Intracellular loss of free alpha chains in beta thalassemia

Chemical methods for protein site-specific ubiquitination

Ubiquitination is an important protein post-translational modification regulating many cellular processes in eukaryotes. Ubiquitination is catalyzed by a three-enzyme cascade resulting in the conjugation of the C-terminal carboxylate of ubiquitin (Ub) to the ε-amino group of a lysine residue in the acceptor protein via an isopeptide bond. In vitro enzymatic ubiquitination utilizing Ub ligases has been successfully employed to generate Ub dimers and polymers. However, limitations of the enzymatic approach exist, particularly due to the requirement of specific Ub ligase for any given target protein and the low catalytic efficiency of the Ub ligase. To achieve an in-depth understanding of the molecular mechanism of Ub signaling, new methods are needed to generate mono- and poly-ubiquitinated proteins at a specific site with defined polyubiquitin chain linkage and length. Chemical methods offer an attractive solution to the above-described challenges. In this review, we summarize the recently developed chemical methods for generating ubiquitinated proteins using synthetic and semisynthetic approaches. These new tools and approaches, as an important part of the Ub toolbox, are crucial to our understanding and exploitation of the Ub system for novel therapeutics.

Diverse fate of ubiquitin chain moieties: The proximal is degraded with the target, and the distal protects the proximal from removal and recycles

One of the enigmas in the ubiquitin (Ub) field is the requirement for a poly-Ub chain as a proteasomal targeting signal. The canonical chain appears to be longer than the distance between the two Ub-binding proteasomal receptors. Furthermore, genetic manipulation has shown that one receptor subunit is sufficient, which suggests that a single Ub can serve as a degradation signal. To shed light on this mystery, we chemically synthesized tetra-Ub, di-Ub (K⁴⁸-based), and mono-Ub adducts of HA-α-globin, where the distal or proximal Ub moieties were tagged differentially with either Myc or Flag. When incubated in a crude cell extract, the distal Ub moiety in the tetra-Ub adduct was mostly removed by deubiquitinating enzymes (DUBs) and reconjugated to other substrates in the extract. In contrast, the proximal moiety was most likely degraded with the substrate. The efficacy of degradation was proportionate to the chain length; while tetra-Ub globin was an efficient substrate, with mono-Ub globin, we observed rapid removal of the Ub moiety with almost no degradation of the free globin. Taken together, these findings suggest that the proximal moieties are necessary for securing the association of the substrate with the proteasome along the proteolytic process, whereas the distal moieties are important in protecting the proximal moieties from premature deubiquitination. Interestingly, when the same experiment was carried out using purified 26S proteasome, mono- and tetra-Ub globin were similarly degraded, highlighting the roles of the entire repertoire of cellular DUBs in regulating the degradation of proteasomal substrates.

Activity-Based Probes Developed by Applying a Sequential Dehydroalanine Formation Strategy to Expressed Proteins Reveal a Potential α-Globin-Modulating Deubiquitinase

We report a general and novel semisynthetic strategy for the preparation of ubiquitinated protein-activity-based probes on the basis of sequential dehydroalanine formation on expressed proteins. We applied this approach to construct a physiologically and therapeutically relevant ubiquitinated α-globin probe, which was used for the enrichment and proteomic identification of α-globin-modulating deubiquitinases. We found USP15 as a potential deubiquitinase for the modulation of α-globin, an excess of which aggravates β-thalassemia symptoms. This development opens new opportunities for activity-based-probe design to shed light on the important aspects underlying ubiquitination and deubiquitination in health and disease.

Synthetic Uncleavable Ubiquitinated Proteins Dissect Proteasome Deubiquitination and Degradation, and Highlight Distinctive Fate of Tetraubiquitin

Various hypotheses have been proposed regarding how chain length, linkage type, position on substrate, and susceptibility to deubiquitinases (DUBs) affect processing of different substrates by proteasome. Here we report a new strategy for the chemical synthesis of ubiquitinated proteins to generate a set of well-defined conjugates bearing an oxime bond between the chain and the substrate. We confirmed that this isopeptide replacement is resistant to DUBs and to shaving by proteasome. Analyzing products generated by proteasomes ranked how chain length governed degradation outcome. Our results support that (1) the cleavage of the proximal isopeptide bond is not a prerequisite for proteasomal degradation, (2) by overcoming trimming at the proteasome, tetraUb is a fundamentally different signal than shorter chains, and (3) the tetra-ubiquitin chain can be degraded with the substrate. Together these results highlight the usefulness of chemistry to dissect the contribution of proteasome-associated DUBs and the complexity of the degradation process.

Nonenzymatic polyubiquitination of expressed proteins

Ubiquitination is one of the most ubiquitous posttranslational modifications in eukaryotes and is involved in various cellular events such as proteasomal degradation and DNA repair. The overwhelming majority of studies aiming to understand ubiquitination and deubiquitination have employed unanchored ubiquitin chains and mono-ubiquitinated proteins. To shed light on these processes at the molecular level, it is crucial to have facile access to ubiquitin chains linked to protein substrates. Such conjugates are highly difficult to prepare homogenously and in workable quantities using the enzymatic machinery. To address this formidable challenge we developed new chemical approaches to covalently attach ubiquitin chains to a protein substrate through its Cys residue. A key aspect of this approach is the installation of acyl hydrazide functionality at the C-terminus of the proximal Ub, which allows, after ubiquitin chain assembly, the introduction of various reactive electrophiles for protein conjugation. Employing α-globin as a model substrate, we demonstrate the facile conjugation to K48-linked ubiquitin chains, bearing up to four ubiquitins, through disulfide and thioether linkages. These bioconjugates were examined for their behavior with the USP2 enzyme, which was found to cleave the ubiquitin chain in a similar manner to unanchored ones. Furthermore, proteasomal degradation study showed that di-ubiquitinated α-globin is rapidly degraded in contrast to the mono-ubiquitinated counterpart, highlighting the importance of the chain lengths on proteasomal degradation. The present work opens unprecedented opportunities in studying the ubiquitin signal by enabling access to site-specifically polyubiquitinated proteins with an increased size and complexity.

Integrated protein quality-control pathways regulate free α-globin in murine β-thalassemia

Cells remove unstable polypeptides through protein quality-control (PQC) pathways such as ubiquitin-mediated proteolysis and autophagy. In the present study, we investigated how these pathways are used in β-thalassemia, a common hemoglobinopathy in which β-globin gene mutations cause the accumulation and precipitation of cytotoxic α-globin subunits. In β-thalassemic erythrocyte precursors, free α-globin was polyubiquitinated and degraded by the proteasome. These cells exhibited enhanced proteasome activity, and transcriptional profiling revealed coordinated induction of most proteasome subunits that was mediated by the stress-response transcription factor Nrf1. In isolated thalassemic cells, short-term proteasome inhibition blocked the degradation of free α-globin. In contrast, prolonged in vivo treatment of β-thalassemic mice with the proteasome inhibitor bortezomib did not enhance the accumulation of free α-globin. Rather, systemic proteasome inhibition activated compensatory proteotoxic stress-response mechanisms, including autophagy, which cooperated with ubiquitin-mediated proteolysis to degrade free α-globin in erythroid cells. Our findings show that multiple interregulated PQC responses degrade excess α-globin. Therefore, β-thalassemia fits into the broader framework of protein-aggregation disorders that use PQC pathways as cell-protective mechanisms.

Protein quality control during erythropoiesis and hemoglobin synthesis

Erythrocytes must regulate hemoglobin synthesis to limit the toxicities of unstable free globin chain subunits. This regulation is particularly relevant in β-thalassemia, in which β-globin deficiency causes accumulation of free α-globin, which forms intracellular precipitates that destroy erythroid precursors. Experimental evidence accumulated over more than 40 years indicates that erythroid cells can neutralize moderate amounts of free α-globin through generalized protein quality control mechanisms, including molecular chaperones, the ubiquitin-proteasome system, and autophagy. In many ways, β-thalassemia resembles protein aggregation disorders of the nervous system, liver, and other tissues, which occur when levels of unstable proteins overwhelm cellular compensatory mechanisms. Information gained from studies of nonerythroid protein aggregation disorders may be exploited to further understand and perhaps treat β-thalassemia.

Loss of alpha-hemoglobin-stabilizing protein impairs erythropoiesis and exacerbates beta-thalassemia

Hemoglobin (Hb) A production during red blood cell development is coordinated to minimize the deleterious effects of free alpha- and beta-Hb subunits, which are unstable and cytotoxic. The alpha-Hb-stabilizing protein (AHSP) is an erythroid protein that specifically binds alpha-Hb and prevents its precipitation in vitro, which suggests that it may function to limit free alpha-Hb toxicities in vivo. We investigated this possibility through gene ablation and biochemical studies. AHSP(-/-) erythrocytes contained hemoglobin precipitates and were short-lived. In hematopoietic tissues, erythroid precursors were elevated in number but exhibited increased apoptosis. Consistent with unstable alpha-Hb, AHSP(-/-) erythrocytes contained increased ROS and evidence of oxidative damage. Moreover, purified recombinant AHSP inhibited ROS production by alpha-Hb in solution. Finally, loss of AHSP worsened the phenotype of beta-thalassemia, a common inherited anemia characterized by excess free alpha-Hb. Together, the data support a model in which AHSP binds alpha-Hb transiently to stabilize its conformation and render it biochemically inert prior to Hb A assembly. This function is essential for normal erythropoiesis and, to a greater extent, in beta-thalassemia. Our findings raise the possibility that altered AHSP expression levels could modulate the severity of beta-thalassemia in humans.

Differential Regulatory and Compensatory Responses in Hematopoiesis/Erythropoiesis in α- and β-Globin Hemizygous Mice

Characterization of hematopoiesis/erythropoiesis in thalassemias from multipotent primitive cells to mature erythrocytes is of fundamental importance and clinical relevance. We investigated this process in alpha- and beta-globin hemizygous mice, lacking the two adult tandemly organized genes from either the alpha- or beta-globin locus. Although both mice backcrossed on a homogeneous background exhibited similar reduced red blood cell (RBC) survival, beta-globin hemizygous mice had less severe reticulocyte loss and globin chain imbalance, suggesting an apparently milder thalassemia than for alpha-globin hemizygous mice. In contrast, however, beta-globin hemizygous mice displayed a more marked perturbation of hematologic parameters. Quantification of erythroid precursor subpopulations in marrow and spleen of beta-globin hemizygous mice showed more severely impaired maturation from the basophilic to orthochromatophilic erythroblasts and substantial loss of these late precursors probably as a consequence of a greater susceptibility to an excess of free alpha-chain than beta-chain. Hence, only erythroid precursors exhibiting stochastically moderate chain imbalance would escape death and mature to reticulocyte/RBC stage, leading to survival and minimal loss of reticulocytes in the beta-globin hemizygous mice. Furthermore, in response to the ineffective erythropoiesis in beta-globin hemizygous mice, a dynamic compensatory hematopoiesis was observed at earlier differentiation stage as evidenced by a significant increase of erythroid progenitors (erythroid colony-forming units approximately 100-fold) as well as of multipotent primitive cells (day 12 spleen colony-forming units approximately 7-fold). This early compensatory mechanism was less pronounced in alpha-globin hemizygous mice. The expansion of multipotent primitive and potentially stem cell populations, taken together with ineffective erythropoiesis and increased reticulocyte/RBC destruction could confer major cumulative advantage for gene targeting/bone marrow transplantation. Therefore, this study not only corroborated the strong potential effectiveness of transplantation for thalassemic hematopoietic therapy but also demonstrated the existence of a differential regulatory response for alpha- and beta-thalassemia.

An abundant erythroid protein that stabilizes free alpha-haemoglobin

The development of red blood cells (erythrocytes) is distinguished by high-level production of the oxygen carrier, haemoglobin A (HbA), a heterotetramer of alpha- and beta-haemoglobin subunits. HbA synthesis is coordinated to minimize the accumulation of free subunits that form cytotoxic precipitates. Molecular chaperones that regulate globin subunit stability, folding or assembly have been proposed to exist but have never been identified. Here we identify a protein stabilizing free alpha-haemoglobin by using a screen for genes induced by the essential erythroid transcription factor GATA-1 (refs 4, 5). Alpha Haemoglobin Stabilizing Protein (AHSP) is an abundant, erythroid-specific protein that forms a stable complex with free alpha-haemoglobin but not with beta-haemoglobin or haemoglobin A (alpha(2)beta(2)). Moreover, AHSP specifically protects free alpha-haemoglobin from precipitation in solution and in live cells. AHSP-gene-ablated mice exhibit reticulocytosis and abnormal erythrocyte morphology with intracellular inclusion bodies that stain positively for denatured haemoglobins. Hence, AHSP is required for normal erythropoiesis, probably acting to block the deleterious effects of free alpha-haemoglobin precipitation. Accordingly, AHSP gene dosage is predicted to modulate pathological states of alpha-haemoglobin excess, such as beta-thalassaemia.

Ubiquitin Aldehyde Increases Adenosine Triphosphate–Dependent Proteolysis of Hemoglobin α-Subunits in β-Thalassemic Hemolysates

Two major causes of the anemia in β-thalassemia are a deficiency in hemoglobin (Hb) β-subunit (and consequently HbA) synthesis and, due to the resulting excess of Hb α-subunits, erythroid cell hemolysis. The hemolytic component might be ameliorated by increasing the intracellular proteolysis of the excess α-subunits. Isolated 3H-labeled α-chains are known to be degraded primarily by the adenosine triphosphate (ATP)- and ubiquitin (Ub)-dependent proteolysis pathway in unfractionated β-thalassemic hemolysates. Our objective was to increase this degradation by targeted intervention. Ub aldehyde (Ubal), a synthetic inhibitor of isopeptidases (proteases that hydrolyze the bond between the Ub polypeptide and its protein adduct), was added to reaction mixtures containing a hemolysate from the blood cells of one of four β-thalassemic donors and 3H-α-chains or 3H-α-globin as a substrate. Optimum enhancement of ATP-dependent degradation occurred at 0.4 to 1.5 μmol/L Ubal and ranged from 29% to 115% for 3H-α-chains and 47% to 96% for 3H-α-globin among the four hemolysates. We suggest that Ubal stimulates 3H-α-subunit proteolysis by inhibition of an isopeptidase(s) that deubiquitinates, or “edits,” Ub-3H-α-subunit conjugates, intermediates in the degradative pathway. In control studies, similarly low Ubal concentrations did not enhance the degradation of 3H-α2β2 (HbA) tetramers or inhibit the activities of methemoglobin reductase and four selected glycolysis pathway enzymes. These and other results may be the basis for a therapeutic approach to β-thalassemia.

Ubiquitin Aldehyde Increases Adenosine Triphosphate–Dependent Proteolysis of Hemoglobin α-Subunits in β-Thalassemic Hemolysates

Two major causes of the anemia in β-thalassemia are a deficiency in hemoglobin (Hb) β-subunit (and consequently HbA) synthesis and, due to the resulting excess of Hb α-subunits, erythroid cell hemolysis. The hemolytic component might be ameliorated by increasing the intracellular proteolysis of the excess α-subunits. Isolated 3H-labeled α-chains are known to be degraded primarily by the adenosine triphosphate (ATP)- and ubiquitin (Ub)-dependent proteolysis pathway in unfractionated β-thalassemic hemolysates. Our objective was to increase this degradation by targeted intervention. Ub aldehyde (Ubal), a synthetic inhibitor of isopeptidases (proteases that hydrolyze the bond between the Ub polypeptide and its protein adduct), was added to reaction mixtures containing a hemolysate from the blood cells of one of four β-thalassemic donors and 3H-α-chains or 3H-α-globin as a substrate. Optimum enhancement of ATP-dependent degradation occurred at 0.4 to 1.5 μmol/L Ubal and ranged from 29% to 115% for 3H-α-chains and 47% to 96% for 3H-α-globin among the four hemolysates. We suggest that Ubal stimulates 3H-α-subunit proteolysis by inhibition of an isopeptidase(s) that deubiquitinates, or “edits,” Ub-3H-α-subunit conjugates, intermediates in the degradative pathway. In control studies, similarly low Ubal concentrations did not enhance the degradation of 3H-α2β2 (HbA) tetramers or inhibit the activities of methemoglobin reductase and four selected glycolysis pathway enzymes. These and other results may be the basis for a therapeutic approach to β-thalassemia.

Fate of alpha-hemoglobin chains and erythrocyte defects in beta-thalassemia

The fate of alpha-hemoglobin chains and the cause of membrane protein defects in thalassemic erythrocytes have been studied in: (1) human beta-thalassemia syndromes, (2) mouse beta-thalassemia, and (3) normal human erythrocytes loaded with purified alpha-hemoglobin chains. The similarity and differences observed in these three systems underline the importance of insoluble alpha chains and the direct relationship between the amount of these chains and the membrane protein defects. Indeed, in addition to the alpha/non-alpha ratio of globin chain synthesis, the proteolysis and instability of alpha chains are major factors in modulating the cellular defects.

Thalassemia: molecular pathology and management

Recent advances in molecular biology have allowed us to develop an almost complete picture of the molecular pathology of the thalassemia syndromes. The different classes of mutations that are responsible for the thalassemia syndromes will be discussed along with the special insights they have provided into the controls of eukaryotic gene expression. While management of these disorders has not kept pace with our understanding of their cause, there have been notable advances in treatment. Perhaps even more exciting is what the future holds, as the continued march of molecular biology is melded with novel approaches to the definitive treatment of thalassemias.

Hemoglobin Mississippi (beta 44ser----cys). Studies of the thalassemic phenotype in a mixed heterozygote with beta +-thalassemia

Hemoglobin Mississippi (HbMS: beta 44ser----cys) has anomalous properties that include disulfide linkages with normal beta-, delta-, gamma-, and alpha-chains, and the formation of high molecular weight multimers. While heterozygotes for HbMS are clinically and hematologically normal and carriers of the beta +-thalassemia gene in our family had mild microcytic anemia, the proband with HbMS-beta +-thalassemia had a hemoglobin level of 7 g/dl, mean corpuscular volume (MCV) of 68 fl, reticulocytes of 2-6%, HbF of 18%, marked anisocytosis and poikilocytosis, and splenomegaly, all features of thalassemia intermedia. With oxidant stress, her erythrocytes developed multiple dispersed Heinz bodies, but HbMS was only mildly unstable. HbMS was susceptible to proteolytic degradation in the presence of ATP. The unexpectedly severe clinical findings in HbMS-beta +-thalassemia may result from the proteolytic digestion of HbMS, as well as the excessive alpha-chains characteristic of beta +-thalassemia, which combined provide the increment of cellular damage that results in the phenotype of thalassemia intermedia.

Thalassemia in southeast Asia: determination of different degrees of severity of anemia in thalassemia

beta (0)-Thalassemia/Hb E in Southeast Asia varies greatly in severity, with hemoglobin levels ranging from 2.5 to 13.5 g/dl, averaging 7.7 g/dl. Results of systematic investigations to find out what determines different levels of severity are reviewed. Concomitant inheritance of alpha-thalassemia significantly decreases the severity. Different degrees of severity in the majority of cases, however, is not due to alpha-thalassemia. Concordance of hemoglobin levels among patients who are sibs prevails, suggesting polygenic factor determinants. Potential factors ruled out as determinants for different levels of severity are discriminating fetal hemoglobin production, erythrocyte superoxide dismutase activity, reticulo-endothelial function, and failure of erythropoiesis compensation. Red cell survival and globin synthesis studies indicate that different degrees of excess of alpha-chains leading to different red cell pathology and survival are responsible for variable severity. Degrees of excess of alpha-chains in this circumstance are probably mainly determined by erythrocyte proteolytic activity. The relationship between the hemoglobin levels and erythrocyte cytosol proteolytic activity in 15 beta(0) -thalassemia/Hb E disease patients in whom a deletional type of alpha-thalassemia had been ruled out by DNA mapping is striking, with a correlation coefficient of 0.78. This finding suggests that modulation of erythrocyte proteolysis is another approach for treatment of thalassemia.

Electrostatic interactions in the assembly of haemoglobin

Haemoglobin is the prototype of an allosteric protein in which cooperative behaviour depends on interaction between unlike subunits. Here we present haematological and biochemical evidence that electrostatic interactions are an important determinant of haemoglobin assembly. Individuals heterozygous for positively charged beta-globin variants have a significantly lower proportion of abnormal haemoglobin than those with negatively charged variants. Moreover, these differences become more pronounced when alpha-thalassaemia is also present. Kinetic experiments using isolated chains indicate that the rate of assembly of the heterotetramer is influenced by alterations in surface charge. A simple electrostatic model is proposed in an attempt to explain these haematological and experimental findings.

Evidence for increased proteolysis in intact beta thalassemia erythroid cells

Much excess alpha chain is synthesized, but little accumulates in the erythroid cells of patients with homozygous beta thalassemia. To determine if the proteases known to exist in erythroid cells play a role in the destruction or alteration of any of this excess alpha chain, thalassemic and nonthalassemic erythroid cells were incubated for 90 minutes with 3H-leucine. The cells were then washed, and incubated twice for 15 minutes in 100 volumes of cold leucine-rich media, a procedure which eliminates almost all intracellular TCA soluble radioactivity. After these incubations levels of TCA soluble and TCA precipitable radioactivity in the cell lysates were determined, and the cells incubated for 120 minutes more in two volumes of leucine-rich media. At the end of this incubation, total TCA soluble and precipitable radioactivity was again determined in the cell lysate, and also in the two hour incubation media. The total increase in TCA soluble radioactivity in the cells and their media was divided by the 0 time TCA precipitable radioactivity, to determine the percent proteolysis labelled globin chains. In five control patients percent proteolysis ranged from 0 to 3.10 (mean = 1.50); in four severe and three mild thalassemia patients percent proteolysis ranged from 5.80 to 14.1 (mean = 11.0). The difference between the control and thalassemic groups was significant at a p of less than 0.001. This data is the first direct evidence that more proteolysis takes place in intact thalassemic cells than in non-thalassemic cells.

Proteolysis in thalassemia: studies with protease inhibitors

Intracellular proteolysis was studied in both thalassemic and normal reticulocytes. The main experiment comprised a short incubation of RBC with [3H] leucine, extensive washings, and further incubation in the presence of cold leucine and protein synthesis inhibitors. In the aliquots removed at various time intervals, the TCA-soluble radioactivity increased in contrast with the TCA-precipitable cpm, which decreased with no exception on prolonged incubation. Results were more pronounced when the initial incubation was carried out in the presence of a valine analog and were markedly inhibited when phenanthroline was added during the second phase of the experiment. Cell-free system gave similar results. "globin" containing analog residues were degraded easier than free alpha- and beta-globin chains; free alpha or beta Hb chains are not proteolysed. Thalassemic and normal reticulocytes did not show any significant differences. It is proposed that the thalassemic RBC contain efficient proteolytic mechanisms up to the end of their maturation, but these prove incapable of proteolysing the excess alpha chains completely. Chemical manipulations aiming to make those chains more digestible constitute another approach to the treatment of thalassemia.

Globin chain biosynthesis in iron deficiency

Globin chain synthesis was studied in seven severely iron-deficient patients before and after treatment with iron. There was no appreciable difference between the individual pre- and post-treatment alpha/beta specific activity ratios and the mean alpha/beta ratio for each group was 1.00+/-SD 0.04. In a further six untreated iron-deficient patients the mean alpha/beta ratio was 1.00+/-SD 0.04. There was therefore no evidence that iron deficiency caused a reduction in alpha/beta ratio. Three patients with beta thalassaemia trait and coexistent iron deficiency had lower alpha/beta ratios before treatment than after treatment with iron. It appeared that iron deficiency had caused reduced alpha chain synthesis in this group. Preliminary experiments have shown that the alpha/beta specific activity ratio of purified haemoglobin A is decreased in iron deficiency, indicating an increase in the size of the free alpha chain pool. It is suggested that iron deficiency may interfere with the proteolytic mechanism normally responsible for the destruction of excess alpha chains. In combined iron deficiency and beta thalassaemia trait, the resulting increase in free alpha chains might act by negative feedback to inhibit further alpha chain synthesis (Blum et al, 1970) thereby reducing the pre-treatment alpha/beta ratio.

Sickle beta-thalassemia: identical twins differing in severity implicate nongenetic factors influencing course

25-yr old female identical twins of Italian-American origin concordant for sickle beta-thalassemia were studied to explain their clinical differences. One of them has been severely affected from childhood with one aplastic crisis, an earlier onset of vaso-occlusive crises, and recent cardiac decompensation; the other twin shows no cardiac decompensation. Similar are their degree of anemia, RBC indices, blood volumes, absence of splenic sequestration, depression of pO2, elevation of p50 and 2,3-DPG, hemoglobin composition, and peripheral blood globin-synthetic rates. Regarding differences, the more severely affected has a shorter 51Cr RBC life span, a greater menstrual blood loss, and is more overweight, whereas the less severely affected has functional asplenia by 99mTc scanning and a larger proportion of RBC with decreased cellular deformability. We conclude that in sickle beta-thalassemia: (1) genotype alone does not determine the clinical course; (2) significant differences in clinical course can occur with almost identical hemoglobin composition and globin synthetic rates; (3) cellular deformability changes do not correlate exactly with clinical course; and (4) functional asplenia and leanness may be advantageous.

Heterozygous beta thalassaemia of unusual severity

The proband of each of three families of Northern European or Italian extraction had an unusual form of heterozygous beta-thalassaemia, confirmed by haematological, genetic and peripheral blood globin synthesis studies. The unusual severity of this disorder was indicated by chronic haemolysis leading to splenectomy and cholecystectomy, by numerous nucleated red cells and reticulocytes in the peripheral blood, and by leg ulcers in one family. The diversity of clinical expression in many family members with heterozygous beta-thalassaemia was striking. Bone marrow examination in the probands showed numerous large inclusion bodies of the type usually found only in thalassaemia major. In addition, there was unbalanced globin synthesis in the bone marrow, in contrast to the more balanced synthesis found in asymptomatic beta-thalassaemia trait. The amount of newly synthesized alpha-chain found in the free alpha-chain pool was markedly elevated. The unbalanced globin synthesis and alpha-chain inclusions in the bone marrow cells suggest that the severity of the disorder in these patients may be due to the inability of their red cell precursors to fully compensate for the thalassaemic defect or to remove excess alpha-chains. The diversity of clinical expression suggests the influence of undefined acquired or genetic factors on the expression of beta-thalassaemia in these families.

Globin synthesis in fractionated Normoblasts of beta-thalassemia heterozygotes

Globin chain synthesis was examined in erythroid cells of increasing maturity, fractionated from the whole bone marrow of beta-thalassemia heterozygotes by a density gradient centrifugation procedure. In experiments using total cell "globin," a gradient of alpha/beta chain ratios was observed, increasing with erythroid cell maturation from unity in the basophilic cells up to 2.0 in reticulocytes. Gel filtration of the lysates from these marrow fractions revealed the presence of free alpha chains even in the most immature cells, the amount of which increased with erythroid cell age; the total alpha/beta ratio derived from gel filtration experiments showed a gradient similar to that observed in the total globin experiments. However, the alpha/beta ratio of the hemoglobin fraction obtained by gel filtration remained constant throughout maturation at an average of 0.65. This latter finding is incompatible with balanced synthesis at any stage of red cell development and excludes the possibility that total beta chain production is higher in the early cells than in the later cells or that alpha chain production in the early cells is reduced to the level of beta chain synthesis. Furthermore, in a Hb S/beta-thalassemia marrow examined, the beta A/beta S ratio remained constant throughout maturation while the alpha/non-alpha ratio showed an increase like that observed in the simple beta-thalassemia heterozygotes. This argues strongly against increased synthesis from either the thalassemic or nonthalassemic beta chain gene being responsible for the balanced synthesis in the immature cells. These findings lead us to suggest that, in beta-thalassemia heterozygotes, a large alpha chain pool is present throughout erythroid cell maturation and that the observed increase in alpha/beta ratios is a function of the ability of those cells to degrade the excess alpha chains.

Free alpha-globin pool in human bone marrow

A pool of free alpha-globin chains was found in the bone marrow samples from three controls, two patients with beta-thalassemia trait, three with sickle beta-thalassemia, three with hemoglobin (Hb) Lepore trait, one with alphabeta-thalassemia, four with homozygous beta-thalassemia, and one doubly heterozygous for Hb Lepore and beta-thalassemia. The average percentage of newly synthesized alpha-chains found in the free alpha-globin pool was 6.2% in the controls and 33.0% in the patients heterozygous for thalassemia or Hb Lepore. These controls and patients had balanced beta- and alpha-globin synthesis in the bone marrow. In the homozygous patients and in the one patient doubly heterozygous for thalassemia and Hb Lepore, there was a marked deficit of beta-chain synthesis in the bone marrow and also a large pool of newly synthesized free alpha-chains. The function of this pool of free alpha-chains is not known, but it may be involved in the regulation of globin chain synthesis in normal patients and in the compensatory synthesis of beta-chains that occurs in the bone marrow of patients heterozygous for thalassemia or for Hb Lepore.

Equal synthesis of - and -globin chains in erythroid precursors in heterozygous -thalassemia

In patients with heterozygous beta-thalassemia, the beta/alpha synthetic ratio in marrow erythroid cells incubated in vitro is 1, whereas in reticulocytes the ratio is 0.5. These ratios reflect the equal synthesis of the two chains on the polyribosomes of the bone marrow and unequal synthesis on the polyribosomes of the peripheral blood reticulocytes. alpha- and beta-chain synthesis is also equal in marrow cells in vivo. Equal synthesis is probably due both to a decrease in alpha-chain synthesis and an increase in beta-chain synthesis in bone marrow erythroid cells and may contribute to the absence of overt hemolysis due to excess alpha-globin chain accumulation in heterozygous beta-thalassemia.