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<< Sickle Cell Disease And Other Hemoglobinopathies: Approaches To Emergency Diagnosis And Treatment

Terminology

The hemoglobin molecule contains four heme units and four globin chains. There are four different types of globin chains: alpha, beta, gamma, and delta. Of the four globin chains, two are always alpha, and the other two are either beta (in normal adult hemoglobin—HbA), gamma (in fetal hemoglobin—HbF), or delta (in minor adult hemoglobin—HbA2).1

SCD is a molecular disease. It results from a mutation that substitutes valine for glutamic acid at position 6 of the beta subunit to produce HbS. People who inherit the abnormal gene from both parents have the homozygous SS genotype and the most severe form of SCA. These patients produce no normal hemoglobin A, but instead produce mostly HbS and small amounts of HbF and HbA2.1,6 Those who inherit the gene from only one parent are heterozygous and have sickle cell trait. These patients usually have more than 80% normal hemoglobin, produce insignificant amounts of HbS, and usually remain asymptomatic.7

Hemoglobin C differs from HbS in the substitution of lysine rather than valine in place of glutamic acid at position 6. When it is combined with the HbS gene, it produces HbSC disease.1 SCD may co-exist with various thalassemias. HbSC disease and the sickle-thalassemia variants are clinically less severe than SCA.

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Last Modified: 07/23/2017
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