Development of Abnormal Hemoglobins
Abnormal hemoglobins develop through genetic mutations in the globin genes that either alter hemoglobin structure (structural variants) or reduce hemoglobin production (thalassemias), with most following autosomal recessive or autosomal dominant inheritance patterns.
Genetic Mechanisms
Structural Hemoglobin Variants
The most common mechanism involves single amino acid substitutions in the beta, alpha, or gamma globin genes 1. These point mutations create structurally abnormal hemoglobin molecules:
- Sickle Cell Disease (HbS): A single amino acid change in the beta globin gene causes polymerization of deoxygenated hemoglobin, leading to sickle-shaped red blood cells 1
- HbC, HbE, HbD-Punjab, HbO-Arab: Other structural variants resulting from different amino acid substitutions 1
M-Hemoglobins (Methemoglobin Variants)
Autosomal dominant variants in alpha-globin (HBA1, HBA2), beta-globin (HBB), or gamma-globin (HBG1, HBG2) genes cause structural abnormalities where heme iron auto-oxidizes 2. The mechanism is specific:
- Tyrosine substitutes for either proximal histidine (position 87 in α-chain, 92 in β-chain) or distal histidine (position 58 in α-chain, 63 in β-chain)
- This creates an iron-phenolate complex that resists reduction to the divalent state
- At least 13 different HbM variants exist (Boston, Saskatoon, Iwate, Hyde Park, F-M-Osaka, FM-Fort Ripley) 2
Unstable Hemoglobins
Approximately 200 unstable hemoglobins have been identified, with autosomal dominant inheritance 3. These arise from:
- Amino acid substitutions that disrupt the alpha-helix of globin
- Alterations in the steric configuration of the heme pocket by inserting polar amino acids into the hemoglobin molecule's interior
- Deletion of amino acids in the helix 3
The unstable molecules spontaneously oxidize to methemoglobin with subsequent hemichrome formation and precipitation as Heinz bodies 3.
Thalassemias (Quantitative Defects)
Beta-Thalassemia
Mutations in the beta globin gene reduce or eliminate beta-chain production 1:
- β⁰-thalassemia: No hemoglobin A produced
- β⁺-thalassemia: Reduced hemoglobin A production
- Combined forms with HbS create sickle β-thalassemia variants 1
Alpha-Thalassemia
Deletions or mutations affecting one or both alpha-globin genes on chromosome 16 4:
- α⁰-thalassemia: Both linked alpha-globin genes deleted (--/)
- α⁺-thalassemia: Single alpha-globin gene deletion (-α/)
- HbH disease: Most common thalassemia intermedia, typically from double heterozygosity (--/-α) 4
Enzyme Deficiency-Related Abnormalities
Biallelic mutations in the CYB5R3 gene cause NADH cytochrome b5 reductase deficiency, leading to hereditary methemoglobinemia 2:
- Over 80 different disease-causing variants reported
- Type I: Missense variants producing unstable enzyme in red blood cells only
- Type II: More severe systemic enzyme deficiency 2
Clinical Expression Modifiers
Important caveat: Even identical genotypes show variable clinical severity 1, 4. This unpredictability suggests:
- Additional genetic modifiers influence phenotype
- Environmental factors (smoking, oxidant exposure, infections, fever) affect expression 5
- Fetal hemoglobin levels modulate disease severity—symptoms emerge as HbF declines in infancy 1
Inheritance Patterns Summary
- Autosomal recessive: Sickle cell disease, thalassemias, CYB5R3 deficiency 1, 2
- Autosomal dominant: M-hemoglobins, unstable hemoglobins 2, 3
- X-linked: ABCB7 defects causing sideroblastic anemia with ataxia 6
Critical pitfall: Approximately one-third of unstable hemoglobins are spontaneous mutations with no family history 5. Many are neutral substitutions undetectable by routine electrophoresis, requiring specialized instability testing (heat, isopropanol, redox dye exposure) 5.