A recent study, published in eLife, has redefined our understanding of the S-phase checkpoint which has important implications for our knowledge of checkpoint function in cancers.
During cell division, the entire genome must be replicated once and only once. In eukaryotes, this is done by linking DNA replication control to the cell cycle. Multiple checkpoints exist that regulate DNA synthesis and genome integrity before (G1 checkpoint), during (S-phase checkpoint) and after S-phase (G2/M checkpoint). These checkpoints are mediated by several kinases including ATM/ATR and Chk1/Chk2. In G1 phase, DNA replication is delayed by G1/S cyclin–CDK activity if DNA damage is detected. Similarly, in S-phase, replication rates are slowed when firing origins stall, for example due to DNA lesions. In budding yeast, this occurs by Rad53-dependent inhibition of the initiation factors Sld3 and Dbf4.
It is proposed that a key feature of DNA damage checkpoints is that the response is tailored to the cell cycle phase in which the DNA damage occurred. However, to date, there is very little evidence suggesting that substrate specificity of checkpoint kinases change during the cell cycle.
In this study, researchers explored the specificity of Rad53 towards the replication substrates Sld3 and Dbf4 across the cell cycle in budding yeast, Saccharomyces cerevisiae. They found that Rad53 phosphorylates both of these substrates throughout the cell cycle at the same sites as in S-phase. As a result, the team hypothesised that these substrates although deemed targets of the S-phase checkpoint, in fact, Rad53 may also prevent aberrant origin firing outside of S-phase.
They showed that Rad53-dependent inhibition of Sld3 and Dbf4 limits re-initiation of replication in G2/M phase, preventing gene amplification. It also prevents premature firing of all origins, not just late origins, at the G1/S transition.
This study redefines our understanding of the cell cycle. It provides a novel mechanism that restricts replication initiation to a specific window of the cell cycle after DNA damage. Understanding the complexities of the cell cycle can provide a potential mechanistic rationale for targeting specific components in cancer. For example, targeting of p53/Rb cancers using Chk1 and ATR inhibitors.
Image credit: By man_at_mouse