Hemoglobinopathy is a kind of genetic defect that results in abnormal structure of one of the globin chains of the hemoglobin molecule. It is a collection of a number of diseases, including sickle-cell disease and thalassemia. Symptoms vary for the different diseases: in sickle cell disease the red blood cells tend to assume a different shape under anaerobic conditions, leading to organ damage and circulatory problems, while in thalassemia there is ineffective production of red blood cells (ineffective erythropoiesis). more...
Some hemoglobinopathies (and also related diseases like glucose-6-phosphate dehydrogenase deficiency) seem to have given an evolutionary benefit, especially to heterozygotes, in areas where malaria is endemic. Malaria parasites live inside red blood cells, but subtly disturb normal cellular function. In patients predisposed for rapid clearance of red blood cells, this may lead to early destruction of cells infected with the parasite and increased chance of survival for the carrier of the trait.
Despite the malaria link, Caucasians can be affected by hemoglobinopathies (thalassemia occurs in the Mediterranean countries), as can people from South America and India.
Diagnosis The diagnosis of each hemoglobinopathy is best approached using alkaline electophoresis (pH 8.6) and acid electophoresis (pH 6.2) in which is red cell lysate is put into cellulose acetate or agar support medium and placed in an electric field. Each hemoglobin band has a characteristic migration sequence based on mainly size and charge of the hemoglobin-agaropectin complex. Migration generally goes from the anode (-) to the cathode (+). These methods reliably separate Hemoglobin A (alpha2-beta2) from Hemoglobin S (alpha2-betaS2), Hemoglobin C (alpha2-betaC2), and others. Rare hemoglobin variants can be also isolated using these tests in combination with high performace liquid chromatography (HPLC). Other tests which are more esoteric exist such as globin chain electophoresis, isoelectric focusing, and DNA sequencing/amino acid sequencing, with the prior two tests showing greater resolution but still rely on electrophoresis for separation.
Globin chain electrophoreis is a method in which hemoglobin lysate is mixed with hydrochloric acid and acetone, the heme group is removed by repeated washing of the precipitated globin by acetone. The globin chains are dissociated into monomers by urea and then separated on the basis of charge differences by electophoresis at both acid (pH 6.2) and alkaline (pH 8.9) environments. This method is used as an extension to HPLC when both alpha chain variants and beta chain variants are present within the same individual (dual heterozygote).
Isoelectric focusing is an electrophoretic method which utilizes carrier ampholytes (small proteins which carry both charge and pH). These compounds have molecular weights of 300-1000 Daltons. The ampholytes are incorporated into the support medium (agar) and they establish a pH gradient when charged. High voltages are used to separate the ampholytes due to large concentrations within the medium. Each hemoglobin will travel until it's isoelectric point (zero charge) where migration stops. Isoelectric focusing gives better resolution than alkaline and acid electrophoresis and produce sharper bands. The resolution, however, does have a downside in that minor glycosylated hemoglobins and aging hemoglobins (methemoglobin, glycerated hemoglobin) may cause confusion.
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