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Unprotected Replication Forks Are Converted into Mitotic Sister Chromatid Bridges

DNA Recombination, Replication and Genome Stability (CNRS/Institut Curie) group, leaded by Sarah Lambert, published in Molecular Cell. Research reveals how stressed forks, unprotected by the recombination machinery, trigger mitotic defect and genome instability in a yeast model organism. Their findings open new avenues to understand how unprotected forks, occurring in breast cancer cell lines mutated for BRCA2, are key determinants of the efficacy of chemotherapies.

In the absence of the recombination machinery, dysfunctional replication forks are unprotected and accumulate large amount of ssDNA (in red). Unprotected forks are then unable to merge with opposite forks (green fork) resulting in mitotic sister chromatids bridges. The left panel shows the site of a dysfunctional fork (revealed by GFP-LacI-bound LacO arrays in green) and containing single stranded DNA (revealed by RPA-mCherry in Red) and embedded in an anaphase bridge in anaphase.

Abstract
“Replication stress and mitotic abnormalities are key features of cancer cells. Temporarily paused forks are stabilized by the intra-S phase checkpoint and protected by the association of Rad51, which prevents Mre11-dependent resection. However, if a fork becomes dysfunctional and cannot resume, this terminally arrested fork is rescued by a converging fork to avoid unreplicated parental DNA during mitosis. Alternatively, dysfunctional forks are restarted by homologous recombination. Using fission yeast, we report that Rad52 and the DNA binding activity of Rad51, but not its strand-exchange activity, act to protect terminally arrested forks from unrestrained Exo1-nucleolytic activity. In the absence of recombination proteins, large ssDNA gaps, up to 3 kb long, occur behind terminally arrested forks, preventing efficient fork merging and leading to mitotic sister chromatid bridging. Thus, Rad52 and Rad51 prevent temporarily and terminally arrested forks from degrading and, despite the availability of converging forks, converting to anaphase bridges causing aneuploidy and cell death.”

Unprotected Replication Forks Are Converted into Mitotic Sister Chromatid Bridges
Anissia Ait Saada, Ana Teixeira-Silva, Ismail Iraqui, Audrey Costes, Julien Hardy, Giulia Paoletti, Karine Fréon and Sarah A.E. Lambert
http://dx.doi.org/10.1016/j.molcel.2017.04.002