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1 Division of Digestive Diseases, Department of Medicine, 2 Biochemistry, Cell and Developmental Biology Graduate Program, and 3 Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia; and 4 Institute for Quantitative Social Science, Harvard University, Cambridge, Massachusetts
Requests for reprints: Vincent W. Yang, Division of Digestive Diseases, Department of Medicine, Emory University School of Medicine, 201 Whitehead Research Building, 615 Michael Street, Atlanta, GA 30322. Phone: 404-727-5638; Fax: 404-727-5767; E-mail: vyang{at}emory.edu.
| Abstract |
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| Introduction |
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-H2AX, the phosphorylated form of histone H2AX that forms around sites of DNA breaks. We thus wondered whether DNA breaks might be generated during mitotic arrest in human cancer cells. | Materials and Methods |
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-Irradiation was done with a Cs-137 Gammacell. Stealth Select siRNAs targeted to CENP-E, control siRNA, and LipofectAMINE RNAiMax were obtained from, and used according to the instructions of, Invitrogen. All analyses were done 24 h after transfection. Immunodetection. For immunocytochemistry, cells were fixed with 2% formaldehyde/PBS and permeabilized with –20°C methanol. Antibody incubations were 1 h at room temperature, and DNA was counterstained with Hoechst. Images were acquired with a Zeiss Axiovert 100M confocal microscope, except images of cells with spontaneous spindle defects, which were acquired with a Zeiss Axioskop 2 Plus microscope. For flow cytometry, cells were harvested by trypsinization and fixed overnight at –20°C in 70% ethanol. Antibody incubations were 1 h at room temperature, and DNA was counterstained with propidium iodide. Data were acquired using a FACSCalibur (Becton Dickinson) and analyzed with CellQuest. Immunoblotting was done as previously described (6). Antibodies used and image quantification methods are detailed in Supplementary Methods.
Time-lapse imaging. Phase-contrast images of cells grown inside a 37°C, 5% CO2 chamber were automatically obtained at 6-min intervals in multiple locations using an Olympus IX81 microscope. All images were analyzed with Slidebook.
Cytogenetic analyses. Chromosome spreads were prepared using standard cytogenetic techniques; DNA was stained with 4',6-diamidino-2-phenylindole; and images were obtained with a Zeiss Axioskop 2 Plus microscope. Scoring of chromosome aberrations was done according to the classification of Savage (7).
| Results and Discussion |
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-H2AX foci. During 36 h of treatment, nocodazole produced a transient increase in mitotic index that peaked at 12 h and was followed by mitotic slippage (Supplementary Fig. S1). Whereas control prometaphase cells exhibited few
-H2AX foci, nocodazole-arrested prometaphase cells showed a time-dependent increase in
-H2AX foci per cell (Fig. 1A and B
). A similar increase was observed after treatment with the microtubule stabilizer paclitaxel and the Eg5 mitotic kinesin inhibitor monastrol (Fig. 1A), indicating that agents that induce mitotic arrest through different mechanisms also induce
-H2AX foci. Notably, cells exposed to drug treatment, but not yet in mitosis, displayed no increase in
-H2AX foci (Supplementary Fig. S2), indicating that only cells that have entered mitosis exhibit the response.
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-H2AX foci recruit additional proteins involved in the DNA damage response. However, it has previously been shown in HeLa cells that whereas
-H2AX foci form in irradiated mitotic cells, other DNA damage response proteins are only recruited once cells return to interphase (8). Because we also observed this lack of recruitment of DNA damage response proteins in mitotic HCT116 cells (data not shown), we examined the cells after 36 h of nocodazole treatment, at which point virtually all cells had undergone mitotic slippage (Supplementary Fig. S1). Indeed, these postmitotic cells had increased
-H2AX foci (Fig. 1D), which recruited the DNA damage response protein 53BP1 and a phospho-activated form of ATM (pATM; Fig. 1C). Thus,
-H2AX foci acquired during mitotic arrest, like those produced by
-irradiation, recruit other DNA damage response proteins once cells have exited mitosis.
Pharmacologic induction of mitotic arrest is accompanied by some degree of cell death in most cell lines, and
-H2AX formation can participate in apoptosis (1, 9). We thus investigated whether
-H2AX formed during mitotic arrest represents ongoing cell death. Live-cell videomicroscopy indicated that <1% of cells died in a 30-h period after release from a 6-h nocodazole arrest (Fig. 2B
and Supplementary Movie 1), whereas 25% of cells died if continuously treated with the drug (Fig. 2B and Supplementary Movie 2). Given that the average
-H2AX per cell is increased 2.5-fold after 6 h of nocodazole (Fig. 1B) but 99% of these cells survive following drug washout, acquisition of
-H2AX foci must precede any eventual commitment to cell death. Consistent with this, only cells with apoptotic nuclear morphology, which comprised <10% of the total after 18 h nocodazole, exhibited caspase-3 cleavage, cytoplasmic localization of cytochrome c, or activation of BAX, whereas mitotic cells with normal nuclear morphology and abundant
-H2AX foci displayed none of these events (Fig. 2A). Moreover, in contrast to the
-H2AX foci seen in mitotic cells, apoptotic cells exhibited a distinct
-H2AX staining pattern at the periphery of apoptotic chromatin (Fig. 2A). Finally, cotreatment of nocodazole-arrested cells with the caspase inhibitor Q-VD-OPh (10) produced a 98% inhibition of apoptotic cells but no inhibition of
-H2AX foci (Fig. 2C). Taken together, these data indicate that whereas death indeed follows mitotic arrest in a fraction of HCT116 cells, such death is neither the cause of nor the obligatory conclusion to
-H2AX foci acquired during mitotic arrest.
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-H2AX data indicating that breaks arise during mitotic arrest, and thus before subsequent DNA replication. However, some chromatid-type aberrations were also observed (Fig. 3B, top inset), raising the possibility that additional DNA lesions, such as single-stranded breaks, base damages, or postreplication double-strand breaks, might be acquired by the cells (7, 11). Notably, aberrations were observed not only in tetraploid cells produced through mitotic slippage (Fig. 3C) but also in near-diploid cells generated from cell division following drug washout (Fig. 3B), indicating that breaks occur even in cells that recover from transient arrest. Thus, DNA breaks incurred during mitotic arrest manifest as structural karyotype changes in subsequent cell cycles.
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-H2AX foci (Fig. 4C). Thus, genetic induction of mitotic arrest, like that elicited by antimitotic agents, provokes
-H2AX foci in HCT116 cells.
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-H2AX immunocytochemistry, we noticed that occasional untreated mitotic HCT116 cells bore characteristics of prolonged mitotic arrest: a prometaphase arrangement of chromosomes and abundant
-H2AX foci. We thus wondered whether these cells might contain spontaneous spindle defects, which can disrupt mitotic progression (15, 16). Of 4,000 mitotic cells examined, prometaphase cells with marked elevation of
-H2AX foci comprised 2.2% of the total, and 69% of these contained either monopolar or multipolar spindles (Fig. 4D). In contrast, only 5.3% of prometaphase cells with minimal
-H2AX foci contained mitotic spindle abnormalities. Thus, the frequency of prometaphase
-H2AX elevation was increased 13-fold in cells with spontaneous spindle defects (
2 P < 0.0001). Of note, prophase cells with nascent monopolar or multipolar spindles did not possess elevated
-H2AX foci (Supplementary Fig. S3), suggesting that
-H2AX–intense mitotic cells acquire their foci during, and not before, prometaphase. These data indicate that prometaphase acquisition of
-H2AX foci can occur spontaneously in HCT116 cells and may arise during mitotic arrest provoked by spindle abnormalities.
Finally, we explored the extent to which this phenomenon occurs in other human cells. Of eight colorectal cancer lines (including HCT116) exposed to nocodazole, six exhibited a
-H2AX increase in some or most mitotic cells (Supplementary Fig. S4). Increased
-H2AX was also observed in HeLa cervical cancer cells and BG-1 ovarian cancer cells (Supplementary Fig. S4). Thus, DNA damage during mitotic arrest is a common occurrence in human cancer cells. Additionally, we observed a modest
-H2AX increase in IMR90 primary human fibroblasts (Supplementary Fig. S4), suggesting that even nontransformed human cells can acquire some DNA damage during mitotic arrest. However, the degree of
-H2AX induction in primary cells was lower than in the majority of cancer lines, suggesting that malignant cells may be more prone to this damage.
Our study shows that many human cells acquire DNA damage during mitotic arrest. While the mechanism responsible for this damage remains an area for future work, our results have several important implications. First, although unscheduled DNA breaks are known to derive from sources such as replication, oxidative stress, and exogenous mutagens (17), it was hitherto unknown that such breaks could accumulate during mitotic arrest. As such, our findings describe a novel source of DNA damage. Second, because cancer cells frequently contain mitotic defects that induce arrest (1, 18), and because DNA breaks can promote tumorigenesis (17), our findings suggest that one way mitotic arrest may promote tumorigenesis is through DNA damage. Along these lines, it is interesting to note that inactivation of the tumor suppressor hCDC4 and oncogenic overexpression of MAD2 are accompanied not only by prolongation of mitosis and aneuploidy but also by evidence of DNA damage (3, 4). Our results suggest that breaks acquired during mitotic arrest could contribute to the damage observed in these systems. In this way, mitotic arrest may be a source of structural, as well as numerical, chromosomal instability in human cells.
At the same time, this and other studies indicate that cells that endure prolonged or complete inhibition of mitosis exhibit reduced reproductive capacity (1). Indeed, although cell death was not the compulsory fate of HCT116 cells that acquired damage during mitotic arrest, we did observe subsequent death in a fraction of cells. Given that
-H2AX foci increased with time of mitotic arrest, and that the cytotoxicity of DNA damage is dose dependent, a third implication of our findings is that extensive DNA damage imposed by prolonged or complete inhibition of mitosis may promote cell death or growth arrest. In this way, our data support and extend the idea offered by Wong and Stearns (5) that the growth arrest following pharmacologic induction of mitotic arrest (formerly known as the "tetraploidy checkpoint") may, at least in some contexts, be a response to DNA damage. Moreover, this consequence may have clinical implications: given the variable degree of
-H2AX formed on drug-induced arrest in different cancer cell lines, it is plausible that the sensitivity of a tumor to antimitotic chemotherapy could be influenced by its propensity to sustain DNA damage during mitotic arrest. By extension, efforts to identify markers of this propensity might aid attempts to tailor antimitotic chemotherapy to susceptible tumors, as is currently being tried with markers of numerical chromosomal instability (19).
Lastly, our findings do not support a recent report that microtubule-destabilizing agents elicit DNA breaks during G1 phase, and not mitosis (20). Although we do corroborate these authors' finding that such agents induce breaks, our data indicate that these breaks arise on mitotic chromosomes. This explains why even cells that divide following mitotic arrest possess DNA damage, and it indicates that some yet undefined property of mitosis itself commonly confers vulnerability to DNA damage in human cancer cells.
| Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
We thank D. Pallas and J. Lee for discussion and assistance with live-cell microscopy and P. Doetsch, D. Jones, A. Corbett, H. Bastians, G. Davis, B. Craige, and M. Wiltenburg for discussion and support.
| Footnotes |
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Received 9/ 4/07. Revised 10/15/07. Accepted 10/22/07.
| References |
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-irradiation-induced DNA damage. Oncogene 2005;24:4017–25.[Medline]This article has been cited by other articles:
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Z. Storchova and C. Kuffer The consequences of tetraploidy and aneuploidy J. Cell Sci., December 1, 2008; 121(23): 3859 - 3866. [Abstract] [Full Text] [PDF] |
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D. Olmos, C. Swanton, and J. de Bono Targeting Polo-Like Kinase: Learning Too Little Too Late? J. Clin. Oncol., December 1, 2008; 26(34): 5497 - 5499. [Full Text] [PDF] |
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R. Agarwal and G. Deep Kava, a Tonic for Relieving the Irrational Development of Natural Preventive Agents Cancer Prevention Research, November 1, 2008; 1(6): 409 - 412. [Full Text] [PDF] |
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