Laboratory diagnosis of structural hemoglobinopathies and thalassemias by capillary isoelectric focusing

Structural hemoglobinopathies and thalassemias are congenital hemoglobin (Hb) disorders that cause anemia, morbidity, and mortality resulting from abnormal Hb function. Structurally different normal Hb variants include HbA(2), HbF (fetal hemoglobin), and HbA (adult hemoglobin). Each is a protein tetramer consisting of two α-globins, and either two δ-, γ-, or β-globins, respectively. Fetal Hb (α(2)γ(2)) predominates in neonates (60-95% of total Hb), but declines to <1% in older children and adults. HbA(2) (α(2)δ(2)) is a minor constituent with apparently normal function that is not detected in neonates, but increases to about 2-3% of HbA (α(2)β(2)) in older children and adults. Mutations in the genes that regulate the structure and synthesis of α-, β-, γ- and δ-globins produce abnormal and often dysfunctional Hb variants (structural hemoglobinopathy), or cause decreased synthesis of normal Hb variants (thalassemia) (1,2). DNA mutations that cause structural hemoglobinopathies are most commonly diagnosed by indirect assays that identify abnormal gene products (i.e., Hb variants) rather than abnormal genes. Over 600 abnormal structural Hb variants have been reported (3), most of which (95%) differ from normal HbA by replacement of a single amino acid (2). Although some structural mutations are benign, many (50% of β-variants and 20% of α-variants) alter Hb solubility, stability, or oxygen affinity in ways that adversely affect Hb function. In contrast, thalassemia syndromes are caused either by deletions of entire genes or by mutations that affect the production or processing of normal globin mRNAs.