Alpha-thalassemia

An update on electrophoretic and chromatographic methods in the diagnosis of hemoglobinopathies

This review primarily deals with methods for separations of hemoglobins. An introduction considers electrophoretic methods as well as those involving isoelectric focusing and chromatography. The main advantages or disadvantages of each procedure are discussed after each technical description. The chromatographic methods are mainly limited to those used in clinical biochemistry. The second section treats the main diagnostic problems typically met with in the field of the hemoglobinopathies and deals successively with the diagnosis of hemoglobinopathies in the adult and the newborn. Numerous variants have been described in the adult, and among them Hb-S and Hb-C variants are the most frequent. Unstable or high oxygen affinity variants of hemoglobin are also considered. Finally, a new strategy for diagnosis is proposed. A special section is devoted to the diagnosis of thalassemia syndromes. The prenatal diagnosis of hemoglobinopathies is also discussed in some detail with a view to preventing the birth of homozygous children. This update ends with a chapter on the interest of the assay of hemoglobins A1c in the pathology of diabetes mellitus.

Thalassemia: recent insights into molecular mechanisms

Recent advances in defining the molecular basis for the thalassemia syndromes are discussed. We now realize that the causes of the thalassemia phenotype are diverse and include gene deletions, nuclear RNA processing defects, nonsense mutations, fusion genes, termination codon mutants, and unstable globin chains.

The laboratory evaluation of microcytic red blood cells

Microcytic red blood cell states are common clinical problems in both adult and pediatric age groups. The recent widespread availability of electronic blood cell counters for performing routine blood counts has increased the detection of microcytic red blood cells. Physicians must workup both symptomatic and asymptomatic patients with microcytic red blood cells before they can initiate proper therapy and/or counseling. The purpose of this review is threefold: (1) to discuss the causes of microcytic red blood cells in terms of disorders of decreased heme production vs. disorders of decreased globin production, (2) to review the clinical laboratory tests useful in differentiating microcytic red blood cell states, and (3) to present a practical approach for the laboratory workup of microcytic red blood cells.

Carboxymethyl-cellulose microchromatography for the quantitation of hemoglobin Bart's (gamma 4) and its use in the detection of the alpha-thalassemia conditions

A modification of an existing (micro) CM-cellulose chromatographic procedure is introduced for the quantitation of hemoglobin Bart's (or gamma 4) in blood samples of newborn babies. Normal newborn with four active alpha chain genes (alpha alpha/alpha alpha) have small amounts (average 0.55%) of this abnormal hemoglobin while increased percentages are present in newborn with an alpha-thalassemia-2 heterozygosity (alpha 0 alpha/alpha alpha; average 1.55%) or an alpha-thalassemia-2 homozygosity (alpha 0 alpha/alpha 0 alpha; average 4.65%). The identification of hemoglobin Bart's in normal newborn was made by high-performance liquid chromatography, and the absence of contaminating non-hemoglobin proteins was confirmed by electrophoresis and additional chromatographic experiments. This rapid procedure is useful for the detection and differentiation at time of birth of the different alpha chain deficiencies which are common among various populations in the world.

The molecular basis of alpha-thalassemias: frequent occurrence of dysfunctional alpha loci among non-Asians with Hb H disease

Study of Asians has previously indicated that deletion of alpha-globin structural genes is the predominant lesion in alpha-thalassemias and that Hb H disease occurs when three of four normal alpha loci per cell are deleted. To test the generality of this model, Hb H disease DNAs of both Asian and non-Asian origin were analyzed by restriction endonuclease mapping using the technique of Southern (1975). Whereas in normal DNA, alpha sequences are present in a single Eco Rl fragment of cellular DNA approximately 22.5 kb long, fragments of 22.5, 20 and 2.6 kb were found in various Hb H disease DNAs. The 20 kb Eco Rl fragment alone, in which a single alpha-globin structural locus resides, was found in Asian Hb H disease DNA. This finding is consistent with the deletion model of alpha-thalassemia. In contrast, seven of eight non-Asian Hb H disease DNAs displayed a more complex molecular composition. The fragment patterns observed were 22.5 kb alone, 22.5 plus 2.6 kb, 20 plus 2.6 kb and 20 kb alone. Non-Asian Hb H disease DNAs contained one, two or three alpha loci per cell in contrast to the one locus predicted by the simple deletion model of alpha-thalassemia. The data are best explained by the existence of defective alpha loci in certain individuals with alpha-thalassemia, particularly outside the Asian population. Restriction mapping of the 20 kb Eco Rl fragment found in Asian and some non-Asian Hb H disease DNAs demonstrated a striking similarity in the placement of restriction sites about the single alpha gene compared with sites about the two genes in the 22.5 kb Eco Rl fragment seen in normal DNA. These data are consistent with origin of the 20 kb fragment from the 22.5 kb normal Eco Rl fragment by either unequal crossing-over or a deletion event. The molecular heterogeneity and frequent occurrence of defective alpha loci in non-Asian Hb H disease DNAs described here may explain, in part, the clinical heterogeneity of alpha-thalassemias and the absence of the homozygous deletion state (hydrops fetalis) in non-Asians. Further study of cellular DNA fragments containing the defective alpha loci identified in this work may indicate the types of specific mutations responsible for abnormal globin gene expression and complement similar studies on abnormal beta genes in beta-thalassemias.