The diagnosis of β-thalassemia major can be strongly suspected on clinical grounds. Hb electrophoresis shows a profound reduction or absence of HbA and increased levels of HbF. The HbA2 level may be normal or increased. Similar but less severe changes are noted in patients affected by β-thalassemia intermedia. Prenatal diagnosis of β-thalassemia is challenging, but can be made in specialized centers by DNA analysis. In fact, thalassemia was the first disease diagnosed by DNA-based tests, opening the way for the field of molecular diagnostics. The diagnosis of β-thalassemia minor is made by Hb electrophoresis, which typically shows a reduced level of HbA (α2β2) and an increased level of HbA2 (α2δ2). HbH disease can be diagnosed by detection of β4 tetramers by electrophoresis.
Red cells are constantly exposed to both endogenous and exogenous oxidants, which are normally inactivated by reduced glutathione (GSH). Abnormalities affecting enzymes responsible for the synthesis of GSH leave red cells vulnerable to oxidative injury and hemolysis. By far the most common of these conditions is glucose-6-phosphate dehydrogenase (G6PD) deficiency. The G6PD gene is on the X chromosome. More than 400 G6PD variants have been identified, but only a few are associated with disease. One is G6PD A− , which is carried by approximately 10% of black males in the United States. G6PD A− has a normal enzymatic activity but a decreased half-life. Because red cells do not synthesize proteins, older G6PD A− red cells become progressively deficient in enzyme activity and GSH. This in turn renders older red cells more sensitive to oxidant damage.
G6PD deficiency is associated with transient episodes of intravascular hemolysis caused by exposure to an environmental factor (usually infectious agents or drugs) that produces oxidant stress. Incriminated drugs include antimalarials (e.g., primaquine), sulfonamides, nitrofurantoin, phenacetin, aspirin (in large doses), and vitamin K derivatives. More commonly episodes of hemolysis are triggered by infection, which induce phagocytes to generate oxidants as part of the host response. These oxidants, such as hydrogen peroxide, are normally sopped up by GSH, which is converted to oxidized GSH in the process. Because regeneration of GSH is impaired in G6PD-deficient cells, oxidants are free to “attack” other red cell components including globin chains. Oxidized hemoglobin denatures and precipitates, forming intracellular inclusions called Heinz bodies, which can damage the red cell membrane so severely that intravascular hemolysis results. Other cells with lesser damage lose their deformability and suffer further injury when splenic phagocytes attempt to “pluck out” the Heinz bodies, creating bite cells (
Fig. 12.6 ). Such cells become trapped on recirculation to the spleen and are destroyed by phagocytes (extravascular hemolysis).
Fig. 12.6 Glucose-6-phosphatedehydrogenasedeficiencyafteroxidantdrugexposure—peripheralbloodsmear.(Inset)Redcellswithprecipitatesofdenaturedglobin(Heinzbodies)shownbysupravitalstaining.Asthesplenicmacrophagespluckouttheseinclusions,“bitecells”similartotheoneinthissmearareproduced.(Courtesy of Dr. Robert W. McKenna, Depart-ment of Pathology, University of Texas Southwestern Medical School, Dallas,Texas.)