B-Myb is Critical for Proper Duplication During an Unperturbed s Phase in Mouse Embryonic Stem Cells.

A common feature of early embryo cells from the inner cell mass (ICM) and of embryonic stem cells (ESCs), is an absolute dependence on an atypical cell cycle in which the G1 phase is shortened in order to preserve their self-renewing and pluripotent nature. The transcription factor B-Myb has been attributed a role in proliferation, in particular during the G2/M phases of the cell cycle. Intriguingly, B-Myb levels in ICM/ESCs are greater than 100 times compared to those in normal proliferating cells, suggesting a particularly important function for this transcription factor in pluripotent stem cells. B-Myb is essential for embryo development beyond the pre-implantation stage, but its role in ICM/ESCs remains unclear. Using a combination of mouse genetics, single DNA-fibre analyses and high-resolution 3D imaging, we demonstrate that B-Myb has no influence on the expression of pluripotency factors, but instead B-Myb ablation leads to stalling of replication forks and super-activation of replication factories that result in disorganisation of the replication program and an increase in double-strand breaks. These effects are partly due to aberrant transcriptional regulation of cell-cycle proliferation factors, namely c-Myc and FoxM1, which dictate normal S-phase progression. We conclude that B-Myb acts crucially during the S-phase in ESCs by facilitating proper progression of replication, thereby protecting the cells from genomic damage. Our findings have particular relevance in the light of the potential therapeutic application of ESCs and the need to maintain their genomic integrity.