Alpha hemoglobin stabilizing protein: Its causal relationship with the severity of beta thalassemia

Increased Expression of α-Hemoglobin Stabilizing Protein (AHSP) mRNA in Erythroid Precursor Cells Isolated from β-Thalassemia Patients Treated with Sirolimus (Rapamycin)

Background/Objectives: in β-thalassemia, important clinical complications are caused by the presence of free α-globin chains in the erythroid cells of β-thalassemia patients. These free α-globin chains are present in excess as a result of the lack of β-globin chains to bind with; they tend to aggregate and precipitate, causing deleterious effects and overall cytotoxicity, maturation arrest of the erythroid cells and, ultimately, ineffective erythropoiesis. The chaperone protein α-hemoglobin-stabilizing protein (AHSP) reversibly binds with free α-globin; the resulting AHSP-αHb complex prevents aggregation and precipitation. Sirolimus (rapamycin) has been previously demonstrated to induce expression of fetal hemoglobin and decrease the excess of free α-globin chain in the erythroid cells of β-thalassemia patients. The objective of this study was to verify whether sirolimus is also able to upregulate AHSP expression in erythroid precursor cells (ErPCs) isolated from β-thalassemia patients. Methods: the expression of AHSP genes was analyzed by measuring the AHSP mRNA content by real-time quantitative PCR (RT-qPCR) and the AHSP protein production by Western blotting. Results: AHSP gene expression was found to be higher in ErPCs of β-thalassemia patients in comparison to ErPCs isolated from healthy subjects. In addition, AHSP expression was further induced by treatment of β-thalassemia ErPCs with sirolimus. Finally, AHSP mRNA was expressed at an increased level in ErPCs of sirolimus-treated β-thalassemia patients participating in the NCT03877809 Sirthalaclin clinical trial. Conclusions: this exploratory study suggests that AHSP expression should be considered as an endpoint in clinical trials based on sirolimus.

Current status of beta-thalassemia and its treatment strategies

Background

Thalassemia is an inherited hematological disorder categorized by a decrease or absence of one or more of the globin chains synthesis. Beta‐thalassemia is caused by one or more mutations in the beta‐globin gene. The absence or reduced amount of beta‐globin chains causes ineffective erythropoiesis which leads to anemia.

Methods

Beta‐thalassemia has been further divided into three main forms: thalassemia major, intermedia, and minor/silent carrier. A more severe form among these is thalassemia major in which individuals depend upon blood transfusion for survival. The high level of iron deposition occurs due to regular blood transfusion therapy.

Results

Overloaded iron raises the synthesis of reactive oxygen species (ROS) that are noxious and prompting the injury to the hepatic, endocrine, and vascular system. Thalassemia can be analyzed and diagnosed via prenatal testing (genetic testing of amniotic fluid), blood smear, complete blood count, and DNA analysis (genetic testing). Treatment of thalassemia intermediate is symptomatic; however; it can also be accomplished by folic supplementation and splenectomy.

Conclusion

Thalassemia major can be cured through regular transfusion of blood, transplantation of bone marrow, iron chelation management, hematopoietic stem cell transplantation, stimulation of fetal hemoglobin production, and gene therapy.

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.

Molecular Characterization of β-Thalassemia Intermedia in Southeast Iran

Inheritance of mild mutations within the β-globin gene and coinheritance of α-thalassemia (α-thal) are known as two important genetic modifiers in β-thalassemia (β-thal) intermedia (β-TI). We aimed to evaluate the spectrum of β- and α-thal mutations in β-TI patients in Southeast Iran. Common β- and α-globin gene mutations were detected by amplification refractory mutation system-polymerase chain reaction (ARMS-PCR) and multiplex gap-PCR, respectively. There were 26 male (57.8%) and 19 female (42.2%) patients. HBB: c.92 + 5T > C [IVS-I-5 (G > C)] and HBB: c.-138C + 1G > A [IVS-II-I (G > A)] represented the prevalent alleles with respective frequencies of 60.0 and 10.0%. Other β-globin mutations included HBB: c.-138C > T [-88 (C > T)], HBB: c.27_28insG [frameshift codons (FSC) 8/9 (+G)], HBB: c.46delT [codon 15 (-T)], HBB: c.93-22_95del (IVS-I, 25 del), and the 619 bp deletion (NG_000007.3: g.71609_72227del619). The predominant genotypic combinations were β(0)/β(0) (68.9%), β(0)/β(+ )(8.9%) and β(+)/β(+ )(2.2%). Coinheritance of α-thal was observed in 33.0% of the patients, with the -α(3.7) (rightward) (NG_000006.1: g.34164_37967del3804) as the most common deletion (86.0%). One patient was diagnosed with the -α(4.2) (leftward) (AF221717) and one with the - -(MED) (g.24664_41064del16401) deletions, while no patients carried the -(α)(20.5) (g.15164_37864del22701), α(-5 nt) (HBA2: c.95 + 2_95_6delTGAGG) or codon 19 (-G) (HBA2: c.56delG) mutations. The alleviating molecular mechanism was not explainable by β(+ )or concurrent α-thal in more than half of our β-TI patients. This encourages conducting more studies to identify other contributing factors, especially Hb F-inducing genetic modifiers.