MDC1 Interacts with TOPBP1 to Maintain Chromosomal Stability during Mitosis.

In mitosis, cells inactivate DNA double-strand break (DSB) repair pathways to preserve genome stability. However, some early signaling events still occur, such as recruitment of the scaffold protein MDC1 to phosphorylated histone H2AX at DSBs. Yet, it remains unclear whether these events are important for maintaining genome stability during mitosis. Here, we identify a highly conserved protein-interaction surface in MDC1 that is phosphorylated by CK2 and recognized by the DNA-damage response mediator protein TOPBP1. Disruption of MDC1-TOPBP1 binding causes a specific loss of TOPBP1 recruitment to DSBs in mitotic but not interphase cells, accompanied by mitotic radiosensitivity, increased micronuclei, and chromosomal instability. Mechanistically, we find that TOPBP1 forms filamentous structures capable of bridging MDC1 foci in mitosis, indicating that MDC1-TOPBP1 complexes tether DSBs until repair is reactivated in the following G1 phase. Thus, we reveal an important, hitherto-unnoticed cooperation between MDC1 and TOPBP1 in maintaining genome stability during cell division.

The CIP2A-TOPBP1 complex safeguards chromosomal stability during mitosis.

The accurate repair of DNA double-strand breaks (DSBs), highly toxic DNA lesions, is crucial for genome integrity and is tightly regulated during the cell cycle. In mitosis, cells inactivate DSB repair in favor of a tethering mechanism that stabilizes broken chromosomes until they are repaired in the subsequent cell cycle phases. How this is achieved mechanistically is not yet understood, but the adaptor protein TOPBP1 is critically implicated in this process. Here, we identify CIP2A as a TOPBP1-interacting protein that regulates TOPBP1 localization specifically in mitosis. Cells lacking CIP2A display increased radio-sensitivity, micronuclei formation and chromosomal instability. CIP2A is actively exported from the cell nucleus in interphase but, upon nuclear envelope breakdown at the onset of mitosis, gains access to chromatin where it forms a complex with MDC1 and TOPBP1 to promote TOPBP1 recruitment to sites of mitotic DSBs. Collectively, our data uncover CIP2A-TOPBP1 as a mitosis-specific genome maintenance complex.

Mechanisms of genome stability maintenance during cell division.

During mitosis, chromosomes undergo extensive structural changes resulting in the formation of compact cylindrical bodies and in the termination of the bulk of DNA-dependent metabolic activities. Therefore, DNA lesions that interfere with processes such as DNA replication and transcription in interphase are not expected to pose a major threat to genome stability in mitosis. There are, however, a few exceptions. DNA replication and repair intermediates that physically interconnect the sister chromatids jeopardize faithful chromosome segregation and need to be resolved before the onset of anaphase. In addition, dicentric chromosomes can form chromatin bridges and induce breakage-fusion-breakage cycles with dire consequences for genome stability. Finally, chromosome breaks that escape the G2/M DNA damage checkpoint or emerge early in mitosis may result in lagging acentric DNA fragments that mis-segregate and form micronuclei when cells exit from mitosis. Both chromatin bridges and micronuclei are potential sources of a mutational cascade that results in massive chromosomal instability and significantly contributes to genomic complexity. Here, we review recent progress in our understanding of the origins and consequences of chromosome bridges and micronuclei and the mechanisms by which cells suppress them.

The CIP2A-TOPBP1 axis safeguards chromosome stability and is a synthetic lethal target for BRCA-mutated cancer.

BRCA1/2-mutated cancer cells adapt to the genome instability caused by their deficiency in homologous recombination (HR). Identification of these adaptive mechanisms may provide therapeutic strategies to target tumors caused by the loss of these genes. In the present study, we report genome-scale CRISPR-Cas9 synthetic lethality screens in isogenic pairs of BRCA1- and BRCA2-deficient cells and identify CIP2A as an essential gene in BRCA1- and BRCA2-mutated cells. CIP2A is cytoplasmic in interphase but, in mitosis, accumulates at DNA lesions as part of a complex with TOPBP1, a multifunctional genome stability factor. Unlike PARP inhibition, CIP2A deficiency does not cause accumulation of replication-associated DNA lesions that require HR for their repair. In BRCA-deficient cells, the CIP2A-TOPBP1 complex prevents lethal mis-segregation of acentric chromosomes that arises from impaired DNA synthesis. Finally, physical disruption of the CIP2A-TOPBP1 complex is highly deleterious in BRCA-deficient tumors, indicating that CIP2A represents an attractive synthetic lethal therapeutic target for BRCA1- and BRCA2-mutated cancers.

Landscape of CDKN1B Mutations in Luminal Breast Cancer and Other Hormone-Driven Human Tumors.

The CDKN1B gene encodes for the p27Kip1 protein, firstly characterized as a cyclin dependent kinase (CDK)-inhibitor. Germline CDKN1B pathogenic variants have been described in hereditary tumors, such as multiple endocrine neoplasia (MEN)-like syndromes and familial prostate cancer. Despite its central role in tumor progression, for a long time it has been proposed that CDKN1B was very rarely somatically mutated in human cancer and that its expression levels were almost exclusively regulated at post-transcriptional level. Yet, the advent of massive parallel sequencing has partially subverted this general understanding demonstrating that, at least in some types of cancer, CDKN1B is mutated in a significant percentage of analyzed samples. Recent works have demonstrated that CDKN1B can be genetically inactivated and this occurs particularly in sporadic luminal breast cancer, prostate cancer and small intestine neuroendocrine tumors. However, a clear picture of the extent and significance of CDKN1B mutations in human malignances is still lacking. To fill this gap, we interrogated the COSMIC, ICGC, cBioPortal, and TRANSFAC data portals and current literature in PubMed, and reviewed the mutational spectrum of CDKN1B in human cancers, interpreting the possible impact of these mutations on p27Kip1 protein function and tumor onset and progression.