POSTDOC Centre For Regenerative Medicine and Health, Hong Kong Institute of Science & Innovation，Chinese Academy of Sciences Hong Kong Special Administrative Region, Hong Kong
Abstract: DDX21 is a nucleolar RNA helicase known to regulate RNA metabolism, including ribosomal RNA (rRNA) transcription and processing, ribosomal protein gene transcription, mRNA splicing, RNA modifications, and resolving R-loop structure. Knockout (KO) of Ddx21 in mice is embryonic lethal, suggesting that the helicase is essential for embryonic survival. To elucidate the function of DDX21 in embryonic stem cells (ESCs), we performed gene knockout and knockdown in ESCs. No homozygous ESC KO clones could survive during expansion. DDX21 knockdown (DDX21-KD) ESCs showed poor clonogenicity, which can be rescued by overexpressing wild-type DDX21 but not helicase-dead DDX21, suggesting that the helicase activity is required for the ESC self-renewal and proliferation. Transcriptomic analysis indicated the downregulation of ribosome biogenesis, nuclear division, and activation of the p53 pathway. Inhibition of p53 could also rescue ESC clonogenicity. Thus, loss of DDX21 may trigger a p53-dependent nucleolar stress that attenuates ESC proliferation. We also found a change in the nuclear heterochromatin in DDX21-KD cells. Analysis of chromatin accessibility by ATAC-seq revealed a decreased open chromatin near the transcription start sites. We thus hypothesized that DDX21 regulates ESC self-renewal through an epigenetic mechanism. By co-immunoprecipitation, we demonstrated that DDX21 could bind the Polycomb repressive complex 2 (PRC2). As PRC2 is known to maintain stem cell pluripotency by repressing lineage-specific gene expressions, our results suggested that DDX21 plays a role in maintaining an epigenetic state for ESC self-renewal by interacting with PRC2 complex, besides its established function in ribosome biogenesis.
