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We cultured human umbilical cord blood erythropoietic precursors in methyl cellulose clonal assay and analyzed the synthetic rates of Hb A and Hb F in individual erythropoietic bursts. Hemoglobin was labeled with 14C-amino acids in culture, separated by slab gel isoelectric focusing techniques, and quantitated by fluorographic methods. Almost all bursts exhibited both Hb A and Hb F in varying ratios. Frequencies of the individual bursts differing in percentage Hb F biosynthesis had normal distributions. Natal erythropoietic precursors appeared to be randomly committed to Hb F synthesis.

The influence of erythropoietin (Ep) and burst-promoting activity (BPA) on the relative rates of synthesis of fetal and adult hemoglobins (HbF and HbA) were investigated in clonal cell culture. In the case of crude Ep, a positive dose-response relationship between the concentration and the relative rate of HbF biosynthesis by circulating erythropoietic precursors was clearly observed. In contrast, increasing the concentration of purified Ep enhanced the synthesis of HbF and HbA equally. BPA from bone marrow conditioned media and T-cell conditioned media enhanced the relative rate of HbF biosynthesis and burst number at limiting serum concentrations (10%-15%). We, therefore, conclude that BPA is important in the modulation of HbF biosynthesis in culture.

Hemoglobin biosynthesis in rabbits with phenylhydrazine-induced anemia has been morphologically studied, comparing late bone marrow with peripheral blood erythroid cells of the same maturing degree. They were analysed through thin sections, as to their organelle type content and behavior regarding iron incorporation. Hemosomes are organelles taken as sites of final hemoglobin biosynthesis, and the ratio between their mean number in blood reticulocyte sections and that in late marrow erythroid cell sections reach about 10: 1. Late marrow erythroid cells contain practically twice the mean number of mitochondria in relation to this mean number in blood reticulocytes. Mitochondria are considered as potential acting entities for hemoglobin biosynthesis, since they participate in hemosome formation. Iron incorporation activity increases more than twice in peripheral blood reticulocytes, as compared to the mean in late marrow erythroid cells. These analytical results suggest that an enhancement of hemoglobin synthesis activity occurs when erythroid cells enter the peripheral blood.

Molecular analysis of normal and abnormal human globin genes and their gene products has recently provided information on the precise genetic events that result in hemoglobinopathies. In the case of structurally abnormal hemoglobins, the following mechanisms can be invoked: single nucleotide base substitutions leading to amino acid replacement or chain termination variants; nucleotide deletions (or additions) leading to deletion and frameshift variants; and nonhomologous crossing over leading to the production of fused globin chains. The molecular basis of the thalassemia syndromes, disorders characterized by absent or decreased synthesis of alpha- or beta-globin chains, is quite heterogeneous. In some cases globin gene deletions have been demonstrated; whereas in others there is probably either a defect in globin gene transcription or a defect in nuclear globin messenger RNA (mRNA) processing, mRNA transport or globin mRNA stability. In one form of beta(0)-thalassemia a nonsense mutation has recently been demonstrated, and other cases are also associated with some as yet undetermined functional abnormality of beta-globin mRNA.