CIP2A Mediates the Recruitment of the SLX4-MUS81-XPF Tri-Nuclease Complex in Mitosis and Protects Against Replication Stress

DNA replication stress frequently elicits problems in mitosis because incompletely replicated chromosomes or replication intermediates physically link sister chromatids together and prevent their proper segregation during anaphase. We and others recently discovered a mitotic role for the CIP2A oncoprotein, which is critically implicated in chromosome stability maintenance and chromosome fragment clustering during mitosis. In addition, in homologous recombination deficient (HRD) cells, CIP2A is essential and may thus constitute a new drug target in HRD cancers. Yet, the precise mechanisms by which CIP2A suppresses chromosomal instability during mitosis and thus allows for the survival of HRD cancer cells remain largely elusive. Here we characterize CIP2A’s role in DNA replication stress responses. We show that upon replication stress, wild-type cells show an elevated accumulation of CIP2A foci during mitosis, indicating its involvement in the mitotic response to replication stress. Defective DNA replication leads to the accumulation of under-replicated DNA, which can be carried into mitosis. We demonstrate that in the absence of CIP2A, cells fail to recruit the SLX4-MUS81-XPF (SMX) tri-nuclease complex to sites of under-replicated DNA in mitosis, resulting in a high incidence of lagging chromosomes during anaphase and subsequent micronuclei formation. In a subset of cell lines, we also observed CIP2A-dependent mitotic DNA synthesis (MiDAS) upon replication stress. However, our data suggest that MiDAS and SMX recruitment are not functionally linked. This novel role of CIP2A in managing under-replicated DNA may provide insights into the molecular mechanisms underlying therapeutic vulnerabilities in cancer cells.