October 28, 2016

Resolving replication stress

Ulrich Rass and his group at the FMI have elucidated novel roles of two proteins - Dna2 and Yen1 - in DNA replication. Both of these proteins, at different stages of cell division, ensure completion of replication and faithful chromosome segregation by resolving aberrant replication intermediates that arise under replication stress. These insights have implications for our understanding of cancer, where replication stress leads to genomic instability, driving cancer development.

With two meters’ worth of DNA compacted into each cell and trillions of cells in our body, DNA replication is a daunting feat of information transfer. Replication is initiated at many sites along chromosomes, and individual replication forks travel at high speed, each incorporating about 100 nucleotides per second into nascent DNA strands. Given this complexity, replication can of course go awry, and numerous processes are in place to ensure that stress is resolved and accidents are cleaned up.

High levels of replication stress are seen in hyperproliferating cancer cells. While this may drive genetic changes required for cancer development, it also provides an opportunity for therapeutic intervention: the cellular response to replication stress can be targeted so as to use stress overload to kill cancer cells.

In the last 50 years, the molecular processes controlling DNA replication have attracted a lot of interest. Many factors have been identified, and interactions elucidated. However, it has proved difficult to dissect the function and interplay of the numerous proteins that protect cells from replication stress.

Here, a study by Ulrich Rass, Group leader at the Friedrich Miescher Institute for Biomedical Research, offers important new insights.

The scientists investigated the interplay between two proteins involved in DNA replication and the replication stress response - Yen1 and Dna2. Yen1 is a structure-specific nuclease known to resolve so-called Holliday junctions (DNA structures that persist after DNA repair processes involving homologous recombination). Dna2, containing a nuclease and a helicase domain, is thought to promote lagging-strand DNA synthesis by trimming Okazaki fragments to size.

Rass and his group showed that these two proteins, despite being active at different points during cell division, complement each other to ensure that DNA replication is completed, and that chromosome segregation can safely occur at the end of the cell cycle.

As the scientists demonstrated, the helicase activity of Dna2 removes aberrant DNA intermediates arising at perturbed replication forks. Unfortunately, in some cases, Dna2 helicase fails to respond properly to replication fork stalling. In this case, physical links persist between the nascent sister chromatids, preventing proper chromosome segregation into two daughter cells. As Rass explains, this is where Yen1 comes into play: “We showed that Yen1 not only plays a role in homologous recombination repair, it also resolves similar DNA structures that arise when a replication fork stalls. In addition, while many studies have focused on the nuclease activity of Dna2, we are the first to pinpoint a role for the helicase activity of this protein in DNA replication.” Both proteins protect dividing cells from some of the effects of replication stress.

This is particularly relevant also for cancer: here, activated oncogenes lead to increased proliferation, which in turn induces replication stress. If this stress is not resolved by factors such as Dna2 or Yen1, it leads to genomic instability, which likely drives cancer development. Rass comments: "An in-depth understanding of the processes occurring during replication stress is therefore clearly needed, and our insights into the functioning of Dna2 and Yen1 contribute to this."

Original publication
Ölmezer G, Levikova M, Klein D, Falquet B, Fontana GA, Cejka P, Rass U (2016) Replication intermediates that escape Dna2 activity are processed by Holliday junction resolvase Yen1. Nature Communications doi:10.1038/ncomms13157 [Epub ahead of print]
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More about Ulrich Rass
Ulrich Rass and his group are interested in cellular DNA repair mechanisms which resolve the many types of DNA damage that constantly affect the genome. He focuses in particular on the repair of chromosomal breaks and the molecular basis of the replication stress response.
» More about Ulrich Rass

FMI The replication machinery opens DNA like a zipper. If replication forks get stuck, they are helped on by Dna2, or can be resolved by Yen1 at mitosis to promote chromosome segregation. Picture from the Basel Fasnacht 2012 with the slogan "Es glemmt" ("stuck").