Replication stress dependent cell fate reprogramming in embryonic stem cells

DNA replication stress (RS) caused by perturbations to replication fork progression leads to DNA damage and genome instability, as commonly found in cancer cells. Tumors are generally affected by a variety of perturbations in regulatory mechanisms that in many cases result in aberrant activation of different developmental pathways, leading to an undifferentiated phenotype. However, RS effects on cell fate determination are poorly understood. We recently found that RS induces the transcription of ~3000 genes in mouse embryonic stem cells (mESCs) (Atashpaz et al, Elife 2020). A subset of these genes includes 2-cell stage factors Dux and Zscan4, whose expression is strongly dependent on the RS-response kinases ATR and CHK1, responsible for restraining replication fork progression. Strikingly, we showed that persistent RS expands mESCs fate, promoting their reprogramming to a 2-cell stage like status, as shown by their ability to colonize the trophectoderm (TE) in addition to the embryo proper. Accordingly, we recently found that RS induces TE markers in mESCs, including Cdx2, Gata3, Eomes and Fgf2r, promoting differentiation towards trophoblast-like stem cells that support the formation of 3D embryolike structures. We also show that RS fosters mESCs trophoblast giant cells (TGCs) terminal differentiation. We propose that this novel pathway may divert the fate of blastomeres exposed to RS towards extraembryonic compartments, thus preventing somatic and germline incorporation of damaged cells. Reactivation of this pathway in adult cells might contribute to cancer cell transformation. We will discuss these results in the context of novel RS-dependent mechanisms that promote stem cell fate transitions.