A molecular tool to assess the pathological relevance of alpha-globin DNA variants

Molecular and cellular analysis of three novel alpha2-globin gene promoter mutations [HBA2: c.-59C>T], [HBA2: c.-81C>A] and [HBA2: c.-91G>A] reveal varying patterns of transcriptional and translational activities

While point mutations affecting the promoter region of β-globin gene are widely described, there are no well characterised reports of any point mutations currently found in the promoter of the α2-globin (HBA2) gene. We present clinical and experimental data for three novel HBA2 gene core and proximal promoter mutations. Using an in vitro system designed to assess the impact of point mutations, the three novel [HBA2:c.-59C>T], [HBA2:c.-81C>A] and [HBA2:c.-91G>A] promoter mutations identified in three unrelated patients were analysed for HBA2 gene transcriptional and translational activities. Following the generation and transfection of expression vectors carrying each mutation, the HBA2 transcription activity of the promoters from each mutant was analysed with quantitative real time-PCR (qReTi-PCR) technique. Immunofluorochemistry (IFC) was used to analyse HBA2 protein synthesis. The analyses showed that [HBA2:c.-59C>T] and [HBA2:c.-91G>A] mutant constructs caused significant reduction in the HBA2 transcription levels by 53.7% (p = 0.0008) and 36.2% (p = 0.004), respectively, resulting in markedly lower HBA2 protein labelling when compared to the wild type as shown with subsequent IFC analysis. Conversely, the [HBA2:c.-81C>A] construct showed no significant changes in either transcription (p = 0.089) or in protein labelling when compared to the wild type. The equal pAmp transcription levels found in each group confirmed that the observed labelling differences were not due to varying transfection efficiencies. This study emphasises the importance of in vitro studies to establish the impact of base substitutions on the level of gene expression, and the value of these studies in clinicopathological correlation so that appropriate advice can be given in genetic counselling.

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.

Molecular and cellular analysis of a novel HBA2 mutation (HBA2: c.94A > G) shows activation of a cryptic splice site and generation of a premature termination codon

In this study, we describe the clinical features and provide experimental analyses of a novel point mutation affecting the penultimate nucleotide of the first exon of the HBA2 (HBA2: c.94A > G) gene identified in a 26-year-old female who also carries a heterozygous Hb E (HBB: c.79G > A) variant. The aim of the study was to investigate the impact of this point mutation on the transcriptional activity of the HBA2 gene using a combination of an initial in silico prediction followed by in vitro mutagenesis and transcriptional activity assessment. The analyses revealed that the HBA2: c.94A > G point mutation causes the activation of a cryptic splice site located 49 bp upstream of the exon1-intron1 boundary in both HBA2 long and short isoforms, thus generating a frameshift and a premature termination codon between codons 48 and 49 in the second exon. A rapid degradation of the aberrantly spliced transcripts by the nonsense mediated decay (NMD) surveillance system is highly indicative of an α-thalassemia (α-thal) phenotype. However, the abnormal mRNA may not be entirely degraded since the proband presents a slight splenomegaly that could be the sign of extra vascular hemolysis.