Hemoglobinopathies are genetic disorders affecting the structure or production of the hemoglobin molecule.
Hemoglobin molecules consist of polypeptide chains whose chemical structure is genetically controlled. Different hemoglobins, as distinguished by electrophoretic mobility, are alphabetically designated in order of discovery (eg, A, B, C), although the first abnormal hemoglobin to be identified, sickle cell hemoglobin, was designated hemoglobin S.
Structurally different hemoglobins with the same electrophoretic mobility are named for the city or location in which they were discovered (eg, Hb S Memphis, Hb C Harlem). Standard description of a patient’s hemoglobin composition places the hemoglobin of greatest concentration first (eg, AS in sickle cell trait).
The normal adult hemoglobin molecule (Hb A) consists of 2 pairs of chains designated alpha and beta. Normal adult blood also contains ≤ 2.5% hemoglobin A2 (composed of alpha and delta chains) and < 1.4 % hemoglobin F (fetal hemoglobin, composed of alpha and gamma chains—see also Hemoglobinopathies in Pregnancy). Hemoglobin F predominates during gestation and gradually decreases after birth, particularly in the first months of life; its concentration increases in certain disorders of hemoglobin synthesis and in aplastic anemia and myeloproliferative neoplasms.
Some hemoglobinopathies result in anemias that are severe in patients who are homozygous but mild in those who are heterozygous. Some patients are compound heterozygotes for 2 different hemoglobinopathies and have anemia of varying severity.
In the US, important anemias are caused by genetic mutations resulting in sickle cell disease (Hb S disease) or Hb C disease and the thalassemias. Immigration of people of Southeast Asian descent has made Hb E disease common.