During embryogenesis in Xenopus, the midblastula transition (MBT) occurs upon completion of the 12th cell division and marks a number of critical changes, including the introduction of gap phases into the cell cycle and the beginning of transcription. Moreover, early embryonic cell cycles lack checkpoints that halt the cell cycle in response to DNA damage, whereas the DNA damage checkpoint functions normally in post-MBT embryos. Interestingly, introduction of a threshold amount of undamaged plasmid or sperm DNA allows a DNA damage checkpoint response to be activated in both per-MBT embryos and egg extracts, suggesting a previously undefined role for undamaged threshold DNA in DNA damage checkpoint signal transduction. We show that undamaged threshold DNA directly participates in checkpoint signaling, as judged by several dynamic changes, including histone modification, ATM phosphorylation and loading onto chromatin, and Chk1/Chk2 phosphorylation and release from nuclear DNA. These responses on physically separated threshold DNA are triggered by an ATM-dependent soluble signal initiated by damaged DNA and required for efficient checkpoint activation. The signal persists in egg extracts even after damaged DNA is removed from the system, indicating the absence of damaged DNA is not sufficient to end the checkpoint response. The results identify a novel mechanism by which undamaged DNA enhances checkpoint signaling, and provide an example of how the transition to cell cycle checkpoint activation during development is accomplished by maternally programmed increases in the DNA-to-cytoplasmic ratio.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA