Reporter CRISPR screens decipher cis- and trans-regulatory principles at the Xist locus [Xist_CRISPR_screens]

Developmental genes are controlled by several cis-acting regulatory elements (REs), which in turn respond to multiple trans-acting transcription factors (TFs). Understanding this regulatory complexity has remained a challenge. Here, we combine pooled CRISPR screens with different phenotypic readouts to dissect how a cis-regulatory landscape integrates information from a large number of TFs. We apply our approach to the murine X-inactivation center, which harbors the Xist gene. We identify a large set of Xist-controlling TFs and map their TF-RE wiring at the Xist locus. We show that a transiently expressed group of XX-biased TFs, which includes the X-linked factor ZIC3, control Xist's promoter-proximal REs. These factors promote initial Xist upregulation in a binary fashion, potentially ensuring female-specific Xist expression. A second set of developmental TFs, which include the epiblast master regulator of the epiblast OTX2, are upregulated slightly later and activate distal REs associated with the lncRNA genes Jpx, Ftx and Xert. We show that this second regulatory axis is required to ensure sufficiently high Xist RNA levels for efficient establishment of X-chromosome inactivation. Our unbiased, systematic approach provides a framework for the comprehensive dissection of regulatory interactions across genomic loci. Overall design: CRISPRi screens targeting transcription factor (TF) genes. Xist-High, -Low and -Negative populations were sorted using flow cytometry following FlowFISH staining for Xist RNA. The TFi library targets 570 TF genes using 11058 sgRNAs. The TFiMini library targets 119 TF genes (FDR <0.2 from the TFi screen) with 1270 individual sgRNAs. Two replicates were collected per screen