Diameter of haemoglobin S fibres in sickled cells

Molecular Dynamics of Hemoglobin Reveals Structural Alterations and Explains the Interactions Driving Sickle Cell Fibrillation

In sickle cell anemia, deoxyhemoglobin deforms RBCs by forming fibrils inside that disintegrate on oxygenation. We studied 100 ns long all-atom molecular dynamics (MD) for sickle and normal hemoglobin fibril models to understand this process, complemented by multiple 1 μs MD for a single tetramer of sickle and normal hemoglobin in deoxy and oxy states. We find that the presence of hydrophobic residues without a bulky side chain at β-6 in hemoglobin is the reason for the stability of the fibrils. Moreover, the free energy landscapes from MD of hemoglobin starting in the tensed (T) state capture the putative transition from T to relaxed (R) state, associated with oxygen binding. The three conformational wells in the landscapes are characterized by the quaternary changes where one αβ dimer rotates with respect to the other. The conformational changes from the oxygenation of sickle hemoglobin hinder the intermolecular contacts necessary for fibril formation.

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.

Pluripotent stem cells in research and treatment of hemoglobinopathies

Pluripotent stem cells (PSCs) hold great promise for research and treatment of hemoglobinopathies. In principle, patient-specific induced pluripotent stem cells could be derived from a blood sample, genetically corrected to repair the disease-causing mutation, differentiated into hematopoietic stem cells (HSCs), and returned to the patient to provide a cure through autologous gene and cell therapy. However, there are many challenges at each step of this complex treatment paradigm. Gene repair is currently inefficient in stem cells, but use of zinc finger nucleases and transcription activator-like effector nucleases appear to be a major advance. To date, no successful protocol exists for differentiating PSCs into definitive HSCs. PSCs can be directly differentiated into primitive red blood cells, but not yet in sufficient numbers to enable treating patients, and the cost of clinical scale differentiation is prohibitively expensive with current differentiation methods and efficiencies. Here we review the progress, promise, and remaining hurdles in realizing the potential of PSCs for cell therapy.

Genetic predisposition to β-thalassemia and sickle cell anemia in Turkey: a molecular diagnostic approach

The thalassemia syndromes are a diverse group of inherited disorders that can be characterized according to their insufficient synthesis or absent production of one or more of the globin chains. They are classified in to α, β, γ, δβ, δ, and εγδβ thalassemias depending on the globin chain(s) affected. The β-thalassemias refer to that group of inherited hemoglobin disorders, which are characterized by a reduced synthesis (β(+)-thalassemia) or absence (β(0)-thalassemia) of beta globin (β-globin) chain production (1). Though known as single-gene disorders, hemoglobinopathies such as β-thalassemia and sickle cell anemia are far from being fully resolved in terms of cure, considering the less complex nature of the beta globin (β-globin) gene family compared to more complex multifactorial genetic disorders such as cancer. Currently, there are no definitive therapeutic options for patients with β-thalassemia and sickle cell anemia, and new insights into the pathogenesis of these devastating diseases are urgently needed. Here we address in detail the overall picture utilizing molecular diagnostic approaches that contribute to unraveling the population-specific mutational analysis of β-globin gene. We also present approaches for molecular diagnostic strategies that are applicable to β-thalassemia, sickle cell anemia, and other genetic disorders.

Gene replacement therapy for sickle cell disease and other blood disorders

Previous studies have demonstrated that sickle cell disease (SCD) can be corrected in mouse models by transduction of hematopoietic stem cells with lentiviral vectors containing anti-sickling globin genes followed by transplantation of these cells into syngeneic recipients. Although self-inactivating (SIN) lentiviral vectors with or without insulator elements should provide a safe and effective treatment in humans, some concerns about insertional mutagenesis persist. An ideal correction would involve replacement of the sickle globin gene (betaS) with a normal copy of the gene (betaA). We recently derived embryonic stem (ES) cells from a novel knockin mouse model of SCD and tested a protocol for correcting the sickle mutation by homologous recombination. Animals derived after gene replacement produced high levels of normal human hemoglobin (HbA), and the pathology associated with SCD was corrected. These experiments provided a foundation for similar studies in which our group collaborated with Rudolf Jaenisch's laboratory to correct SCD by gene replacement in iPS (induced pluripotent stem) cells derived by direct reprogramming of sickle skin fibroblasts. Corrected iPS cells were differentiated into hematopoeitic progenitors that were transplanted into irradiated sickle recipients. The transplanted animals produced high levels of normal human HbA, and the pathology of SCD was corrected. These proof-of-principle studies provide a foundation for the development of gene replacement therapy for human patients with SCD and other blood disorders.

Correction of sickle cell disease by homologous recombination in embryonic stem cells

Previous studies have demonstrated that sickle cell disease (SCD) can be corrected in mouse models by transduction of hematopoietic stem cells with lentiviral vectors containing antisickling globin genes followed by transplantation of these cells into syngeneic recipients. Although self-inactivating (SIN) lentiviral vectors with or without insulator elements should provide a safe and effective treatment in humans, some concerns about insertional mutagenesis persist. An ideal correction would involve replacement of the sickle globin gene (beta(S)) with a normal copy of the gene (beta(A)). We recently derived embryonic stem (ES) cells from a novel knock-in mouse model of SCD and tested a protocol for correcting the sickle mutation by homologous recombination. In this paper, we demonstrate the replacement of the human beta(S)-globin gene with a human beta(A)-globin gene and the derivation of mice from these cells. The animals produce high levels of normal human hemoglobin (HbA) and the pathology associated with SCD is corrected. Hematologic values are restored to normal levels and organ pathology is ameliorated. These experiments provide a foundation for similar studies in human ES cells derived from sickle cell patients. Although efficient methods for production of human ES cells by somatic nuclear transfer must be developed, the data in this paper demonstrate that sickle cell disease can be corrected without the risk of insertional mutagenesis.

Correction of a mouse model of sickle cell disease: lentiviral/antisickling beta-globin gene transduction of unmobilized, purified hematopoietic stem cells

Although sickle cell anemia was the first hereditary disease to be understood at the molecular level, there is still no adequate long-term treatment. Allogeneic bone marrow transplantation is the only available cure, but this procedure is limited to a minority of patients with an available, histocompatible donor. Autologous transplantation of bone marrow stem cells that are transduced with a stably expressed, antisickling globin gene would benefit a majority of patients with sickle cell disease. Therefore, the development of a gene therapy protocol that corrects the disease in an animal model and is directly translatable to human patients is critical. A method is described in which unmobilized, highly purified bone marrow stem cells are transduced with a minimum amount of self-inactivating (SIN) lentiviral vector containing a potent antisickling beta-globin gene. These cells, which were transduced in the absence of cytokine stimulation, fully reconstitute irradiated recipients and correct the hemolytic anemia and organ pathology that characterize the disease in humans. The mean increase of hemoglobin concentration was 46 g/L (4.6 g/dL) and the average lentiviral copy number was 2.2; therefore, a 21-g/L /vector copy increase (2.1-g/dL) was achieved. This transduction protocol may be directly translatable to patients with sickle cell disease who cannot tolerate current bone marrow mobilization procedures and may not safely be exposed to large viral loads.

Participation and strength of interaction of lysine 95(beta) in the polymerization of hemoglobin S as determined by its site-directed substitution by isoleucine

The role of Lys-95(beta), which is on the exterior of the hemoglobin (HbS) tetramer, in the aggregation process has been addressed because there is a lack of agreement on its importance. The early studies on the aggregation of HbS in the presence of other mutant hemoglobins are consistent with the subsequent electron microscopic studies in demonstrating the participation of Lys-95(beta) in gelation; the results of the crystal structure do not agree with these conclusions. Therefore, with the objective of clarifying its role we have carried out site-directed substitution of Lys-95(beta) to an isoleucine residue. The mutation was introduced by polymerase chain reaction recombination methodology, and the absence of other mutations in the beta-globin gene was established by sequencing the gene in its entirety. The recombinant mutant hemoglobin was expressed in yeast and characterized by peptide mapping and sequencing, which demonstrated that the only different tryptic peptide had the Ile substitution at position 95(beta). The recombinant hemoglobin had the correct amino acid composition and molecular weight by mass spectrometric analysis. It was also pure as judged by isoelectric focusing. It was fully functional because it had an average Hill coefficient of 3.1 and responded normally to the allosteric regulators, chloride, 2,3-diphosphoglycerate, and inositol hexaphosphate. Of particular interest was the finding that this hemoglobin mutant aggregated at a concentration of about 40 g/dl, nearly twice that at which HbS itself aggregated (24 g/dl). Therefore, Lys-95(beta) has a very important role in the aggregation process and is a good candidate site for the design of a therapeutic agent for sickle cell anemia.

Recombinant human hemoglobins designed for gene therapy of sickle cell disease

Two human hemoglobins designed to inhibit the polymerization of sickle hemoglobin (Hb S; alpha 2 beta S2) have been produced. Mutations that disrupt the ability of Hb S to form polymers were introduced into the normal human beta-globin gene by site-specific mutagenesis. These mutations affect the axial and lateral contacts in the sickle fiber. The recombinant hemoglobin designated anti-sickling hemoglobin 1 (Hb AS1) contains the mutations beta 22 glutamic acid to alanine and beta 80 asparagine to lysine. Hb AS2 has the same beta 22 glutamic acid to alanine mutation combined with beta 87 threonine to glutamine. Human alpha- and beta AS-globin genes were separately fused downstream of beta-globin locus control region sequences and these constructs were coinjected into fertilized mouse eggs. Transgenic mouse lines that synthesize high levels of each anti-sickling hemoglobin were established and anti-sickling hemoglobins were purified from hemolysates and characterized. Both AS hemoglobins bind oxygen cooperatively and the oxygen affinities of these molecules are in the normal range. Delay time experiments demonstrate that Hb AS2 is a potent inhibitor of Hb S polymerization; therefore, locus control region beta AS2-globin gene constructs may be suitable for future gene therapy of sickle cell disease.

Human sickle hemoglobin in transgenic mice

DNA molecules that contain the human alpha- and beta s-globin genes inserted downstream of erythroid-specific, deoxyribonuclease I super-hypersensitive sites were coinjected into fertilized mouse eggs and a transgenic mouse line was established that synthesizes human sickle hemoglobin (Hb S). These animals were bred to beta-thalassemic mice to reduce endogenous mouse globin levels. When erythrocytes from these mice were deoxygenated, greater than 90 percent of the cells displayed the same characteristic sickled shapes as erythrocytes from humans with sickle cell disease. Compared to controls the mice have decreased hematocrits, elevated reticulocyte counts, lower hemoglobin concentrations, and splenomegaly, which are all indications of the anemia associated with human sickle cell disease.

Pairings and polarities of the 14 strands in sickle cell hemoglobin fibers

Sickle cell anemia results from the formation of hemoglobin S fibers in erythrocytes, and a greater understanding of the structure of these fibers should provide insights into the basis of the disease and aid in the development of effective antisickling agents. Improved reconstructions from electron micrographs of negatively stained single hemoglobin S fibers or embedded fiber bundles reveal that the 14 strands of the fiber are organized into pairs. The strands in each of the seven pairs are half-staggered, and from longitudinal views the polarity of each pair can be determined. The positions of the pairs and their polarities (three in one orientation; four in the opposite orientation) suggest a close relationship with the crystals of deoxyhemoglobin S composed of antiparallel pairs of half-staggered strands.

Fibers, crystals, and other forms of HbS polymers in deoxygenated sickle erythrocytes

We have examined by electron microscopy the formation of fibers and crystals from sickle hemoglobin within sickle erythrocytes following deoxygenation during capillary storage from 1 to 132 days. Intracellular fibers were found on the first day and throughout the period of study. The fibers exhibited a diameter (mean +/- SD) of 17.4 +/- 0.62 nm and were aligned in the cell with a fiber-to-fiber spacing of 18.6 nm (x-axis) by 22.7 nm (y-axis). Between 65 and 132 days, extracellular hemoglobin crystals developed, with a lattice periodicity of 9.63 +/- 0.6 nm. Fibers and crystals coexist as separate structures. These results suggest that crystal formation upon storage of packed deoxygenated sickle erythrocytes may proceed via a phase of fiber dissolution followed by hemoglobin reassembly into extracellular crystals, rather than by a progressive alignment and direct fusion of existing fibers.

X-ray diffraction studies of 14-filament models of deoxygenated sickle cell hemoglobin fibers. Models based on electron micrograph reconstructions

The transforms of a large number of models of deoxygenated sickle hemoglobin fibers, related to that derived from image reconstruction of electron micrographs, have been calculated and compared with X-ray diffraction data of 15 A resolution. The model of the fiber, determined from the reconstructed image, is a helix consisting of 14 filaments that associate in a specific mode to form seven pairs, or protofilaments. Pairs were identified through the pattern of filament loss in partially disassembled fibers and by the separation between molecules, in adjacent filaments, of half a molecular diameter, along the fiber axis. An alternative mode of filament association can be derived also from the surface lattice of the reconstruction, which meets these criteria for the pairing of molecular filaments. Both pairing modes have been used in the search for structures whose transforms show the best agreement with the diffraction data. Models were generated by the systematic translation of six protofilaments, taken in symmetry related pairs, in steps of 3.5 A along the fiber axis relative to a fixed central protofilament. Each translation of a protofilament corresponds to a different fiber model, whose transform was compared with observed data. In all, over 11,000 transforms were calculated. Of all the models considered, three have been found whose residuals are minimal. At 30 A resolution, similar to that of electron micrographs, the model derived from image reconstruction and the three found through our search procedure are indistinguishable. At 15 A, however, the transforms of these models show better agreement with the observed data than the transform of the reconstructed image. Comparison of residuals shows that the model derived from the reconstructed image can be rejected with 99.5% probability relative to the model, with the same pairing scheme, found by our search procedures. The two other models, derived from the alternative pairing scheme, are also more credible than the reconstructed image, but at a lower confidence level. Each of our three models is equally acceptable. Their existence may reflect structural polymorphism of the fiber.

Kinetics of nucleation-controlled polymerization. A perturbation treatment for use with a secondary pathway

We present a perturbation method for analyzing nucleation-controlled polymerization augmented by a secondary pathway for polymer growth. With this method, the solution to the kinetic equations assumes a simple analytic closed form that can easily be used in fitting data. So long as the formation of polymers by the secondary pathway depends linearly on the concentration of monomers polymerized, the form of the solutions is the same. This permits the analysis of augmented growth models with a minimum number of modeling assumptions, and thus makes it readily possible to distinguish between a variety of secondary processes (heterogeneous nucleation, lateral growth, and fragmentation). In addition, the parameters of the homogeneous process, such as the homogeneous nucleus size, can be determined independent of the nature of the secondary mechanism. We describe applications of this method to the polymerization of actin, collagen, and sickle hemoglobin. We present an extensive analysis of data on actin polymerization (Wegner, A., and P. Savko, 1982, Biochemistry, 21:1909-1913) to illustrate the use of the method. Although our conclusions generally agree with theirs, we find that lateral growth describes the secondary pathway better than the fragmentation model originally proposed. We also show how this method can be used to study the degree of polymerization, the parentage of polymers, and the behavior of polymers in cycling experiments.

Sickle cell hemoglobin fiber structure altered by alpha-chain mutation

Hybrid hemoglobin molecules prepared with beta chains from hemoglobin S (beta 6 Glu leads to Val) and alpha chains from hemoglobin Sealy (alpha 47 Asp leads to His) form fibers with a novel structure. In contrast to the typical fibers of hemoglobin S with an average diameter of 22 nm and a solid cross section composed of 10 outer filaments surrounding a 4-filament core, the fibers of the alpha Sealy2 beta S2 hybrid are much larger, with a mean diameter of 32 nm and a unique double-hollow arrangement of filaments. Sealy--S fibers can be described by a model in which the two pairs of filaments most readily lost from fibers of hemoglobin S are missing to form the hollow regions, with an additional sheath of filaments added to form the overall larger structure.

Patterns in the quinary structures of proteins. Plasticity and inequivalence of individual molecules in helical arrays of sickle cell hemoglobin and tubulin

The four recognized levels of organization of protein structure (primary through quaternary) are extended to add the designation quinary structure for the interactions within helical arrays, such as found for sickle cell hemoglobin fibers or tubulin units in microtubules. For sickle cell hemoglobin the main quinary structure is a 14-filament fiber, with a number of other minor forms also encountered. Degenerate forms of the 14-filament fibers can be characterized that lack specific pairs of filaments; evidence is presented which suggests an overall organization of the 14 filaments in pairs, with particular pairs aligned in an antiparallel orientation. For tubulin, a range of quinary structures can be detected depending on the number of protofilaments and whether adjacent protofilaments composed of alternating alpha- and beta-subunits are aligned with contacts between like or unlike subunits and with parallel or antiparallel polarity. Thus, in contrast to quarternary structure, which generally involves a fixed number of subunits, the quinary structures of proteins can exhibit marked plasticity and inequivalence in the juxtaposition of constituent molecules.

Determination of deoxyhemoglobin S polymer in sickle erythrocytes upon deoxygenation

We have used 13C/1H magnetic double-resonance spectroscopy to measure the amount of sickle hemoglobin polymer within sickle erythrocytes as a function of oxygen saturation. We previously showed that the methods of cross-polarization and scalar decoupling could be used to measure accurately the polymer fraction in deoxygenated sickle hemoglobin solutions [Noguchi, C.T., Torchia, D.A. & Schechter, A.N. (1979) Proc. Natl. Acad. Sci. USA 76, 4936-4940]. Our measurements show that the amount of intracellular deoxyhemoglobin S polymer increases monotonically with decreasing oxygen saturation. Polymer can be detected at oxygen saturation values above 90%. This result can be theoretically explained by the excluded volume effect of the oxyhemoglobin S in the cell. The very high total intracellular hemoglobin concentration (34 g/dl) reduces the amount of soluble deoxyhemoglobin S to about 3 g/dl at 90% oxygen saturation. The agreement between theory and experiment indicates that the equilibrium properties of intracellular polymerization can be described by the analyses resulting from studies of concentrated sickle hemoglobin solutions. The curve for polymer formation as a function of oxygen saturation is roughly hyperbolic whereas that for cell sickling is sigmoidal; the difference is most apparent for measurements at pH 7.65. Intracellular polymer formation may in general have a different relationship to oxygen saturation than cell sickling and may be a more meaningful parameter of the pathophysiological process in sickle cell anemia than cell morphology. In addition, measurements of intracellular polymer should be useful in evaluating potential therapeutic agents.