Understanding the limitations of radiation-induced cell cycle checkpoints

Abstract

The DNA damage response pathways involve processes of double-strand break (DSB) repair and cell cycle checkpoint control to prevent or limit entry into S phase or mitosis in the presence of unrepaired damage. Checkpoints can function to permanently remove damaged cells from the actively proliferating population but can also halt the cell cycle temporarily to provide time for the repair of DSBs. Although efficient in their ability to limit genomic instability, checkpoints are not foolproof but carry inherent limitations. Recent work has demonstrated that the G1/S checkpoint is slowly activated and allows cells to enter S phase in the presence of unrepaired DSBs for about 4–6 h post irradiation. During this time, only a slowing but not abolition of S-phase entry is observed. The G2/M checkpoint, in contrast, is quickly activated but only responds to a level of 10–20 DSBs such that cells with a low number of DSBs do not initiate the checkpoint or terminate arrest before repair is complete. Here, we discuss the limitations of these checkpoints in the context of the current knowledge of the factors involved. We suggest that the time needed to fully activate G1/S arrest reflects the existence of a restriction point in G1-phase progression. This point has previously been defined as the point when mitogen starvation fails to prevent cells from entering S phase. However, cells that passed the restriction point can respond to DSBs, albeit with reduced efficiency.

Mitotic progression following DNA damage enables pattern recognition within micronuclei

“…Checkpoint adaptation and insensitivity has been described in a wide range of eukaryotic organisms 27, 28 . Our data support a model in which imperfect cell cycle checkpoints allow passage through mitosis and accumulation of micronuclei where pattern recognition occurs (Fig 4f).…”

Lymphocyte lineage-specific and developmental stage specific mechanisms suppress cyclin D3 expression in response to DNA double strand breaks

“…In response to DSBs or other types of damage, cells activate mechanisms to inhibit cyclin:CDK complexes and thereby block cell cycle progression at G1/S or G2/M checkpoints. 4 The repair of damage inactivates these checkpoints, enabling resumption of cellular proliferation. The ability of cells to repress activities of cyclin:CDK complexes following stress is critical to maintain cellular viability and function and suppress malignant cellular transformation.…”

“…Mammalian cells experiencing DSBs in G1 activate the ATM kinase to restrict S phase entry until DSBs are repaired or apoptosis is induced. 4 In all non-malignant mammalian cell types analyzed, ATM activates complementary pathways that inhibit phosphorylation of CDK2 substrates and thereby block cell cycle progression in late G1 at the G1/S checkpoint. ATM inactivates the Cdc25a phosphatase that removes inhibitory phosphates from CDK2 proteins.…”

“…The ability of cells to repress activities of cyclin:CDK complexes following stress is critical to maintain cellular viability and function and suppress malignant cellular transformation. 4 Mitogenic stimuli cause proliferation of mammalian cells by inducing expression of one or more D-type cyclins, which bind and activate CDK4 and/or CDK6 to drive G1 phase progression and commit cells to enter the cell cycle. [1][2][3] Cyclin D:CDK4/6 complexes phosphorylate the Retinoblastoma (Rb) family proteins that sequester and inhibit E2F transcription factors.…”

“…ATM inactivates the Cdc25a phosphatase that removes inhibitory phosphates from CDK2 proteins. 4 ATM activates the p53 transcription factor, which transcriptionally induces expression of the p21 CDK inhibitor (CKI) that binds and inhibits Cyclin E:CDK2 complexes. 4 The p53-independent arm of the G1/S checkpoint is activated more rapidly than the p53-dependent arm, which requires transcription and is more important for G1/S checkpoint maintenance.…”