RECQ5 Helicase Cooperates with MUS81 Endonuclease in Processing Stalled Replication Forks at Common Fragile Sites during Mitosis

Abstract

The MUS81-EME1 endonuclease cleaves late replication intermediates at common fragile sites (CFSs) during early mitosis to trigger DNA-repair synthesis that ensures faithful chromosome segregation. Here, we show that these DNA transactions are promoted by RECQ5 DNA helicase in a manner dependent on its Ser727 phosphorylation by CDK1. Upon replication stress, RECQ5 associates with CFSs in early mitosis through its physical interaction with MUS81 and promotes MUS81-dependent mitotic DNA synthesis. RECQ5 depletion or mutational inactivation of its ATP-binding site, RAD51-interacting domain, or phosphorylation site causes excessive binding of RAD51 to CFS loci and impairs CFS expression. This leads to defective chromosome segregation and accumulation of CFS-associated DNA damage in G1 cells. Biochemically, RECQ5 alleviates the inhibitory effect of RAD51 on 3'-flap DNA cleavage by MUS81-EME1 through its RAD51 filament disruption activity. These data suggest that RECQ5 removes RAD51 filaments stabilizing stalled replication forks at CFSs and hence facilitates CFS cleavage by MUS81-EME1.

Genomic instability in fragile sites—still adding the pieces

“…These factors together lead to incomplete DNA replication of the region under stress conditions . The entry of cells with un‐replicated regions into mitosis triggers DNA repair synthesis to minimize chromosome mis‐segregation . The remaining un‐replicated regions are hotspots for genomic instability and chromosomal rearrangements.…”

“…41,42 The entry of cells with un-replicated regions into mitosis triggers DNA repair synthesis to minimize chromosome missegregation. 43,44 The remaining un-replicated regions are hotspots for genomic instability and chromosomal rearrangements. This section focuses on the potential genetic and epigenetics sources of CFS instability.…”

DNA replication stress and cancer chemotherapy

“…If such cells proceed into mitosis, sister chromatids are interlinked as replication intermediates and can be visualized as UFBs during cytokinesis . When UFBs are properly processed by endonuclease Mus81, these under‐replicated genomic regions form visible gaps or breaks on condensed chromosomes during mitosis . Thus, difficult‐to‐replicate regions in mammalian cells are defined as chromosomal fragile sites because these regions are prone to forming gaps and breaks, particularly following DRS .…”

RECQL5: Another DNA helicase potentially involved in hereditary breast cancer susceptibility

“…RECQL5 (17q25.1) encodes for three protein isoforms (α, β, γ) among which the most studied is RECQL5β that has 991 residues, ubiquitous tissue expression, and nuclear location (Bohr, ). RECQL5 is implicated in DNA replication, transcription, and repair (Kanagaraj et al, ; Paliwal, Kanagaraj, Sturzenegger, Burdova, & Janscak, ; Popuri, Tadokoro, Croteau, & Bohr, , Saponaro et al, ) and specifically involved in the regulation of the homologous recombination (HR) DNA repair pathway (Di Marco et al, ; Hu et al, ; Pellatt et al, ), in which BRCA1, BRCA2, and most of the known HBOC susceptibility genes are implicated. In addition, it has been shown that RECQL5 suppresses tumor formation, and knock‐out mice homozygote for Recql5 deletion have increased cancer predisposition (Hu et al, ; Hu, Lu, & Luo, ).…”