10 Thalassemia Syndromes & Other Hemoglobinopathies
- Thalassemias: ↓/absent globin chains → imbalanced synthesis, hemolysis & anemia
- α-thal trait: 1-2 gene deletions (HBA1/HBA2), mild microcytic anemia
- β-thal major: transfusion-dependent, Hb <7 g/dL, severe hemolysis
- TDT vs NTDT: TDT requires regular transfusions; NTDT survives w/o
- Iron overload: major TDT complication; chelation therapy essential
- Hemoglobinopathies: structural variants (sickle, HbE, HbC) cause hemolysis
- Novel therapies: luspatercept, lentiviral gene therapy, CRISPR BCL11A editing
10.1 Introduction
Hemoglobin A (HbA) = α₂β₂ dimer w/ heme. Locus control regions (LCRs) regulate gene expression.
Thalassemia = monogenic globin gene disorder; mostly deletions, rarely mutations. Imbalance → ineffective erythropoiesis (IE), hemolysis, anemia.
Hemoglobinopathies = structural globin variants; abnormal Hb → hemolysis, sickling, O₂ affinity changes.
10.2 Classification
- Quantitative: ↓/absent globin (thalassemias)
- Qualitative: structurally abnormal globin (hemoglobinopathies)
10.3 Thalassemia Pathophysiology
| Mechanism | Effect |
|---|---|
| β-globin ↓ | α-chain precipitation in RBC precursors |
| Redox damage | Ineffective erythropoiesis (IE) |
| ↑ Phosphatidylserine | RBC hemolysis |
| IE → ↑ EPO | ↑ erythropoiesis (ineffective) |
| Bone marrow expansion | Skeletal changes, EMH |
| ↑ Iron absorption | Iron overload + transfusions |
10.4 α-Thalassemia
10.4.1 Genetics
Four functional α-globin genes: 2 HBA1 & 2 HBA2 (Chr 16). Deletions > point mutations.
| Genotype | Deletions | Phenotype |
|---|---|---|
| Normal | 0 | Normal |
| Silent carrier | 1 (−α/αα) | Asymptomatic, normal labs |
| α-thal trait | 2 (−−/αα or −α/−α) | Mild microcytic anemia |
| HbH disease | 3 (−−/−α) | Moderate hemolytic anemia |
| Hydrops fetalis | 4 (−−/−−) | Intrauterine death |
10.4.2 Silent Carrier & Trait
Silent carrier (−α/αα): 1 gene deletion. Asymptomatic; labs normal.
α-thal trait (−−/αα or −α/−α): - Mild microcytic anemia, Hb 11-13 g/dL - ↑ RBC count, ↓ MCV, normal/↑ HbA2 - Hb electrophoresis normal - High prevalence: African, Mediterranean, Asian populations - Asymptomatic; no therapy - Distinguish from iron deficiency (Fe studies normal)
10.4.3 HbH Disease
HbH = 3 α-gene deletions (−−/−α): - Moderate hemolytic anemia w/ ↑ RBC indices - β₄ tetramers form Heinz bodies (supravital stain) - Periodic transfusions if severe
10.5 β-Thalassemia
10.5.1 Genetics
>350 mutations in β-globin gene: point mutations, deletions, splice defects, regulatory mutations.
- β0-thalassemia: complete β-globin loss
- β+-thalassemia: ↓ β-globin synthesis
- Severity depends on: mutation type/number, β-globin reduction degree, HbF level
| Genotype | β-Status | Phenotype |
|---|---|---|
| β/β | Normal | Trait (asymptomatic) |
| β0/β0 or β+/β+ | Absent/severe ↓ | Major or intermediate |
| β0/β+ | Mixed | Intermediate |
10.5.2 β-Thalassemia Minor (Trait)
β-thal trait (heterozygous): - Asymptomatic, mild microcytic anemia - Hb 11-13 g/dL, ↑ RBC count, ↓ MCV (60-70 fL) - ↑ HbA2 (3.5-5%), ↑ HbF (mild) - Hb electrophoresis normal - No therapy; distinguish from iron deficiency
10.5.3 β-Thalassemia Intermedia
β-thal intermedia (β+/β+ or β0/β+ w/ HbE): - Moderate-severe hemolytic anemia, Hb 7-10 g/dL - ↑ jaundice, modest splenomegaly - Occasional transfusions needed - IE → bone marrow expansion, skeletal changes
10.5.4 β-Thalassemia Major
β-thal major (β0/β0 or severe β+/β+): - Severe hemolytic anemia, presents 6-12 months (after HbF ↓) - Regular transfusions required for survival - Hb <7 g/dL untransfused - Hepatosplenomegaly (EMH & hemosiderosis) - Skeletal: frontal bossing, maxillary prominence, target cells - Pancytopenia if splenectomy - High ferritin, iron overload (transfusions + GI absorption) - Delayed puberty, growth retardation - HbF variably ↑ (10-90%) - Hb electrophoresis: ↑ HbF & HbA2 - Diagnosis: clinical presentation, family history, genetic testing
10.6 Clinical (Phenotypic) Classification
| Feature | TDT | NTDT |
|---|---|---|
| Regular transfusions | Yes | No/rarely |
| Age of diagnosis | Infancy-childhood | Variable |
| Hb (untransfused) | <7 g/dL | >7 g/dL |
| Splenomegaly | Variable | ↑ risk |
| Iron overload | Transfusions & GI | GI only |
| Growth/development | Often delayed | Variable |
10.6.1 TDT (Transfusion-Dependent Thalassemia)
- Homozygous β0 or β+ (or β-like); e.g., HbE/β0-thal
- Goal: regular transfusions to maintain Hb >12 g/dL
- Prevents severe anemia complications; allows normal growth
Diagnosis: Hb <7.5 g/dL, anemia <2 yrs, ↑ HbA2/HbF, growth retardation, frequent infections
10.6.2 NTDT (Non-Transfusion-Dependent Thalassemia)
- Do NOT require regular transfusions for survival
- Ranges from mild to moderate anemia
- Includes β-thal intermedia, milder β+, some HbE/β-thal
- Phenotype varies w/ genotype
10.7 Diagnosis
10.7.1 Clinical Criteria
- Hb <7.5 g/dL
- Anemia before 2 years
- ↑ HbA2 & HbF
- Growth retardation
- Frequent infections (post-splenectomy)
10.7.2 Laboratory Testing
Hb electrophoresis: ↑ HbA2 (3.5-5.5% trait; >5% major), variable HbF (10-90% major; <5% trait)
High-performance liquid chromatography (HPLC): detects HbA2, HbF, abnormal Hbs
Genetic testing: confirms diagnosis, identifies mutations
10.8 Iron Overload & Chelation
Each RBC unit = 200-250 mg iron. Total body iron ~3000-3500 mg adult. Normal excretion <1 mg/day.
10.8.1 Iron Organ Toxicity
- Cardiac: arrhythmias, dilated cardiomyopathy, conduction abnormalities (leading TDT death)
- Liver: cirrhosis, fibrosis, portal hypertension
- Pancreas: diabetes, ↓ insulin
- Pituitary: hypogonadism, ↓ GH
- Endocrine: hypothyroidism, hypoparathyroidism
10.8.2 Iron Chelators
| Chelator | Route | Dose | Schedule | Monitoring |
|---|---|---|---|---|
| DFO | IV/SC | 20-60 mg/kg/day | 5-7×/week | Ferritin, cardiac MRI (T2*), audio/optho |
| DFX | PO | 20-40 mg/kg/day | Daily | Ferritin, Cr, eGFR, audio/optho |
| DFP | PO | 75-100 mg/kg/day | 3-4 divided | LIC, cardiac T2*, CBC, audio |
NTDT iron load may exceed ferritin estimate (↑ GI absorption). Start chelation if ferritin >1000 ng/mL or iron-related organ dysfunction. MRI: cardiac T2* <20 ms = significant cardiac iron. LIC assessment guides treatment.
10.9 Transfusion & Splenectomy
Episodic/regular transfusions = TDT mainstay. Suppress bone marrow expansion & IE.
Transfusion benefits: ↓ thrombosis, leg ulcers, silent infarcts, EMH masses.
Splenectomy (common NTDT): - ↓ transfusion needs ~25%+ - ↑ infection risk; requires vaccination (pneumococcal, meningococcal, Hib) - Prophylactic penicillin post-op
10.10 Complications of Thalassemia
- Pulmonary hypertension: chronic hemolysis, endothelial dysfunction, hypoxemia
- Thrombosis: ↑ risk post-splenectomy; cerebral/venous infarcts, PE
- Cardiac disease: iron overload → cardiomyopathy, conduction blocks, arrhythmias
- Endocrine dysfunction: iron & chronic disease → growth failure, hypogonadism, hypothyroidism, diabetes
- Liver disease: cirrhosis from iron & chronic infection (HBV, HCV)
- Leg ulcers & pseudotumors: hemolysis & bone disease → vaso-occlusion
10.11 Novel Therapies
10.11.1 Enhancing Effective Erythropoiesis
Luspatercept (activin trap): - Binds activin A & B → ↑ terminal erythroid differentiation - ↑ Hb production in sickle & β-thal w/ HbF - ↓ transfusion burden; FDA approved
Sotatercept: TGF-β pathway target; similar mechanism; Phase 2 data promising
10.11.2 Gene Therapy & Editing (Curative Intent)
Lentiviral vector gene therapy: - Autologous CD34+ transduced w/ normal HBB - Reduced-intensity conditioning required - Sustained HbF, ↓ transfusion needs - FDA approved select TDT; long-term durability unknown
CRISPR gene editing: - Knockout BCL11A (γ-globin repressor) → ↑ HbF - Ex vivo patient HSCs, reinfused - Phase 1/2: ↑ HbF, ↓ transfusions (TDT & NTDT) - Long-term safety profile pending
10.12 Bibliography
Guideline recommendations, clinical trials, & key references available in source materials; extensive literature available on thalassemia management, complications, & novel therapeutic approaches.