Contributions
Abstract: PB1819
Type: Publication Only
Session title: Thalassemias
Background
In thalassemias there are structural variants that, however, present a phenotype of α-thalassemia called thalassemic hemoglobinopathies, they are included in the so-called α-thalassemia no deletion. Non-deletion thalassemia represents 12% of all α-thalassemia in our country and 36% is due to thalassemic hemoglobinopathies
Aims
We present a new mutation in CD65 of the HBA1 gene, where the change AAG>TAG (Ala>Pro) generates a thalassemic hemoglobinopathy.
Methods
The propositus, a 37-year-old man from Madrid, was studied because he presented sustained microcytosis without iron deficiency. Hb A2 and Hb F levels were measured by ion exchange HPLC (VARIANT II). Hemoglobin was studied by capillary zone electrophoresis and ion exchange HPLC. The most frequent α-thalassemia mutations were ruled out by multiplex PCR (Alpha-Globin StripAssay kit) and molecular characterization was performed by automatic sequencing of the α globin genes.
Results
The propositus presented microcytosis (MCV 78.3 fL) with hypochromia (HCM 26.3 pg), without anemia (Hb 14.1 g /dL) and normal reticulocytes (1.01%). No abnormal hemoglobins were detected and Hb A2 and Hb F levels were within normality (2.6% and 0.8% respectively). Molecular characterization of the globin α1 gene, by automatic sequencing, identified a new HBA1: c.196G>C transversion mutation, which resulted in an amino acid change from Ala>Pro at codon 65 of exon 2 in the heterozygous state [α165(E14)Ala>Pro;HBA1:c.196G>C] which we have named Hb Maruchi.
Conclusion
The binding of heme to globin involves a very specific stereochemical adjustment that helps stabilize the tertiary conformation of the subunit. The heme group inserts into a cleft between the E and F helices. While the vinyl groups are asymmetrically located in the hydrophobic interior of the cleft, the propionic acids of porphyrin are oriented toward the hydrophilic surface, interacting with certain nonpolar amino acids in the CD, E, F and FG regions of the α and β globin chains. Most of these nonpolar amino acids are invariant residues. Therefore, it is not surprising that substitutions of any of the residues located in these regions can result in a decrease in the stability of the heme-globin bond and, subsequently, of the globin chain. The CD65 of the HBA1 gene encodes an Ala corresponding to position E14 of the α1 globin chain, located inside the pocket of the heme group. In Hb Maruchi this Ala is replaced by a Pro, which causes instability of the α1 globin chain, this instability is enhanced by the fact that Pro is an amino acid that cannot participate in the configuration of the alpha helix of the secondary structure deforming said structure. This new variant causes thalassemia instead of hemolysis because its α chains are unstable, catabolizing very rapidly before the formation of αβ dimers. The α1Maruchi gene does not produce functional α chains, hence the thalassemia phenotype. This variant is so unstable and catabolizes so rapidly that its detection by electrophoretic and chromatographic methods is impossible. The diagnosis of this type of mutations is important because the association with α0-thalassemia can lead to more severe forms such as Hb H disease or hydrops fetalis.
Keyword(s): Hemoglobin variants, Hemoglobinopathy, Thalassemia
Abstract: PB1819
Type: Publication Only
Session title: Thalassemias
Background
In thalassemias there are structural variants that, however, present a phenotype of α-thalassemia called thalassemic hemoglobinopathies, they are included in the so-called α-thalassemia no deletion. Non-deletion thalassemia represents 12% of all α-thalassemia in our country and 36% is due to thalassemic hemoglobinopathies
Aims
We present a new mutation in CD65 of the HBA1 gene, where the change AAG>TAG (Ala>Pro) generates a thalassemic hemoglobinopathy.
Methods
The propositus, a 37-year-old man from Madrid, was studied because he presented sustained microcytosis without iron deficiency. Hb A2 and Hb F levels were measured by ion exchange HPLC (VARIANT II). Hemoglobin was studied by capillary zone electrophoresis and ion exchange HPLC. The most frequent α-thalassemia mutations were ruled out by multiplex PCR (Alpha-Globin StripAssay kit) and molecular characterization was performed by automatic sequencing of the α globin genes.
Results
The propositus presented microcytosis (MCV 78.3 fL) with hypochromia (HCM 26.3 pg), without anemia (Hb 14.1 g /dL) and normal reticulocytes (1.01%). No abnormal hemoglobins were detected and Hb A2 and Hb F levels were within normality (2.6% and 0.8% respectively). Molecular characterization of the globin α1 gene, by automatic sequencing, identified a new HBA1: c.196G>C transversion mutation, which resulted in an amino acid change from Ala>Pro at codon 65 of exon 2 in the heterozygous state [α165(E14)Ala>Pro;HBA1:c.196G>C] which we have named Hb Maruchi.
Conclusion
The binding of heme to globin involves a very specific stereochemical adjustment that helps stabilize the tertiary conformation of the subunit. The heme group inserts into a cleft between the E and F helices. While the vinyl groups are asymmetrically located in the hydrophobic interior of the cleft, the propionic acids of porphyrin are oriented toward the hydrophilic surface, interacting with certain nonpolar amino acids in the CD, E, F and FG regions of the α and β globin chains. Most of these nonpolar amino acids are invariant residues. Therefore, it is not surprising that substitutions of any of the residues located in these regions can result in a decrease in the stability of the heme-globin bond and, subsequently, of the globin chain. The CD65 of the HBA1 gene encodes an Ala corresponding to position E14 of the α1 globin chain, located inside the pocket of the heme group. In Hb Maruchi this Ala is replaced by a Pro, which causes instability of the α1 globin chain, this instability is enhanced by the fact that Pro is an amino acid that cannot participate in the configuration of the alpha helix of the secondary structure deforming said structure. This new variant causes thalassemia instead of hemolysis because its α chains are unstable, catabolizing very rapidly before the formation of αβ dimers. The α1Maruchi gene does not produce functional α chains, hence the thalassemia phenotype. This variant is so unstable and catabolizes so rapidly that its detection by electrophoretic and chromatographic methods is impossible. The diagnosis of this type of mutations is important because the association with α0-thalassemia can lead to more severe forms such as Hb H disease or hydrops fetalis.
Keyword(s): Hemoglobin variants, Hemoglobinopathy, Thalassemia