The alpha-hemoglobin stabilizing protein and expression of unstable alpha-Hb variants

Hemoglobin Balkh, a Novel Mutation in Codon 132 of α2-Globin Gene [α132(H15) (+T) or HBA2:C.396dup (p.Val134fs)]: A Case Report and Insight into the Pathophysiology

We report a novel mutation on α2-globin gene leading to an elongated α-chain. This novel frameshift mutation was detected in a 13-year-old boy from Balkh province, Afghanistan. DNA analysis identified an insertion of thymine (T) at codon 132 [HBA2:c.396dup (p.Val134fs)]. We named the novel hemoglobin variant 'Hemoglobin Balkh' after the geographic location from which the patient originated. This novel variant was found in association with α3.7 kb α-globin gene deletion, suggesting a compound heterozygous state that contributes to the patient's clinical presentation.

Effect of Mutations on mRNA and Globin Stability: The Cases of Hb Bernalda/Groene Hart and Hb Southern Italy

We identified two unstable variants in the third exon of α-globin genes: Hb Bernalda/Groene Hart (HBA1:c.358C>T), and Hb Caserta (HBA2:c.79G>A) in cis to Hb Sun Prairie (HBA2:c.391G>C), also named Hb Southern Italy. These mutations occurred in the H helix of the α-globin that is involved in heme contacting, specific recognition of α-hemoglobin-stabilizing protein (AHSP), and α₁β₁ interactions. The carriers showed α-thalassemia phenotype, but one also jaundice and cholelithiasis. Molecular identification of clusters of families in Southern Italy encouraged molecular characterization of mRNA, globin chain analyses, molecular modeling studies, and comparison with globin variants to understand the mechanisms causing the α-thalassemia phenotype. A normal amount of Hb Bernalda/Groene Hart mRNA were found, and molecular modeling highlighted additional H bonds with AHSP. For Hb Southern Italy, showing an unexpected α/β biosynthetic ratio typical of the β-thalassemia type, two different molecular mechanisms were shown: Reduction of the variant mRNA, likely due to the No-Go Decay for the presence of unused triplet ACG at cod 26, and protein instability due to the impairment of AHSP interaction. The UDP glucuronosyltransferase 1A (UGT1A1) genotyping was conclusive in the case of jaundice and cholelithiasis. Multiple approaches are needed to properly identify the mechanisms leading to unstable variants and the effect of a mutation.

Further Characterization of Hb Bronovo [α103(G10)His→Leu; HBA2: c.311A>T] and First Report of the Homozygous State

Hb Bronovo [α103(G10)His→Leu, HBA2: c.311A>T] is an α-globin variant that interferes with and decreases binding efficiency to α hemoglobin (Hb) stabilizing protein (AHSP), a chaperone molecule. The histidine residue at position 103 is integral to the AHSP hydrogen bond formation where disruption results in an increased quantity of cytotoxic free α-globin chains, thereby creating a similar pathophysiology as β-thalassemia (β-thal). We report a family with Hb Bronovo, including a homozygous proband, which resulted from maternal uniparental disomy (UPD). Although not detected by routine studies in previous reports, the variant protein is visible by intact mass spectrometry (MS).

Alpha-hemoglobin-stabilizing protein (AHSP): a modulatory factor in β-thalassemia

Hemoglobin (Hb) is an iron-containing metalloprotein that transports oxygen molecules from the lungs to the rest of the human body. Among the different variants of Hb, HbA1 is the most common and is composed of two alpha (αHb) and two beta globin chains (βHb) constructing a heterotetrameric protein complex (α₂β₂). Due to the higher number of AHSP genes, there is a tendency to produce approximately twice as much of α subunit as β subunit. Therefore, there is a chance of presenting excess α subunit leftover in human blood plasma; excess subunits subsequently bind with each other and aggregates β-thalassemia occurs due to lack of or reduced numbers of βHb subunit. Alpha-hemoglobin-stabilizing protein (AHSP) is a scavenger protein which acts as a molecular chaperon by reversibly binding with free αHb forming a complex (AHSP-αHb) that prevents aggregation and precipitation preventing deleterious effects towards developing serious human diseases including β-thalassemia. Clinical severity worsens if mutations in AHSP gene co-occur in patients with β-thalassemia. Considering the mechanism of action of AHSP and its contribution to ameliorating β-thalassemia severity, it could potentially be used as a modulatory agent in the treatment of β-thalassemia.

Non-immune Hemolysis: Diagnostic Considerations

Non-immune hemolytic anemia (NIHA) is characterized by positive routine hemolytic tests but negative anti-human immunoglobulin (Coombs) test. Hereditary non-immune hemolysis includes disorders of erythrocytic enzymes, membrane, hemoglobin (qualitative and quantitative disorders), as well as the rare hereditary forms of thrombotic microangiopathies. Acquired NIHA includes paroxysmal nocturnal hemolysis (PNH), infections, drug and metal intoxications with as a target red blood cells or endothelium of capillaries, the rare acquired forms of thalassemia or erythrocytic membrane disorders, and hemolysis secondary to a dysfunctioning artificial (prosthetic) cardiac valve. Identification of the specific cause of NIHA is sometimes difficult and requires not only a good knowledge of this entity but mainly a qualified specialized hematologic laboratory. An algorithm to be used in every new patient consulting for NIHA is proposed in the last part of this article.

Molecular characterization of Hb Hamilton Hill (HBA2: c.388delC), a novel HBA2 variant generating a premature termination codon and truncated HBA2 chain

In recent years, the identification of α-thalassemias caused by nondeletional mutations has increased significantly due to the advancement of sensitive molecular genetics tools. We report clinical and experimental data for a novel frameshift mutation caused by a single base deletion at position 388 in exon 3 of the α2-globin gene (HBA2: c.388delC; Hb Hamilton Hill), resulting in the phenotype of α-thalassemia (α-thal). Hb Hamilton Hill was identified in an adult female of unknown ethnicity investigated for unexplained microcytosis. Direct DNA sequencing of the HBA2 gene revealed a heterozygous mutation, HBA2: c.388delC, and the molecular effect of this mutation was assessed experimentally using our previously described in vitro model. The experimental analysis involved transfection of a human bladder carcinoma (5637) cell line with expression vectors carrying either HBA2-wild type (HBA2-WT) or HBA2: c.388delC followed by total RNA purification and cDNA synthesis. Both wild type and mutant gene expression was studied and compared at the transcriptional and translational levels using quantitative real time polymerase chain reaction (qReTi-PCR) and immunofluorochemistry (IFC), respectively. Our experimental data showed a significant reduction by 25.0% (p = 0.04) in the transcriptional activity generated from HBA2: c.388delC compared to HBA2-WT. As a result of this base deletion, a frameshift in the open reading frame generates a premature termination codon (PTC) at codon 132 of exon 3 resulting in the formation of a truncated α-globin chain. The truncated α-globin chain, observed by the IFC technique, is most likely unstable and undergoes a rapid turnover resulting in the thalassemic phenotype.

[Association between hemoglobin Groene Hart and hemoglobin J-Paris-I: first case in Spain]

BACKGROUND AND OBJECTIVE

Thalassemias are the most frequent monogenic disorder around the world. α-thalassemias are due to a deficiency of synthesis in the alpha-globin chain of the hemoglobin (Hb). Hb Groene Hart is a hyperunstable variant. In this work, we have studied 24 cases affected by Hb Groene Hart, one of them associated with Hb J-Paris-I.

PATIENTS AND METHODS

Twenty-four patients from 17 unrelated families were included in this study. The characterization was done by sequencing.

RESULTS

α1 gene sequencing showed the mutation CCT→TCT (Pro→Ser) at codon 119 (Hb Groene Hart) in all patients. In one case, there was an association with Hb J-Paris-I.

CONCLUSIONS

In the Hb Groene Hart, the residue 119 of alpha-globin chain is affected. This amino acid has a key role in preserving the stability of alpha-globin chain. It is also remarkable the presence of this variant in both the immigrant and native population. Thus, the identification of Hb Groene Hart carriers should be considered in the screening of α-thalassemia in Spain, as it is done in Northern Africa.

α-Thalassemia associated with hb instability: a tale of two features. the case of Hb Rogliano or α1 Cod 108(G15)Thr→Asn and Hb Policoro or α2 Cod 124(H7)Ser→Pro

We identified two new variants in the third exon of the α-globin gene in families from southern Italy: the Hb Rogliano, α1 cod108 ACC>AAC or α1[α108(G15)Thr→Asn] and the Hb Policoro, α2 cod124 TCC>CCC or α2[α124(H7)Ser→Pro]. The carriers showed mild α-thalassemia phenotype and abnormal hemoglobin stability features. These mutations occurred in the G and H helices of the α-globin both involved in the specific recognition of AHSP and β1 chain. Molecular characterization of mRNA, globin chain analyses and molecular modelling studies were carried out to highlight the mechanisms causing the α-thalassemia phenotype. The results demonstrated that the α-thalassemia defect associated with the two Hb variants originated by different defects. Hb Rogliano showed an intrinsic instability of the tetramer due to anomalous intra- and inter-chain interactions suggesting that the variant chain is normally synthesized and complexed with AHSP but rapidly degraded because it is unable to form the α1β1 dimers. On the contrary in the case of Hb Policoro two different molecular mechanisms were shown: the reduction of the variant mRNA level by an unclear mechanism and the protein instability due to impairment of AHSP interaction. These data highlighted that multiple approaches, including mRNA quantification, are needed to properly identify the mechanisms leading to the α-thalassemia defect. Elucidation of the specific mechanism leads to the definition of a given phenotype providing important guidance for the diagnosis of unstable variants.

[Role of alpha-hemoglobin molecular chaperone in the hemoglobin formation and clinical expression of some hemoglobinopathies]

Alpha-hemoglobin stabilizing protein (AHSP), described as a chaperone of alpha-hemoglobin (α-Hb), is synthesized at a high concentration in the erythroid precursors. AHSP specifically recognizes the G and H helices of α-Hb and forms a stable complex with free α-Hb until its association with the partner β-subunits. Unlike the free β-Hb which are soluble and form homologous tetramers, freshly synthesized α-Hb chains are highly unstable molecular species which precipitate and generate reactive oxygen species within the erythrocyte precursors of the bone marrow leading to apoptosis and ineffective erythropoiesis. AHSP protects the free α-Hb chains in maintaining it in the soluble state. In this review, we report data from the literature and our laboratory concerning the key role of AHSP in the biosynthesis of Hb and its possible involvement in some disorders of the red blood cell as well as the hemoglobinopathies and we discuss its use as a prognostic tool in thalassemia syndromes.

Role of α-globin H helix in the building of tetrameric human hemoglobin: interaction with α-hemoglobin stabilizing protein (AHSP) and heme molecule

Alpha-Hemoglobin Stabilizing Protein (AHSP) binds to α-hemoglobin (α-Hb) or α-globin and maintains it in a soluble state until its association with the β-Hb chain partner to form Hb tetramers. AHSP specifically recognizes the G and H helices of α-Hb. To investigate the degree of interaction of the various regions of the α-globin H helix with AHSP, this interface was studied by stepwise elimination of regions of the α-globin H helix: five truncated α-Hbs α-Hb1-138, α-Hb1-134, α-Hb1-126, α-Hb1-123, α-Hb1-117 were co-expressed with AHSP as two glutathione-S-transferase (GST) fusion proteins. SDS-PAGE and Western Blot analysis revealed that the level of expression of each truncated α-Hb was similar to that of the wild type α-Hb except the shortest protein α-Hb1-117 which displayed a decreased expression. While truncated GST-α-Hb1-138 and GST-α-Hb1-134 were normally soluble; the shorter globins GST-α-Hb1-126 and GST-α-Hb1-117 were obtained in very low quantities, and the truncated GST-α-Hb1-123 provided the least material. Absorbance and fluorescence studies of complexes showed that the truncated α-Hb1-134 and shorter forms led to modified absorption spectra together with an increased fluorescence emission. This attests that shortening the H helix leads to a lower affinity of the α-globin for the heme. Upon addition of β-Hb, the increase in fluorescence indicates the replacement of AHSP by β-Hb. The CO binding kinetics of different truncated AHSP^WT/α-Hb complexes showed that these Hbs were not functionally normal in terms of the allosteric transition. The N-terminal part of the H helix is primordial for interaction with AHSP and C-terminal part for interaction with heme, both features being required for stability of α-globin chain.

Hb Iberia [α104(G11)Cys → Arg,TGC>CGC (α2) (HBA2:c.313T>C)], a new α-thalassemic hemoglobin variant found in the Iberian Peninsula: report of six cases

We report a new structural defect of the α2-globin chain presenting with moderate microcytic hypochromic anemia, in six individuals from three unrelated families, living in Portugal and Spain. α-Globin gene deletions were ruled out by gap-polymerase chain reaction (gap-PCR) and multiplex ligation-dependent probe amplification (MLPA). Direct sequencing of the α2-globin gene revealed a substitution of codon 104 [α104(G11)Cys→Arg, TGC>CGC (α2) (HBA2:c.313T>C)]. This new variant, not detectable by high performance liquid chromatography (HPLC) or electrophoresis, was called Hb Iberia, as it was observed for the first time in families from the Iberian Peninsula. Although the mutant allele is transcribed, as indicated by the balanced mRNA α/β ratio, the abnormal α2 chain could not form a stable tetramer as the cysteine and arginine residues, located at the α1β1 contact, differ in size, charge and hydrophobicity. Hb Iberia is the third mutation described at codon 104 on the α-globin genes, namely, Hb Sallanches (α2, TGC>TAC) and Hb Oegstgeest (α1, TGC>AGC), also characterized as unstable hemoglobins (Hbs), present on an α-thalassemic phenotype.

Hemoglobin variants: biochemical properties and clinical correlates

Diseases affecting hemoglobin synthesis and function are extremely common worldwide. More than 1000 naturally occurring human hemoglobin variants with single amino acid substitutions throughout the molecule have been discovered, mainly through their clinical and/or laboratory manifestations. These variants alter hemoglobin structure and biochemical properties with physiological effects ranging from insignificant to severe. Studies of these mutations in patients and in the laboratory have produced a wealth of information on hemoglobin biochemistry and biology with significant implications for hematology practice. More generally, landmark studies of hemoglobin performed over the past 60 years have established important paradigms for the disciplines of structural biology, genetics, biochemistry, and medicine. Here we review the major classes of hemoglobin variants, emphasizing general concepts and illustrative examples.

Insights into hemoglobin assembly through in vivo mutagenesis of α-hemoglobin stabilizing protein

α-Hemoglobin stabilizing protein (AHSP) is believed to facilitate adult Hemoglobin A assembly and protect against toxic free α-globin subunits. Recombinant AHSP binds multiple forms of free α-globin to stabilize their structures and inhibit precipitation. However, AHSP also stimulates autooxidation of αO(2) subunit and its rapid conversion to a partially unfolded bishistidyl hemichrome structure. To investigate these biochemical properties, we altered the evolutionarily conserved AHSP proline 30 in recombinantly expressed proteins and introduced identical mutations into the endogenous murine Ahsp gene. In vitro, the P30W AHSP variant bound oxygenated α chains with 30-fold increased affinity. Both P30W and P30A mutant proteins also caused decreased rates of αO(2) autooxidation as compared with wild-type AHSP. Despite these abnormalities, mice harboring P30A or P30W Ahsp mutations exhibited no detectable defects in erythropoiesis at steady state or during induced stresses. Further biochemical studies revealed that the AHSP P30A and P30W substitutions had minimal effects on AHSP interactions with ferric α subunits. Together, our findings indicate that the ability of AHSP to stabilize nascent α chain folding intermediates prior to hemin reduction and incorporation into adult Hemoglobin A is physiologically more important than AHSP interactions with ferrous αO(2) subunits.

Alpha-hemoglobin-stabilizing protein: an erythroid molecular chaperone

Alpha-hemoglobin-stabilizing protein (AHSP) is an erythroid-specific protein that acts as a molecular chaperone for the free α chains of hemoglobin. Evidence strongly suggests that AHSP participates in hemoglobin synthesis and may act to neutralize the cytotoxic effects of excess free alpha-globin subunits that accumulate both in normal and beta-thalassemic erythroid precursor cells. As such, AHSP seems to be essential for normal erythropoiesis, and impaired upregulation of AHSP may lead to premature erythroid cell death, resulting in ineffective erythropoiesis. Reduced AHSP mRNA expression has been associated with clinical variability in some cases of β-thalassemia. It has been shown that αHb variants may also impair AHSP-αHb interactions, leading to pathological conditions that resemble α-thalassemia syndromes. The aim of this paper is to summarize current information concerning the structure and function of AHSP, focusing on its role in normal erythropoiesis and its relevance in health and disease.

The role of alpha-hemoglobin stabilizing protein in redox chemistry, denaturation, and hemoglobin assembly

Hemoglobin biosynthesis in erythrocyte precursors involves several steps. The correct ratios and concentrations of normal alpha (alpha) and beta (beta) globin proteins must be expressed; apoproteins must be folded correctly; heme must be synthesized and incorporated into these globins rapidly; and the individual alpha and beta subunits must be rapidly and correctly assembled into heterotetramers. These events occur on a large scale in vivo, and dysregulation causes serious clinical disorders such as thalassemia syndromes. Recent work has implicated a conserved erythroid protein known as Alpha-Hemoglobin Stabilizing Protein (AHSP) as a participant in these events. Current evidence suggests that AHSP enhances alpha subunit stability and diminishes its participation in harmful redox chemistry. There is also evidence that AHSP facilitates one or more early-stage post-translational hemoglobin biosynthetic events. In this review, recent experimental results are discussed in light of several current models describing globin subunit folding, heme uptake, assembly, and denaturation during hemoglobin synthesis. Particular attention is devoted to molecular interactions with AHSP that relate to alpha chain oxidation and the ability of alpha chains to associate with partner beta chains.