Expression patterns of fetal hemoglobin in sickle cell erythrocytes are both patient- and treatment-specific during childhood

Hydroxyurea Decouples Persistent F-Cell Elevation and Induction of γ-Globin

Mechanisms that control the fetal-to-adult hemoglobin switch are attractive therapeutic targets in sickle cell disease. In this study, we investigated developmental γ-globin silencing in the Townes humanized knock-in mouse model, which harbors a construct containing the human γ-, β^A-, and β^S-globin genes, and examined the utility of this model in evaluation of pharmacologic induction of fetal hemoglobin (HbF). We studied mouse pups on the day of delivery (P0) to 28 days after birth (P28). Regardless of the hemoglobin genotype (SS, AS, or AA), the proportion of F cells in peripheral blood was 100% at P0, declined sharply to 20% at P2, and was virtually undetectable at P14. Developmental γ-globin silencing in Townes mice was complete at P4 in association with significantly increased BCL11A expression in the primary erythropoietic organs of the mouse. Hydroxyurea given at P2 significantly sustained elevated percentages of F cells in mice at P14. However, the percentage of F cells declined at P14 for treatment begun at P4. A lack of augmentation of γ-globin mRNA in erythroid tissues suggests that the apparent increase in HbF in red cells caused by hydroxyurea was not due to sustained or re-activation of γ-globin transcription, but was instead a function of erythropoiesis suppression. Thus, we provide new details of the hemoglobin switch in the Townes murine model that recapitulates postnatal γ- to β-globin switch in humans and identify the myelosuppressive toxicity of hydroxyurea as a superseding factor in interpreting pharmacologic induction of HbF.

Automated Oxygen Gradient Ektacytometry: A Novel Biomarker in Sickle Cell Anemia

Sickle cell anemia (SCA) is a hereditary hemoglobinopathy with a variable phenotype. There is no single biomarker that adequately predicts disease severity and can be used to monitor treatment response in patients in clinical trials and clinical care. The use of clinical outcomes, such as vaso-occlusive crises (VOC), requires long and expensive studies, sometimes with inconclusive results. To address these limitations, there are several biomarkers under study to improve the ability to predict complications and assess treatment response in both clinical and research settings. Oxygen gradient ektacytometry, also called as oxygenscan, is an assay that measures the effects of deoxygenation and reoxygenation on red blood cell (RBC) deformability and is gaining popularity in SCA research, because it captures the dynamic sickling capacity of a patient’s RBCs as they are subjected to an oxygen gradient under steady shear stress. We describe here the oxygenscan methodology and evaluate the correlation between oxygenscan parameters and more well-known biomarkers of SCA such as fetal hemoglobin (HbF), F-cells, and dense red blood cells (DRBCs). Our data indicate that the oxygenscan curve is affected by all these parameters and the result incorporates the effects of %HbF, %F-cells, RBC hydration, and RBC membrane deformability.

Shape oscillations of single blood drops: applications to human blood and sickle cell disease

Sickle cell disease (SCD) is an inherited blood disorder associated with severe anemia, vessel occlusion, poor oxygen transport and organ failure. The presence of stiff and often sickle-shaped red blood cells is the hallmark of SCD and is believed to contribute to impaired blood rheology and organ damage. Most existing measurement techniques of blood and red blood cell physical properties require sample contact and/or large sample volume, which is problematic for pediatric patients. Acoustic levitation allows rheological measurements in a single drop of blood, simultaneously eliminating the need for both contact containment and manipulation of samples. The technique shows that the shape oscillation of blood drops is able to assess blood viscosity in normal and SCD blood and demonstrates an abnormally increased viscosity in SCD when compared with normal controls. Furthermore, the technique is sensitive enough to detect viscosity changes induced by hydroxyurea treatment, and their dependence on the total fetal hemoglobin content of the sample. Thus this technique may hold promise as a monitoring tool for assessing changes in blood rheology in sickle cell and other hematological diseases.

Absolute Reticulocyte Count Acts as a Surrogate for Fetal Hemoglobin in Infants and Children with Sickle Cell Anemia

Hemoglobin switching is largely complete in humans by six months of age. Among infants with sickle cell anemia (HbSS, SCA), reticulocytosis begins early in life as fetal hemoglobin (HbF) is replaced by sickle hemoglobin (HbS). The objective of this study was to determine if absolute reticulocyte count (ARC) is related to HbF levels in a cohort of pediatric SCA patients. A convenience sample of 106 children with SCA between the ages of 1 month and 20 years who were not receiving hydroxyurea or monthly blood transfusions were enrolled in this observational study. Hematologic data, including ARC and HbF levels, were measured at steady state. F-cells were enumerated by flow cytometry. Initial studies compared infants with ARC greater than or equal to 200 K/μL (ARC ≥ 200) based upon the previously reported utility of this threshold as a predictive marker for SCA severity. Mean HbF and F-cell levels were significantly lower in the ARC ≥ 200 group when compared to the ARC < 200 group. Both HbF and F-cell percentages were negatively correlated to ARC in infants and in children between the ages of 1 and 9 years. However, the inverse relationship was lost after the age of 10 years. Overall, decreased expression and distribution of HbF during childhood SCA is well-correlated with increased reticulocyte production and release into the peripheral blood. As such, these data further support the clinical use of reticulocyte enumeration as a disease severity biomarker for childhood sickle cell anemia.

LIN28A expression reduces sickling of cultured human erythrocytes

Induction of fetal hemoglobin (HbF) has therapeutic importance for patients with sickle cell disease (SCD) and the beta-thalassemias. It was recently reported that increased expression of LIN28 proteins or decreased expression of its target let-7 miRNAs enhances HbF levels in cultured primary human erythroblasts from adult healthy donors. Here LIN28A effects were studied further using erythrocytes cultured from peripheral blood progenitor cells of pediatric subjects with SCD. Transgenic expression of LIN28A was accomplished by lentiviral transduction in CD34(+) sickle cells cultivated ex vivo in serum-free medium. LIN28A over-expression (LIN28A-OE) increased HbF, reduced beta (sickle)-globin, and strongly suppressed all members of the let-7 family of miRNAs. LIN28A-OE did not affect erythroblast differentiation or prevent enucleation, but it significantly reduced or ameliorated the sickling morphologies of the enucleated erythrocytes.

Emerging science of hydroxyurea therapy for pediatric sickle cell disease

Hydroxyurea (HU) is the sole approved pharmacological therapy for sickle cell disease (SCD). Higher levels of fetal hemoglobin (HbF) diminish deoxygenated sickle globin polymerization in vitro and clinically reduce the incidence of disease morbidities. Clinical and laboratory effects of HU largely result from induction of HbF expression, though to a highly variable extent. Baseline and HU-induced HbF expression are both inherited complex traits. In children with SCD, baseline HbF remains the best predictor of drug-induced levels, but this accounts for only a portion of the induction. A limited number of validated genetic loci are strongly associated with higher baseline HbF levels in SCD. For induced HbF levels, genetic approaches using candidate single-nucleotide polymorphisms (SNPs) have identified some of these same loci as being also associated with induction. However, SNP associations with induced HbF are only partially independent of baseline levels. Additional approaches to understanding the impact of HU on HbF and its other therapeutic effects on SCD include pharmacokinetic, gene expression-based, and epigenetic analyses in patients and through studies in existing murine models for SCD. Understanding the genetic and other factors underlying the variability in therapeutic effects of HU for pediatric SCD is critical for prospectively predicting good responders and for designing other effective therapies.

Increased reticulocytosis during infancy is associated with increased hospitalizations in sickle cell anemia patients during the first three years of life

Objective

Among older children with sickle cell anemia, leukocyte counts, hemoglobin, and reticulocytosis have previously been suggested as disease severity markers. Here we explored whether these blood parameters may be useful to predict early childhood disease severity when tested in early infancy, defined as postnatal ages 60–180 days.

Study Design

Data from fifty-nine subjects who were followed at Children’s National Medical Center’s Sickle Cell Program for at least three years was retrospectively analyzed. Comparisons were made between white blood cell counts, hemoglobin and reticulocyte levels measured at ages 60–180 days and the clinical course of sickle cell anemia during infancy and childhood.

Results

A majority of subjects had demonstrable anemia with increased reticulocytosis. Only increased absolute reticulocyte levels during early infancy were associated with a significant increase in hospitalization during the first three years of life. Higher absolute reticulocyte counts were also associated with a markedly shorter time to first hospitalizations and a four-fold higher cumulative frequency of clinical manifestations over the first three years of life. No significant increase in white blood cell counts was identified among the infant subjects.

Conclusions

These data suggest that during early infancy, increased reticulocytosis among asymptomatic SCA subjects is associated with increased severity of disease in childhood.

Candidate sequence variants and fetal hemoglobin in children with sickle cell disease treated with hydroxyurea

BACKGROUND

Fetal hemoglobin level is a heritable complex trait that strongly correlates swith the clinical severity of sickle cell disease. Only few genetic loci have been identified as robustly associated with fetal hemoglobin in patients with sickle cell disease, primarily adults. The sole approved pharmacologic therapy for this disease is hydroxyurea, with effects largely attributable to induction of fetal hemoglobin.

METHODOLOGY/PRINCIPAL FINDINGS

In a multi-site observational analysis of children with sickle cell disease, candidate single nucleotide polymorphisms associated with baseline fetal hemoglobin levels in adult sickle cell disease were examined in children at baseline and induced by hydroxyurea therapy. For baseline levels, single marker analysis demonstrated significant association with BCL11A and the beta and epsilon globin loci (HBB and HBE, respectively), with an additive attributable variance from these loci of 23%. Among a subset of children on hydroxyurea, baseline fetal hemoglobin levels explained 33% of the variance in induced levels. The variant in HBE accounted for an additional 13% of the variance in induced levels, while variants in the HBB and BCL11A loci did not contribute beyond baseline levels.

CONCLUSIONS/SIGNIFICANCE

These findings clarify the overlap between baseline and hydroxyurea-induced fetal hemoglobin levels in pediatric disease. Studies assessing influences of specific sequence variants in these and other genetic loci in larger populations and in unusual hydroxyurea responders are needed to further understand the maintenance and therapeutic induction of fetal hemoglobin in pediatric sickle cell disease.