Individual red blood cell fetal hemoglobin quantification allows to determine protective thresholds in sickle cell disease

Polymerization of the sickle hemoglobin (HbS) is a key determinant of sickle cell disease (SCD), an inherited blood disorder. Fetal hemoglobin (HbF) is a major modulator of the disease severity by both decreasing HbS intracellular concentration and inhibiting its polymerization. However, heterocellular distribution of HbF is common in SCD. For HbS polymerization inhibition, the hypothesis of an "HbF per red blood cell (HbF/RBC) threshold" requires accurate measurement of HbF in individual RBC. To date, HbF detection methods are limited to a qualitative measurement of RBC populations containing HbF - the F cells, which are variable. We developed an accurate method for HbF quantification in individual RBC. A linear association between mean HbF content and mean RBC fluorescence by flow cytometry, using an anti-Human-HbF antibody, was obtained from non-SCD subjects presenting homogeneous HbF distribution. This correlation was then used to measure HbF/RBC. Hydroxyurea (HU) improves SCD clinical manifestations, mainly through its ability to induce HbF synthesis. The HbF distribution was analyzed in 14 SCD patients before and during HU treatment. A significant decrease in RBC population containing less than 2 pg of HbF/RBC was observed. Therefore, we tested associations for %RBC above different HbF/RBC thresholds and showed a decrease in the pathognomonic vaso-occlusive crisis incidence from the threshold of 4 pg. This quantity was also correlated with the level of sickle RBC after in vitro deoxygenation. This new method allows the comparison of HbF/RBC distributions and could be a useful tool to characterize baseline patients HbF distribution and therapeutic response to HbF inducers.

Dense red blood cell and oxygen desaturation in sickle-cell disease

Production of abnormal hemoglobin (HbS) in sickle-cell disease (SCD) results in its polymerization in deoxygenated conditions and in sickled-RBC formation. Dense RBCs (DRBCs), defined as density >1.11 and characterized by increased rigidity are absent in normal AA subjects, but present at percentages that vary of a patient to another remaining stable throughout adulthood for each patient. Polymerized HbS has reduced affinity for oxygen, demonstrated by the rightward shift of the oxygen-dissociation curve, leading to disturbances in oxygen transport. Ninety-two SCD patients' total RBCs were separated into LightDRBC (LRBC) (d < 1.11 g/mL) and DRBC fractions. Venous blood partial oxygen pressure and RBC-fraction-deoxygenation and -reoxygenation Hb-oxygen-equilibrium curves were determined. All patients took a 6-minute walking test (6MWT); 10 had results before and after >6 months on hydroxyurea. 6MWT time with SpO2  < 88% (TSpO2  < 88) assessed the physiological impact of exertion. Elevated mean corpuscular hemoglobin (Hb) concentrations, decreased %HbF, and 2,3-bisphosphoglycerates in DRBCs modulated Hb-oxygen affinity. Deoxygenation and reoxygenation Hb-oxygen equilibrium curves differed between normal Hb AA and SS RBCs and between LRBCs and DRBCs, with rightward shifts confirming HbS-polymerization's role in affinity loss. In bivariate analyses, 50% Hb saturation correlated positively with %DRBCs (P < 0.0001, r(2)  = 0.34) and negatively with %HbF (P < 0.0001, r(2)  = 0.25). The higher the %DRBCs, the longer the TSpO2 88 (P = 0.04). Hydroxyurea was associated with significantly shorter TSpO2  < 88 (P = 0.01). We report that the %DRBCs directly affects SCD patients' SpO2 during exertion; hydroxyurea improves oxygen affinity and lowers the %DRBCs. Am. J. Hematol. 91:1008-1013, 2016. © 2016 Wiley Periodicals, Inc.

Six Months of Hydroxyurea Reduces Albuminuria in Patients with Sickle Cell Disease

The earliest symptom of glomerular injury in patients with sickle cell disease (SCD) is microalbuminuria. The effect of hydroxyurea (HU) on urine albumin-to-creatinine ratio (ACR) is unclear and should be determined, because increasing numbers of patients with SCD take this drug to improve red blood cell function. In this cohort study of 58 SS-homozygous adults with SCD who initiated HU therapy, we evaluated ACR changes and relationships of these changes with demographic, clinical, and biologic parameters at HU initiation (baseline) and 6 months later (follow-up). Between baseline and follow-up, ACR declined significantly for the entire population (3.0-1.7 mg/mmol; P<0.01), but this was primarily driven by the ACR reduction in the microalbuminuria subgroup (8.1-2.3 mg/mmol; P=0.03; n=23). According to bivariate analyses on 39 patients who did not receive a blood transfusion during the study period, the baseline to follow-up ACR decline was strongly associated with decreases in levels of hemolysis markers, percentage of dense red blood cells, and systolic BP. Bivariate analysis also revealed a close association between the ACR decrease and high baseline levels of hemolysis markers and percentage of dense red blood cells. These results show that urine ACR decreased significantly after 6 months of HU and confirm a close relationship between ACR and hemolysis evolution in patients with SCD.

Biological parameters predictive of percent dense red blood cell decrease under hydroxyurea

Background

Dense red blood cells (DRBCs) are associated with chronic clinical manifestations of sickle-cell–disease (SCD). Hydroxyurea (HU) decreases the percent (%) DRBCs, thereby improving its therapeutic benefits, especially the prevention of SCD clinical complications, but parameters influencing %DRBCs remain unknown. The purpose of this study was to determine predictive biological parameters of %DRBC decline under HU.

Methods

Factors affecting the %DRBC decrease in SCD patients HU-treated for ≥6 months were analyzed. Biological parameters and the %DRBCs were determined before starting HU and after ≥6 months of HU intake. Bivariate analyses evaluated the impact of each biological parameter variation on %DRBC changes under treatment. Multivariate analyses assessed the correlations between the decreased %DRBCs and biological parameters.

Results

The %DRBCs declined by 40.95% after ≥6 months on HU. That decrease was associated with less hemolysis, however in several analyses on this group of patients we did not find a statistically significant correlation between decrease in %DRBCs and increase in HbF. Initial %DRBC values were the most relevant parameter to predict %DRBC decline.

Conclusion

Our results strengthen the known HU efficacy in SCD management statistically independently of the classical HbF biological response. Decreasing %DRBCs is essential to limiting chronic SCD symptoms related to DRBCs and predictive factors might help prevent those manifestations. The results of this study provide new perspectives on indication for HU use, i.e., to prevent SCD-induced organ damage.