Safeguarding Entry into Mitosis: the Antephase Checkpoint

Maintenance of genomic stability is needed for cells to survive many rounds of division throughout their lifetime. Key to the proper inheritance of intact genome is the tight temporal and spatial coordination of cell cycle events. Moreover, checkpoints are present that function to monitor the proper execution of cell cycle processes. For instance, the DNA damage and spindle assembly checkpoints ensure genomic integrity by delaying cell cycle progression in the presence of DNA or spindle damage, respectively. A checkpoint that has recently been gaining attention is the antephase checkpoint that acts to prevent cells from entering mitosis in response to a range of stress agents. We review here what is known about the pathway that monitors the status of the cells at the brink of entry into mitosis when cells are exposed to insults that threaten the proper inheritance of chromosomes. We highlight issues which are unresolved in terms of our understanding of the antephase checkpoint and provide some perspectives on what lies ahead in the understanding of how the checkpoint functions.Segregation of sister chromosomes during the metaphase-to-anaphase transition is a dramatic event that results in the inheritance of a complete set of chromosomes by each daughter cell undergoing cell division. This process, which occurs during mitosis, requires the temporal and spatial coordination of a myriad of proteins. As many excellent reviews on the process of chromosome segregation have been published (9, 37, 84, 97, 136), we give here an overview of the process.In essence, duplicated chromosomes are condensed and then lined up at the metaphase plate, where the sister chromatids are subsequently pulled apart by microtubules attached to the kinetochores. The duplicated chromosomes are condensed by condensin I and II complexes that function to pack interphase chromatin so that it can then be neatly divided into daughter cells (6, 48, 50) (see below). Yet other protein complexes essential for ensuring genomic integrity during nuclear separation are the cohesins which maintain cohesion between sister chromatids (17, 85). The cohesins are loaded onto the duplicated chromosomes toward the end of mitosis in the preceding round of cell division or in late G₁/early S phase in the new round of cell division (9, 90, 111, 130). The presence of the cohesins helps keep the sister chromatids together until the kinetochores are correctly attached to spindle microtubules emanating from both microtubule-organizing centers (i.e., the spindle pole bodies in Saccharomyces cerevisiae or the centrosomes in higher eukaryotes) in a process known as bi-orientation (122). Upon proper attachment of the mitotic spindles to the kinetochores, the sister chromatids separate as cohesins are destroyed through proteolysis by separase, a CD clan protease (129). Chromosome separation occurs as the spindle microtubules pull the chromosomes toward opposite ends of the dividing cells. This process of chromosome segregation is highly complex and requires tight regulation in order that genomic stability is maintained over successive rounds of cell division (1).In addition to the tight coordination of events during chromosome segregation, the genomic integrity of dividing cells is kept in check by the presence of checkpoints (Fig. ​(Fig.1)1) that are needed to prevent the propagation of transformed cells (44). In mitosis, the spindle assembly checkpoint pathway plays a critical role in the surveillance of spindle integrity and elicits a delay in the metaphase-to-anaphase transition in the presence of spindle damage (83). The requirement for an intact spindle assembly checkpoint to maintain genomic integrity as cells undergo division is underscored by the correlations between mutations in the spindle assembly checkpoint genes and chromosome instability (15, 16, 72). Key players at the spindle assembly checkpoint include MAD2 and BUB1 (83).Open in a separate windowFIG. 1.Cell cycle checkpoint pathways impinging upon the cell division cycle. The cell division cycle is monitored throughout by various checkpoints, including the DNA replication (blue box) and DNA damage (red box) checkpoints, as well as the spindle assembly checkpoint (gray box). In addition, the antephase checkpoint (green box) plays an important role in preventing mitotic entry in the presence of various stress conditions (see text).Of late, interest has been gathering around a checkpoint that is presumably present in antephase and delays entry into mitosis. This checkpoint, named the “antephase checkpoint” by Matsusaka and Pines (71), is distinct from the G₂ checkpoints, which are activated in response to DNA damage (4, 5) and unreplicated DNA (100, 101). Also, a decatenation checkpoint that monitors the status of chromosome decatenation by topoisomerase II appears to act in a manner distinct from that of the antephase checkpoint (24). The antephase checkpoint has been proposed to function in response to a range of stress agents to delay entry into mitosis (97).In this review, we highlight the initial experiments which led to the idea of the existence of an antephase checkpoint which functions to prevent chromosome condensation, thereby safeguarding entry into mitosis in the presence of perturbations as cells prepare for chromosome condensation and segregation. We also review the players implicated in the checkpoint, such as CHFR (checkpoint with FHA and RING domains) (109) and p38 stress kinase (66), and discuss their roles in modulating the antephase checkpoint and the correlations between mutations or alterations in these genes with tumor formation. Lastly, we look at how the antephase checkpoint is likely to function, based on the current understanding of entry into mitosis and chromosome condensation.