Dev114843 118..127

The pluripotent mammalian epiblast undergoes unusually fast cell proliferation. This rapid growth is expected to generate a high transcriptional demand, but the underlying mechanisms remain unknown. We show here that the chromatin remodeler Chd1 is required for transcriptional output and development of the mouse epiblast. Chd1 embryos exhibit proliferation defects and increased apoptosis, are smaller than controls byE5.5 and fail to grow, to become patterned or to gastrulate. Removal of p53 allows progression of Chd1 mutants only to E7.0-8.0, highlighting the crucial requirement for Chd1 during early post-implantation development. Chd1 embryonic stem cells (ESCs) have a self-renewal defect and a genome-wide reduction in transcriptional output at both known mRNAs and intergenic transcripts. These transcriptional defects were only uncovered when cell number-normalized approaches were used, and correlate with a lower engagement of RNAP II with transcribed genes in Chd1 ESCs. We further show that Chd1 directly binds to ribosomal DNA, and that bothChd1 epiblast cells in vivo and ESCs in vitro express significantly lower levels of ribosomal RNA. In agreement with these findings, mutant cells in vivo and in vitro exhibit smaller and more elongated nucleoli. Thus, the RNA output by bothPol I and II is reduced inChd1 cells. Our data indicate that Chd1 promotes a globally elevated transcriptional output required to sustain the distinctly rapid growth of the mouse epiblast.