Synergistic effect of two β globin gene cluster mutations leading to the hereditary persistence of fetal hemoglobin (HPFH) phenotype

Co-inheritance of gamma and beta globin gene mutations in a compound heterozygous state is rare but of clinical interest as it provides an important data on understanding the HbF expression. Hematological analysis was carried out (Sysmex KX-21). F-cells were enumerated using flow cytometry. Beta globin gene was analysed by CRDB technique and by DNA sequencing. Gamma globin promoter region was sequenced and expression studies were carried out using real time Taqman assay. We report a family, where two inherited defects of the β globin gene cluster segregate. The proband and her sibling were compound heterozygotes for a novel Gγ promoter mutation and the 619 bp deletion a common Indian β thalassemia mutation. Molecular characterization revealed that the father (HbA2 5.1%, HbF 5.4%), proband (HbA2 3.6%, HbF 31.7%) and her brother (HbA2 3.9%, HbF 23.6%) were heterozygous for the 619 bp deletion. The mother (HbA2 2.1%, HbF 3.4%) had a normal β globin gene. As both the children showed high HbF levels, the γ globin gene work up was carried out. The Gγ-globin gene promoter analysis revealed that the mother and the two children were heterozygous for a 5 bp deletion -ATAAG (-533 to -529) that resides in the GATA binding site. These findings suggest that the 5 bp deletion in the Gγ globin promoter has a functional role in silencing the γ-globin gene expression in adults by disrupting GATA-1 binding and the associated repressor complex and results in the up-regulation of gamma globin gene expression. When co-inherited with β -thalassemia trait it leads to a phenotype of HPFH.