Genetics of haemoglobin H

The human alpha-globin gene. The protein products of the duplicated genes are identical

In order to determine whether any heterogeneity exists in the human alpha-globin chain, i.e. whether the products of the duplicated genes are identical, we have determined the total sequence of 14 alpha-globin chains: seven of these were abnormal, while six were normal chains from the same individuals, with one additional sample which consisted of the alpha chains from a normal control. In the individuals heterozygous for an alpha-chain abnormality, the product of a single alpha-gene could be isolated from that of the three others using the differing physicochemical properties of the mutant haemoglobins. In the special situation of a double heterozygosity for an alpha-chain abnormality, the products of the two mutated genes were separated from each other and from the mixture of the products of the two normal genes. They were then investigated independently, this approach increased the precision of our work. During the course of the present investigation, sequence determinations were mainly performed on large fragments of the chains, which were purified exclusively by gel chromatography. In this way mixtures of products of several genes could be studied, thereby overcoming the risk of losing peptides differing slightly in sequence. Such loss may often occur when using ion-exchange procedures to purify small peptides. Our results show the absence of any heterogeneity at the level of the gene products of the duplicated alpha loci. Thus the human alpha-globin chain has to be considered as homogeneous.

Haemoglobin Bart's in Saudi Arabia

The haemoglobin (Hb) patterns of 345 Shiite Saudi Arab cord bloods were examined by alkaline starch-gel electrophoresis. A fast-moving component, identified by structural analysis as Hb Bart's, was found in 52% of cases, the highest incidence of this variant yet recorded. The levels of Hb Bart's ranged from 0.5 to 16% of the total haemoglobin. The relative rates of synthesis of the alpha, beta and gamma-chains, measured by [3H]leucine incorporation, were estimated in 12 newborn Arab infants. There was an excellent correlation between the amount of Hb Bart's and the alpha/non-alpha-globin-chain production ratio. Furthermore there was a significant correlation between the level of Hb Bart's and morphological abnormalities of the red cells and the mean cell haemoglobin (MCH). These findings indicate that elevated levels of Hb Bart's in this population are due to the presence of alpha thalassaemia. The absence of hydrops fetalis and the rarity of Hb-H disease despite the intense inbreeding in this population, points to an alpha-thalassaemia genotype that is, in terms of phenotypic expression, intermediate between the heterozygous state for alpha-thalassaemia I and Hb-H disease. A possible molecular basis for this genotype is suggested.

An unusual hemoglobin anomaly and its relation to alpha-thalassemia and hemoglobin-H disease

A Chinese family with hemoglobin H in the propositus has been reinvestigated. Although the original propositus is now deceased, a sister has the same hematological manifestations. Her hemoglobin, like that of the deceased sister, contains hemoglobins A, H, and Bart's. In addition, however, two minor components have been detected. These minor components appear to have abnormal alpha-chains and are also present in the maternal grandmother, the mother, a maternal aunt, and three other siblings but only in about one-tenth the amount. One of the minor components may be the same as Hb-Thai (25). The father has the characteristics of classical alpha-thalassemia. These results are discussed in relation to current concepts of alpha-thalassemia as they relate to "silent" and "classical" alpha-thalassemia and to possible multiple alpha-chain loci.

Globin chain synthesis in the alpha thalassemia syndromes

Whole blood samples of patients with various forms of alpha thalassemia including hemoglobin H disease, alpha thalassemia trait, and the "silent carrier" state were incubated with leucine-(14)C for definition of relative rates of production of alpha and beta chains in these disorders. The chains were separated by carboxymethyl cellulose chromatography in the presence of 8 M urea and dithiothreitol. Their absorptions at 280 mmu were determined and their radioactivities measured in a liquid scintillation spectrometer. After correction for differences in extinction coefficients, the specific activities of the widely separated alpha and beta peaks were determined. In 11 nonthalassemic individuals, the alpha/beta specific activity ratios were found to be 1.02+/-0.07; in nine patients with alpha thalassemia trait, 0.77+/-0.05; in six patients with hemoglobin H disease, 0.41+/-0.11; and in four "silent carriers," 0.88 with a range of 0.82-0.95. The results show that in peripheral blood, alpha chain production relative to beta chain production is indeed limited in the alpha thalassemia syndromes. Hemoglobin H disease results from doubly heterozygous inheritance of a gene resulting in moderate depression of alpha chain production (alpha thalassemia trait) and a gene resulting in very mild depression of alpha chain production (the "silent carrier" syndrome."