First Description in Tunisia of a Point Mutation at Codon 119 (CCT→TCT) in the α1-Globin Gene: Hb Groene Hart in Association with the − α3.7Deletion

[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.

Description of the phenotypes of 63 heterozygous, homozygous and compound heterozygous patients carrying the Hb Groene Hart [α119(H2)Pro→Ser; HBA1: c.358C>T] variant

We here report the phenotypes and genotypes of 63 patients of North African origin, carriers of Hb Groene Hart [Hb GH, α119(H2)Pro → Ser; HBA1: c.358C>T], an α(+)-thalassemia (α(+)-thal) hemoglobin (Hb) variant. Fifty patients were heterozygous, five were homozygous and eight also carried the common -α(3.7) (rightward) deletion in compound heterozygosity. The expression of the α(GH)-globin chain is increased in the following order: heterozygous, compound heterozygous and homozygous. Parallel significant changes of mean corpuscular Hb (MCH) and mean corpuscular volume (MCV) were also observed. Our large cohort of Hb GH carriers could have been obtained by the systematic realization of globin chain separation by reversed phase liquid chromatography (RP-LC) in our routine Hb testing.

Two complex associations of an HBD mutation and a rare α hemoglobinopathy

We present two case reports in which an HBD mutation is present with a rare α hemoglobinopathy that substantially complicates the associated phenotype. In the first case, a new δ-globin variant, Hb A2-Pierre-Bénite [δ83(EF7)Gly→Arg; HBD: c.250G>C] is associated with Hb Groene Hart [α119(H2)Pro→Ser (α1); HBA1: c.358C>T], an α-thalassemic variant. In the second case, a δ(+)-thalassemic variant, δ4(A1)Thr→Ile; HBD: c.14C>T, is associated with a newly described deletion of the hypersensitive site 40 (HS-40) region on the α-globin gene cluster. In both patients, a δ-globin mutation was suspected because of an abnormally low Hb A2 level, whereas the α hemoglobinopathy was sought to explain the slight microcytosis and hypochromia presented by the probands.

Novel α2 gene deletion (c.349_359 del GAGTTCACCCC) identified in association with the -α3.7 deletion

We report a novel α2 gene mutation identified in compound heterozygosity with the common -α(3.7) deletion. The mutation (c.349_359 del GAGTTCACCCC), causes a frameshift after codon α115 with the predicted extension of the αchain from 141 to 166 residues. The new polypeptide would be expected to have a mass of 17,463 Da, but no such product was detected in hemolysates, confirming the mutation's thalassemic phenotype. Mechanistically, the deletion is probably caused by replication slippage during DNA synthesis as the sequence is bracketed by a CC repeat. The lack of protein expression is probably caused by loss of critical binding sites to the chaperone, α Hb stabilizing protein (AHSP).

Analysis of human alpha globin gene mutations that impair binding to the alpha hemoglobin stabilizing protein

Alpha hemoglobin stabilizing protein (AHSP) reversibly binds nascent alpha globin to maintain its native structure and facilitate its incorporation into hemoglobin A. Previous studies indicate that some naturally occurring human alpha globin mutations may destabilize the protein by inhibiting its interactions with AHSP. However, these mutations could also affect hemoglobin A production through AHSP-independent effects, including reduced binding to beta globin. We analyzed 6 human alpha globin variants with altered AHSP contact surfaces. Alpha globin amino acid substitutions H103Y, H103R, F117S, and P119S impaired interactions with both AHSP and beta globin. These mutations are destabilizing in biochemical assays and are associated with microcytosis and anemia in humans. By contrast, K99E and K99N alpha globins bind beta globin normally but exhibit attenuated binding to AHSP. These mutations impair protein folding and expression in vitro and appear to be mildly destabilizing in vivo. In Escherichia coli and erythroid cells, alpha globin K99E stability is rescued on coexpression with AHSP mutants in which binding to the abnormal globin chain is restored. Our results better define the biochemical properties of some alpha globin variants and support the hypothesis that AHSP promotes alpha globin chain stability during human erythropoiesis.

Detection of a thalassemic alpha-chain variant (Hemoglobin Groene Hart) by reversed-phase liquid chromatography

BACKGROUND

Hemoglobin (Hb) Groene Hart [alpha119 (H2)Pro-->Ser (alpha1)], also known as Hb Bernalda, is a nondeletional alpha-thalassemic Hb variant that is frequent in southern Italy and North Africa. This variant is not supposed to be produced in the erythrocytes of carriers. The alpha-thalassemic behavior of this variant has been explained as an impaired interaction between the alpha-globin chain and the alpha-Hb-stabilizing protein.

METHODS

To separate globin chains, we developed a modified reversed-phase liquid chromatography (RPLC) procedure that uses acetonitrile-water solvents containing up to 3 mL/L trifluoroacetic acid. After RPLC, we characterized the isolated globin chains by electrospray ionization (ESI) mass spectrometry (MS) and analyzed their tryptic peptides with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS and nano-LC-ESI-MS/MS.

RESULTS

RPLC detected an abnormal peak with a retention time substantially greater than that of the wild-type alpha(A)-globin chain. We identified this variant as Hb Groene Hart and found it in the hemolysates of 11 unrelated patients (1 homozygote, 9 heterozygotes, and 1 heterozygote associated with the -alpha(3.7) deletion). These patients possessed abnormal hematologic features suggesting an alpha-thalassemia phenotype. Molecular modeling suggested that the increase in hydrophobicity was due to opening of the GH interhelical segment following replacement of amino acid residue 119 with a nonhelix breaker residue.

CONCLUSIONS

This method allows the detection of Hb variants at low concentrations, and adjusting the composition of the organic solvents enables the method to identify Hb variants with large changes in hydrophobicity.

The first case of Hb Groene Hart [alpha119(H2)Pro-->Ser, CCT-->TCT (alpha1)] homozygosity confirms that a thalassemia phenotype is associated with this abnormal hemoglobin variant

Hb Groene Hart [alpha119(H2)Pro-->Ser, CCT-->TCT (alpha1)] has been reported in heterozygotes of Moroccan origin and also in association with the common -alpha(3.7) deletion. In all cases, the mutated protein was not detectable but was apparently associated with a mild alpha-thalassemia (thal) phenotype, presumably due to a modification of the alpha-globin chain domain that is recognized by the a hemoglobin stabilizing protein (AHSP). The present case of Hb Groene Hart homozygosity, confirms that the alpha-thal phenotype is associated with this alpha-globin chain. Hb Groene Hart must be quite frequent not only in Morocco but probably also among the northern African coastal population.

Impaired binding of AHSP to α chain variants: Hb Groene Hart illustrates a mechanism leading to unstable hemoglobins with α thalassemic like syndrome

Alpha hemoglobin stabilizing protein (AHSP) is a small protein of 102 residues induced by GATA-1, Oct-1- and EKLF. It is synthesized at a high level in the red blood cell precursors and acts as a chaperone protecting the alpha hemoglobin (alpha-Hb) chains against precipitation. AHSP and alpha-Hb form a heterodimer complex. In the absence of AHSP, alpha-Hb oxidizes and precipitates within the erythrocyte precursors of the bone marrow leading to apoptosis and defective erythropoiesis. In vitro the binding of AHSP to ferrous alpha-Hb accelerates oxidation of the heme iron in alpha-Hb, but the complex is more resistant to protein unfolding. AHSP could act as a modulating factor in beta-thalassemia. Recent studies showed more severe thalassemic syndromes in patients with decreased levels of AHSP and in one patient who carried a structurally abnormal AHSP. Some alpha-Hb variants with structural abnormality located in the contact area between alpha-Hb and AHSP exhibit an instability and a thalassemic like syndrome. We suggest that this could result from a disturbed interaction between alpha-Hb variants and AHSP. To study this interaction, we constructed the pGEX-alpha-AHSP vector that co-expressed human alpha-Hb and AHSP. Using this approach, we investigated the alpha42 (C7), alpha104 (G11) and alpha119 (H2) sites, where variants with some thalassemic features have been described. Results obtained with recombinant Groene Hart alpha-Hb and Diamant alpha-Hb, in which proline 119 is replaced by a serine and a leucine, respectively, showed clearly an impaired interaction with AHSP. In contrast, the alpha mutants at the sites 42 and 104 exhibit a normal interaction with AHSP. The CO rebinding kinetics of the AHSP/alpha-Hb(42mutant) complexes were similar to those previously obtained with the AHSP/alpha-Hb(WT) complex, which shows a modified rate that is intermediate to the classical Hb allosteric states.