Molecular and cellular pathogenesis of hemoglobin SC disease.

Solution and cell studies were performed to ascertain why individuals with hemoglobin (Hb) SC have disease whereas those with Hb AS do not. The polymerization of deoxygenated mixtures containing sickle cell Hb (Hb S; alpha 2 beta 2(6)Glu leads to Val) and Hb C (alpha 2 beta 2(6)Glu leads to Lys) was investigated by measurements of delay times and solubilities. In mixtures containing more than 40% Hb S, polymerization takes place by the same mechanism as in solutions of Hb S alone, with no evidence for independent crystallization of Hb C. A detailed comparison of Hb S/Hb C and Hb S/Hb A mixtures with identical concentrations and proportions of Hb S show that there is no significant difference in the tendency of Hb C and Hb A to copolymerize with Hb S. In 50:50 Hb S/Hb C mixtures, polymerization is about 15 times more rapid than in 40:60 Hb S/Hb A mixtures at the same total Hb concentration. Measurements on density-fractionated erythrocytes show that SC cells contain a higher total Hb concentration and a more uniform distribution of reticulocytes compared to normal (AA) or sickle trait (AS) cells. The concentration distribution for C trait (AC) cells is much closer to that of SC cells than to AS or AA cells. It appears, therefore, that the presence of Hb C results in the SC cell beginning its life with an abnormally high Hb concentration. From these findings we conclude that both the larger proportion of Hb S and the higher intracellular Hb concentration contribute to the pathogenesis of Hb SC disease.     

Polymerization of recombinant hemoglobin F gamma E6V and hemoglobin F gamma E6V, gamma Q87T alone, and in mixtures with hemoglobin S

To further understand determinants for Hemoglobin (Hb) S polymerization, as well as the inhibitory mechanism of Hb F on Hb S polymerization, Hb F variants containing Val-gamma 6 (Hb F gamma E6V) or Val-gamma 6, Thr-gamma 87 (Hb F gamma E6V, gamma Q87T) were expressed in yeast. The oxy form of Hb F gamma E6V was about 10-fold less stable to mechanical agitation than native oxy Hb F, which is similar to stability differences comparing oxy Hb S and oxy Hb A. Deoxy Hb F gamma E6V showed approximately 20-fold decreased solubility compared with native deoxy Hb F in high phosphate buffer and formed gels like deoxy Hb S in low phosphate buffer, indicating that the Val- gamma 6 substitution decreases solubility of Hb F like Val-beta 6 in deoxy Hb S. Oversaturated deoxy Hb F gamma E6V polymerized without a delay time in low and high phosphate buffers, in contrast to deoxy Hb S, which is accompanied by a distinct delay time before polymerization. Deoxy Hb F gamma E6V, gamma Q87T also polymerized without a delay time like deoxy Hb F gamma E6V. These results suggest that deoxy Hb F gamma E6V gamma Q87T polymers are different from those of deoxy Hb S, and that contact sites differ from those of deoxy Hb S, even though both have the same primary donor (A3) and acceptor sites in the EF helix. These results also suggest that other amino acids in addition to beta 6 Val and amino acids in the F helix are critical for nucleation- controlled polymerization of deoxy Hb S. 1:1 mixtures of deoxy Hb S and either Hb F variant polymerized with a delay time when the concentrations for the Hb S/Hb F gamma E6V and Hb S/Hb F gamma E6V, gamma Q87T mixtures were about 2- and 1.5-fold, respectively, higher than that for Hb S. Logarithmic plots of delay time versus concentration for Hb S/Hb F gamma E6V mixtures showed the same straight line as the line for Hb S/Hb S beta T87Q mixtures, but values for Hb S/Hb F gamma E6V, gamma Q87T mixtures were intermediate between those for Hb S and Hb S/Hb F gamma E6V mixtures. A 1:1 mixture of deoxy Hb A and Hb F gamma E6V, gamma Q87T also polymerized, but exhibited biphasic kinetics, when the concentration was increased to more than 3.5-fold higher than that required for Hb S polymer formation. These results suggest that Gin-gamma 87 is a critical amino acid for exclusion of FS hybrids (alpha 2 beta S gamma) from nuclei formation with Hb S. Our findings also show that Val-gamma 6 in hybrids that form in mixtures of the Hb F variants with either Hb S or Hb A interacts with the hydrophobic acceptor pocket on the EF helix of an adjacent tetramer containing Thr-beta 87.     

Ionic strength dependence of the polymer solubilities of deoxyhemoglobin S + C and S + A mixtures

Factors contributing to the clinical differences between sickle cell- hemoglobin C disease (SC) and the benign sickle cell trait (AS) include the higher proportion of hemoglobin (Hb) S and the higher cell Hb concentrations in SC compared with AS red cells. Reports differ, however, about whether Hb C copolymerizes more than Hb A with Hb S when measured by minimum gelling concentrations (MGCs) and polymer solubilities of the deoxy-Hb mixtures. We now show that the MGCs and solubilities of equimolar mixtures of Hb S + Hb C vary much more with the ionic strength (mu) of the solution than those of Hb S + Hb A mixtures. At mu less than or equal to 0.20, but not at mu greater than 0.25, Hb S + Hb C solubilities were significantly lower than those of Hb S + Hb A. These differences which may reflect a greater effect of the beta 6Lys+ in Hb C at lower mu, can account for the reported discrepancies. The solubility differences were similar in the presence or absence of asymmetric hybrids, and since the intratetramerically cross-linked hybrids alpha 2 beta s beta A and alpha 2 beta s beta c had similar solubilities, they did not indicate the usual mechanism, involving greater incorporation of alpha 2 beta s beta c into the polymers. The small solubility differences between the two Hb mixtures at physiologic (red cell) concentrations of Hb and 2,3- diphosphoglycerate probably play a minor role in the clinical differences between SC and AS states.     

A new form of hereditary persistence of fetal hemoglobin in blacks and its association with sickle cell trait.

A new form of hereditary persistence of fetal hemoglobin (HPFH) producing 3%-8% Hb F in heterozygotes and an elevation of F-cell counts as measured by both the Kleihauer test and an antibody fluorescent procedure was found during the study of a black family. Individuals with this anomaly also had sickle cell trait. A sickle cell homozygote who had apparently inherited the HPFH determinant had 20.3% Hb F. Both types of gamma-chains were present in equal proportions in the Hb F of these individuals. A population study revealed other AS individuals with increased Hb F synthesis, three of whom were sibs. The presence of this previously unrecognized form of HPFH might explain the mild clinical manifestations and the hemoglobin phenotypes of sickle cell homozygotes with unusual elevations of Hb F.     

Solubilization of hemoglobin S by other hemoglobins.

The polymerization of mixtures of Hb S with hemoglobins A, A2, and F has been investigated by analysis of the proportions of S and non-S hemoglobin both in the supernate and in the pellet after centrifugation. In all cases the non-S hemoglobin was incorporated into the polymer even in the absence of hybrids in the order A > A2 > F. The solubility of Hb S is substantially increased by the other hemoglobins, especially by Hb F, which would account for its antisickling effect. It appears that the excluded volume effect of the other hemoglobin on Hb S is largely counterbalanced by the solubilizing effect arising from the interaction between the two hemoglobins in solution. The ability of hybrid hemoglobins to gel was demonstrated directly with tetramers in which alpha beta s dimers were covalently linked to alpha beta A, alpha delta A2, and alpha gamma F dimers.     

Structural bases of the inhibitory effects of hemoglobin F and hemoglobin A2 on the polymerization of hemoglobin S.

We have previously found that the inhibitory effect of hemoglobin F (Hb F) on the polymerization of Hb S proceeds via the formation of asymmetrical hybrid tetramers of the type alpha2betasgamma. Examination of the gelling properties of binary mixtures of Hb S and several Hb variants now shows that, among the gamma chain amino acid residues that differ from those of the beta chain, residues gamma80 (EF4) and gamma87 (F3) are at least partly responsible for this inhibition. Furthermore, we find that mixing Hb A2(alpha2delta2) with Hb S strongly inhibits gelling to an extent similar to that seen with Hb S/Hb F mixtures; this inhibition is attributable to amino acid differences between the delta and beta chain sequences at positions delta22 (B4) and delta87 (F3). Therefore, residues 22, 80, and 87 of the beta chain appear to be involved in intermolecular contact sites that stabilize the deoxy Hb S polymers.     

Hemoglobin S Antilles: a variant with lower solubility than hemoglobin S and producing sickle cell disease in heterozygotes.

We have found a sickling variant, Hb S Antilles, alpha 2 beta 2(6 Glu----Val, 23 Val----Ile), that has the same electrophoretic mobility as Hb S but a distinct isoelectric focus and produces sickling in the carriers of the Hb A/S Antilles trait. The carriers' erythrocytes tend to sickle at O2 partial pressures similar to those that induce sickling in Hb S/C disease. Pure deoxy-Hb S Antilles is 50% as soluble as deoxy-Hb S (saturating concentration = 11 g X dl-1 compared to 18.4 for Hb S). Dilute solutions of pure Hb S Antilles have a lower oxygen affinity than those of Hb A or Hb S (partial pressure for 50% binding is 9 mm Hg compared to 5.5 mm Hg for Hb A or S at pH 7.00). A/S Antilles erythrocytes have a much lower oxygen affinity than A/S cells; this is further decreased in dense cells fractionated on a Percoll density gradient. Their oxygen equilibrium curves had anomalous shapes like those of S/S cells. Fiber formation in the erythrocytes of Hb S Antilles carriers is clearly due to its low solubility and oxygen affinity, showing that heterozygosity for this hemoglobin presents another sickle cell syndrome and suggesting that Hb S heterozygotes who exhibit symptoms of sickle cell disease should be carefully screened for double mutations.     

Microchromatographic quantitation of fetal hemoglobin in patients with sickle cell disease.

A microchromatographic procedure (Isolab Fast Hb Test System) which was developed for the quantitation of Hb AI (10) has been found useful for the quantitation of Hb F in samples that contain Hb S and/or Hb C but no Hb A (% Fmicro). This method has been evaluated through analyses of known mixtures of Hb F and Hb S. The Hb Fmicro levels in patients with sickle cell anemia and related conditions were compared with results obtained by alkali denaturation (% FAD) and conventional DEAE-cellulose chromatography (% FDE). This microchromatographic technique is a fast, simple, and sensitive method for Hb F quantitation in patients with Sickle cell disease.     

Upstream promoter mutation associated with a modest elevation of fetal hemoglobin expression in human adults

In hereditary persistence of fetal hemoglobin, Hb F (alpha 2 gamma 2) is elevated after birth. Screening of sickle cell patients has revealed a family with elevated Hb F and high A gamma values. The propositus was a sickle cell patient with approximately 25% Hb F and 68.4% A gamma. He was heterozygous for the Benin (#19) and Mor beta S haplotypes. Five AS relatives with the Mor haplotype had 2.5% +/- 0.9% fetal hemoglobin and 92.8% +/- 2.8% A gamma, whereas two with the Benin haplotype had normal fetal hemoglobin (0.5%). The Mor haplotype is thus associated with the elevated Hb F in this family. The 13-kilobase (kb) Bg/II fragment containing the G gamma and A gamma genes of the Mor haplotype was cloned, and the G gamma and A gamma promoters sequenced from -383 to beyond the Cap sites. The Mor G gamma gene was normal, but the A gamma gene had a unique C----T mutation at -202. A different mutation at -202 of G gamma (C----G) was previously detected by other researchers in association with considerably higher Hb F in AS cases (15% to 25%). These data suggest either that -202 mutations affect the G gamma and A gamma promoters differently or that different nucleotide substitutions at -202 have divergent effects. Alternatively, additional unknown mutations could cause the differences in gene expression.     

Fetal hemoglobin levels and beta s globin haplotypes in an Indian populations with sickle cell disease

To further explore the cause for variation in hemoglobin F (Hb F) levels in sickle cell disease, the beta globin restriction-fragment length polymorphism haplotypes were determined in a total of 303 (126 SS, 141 AS, 17 S beta degrees, 7 A beta, degrees and 12 AA) Indians from the state of Orissa. The beta s globin gene was found to be linked almost exclusively to a beta S haplotype ( -++-), which is also common in Saudi Arabian patients from the Eastern Province (referred to as the Asian beta s haplotype). By contrast, the majority of beta A and beta degree thalassemia globin genes are linked to haplotypes common in all European and Asian populations (+-----[+/-]; --++-++). Family studies showed that there is a genetic factor elevating Hb F levels dominantly in homozygotes (SS). This factor appears to be related to the Asian beta s globin haplotype, and a mechanism for its action is discussed. There is also a high prevalence of an independent Swiss type hereditary persistence of fetal hemoglobin (HPFH) determinant active in both the sickle cell trait and in sickle cell disease.     

Diagnosis and characterization of Hb C/Hb Iowa: a rare but easily misidentified compound heterozygous condition

Hb Iowa is a rare hemoglobin (Hb) variant with a Gly --> Ala substitution at position 119 of beta-globin. It was previously reported only in an African American infant who was also heterozygous for Hb S [beta6(A3)Glu --> Val] and her mother (Hb A/Iowa). Here we describe the second report of Hb Iowa, the first in conjunction with Hb C [beta6(A3)Glu --> Lys]. The patient was an African American girl, originally diagnosed as homozygous Hb C during neonatal screening. When seen in our clinic, hematological data for both the child and her mother (Hb C trait) indicated mild anemia with slightly low mean corpuscular volume (MCV) but normal red blood cell (RBC) count. Analysis of blood from the child by capillary isoelectric focusing (cIEF) identified Hb C and an unknown Hb variant with an isoelectric point (pI) intermediate to that of Hbs F and A. The unknown variant was identified as Hb Iowa by DNA sequence analysis of the beta-globin gene (GGC --> GCC). Both reported cases of Hb Iowa indicated that there are no abnormal hematological manifestations associated with this rare Hb variant. In both cases, however, Hb Iowa was mistaken for Hb F during routine neonatal screening by high performance liquid chromatography (HPLC) and/or gel IEF. Neonatal misidentification of Hb Iowa led to misdiagnosis of sickle cell disease and Hb C disease, respectively. In our patient, Hb Iowa was also misidentified as Hb A at 2 years of age by a commercial reference laboratory using cellulose acetate and citrate agar gel electrophoresis. This led to an incorrect diagnosis of Hb C trait. These results show that commonly used analytical methods can easily misidentify Hb Iowa as Hbs F or A in neonates, or older individuals, resulting in incorrect identification of the Hb phenotype. We conclude that the presence of Hb Iowa, or other variants with similar pIs, should be considered in cases where the results of follow-up testing conflict with neonatal diagnosis of sickle cell or Hb C disease, or where clinical presentation does not agree with diagnosis of either homozygous or heterozygous Hb S or Hb C.     

Comparison of microcolumn chromatography and laser densitometry of isoelectric focusing strip for the quantitation of Hb A2 and Hb S

Anion exchange microcolumn chromatography is the ICSH (1978) recommended procedure for the quantitation of Hb A2 and Hb S. The use of isoelectric focusing (IEF) followed by laser densitometry of the separated haemoglobin bands is evaluated as a quick and reliable method for quantitation for Hb A2 and Hb S. A paired comparison of 35 specimens, 25 normal and 10 with beta thalassaemia trait, were quantitated for Hb A2. The two groups were well separated by both procedures and Hb A2 levels were similar (r = +0.93, P less than 0.001). In addition, paired analyses of 30 specimens, 20 sickle cell trait and 10 with sickle cell trait combined with alpha thalassaemia, were quantitated for Hb S. The two groups were also well separated by both procedures and Hb S levels were similar (r = +0.94, P less than 0.001). In conclusion, IEF followed by laser densitometry appears to be a reliable, quick procedure for the screening of populations at risk from beta thalassaemia and populations at risk from sickle cell trait, with or without alpha thalassaemia interaction.     

The relationship between fetal hemoglobin level and glycosylation in sickle cell disease

Glycosylated hemoglobin (glyco Hb) was determined by affinity chromatography and Hb S1 and Hb F by Bio-Rex 70 chromatography in patients with sickle cell anemia, SC disease, S beta+-thalassemia, and S beta 0-thalassemia. SC and S beta-thalassemia patients had normal levels of glyco Hb whereas SS patients had significantly lower levels. Within each group of patients a direct correlation existed between Hb F and glyco Hb or Hb S1 levels. A similar relationship was noticed when glyco Hb and Hb F levels were compared in red cell populations of various densities (ages). Hb F seems to influence glycosylation through its effect on red cell survival.     

Pharmacological induction of fetal hemoglobin synthesis using histone deacetylase inhibitors

A number of pharmacological agents are currently available for the induction of the fetal hemoglobin (Hb F) to treat the patients with sickle cell disease and beta-thalassemia. In the present review, we summarized the investigation and development of these Hb F-inducing agents and introduced histone deacetylase inhibitors as the new strategy to induce Hb F to treat the hemoglobin disorders     

Double heterozygous for hemoglobin S and hemoglobin E — a case report from central India

Double heterozygosity for HbS and HbE is rare. HbS and HbE are seen in SC, ST and OBC communities from this part of country. Inter caste marriages amongst these communities have resulted into this compound heterozygous condition. Double heterozygous state for HbS and HbE is clinically silent as compared to HbS-β Thalassaemia and HbSS cases. At Regional Hemoglobinopathy Detection and Management Center, we report a case of 15-year-old male, Teli (OBC) by caste who came for screening for sickle cell disorder. Sickling, solubility test and Hb electrophoresis on agar gel at alkaline pH was carried out. His sickling and solubility tests were positive and on hemoglobin electrophoresis it showed two bands one at Hb A₂ position and another at HbS position. For further confirmation sample was subjected for quantitation of haemoglobin on high performance liquid chromatography (HPLC), Bio-Rad. On quantitation he was having HbS 59.8%, HbE 33.5% and HbF 3.2% confirming his double heterozygous state for HbS and HbE. On family screening his father turned out to be sickle cell trait and mother as hemoglobin E trait.     

Glycosylated hemoglobin levels in a benign form of sickle cell anemia in Saudi Arabia

Glycosylated hemoglobin was determined by the thiobarbituric acid method in sickle cell anemia patients from the Eastern Province of Saudi Arabia. The level of glycosylated hemoglobin in a Saudi SS sample (4.36%, SD 0.83) is 90% of that of the sample of normals (4.85%, SD 0.51). This is in contrast with the reported value of glycosylated hemoglobin in an American Black SS sample (3.9%, SD 0.6), which is only 60% of that of the sample of normals (6.6%, SD 0.7). The fetal hemoglobin level in Saudi sickle cell patients was 12.03% (SD 4.84), which is significantly different from that of Americans of African origin at p = 0.001. There was no significant correlation (r = 0.236) between the percentages of glycosylated Hb and Hb F at the 10% confidence level. The reported positive relationship between the percentages of glycosylated Hb and Hb F in American Blacks seems to be valid in the Saudi population only up to the level of 10-12% of fetal hemoglobin. Above this threshold of Hb F no further alleviating effect is seen. The 2,3-diphosphoglycerate value in Saudi Hb SS adults was 21.7 mumol/g (SD 7.4) and accordingly only twice as high as that of normal individuals. The benign clinical course exhibited by Saudi sickle cell patients is reflected by the survival of the RBC as indexed by its content of glycosylated Hb and 2,3-diphosphoglycerate. Moreover 10-12% of fetal hemoglobin in the RBC seems sufficient to ameliorate the severity of this disease in patients from the Eastern Province of Saudi Arabia.     

High fetal hemoglobin production in sickle cell anemia in the eastern province of Saudi Arabia is genetically determined

Homozygous sickle cell disease in the eastern province of Saudi Arabia is clinically mild. Circulating fetal hemoglobin levels of 16.0 +/- 7.4% were found in these anemic patients, but only 1.09 +/- 0.97% in their sickle trait parents. To determine whether these sickle cell anemia patients inherit an increased capacity to synthesize fetal hemoglobin, a radioimmunoassay of fetal and adult hemoglobin was performed on erythroid progenitor (BFU-E)-derived erythroblasts from Saudi Arabian sickle cell patients and their parents. Mean fetal hemoglobin content per BFU-E-derived erythroblast from Saudi Arabian sickle cell patients was 6.2 +/- 2.4 pg/cell or 30.4 +/- 8.6% fetal hemoglobin (normal 1.1 +/- 0.7 pg/cell and 5.1 +/- 1.8%). Linear regression analysis of % HbF in peripheral blood versus % HbF per BFU-E- derived cell showed a positive correlation with an r of 0.65. The variance of the intrinsic capacity to produce HbF may account for almost 40% (r2) of the variance of circulating fetal hemoglobin but other factors, particularly selective survival of F cells, must also contribute significantly. Despite virtually normal HbF levels in sickle trait parents of these Saudi patients, mean fetal hemoglobin production per BFU-E-derived erythroblast in these individuals was elevated to 3.42 +/- 1.79 pg/cell or 16.1 +/- 6.4% fetal hemoglobin, and the magnitude of fetal hemoglobin production found in parents correlated with that of the patients. These data indicate that the high fetal hemoglobin in Saudi sickle cell disease is genetically determined but expressed only during accelerated erythropoiesis. Further evidence of such genetic determination was provided by analysis of DNA polymorphisms within the beta-globin gene cluster on chromosome 11. This revealed a distinctive 5' globin haplotype (+ + - + +) on at least one chromosome 11 in all high F SS and AS tested. The precise relationship of this haplotype to HbF production in this population remains to be defined.     

Newborn screening shows a high incidence of sickle cell anemia in Central India

There is limited data on the incidence of sickle cell anemia in Central India; we therefore conducted a study to estimate the incidence of this disease in Central India. Mothers who delivered a live baby at the Government Medical College, Nagpur, India were screened for the presence of the sickle cell hemoglobin {Hb S: [β6 (A3) Glu→Val, GAG>GTG]} using the solubility test within 48 hours of delivery. Infants of mothers who showed the presence of Hb S then underwent Hb analysis by high performance liquid chromatography (HPLC). A total of 8243 mothers was screened, 1178 of whom were positive. One thousand, one hundred and sixty-two infants of mothers with a positive solubility test underwent Hb analysis by HPLC; 530 infants were normal, while 536 were heterozygous for Hb S (sickle cell trait), 88 babies were homozygous for Hb S (sickle cell anemia), while another eight babies had other Hb abnormalities. The incidence of sickle cell anemia was highest in the Scheduled caste group (1:50). We concluded that the incidence of sickle cell anemia is high in central India.     

Oxidative reactions of normal and abnormal hemoglobins in the presence of phosphatidylserine vesicles

We have monitored a sequence of reactions involving hemoglobin (Hb) oxidation in the presence of phospholipid bilayers in carefully characterized buffer systems in order to delineate molecular events that may have cytotoxic effects, some of which may even lead to cellular death of erythrocytes. We followed reactions of subunit cross-linking, heme iron oxidation, heme destruction and iron release in normal (Hb A) and sickle [Hb S or beta6(A3)Glu-->Val] Hbs under different experimental conditions. Our results show that, in the presence of lipid surfaces, the reaction rates for heme Fe+2 to Fe+3 oxidation in Hb A and Hb S molecules were both enhanced by lipid surfaces. However, the extent of the enhancement in Hb A and Hb S was quite different in T5K6.5, a Tris buffer with low ionic strength and low pH. In T5K6.5, the rate constants were 0.8 h(-1) for Hb A and 4.3 h(-1) for Hb S, a 5-fold difference. This finding supports a published suggestion that sickle Hb exhibits abnormal heme oxidation in erythrocytes of sickle cell disease patients. We found that the rates were quite similar in P110N7.4, a phosphate buffer near physiological conditions. Even in T5K6.5, the lipid surface did not enhance Hb S reactions involving subunit cross-linking, heme destruction, or iron release. These findings suggest that heme oxidation and related reactions may not be responsible for detrimental cellular events in sickle erythrocytes under physiological conditions, as suggested in the literature.     

A human embryonic hemoglobin inhibits Hb S polymerization in vitro and restores a normal phenotype to mouse models of sickle cell disease

The principle that developmentally silenced globin genes can be reactivated in adults with defects in beta-globin gene expression has been well established both in vitro and in vivo. In practice, levels of developmental stage-discordant fetal gamma globin that can be achieved by using currently approved therapies are generally insufficient to fully resolve typical clincopathological features of sickle cell disease. The therapeutic potential of another developmentally silenced globin--embryonic epsilon globin--has been difficult to evaluate in the absence of a convenient expression system or an appropriate experimental model. The current work analyzes the antisickling properties of an epsilon -globin-containing heterotetramer (Hb Gower-2) both in vitro as well as in vivo in a well-established mouse model of sickle cell anemia. These animals, expressing 100% human Hb S, display a chronic hemolytic anemia with compensatory marrow and extramedullary erythropoiesis, abundant circulating sickled erythrocytes, and chronic tissue damage evidenced by parallel histopathological and functional deficits. By comparison, related mice that coexpress Hb S as well as Hb Gower-2 exhibit normal physiological, morphological, histological, and functional attributes. Subsequent in vitro analyses substantiate results from whole-animal studies, indicating that the polymerization of deoxygenated Hb S can be significantly slowed by relatively small quantities of Hb Gower-2. Together, the in vivo and in vitro analyses suggest that reactivation of epsilon-globin gene expression would be therapeutically beneficial to adults with sickle phenotypes, and provide a rationale for detailed investigations into the molecular basis for its developmental silencing.