Summary Enhancers, critical determinants of cellular identity, are commonly identified by correlative chromatin marks and gain-of-function potential, though only loss-of-function studies can demonstrate their requirement in the native genomic context. Previously we identified an erythroid enhancer of BCL11A, subject to common genetic variation associated with fetal hemoglobin (HbF) level, whose mouse ortholog is necessary for erythroid BCL11A expression. Here we develop pooled CRISPR-Cas9 guide RNA libraries to perform in situ saturating mutagenesis of the human and mouse enhancers. This approach reveals critical minimal features and discrete vulnerabilities of these enhancers. Despite conserved function of the composite enhancers, their architecture diverges. The crucial human sequences appear primate-specific. Through editing of primary human progenitors and mouse transgenesis, we validate the BCL11A erythroid enhancer as a target for HbF reinduction. The detailed enhancer map will inform therapeutic genome editing. The screening approach described here is generally applicable to functional interrogation of noncoding genomic elements.

Developmental silencing of fetal globins serves as both a paradigm of spatiotemporal gene regulation and an opportunity for β-hemoglobinopathy therapeutic intervention. The NuRD chromatin complex participates in γ-globin repression. Here we use pooled CRISPR screening to comprehensively disrupt NuRD protein coding sequences in human adult erythroid precursors. We find essential for fetal hemoglobin (HbF) control a nonredundant subcomplex of NuRD protein family paralogs, whose composition we corroborate by affinity chromatography and proximity labeling mass spectrometry proteomics. Mapping top functional guide RNAs identifies key protein interfaces where in-frame alleles result in loss-of-function due to destabilization or altered function of subunits. We ascertain mutations of CHD4 that dissociate its requirement for cell fitness from HbF repression in both primary human erythroid precursors and transgenic mice. Finally we demonstrate that sequestering CHD4 from NuRD phenocopies these mutations. This work indicates a generalizable approach to discover protein complex features amenable to rational biochemical targeting.