Dissecting polycomb complexes for enhanced fetal hemoglobin production.

Polycomb repressive complex 1 (PRC1) and PRC2 regulate diverse developmental processes, including the fetal-to-adult switch in hemoglobin (Hb) production, a process whose reversal is a goal for the treatment of sickle cell disease and β-thalassemia. PRC inhibitors show promise for various disorders, but use is limited because of pleiotropic PRC activities. We explored whether fetal Hb (HbF) can be reactivated in adult erythroid cells by selective perturbations of PRC1 or PRC2 components without complete loss of PRC function. A high-density CRISPR-CRISPR-associated protein 9 (Cas9) mutagenesis screen identified a region in EZH2 in which Cas9 induced exon 14 skipping (EZH2Δ14). EZH2Δ14, which lacks a portion of the CXC domain, relieves HbF repression while largely maintaining cellular fitness. EZH2Δ14 retains H3K27 methylation and repression of a PRC target gene subset. Experiments in cells derived from mice bearing human β-globin genes confirm that pathways mediating EZH2 control of HbF expression can function in a mouse model of HBG switching. These findings demonstrate that partial disruption of PRC can yield selective phenotypes, highlighting the therapeutic potential of targeting nonenzymatic domains within chromatin-modifying complexes.