Chk2 sustains PLK1 activity in mitosis to ensure proper chromosome segregation

Polo-like kinase 1 (PLK1) protects against genome instability by ensuring timely and accurate mitotic cell division. PLK1 activity is tightly regulated throughout the cell cycle. Although the pathways that initially activate PLK1 in G2 are well-characterized, the factors that directly regulate PLK1 in mitosis remain poorly understood. Here, we identify that human PLK1 activity is sustained by the DNA damage response kinase Checkpoint kinase 2 (Chk2) in mitosis. Chk2 directly phosphorylates PLK1 T210, a residue on its T-loop whose phosphorylation is essential for full PLK1 kinase activity. Loss of Chk2-dependent PLK1 activity causes increased mitotic errors, including chromosome misalignment, chromosome missegregation, and cytokinetic defects. Moreover, Chk2 deficiency increases sensitivity to PLK1 inhibitors, suggesting that Chk2 status may be an informative biomarker for PLK1 inhibitor efficacy. This work demonstrates that Chk2 sustains mitotic PLK1 activity and protects genome stability through discrete functions in interphase DNA damage repair and mitotic chromosome segregation.

ATR promotes clearance of damaged DNA and damaged cells by rupturing micronuclei

The human ataxia telangiectasia mutated and Rad3-related (ATR) kinase functions in the nucleus to protect genomic integrity. Micronuclei (MN) arise from genomic and chromosomal instability and cause aneuploidy and chromothripsis, but how MN are removed is poorly understood. Here, we show that ATR is active in MN and promotes their rupture in S phase by phosphorylating Lamin A/C at Ser395, which primes Ser392 for CDK1 phosphorylation and destabilizes the MN envelope. In cells harboring MN, ATR or CDK1 inhibition reduces MN rupture. Consequently, ATR inhibitor (ATRi) diminishes activation of the cytoplasmic DNA sensor cGAS and compromises cGAS-dependent autophagosome accumulation in MN and clearance of micronuclear DNA. Furthermore, ATRi reduces cGAS-mediated senescence and killing of MN-bearing cancer cells by natural killer cells. Thus, in addition to the canonical ATR signaling pathway, an ATR-CDK1-Lamin A/C axis promotes MN rupture to clear damaged DNA and cells, protecting the genome in cell populations through unexpected cell-autonomous and cell-non-autonomous mechanisms.

Molecular mechanism of hyperactivation conferred by a truncation of TRPA1

A drastic TRPA1 mutant (R919*) identified in CRAMPT syndrome patients has not been mechanistically characterized. Here, we show that the R919* mutant confers hyperactivity when co-expressed with wild type (WT) TRPA1. Using functional and biochemical assays, we reveal that the R919* mutant co-assembles with WT TRPA1 subunits into heteromeric channels in heterologous cells that are functional at the plasma membrane. The R919* mutant hyperactivates channels by enhancing agonist sensitivity and calcium permeability, which could account for the observed neuronal hypersensitivity-hyperexcitability symptoms. We postulate that R919* TRPA1 subunits contribute to heteromeric channel sensitization by altering pore architecture and lowering energetic barriers to channel activation contributed by the missing regions. Our results expand the physiological impact of nonsense mutations, reveal a genetically tractable mechanism for selective channel sensitization, uncover insights into the process of TRPA1 gating, and provide an impetus for genetic analysis of patients with CRAMPT or other stochastic pain syndromes.

ATR protects centromere identity by promoting DAXX association with PML nuclear bodies

Centromere protein A (CENP-A) defines centromere identity and nucleates kinetochore formation for mitotic chromosome segregation. Here, we show that ataxia telangiectasia and Rad3-related (ATR) kinase, a master regulator of the DNA damage response, protects CENP-A occupancy at interphase centromeres in a DNA damage-independent manner. In unperturbed cells, ATR localizes to promyelocytic leukemia nuclear bodies (PML NBs), which house the histone H3.3 chaperone DAXX (death domain-associated protein 6). We find that ATR inhibition reduces DAXX association with PML NBs, resulting in the DAXX-dependent loss of CENP-A and an aberrant increase in H3.3 at interphase centromeres. Additionally, we show that ATR-dependent phosphorylation within the C terminus of DAXX regulates CENP-A occupancy at centromeres and DAXX localization. Lastly, we demonstrate that acute ATR inhibition during interphase leads to kinetochore formation defects and an increased rate of lagging chromosomes. These findings highlight a mechanism by which ATR protects centromere identity and genome stability.

A mitosis-specific and R loop-driven ATR pathway promotes faithful chromosome segregation

The ataxia telangiectasia mutated and Rad3-related (ATR) kinase is crucial for DNA damage and replication stress responses. Here, we describe an unexpected role of ATR in mitosis. Acute inhibition or degradation of ATR in mitosis induces whole-chromosome missegregation. The effect of ATR ablation is not due to altered cyclin-dependent kinase 1 (CDK1) activity, DNA damage responses, or unscheduled DNA synthesis but to loss of an ATR function at centromeres. In mitosis, ATR localizes to centromeres through Aurora A-regulated association with centromere protein F (CENP-F), allowing ATR to engage replication protein A (RPA)-coated centromeric R loops. As ATR is activated at centromeres, it stimulates Aurora B through Chk1, preventing formation of lagging chromosomes. Thus, a mitosis-specific and R loop-driven ATR pathway acts at centromeres to promote faithful chromosome segregation, revealing functions of R loops and ATR in suppressing chromosome instability.

Abstract 1410: Proliferation during epithelial-to-mesenchymal transition induces genomic instability

Epithelial to mesenchymal transition (EMT), a morphogenetic process required for proper embryonic development, is adopted by cancer cells during tumor progression. We show that induction of EMT by TGFß or other EMT-inducers such as Snail leads to genomic instability, associated with failed cytokinesis and chromosome missegregation resulting in aneuploidy and polyploidy. These defects are dependent on persistent proliferation of cells undergoing EMT and are absent in normal cells that growth arrest during EMT. While EMT and the associated mitotic abnormalities are reversible upon removal of the EMT-inducer, the resulting chromosomal abnormalities are inherited. TGFß-induced genomic instability is associated with the acquisition of tumorigenic phenotypes and the resulting tumors are enriched for a genomically-altered tumor cell population, which exhibits differential drug sensitivity. In breast and prostate cancer cells circulating in the blood, TGFß and EMT gene signatures are significantly correlated with aneuploid gene signatures. Analysis of single circulating tumor cells (CTC) from women with metastatic breast cancer reveals increased in genomic instability in mesenchymal CTCs. Together, these findings identify a novel mechanism whereby tumor microenvironment-derived signals impact heritable genetic changes within cancer cells.

Citation Format: Valentine Comaills, Lilian Kabeche, Robert Morris, Min Yu, Marissa Wells Madden, Joseph A. LiCausi, Nicola Aceto, Yu Zheng, David T. Miyamoto, Sridhar Ramaswamy, Lee Zou, Daniel A. Haber, Shyamala Maheswaran. Proliferation during epithelial-to-mesenchymal transition induces genomic instability [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1410. doi:10.1158/1538-7445.AM2017-1410

A bifurcated signaling cascade of NIMA-related kinases controls distinct kinesins in anaphase

A signaling module of NIMA-related kinases (Neks) regulates two kinesins, Mklp2 and Kif14, to spatiotemporally coordinate their subcellular localizations and activities. This is important for faithful completion of cytokinesis and reveals novel mechanisms by which Neks regulate late mitosis.

In mitosis, cells undergo a precisely orchestrated series of spatiotemporal changes in cytoskeletal structure to divide their genetic material. These changes are coordinated by a sophisticated network of protein–protein interactions and posttranslational modifications. In this study, we report a bifurcation in a signaling cascade of the NIMA-related kinases (Neks) Nek6, Nek7, and Nek9 that is required for the localization and function of two kinesins essential for cytokinesis, Mklp2 and Kif14. We demonstrate that a Nek9, Nek6, and Mklp2 signaling module controls the timely localization and bundling activity of Mklp2 at the anaphase central spindle. We further show that a separate Nek9, Nek7, and Kif14 signaling module is required for the recruitment of the Rho-interacting kinase citron to the anaphase midzone. Our findings uncover an anaphase-specific function for these effector kinesins that is controlled by specific Nek kinase signaling modules to properly coordinate cytokinesis.

Mitotic DNA Damage Response: At the Crossroads of Structural and Numerical Cancer Chromosome Instabilities

DNA double-strand breaks (DSBs) prevent cells from entering mitosis allowing cells to repair their genomic damage. Little is known about the response to DSBs once cells have already committed to mitosis. Here, we review the genome-protective role of the mitotic DNA damage response (DDR) and evidence suggesting that its untimely activation induces chromosome segregation errors and paradoxically undermines genomic integrity. In contrast to normal cells, cancer cells coopt this pathway to propagate structural and numerical chromosomal instabilities. Cells derived from genomically unstable tumors exhibit evidence for a partially activated DDR during mitosis, which leads to ongoing chromosome segregation errors. Thus, a thorough understanding of the consequences of mitotic DNA damage is key to our ability to devise novel anticancer therapeutic strategies.

Genomic Instability Is Induced by Persistent Proliferation of Cells Undergoing Epithelial-to-Mesenchymal Transition

TGF-β secreted by tumor stroma induces epithelial-to-mesenchymal transition (EMT) in cancer cells, a reversible phenotype linked to cancer progression and drug resistance. However, exposure to stromal signals may also lead to heritable changes in cancer cells, which are poorly understood. We show that epithelial cells failing to undergo proliferation arrest during TGF-β-induced EMT sustain mitotic abnormalities due to failed cytokinesis, resulting in aneuploidy. This genomic instability is associated with the suppression of multiple nuclear envelope proteins implicated in mitotic regulation and is phenocopied by modulating the expression of LaminB1. While TGF-β-induced mitotic defects in proliferating cells are reversible upon its withdrawal, the acquired genomic abnormalities persist, leading to increased tumorigenic phenotypes. In metastatic breast cancer patients, increased mesenchymal marker expression within single circulating tumor cells is correlated with genomic instability. These observations identify a mechanism whereby microenvironment-derived signals trigger heritable genetic changes within cancer cells, contributing to tumor evolution.

Shugoshin-1 balances Aurora B kinase activity via PP2A to promote chromosome bi-orientation

Correction of faulty kinetochore-microtubule attachments is essential for faithful chromosome segregation and dictated by the opposing activities of Aurora B kinase and PP1 and PP2A phosphatases. How kinase and phosphatase activities are appropriately balanced is less clear. Here, we show that a centromeric pool of PP2A-B56 counteracts Aurora B T-loop phosphorylation and is recruited to centromeres through Shugoshin-1 (Sgo1). In non-transformed RPE-1 cells, Aurora B, Sgo1, and PP2A-B56 are enriched on centromeres and levels diminish as chromosomes establish bi-oriented attachments. Elevating Sgo1 levels at centromeres recruits excess PP2A-B56, and this counteracts Aurora B kinase activity, undermining efficient correction of kinetochore-microtubule attachment errors. Conversely, Sgo1-depleted cells display reduced centromeric localization of Aurora B, whereas the remaining kinase is hyperactive due to concomitant reduction of centromeric PP2A-B56. Our data suggest that Sgo1 can tune the stability of kinetochore-microtubule attachments through recruitment of PP2A-B56 that balances Aurora B activity at the centromere.

Numerical chromosomal instability mediates susceptibility to radiation treatment

The exquisite sensitivity of mitotic cancer cells to ionizing radiation (IR) underlies an important rationale for the widely used fractionated radiation therapy. However, the mechanism for this cell cycle-dependent vulnerability is unknown. Here we show that treatment with IR leads to mitotic chromosome segregation errors in vivo and long-lasting aneuploidy in tumour-derived cell lines. These mitotic errors generate an abundance of micronuclei that predispose chromosomes to subsequent catastrophic pulverization thereby independently amplifying radiation-induced genome damage. Experimentally suppressing whole-chromosome missegregation reduces downstream chromosomal defects and significantly increases the viability of irradiated mitotic cells. Further, orthotopically transplanted human glioblastoma tumours in which chromosome missegregation rates have been reduced are rendered markedly more resistant to IR, exhibiting diminished markers of cell death in response to treatment. This work identifies a novel mitotic pathway for radiation-induced genome damage, which occurs outside of the primary nucleus and augments chromosomal breaks. This relationship between radiation treatment and whole-chromosome missegregation can be exploited to modulate therapeutic response in a clinically relevant manner.

DNA-damage response during mitosis induces whole-chromosome missegregation

Many cancers display both structural (s-CIN) and numerical (w-CIN) chromosomal instabilities. Defective chromosome segregation during mitosis has been shown to cause DNA damage that induces structural rearrangements of chromosomes (s-CIN). In contrast, whether DNA damage can disrupt mitotic processes to generate whole chromosomal instability (w-CIN) is unknown. Here, we show that activation of the DNA-damage response (DDR) during mitosis selectively stabilizes kinetochore-microtubule (k-MT) attachments to chromosomes through Aurora-A and PLK1 kinases, thereby increasing the frequency of lagging chromosomes during anaphase. Inhibition of DDR proteins, ATM or CHK2, abolishes the effect of DNA damage on k-MTs and chromosome segregation, whereas activation of the DDR in the absence of DNA damage is sufficient to induce chromosome segregation errors. Finally, inhibiting the DDR during mitosis in cancer cells with persistent DNA damage suppresses inherent chromosome segregation defects. Thus, the DDR during mitosis inappropriately stabilizes k-MTs, creating a link between s-CIN and w-CIN.

SIGNIFICANCE

The genome-protective role of the DDR depends on its ability to delay cell division until damaged DNA can be fully repaired. Here, we show that when DNA damage is induced during mitosis, the DDR unexpectedly induces errors in the segregation of entire chromosomes, thus linking structural and numerical chromosomal instabilities.

STAG2 promotes error correction in mitosis by regulating kinetochore-microtubule attachments

Mutations in the STAG2 gene are present in ∼20% of tumors from different tissues of origin. STAG2 encodes a subunit of the cohesin complex, and tumors with loss-of-function mutations are usually aneuploid and display elevated frequencies of lagging chromosomes during anaphase. Lagging chromosomes are a hallmark of chromosomal instability (CIN) arising from persistent errors in kinetochore-microtubule (kMT) attachment. To determine whether the loss of STAG2 increases the rate of formation of kMT attachment errors or decreases the rate of their correction, we examined mitosis in STAG2-deficient cells. STAG2 depletion does not impair bipolar spindle formation or delay mitotic progression. Instead, loss of STAG2 permits excessive centromere stretch along with hyperstabilization of kMT attachments. STAG2-deficient cells display mislocalization of Bub1 kinase, Bub3 and the chromosome passenger complex. Importantly, strategically destabilizing kMT attachments in tumor cells harboring STAG2 mutations by overexpression of the microtubule-destabilizing enzymes MCAK (also known as KIF2C) and Kif2B decreased the rate of lagging chromosomes and reduced the rate of chromosome missegregation. These data demonstrate that STAG2 promotes the correction of kMT attachment errors to ensure faithful chromosome segregation during mitosis.

Cyclin A regulates kinetochore microtubules to promote faithful chromosome segregation

The most conspicuous event in the cell cycle is the alignment of chromosomes in metaphase. Chromosome alignment fosters faithful segregation through the formation of bi-oriented attachments of kinetochores to spindle microtubules. Strikingly, numerous kinetochore-microtubule (k-MT) attachment errors are present in early mitosis (prometaphase)¹, and the persistence of those errors is the leading cause of chromosome mis-segregation in aneuploid human tumor cells that continually mis-segregate whole chromosomes (e.g. chromosomal instability)^2–7. How robust error correction is achieved in prometaphase to ensure error-free mitosis remains unknown. Here we show that k-MT attachments in prometaphase cells are significantly less stable than in metaphase cells. The switch to more stable k-MT attachments in metaphase requires the proteasome-dependent destruction of cyclin A in prometaphase. Persistent cyclin A expression prevents k-MT stabilization even in cells with aligned chromosomes. In contrast, k-MTs are prematurely stabilized in cyclin A-deficient cells. Consequently, cells lacking cyclin A display higher rates of chromosome mis-segregation. Thus, the stability of k-MT attachments increases decisively in a coordinated fashion among all chromosomes as cells transit from prometaphase to metaphase. Cyclin A creates a cellular environment that promotes microtubule detachment from kinetochores in prometaphase to ensure efficient error correction and faithful chromosome segregation.

Cdk1 and Plk1 mediate a CLASP2 phospho-switch that stabilizes kinetochore-microtubule attachments

Accurate chromosome segregation during mitosis relies on a dynamic kinetochore (KT)-microtubule (MT) interface that switches from a labile to a stable condition in response to correct MT attachments. This transition is essential to satisfy the spindle-assembly checkpoint (SAC) and couple MT-generated force with chromosome movements, but the underlying regulatory mechanism remains unclear. In this study, we show that during mitosis the MT- and KT-associated protein CLASP2 is progressively and distinctively phosphorylated by Cdk1 and Plk1 kinases, concomitant with the establishment of KT-MT attachments. CLASP2 S1234 was phosphorylated by Cdk1, which primed CLASP2 for association with Plk1. Plk1 recruitment to KTs was enhanced by CLASP2 phosphorylation on S1234. This was specifically required to stabilize KT-MT attachments important for chromosome alignment and to coordinate KT and non-KT MT dynamics necessary to maintain spindle bipolarity. CLASP2 C-terminal phosphorylation by Plk1 was also required for chromosome alignment and timely satisfaction of the SAC. We propose that Cdk1 and Plk1 mediate a fine CLASP2 "phospho-switch" that temporally regulates KT-MT attachment stability.