In a recent study, researchers identified features of globin gene regulation in red blood cells (RBCs) that may shed light on a potential mechanism of fetal hemoglobin (HbF) induction, with potential implications for treatment approaches for beta-hemoglobinopathies like sickle cell disease (SCD) and beta-thalassemia. The study findings were reported in Nature.
“Clinical and laboratory observations have shown that HbF expression can ameliorate both SCD and β-thalassaemia by enhancing the survival of circulating RBCs and their bone marrow precursors,” the researchers explained in their report. Mechanisms related to increased HbF expression have not been fully understood, but hypoxia is a stressor associated with this process.
The researchers undertook the study with a notion that protein ubiquitination is involved in regulation of gene expression. Based on this, they performed a screening analysis using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 to identify ubiquitin-proteasome components that may be involved in regulating HbF expression. They used erythroid cell lines or primary erythroblasts for this and other procedures in this study.
In contrast to adult hemoglobin, HbF contains gamma-globin. In their analyses, the researchers identified a factor called hypoxia-inducible factor (HIF) 1-alpha as a ubiquitination target of von Hippel-Lindau (VHL) E3 ubiquitin ligase. HIF1-alpha and related protein subunits are involved in adaptation to hypoxia. Disruption of the VHL E3 ubiquitin ligase appeared to stabilize HIF1-alpha and promote transcription of genes encoding gamma-globin, leading to increased HbF synthesis. The researchers also identified second substrate of VHL E3 ubiquitin ligase, which was MYOM1, but assays they conducted involving this protein did not suggest an impact on gamma-globin expression.
HIF1-alpha and another protein subunit called HIF1-beta form a heterodimer that binds to DNA elements in a region downstream of the gene pair, HBG1 and HBG2, which together encode gamma-globin. Analyses showed that binding of this heterodimer appeared to lead to a series of steps that promoted interactions involving the g-globin genes and an enhancer element.
The researchers also subjected cells to hypoxia (with 1% oxygen) or to prolyl hydroxylase inhibitor treatment. Prolyl hydroxylase inhibition disrupts targeting of HIF1-alpha for ubiquitination. In both of these conditions, HbF expression appeared to be increased.
“The induction of HbF by VHL disruption revealed a new facet of globin gene regulation,” the researchers wrote in their report. “More broadly, we have shown that the loss of VHL in erythroid cells results in an extensive HIF-related programme of gene expression, including the activation of canonical targets that regulate metabolic adaptation to hypoxia.”
Disclosures: Some authors have declared affiliations with or received grant support from the pharmaceutical industry. Please refer to the original study for a full list of disclosures.
Feng R, Mayuranathan T, Huang P, et al. Activation of γ-globin expression by hypoxia-inducible factor 1α. Nature. 2022;610(7933):783-790. doi:10.1038/s41586-022-05312-w