Alpha thalassaemia in American blacks: a study of a family with five cases of haemoglobin H disease

Studies on the in vitro and in vivo expression of a dysfunctional alpha-globin gene

In a black family with members having alpha-thalassemia and hemoglobin H (HbH) disease, a deletion of an AG dinucleotide at the 3' end of exon 1 near the junction with intron 1 was shown previously to produce a dysfunctional alpha-thalassemia gene with a reading frame-shift and a nonsense codon (Safaya S, Rieder RF: J Biol Chem 263:4328-4332, 1988). We have found that the same mutation is responsible for alpha-thalassemia and HbH disease in a second unrelated black family (Bellevue R, Dosik H, Rieder RF: Br J Haematol 41:193-202, 1979). Despite the loss of two nucleotides from the consensus sequence at the 5' splice donor site of intron 1, studies employing an in vitro plasmid-based expression system indicated that the mutant alpha-globin mRNA was spliced normally and expressed in amounts equal to normal alpha-globin mRNA in COS-7 cells. The correct processing of the mRNA in these studies is probably due to the presence of a tandem repeat of the affected AG dinucleotide. However, in reticulocytes from subjects bearing the mutant gene, we were unable to detect any of the abnormal mRNA. These findings suggest that there is accelerated post-transcriptional loss of mRNA bearing a premature terminator codon.

Dysfunctional alpha-globin genes in hemoglobin H disease in blacks: variation in restriction fragment size permits the detection of the -alpha/-alpha T genotype

Hemoglobin H (HbH) disease is most often due to deletion of three of the four alpha-globin genes (genotype --/--alpha). In black subjects although the -alpha/chromosome is common, the --/haplotype is very rare and few examples of HbH disease have been detected. We have studied three black siblings with HbH by restriction endonuclease mapping of the alpha-like gene complex (5'-zeta-psi zeta-psi alpha 2-psi alpha 1-alpha 2-alpha 1-3') using zeta- and alpha- specific probes. The presence of size differences in the previously described hypervariable region between the zeta and psi zeta genes results in a restriction fragment length polymorphism which permitted the detection of single alpha genes on both number 16 chromosomes in these subjects. Quantitative DNA hybridization by a slot-blot technique confirmed that their genomes contained two alpha-globin genes. The results establish that in these black subjects HbH disease is associated with dysfunctional alpha-globin genes (genotype: -alpha/-alpha T).

Interaction of chromosomes bearing 1, 2 or 3 alpha-globin genes in an American black family with alpha-thalassemia

Utilizing restriction endonuclease mapping and molecular hybridization we have determined the number and arrangement of the alpha-globin genes in members of an American Black family in which alpha-thalassemia is present. In addition to chromosomes bearing 0, 1 or 2 alpha-genes, an unusual chromosome bearing 3 alpha-globin genes was detected in 3 family members. In 2 family members the 3 alpha-globin gene chromosome was present opposite a chromosome containing a single alpha-globin gene; these cases represent the first reports of the alpha alpha alpha/-alpha genotype. The presence of the stigmata of "mild" alpha-thalassemia trait in one of these subjects indicates that the 3 alpha-gene chromosome probably does not direct the synthesis of significantly more alpha-globin chains than does the 2 alpha-gene chromosome.

Soluble and stroma-bound globin chains in mild and severe beta thalassemia

In mild beta thalassemia hemoglobin levels are much higher than in severe beta thalassemia, yet black patients with the mild syndrome are as deficient in beta chain synthesis as severe thalassemia patients. The following studies were undertaken to determine if differences in the precipitation of excess alpha chain distinguishes these two groups of beta thalassemia homozygotes. The percentages of radioactive globin chain bound to erythroid cell stroma were determined in mild and severe thalassemia patients. Only the cells of splenectomized severe patients contained large amounts of stroma-bound globin radioactivity, the percentages in five such patients ranging from 6.7 to 13.9 (mean = 9.9). In four splenectomized patients with mild thalassemia the percentages ranged from 1.0 to 3.5 (mean = 1.9; p less than 0.005). This is the first observed difference in globin chain metabolism between these two groups of patients.

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.