Mouse alpha chains inhibit polymerization of hemoglobin induced by human beta S or beta S Antilles chains

Progressive glomerular and tubular damage in sickle cell trait and sickle cell anemia mouse models

Homozygosity for the hemoglobin (Hb) S mutation (HbSS, sickle cell anemia) results in hemoglobin polymerization under hypoxic conditions leading to vaso-occlusion and hemolysis. Sickle cell anemia affects 1:500 African Americans and is a strong risk factor for kidney disease, although the mechanisms are not well understood. Heterozygous inheritance (HbAS; sickle cell trait) affects 1:10 African Americans and is associated with an increased risk for kidney disease in some reports. Using transgenic sickle mice, we investigated the histopathologic, ultrastructural, and gene expression differences with the HbS mutation. Consistent with progressive glomerular damage, we observed progressively greater urine protein concentrations (P = 0.03), glomerular hypertrophy (P = 0.002), and glomerular cellularity (P = 0.01) in HbAA, HbAS, and HbSS mice, respectively. Ultrastructural studies demonstrated progressive podocyte foot process effacement, glomerular basement membrane thickening with reduplication, and tubular villous atrophy with the HbS mutation. Gene expression studies highlighted the differential expression of several genes involved in prostaglandin metabolism (AKR1C18), heme and iron metabolism (HbA-A2, HMOX1, SCL25A37), electrolyte balance (SLC4A1, AQP6), immunity (RSAD2, C3, UBE2O), fatty acid metabolism (FASN), hypoxia hall-mark genes (GCK, SDC3, VEGFA, ETS1, CP, BCL2), as well as genes implicated in other forms of kidney disease (PODXL, ELMO1, FRMD3, MYH9, APOA1). Pathway analysis highlighted increased gene enrichment in focal adhesion, extracellular matrix-receptor interaction, and axon guidance pathways. In summary, using transgenic sickle mice, we observed that inheritance of the HbS mutation is associated with glomerular and tubular damage and identified several candidate genes and pathways for future investigation in sickle cell trait and sickle cell anemia-related kidney disease.

Heme degradation and oxidative stress in murine models for hemoglobinopathies: thalassemia, sickle cell disease and hemoglobin C disease

Red blood cells with abnormal hemoglobins (Hb) are frequently associated with increased hemoglobin autoxidation, accumulation of iron in membranes, increased membrane damage and a shorter red cell life span. The mechanisms for many of these changes have not been elucidated. We have shown in our previous studies that hydrogen peroxide formed in association with hemoglobin autoxidation reacts with hemoglobin and initiates a cascade of reactions that results in heme degradation with the formation of two fluorescent emission bands and the release of iron. Heme degradation was assessed by measuring the fluorescent band at ex 321 nm. A 5.6 fold increase in fluorescence was found in red cells from sickle transgenic mice that expressed exclusively human globins when compared to red cells from control mice. When sickle transgenic mice co-express the gamma M transgene, that expresses HbF and inhibits polymerization, heme degradation is decreased. Mice expressing exclusively hemoglobin C had a 6.9 fold increase in fluorescence compared to control. Heme degradation was also increased 3.5 fold in beta-thalassemic mice generated by deletion of murine beta(major). Membrane bound IgG and red cell metHb were highly correlated with the intensity of the fluorescent heme degradation band. These results suggest that degradation of the heme moiety in intact hemoglobin and/or degradation of free heme by peroxides are higher in pathological RBCs. Concomitant release of iron appears to be responsible for the membrane damage that leads to IgG binding and the removal of red cells from circulation.

Hemoglobinopathies

The outlook for patients with sickle cell disease has improved steadily during the last two decades. In spite of these improvements, curative therapies are currently available only to a small minority of patients. The main theme of this chapter is to describe new therapeutic options that are at different stages of development that might result in further improvements in the outlook for patients with these disorders.

Dr. Joseph DeSimone and his colleagues had previously made the important observation that the hypomethylating agent 5-azacytidine can reverse the switch from adult to fetal hemoglobin in adult baboons. Although similar activity was demonstrated in patients with sickle cell disease and β-thalassemia, concern about the toxicity of 5-azacytidine prevented its widespread use in these disorders. In Section I, Dr. DeSimone discusses the role of DNA methylation in globin gene regulation and describe recent clinical experience with decitabine (an analogue of 5-azacytidine) in patients with sickle cell disease. These encouraging studies demonstrate significant fetal hemoglobin inducing activity of decitabine in patients who fail to respond to hydroxyurea.

In Section II, Dr. George Atweh continues the same theme by describing recent progress in the study of butyrate, another inducer of fetal hemoglobin, in patients with sickle cell disease and β-thalassemia. The main focus of his section is on the use of a combination of butyrate and hydroxyurea to achieve higher levels of fetal hemoglobin that might be necessary for complete amelioration of the clinical manifestations of these disorders. Dr. Atweh also describes novel laboratory studies that shed new light on the mechanisms of fetal hemoglobin induction by butyrate.

In Section III, Dr. Ronald Nagel discusses the different available transgenic sickle mice as experimental models for human sickle cell disease. These experimental models have already had a significant impact on our understanding of the pathophysiology of sickle cell disease. Dr. Nagel describes more recent studies in which transgenic sickle mice provide the first proof of principle that globin gene transfer into hematopoietic stem cells inhibits in vivo sickling and ameliorates the severity of the disease.

Although stroke in adult patients with sickle cell disease is not as common as in children, adult hematologists, like their pediatric colleagues, need to make management decisions in adult patients with a stroke or a history of stroke. Dr. Robert Adams has led several large clinical studies that investigated the role of transfusions in the prevention of stroke in children with sickle cell disease. Much less is known, however, about the prevention of first or subsequent strokes in adult patients with sickle cell disease. In Section IV, Dr. Adams provides some general guidelines for the management of adult patients with stroke while carefully distinguishing between recommendations that are evidence-based and those that are anecdotal in nature.

Cell culture and animal models to screen for promising fetal hemoglobin-stimulating compounds

Sickle cell anemia (SCA) and the thalassemias are globally the most common class of inherited single-gene disorders. Current treatment options are limited, especially in developing countries. More practical and cheaper therapies are urgently needed. Since high fetal hemoglobin (HbF) levels ameliorate the clinical symptoms of these diseases, one current approach is to use pharmacological agents to reactivate the gamma-globin genes and stimulate the production of HbF. Several in vitro and in vivo experimental models developed for this purpose are the subject of this review. The models include in vitro established erythroid-like cell lines and primary cultures (both in semisolid and liquid media) of erythroid progenitor cells obtained from normal donors and patients with SCA and beta-thalassemia, as well as in vivo models in genetically modified (transgenic) and unmodified animals. These experimental systems are useful for large-scale screening of compounds for HbF-stimulating potential, for determining the mechanism of action of potent compounds at the cellular and molecular levels, and for studying the pharmacology, pharmacokinetics, and toxicology of the drugs. These models are essential to find, test, and develop new drugs that will be effective and safe for clinical use in adults and children.

Second generation knockout sickle mice: the effect of HbF

Sickle transgenic mice expressing exclusively human globins are desirable for studying pathophysiology and testing gene therapy strategies, but they must have significant pathology and show evidence of amelioration by antisickling hemoglobins. Mice were generated that expressed exclusively human sickle hemoglobin with 3 levels of HbF using their previously described sickle constructs (cointegrated human miniLCRalpha2 and miniLCRbeta(S) [PNAS 89:12150, 1992]), mouse alpha- and beta-globin-knockouts, and 3 different human gamma-transgenes. It was found that, at all 3 levels of HbF expression, these mice have balanced chain synthesis, nearly normal mean corpuscular hemoglobin, and, in some cases, F cells. Mice with the least adult HbF expression were the most severe. Progressive increase in HbF from less than 3% to 20% to 40% correlated with progressive increase in hematocrit (22% to 34% to 40%) and progressive decrease in reticulocyte count (from 60% to 30% to 13%). Urine concentrating ability was normalized at high HbF, and tissue damage detected by histopathology and organ weight were ameliorated by increased HbF. The gamma-transgene that produces intermediate levels of HbF was introduced into knockout sickle mice described by Pàszty and coworkers that express the miniLCRalpha1(G)gamma(A)gammadeltabeta(S) transgene and have fetal but not adult expression of HbF. It was found that the level of HbF required to ameliorate low hematocrit and normalize urine concentrating defect was different for the miniLCRalpha2beta(S) and miniLCRalpha1(G)gamma(A)gammadeltabeta(S) mice. We conclude that knockout mice with the miniLCRalpha2beta(S) transgene and postnatal expression of HbF have sufficiently faithful sickle pathology to serve as a platform for testing antisickling interventions.

Hemoglobin C in transgenic mice: effect of HbC expression from founders to full mouse globin knockouts

When present in the homozygous form, hemoglobin C (HbC, CC disease) increases red cell density, a feature that is the major factor underlying the pathology in patients with SC disease (Fabry et al., JCI 70, 1315, 1982). The basis for the increased red cell density has not yet been fully defined. We have generated a HbC mouse in which the most successful founder expresses 56% human alpha and 34% human beta(C). We introduced knockouts (KO) of mouse alpha- and beta-globins in various combinations. In contrast to many KO mice, all partial KOs have normal MCH. Full KOs that express exclusively HbC and no mouse globins have minimally reduced MCH (13. 7 +/- 0.3 pg/cell vs 14.5 +/- 1.0 for C57BL/6) and a ratio of beta- to alpha-globin chains of 0.88 determined by chain synthesis; hence, these mice are not thalassemic. Mice with beta(C) > 30% have increased MCHC, dense reticulocytes, and increased K:Cl cotransport. Red cell morphology studied by SEM is strikingly similar to that of human CC cells with bizarre folded cells. We conclude that red cells of these mice have many properties that closely parallel the pathology of human disease in which HbC is the major determinant of pathogenesis. These studies also establish the existence of the interactions with other gene products that are necessary for pleiotropic effects (red cell dehydration, elevated K:Cl cotransport, morphological changes) that are also present in these transgenic mice, validating their usefulness in the analysis of pathophysiological events induced by HbC in red cells.

Interspecies hybrid HbS: complete neutralization of Val6(beta)-dependent polymerization of human beta-chain by pig alpha-chains

Interspecies hybrid HbS (alpha(2)(P)beta(2)(S)), has been assembled in vitro from pig alpha-globin and human beta(S)-chain. The alpha(2)(P)beta(2)(S) retains normal tetrameric structure (alpha(2)beta(2)) of human Hb and an O(2) affinity comparable to that of HbS in 50 mM Hepes buffer; but, its O(2) affinity is slightly higher than that of HbS in the presence of allosteric effectors (chloride, DPG and phosphate). The (1)H-NMR spectroscopy detected distinct differences between the heme environments and alpha(1)beta(1) interfaces of pig Hb and HbS, while their alpha(1)beta(2) interfaces appear very similar. The interspecies hybrid alpha(2)(H)beta(2)(P) resembles pig Hb; the pig beta-chain dictated the conformation of the heme environment of the human alpha-subunit, and to the alpha(1)beta(1) interfaces of the hybrid. In the alpha(2)(P)beta(2)(S) hybrid, beta(S)-chain dictated the conformation of human heme environment to the pig alpha-chain in the hybrid; but the conformation of alpha(1)beta(1) interface of this hybrid is close to, but not identical to that of HbS. On the other hand, the alpha(1)beta(2) interface conformation is identical to that of HbS. More important, the alpha(2)(P)beta(2)(S) does not polymerize when deoxygenated; pig alpha-chain completely neutralizes the beta(S)-chain dependent polymerization. The polymerization inhibitory propensity of pig alpha-chain is higher when it is present in the cis alpha(P)beta(S) dimer relative to that in a trans alpha(P)beta(A) dimer. The semisynthetically generated chimeric pig-human and human-pig alpha-chains by exchanging the alpha(1-30) segments of human and pig alpha-chains have established that the sequence differences of pig alpha(31-141) segment can also completely neutralize the polymerization. Comparison of the electrostatic potential energy landscape of the alpha-chain surfaces of HbS and alpha(2)(P)beta(2)(S) suggests that the differences in electrostatic potential energy surfaces on the alpha-chain of alpha(2)(P)beta(2)(S) relative to that in HbS, particularly the ones involving CD region, E-helix and EF-corner of pig alpha-chain are responsible for the polymerization neutralization activity. The pig and human-pig chimeric alpha-chains can serve as blueprints for the design of a new generation of variants of alpha-chain(s) suitable for the gene therapy of sickle cell disease.

Inhibition of beta(S)-chain dependent polymerization by synergistic complementation of contact site perturbations of alpha-chain: application of semisynthetic chimeric alpha-chains

Mouse alpha(1-30)-horse alpha(31-141) chimeric alpha-chain, a semisynthetic super-inhibitory alpha-chain, inhibits beta(S)-chain dependent polymerization better than both parent alpha-chains. Although contact site sequence differences are absent in the alpha(1-30) region of the chimeric chain, the four sequence differences of the region alpha(17-22) could induce perturbations of the side chains at alpha(16), alpha(20) and alpha(23), the three contact sites of the region. A synergistic complementation of such contact site perturbation with that of horse alpha(31-141) probably results in the super-inhibitory activity of the chimeric alpha-chain. The inhibitory contact site sequence differences, by themselves, could also exhibit similar synergistic complementation. Accordingly, the polymerization inhibitory activity of Hb Le-Lamentin (LM) mutation [His20(alpha)-->Gln], a contact site sequence difference, engineered into human-horse chimeric alpha-chain has been investigated to map such a synergistic complementation. Gln20(alpha) has little effect on the O(2) affinity of HbS, but in human-horse chimeric alpha-chain it reduces the O(2) affinity slightly. In the chimeric alpha-chain, Gln20(alpha) increased sensitivity of the betabeta cleft for the DPG influence, reflecting a cross-talk between the alpha(1)beta(1) interface and betabeta cleft in this semisynthetic chimeric HbS. In the human alpha-chain frame, the polymerization inhibitory activity of Gln20(alpha) is higher compared with horse alpha(1-30), but lower than mouse alpha(1-30). Gln20(alpha) synergistically complements the inhibitory propensity of horse alpha(31-141). However, the inhibitory activity of LM-horse chimeric alpha-chain is still lower than that of mouse-horse chimeric alpha-chain. Therefore, perturbation of multiple contact sites in the alpha(1-30) region of the mouse-horse chimeric alpha-chain and its linkage with the inhibitory propensity of horse alpha(31-141) has been now invoked to explain the super-inhibitory activity of the chimeric alpha-chain. The 'linkage-map' of contact sites can serve as a blueprint for designing synergistic complementation of multiple contact sites into alpha-chains as a strategy for generating super-inhibitory antisickling hemoglobins for gene therapy of sickle cell disease.

Transgenic knockout mice exclusively expressing human hemoglobin S after transfer of a 240-kb betas-globin yeast artificial chromosome: A mouse model of sickle cell anemia

Sickle cell anemia (SCA) and thalassemia are among the most common genetic diseases worldwide. Current approaches to the development of murine models of SCA involve the elimination of functional murine alpha- and beta-globin genes and substitution with human alpha and betas transgenes. Recently, two groups have produced mice that exclusively express human HbS. The transgenic lines used in these studies were produced by coinjection of human alpha-, gamma-, and beta-globin constructs. Thus, all of the transgenes are integrated at a single chromosomal site. Studies in transgenic mice have demonstrated that the normal gene order and spatial organization of the members of the human beta-globin gene family are required for appropriate developmental and stage-restricted expression of the genes. As the cis-acting sequences that participate in activation and silencing of the gamma- and beta-globin genes are not fully defined, murine models that preserve the normal structure of the locus are likely to have significant advantages for validating future therapies for SCA. To produce a model of SCA that recapitulates not only the phenotype, but also the genotype of patients with SCA, we have generated mice that exclusively express HbS after transfer of a 240-kb betas yeast artificial chromosome. These mice have hemolytic anemia, 10% irreversibly sickled cells in their peripheral blood, reticulocytosis, and other phenotypic features of SCA.

Inhibition of sickle beta-chain (betaS)-dependent polymerization by nonhuman alpha-chains. A superinhibitory mouse-horse chimeric alpha-chain

Horse alpha-chain inhibits sickle beta-chain-dependent polymerization; however, its inhibitory potential is not as high as that of mouse alpha-chain. Horse alpha-(1-30) and alpha-(31-141) segments make, respectively, minor and major contributions to the inhibitory potential of horse alpha-chain. The sum of the inhibitory potential of the two segments does not account for the inhibitory potential of the full-length horse alpha-chain. Although the polymerization inhibitory potential of horse alpha-chain is lower than mouse alpha-chain, the inhibitory potential of horse alpha-(31-141) is comparable to that of mouse alpha-(31-141). When mouse alpha-(1-30) is stitched to horse alpha-(31-141), the product is a chimeric alpha-chain with an inhibitory potential greater than mouse alpha-chain. In contrast, the stitching of horse alpha-(1-30) with mouse alpha-(31-141) had no additional inhibitory potential. Molecular modeling studies of HbS containing the mouse-horse chimeric alpha-chain indicate altered side-chain interactions at the alpha1beta1 interface when compared with HbS. In addition, the AB/GH corner perturbations facilitate a different stereochemistry for the interaction of the epsilon-amino group of Lys-16(alpha) with the beta-carboxyl group of Asp-116(alpha), resulting in a decrease in the accessibility of the side chain of Lys-16(alpha) to the solvent. Based on molecular modeling, we speculate that these perturbations by themselves, or in synergy with the altered conformational aspects of the alpha1beta1 interactions, represent the molecular basis of the superinhibitory potential of the mouse-horse chimeric alpha-chains.

Knockout-transgenic mouse model of sickle cell disease

When transgenic mice that expressed human sickle hemoglobin were mated with mice having knockout mutations of the mouse alpha- and beta-globin genes, animals were produced that synthesized only human hemoglobin in adult red blood cells. Similar to many human patients with sickle cell disease, the mice developed a severe hemolytic anemia and extensive organ pathology. Numerous sickled erythrocytes were observed in peripheral blood. Although chronically anemic, most animals survived for 2 to 9 months and were fertile. Drug and genetic therapies can now be tested in this mouse model of sickle cell disease.

Magnetic resonance evidence of hypoxia in a homozygous alpha-knockout of a transgenic mouse model for sickle cell disease

All transgenic mouse models for sickle cell disease express residual levels of mouse globins which complicate the interpretation of experimental results. We now report on a mouse expressing high levels of human betaS and 100% human alpha-globin. These mice were created by breeding the alpha-knockout and the mouse beta(major)-deletion to homozygosity in mice expressing human alpha- and betaS-transgenes. These betaS-alpha-knockout mice have accelerated red cell destruction, altered hematological indices, ongoing organ damage, and pathology under ambient conditions which are comparable with those found in alphaH betaS-Ant[betaMDD] mice without introduction of additional mutations which convert betaS into a "super-betaS" such as the doubly mutated betaS-Antilles. This is of particular importance for testing strategies for gene therapy of sickle cell disease. Spin echo magnetic resonance imaging at room air and 100% oxygen demonstrated the presence of blood hypoxia (high levels of deoxygenated hemoglobin) in the liver and kidneys that was absent in control mice. We demonstrate here that transgenic mice can be useful to test new noninvasive diagnostic procedures, since the magnetic resonance imaging technique described here potentially can be applied to patients with sickle cell disease.

Lethal alpha-thalassaemia created by gene targeting in mice and its genetic rescue

Mutations at the alpha-globin locus are the most common class of mutations in humans, with deletion of all four adult alpha-globin genes resulting in the perinatal lethal condition haemoglobin Barts hydrops fetalis. Using gene targeting in mice, we have deleted a 16 kilobase region encompassing both adult alpha-globin genes. Animals homozygous for this deletion become hydropic and die late in gestation mimicking humans with hydrops fetalis. Introduction of a human alpha-globin transgene rescued these animals from perinatal death thus demonstrating the utility of this murine model in the development of cellular and gene based approaches for treating this human genetic disease.

Symmetric interspecies hybrids of mouse and human hemoglobin: molecular basis of their abnormal oxygen affinity

Interspecies hybrids of HbA and Hb from mouse C57BL/10 [alpha 2M beta 2H and alpha 2H beta 2M (H = human, M = mouse)], representing 19 and 27 sequence differences per alpha beta dimers (as compared with human alpha beta dimer) have been generated in vitro. The efficiency of the assembly of the interspecies hybrids by the alloplex intermediate pathway is about twofold higher than the low-pH-mediated subunit approach. The interspecies hybrids exhibit a cooperative O2 binding. The intrinsic O2 affinity of mouse Hb is slightly lower than HbA, while the 2,3-diphosphoglycerate (DPG) effect is comparable. Interestingly, the interspecies hybrid alpha 2M beta 2H has high O2 affinity (compared to either human or mouse Hb), while the interspecies hybrid alpha 2H beta 2M exhibits a very low O2 affinity. These results suggest that the mouse beta chain generates a tetramer with very low oxygen affinity. However, the complementarity of the mouse alpha and beta chains generates a set of unique interactions that compensate for the low-oxygen-affinity propensity of the mouse beta chain. DPG binds the tetramer in the central cavity formed by the two beta subunits, hence the DPG effects on the interspecies hybrids should be as in the parent molecule. However, the results of the present study demonstrate that the DPG binding pocket is influenced by the nature of the alpha chain present in the tetramer. The mouse alpha chain reduces considerably the DPG right shift of the O2 affinity of the human beta-chain containing hybrid.(ABSTRACT TRUNCATED AT 250 WORDS)

Transgenic animal models of sickle cell disease

An animal model which allows study of chronic processes occurring in sickle cell disease has finally been realized with the development of several lines of transgenic mice which express high levels of beta s or beta s-variants in their red cells. The red cells of all mouse lines exhibit characteristic sickle shapes on deoxygenation and most lines have enlarged spleens and mildly elevated reticulocytes in adult mice; both of these properties are suggestive of enhanced red cell destruction and erythropoiesis. However, all lines examined to date have normal hemoglobin levels in adult mice. In one mouse line under ambient conditions, retinopathy and abnormal renal function have been observed and in the same line under hypoxic conditions, anemia, irreversibly sickled cell formation, and urine concentrating defect have been observed. The current mouse lines will allow some aspects of sickle cell disease to be studied, but significant improvements can still be made in the transgenic mouse model.

High expression of human beta S- and alpha-globins in transgenic mice: erythrocyte abnormalities, organ damage, and the effect of hypoxia

A line of transgenic mice with two cointegrated transgenes, the human beta S- and alpha 2-globin genes, linked to the beta-globin locus control region was produced and bred with mice carrying a deletion of the mouse beta major-globin gene. In transgenic mice homozygous for the beta major deletion (alpha H beta S[beta MDD]; where alpha H is human alpha-globin and MD is mouse deletion), 72.5 +/- 2.4% (mean +/- SD) of the beta-chains are beta S and the ratio of alpha H- to beta S-globin was 0.73. Introduction of a heterozygous mouse alpha-globin deletion into mice homozygous for the beta major deletion (alpha H beta S[alpha MD beta MDD]) resulted in 65.1 +/- 8.5% beta S and a human alpha/beta ratio of 0.89 +/- 0.2. Sickling occurs in 95% of erythrocytes from alpha H beta S[beta MDD] mice after slow deoxygenation. Transmission electron microscopy revealed polymer fiber formation but not fascicles of fiber. Increased organ weight was noted in lung, spleen, and kidney of transgenic mice vs. controls that may be due to hypertrophy or increased blood volume in the lungs and/or increased tissue water content. The hemoglobin content of lung, spleen, and kidney was also elevated in transgenic animals due to trapped hemoglobin and/or increased blood volume. When transgenic and control mice were examined by magnetic resonance imaging at 9.4 tesla, some transgenic animals had enlarged kidneys with prolonged relaxation time, consistent with increased organ weight and water content. The glomerular filtration rate was elevated in transgenic animals, which is characteristic of young sickle cell patients. Furthermore, exposure to hypoxia resulted in significantly decreased hematocrit, increased erythrocyte density, and induced a urine-concentrating defect. We conclude that the transgenic mouse line reported here has chronic organ damage and further hematological and organ dysfunction can be induced by hypoxia.

Hypoxia-induced in vivo sickling of transgenic mouse red cells

To develop an animal model for sickle cell anemia, we have created transgenic mice that express a severe naturally occurring human sickling hemoglobin, Hb S Antilles. Due to its low solubility and oxygen affinity, Hb S Antilles has a greater propensity to cause red cell sickling than Hb S. To make transgenic animals that express a high level of Hb S Antilles, the erythroid-specific DNAse I hypersensitive site II from the human beta-globin cluster was linked independently to the human alpha 2-globin gene and to the beta S Antilles gene. Embryos were injected with both constructs simultaneously and seven transgenic mice were obtained, three of which contained both the human alpha and the human beta S Antilles transgene. After crossing the human transgenes into the mouse beta-thalassemic background a transgenic mouse line was derived in which approximately half the beta-globin chains in the murine red cells were human beta S Antilles. Deoxygenation of the transgenic red cells in vitro resulted in extensive sickling. An increase of in vivo sickling was achieved by placing these transgenic mice in a low oxygen environment. This murine model for red cell sickling should help to advance our understanding of sickle cell disease and may provide a model to test therapeutic interventions.

A transgenic mouse model of sickle cell disorder

A single base-pair mutation (beta s) in codon 6 of the human beta-globin gene, causing a single amino-acid substitution, is the cause of sickle cell anaemia. The mutant haemoglobin molecule, HbS, polymerizes when deoxygenated and causes deformation of the erythrocytes to a characteristic 'sickled' shape. Sickling of cells in small vessels causes painful crises and other life-threatening complications. Although the molecular basis for sickle cell anaemia has been known for 30 years, no definitive treatment is available. An animal model of sickle cell anaemia would not only allow a detailed analysis of the factors that initiate erythrocyte sickling in vivo and of the pathophysiology of the disease, but would also permit the development of novel approaches to the treatment of the disease. By using the dominant control region sequences from the human beta-globin locus, together with human alpha- and beta s-globin genes, we have obtained three transgenic mice with HbS levels ranging from 10 to 80% of total haemoglobin in their red cells. As observed in homozygous and heterozygous Hbs patients, the erythrocytes of this mouse sickle readily on deoxygenation. Irreversibly sickled cells, which are characteristic of sickle-cell patients homozygous for beta s, are also observed in the peripheral blood of the mouse with high levels of HbS.

High-level erythroid expression of human alpha-globin genes in transgenic mice

The human alpha 1-globin gene was fused downstream of two erythroid-specific DNase I super-hypersensitive sites that are normally located upstream of the human beta-globin locus. This construct was injected into fertilized mouse eggs, and expression was analyzed in 16-day fetal livers and brains. All 11 fetuses that contained intact copies of the transgene expressed correctly initiated human alpha-globin mRNA in the erythroid fetal liver but not in brain. Levels of expression ranged from 4% to 337% of endogenous mouse beta-globin mRNA. A human alpha-globin construct that did not contain super-hypersensitive sites was not expressed. These results demonstrate that human beta-globin locus activation sequences can stimulate high levels of human alpha-globin gene expression in erythroid tissue of transgenic mice. The results also provide a foundation for experiments designed to coexpress human alpha- and beta-globin genes in transgenic mice and suggest a feasible approach for production of a mouse model for human sickle cell disease.