Effect of FACT depletion on the cell composition of bone marrow and intestine in mice

Preservation of nucleosomes during replication has been extensively studied, while the maintenance of nucleosomes during transcription has gotten less attention. The histone chaperone FACT is involved in transcription elongation, although whether it disassembles or assembles nucleosomes during the passage of RNA polymerase is still unclear. We deleted the FACT subunit in adult mice to clarify the function of FACT in mammals. FACT loss was lethal due to bone marrow and intestinal failure, where the earliest progenitors completely vanished, while several other cell types were increased or decreased. Using cells isolated from several tissues, we showed that FACT loss was lethal only for stem cells but not cells differentiated in vitro. FACT depletion led to increased chromatin accessibility in a transcription-dependent manner, suggesting that nucleosomes are lost from transcribed regions in the absence of FACT. The most prominent response to the loss of chromatin integrity was the activation of interferon signaling and the accumulation of immunocytes in sensitive organs. FACT maintained chromatin integrity during transcription in mammalian adult stem cells, suggesting that chromatin transcription in these cells is different from more differentiated cells. Overall design: Ssrp1fl/fl; CreERT2+/+ mice of 10-15 weeks of age were treated with vehicle or tamoxifen (2mg, IP, every day for 5 days). 24 hours after the end of treatment bone marrow and small and large intestine (2cm before and after secum, excluding secum) were collected from mice. Samples from two mice from the same treatement group were pooled together.