Molecular basis of a high Hb A(2)/Hb Fβ-thalassemia trait: a retrospective analysis, genotype-phenotype interaction, diagnostic implication, and identification of a novel interaction with α-globin gene triplication

BACKGROUND: β(0)-thalassemia deletion removing 5´β-globin promoter usually presents phenotype with high hemoglobin (Hb) A(2) and Hb F levels. We report the molecular characteristics and phenotype-genotype correlation in a large cohort of the β(0)-thalassemia with 3.4 kb deletion. METHODS: A total of 148 subjects, including 127 heterozygotes, 20 Hb E-β-thalassemia patients, and a double heterozygote with α-globin gene triplication, were recruited. Hb and DNA analysis were performed to identify thalassemia mutations and four high Hb F single nucleotide polymorphisms (SNPs) including four base pair deletion (-AGCA) at (A)γ-globin promoter, rs5006884 on OR51B6 gene, −158 (G)γ-XmnI, BCL11A binding motifs (TGGTCA) between 3´(A)γ-globin gene and 5´δ-globin gene. RESULTS: It was found that heterozygous β(0)-thalassemia and Hb E-β(0)-thalassemia with 3.4 kb deletion had significantly higher Hb, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin and Hb F values as compared with those with other mutations. Co-inheritance of heterozygous β(0)-thalassemia with 3.4 kb deletion and α-thalassemia was associated with even higher MCV and MCH values. The Hb E-β(0)-thalassemia patients carried a non-transfusion-dependent thalassemia phenotype with an average Hb of around 10 g/dL without blood transfusion. A hitherto undescribed double heterozygous β(0)-thalassemia with 3.4 kb deletion and α-globin gene triplication presented as a plain β-thalassemia trait. Most of the subjects had wild-type sequences for the four high Hb F SNPs examined. No significant difference in Hb F was observed between those of subjects with and without these SNPs. Removal of the 5´β-globin promoter may likely be responsible for this unusual phenotype. CONCLUSIONS: The results indicate that β(0)-thalassemia with 3.4 kb deletion is a mild β-thalassemia allele. This information should be provided at genetic counseling and prenatal thalassemia diagnosis.