Hemoglobinopathies

I. Introduction
These conditions comprise a very large number of genetic biochemical/ physiological entities, most of which are academic curiosities whose major effect on medicine is to add to the surfeit of useless scientific information. However, several of these conditions (e.g., sickle cell anemia, hemoglobin SC disease, and some thalassemias) are common major life-threatening diseases, and some others (e.g., most thalassemias, hemoglobin E disease, and hemoglobin O disease) are conditions that produce clinically noticeable -- if not serious -- effects and can cause the unaware physician a lot of frustration and the hapless patient a lot of expense and inconvenience. We will study a few hemoglobinopathies and thalassemias of special importance. It should be kept in mind, though, that there are literally hundreds of diseases in these categories.
II. Definitions
Hemoglobinopathy: A genetic defect that results in abnormal structure of one of the globin chains of the hemoglobin molecule. Although the suffix "-pathy" would conjure an image of "disease," most of the hemoglobinopathies are not clinically apparent. Others produce asymptomatic abnormal hematologic laboratory findings. A very few produce serious disease. The genetic defect may be due to substitution of one amino acid for another (as with the very common Hb S and Hb C and the great majority of the other abnormal hemoglobins), deletion of a portion of the amino acid sequence (Hb Gun Hill), abnormal hybridization between two chains (Hb Lepore), or abnormal elongation of the globin chain (Hb Constant Spring). The abnormal chain that results may be the chain (Hb GPhiladelphia), chain (Hb S, Hb C), chain (Hb FTexas), or chain (Hb A2Flatbush). These abnormal hemoglobins can have a variety of physiologically significant effects, discussed below in greater depth, but the most severe hemoglobinopathies (Hb S and Hb C diseases) are characterized by hemolysis.
Thalassemia: A genetic defect that results in production of an abnormally low quantity of a given hemoglobin chain or chains. The defect may affect the , , , or chain, or may affect some combination of the , , and chain in the same patient (but never the and chain together). The result is an imbalance in production of globin chains and the production of an inadequate number of red cells. The cells which are produced are hypochromic/microcytic and contain a surfeit of the unaffected chains which cannot stoichiometrically "mate" with the inadequate supply of thalassemic chains. These "bachelor" chains can produce adverse effects on the red cell and lead to destruction of the red cell in the marrow (ineffective erythropoiesis) and in the circulation (hemolysis). Note that these two definitions are not mutually exclusive -- some hemoglobinopathies may also be thalassemias, in that a structurally abnormal hemoglobin (hemoglobinopathy) may also be underproduced (thalassemia). Some, but not all, hemoglobinopathies and thalassemias are hemolytic anemias. These nosologic concepts are summarized by the Venn diagram below.