Fetal Hemoglobin In Sickle Cell Anemia: A Glass Half Full?

HbF modulates the phenotype of sickle cell anemia by inhibiting deoxyHbS polymerization. HbF is confined to erythrocytes called F-cells that can be detected by FACS when these cells contain sufficient HbF. Measuring the amount of HbF/F-cell is difficult and not clinically available. African-Americans with sickle cell anemia have 2-80% F-cells with an average HbF/F-cell of 6.4±1.6 pg. The distribution of HbF/F-cell is highly individual regardless of HbF level. People with HbS-gene deletion hereditary persistence of HbF (HPFH) have a mean HbF of 30%, and HbF is evenly distributed among their erythrocytes. Polymer is not present in these cells either experimentally or after calculating the HbS polymer fraction at 70% O₂ saturation. Therefore, each cell contains about 10 pg. of HbF. DeoxyHbS polymerization is prevented at physiologic venous and capillary O₂ saturations of 40-70% when HbF/F-cell is 9-12 pgs. We call this the “protective” level of HbF. F-cells need not contain “protective” levels of HbF. Some β-globin gene cluster haplotypes are associated with high HbF. Carriers of these haplotypes can have milder disease. Nevertheless, even patients with high HbF can have frequent painful episodes, acute chest syndrome and osteonecrosis. Patients with HbS-δβ thalassemia have 15 to 25% HbF but are anemic and have vasoocclusive complications, albeit less often than in sickle cell anemia. Hydroxyurea reduces the morbidity and mortality of sickle cell anemia, an effect likely to be mediated by its induction of HbF. Patients treated with hydroxyurea are better and probably live longer, but adults are anemic and rarely asymptomatic. In all these patient groups, HbF is unevenly distributed among erythrocytes. In contrast, people with HbS-HPFH are nearly asymptomatic and not anemic. The failure of HbF to modulate uniformly all complications of sickle cell disease might be related to the heterogeneous concentration of HbF in sickle erythrocytes. HbF is associated with protection from the development of certain disease subphenotypes but has limited prognostic value in individuals. In many cross-sectional studies, high HbF was associated with a reduced rate of acute painful episodes, fewer leg ulcers, less osteonecrosis, less frequent acute chest syndromes and reduced disease severity. HbF had a weak or no clear association with priapism, urine albumin excretion, stroke and silent cerebral infarction, systemic blood pressure and tricuspid regurgitant velocity. Perhaps this is because intravascular hemolysis of cells with little or no HbF causes nitric oxide scavenging, or because these complications are less dependent on HbS polymerization. No study provides information on the concentration of HbF/F-cell other than providing the relatively meaningless calculated mean value. Rather than the total number of F-cells or the concentration of HbF in the hemolysate, HbF/F-cell and the proportion of F-cells that have “protective” HbF is the most critical predictor of the likelihood of some disease subphenotypes. Hypothetical distributions of HbF-cells with different levels of HbF/F-cell can be plotted for different concentrations of HbF. With mean HbF levels of 5%, 10% and 20%, and HbF content per cell of 1.5, 3 and 6 pg., assuming a fixed mean, the variance was changed to show how the distribution of HbF per cell can greatly vary, even if the mean is constant. For example, with 20% HbF, as few as 1% and as many as 24% of cells have “protective” HbF. When HbF is lower, few or no “protected” cells can be present. Due to the heterogeneous concentrations of HbF, HbS can polymerize in some F-cells that have sub-polymer inhibiting concentrations of HbF. Inducing high levels of HbF is one approach to treating sickle cell disease. Inactivating BCL11A, a repressor of γ-globin gene expression, abrogates sickle cell disease in transgenic sickle mice. Their HbF was distributed homogeneously, and their phenotype mimicked HbS-HPFH. If it becomes possible in humans to target BCL11A or its pathway with agents that affect gene transcription, will it result in pancellular HbF? Broadening the distribution of HbF amongst sickle erythrocytes with drugs like hydroxyurea that effect the kinetics of erythropoiesis, coupled with an agent whose primary mechanism of action is to increase the transcription of the γ-globin genes, might be the most fruitful approach to HbF induction therapy and more efficacious than single agent treatment.