Investigating PUM1 mediated post-transcriptional regulation of human hemoglobin switching and erythropoiesis

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Project Summary: The fetal to adult hemoglobin switching around birth involves an expression shift from γ-globin to β-globin in erythroid cells. Effective re-expression of fetal γ-globin can ameliorate sickle cell anemia and β-thalassemia. Hence, identification of inducers of fetal hemoglobin (HbF) is an active area of research and holds immense therapeutic potential. Unlike the transcriptional and epigenetic regulation, post-transcriptional regulation of β- globin switching is poorly understood, with few reports on its physiological and clinical relevance. Our preliminary studies have identified Pumilo-1 (PUM1), an RNA binding protein with no previously reported functions in erythropoiesis, as a post-transcriptional regulator of β-globin switching. PUM1, whose expression is regulated by the erythroid master transcription factor, Erythroid Krüppel-like factor (EKLF/KLF1), peaks during erythroid differentiation, binds fetal γ-globin mRNA, and reduces γ-globin (HBG1) mRNA stability and translational efficiency, which culminates in reduced γ-globin protein levels. Knockdown of PUM1 leads to a robust increase in fetal γ-globin (~22% HbF of total hemoglobin), without affecting adult β-globin levels in human erythroid cells. Importantly, knocking down PUM1 does not limit erythropoiesis progression, providing a potentially safe and effective treatment strategy in sickle cell anemia and β-thalassemia. In support of this idea, we report elevated fetal hemoglobin levels in the absence of anemia, in an individual with a novel heterozygous PUM1 mutation in the RNA binding domain (p.(His1090Profs*16); c.3267_3270delTCAC), suggesting that PUM1 mediated post- transcriptional regulation is a critical player during human hemoglobin switching. Based on these preliminary results, we hypothesize that impairing the RNA binding functions of PUM1 in human adult erythroid cells can induce fetal hemoglobin levels, without deleterious effects on other aspects of erythropoiesis; this induction could ameliorate the disease phenotypes such as sickling in cultured sickle cell anemia erythroid cells. To test this hypothesis, we propose the following aims: 1) Decipher the regulatory elements that enable PUM1 mediated fetal hemoglobin silencing in human erythroid cells, 2) Study the impact of PUM1 knockdown on human erythroid differentiation, 3) Evaluate the capacity of PUM1 knockdown to relieve sickling in sickle cell anemia erythroid cells. Our studies will for the first time describe the post-transcriptional functions of PUM1 in hemoglobin switching and erythropoiesis and reveal the impact of a novel human PUM1 mutation p.(His1090Profs*16), which we have identified in a patient with elevated HbF. Further, these studies will advance our mechanistic understanding of the post-transcriptional silencing of human fetal hemoglobin, which is poorly understood. Finally, since PUM1 functions as a cytoplasmic post-transcriptional regulator, our studies will determine if disrupting PUM1 to induce fetal hemoglobin could serve as a potential non-gene altering therapeutic target towards ameliorating β- thalassemia and sickle cell anemia.