Activation of gamma-globin expression by hypoxia-inducible factor 1 alpha

Around birth, globin expression in human red blood cells (RBCs) shifts from gamma-globin to beta-globin, which results in fetal haemoglobin (HbF, alpha(2)gamma(2)) being gradually replaced by adult haemoglobin (HbA, alpha(2)beta(2))(1). This process has motivated the development of innovative approaches to treat sickle cell disease and beta-thalassaemia by increasing HbF levels in postnatal RBCs2. Here we provide therapeutically relevant insights into globin gene switching obtained through a CRISPR-Cas9 screen for ubiquitin-proteasome components that regulate HbF expression. In RBC precursors, depletion of the von Hippel-Lindau (VHL) E3 ubiquitin ligase stabilized its ubiquitination target, hypoxia-inducible factor 1 alpha (HIF1 alpha)(3,4), to induce gamma-globin gene transcription. Mechanistically, HIF1 alpha-HIF1 beta heterodimers bound cognate DNA elements in BGLT3, a long noncoding RNA gene located 2.7 kb downstream of the tandem gamma-globin genes HBG1 and HBG2. This was followed by the recruitment of transcriptional activators, chromatin opening and increased long-range interactions between the gamma-globin genes and their upstream enhancer. Similar induction of HbF occurred with hypoxia or with inhibition of prolyl hydroxylase domain enzymes that target HIF1 alpha for ubiquitination by the VHL E3 ubiquitin ligase. Our findings link globin gene regulation with canonical hypoxia adaptation, provide a mechanism for HbF induction during stress erythropoiesis and suggest a new therapeutic approach for beta-haemoglobinopathies.