The disorders of β-hemoglobin, sickle cell disease (SCD) and β-thalassemia, are major causes of morbidity and mortality worldwide. These diseases are the most common genetic disorders in the world.
SCD is due to a single base-pair point mutation in the β-globin gene resulting in the substitution of valine for glutamic acid in the β-globin chain. The pathophysiology is directly related to polymerization of deoxygenated hemoglobin, leading to a cascade of pathologic events including erythrocyte sickling, vaso-occlusion, and hemolysis.
In β-thalassemia, insufficient production of the β-globin molecule results in an excess of free α-globin chains that can precipitate within erythroid precursors, impairing their maturation and leads to death of these precursors and ineffective production of erythroid cells. As a result, a significant anemia occurs and the consequent expansion of erythroid precursors can lead to secondary problems in bones and other organs.
The hemoglobin molecule is a tetramer composed of two subunits of α-like globin peptides and two subunits of the β-like globin peptides, along with heme moieties. β-globin switching from fetal γ-globin (HBG1 and HBG2) to adult β-globin is a developmental process that occurs in erythrocytes at around the time of birth. Fetal hemoglobin (HbF) induction in adult erythrocytes is an effective therapeutic strategy for SCD and β-thalassemia.
Pumilio1 (PUM1) is a novel target of the erythroid master transcription factor erythroid Krüppel-like factor (EKLF). PUM1 is a member of Pumilio-Fem3-binding factor (PUF) family of sequence specific RNA-binding proteins, acts as a posttranscriptional repressor by binding to the 3' untranslated region (3'-UTR) of messenger RNA (mRNA). It peaks during terminal erythroid differentiation and binds to fetal γ-globin (HBG1) mRNA and impairs its stability and translation. HBG1 has 2 core PUM1 consensus binding sites, but HBG2 and adult globins do not. Knockdown of PUM1 leads to a robust increase in HBF (∼22%) without affecting β-globin levels in human erythroid cells. Moreover, targeting PUM1 does not affect erythropoiesis, which provides a potentially safe and effective therapeutic strategy for SCD and β-thalassemia. Also it was found that elevated HbF levels in the absence of anemia in an individual with a novel heterozygous PUM1 mutation in the RNA-binding domain, which suggests that PUM1 is a critical player during human hemoglobin switching.
