Hemoglobinopathies

Classification of hemoglobinopathies

In medical genetics, all hemoglobinopathies are generally divided into two large groups depending on at what stage the normal physiology of the molecule is disrupted.

1. Qualitative (structural) disorders. In this case, the primary structure of the protein itself changes. The mutation leads to the synthesis of abnormal hemoglobin, which loses its biochemical stability and cannot properly perform transport functions. A classic example of such pathology is sickle cell anemia.
2. Quantitative disorders. In such conditions, the primary structure of hemoglobin remains completely normal, but the rate of globin chain synthesis drops sharply. Cells simply do not receive the required amount of protein for adequate gas exchange. A typical representative of this group is thalassemia.

Thalassemia: quantitative deficiency

The essence of thalassemia lies in the reduced rate of production of polypeptide chains of normal adult hemoglobin — HbA. The severity and prognosis of the disease directly depend on the genetic inheritance pattern.

• Homozygous form (also known as major thalassemia or Cooley's anemia). Characterized by a critical drop in normal HbA hemoglobin levels. This is an extremely severe condition: infants with this pathology typically die in the first months after birth.
• Heterozygous form. Proceeds much more mildly. Patients exhibit a moderate decrease in total hemoglobin, changes in the natural shape of erythrocytes, and a shortened lifespan in the bloodstream. The prognosis here is more favorable: patients can survive up to 5–8 years, and in mild cases of the disease, even reach adulthood.

Sickle cell anemia: structural anomaly

This disease arises due to a missense gene mutation. In the gene responsible for encoding the hemoglobin β-chain, a critical error occurs in the sixth triplet. As a result, hydrophilic glutamic acid is replaced by hydrophobic valine.

This molecular shift leads to the synthesis of pathological hemoglobin — HbS. Under conditions of oxygen deficiency (low $O_2$ partial pressure), erythrocytes with such hemoglobin lose their normal biconcave shape and become sickle-shaped.

The level of mutant HbS hemoglobin strictly depends on the patient's genotype:

• In heterozygotes, it ranges from 20% to 45%.
• In homozygotes, it reaches 60–99%.

An interesting epidemiological phenomenon: in regions endemic for malaria (Central Africa, Mediterranean countries, the Middle East, and India), a paradoxically high carriage rate of this mutant gene is observed. The reason lies in natural selection — heterozygous carriers of the HbS gene possess innate resistance to the malaria pathogen.

Molecular pathogenesis: from gene to cell

To understand how a single point mutation in the β-globin gene breaks the entire oxygen transport mechanism, let's examine the process step by step.

1. DNA level (gene). Normally, the DNA triplet appears as GAG (complementary strand — CTC). Upon mutation, a point substitution of the adenine (A) nucleotide for thymine (T) occurs. The triplet turns into the pathological GTG (complementary — CAC).
2. Transcription (mRNA synthesis). The normal messenger RNA codon is GAG. The mutant codon is GUG.
3. Translation (protein assembly). Normal mRNA gives the command to incorporate glutamic acid into the polypeptide chain. Mutant mRNA incorporates valine. The replacement of a hydrophilic amino acid with a hydrophobic one radically alters the spatial structure of the protein.
4. Cellular level. Healthy erythrocytes have a biconcave shape, ideal for passing through capillaries. Mutant cells twist, taking the shape of a crescent (sickle).
5. Physiological outcome. While adequate oxygen transport occurs normally, $O_2$ transport is critically impaired in sickle cell anemia, and massive hemolysis (destruction) of erythrocytes occurs.

Class and exam questions

What else to study on this topic

• Mechanisms of the effect of low $O_2$ partial pressure on the polymerization of mutant HbS.
• Causes of shortened erythrocyte lifespan in heterozygous thalassemia.
• Differential laboratory diagnostics of homozygous and heterozygous forms of hemoglobinopathies.
• Pathogenesis of intravascular hemolysis upon altered erythrocyte morphology.

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Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.