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.     

Insights into the Structural Dynamics of Nucleocytoplasmic Transport of tRNA by Exportin-t

Exportin-t (Xpot) transports mature 5′- and 3′-end processed tRNA from the nucleus to the cytoplasm by associating with a small G-protein Ran (RAs-related nuclear protein), in the nucleus. The release of tRNA in cytoplasm involves RanGTP hydrolysis. Despite the availability of crystal structures of nuclear and cytosolic forms of Xpot, the molecular details regarding the sequential events leading to tRNA release and subsequent conformational changes occurring in Xpot remain unknown. We have performed a combination of classical all-atom and accelerated molecular dynamics simulations on a set of complexes involving Xpot to study a range of features including conformational flexibility of free and cargo-bound Xpot and functionally critical contacts between Xpot and its cargo. The systems investigated include free Xpot and its different complexes, bound either to Ran (GTP/GDP) or tRNA or both. This approach provided a statistically reliable estimate of structural dynamics of Xpot after cargo release. The mechanistic basis for Xpot opening after cargo release has been explained in terms of dynamic structural hinges, about which neighboring region could be displaced to facilitate the nuclear to cytosolic state transition. Post-RanGTP hydrolysis, a cascade of events including local conformational change in RanGTP and loss of critical contacts at Xpot/tRNA interface suggest factors responsible for eventual release of tRNA. The level of flexibility in different Xpot complexes varied depending on the arrangement of individual HEAT repeats. Current study provides one of the most comprehensive and robust analysis carried out on this protein using molecular dynamics schemes.