CRISPRa-mediated entanglement of the Dux-MERVL axis

Transposable elements (TEs) provide sequences that are powerful cis-regulatory drivers of gene expression programmes. This is particularly apparent during early development when many TEs become de-repressed. MERVL elements are highly yet transiently upregulated in mouse totipotent 2-cell (2C) embryos during major zygotic genome activation (ZGA), and in 2C-like cells in vitro. One of the most powerful activators of MERVL is the pioneer transcription factor, Dux. However, apparent differences lie in the requirement for Dux versus MERVL activation in development, for unclear reasons. Moreover, sustained Dux activation causes cell toxicity in multiple cell types, which may or may not be linked to MERVL activation. Using a CRISPR-activation, 2C-GFP reporter system, we have unpicked the relative role of Dux and MERVL in ZGA, totipotent-like characteristics and cell toxicity. We find that direct MERVL activation comprises only a portion of the Dux-dependent transcriptome, and which is sufficient for some – but not all – totipotency features. Conversely, Dux-induced pathology is independent of MERVL activation and involves induction of the pro-apoptotic factor, Noxa. Our study highlights the complexity of the Dux-MERVL transcriptional network and uncovers a new player in Dux-driven pathology.