ATF4 mediates fetal globin upregulation in response to reduced β-globin

Fetal development and anemias such as {beta}-hemoglobinopathies trigger rapid production of red blood cells in a process known as stress erythropoiesis. Cellular stress prompts differentiating erythroid precursors to express high levels of fetal {gamma}-globin, which has suggested strategies to treat hemoglobinopathies such as thalassemia and sickle cell disease. However, the mechanisms underlying {gamma}-globin production during cellular stress are still poorly defined. Here we use CRISPR-Cas genome editing and CRISPRi transcriptional repression to model the stress caused by reduced levels of adult {beta}-globin. We find that loss of {beta}-globin is sufficient to induce widespread globin compensation, including robust re-expression of {gamma}-globin. Time-course RNA-seq of differentiating isogenic erythroid precursors identified the ATF4 transcription factor as a causal regulator of this response. ChIP-seq of multiple erythroid precursor genotypes and differentiation states revealed that {beta}-globin knockout leads to reduced engagement of ATF4 targets involved in the unfolded protein response. This ATF4 program indirectly regulates the levels of BCL11A, a key repressor of {gamma}-globin. Identification of ATF4 as a key regulator of globin compensation adds mechanistic insight to the poorly understood phenomenon of stress-induced globin compensation and could be relevant for proposed gene editing strategies to treat hemoglobinopathies.