Neelsen et al. describe how certain oncogenes can perturb DNA replication and induce DNA damage.
Several oncogenes, including Cyclin E and Cdc25A, push cells precipitously into S phase, resulting in numerous DNA double-strand breaks (DSBs). But how S phase deregulation disrupts replication and induces DNA damage is unclear.
To find out, Neelsen et al. overexpressed either Cyclin E or Cdc25A in human cells. Within hours, both oncogenes slowed the progression of replication forks, inducing the formation of unusual replication intermediates called “reversed forks,” in which newly synthesized DNA strands anneal to each other instead of their parental strands. Reversed forks are presumably caused by topological stress, which might arise because the oncogenes deregulate replication initiation and thus increase interference with transcription.
Though Cyclin E and Cdc25A caused similar replication defects, the two oncogenes induced DNA damage at different rates. Cyclin E–expressing cells took several cell cycles to accumulate significant numbers of DSBs because they could transiently delay mitotic entry and resolve most unusual replication intermediates. Cdc25A-expressing cells, on the other hand, are defective in the DNA damage checkpoint and therefore enter mitosis prematurely. These cells rapidly accumulated DSBs due, in part, to the activation of a mitotic nuclease called MUS81, which appears to target unresolved reversed forks, perhaps so that sister chromatids can be separated.
DSBs therefore do not arise directly from oncogene-induced replication defects but instead result from nuclease-mediated processing during mitosis. The DNA damage checkpoint limits this processing by allowing cells to fix their replication problems before mitotic entry. Because MUS81 depletion only partially suppresses DSB formation, Neelsen et al. now want to investigate other factors that may contribute to oncogene-induced replication stress and genotoxicity.
Text by Ben Short