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Pathophysiology

Thalassemia is a hemoglobinopathy that is among the most common inherited disorders of hemoglobin production. The normal adult hemoglobin molecule (Hb A) consists of 2 pairs of chains designated alpha and beta. Normal adult blood also contains ≤ 2.5% Hb A2 (composed of alpha and delta chains) and < 2% hemoglobin F (fetal hemoglobin), which has gamma chains in the place of beta chains Thalassemia results from unbalanced hemoglobin synthesis caused by decreased production of at least one globin polypeptide chain (beta, alpha, gamma, delta).

Alpha-thalassemia

Alpha-thalassemia results from decreased production of alpha-polypeptide chains due to a deletion of one or more alpha genes. People normally have four alpha alleles (two on each of a pair of chromosomes) because the alpha gene is duplicated. Disease classification is based on the number and location of deletions:

  • Alpha + thalassemia: Loss of a single gene on one chromosome (alpha/–)
  • Alpha 0 thalassemia: Loss of both genes on the same chromosome (–/–)

Beta-thalassemia

Beta-thalassemia results from decreased production of beta-polypeptide chains due to either mutations or deletions in the beta globin gene, leading to impaired production of hemoglobin (Hb) A. Mutations or deletions may result in partial loss (beta + allele) or complete loss (beta 0 allele) of beta globin function. There are two beta globin genes, and patients may have heterozygous, homozygous, or compound heterozygous mutations. In addition, patients may be heterozygous or homozygous for abnormalities in 2 different globin genes (eg, beta and delta).

Beta-delta-thalassemia is a less common form of beta-thalassemia in which production of both the delta chain as well as the beta chain is impaired. These mutations may be heterozygous or homozygous.