Cation Homeostasis in Red Cells From Patients With Sickle Cell Disease Heterologous for HbS and HbC (HbSC Genotype)

Emerging drug targets for sickle cell disease: shedding light on new knowledge and advances at the molecular level

INTRODUCTION

In sickle cell disease (SCD), a single amino acid substitution at β6 of the hemoglobin (Hb) chain replaces glutamate with valine, forming HbS instead of the normal adult HbA. Loss of a negative charge, and the conformational change in deoxygenated HbS molecules, enables formation of HbS polymers. These not only distort red cell morphology but also have other profound effects so that this simple etiology belies a complex pathogenesis with multiple complications. Although SCD represents a common severe inherited disorder with life-long consequences, approved treatments remain inadequate. Hydroxyurea is currently the most effective, with a handful of newer treatments, but there remains a real need for novel, efficacious therapies.

AREAS COVERED

This review summarizes important early events in pathogenesis to highlight key targets for novel treatments.

EXPERT OPINION

A thorough understanding of early events in pathogenesis closely associated with the presence of HbS is the logical starting point for identification of new targets rather than concentrating on more downstream effects. We discuss ways of reducing HbS levels, reducing the impact of HbS polymers, and of membrane events perturbing cell function, and suggest using the unique permeability of sickle cells to target drugs specifically into those more severely compromised.

Unidirectional fluxes of monovalent ions in human erythrocytes compared with lymphoid U937 cells: Transient processes after stopping the sodium pump and in response to osmotic challenge

Recently, we have developed software that allows, using a minimum of required experimental data, to find the characteristics of ion homeostasis and a list of all unidirectional fluxes of monovalent ions through the main pathways in the cell membrane both in a balanced state and during the transient processes. Our approach has been successfully validated in human proliferating lymphoid U937 cells during transient processes after stopping the Na/K pump by ouabain and for staurosporine-induced apoptosis. In present study, we used this approach to find the characteristics of ion homeostasis and the monovalent ion fluxes through the cell membrane of human erythrocytes in a resting state and during the transient processes after stopping the Na/K pump with ouabain and in response to osmotic challenge. Due to their physiological significance, erythrocytes remain the object of numerous studies, both experimental and computational methods. Calculations showed that, under physiological conditions, the K+ fluxes through electrodiffusion channels in the entire erythrocyte ion balance is small compared to the fluxes through the Na/K pump and cation-chloride cotransporters. The proposed computer program well predicts the dynamics of the erythrocyte ion balance disorders after stopping the Na/K pump with ouabain. In full accordance with predictions, transient processes in human erythrocytes are much slower than in proliferating cells such as lymphoid U937 cells. Comparison of real changes in the distribution of monovalent ions under osmotic challenge with the calculated ones indicates a change in the parameters of the ion transport pathways through the plasma membrane of erythrocytes in this case. The proposed approach may be useful in studying the mechanisms of various erythrocyte dysfunctions.

The pleiotropic effects of α-thalassemia on HbSS and HbSC sickle cell disease: Reduced erythrocyte cation co-transport activity, serum erythropoietin, and transfusion burden, do not translate into increased survival

α-Thalassemia is one of the most important genetic modulators of sickle cell disease (SCD). Both beneficial and detrimental effects have been described previously. We use a 12-year data set on a large cohort of patients with HbSS (n = 411) and HbSC (n = 146) to examine a wide range of these clinical and laboratory associations. Our novel findings are that α-thalassemia strongly reduces erythrocyte potassium chloride co-transporter (KCC) activity in both HbSS and HbSC (p = .035 and p = .00045 respectively), suggesting a novel mechanism through which α-thalassemia induces a milder phenotype by reducing red cell cation loss. This may be particularly important in HbSC where reduction in mean cell hemoglobin concentration is not seen and where KCC activity has previously been found to correlate with disease severity. Additionally, we show that α-thalassemia not only increases hemoglobin in patients with HbSS (p = .0009) but also reduces erythropoietin values (p = .0005), demonstrating a measurable response to improved tissue oxygenation. We confirm the reno-protective effect of α-thalassemia in patients with HbSS, with reduced proteinuria (p = .003) and demonstrate a novel association with increased serum sodium (p = .0004) and reduced serum potassium values (p = 5.74 × 10^-10 ). We found patients with α-thalassemia had a reduced annualized transfusion burden in both HbSS and HbSC, but α-thalassemia had no impact on annualized admission rates in either group. Finally, in a larger cohort, we report a median survival of 62 years in patients with HbSS (n = 899) and 80 years in those with HbSC (n = 240). α-thalassemia did not influence survival in HbSS, but a nonsignificant trend was seen in those with HbSC.

The protective effect of the spleen in sickle cell patients. A comparative study between patients with asplenia/hyposplenism and hypersplenism

Sickle cell disease (SCD) is caused by a point mutation in the beta-globin gene. SCD is characterized by chronic hemolytic anemia, vaso-occlusive events leading to tissue ischemia, and progressive organ failure. Chronic inflammatory state is part of the pathophysiology of SCD. Patients with SCD have extremely variable phenotypes, from mild disease to severe complications including early age death. The spleen is commonly injured in SCD. Early splenic dysfunction and progressive spleen atrophy are common. Splenomegaly and hypersplenism can also occur with the loss of the crucial splenic function. Acute, life-threatening spleen-related complications in SCD are well studied. The association of laboratory parameters with the spleen status including hyposplenism, asplenia, and splenomegaly/hypersplenism, and their implication in vaso-occlusive crisis and long-term complications in SCD remain to be determined. We evaluated the association between the spleen status with clinical and laboratory parameters in 31 SCD patients: Group a) Patients with asplenia/hyposplenism (N = 22) (including auto-splenectomy and splenectomized patients) vs. Group b) patients with splenomegaly and or hypersplenism (N = 9). Laboratory studies included: Complete Blood Count, reticulocyte count, iron metabolism parameters, C Reactive Protein (CRP), Hb variant distribution, and D-dimer. Metabolic and morphological red blood cell (RBC) studies included: density gradient (by Percoll), glucose consumption, lactate release, and K⁺ leakage, fetal RBC (F-Cells) and F-Reticulocytes, annexinV+, CD71⁺, oxidative stress measured by GSH presence in RBC and finally Howell Jolly Bodies count were all analyzed by Flow Cytometry. Scanning electron microscopy analysis of RBC was also performed. Patients with asplenia/hyposplenism showed significantly higher WBC, platelet, Hematocrit, hemoglobin S, CRP, D-dimer, Gamma Glutamyl Transferase (GGT), cholesterol, transferrin, annexin V+ RBCs, CD71⁺ RBCs, together with a markedly lower F Reticulocyte levels in comparison with splenomegaly/hypersplenism patients. In summary, important differences were also found between the groups in the studied RBCs parameters. Further studies are required to elucidate the effect of the spleen including hyper and hypo-splenia on laboratory parameters and in clinical manifestations, vascular pathology, and long-term complications of SCD. The benefits and risks of splenectomy compared to chronic transfusion need to be evaluated in clinical trials and the standard approach managing hypersplenism in SCD patients should be re-evaluated.

Determinants of severity in sickle cell disease

Sickle cell disease is a very variable condition, with outcomes ranging from death in childhood to living relatively symptom free into the 8^th decade. Much of this variability is unexplained. The co-inheritance of α thalassaemia and factors determining HbF levels significantly modify the phenotype, but few other significant genetic variants have been identified, despite extensive studies. Environmental factors are undoubtedly important, with socio-economics and access to basic medical care explaining the huge differences in outcomes between many low- and high-income countries. Exposure to cold and windy weather seems to precipitate acute complications in many people, although these effects are unpredictable and vary with geography. Many studies have tried to identify prognostic factors which can be used to predict outcomes, particularly when applied in infancy. Overall, low haemoglobin, low haemoglobin F percentage and high reticulocytes in childhood are associated with worse outcomes, although again these effects are fairly weak and inconsistent.

Sickle cell nephropathy: A review of novel biomarkers and their potential roles in early detection of renal involvement

Whether the underlying mutations are homozygous, heterozygous, or co-inherited with other hemoglobinopathies, sickle cell disease is known to afflict the kidneys, leading to the clinical entity known as sickle cell nephropathy (SCN). Although common, SCN remains diagnostically elusive. Conventional studies performed in the context of renal disorders often fail to detect early stage SCN. This makes the quest for early diagnosis and treatment more challenging, and it increases the burden of chronic kidney disease-related morbidity among patients. Novel diagnostic tools have been employed to overcome this limitation. In this study, we discuss various biomarkers of SCN, including those employed in clinical practice and others recently identified in experimental settings, such as markers of vascular injury, endothelial dysfunction, tubulo-glomerular damage, and oxidative stress. These include kidney injury molecule-1, monocyte chemoattractant protein-1, N-acetyl-B-D-glucosaminidase, ceruloplasmin, orosomucoid, nephrin, and cation channels, among others. Furthermore, we explore the potential of novel biomarkers for refining diagnostic and therapeutic approaches and describe some obstacles that still need to be overcome. We highlight the importance of a collaborative approach to standardize the use of promising new biomarkers. Finally, we outline the limitations of conventional markers of renal damage as extensions of the pathogenic process occurring at the level of the organ and its functional subunits, with a discussion of the expected pattern of clinical and biochemical progression among patients with SCN.

Testicular Volume in Children with Sickle Cell Anaemia in Uyo, a South-South Nigerian City

INTRODUCTION

Sickle cell anaemia (SCA) often presents in early childhood with repeated vaso-occlusive crisis that leads to ischaemia, infarction and fibrosis which may result in a reduction in expected testicular volume (TV) at puberty.

METHOD

This was a cross-sectional study of 95 children with SCA aged 1-18 years compared with 95 age-matched controls. Participants responded to an interviewer-administered questionnaire, with their anthropometric measurements taken, pubertal maturity assessed by Tanner staging and testicular ultrasonography done. Changes in TV across the ages were compared graphically and regression analyses were used to determine the factors independently associated with TV. A p-value of <0.05 was considered statistically significant.

RESULTS

In the prepubertal period, the haemoglobin SS (HbSS) participants had larger median ultrasound TV (MUSTV) compared to the haemoglobin AA (HbAA) controls (p = 0.001). This trend reversed in the pubertal period. On regression analysis, the frequency of testicular pain (p = 0.04), weight (p = 0.02) and pubic hair rating (p = 0.03) of the HbSS participants were significant predictors of increased TVs in the HbSS participants, irrespective of pubertal status.

CONCLUSION

The prepubertal MUSTV of the HbSS participants were higher than those of the HbAA controls, while the HbAA controls had higher MUSTV at puberty and beyond. The frequency of testicular pain episodes, pubic hair rating and weight were independent predictors of TV changes in the HbSS participants. Prevention of repeated vaso-occlusive crisis in the prepubertal period may help prevent the reduction in TV and possible hypogonadism. Lay summarySickle cell anaemia (SCA) causes repeated episodes of painful crisis and in boys, these may affect the way their testes grow. The study set out to document testicular sizes on a one-time basis in boys aged 1-18 years with SCA compared with controls of similar ages. The participants responded to structured questions assisted by the researchers and their body measurements were appropriately taken. Their level of sexual maturation was assessed according to the method by Tanner and the sizes of their testes were measured using an ultrasound machine. The research information was analysed and a statistical value less than 0.05 was taken to mean that there was a difference between the measured variables. The mid-testicular sizes of the SCA participants were noted to be higher than that of their controls during the prepubertal period while the non-SCA boys had higher sizes from puberty onwards. The frequency of testicular pain, weight and pubic hair stage of the SCA boys were important contributors to their increased testes sizes, irrespective of pubertal status. Efforts aimed at preventing painful crisis should start during early childhood to forestall future sexual challenges in adulthood.

Is Skeletal Muscle Dysfunction a Limiting Factor of Exercise Functional Capacity in Patients with Sickle Cell Disease?

Patients with sickle cell disease (SCD) have reduced functional capacity due to anemia and cardio-respiratory abnormalities. Recent studies also suggest the presence of muscle dysfunction. However, the interaction between exercise capacity and muscle function is currently unknown in SCD. The aim of this study was to explore how muscle dysfunction may explain the reduced functional capacity. Nineteen African healthy subjects (AA), and 24 sickle cell anemia (SS) and 18 sickle cell hemoglobin C (SC) patients were recruited. Maximal isometric torque (Tmax) was measured before and after a self-paced 6-min walk test (6-MWT). Electromyographic activity of the Vastus Lateralis was recorded. The 6-MWT distance was reduced in SS (p < 0.05) and SC (p < 0.01) patients compared to AA subjects. However, Tmax and root mean square value were not modified by the 6-MWT, showing no skeletal muscle fatigue in all groups. In a multiple linear regression model, genotype, step frequency and hematocrit were independent predictors of the 6-MWT distance in SCD patients. Our results suggest that the 6-MWT performance might be primarily explained by anemia and the self-paced step frequency in SCD patients attempting to limit metabolic cost and fatigue, which could explain the absence of muscle fatigue.

Impact of a submaximal mono-articular exercise on the skeletal muscle function of patients with sickle cell disease

PURPOSE

Sickle cell disease (SCD) patients exhibit a limited exercise tolerance commonly attributed to anaemia, as well as hemorheological and cardio-respiratory abnormalities, but the functional status of skeletal muscle at exercise is unknown. Moreover, the effect of SCD genotype on exercise tolerance and skeletal muscle function has been poorly investigated. The aim of this study was to investigate skeletal muscle function and fatigue during a submaximal exercise in SCD patients.

METHODS

Nineteen healthy individuals (AA), 28 patients with sickle cell anaemia (SS) and 18 with sickle cell-haemoglobin C disease (SC) performed repeated knee extensions exercise (FAT). Maximal isometric torque (Tmax) was measured before and after the FAT to quantify muscle fatigability. Electromyographic activity and oxygenation by near-infrared spectroscopy of the Vastus Lateralis were recorded.

RESULTS

FAT caused a reduction in Tmax in SS (- 17.0 ± 12.1%, p < 0.001) and SC (- 21.5 ± 14.5%, p < 0.05) but not in AA (+ 0.58 ± 29.9%). Root-mean-squared value of EMG signal (RMS) decreased only in SS after FAT, while the median power frequency (MPF) was unchanged in all groups. Oxygenation kinetics were determined in SS and AA and were not different.

CONCLUSION

These results show skeletal muscle dysfunction during exercise in SCD patients, and suggest different fatigue aetiology between SS and SC. The changes in EMG signal and oxygenation kinetics during exercise suggest that the greater skeletal muscle fatigue occurring in SCD patients would be rather due to intramuscular alterations modifications than decreased tissue oxygenation. Moreover, SS patients exhibit greater muscle fatigability than SC.

Methodological aspects of oxygen gradient ektacytometry in sickle cell disease: Effects of sample storage on outcome parameters in distinct patient subgroups

Sickle cell disease (SCD) is a genetic disorder characterized by the production of an abnormal hemoglobin (Hb), which, under deoxygenation, may polymerize and cause a mechanical distortion of red blood cell (RBC) into a crescent-like shape. Recently a method, using ektacytometry principle, has been developed to assess RBC deformability as a function of oxygen tension (pO2) and is called oxygen gradient ektacytometry (oxygenscan). However, standardization of this test is needed to properly assess the tendency of sickling of RBCs under deoxygenation and to allow comparisons between different laboratories. The study compared the oxygenscan responses during blood storage between distinct populations of SCD patients. Blood from 40 non-transfused homozygous SCD patients (HbSS), 16 chronically transfused HbSS patients, and 14 individuals with compound heterozygous hemoglobin SC disease (HbSC) at steady-state was collected in EDTA tubes. Measurements were performed within 4 hours after collection and after 24 hours of storage at 4°C. We showed that storage affected the minimum RBC deformability reached during deoxygenation (EImin) in both non-transfused HbSS and HbSC patients and the maximum RBC deformability (EImax) measured before deoxygenation (i.e., in normoxia) in the three groups. In contrast, the tendency of RBCs to sickle under deoxygenation (i.e., the point of sickling; PoS) remained rather stable between the two time of measurements. Collectively, since the time between blood sampling and analysis affects some key oxygen gradient ektacytometry-derived parameters we recommend that each laboratory performs oxygenscan measurements at a standardized time point.

Curative vs targeted therapy for SCD: does it make more sense to address the root cause than target downstream events?

Sickle cell disease (SCD) places a heavy burden on a global and increasing population predominantly resident in resource-poor and developing countries. Progress continues to be made in preventing childhood mortality, and increasing numbers of chronically ill adults with disease are requiring care for disease sequelae. Curative therapies for SCD are therefore attractive to physicians and investigators focused on SCD. Gene therapies are being developed, and several are now in various stages of early-phase human clinical trials. However, we must also pursue avenues through which we can do the most good for the most people alive today. Such efforts include improving our understanding of disease mechanisms and which disease sequelae most strongly affect survival and interfere with quality of life. The pathways leading to disease sequelae are multiple, complex, and highly interactive. Four drugs have now been approved by the US Food and Drug Administration for SCD; however, each has a distinct mechanism and a measurable but limited effect on the many clinical sequelae of SCD. We therefore need to learn how to approach multi-agent therapy for SCD. The order of addition of each agent to treat a specific patient will need to be guided by response to previous therapy, risk factors identified for specific disease outcomes, and clinical studies to determine more comprehensively how the 4 currently approved drugs might interact and produce (or not) additive effects. Moreover, this will have to be accomplished with defined end points in mind, according to which pose the greatest threats to quality of life as well as survival.

Clinical Diagnosis of Red Cell Membrane Disorders: Comparison of Osmotic Gradient Ektacytometry and Eosin Maleimide (EMA) Fluorescence Test for Red Cell Band 3 (AE1, SLC4A1) Content for Clinical Diagnosis

The measurement of band 3 (AE1, SLC4A1, CD233) content of red cells by eosin-5- maleimide (EMA) staining is swiftly replacing conventional osmotic fragility (OF) test as a tool for laboratory confirmation of hereditary spherocytosis across the globe. Our group has systematically evaluated the EMA test as a method to screen for a variety of anemias in the last 10 years, and compared these results to those obtained with the osmotic gradient ektacytometry (osmoscans) which we have used over three decades. Our overall experience allowed us to characterize the distinctive patterns with the two tests in several congenital erythrocyte membrane disorders, such as hereditary spherocytosis (HS), hereditary elliptocytosis (HE), Southeast Asian Ovalocytosis (SAO), hereditary pyropoikilocytosis (HPP) variants, erythrocyte volume disorders, various red cell enzymopathies, and hemoglobinopathies. A crucial difference between the two methodologies is that osmoscans measure red blood cell deformability of the entire sample of RBCs, while the EMA test examines the band 3 content of individual RBCs. EMA content is influenced by cell size as smaller red cells have lower amount of total membrane than larger cells. The SAO mutation alters the EMA binding site resulting in a lower EMA MCF even as the band 3 content itself is unchanged. Thus, EMA scan results should be interpreted with caution and both the histograms and dot plots should be analyzed in the context of the clinical picture and morphology.

Serum Potassium, Sodium, and Chloride Levels in Sickle Cell Disease Patients and Healthy Controls: A Case-Control Study at Korle-Bu Teaching Hospital, Accra

The activity of Na⁺-K⁺ ATPase is altered in sickle cell disease (SCD), which affects serum electrolyte levels. This alteration is associated with several complications in sickle cell patients. This study evaluated the serum levels of sodium, potassium, and chloride in patients with SCD. The study was a case-control cross-sectional study involving 120 SCD patients in the steady state and 48 ‘healthy’ controls. The SCD patients were made up of 69 HbSS patients and 41 HbSC patients. Serum electrolyte levels (Na⁺, K⁺, and Cl⁻) were measured using a Flame Atomic Absorption Spectrometer (Variant 240FS; Varian Australia Pty Ltd). Serum sodium levels were significantly lower in the sickle cell patients, compared with their ‘healthy’ counterparts (P = .0001). Although the study found significantly higher serum levels of potassium in the SCD patients (P = .0001), there was no significant difference in serum chloride levels between patients with SCD and the controls (P = .098). Serum sodium and chloride levels were not significantly different in both HbSS and HbSC patients (P = .197 and P = .553, respectively). The level of serum potassium in the HbSS patients was, however, significantly higher compared with those with the HbSC genotype (P = .0001). There is higher efflux of K⁺ from the intracellular into the extracellular space in HbSS patients, which may lead to red cell membrane dysfunction and associated complications.

The Effect of Antioxidants on the Properties of Red Blood Cells From Patients With Sickle Cell Anemia

Oxidative damage to red blood cells (RBCs) may contribute to pathogenesis of sickle cell anemia. Reducing the deleterious effects of oxidants by exposing RBCs to a number of antioxidants has been shown to have protective effects against lipid and protein peroxidation. We hypothesize that antioxidants may also have beneficial effects on the abnormal membrane permeability of sickle cells. Increased cation permeability of these cells encourages HbS polymerization by causing RBC dehydration and also leads to externalization of the prothrombotic aminophospholipid phosphatidylserine (PS). Three antioxidants with different mechanisms of action were investigated - dithiothreitol, N-acetylcysteine, and quercetin. All three were found to inhibit the main cation pathways responsible for dehydration - the deoxygenation-induced cation conductance (or Psickle), the Ca2+-activated K+ channel (or Gardos channel), and the K+-Cl- cotransporter. They also reduced Ca2+-induced PS exposure and hemolysis. Findings provide evidence for additional beneficial actions of antioxidants in maintenance of rheology and reducing vascular adhesion and further inform the rationale for their clinical use.

The effect of xanthine oxidase and hypoxanthine on the permeability of red cells from patients with sickle cell anemia

Red cells from patients with sickle cell anemia (SCA) are under greater oxidative challenge than those from normal individuals. We postulated that oxidants generated by xanthine oxidase (XO) and hypoxanthine (HO) contribute to the pathogenesis of SCA through altering solute permeability. Sickling, activities of the main red cell dehydration pathways (Psickle , Gardos channel, and KCl cotransporter [KCC]), and cell volume were measured at 100, 30, and 0 mmHg O2 , together with deoxygenation-induced nonelectrolyte hemolysis. Unexpectedly, XO/HO mixtures had mainly inhibitory effects on sickling, Psickle , and Gardos channel activities, while KCC activity and nonelectrolyte hemolysis were increased. Gardos channel activity was significantly elevated in red cells pharmacologically loaded with Ca2+ using the ionophore A23187, consistent with an effect on the transport system per se as well as via Ca2+ entry likely via the Psickle pathway. KCC activity is controlled by several pairs of conjugate protein kinases and phosphatases. Its activity, however, was also stimulated by XO/HO mixtures in red cells pretreated with N-ethylmaleimide (NEM), which is thought to prevent regulation via changes in protein phosphorylation, suggesting that the oxidants formed could also have direct effects on this transporter. In the presence of XO/HO, red cell volume was better maintained in deoxygenated red cells. Overall, the most notable effect of XO/HO mixtures was an increase in red cell fragility. These findings increase our understanding of the effects of oxidative challenge in SCA patients and are relevant to the behavior of red cells in vivo.

Sickle cell dehydration: Pathophysiology and therapeutic applications

Cell dehydration is a distinguishing characteristic of sickle cell disease and an important contributor to disease pathophysiology. Due to the unique dependence of Hb S polymerization on cellular Hb S concentration, cell dehydration promotes polymerization and sickling. In double heterozygosis for Hb S and C (SC disease) dehydration is the determining factor in disease pathophysiology. Three major ion transport pathways are involved in sickle cell dehydration: the K-Cl cotransport (KCC), the Gardos channel (KCNN4) and Psickle, the polymerization induced membrane permeability, most likely mediated by the mechano-sensitive ion channel PIEZO1. Each of these pathways exhibit unique characteristics in regulation by oxygen tension, intracellular and extracellular environment, and functional expression in reticulocytes and mature red cells. The unique dependence of K-Cl cotransport on intracellular Mg and the abnormal reduction of erythrocyte Mg content in SS and SC cells had led to clinical studies assessing the effect of oral Mg supplementation. Inhibition of Gardos channel by clotrimazole and senicapoc has led to Phase 1,2,3 trials in patients with sickle cell disease. While none of these studies has resulted in the approval of a novel therapy for SS disease, they have highlighted the key role played by these pathways in disease pathophysiology.

Early Markers of Sickle Nephropathy in Children With Sickle Cell Anemia Are Associated With Red Cell Cation Transport Activity

The early stages of sickle cell nephropathy (SCN) manifest in children with sickle cell anemia (SCA) as hyperfiltration and proteinuria. The physiological conditions of the renovascular system are among the most conducive to hemoglobin S polymerization in the body and will magnify small changes in red cell volume thus crucially modulating intracellular concentrations of hemoglobin S. This large cross-sectional study of children with sickle cell anemia measured glomerular filtration rates and microalbuminuria to report prevalence, clinical correlates and uniquely, association with key red cell cation transport mechanisms. One hundred and twelve patients (mean age 10.7 ± 4.1) were recruited. The prevalence of hyperfiltration and microalbuminuria was 98% and 15.1%, respectively. Glomerular filtration rates did not vary with age, but proteinuria became more prevalent with increasing age. Both features associated with markers of hemolysis, while elevated hemoglobin F was protective, but no association was seen with systolic or diastolic blood pressure. In multivariate analysis, both Gardos channel (β = 0.476, P < 0.001) and KCl co-transporter (KCC; β = -0.216, P = 0.009) activity, alongside age (β = 0.237, P = 0.004), remained independently predictive for microalbuminuria. Increased activity of Gardos channel and P_sickle positively associated with microalbuminuria, while increased KCC activity associated with a reduction in microalbuminuria. This study demonstrates a direct link between the abnormally active red cell cation transport systems in sickle cell disease and sickle organopathy. Small variations in the activity of these transport mechanisms predict for SCN and measurement of them may help identify those at risk, while pharmaceutical manipulation of these excessively active systems may ameliorate their risk.

Retinal thickness measurements in sickle cell patients with HbSS and HbSC genotype

OBJECTIVE

Temporal macula thinning has been reported in sickle cell patients, but it remains unclear if there is a difference between HbSS and HbSC genotypes. We aimed to quantitatively compare macular thickness between eyes with HbSS and HbSC genotype.

DESIGN

Retrospective descriptive study.

METHODS

Consecutive patients seen over a 5.5-year period in the Ophthalmology Department at St Thomas' Hospital, London, were identified. Macular optical coherence tomography images were retrospectively analyzed. The retinal thickness in all 9 subfields of the Early Treatment Diabetic Retinopathy Study (ETDRS) grid was compared between HbSS and HbSC eyes. Right eyes and left eyes were analyzed independently, as well as averaged measurements from both eyes. Comparison was made between the 2 genotypes, adjusting for age and sex, and for multiple testing. Scans were excluded in cases of poor fixation or ocular comorbidity affecting retinal thickness.

RESULTS

132 HbSC and 120 HbSS patients were identified. Scans from 166 right and 153 left eyes were included (with approximately equal numbers of HbSS and HbSC genotypes). Mean retinal thickness was lower in HbSS eyes compared with HbSC eyes in all subfields of the ETDRS grid, but in most subfields the difference was <10 microns. Differences reached statistical significance for outer superior, inferior, and temporal subfields and the inner temporal subfield (p < 0.05).

CONCLUSION

Although the HbSC genotype is more strongly associated with proliferative retinopathy, HbSS patients had on average more macular thinning.

Disorders of erythrocyte hydration

The erythrocyte contains a network of pathways that regulate salt and water content in the face of extracellular and intracellular osmotic perturbations. This allows the erythrocyte to maintain a narrow range of cell hemoglobin concentration, a process critical for normal red blood cell function and survival. Primary disorders that perturb volume homeostasis jeopardize the erythrocyte and may lead to its premature destruction. These disorders are marked by clinical, laboratory, and physiologic heterogeneity. Recent studies have revealed that these disorders are also marked by genetic heterogeneity. They have implicated roles for several proteins, PIEZO1, a mammalian mechanosensory protein; GLUT1, the glucose transporter; SLC4A1, the anion transporter; RhAG, the Rh-associated glycoprotein; KCNN4, the Gardos channel; and ABCB6, an adenosine triphosphate-binding cassette family member, in the maintenance of erythrocyte volume homeostasis. Secondary disorders of erythrocyte hydration include sickle cell disease, thalassemia, hemoglobin CC, and hereditary spherocytosis, where cellular dehydration may be a significant contributor to disease pathology and clinical complications. Understanding the pathways regulating erythrocyte water and solute content may reveal innovative strategies to maintain normal volume in disorders associated with primary or secondary cellular dehydration. These mechanisms will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment beyond the erythrocyte.

The super sickling haemoglobin HbS-Oman: a study of red cell sickling, K+ permeability and associations with disease severity in patients heterozygous for HbA and HbS-Oman (HbA/S-Oman genotype)

Studying different sickle cell genotypes may throw light on the pathogenesis of sickle cell disease (SCD). Here, the clinical profile, red cell sickling and K⁺ permeability in 29 SCD patients (15 patients with severe disease and 14 with a milder form) of HbA/S-Oman genotype were analysed. The super sickling nature of this Hb variant was confirmed. The red cell membrane permeability to K⁺ was markedly abnormal with elevated activities of P_sickle , Gardos channel and KCl cotransporter (KCC). Results were consistent with Ca²⁺ entry and Mg²⁺ loss via P_sickle stimulating Gardos channel and KCC activities. The abnormal red cell behaviour was similar to that in the commonest genotype of SCD, HbSS, in which the level of mutated Hb is considerably higher. Although activities of all three K⁺ transporters also correlated with the level of HbS-Oman, there was no association between transport phenotype and disease severity. The super sickling behaviour of HbS-Oman may obviate the need for solute loss and red cell dehydration to encourage Hb polymerisation, required in other SCD genotypes. Disease severity was reduced by concurrent α thalassaemia, as observed in other SCD genotypes, and represents an obvious genetic marker for prognostic tests of severity in young SCD patients of the HbA/S-Oman genotype.