SRP311332 PRJNA715661 GSE169209 RNA Sequencing of H1 WT hESCs, H1 QSER1 KO hESCs, H1 TET1 KO hESCs, H1 QSER1/TET1 DKO hESCs, WT Day10 EBs, QSER1 KO Day10 EBs, TET1 KO Day10 EBs, QSER1/TET1 DKO Day10 EBs, WT PP1, QSER1 KO PP1, TET1 KO PP1, and QSER1/TET1 DKO PP1. RNA Sequencing of H1 WT hESCs, H1 QSER1 KO hESCs, H1 TET1 KO hESCs, H1 QSER1/TET1 DKO hESCs, WT Day10 embryoid bodies (EBs), QSER1 KO Day10 EBs, TET1 KO Day10 EBs, QSER1/TET1 DKO Day10 EBs, WT pancreatic progenitors (PP1), QSER1 KO PP1, TET1 KO PP1, and QSER1/TET1 DKO PP1. DNA methylation is essential to mammalian development, and dysregulation can cause serious pathological conditions. Key enzymes responsible for deposition and removal of DNA methylation are known, but how they cooperate to tightly regulate the methylation landscape remains a central question. Utilizing a knockin DNA methylation reporter, we performed a genome-wide CRISPR/Cas screen in human embryonic stem cells to discover DNA methylation regulators. The top screen hit was an uncharacterized gene QSER1, which proved to be a key guardian of bivalent promoters and poised enhancers of developmental genes, especially those residing in DNA methylation valleys (or canyons). We further demonstrate cooperation of QSER1 and TET1 through genetic and biochemical interactions to inhibit DNMT3-mediated de novo methylation and safeguard developmental programs. Overall design: For RNA Sequencing, total RNA was isolated with the RNeasy Mini Kit (Qiagen, 74136). H1 WT hESCs had n=5 biological replicates. All other conditions had n=3 biological replicates. GSE169209 pubmed 33833093 parent_bioproject PRJNA631028