F-cell Production Research Articles

Fetal hemoglobin levels in sickle cell disease and normal individuals are partially controlled by an X-linked gene located at Xp22.2

Fetal hemoglobin (Hb F) production in sickle cell (SS) disease and in normal individuals varies over a 20-fold range and is under genetic control. Previous studies suggested that variant Hb F levels might be controlled by genetic loci separate from the beta-globin complex on chromosome 11. Using microscopic radial immunodiffusion and flow cytometric immunofluorescent assays to determine the percentage of F reticulocytes and F cells in SS and nonanemic individuals, we observed that F-cell levels were significantly higher in nonanemic females than males (mean +/- SD, 3.8% +/- 3.2% v 2.7% +/- 2.3%). F-cell production as determined by F reticulocyte levels in SS females was also higher than in SS males (17% +/- 10% v 13% +/- 8%). We tested the hypothesis that F-cell production in both normal and anemic SS individuals was controlled by an X-linked locus with two alleles, high (H) and low (L). Using an algorithm to determine the 99.8% confidence interval of a normal distribution in nonanemic individuals, we estimated that males and females with at least one H allele had greater than 3.3% F cells. Comparisons of male-male or female-female SS sib pairs with discordant F reticulocyte levels distinguished two phenotypes in SS males (L, less than 12%; H, greater than 12%) and three phenotypes in SS females (LL, less than 12%; HL, 12% to 24%, HH greater than 24%). Linkage analysis using polymorphic restriction sites along the X chromosome in eight SS and one AA family localized the F-cell production (FCP) locus to Xp22.2, with a maximum lod score (logarithm of odds of linkage v independent assortment) of 4.6 at a recombination fraction of 0.04.

Cooperative Enhancement of F-Cell Formation in Baboons Treated With Erythropoietin and Hydroxyurea

Chronically anemic baboons on a continuous hydroxyurea regimen were treated with pulsed doses of recombinant human erythropoietin (rHuEpo) to test whether the combination of these two compounds, which individually induce F-cell production, can enhance further F-cell output. A low-F-cell-responding animal under chronic hydroxyurea treatment was given three separate pulses of Epo and responded with F-reticulocyte increments that were similar to the sum increments caused by either hydroxyurea alone or rHuEpo alone. The same results were obtained in a high-F-responding animal similarly treated. These findings suggest that rHuEpo and hydroxyurea can increase F cell numbers in an additive fashion. It is speculated that both compounds act through perturbation of erythroid differentiation kinetics.

Cooperative enhancement of F-cell formation in baboons treated with erythropoietin and hydroxyurea

Chronically anemic baboons on a continuous hydroxyurea regimen were treated with pulsed doses of recombinant human erythropoietin (rHuEpo) to test whether the combination of these two compounds, which individually induce F-cell production, can enhance further F-cell output. A low-F-cell-responding animal under chronic hydroxyurea treatment was given three separate pulses of Epo and responded with F- reticulocyte increments that were similar to the sum increments caused by either hydroxyurea alone or rHuEpo alone. The same results were obtained in a high-F-responding animal similarly treated. These findings suggest that rHuEpo and hydroxyurea can increase F cell numbers in an additive fashion. It is speculated that both compounds act through perturbation of erythroid differentiation kinetics.

Stimulation of F-Cell Production in Patients with Sickle-Cell Anemia Treated with Cytarabine or Hydroxyurea

To investigate the mechanism of pharmacologic stimulation of fetal hemoglobin in sickle-cell anemia (hemoglobin S disease), we treated two patients with homozygous disease with various doses of cytarabine (also known as Ara-C) or hydroxyurea and evaluated the effects of each treatment on F-reticulocyte production and on hemopoiesis. The treatments stimulated F-cell production in a dose-related fashion. Treatments that increased F-cell production also increased the patient's hematocrit and caused only minor, transient decreases in white cells. The main effect on erythropoiesis consisted of cytoreduction of the mature erythron (as assessed by measurements of reticulocytes) or a decrease in the compartment of erythroid progenitors (colony-forming units--erythroid and burst-forming units--erythroid). The reduction phase was followed by reticulocyte regeneration, during which most of the increase in the absolute numbers of F reticulocytes took place. Lower doses of cytarabine or hydroxyurea resulted in smaller waves of reticulocyte regeneration and lesser effects on F-reticulocyte production. These results suggest that the main cause of stimulation of fetal hemoglobin in patients with sickle-cell anemia treated with cell cycle-specific compounds is the erythroid regeneration triggered by the drug treatment.

Two siblings with unusually mild homozygous beta-thalassemia: a didactic example of the effect of a nonallelic modifier gene of the expressivity of a monogenic disorder.

Two adult Black sibs with homozygous beta-thalassemia had severe deficiency of beta-chain production but an unusually mild clinical course and almost normal hematocrit values. Their father had the typical hematological findings of beta-thalassemia trait but the mother, in addition to the beta-thalassemia trait, had elevated F cells (42.2%). Elevated F cells were also present in a hematologically normal sister of the affected providing evidence that a gene for heterocellular hereditary persistence of fetal hemoglobulin (HPFH) was also running in this family. The unusually mild clinical course of the two homozygotes is attributed to their inheritance from their mother of the heterocellular HPFH gene. The gene for heterocellular HPFH is nonallelic to the beta-globin locus and apparently acted as a modifier of the homozygous beta-thalassemia phenotype by facilitating F-cell production and thus diminishing the pathophysiological consequences of the beta-thalassemia defect. The effect of heterocellular HPFH on the expression of homozygous beta-thalassemia or Hb S genotypes is an impressive example of the pathophysiological basis of changes in the expressivity of monogenic disorders due to action of nonallelic modifiers.

Fetal Hb production during acute erythroid expansion. I. Observations in patients with transient erythroblastopenia and post-phlebotomy.

In order to study fetal haemoglobin production during acute erythroid expansion we did sequential measurements of Hb F-containing erythrocytes (F-cells) and of relevant haematological parameters in 10 subjects recovering from eyrthroid aplasia, iron deficiency anaemia or following phlebotomy. An increased production of F-cells was consistently observed during the acute marrow expansion, but there were significant differences in the maximum F-cell response among individuals. These differences could not be explained by differences in the degree of anaemia alone, nor could they be correlated with the level of peak reticulocytosis. Two patients who reached the highest F-cell numbers were probably carriers of heterocellular hereditary persistence of Hb F, suggesting that this gene may play a role in determining the magnitude of F-cell production in anaemic patients. It is speculated that the main reason for the consistently observed increase in F cell production during acute marrow expansion is the premature terminal differentiation of earlier erythroid precursos (burst forming units; BFUe). This proposition is in accord with observations in vitro which suggest that Hb F is expressed en eyrthroid clones derived from BFUne's. It is further proposed that differences in the degree of BFUe recruitment may underly the disparities in Hb F response among individuals subjected to similar anaemic stimuli.

Genetic control of F cells in human adults

Hb F levels were determined on samples from 750 normal blood donors. Six individuals (0.8%) had Hb F levels in excess of 1.1%, the upper end of the continuous distribution. Eight individuals at the upper end and 7 individuals at the lower end of the range were selected for family studies. These studies revealed that the control of Hb F levels in adults, as judged by the more sensitive F-cell technique, has a major genetic component. Structural analysis of the Hb F in several cases demonstrated that both G gamma and A gamma chains were present, but in variable proportions. These results are discussed in light of current concepts of adult F-cell production.

Genetic Control of F Cells in Human Adults

AbstractHb F levels were determined on samples from 750 normal blood donors. Six individuals (0.8 %) had Hb F levels in excess of 1.1 %, the upper end of the continuous distribution. Eight individuals at the upper end and 7 individuals at the lower end of the range were selected for family studies. These studies revealed that the control of Hb F levels in adults, as judged by the more sensitive F-cell technique, has a major genetic component. Structural analysis of the Hb F in several cases demonstrated that both Gγ and Aγ chains were present, but in variable proportions. These results are discussed in light of current concepts of adult F-cell production.

Production of Erythrocytes that Contain Fetal Hemoglobin in Anemia

Serial microscopic immunodiffusion assays of F cells, i.e., erythrocytes that contain fetal hemoglobin (HbF), in four individuals recovering from anemia demonstrate initial increases in the percentage of circulating reticulocytes that contain HbF (F reticulocytes) and subsequent increases in the percentage of mature erythrocytes that contain HbF (F erythrocytes). In one individual responding to therapy for iron-deficiency anemia, the average percentage of F reticulocytes increased from 4.8+/-1.1 to 16.0+/-2.8% (mean+/-SD), while the mean level of F erythrocytes increased from 3.5+/-0.7 to 7.2+/-0.6%. Two normal children with transient erythroblastopenia exhibited F reticulocyte percentages of 71.3+/-6.7 and 41.5+/-1.5%, respectively, when erythropoiesis resumed. With recovery these values fell to finally measured values of 33.7+/-4.7 and 12.6+/-1.1%, respectively. In an adolescent with sickle cell anemia, F-reticulocyte percentages fluctuated between 0.6+/-1.1 and 34.0+/-2.8% and paralleled the rise and fall of total reticulocytes associated with therapy for a nasopharyngeal carcinoma. Such findings suggest that first, the production of F cells and non-F cells are separately regulated. Second, F-cell production is preferentially stimulated during escape from erythropoietic suppression and selectively depressed at the start of suppression. Third, during escape from erythropoietic suppression, F-cell production in vivo resembles that reported for in vitro cultures of erythroid stem cells. Fourth, individuals with sickle cell anemia, like individuals without hemoglobinopathies, can change their relative level of F-cell production.