Review
How Cells Respond to DNA Breaks in Mitosis

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Highlights

  • DNA double-strand breaks (DSBs) are one of the most toxic types of DNA lesion. Although much is known about how cells respond to DSBs in interphase, far less is understood about how they respond to such lesions during mitosis.

  • The major DSB repair pathways, nonhomologous end joining and homologous recombination, are inhibited during mitosis and there is no DNA damage checkpoint after late prophase until cells re-enter G1 when mitosis is complete.

  • DSBs that occur during mitosis must be stabilized until the next G1 phase to avoid mis-segregation of acentric fragments and chromosomal instability.

  • The chromatin response to DSBs is rewired during mitosis. Chromatin ubiquitylation is blocked and 53BP1, BRCA1, and their downstream effectors are not recruited. Instead, TOPBP1 is recruited via direct binding to MDC1 and this mechanism is required to maintain chromosomal stability.

  • Micronuclei from a failed DSB response during mitosis can induce chromothripsis and trigger an inflammatory response by activating cGAS-STING signaling.

DNA double-strand breaks (DSBs) are highly toxic lesions that can lead to chromosomal instability if they are not repaired correctly. DSBs are especially dangerous in mitosis when cells go through the complex process of equal chromosome segregation into daughter cells. When cells encounter DSBs in interphase, they are able to arrest the cell cycle until the breaks are repaired before entering mitosis. However, when DSBs occur during mitosis, cells no longer arrest but prioritize completion of cell division over repair of DNA damage. This review focuses on recent progress in our understanding of the mechanisms that allow mitotic cells to postpone DSB repair without accumulating massive chromosomal instability. Additionally, we review possible physiological consequences of failed DSB responses in mitosis.

Section snippets

Differential Responses to DNA Breaks in Interphase and Mitosis

Cells must maintain genome stability during cycles of cell division to pass on their hereditary material intact to the next generation [1]. DNA is vulnerable to endogenous and exogenous sources of damage, so cells have evolved a complex network of biochemical pathways to counteract these threats, collectively called the DNA damage response (DDR). Activation of the DDR involves recognition and repair of DNA lesions, modulation of chromatin structure and transcription, and cell-cycle checkpoint

DNA Damage Checkpoint Signaling in Interphase and Mitosis

Many types of DNA damage can be repaired rapidly without the need for cells to activate cell-cycle checkpoints or sustain other global signaling responses. However, some lesions, such as DNA DSBs, are highly toxic and can be particularly challenging for cells to repair accurately without causing genome instability [1], especially if left unrepaired when cells attempt chromosome segregation during mitosis. The DDR is initiated by three structurally related protein kinases: ATM, ATR, and DNA-PKcs

DSB Repair and the Chromatin Response to DNA Breaks in Interphase and Mitosis

In interphase, most DSBs are repaired via the nonhomologous end-joining (NHEJ) or homologous recombination (HR) pathways [37]. NHEJ re-ligates DNA ends with limited or no DNA end processing and is the predominant DSB repair mechanism that cells use throughout interphase except at collapsed DNA replication forks, where NHEJ is highly toxic so HR pathways must be used instead [38,39]. HR pathways rely on the presence of a homologous donor template for the repair and accurate regeneration of the

Are DSBs Repaired in Mitosis?

The absence of 53BP1 recruitment to sites of DSBs in mitosis removes a major DNA-end resection roadblock that could, in theory, allow hyper-resection in mitosis. However, this is not observed in mitotic cells [77,78], possibly due to the fact that chromosomes are too highly condensed for enzymes involved in long-range resection to overcome. In addition to lack of DNA-end resection, key HR factors such as BRCA1 and RAD51 are also not recruited to mitotic DSB sites [30,79]. The bulk of evidence

Consequences of Defective Responses to DNA Breaks during Mitosis

If a DSB that occurs in mitosis fails to be repaired or tethered, the affected chromosome breaks into two pieces: an acentric chromosome fragment and a centric chromosome fragment, each containing a telomere on one end and free DNA end on the other (Figure 3). Since the acentric fragment is not able to interact with the mitotic spindle, it lingers near the equatorial plate during anaphase. These acentric fragments usually randomly segregate into the cytoplasm of one of the daughter cells and

Under-replicated DNA as an Endogenous Source of Mitotic DSBs

Due to the relative amount of time cells spend in mitosis compared with interphase, the majority of DSBs that cells acquire from exogenous sources would be expected to occur and be repaired in interphase. Clearly however, the fact that mitosis-specific DSB responses exist and are apparently conserved from some single-celled organisms to humans [15] means that DSBs must occur to a significant extent during cell division and be recognized for the maintenance of genome stability. What might the

Concluding Remarks

The key features of the specialized response to DNA breaks in mitosis can be summarized as follows. On entering prophase, cells do not arrest the cell cycle in the presence of DSBs and instead prioritize completion of cell division over repair of DNA breaks. The chromatin response to DSBs is truncated in mitosis. Although H2AX is phosphorylated in mitosis and recognized by MDC1, the recruitment of key downstream factors that regulate DSB repair pathway choice such as 53BP1 and BRCA1 is blocked.

Acknowledgments

Andrew Blackford is funded by a Cancer Research UK Career Development Fellowship (C29215/A20772). We apologize to all colleagues whose important findings could not be cited owing to space limitations.

Glossary

Anaphase-promoting complex/cyclosome (APC/C)
a multisubunit E3 ubiquitin ligase complex that targets various cell-cycle proteins for degradation by the proteasome and thus mainly regulates the metaphase-to-anaphase transition.
Apoptosis
a highly regulated and tightly controlled form of programmed cell death that occurs in multicellular organisms, mediated in response to DNA damage by the tumor suppressor p53.
Cell-cycle checkpoints
control mechanisms in eukaryotic cells that block or slow cell-cycle

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