ERCC1 and MUS81–EME1 promote sister chromatid separation by processing late replication intermediates at common fragile sites during mitosis

Breakage and rearrangement of common fragile sites (CFSs) can cause genomic instability. Rosselli and colleagues demonstrate that the endonuclease subunit ERCC1 and the endonuclease MUS18–EME1 are recruited to FANCD2-binding CFSs in mitosis to mediate their processing. Chromosomal instability (CIN) is a hallmark of tumour initiation and progression 1 . Some genomic regions are particularly unstable under replication stress, notably common fragile sites 2 (CFSs) whose rearrangements in tumour cells contribute to cancer development. Recent work has shown that the Fanconi anaemia (FANC) pathway plays a role in preventing defective chromosome segregation and CIN under conditions of replication stress 3 . Strikingly, FANCD2 is recruited to regions hosting CFSs on metaphase chromosomes 4 . To decipher the mechanisms protecting CFSs in G2/M, we searched for proteins that co-localize with FANCD2 on mitotic chromosomes, and identified XPF–ERCC1 and MUS81–EME1, two structure-specific endonucleases. We show that depletion of either ERCC1 or MUS81–EME1 affects accurate processing of replication intermediates or under-replicated DNA that persist at CFSs until mitosis. Depletion of these endonucleases also leads to an increase in the frequency of chromosome bridges during anaphase that, in turn, favours accumulation of DNA damage in the following G1 phase.