Adaptation to the Ionizing Radiation–Induced G2 Checkpoint Occurs in Human Cells and Depends on Checkpoint Kinase 1 and Polo-like Kinase 1 Kinases

Abstract

Checkpoint adaptation was originally defined in yeast as the ability to divide despite the presence of damaged DNA. An important unanswered question is whether checkpoint adaptation also occurs in human cells. Here, we show that following the ionizing radiation-induced G 2 checkpoint, human osteosarcoma cells entered mitosis with ;-H2AX foci, a marker for unrepaired DNA double-strand breaks. Exit from the G 2 checkpoint was accelerated by inhibiting the checkpoint kinase 1 (Chk1) and delayed by overexpressing wild-type Chk1 or depleting the Polo-like kinase 1 (Plk1). Chk1 and Plk1 controlled this process, at least partly, via independent signaling pathways. Our results suggest that human cells are able to exit the checkpoint arrest and divide before the damage has been fully repaired. Such cell division in the presence of damaged DNA may be detrimental for genetic stability and could potentially contribute to cancer development. (Cancer Res 2006; 66(21): 10253-7)

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).…”

Breaking down cell cycle checkpoints and DNA repair during antigen receptor gene assembly

“…In checkpoint deficient strains (for example RAD9, RAD17 or MEC1 (similar to ATM/ATR)), cells do not even delay the first cycle and they continue to divide despite chromosome loss and genomic instability (Galgoczy and Toczyski, 2001). Adaptation to irreparable DSBs has also been observed in Xenopus oocytes (Yoo et al, 2004) and most recently in human cells after irradiation (Syljuasen et al, 2006). In these systems, and in some tumor cells mitotic reentry requires overexpression of the polo-like kinase Plk1 and the subsequent inhibition of the ATR substrate Chk1 (Bertoni et al, 1999;Strebhardt and Ullrich, 2006;Syljuasen et al, 2006).…”

When genome integrity and cell cycle decisions collide: roles of polo kinases in cellular adaptation to DNA damage

“…Since the process of adaptation has been predominantly studied in the context of DNA doublestrand breaks (DSBs) (see below), we will limit our overview to the cellular responses induced by this type of DNA lesion. However, it is important to keep in mind that adaptation can occur in response to a wide variety of genotoxic lesions (Kubara et al 2012;Syljuasen et al 2006;Yoo et al 2004). For a detailed discussion on checkpoint responses and DNA repair pathways, we direct readers to several recent reviews (Chapman et al 2012;Huertas 2010;Jackson and Bartek 2009;Moynahan and Jasin 2010;Panier and Durocher 2013).…”

“…Typical agents used in adaptation studies include DNA replication-inhibiting drugs, such as aphidicolin (Yoo et al 2004), topoisomerase I inhibitors, such as camptothecin (Kubara et al 2012), or non-specific DNA-damaging agents, such ionizing radiation (Syljuasen et al 2006). These agents are used at doses that would induce enough DNA damage to exceed (at least momentarily) the DNA repair capacity of cells, but not grossly so, in order to avoid the induction of immediate cell death.…”

“…The goal of both systems described above is to create a situation whereby cells could either maintain a checkpoint-induced cell cycle arrest, or take the decision to re-enter a proliferative state despite the presence of significant levels of DNA damage in their genome. This is usually detected by monitoring cell morphology (e.g., yeast budding or microcolony formation on solid medium; Lee et al 1998;Sandell and Zakian 1993;Toczyski et al 1997) or the appearance of changes in cell cycle status (e.g., passage from interphase to mitosis, completion of DNA replication, appearance of chromosome condensation; Kubara et al 2012;Syljuasen et al 2006;Yoo et al 2004). At the molecular level, adaptation is also associated with silencing of the DNA damage signaling machinery (e.g., dephosphorylation of checkpoint effectors; Donnianni et al 2010;Pellicioli et al 2001;Vidanes et al 2010), while markers of DNA damage persist.…”