FLASH/casp8ap2 is indispensable for early embryogenesis but dispensable for proliferation and differentiation of ES cells.

FLICE/caspase-8-associated huge protein (FLASH)/casp8ap2 is involved in various cellular functions, such as cell cycle progression, transcriptional regulation, the regulation of apoptosis, and the regulation of histone gene expression. The down-regulated expression of FLASH has been shown to inhibit cell cycle progression in the S phase in many kinds of mice and human cell lines and the inhibition of cell cycle progression may be attributed to the suppressed expression of replication-dependent histone genes. We here demonstrated that the induced knockout of FLASH never affected cell cycle progression in ES cells, in which the expression of core histone genes was decreased to levels similar to those in human KB cells sensitive to the knockdown of FLASH. In addition, the FLASH conditional knockout ES cells could differentiate normally into not only mesodermal and endodermal cells, but also trophoblasts. In order to investigate the function of FLASH in early embryogenesis in vivo, we also examined a FLASH mutant mouse, in which FLASH mutant allele did not express FLASH mRNA in embryos and most adult organs, except for the testis. FLASH mutant embryos died between E3.5 and E8.5. Furthermore, the in vitro cultivation of FLASH mutant embryos generated by in vitro fertilization showed embryonic lethality at the pre-implantation stage by inhibiting the hatching of embryos and their adherence to substrates. Taken together, these results indicate that FLASH plays an important role in early embryogenesis, but is not essential for either the proliferation or differentiation of ES cells.