MCM proteins: DNA damage, mutagenesis and repair
Introduction
S phase cells are especially susceptible to chromosomal damage because of the potential for replication errors, in addition to injury from exogenous sources. Thus, proteins that are involved in DNA synthesis must play a crucial role in protecting genome integrity. The minichromosome maintenance (MCM) proteins, MCM2–7, are a conserved set of six related proteins that are essential for replication initiation and elongation in eukaryotic cells 1., 2.. MCM proteins form a heterohexomeric complex that associates with the origins of DNA replication to form part of the pre-replicative complex (preRC). Activation of the MCM complex at origins by cyclin-dependent kinases and the Cdc7 (SpHsk1) protein kinase leads to initiation of DNA synthesis. After origin firing, a subset of MCM proteins appears to travel along the chromatin with the replication fork 3., 4.••; MCM function is required for processive DNA replication throughout S phase [5]. The subcomplex of MCM4–6–7 displays weak helicase activity in vitro 6., 7., as does the heterohexameric MCM2–7 complex in the presence of another replication fork component, Cdc45 [8]. Thus, the MCM proteins act during replication initiation, as part of the preRC at origins, and also during elongation, presumably as a helicase at replication forks.
This does not explain the ‘MCM paradox’ 9., 10., however: MCM proteins are estimated to be 40–100-fold more abundant than the number of replication origins and forks in eukaryotic cells [2], but the majority of these do not colocalize with sites of new DNA synthesis 11., 12., 13.. Importantly, even modest reductions in the levels of MCM proteins confer chromosome instability [14]. Thus, there may be multiple pools of MCM proteins with different roles in replication and genome integrity.
In this review, we evaluate evidence that the MCM proteins are central contributors to genome stability during S phase, not only at the replication origin but at the replication fork and beyond.
Section snippets
Loss of MCM function during S phase generates chromosome instability and DNA damage
In yeasts, most temperature-sensitive mcm mutants (mcmts) progress through S phase slowly at restrictive temperature and synthesize nearly a genome-equivalent of DNA before arresting in late S/G2 phase [2]. More severe depletion of MCM protein, by use of a degron allele (in which the protein is actively degraded at restrictive temperature) 5., 15., or in spores germinated from a diploid that is heterozygous for an MCM deletion 14., 16., results in little or no replication of the DNA. In the mcm
S-phase checkpoints target the MCM complex
Cells undergoing DNA synthesis must recognize and respond to both errors in replication and damage caused by exogenous sources. Checkpoint activation and subsequent lesion repair are crucial in preventing amplification or loss of genes or chromosomes, and to promote accurate chromosome segregation in mitosis. During S phase, two main checkpoints respond to alterations in the DNA 30., 31.. The replication checkpoint stabilizes existing forks and prevents additional origins from firing when
MCM proteins affect chromosome structure
In S. pombe, mcmts mutants display increased chromosome loss and recombination, even at permissive temperature, where initiation and progression of DNA replication appear normal [14]. Drosophila mcmts mutants display defects in chromosome condensation and arrest at metaphase 49., 50., while S. cerevisiae mcmts mutant arrest depends in part on the spindle checkpoint [51]. This supports a broader role for MCM proteins in genome stability than simply modulation of origin usage and fork
MCM expression and cell proliferation: markers for cancer
MCM proteins are expressed in cycling cells but are down-regulated and dissociated from chromatin in quiescent cells [57]. One major mechanism by which the MCM proteins promote genome stability is by limiting DNA replication to once per cell cycle [2]. This crucial role makes MCM proteins potential targets for cellular oncogenes, which inhibit or deregulate the replication process. Dysregulation of MCM activity might, therefore, contribute to abnormal cell proliferation and accompanying genome
Conclusions
The MCM complex plays a crucial role in determining the replication potential of cells. The role of the complex in processive DNA replication appears to be as a replicative helicase 1., 10., but the mechanisms by which the MCM proteins act to sense and respond to S-phase damage remains to be determined. Recent work suggests that MCM proteins are not only targets of the S-phase checkpoints, but they also interact directly with components of the checkpoint and repair pathways. Future
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
We thank Eliana Gómez and Daniel Pankratz for comments on the manuscript. Work in SL Forsburg’s laboratory is supported by grants from the American Cancer Society (RSG-00-132-04-CCG) and NIH (R01 GM59321). JM Bailis was supported by the Damon Runyon Cancer Research Foundation (fellowship 1634).
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