Intracellular Hemoglobin S Polymerization and the Clinical Severity of Sickle Cell Anemia

Standardized microfluidic assessment of red blood cell-mediated microcapillary occlusion: Association with clinical phenotype and hydroxyurea responsiveness in sickle cell disease

OBJECTIVES

We present a standardized in vitro microfluidic assay and Occlusion Index (OI) for the assessment of red blood cell (RBC)-mediated microcapillary occlusion and its clinical associations in sickle cell disease (SCD).

METHODS

Red blood cell mediated microcapillary occlusion represented by OI and its clinical associations were assessed for seven subjects with hemoglobin-SC disease (HbSC), 18 subjects with homozygous SCD (HbSS), and five control individuals (HbAA).

RESULTS

We identified two sub-populations with HbSS based on the OI distribution. HbSS subjects with relatively higher OIs had significantly lower hemoglobin levels, lower fetal hemoglobin (HbF) levels, and lower mean corpuscular volume (MCV), but significantly higher serum lactate dehydrogenase levels and absolute reticulocyte counts, compared to subjects with HbSS and lower OIs. HbSS subjects who had relatively higher OIs were more likely to have had a concomitant diagnosis of intrapulmonary shunting (IPS). Further, lower OI associated with hydroxyurea (HU) responsiveness in subjects with HbSS, as evidenced by significantly elevated HbF levels and MCV.

CONCLUSIONS

We demonstrated that RBC-mediated microcapillary occlusion and OI associated with subject clinical phenotype and HU responsiveness in SCD. The presented standardized microfluidic assay may be useful for evaluating clinical phenotype and assessing therapeutic outcomes in SCD, including emerging targeted and curative treatments that aim to improve RBC deformability and microcirculatory health.

Repurposing of genistein as anti-sickling agent: elucidation by multi spectroscopic, thermophoresis, and molecular modeling techniques

Sickle cell disease (SCD) is a major medical problem in which mono-therapeutic interventions have so far shown only limited effectiveness. We studied the repurpose of genistein, which could prevent sickle hemoglobin from polymerizing under hypoxic conditions in this disease. Genistein an important nutraceutical molecule found in soybean. The present study examines the repurposing genistein as an anti- sickling agent. Genistein shows inhibition of Hb S polymerization as well as a sickle reversal. Also, we have explored the interaction of the genistein with sickle hemoglobin (Hb S), using fluorescence, far-UV-CD spectroscopy, MicroScale Thermophoresis (MST), FTIR, combined with molecular modeling computations. The quenching constant decreases with increasing temperature, a characteristic that coincides with the static type of quenching mechanism. Temperature-dependent fluorescence measurements and molecular modeling studies reveal that apart from the hydrogen bonding, electrostatic interactions also play a crucial role in genistein and Hb S complex formation. In silico, distribution prediction of adsorption, digestion, metabolism, excretion, and toxicity (ADME/Tox) based on physical and chemical properties show that genistein is nontoxic and has ideal drug properties. The helicity and thermophoretic mobility of Hb S was a change in the presence of genistein, which leads to the destabilizing the Hb S polymer was examined using CD and MST, respectively. Our results open up the possibility for a promising therapeutic approach for the SCD by repurposed genistein as an anti-sickling agent.Communicated by Ramaswamy H. Sarma.

Potential of isoquercitrin as antisickling agent: a multi-spectroscopic, thermophoresis and molecular modeling approach

Sickle cell disease is an inherited disease caused by point mutation in hemoglobin (β-globin gene). Under oxygen saturation, sickle hemoglobin form polymers, leading to rigid erythrocytes. The transition of the blood vessels is altered and initiated by the adhesion of erythrocytes, neutrophils and endothelial cells. Sickle Hemoglobin (HbS) polymerization is a major cause in red blood cells (RBC), promoting sickling and destruction of RBCs. Isoquercitrin, a medicinal bioactive compound found in various medicinal plants, has multiple health benefits. The present study examines the potential of isoquercitrin as an anti-sickle agent, showing a significant decrease in the rate of polymerization as well as sickling of RBCs. Isoquercitrin-induced graded alteration in absorbance and fluorescence of HbS, confirmed their interaction. A negative value of ΔG° strongly suggests that it is a spontaneous exothermic reaction induced by entropy. Negative ΔH° and positive ΔS° predicted that hydrogen and hydrophobic binding forces interfered with a hydrophobic microenvironment of β6Val leading to polymerization inhibition of HbS. HbS-Isoquercitrin complex exhibits helical structural changes leading to destabilization of the HbS polymer as confirmed by CD spectroscopy. MST and DSC results indicate greater changes in thermophoretic mobility and thermal stability of sickle hemoglobin in the presence of isoquercitrin, respectively. These findings were also supported by molecular simulation studies using DOCK6 and GROMACS. Hence, we can conclude that isoquercitrin interacts with HbS through hydrogen bonding, which leads to polymerization inhibition. Consequently, isoquercitrin could potentially be used as a medication for the treatment of sickle cell disease.Communicated by Ramaswamy H. Sarma.

The spectrum of sickle hemoglobin-related nephropathy: from sickle cell disease to sickle trait

INTRODUCTION

Renal dysfunction is among the most common complication of sickle cell disease (SCD), from hyposthenuria in children to progression to overt chronic kidney disease (CKD) in young adults. Emerging evidence now suggests that sickle hemoglobin-related nephropathy extends to individuals with sickle cell trait (SCT). Areas covered: This review will highlight the pathophysiology, epidemiology, and management recommendations for sickle hemoglobin-related nephropathy in both SCD and SCT. In addition, it will focus on the major demographic and genetic modifiers of renal disease in sickling hemoglobinopathies. Expert commentary: Studies have elucidated the course of renal disease in SCD; however, the scope and age of onset of renal dysfunction in SCT has yet to be determined. In SCD, several modifiers of renal disease - such as α-thalassemia, hemoglobin F, APOL1 and HMOX1 - have been described and provide an opportunity for a precision medicine approach to risk stratify patients who may benefit from early intervention. Extrapolating from this literature may also provide insight into the modifiers of renal disease in SCT. Further studies are needed to determine the optimal treatment for sickle hemoglobin-related nephropathy.

Socio-environmental exposures and health outcomes among persons with sickle cell disease

There is much variability in the expression of sickle cell disease (SCD) and recent works suggest that environmental and social factors may also influence this variability. This paper aims to use geographic information systems technology to examine the association between socio-environmental exposures and health outcomes in all persons who have attended or currently attend the Sickle Cell Unit in Jamaica. Rural patients presented for clinical care at older ages and had less annual visits to clinic. Persons travelled relatively long distances to seek SCD care and those travelling longer had less health maintenance visits. Urban patients had a higher prevalence of significant pain crises (69.4% vs. 55.8%, p value<0.001) and respiratory events (21.2% vs. 14%, p value<0.001). Prevalence of leg ulcers did not vary between rural and urban patients but was higher in males than in females. Females also had lower odds of having respiratory events but there was no sex difference in history of painful crises. Persons with more severe genotypes lived in higher poverty and travelled longer for healthcare services. Persons in areas with higher annual rainfall, higher mean temperatures and living farther from factories had less painful crises and respiratory events. The paper highlights a need for better access to healthcare services for Jamaicans with SCD especially in rural areas of the island. It also reports interesting associations between environmental climatic exposures and health outcomes.

Ultrasonographic Assessment of the Distal Femoral Cartilage Thickness in Patients with Homozygous Sickle Cell Disease

OBJECTIVE

To compare the distal femoral cartilage thickness of patients with sickle cell disease (SCD) with those of healthy subjects using ultrasonography.

METHODS

The study comprised 30 patients with SCD (16 male, 14 female; mean age, 30.1 years) and 30 age- and sex-matched healthy subjects. Demographic features and medications of the patients were recorded. With the knees held in maximum flexion, the femoral cartilage thickness was measured bilaterally with a 7- to 12-MHz linear probe. Using ultrasonography, 3 midpoint measurements were taken from both knees: lateral femoral condyle (LFC), intercondylar area (ICA), and medial femoral condyle (MFC).

RESULTS

Patients with SCD had thinner femoral cartilage thickness values at LFC (P = 0.004), at MFC (P = 0.000), and ICA (P = 0.002) when compared with those of the healthy subjects. Patients with SCD also had lower Hb levels (P = 0.000) levels. Weak positive correlations were determined between Hemoglobin (Hb) levels and ultrasonographic measurements in the SCD group at MFC (r = 0.331, P = 0.010), and ICA (r = 0.289 , P = 0.025 ). Low levels of Hb seem to affect the femoral cartilage thickness.

CONCLUSION

These preliminary findings of decreased femoral cartilage thickness in SCD patients should be complemented with future studies. The possibility of early knee joint degeneration and eventual osteoarthritis in SCD should be kept in mind.

Measurement science in the circulatory system

The dynamics of the cellular and molecular constituents of the circulatory system are regulated by the biophysical properties of the heart, vasculature and blood cells and proteins. In this review, we discuss measurement techniques that have been developed to characterize the physical and mechanical parameters of the circulatory system across length scales ranging from the tissue scale (centimeter) to the molecular scale (nanometer) and time scales of years to milliseconds. We compare the utility of measurement techniques as a function of spatial resolution and penetration depth from both a diagnostic and research perspective. Together, this review provides an overview of the utility of measurement science techniques to study the spatial systems of the circulatory system in health and disease.

Pathophysiology of sickle cell disease is mirrored by the red blood cell metabolome

Emerging metabolomic tools can now be used to establish metabolic signatures of specialized circulating hematopoietic cells in physiologic or pathologic conditions and in human hematologic diseases. To determine metabolomes of normal and sickle cell erythrocytes, we used an extraction method of erythrocytes metabolites coupled with a liquid chromatography-mass spectrometry–based metabolite profiling method. Comparison of these 2 metabolomes identified major changes in metabolites produced by (1) endogenous glycolysis characterized by accumulation of many glycolytic intermediates; (2) endogenous glutathione and ascorbate metabolisms characterized by accumulation of ascorbate metabolism intermediates, such as diketogulonic acid and decreased levels of both glutathione and glutathione disulfide; (3) membrane turnover, such as carnitine, or membrane transport characteristics, such as amino acids; and (4) exogenous arginine and NO metabolisms, such as spermine, spermidine, or citrulline. Finally, metabolomic analysis of young and old normal red blood cells indicates metabolites whose levels are directly related to sickle cell disease. These results show the relevance of metabolic profiling for the follow-up of sickle cell patients or other red blood cell diseases and pinpoint the importance of metabolomics to further depict the pathophysiology of human hematologic diseases.

Mechanism for fetal hemoglobin induction by histone deacetylase inhibitors involves gamma-globin activation by CREB1 and ATF-2

The histone deacetylase inhibitors (HDA-CIs) butyrate and trichostatin A activate gamma-globin expression via a p38 mitogen-activating protein kinase (MAPK)-dependent mechanism. We hypothesized that down-stream effectors of p38 MAPK, namely activating transcription factor-2 (ATF-2) and cyclic AMP response element (CRE) binding protein (CREB), are intimately involved in fetal hemoglobin induction by these agents. In this study, we observed increased ATF-2 and CREB1 phosphorylation mediated by the HDACIs in K562 cells, in conjunction with histone H4 hyperacetylation. Moreover, enhanced DNA-protein interactions occurred in the CRE in the (G)gamma-globin promoter (G-CRE) in vitro after drug treatments; subsequent chromatin immunoprecipitation assay confirmed ATF-2 and CREB1 binding to the G-CRE in vivo. Enforced expression of ATF-2 and CREB produced (G)gamma-promoter trans-activation which was abolished by a 2-base pair mutation in the putative G-CRE. The data presented herein demonstrate that gamma-gene induction by butyrate and trichostatin A involves ATF-2 and CREB1 activation via p38 MAPK signaling.

Responses of normal and sickle cell hemoglobin to S-nitroscysteine: implications for therapeutic applications of NO in treatment of sickle cell disease

Factors which govern transnitrosation reactions between hemoglobin (Hb) and low molecular weight thiols may define the extent to which S-nitrosated Hb (SNO-Hb) plays a role in NO in the control of blood pressure and other NO-dependent reactions. We show that exposure to S-nitrosylated cysteine (CysNO) produces equivalent levels of SNO-Hb for Hb A(0) and sickle cell Hb (Hb S), although these proteins differ significantly in the electron affinity of their heme groups as measured by their anaerobic redox potentials. Dolphin Hb, a cooperative Hb with a redox potential like that of Hb S, produces less SNO-Hb, indicating that steric considerations outweigh effects of altered electron affinity at the active-site heme groups in control of SNO-Hb formation. Examination of oxygen binding at 5-20 mM heme concentrations revealed increases due to S-nitrosation in the apparent oxygen affinity of both Hb A(0) and Hb S, similar to increases seen at lower heme concentrations. As observed at lower heme levels, deoxygenation is not sufficient to trigger release of NO from SNO-Hb. A sharp increase in apparent oxygen affinity occurs for unmodified Hb S at concentrations above 12.5 mM, its minimum gelling concentration. This affinity increase still occurs in 30 and 60% S-nitrosated samples, but at higher heme concentration. This oxygen binding behavior is accompanied by decreased gel formation of the deoxygenated protein. S-nitrosation is thus shown to have an effect similar to that reported for other SH-group modifications of Hb S, in which R-state stabilization opposes Hb S aggregation.

Erythrocyte-active agents and treatment of sickle cell disease

The sickle hemoglobin (HbS)-containing erythrocyte and its membrane represent a logical target for sickle cell disease therapy. Several antisickling agents which interfere with HbS polymerization have been studied over the last 30 years, but none has overcome the challenge of delivering high concentrations inside the sickle red blood cell without toxicity. The sickle erythrocyte membrane has also been targeted for therapeutic developments. Prevention of sickle cell dehydration by use of specific blockers of ion transport pathways mediating potassium loss from the sickle erythrocyte has been shown to be a feasible strategy in vitro, in vivo in transgenic sickle mice, and in patients. Other approaches have focused on improving the hemorheology of sickle erythrocytes and reducing their abnormal adhesion to endothelial cells. These potential treatments could be used alone or in combination with other approved therapies, such as hydroxyurea.

Mathematical modeling and computer simulation of erythrocytapheresis for SCD

BACKGROUND

Erythrocytapheresis is used to prevent acute chest syndrome and stroke in patients with sickle cell disease (SCD). However, such regimens are associated with significant risks, such as iron overload and potential exposure to transfusion-transmitted infectious diseases. Computer modeling of erythrocytapheresis procedures may help optimize treatments and minimize risks.

STUDY DESIGN AND METHODS

Mathematical models based upon material balance equations and patient-specific statistical analyses were developed to estimate HbS levels immediately after erythrocytapheresis and immediately before the next treatment. The equations were incorporated into a software application that was used to model the effects of various treatment values on four patients treated with 90 erythrocytapheresis procedures.

RESULTS

Immediate postprocedure HbS values were accurately estimated with correlations between measured and calculated values ranging from R(2) = 0.83 to 0.96. Estimates of HbS just before the next treatment correlated well in three patients (R(2) = 0.71 to 0.83) but poorly in one (R(2) = 0.28 to 0.46). Varying the treatment values by computer simulation led to a wide variation in the number of RBC units and the net RBC volume transfused.

CONCLUSION

Computer modeling of erythrocytapheresis can be used to optimize chronic treatment regimens for SCD patients and potentially to minimize the risks of overtransfusion.

Reperfusion injury pathophysiology in sickle transgenic mice

Reperfusion of tissues after interruption of their vascular supply causes free-radical generation that leads to tissue damage, a scenario referred to as “reperfusion injury.” Because sickle disease involves repeated transient ischemic episodes, we sought evidence for excessive free-radical generation in sickle transgenic mice. Compared with normal mice, sickle mice at ambient air had a higher ethane excretion (marker of lipid peroxidation) and greater conversion of salicylic acid to 2,3-dihydroxybenzoic acid (marker of hydroxyl radical generation). During hypoxia (11% O2), only sickle mice converted tissue xanthine dehydrogenase to oxidase. Only the sickle mice exhibited a further increase in ethane excretion during restitution of normal oxygen tension after 2 hours of hypoxia. Only the sickle mice showed abnormal activation of nuclear factor–κB after exposure to hypoxia-reoxygenation. Allopurinol, a potential therapeutic agent, decreased ethane excretion in the sickle mice. Thus, sickle transgenic mice exhibit biochemical footprints consistent with excessive free-radical generation even at ambient air and following a transient induction of enhanced sickling. We suggest that reperfusion injury physiology may contribute to the evolution of the chronic organ damage characteristic of sickle cell disease. If so, novel therapeutic approaches might be of value.

Reperfusion injury pathophysiology in sickle transgenic mice

Reperfusion of tissues after interruption of their vascular supply causes free-radical generation that leads to tissue damage, a scenario referred to as “reperfusion injury.” Because sickle disease involves repeated transient ischemic episodes, we sought evidence for excessive free-radical generation in sickle transgenic mice. Compared with normal mice, sickle mice at ambient air had a higher ethane excretion (marker of lipid peroxidation) and greater conversion of salicylic acid to 2,3-dihydroxybenzoic acid (marker of hydroxyl radical generation). During hypoxia (11% O2), only sickle mice converted tissue xanthine dehydrogenase to oxidase. Only the sickle mice exhibited a further increase in ethane excretion during restitution of normal oxygen tension after 2 hours of hypoxia. Only the sickle mice showed abnormal activation of nuclear factor–κB after exposure to hypoxia-reoxygenation. Allopurinol, a potential therapeutic agent, decreased ethane excretion in the sickle mice. Thus, sickle transgenic mice exhibit biochemical footprints consistent with excessive free-radical generation even at ambient air and following a transient induction of enhanced sickling. We suggest that reperfusion injury physiology may contribute to the evolution of the chronic organ damage characteristic of sickle cell disease. If so, novel therapeutic approaches might be of value.

Novel approaches to treatment of sickle cell anaemia

Sickle cell anaemia, a chronic and often debilitating disease, results from homozygosity for a single amino acid substitution in the beta-globin subunit of the haemoglobin molecule. Sickle haemoglobin (HbS), the product of this mutation, polymerises when deoxygenated, thus damaging the red blood cell and causing vaso-occlusive complications and haemolytic anaemia. Most cases of sickle cell anaemia are found in Africa. Until recently, treatment was directed at the management of disease complications. Patients with central nervous system events undergo exchange transfusions followed by chronic transfusion programmes. Patients with painful episodes, which result in many days missed from work and school are treated with narcotics and aggressive hydration. Novel therapy for sickle cell anaemia is designed to prevent complications through targeting disease mechanisms. Hydroxyurea is given to severely affected sickle cell anaemia patients in an attempt to prevent painful episodes, reduce hospital days, improve the patients' overall quality of life, and perhaps to prevent or provide some degree of end-organ damage stabilisation. Other novel therapies, such as bone marrow transplantation and gene therapy, pursue a cure. For these novel therapies to be effective on a global basis they must be amenable to underdeveloped and poorer countries of the world.

A new sickle cell disease phenotype associating Hb S trait, severe pyruvate kinase deficiency (PK Conakry), and an alpha2 globin gene variant (Hb Conakry)

A Guinean woman, heterozygous for haemoglobin (Hb) S, was studied because of episodes of marked anaemia, repeated typical metaphyseal painful crises and haemosiderosis. Her sickling syndrome resulted from the association of Hb S trait with a severe pyruvate kinase deficiency leading to a 2,3-DPG concentration of twice normal levels. Sequence of the PK-R gene revealed an undescribed mutation in the homozygous or hemizygous state within exon 5 (nucleotide 2670 C-->A), leading to the interchange of Ser 130 into Tyr (PK Conakry). In addition, the patient carried a new haemoglobin variant, Hb Conakry [alpha80(F1) Leu-->Val], which seemed to have a mild effect. The high intraerythrocytic 2,3-DPG concentration induced by the PK deficiency resulted in a decreased oxygen affinity which favoured sickling to a level almost similar to that of Hb S/C compound heterozygous patients. This was confirmed by oxygen binding measurements of Hb A/Hb S erythrocytes in which 2,3-DPG content was modified in vitro. Hysteresis between deoxy- and reoxygenation curves, as well as increase in the n(max) value, demonstrated that the extent of HbS polymerization in the propositus was almost the same as that of RBCs from a homozygous sickle cell patient or those of an A/S heterozygous patient with an artificial in vitro increase of 2,3-DPG concentration.