Two novel unstable hemoglobin variants due to in-frame deletions of key amino acids in the β-globin chain

Hb Monza: A novel extensive HBB duplication with preserved α-β subunit interaction and unstable hemoglobin phenotype

BACKGROUND

Unstable hemoglobins are caused by single amino acid substitutions in the HBB gene, often affecting key histidine residues, leading to protein destabilization and hemolytic crises. In contrast, long HBB variants, exceeding 20 bp, are rare and associated with a β-thalassemia phenotype due to disrupted α-β chain interactions. We describe a family wherein four of six members carry a novel 23-amino-acid in-frame duplication of HBB (c.176_244dup), named hemoglobin (Hb) Monza. Despite its length, this duplication manifests as an unstable hemoglobin variant rather than a β-thalassemia phenotype.

METHODS

A static 3D model of the Hb Monza β chain was generated using AlphaFold and SWISS-MODEL. Molecular dynamics (MD) simulations were performed with the Generalized Born implicit solvent model. After energy minimization and heating to 311 K (38°C), a 40 ns production run was conducted.

FINDINGS

3D modeling of Hb Monza revealed minimal structural changes in the Hb β chain, particularly in the key histidine residues and their interaction with the iron atom. Additionally, the static 3D model showed a preserved α-β interaction, explaining the absence of a β-thalassemia clinical phenotype. MD simulations under thermal stress revealed a notable increase in root-mean-square deviation compared to the wild-type β subunit, along with a loss of contacts with the heme, explaining the hemolytic crises during febrile episodes.

CONCLUSION

Despite the long duplication in HBB, Hb Monza retains functional α-β interaction while demonstrating instability under stressful conditions. This unique variant presents with an unstable Hb phenotype rather than a β-thalassemia phenotype.

FUNDING

No financial funding was received.

Molecular and phenotype characterization of an elongated β-globin variant produced by HBB:C.313delA

INTRODUCTION

β-thalassemia is a severe hereditary hemolytic anemia. Due to the diversity of mutations spectrum, β-thalassemia manifests a highly heterogeneous clinical severity. We noted that a previous report characterized HBB:c.313delA, at the end of exon 2, as a β-thalassemia trait rather than dominant β-thalassemia, the classification given to similar mutations. We further explored the impact of this functional variant on globin structure through larger pedigree analysis and in vitro studies.

METHODS

Hematological analysis and molecular genotyping were conducted on the proband and his family members. We evaluated functional effects of the variant on β-globin gene in the proband's nucleated erythrocytes and transfected HEK-293T cells. Three-dimensional construction of protein structure was carried out in silico to demonstrate amino acid changes.

RESULTS

The thalassemia major proband was identified as a compound heterozygote of HBB:c.313delA and HBB:c.126_129delCTTT. Three family members with heterozygotes of HBB:c.313delA displayed microcytic hypochromic anemia. Molecular characterization demonstrated that the frameshift mutation could give rise to retro-positioning of the termination codon, resulting in an elongated β-globin chain with an extension of 10 amino acids. Clinical phenotype and functional experiments indicated that HBB:c.313delA led to β⁰ -thalassemia phenotype.

CONCLUSION

We concluded that the phenotype of HBB:c.313delA was mainly related to the stability of mutant mRNA, the degradation of mutant proteins, and production of inclusion bodies according to a systematic description of clinical phenotype and a series of molecular experiments.

Characterization of a Novel 71.8 kb α0-Thalassemia Deletion and Subsequent Summary of a Practical Procedure for Thalassemia Molecular Diagnosis

Thalassemia is the most common monogenic disorder around the world. Based on the principle of genotype-phenotype correlation, identification of thalassemia mutations is the essential prerequisite for clinical diagnosis and management. Because only common mutations are routinely detected, the identification of rare or undetermined mutations is a challenge for clinical laboratories. Herein, a proband presenting with inconsistent phenotype-genotype correlation after routine molecular screening was investigated by multiplex ligation-dependent probe amplification (MLPA), targeted-next generation sequencing (targeted-NGS), gap-polymerase chain reaction (gap-PCR) and Sanger sequencing. Eventually, a novel 71.8 kb deletion (- -^71.8) was identified and characterized, which included HBZ (ζ), HBA2 (α2), and HBA1 (α1) genes and was causing α⁰-thalassemia (α⁰-thal). Furthermore, we summarized a practical procedure based on accumulated experience in studies and clinical practice, which can be a guide for molecular screening and clinical diagnosis of thalassemia, especially for identification of undetermined or novel mutations.