeric Nightmares

• Defect: ↓ α-globin synthesis from gene deletions (HBA1/HBA2, Chr 16). Severity α number of deleted genes (1-4).

• HbH Disease: Excess β-chains → β₄ (HbH).
• Hb Bart's: Excess γ-chains → γ₄ (Hb Bart's) in fetuses.

⭐ Hb Bart's (γ₄), from 4 α-globin gene deletions, causes hydrops fetalis; lethal postnatally.

Beta Thalassemias - Beta Blockage Breakdown

• Defect in β-globin chain synthesis (HBB gene, Chr 11). Excess α-chains precipitate → ineffective erythropoiesis, hemolysis.
• Types:
  • Minor (Trait): Genotype $β/β^+$ or $β/β^0$. Asymptomatic or mild microcytic anemia. ↑ HbA₂ >3.5%.
  • Intermedia: Variable severity, later presentation.
  • Major (Cooley's Anemia): Genotype $β^0/β^0$. Severe anemia by 4-6 months. Hepatosplenomegaly, bone deformities. Lifelong transfusions & iron chelation. ↑↑ HbF.

⭐ The characteristic "crew-cut" or "hair-on-end" appearance on skull X-ray in β-thalassemia major is due to massive expansion of erythroid marrow.

• 📌 Beta = Bone marrow expansion.

Clinical Features & Complications - Systemic Storm

• Severe anemia: Pallor, poor growth, FTT.
• Extramedullary hematopoiesis (EMH):
  • Massive hepatosplenomegaly.
  • Bone marrow expansion: "Chipmunk facies", frontal bossing, fractures.
• Iron overload (transfusions, ↑GIT absorption):
  • Cardiac: Cardiomyopathy (major cause of death).
  • Hepatic: Cirrhosis.
  • Endocrine: Diabetes, hypogonadism.
• Others: Gallstones, infections.

⭐ Iron overload, a consequence of repeated blood transfusions and increased gastrointestinal absorption, is the primary cause of mortality and morbidity in transfusion-dependent thalassemias, particularly affecting the heart, liver, and endocrine organs.

Diagnosis & Management - Decoding & Defending

• Investigations:

  • CBC: Microcytic hypochromic anemia, ↑RBC count (thal trait), ↑RDW.
  • Peripheral Smear: Target cells, anisopoikilocytosis, basophilic stippling.
  • Iron studies: Normal/↑ serum iron & ferritin (crucial to differentiate from IDA).
  • Hb Electrophoresis/HPLC: Definitive.

    ⭐ An HbA2 level greater than 3.5% on hemoglobin electrophoresis or HPLC is a key diagnostic marker for β-thalassemia trait.

    • β-thal major: Markedly ↑HbF (often >90%), ↓/absent HbA.
    • α-thal (HbH disease): HbH (β4) detected.
  • DNA analysis: Confirmatory, carrier screening, prenatal diagnosis.

• Management Principles:

  • Thalassemia Minor/Trait: Usually no treatment; genetic counseling vital.
  • Thalassemia Major & Intermedia (symptomatic):
    • Regular packed RBC transfusions (maintain pre-transfusion Hb 9-10.5 g/dL).
    • Iron chelation therapy (e.g., Deferasirox, Deferoxamine) to prevent hemosiderosis.
    • Folic acid supplementation.
    • Splenectomy: For hypersplenism, ↑transfusion needs.
    • Allogeneic HSCT: Only curative option.

High‑Yield Points - ⚡ Biggest Takeaways

• Thalassemias: Quantitative defects in globin synthesis (α or β chains).
• α-Thalassemia: Caused by gene deletions (chromosome 16). Hb Bart's (γ4) in severe forms (hydrops fetalis).
• HbH disease (β4): Three α-gene deletions; chronic hemolysis.
• β-Thalassemia: Caused by point mutations (chromosome 11). Excess α-chains precipitate.
• β-Thalassemia Major (Cooley's Anemia): Severe anemia, hepatosplenomegaly, "crew-cut" skull X-ray, requires transfusions.
• β-Thalassemia Minor: Often asymptomatic; ↑ HbA2 is characteristic.
• Peripheral smear: Microcytic, hypochromic anemia with target cells and basophilic stippling common findings across types.