Unresolved recombination intermediates lead to ultra-fine anaphase bridges, chromosome breaks and aberrations

The resolution of joint molecules that link recombining sister chromatids is essential for chromosome segregation. Here, we determine the fate of unresolved recombination intermediates that arise in GEN1(-/-) knock-out cells depleted for MUS81, the two nucleases required for resolution. We find that intermediates persist until mitosis where they form a distinct class of anaphase bridges, which we term homologous recombination ultra-fine bridges, or HR-UFBs. The HR-UFBs are distinct from replication stress-associated UFBs, that arise at common fragile sites, and from centromeric UFBs. HR-UFBs are processed by BLM helicase to generate single-stranded RPA-coated bridges that are broken at mitosis. In the next cell cycle, DNA breaks activate the DNA damage checkpoint and chromosome fusions arise by non-homologous end joining. Consequently, the cells undergo a cell cycle delay and massive cell death. These results lead us to present a model detailing how unresolved recombination intermediates can promote DNA damage and chromosomal instability.