Ihe most common hemoglobin variant, known as hemoglobin S (HbS), results from a single point mutation on the beta chain: Valine is substituted for glutamic acid in the sixth position. Hemoglobin S is little problem when it is carrying oxygen (oxyhemoglobin form), but when it is deoxygenated, HbS polymerizes within the red blood cell causing the sickle shape. Sickled red blood cells increase the viscosity of the blood, leading to sludging or obstruction in the microcirculation. Eventually, the cells may become irreversibly sickled. Conditions that increase the amount of sickling include acidosis and increased 2,3-diphosphoglycerate (both cause the oxygen dissociation curve to shift to the right, which favors the formation of the deoxyhemoglobin form), vascular stasis, dehydration, the presence of higher levels of HbS in the cells, and low oxygen tension. Ihe presence of hemoglobin F (HbF) has a protective role; higher HbF levels are associated with less sickling phenomena. Sickled red blood cells are rapidly hemolyzed resulting in a red blood cell survival of 10 to 20 days, compared with the normal red blood cell lifespan of 120 days.
In the US African-American population, about 8 percent carry the sickle cell gene; these persons have HbAS (a 2bS, sickle cell trait). Inherited in an autosomal codominant pattern, sickle cell disease (SCD) results when the patient inherits the sickle gene from both parents. Ihis is hemoglobin SS (a 2S2) and occurs in 0.14 to 0.20 percent of the US African-American population, resulting in about 50,000 patients with SCD. When the sickle cell gene is inherited from one parent and another abnormal beta-chain gene is inherited from the other parent, heterozygous hemoglobinopathies result, such as hemoglobin SC disease and sickle-b thalassemia (see below).
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