The HRI-regulated transcription factor ATF4 activates BCL11A transcription to silence fetal hemoglobin expression

Reactivation of fetal hemoglobin remains a critical goal in the treatment of patients with sickle cell disease and beta-thalassemia. Previously, we discovered that silencing of the fetal g-globin gene requires the erythroid-specific eIF2 alpha kinase heme-regulated inhibitor (HRI), suggesting that HRI might present a pharmacologic target for raising fetal hemoglobin levels. Here, via a CRISPR-Cas9-guided loss-of-function screen in human erythroblasts, we identify transcription factor ATF4, a known HRI-regulated protein, as a novel gamma-globin regulator. ATF4 directly stimulates transcription of BCL11A, a repressor of gamma-globin transcription, by binding to its enhancer and fostering enhancer-promoter contacts. Notably, HRI-deficient mice display normal Bcl11a levels, suggesting species-selective regulation, which we explain here by demonstrating that the analogous ATF4 motif at the murine Bcl11a enhancer is largely dispensable. Our studies uncover a linear signaling pathway from HRI to ATF4 to BCL11A to g-globin and illustrate potential limits of murine models of globin gene regulation.