Stephen C. West, PhD, FRS, FMedSci
Professor, The Francis Crick Institute

“A Life on Holliday”

DNA repair promoted by homologous recombination leads to the formation of recombination intermediates (Holliday junctions) that physically link sister chromatids. Removal of these recombination intermediates is essential for the segregation of DNA to daughter cells at mitosis, and requires the MUS81-EME1 and GEN1 endonucleases. MUS81-EME1 is activated at prometaphase by formation of the SMX tri-nuclease containing three DNA repair structure-selective endonucleases: SLX1-SLX4, MUS81-EME1 and XPF-ERCC1. Within SMX, SLX4 co-ordinates the SLX1 and MUS81-EME1 nucleases for Holliday junction resolution. SMX formation also activates MUS81-EME1 for the removal of late replication intermediates, a reaction that involves MUS81’s conserved N-terminal HhH domain. Cell cycle-dependent formation of this repair tri-nuclease complex provides an unparalleled mechanism by which human cells utilize existing resources to ensure chromosome segregation.

To further understand their genetic interactions and functions, resolvase-defective knock out human cell lines were generated. Although the GEN1 k/o showed little cellular defect, the additional depletion of MUS81 induced massive cell death and severe cell cycle delay. This phenotype occurs in undamaged cells showing that the resolvases are essential for cell survival. The MUS81/GEN1-deficient cells displayed an elevated frequency of lagging chromosomes and DNA ultra-fine bridges (UFBs), that were distinct from previously described replication stress-induced UFBs and centromeric UFBs. The anaphase bridges are broken at cell division, leading to activation of the DNA damage response and aberrant chromosome fusions.

Our work shows that SMX and GEN1 promote a critical role in the resolution of recombination and replication intermediates, and that loss of function leads to a failure in chromosome segregation, resulting in DNA damage and genome instability.

Reading Material:

1. Haley D.M. Wyatt and Stephen C. West Holliday Junction Resolvases Cold Spring Harb Perspect Biol 2014;6:a023192
2. Ying Wai Chan, Kasper Fugger and Stephen C. West Unresolved recombination intermediates lead to ultra-fine anaphase bridges, chromosome breaks and aberration Nature Cell Biology | VOL 20 | JANUARY 2018 | 92–103

(Faculty host: Karlene Cimprich)