Compensation of chain synthesis by the single chain gene in Hb Lepore trait

A fusion gene in man: DNA sequence analysis of the abnormal globin gene of hemoglobin Miyada

An abnormal globin gene from a patient heterozygous for Hemoglobin Miyada was cloned and sequenced. The results indicated that the 5' flanking region and the 5' side of the gene were identical to those of a beta-globin gene and that the 3' side was identical to that of a delta-globin gene. The part of the gene identical to a beta-globin gene shifted to the part identical to the delta-globin gene somewhere in a homologous sequence region between the third nucleotide of the 17th codon and the second nucleotide of the 22nd codon of these two genes. Thus, results of analysis of the nucleotide sequence support the idea that the abnormal globin gene of Hemoglobin Miyada was generated as a fusion gene by unequal crossing over between a beta- and a delta-globin gene.

Globin synthesis in normal human bone marrow

Globin synthesis has been studied by in vitro labelling with radioactive amino acids in 60 normal human bone-marrow samples. Under the conditions routinely used to fractionate alpha and beta chains by chromatography alpha/beta production ratios ranging from 0.5 to 1.0 were obtained, depending on the method of sample treatment. This variation was due entirely to the presence of non-haem proteins derived from white cells which chromagraphy with globin on CM-cellulose. Purification of globin on Sephadex G100 and fractionation of alpha and beta globin chains by a modified chromatographic system resulted in alpha/beta ratios of unity. The relevance of these findings to the study of marrows in which there is unbalanced globin chain production is discussed.

Imbalanced globin chain synthesis in heterozygous beta-thalassemic bone marrow

Globin synthesis was studied in the bone marrow of seven heterozygous beta-thalassemic subjects. We found evidence of significant imbalance of alpha- and beta-chain production particularly at short times of incubation. There was a progressive decrease in alpha/beta-chain production ratio with increasing incubation time which was due to a decreased rate of net alpha-chain production, indicating that a large proportion of the newly synthesized alpha chains are degraded, particularly in bone marrow, within a few minutes of synthesis, leading to relatively low alpha/beta ratios if these are measured solely at incubation times greater than 10 min. The significant degradation of excess alpha chains explains why inclusion body formation and ineffective erythropoiesis, notable in beta-thalassemia homozygotes where there is gross chain imbalance, are not observed to any marked degree in heterozygotes.

Translational control of hemoglobin synthesis in thalassemic bone marrow

Previous studies of beta-thalassemic reticulocytes have implied a decreased amount of functional beta-mRNA but unimpaired translation of the beta-mRNA present. However, the beta/alpha synthetic ratios in beta-thalassemic marrow are higher than those observed in reticulocytes of the same patients. This could imply that marrow cells contain an abnormally functioning beta-mRNA no longer active in reticulocytes. To test the function of mRNA found in marrow, intact cells were incubated with [(35)S]methionine and the relative amounts of nascent alpha- and beta-chains on polysomes of different sizes were measured by tryptic digestion and determination of the specific activities of the respective peptides. Results showed that in normal and beta-thalassemic marrow, as well as in reticulocytes, beta-chain production, though deficient, occurs predominantly on larger polysomes than the production of alpha-chains. In one patient with severe thalassemia and very little production of beta-chains in marrow or reticulocytes, delta-chain synthesis was found predominantly on larger polysomes than alpha-chain synthesis. These results indicate that in beta-thalassemic as well as in nonthalassemic marrow and reticulocytes, each beta- and delta-mRNA initiates protein synthesis at a rate faster than does each alpha-mRNA, and suggest that the beta-mRNA in contact with polyribosomes is normally functioning but quantitatively deficient in beta-thalassemic marrow as well as in reticulocytes. No translational defect was detected in a similar study performed in reticulocytes of a patient with hemoglobin H disease, suggesting a normally functioning mRNA in contact with polyribosomes in this condition as well. In both thalassemias, unbalanced synthesis of alpha- and beta-chains was more pronounced on polysomes than in completed chains. This difference possibly reflects a compensatory delay in translation of the nonthalassemic chain, which is present in excess.

Free alpha-globin pool in human bone marrow

A pool of free alpha-globin chains was found in the bone marrow samples from three controls, two patients with beta-thalassemia trait, three with sickle beta-thalassemia, three with hemoglobin (Hb) Lepore trait, one with alphabeta-thalassemia, four with homozygous beta-thalassemia, and one doubly heterozygous for Hb Lepore and beta-thalassemia. The average percentage of newly synthesized alpha-chains found in the free alpha-globin pool was 6.2% in the controls and 33.0% in the patients heterozygous for thalassemia or Hb Lepore. These controls and patients had balanced beta- and alpha-globin synthesis in the bone marrow. In the homozygous patients and in the one patient doubly heterozygous for thalassemia and Hb Lepore, there was a marked deficit of beta-chain synthesis in the bone marrow and also a large pool of newly synthesized free alpha-chains. The function of this pool of free alpha-chains is not known, but it may be involved in the regulation of globin chain synthesis in normal patients and in the compensatory synthesis of beta-chains that occurs in the bone marrow of patients heterozygous for thalassemia or for Hb Lepore.