Absence of beta-globin synthesis and excess of alpha-globin synthesis in homozygous beta-thalassemia

beta (+)-Thalassaemia in the Po river delta region (northern Italy): genotype and beta globin synthesis

Six beta(+)-thalassaemic patients from the Po river delta region have been studied. Using synthetic oligonucleotides as specific hybridisation probes, the beta(+) IVS I mutation (G----A at position 108) was demonstrated. This lesion and the enzyme polymorphism pattern in the subjects examined are the same as have been described for other Mediterranean beta(+)-thalassaemias. Antenatal diagnosis through DNA analysis of beta(+)-thalassaemia is therefore possible. The production of beta globin in a beta(+), homozygote and in a beta (+), beta(0) 39 (nonsense mutation at codon 39) double heterozygote is approximately 20% and 10% respectively of total non-alpha globin synthesis. Despite some overlapping of the results, similar beta globin synthesis levels have been obtained in 43 beta(+)-thalassaemia patients. This suggests that in the Po river delta region the most common thalassaemic genes are beta(0) 39 and beta(+) IVS I.

Regulation of hemoglobin beta-chain synthesis in bone marrow erythroid cells by alpha chains

Synthesis of alpha and beta chains of hemoglobin was studied in vitro in intact reticulocytes and bone marrow cells. The cells were from rabbits having a variant form of hemoglobin in which L-isoleucine is in the alpha but not in the beta chains. This characteristic permitted a selective inhibition of alpha-chain synthesis to be produced by addition to the incubation medium of L-O-methylthreonine, an inhibitor of protein synthesis that is a specific antagonist of L-isoleucine. In studies with reticulocytes, 25 mM L-O-methylthreonine produced a 60-70% inhibition of alpha-chain synthesis, but beta-chain synthesis was unaffected even after incubation times for 4 hr. Because reticulocytes contain a pool of uncombined alpha chains which might have obscured the demonstration of an alpha chain-dependent mechanism for beta-chain synthesis, subsequent studies were done with bone marrow cells. The latter had little or no detectable alpha-chain pool. A substantial inhibition of alpha-chain synthesis by the bone marrow cells was produced by the isoleucine antagonist but was also accompanied by a significantly decreased rate of beta-chain synthesis. These findings suggest that the coordinated synthesis of the complementary alpha- and beta-globin chains of hemoglobin may reflect in part a modifying effect of alpha-chain synthesis on the synthesis of beta chains.

Hemoglobin synthesis in beta-thalassemia: the properties of the free alpha-chains

The decrease in hemoglobin A (HbA, alpha(2)beta(2)) synthesis in the erythroid cells of patients with beta-thalassemia is due to a selective defect in beta-chain synthesis. Since alpha-chains continue to be formed at a normal rate in these cells, this results in a marked relative excess of alpha-chain synthesis over beta- and gamma-chain synthesis. The alpha-chains uncombined with beta- or beta-like-chains (delta, gamma) will be referred to as free alpha-chains. The experiments presented in this paper show that these free alpha-chains are capable of combining with beta-chains to form HbA and are, therefore, structurally normal. Alternatively, in the absence of added beta-chains, alpha-chains aggregates of various sizes are formed. Peripheral blood from patients with beta-thalassemia was incubated with radioactive amino acids and hemolysates were prepared. Column chromatography demonstrates that a majority of the free alpha-chains are not present in HbA. They are strongly bound to carboxymethylcellulose resin at pHs from 7.0 to 10.0, and do not elute with HbA. However, when chemically prepared hemoglobin H (Hbbeta(4)) is added to the fresh hemolysates, the free alpha-chains are readily recovered in the HbA peak. This indicates that the free alpha-chains are able to combine normally with beta-chains to form HbA. Freshly labeled hemolysates were also subjected to Sephadex G-100 chromatography. The free alpha-chains eluted as a broad peak migrating between myoglobin and hemoglobin, consistent with their forming alpha-chain aggregates of various mol wt between 16,000 and 64,000. It is suggested that the chromatographic behavior of the free alpha-chains reported herein simply reflects the chemical properties of normal alpha-chains in the absence of adequate numbers of beta- or gamma-chains. The tendency of these free alpha-chains to aggregate may lead to their intracellular precipitation and the subsequent destruction of the cells containing them.

Inhibition of synthesis of fetal hemoglobin by an isoleucine analogue

Reticulocytes from newborn infants with Rh isoimmune hemolytic disease actively incorporated radioactive amino acids in vitro into hemoglobins F and A. Approximately 50% of the reticulocytes appeared capable of synthesis of both of these hemoglobins within the same cell, as demonstrated by the selective elution technique of Betke and Kleihauer. An isoleucine analogue, L-O-methylthreonine, inhibited the incorporation of a variety of amino acids into hemoglobin F, without significantly affecting the synthesis of hemoglobin A. The inhibition was prevented upon concomitant addition of L-isoleucine to the medium. These observations suggest that an independent biosynthetic apparatus is present in the cell for the synthesis of each of these two hemoglobins. Because isoleucine is present only in the gamma chains of hemoglobin F, the inhibitory effect of the analogue on the synthesis of this hemoglobin must represent a selective effect on the production of gamma chains.