The DNA Damage Checkpoint

The cell cycle contains built-in control mechanisms that register defects in the course of the cell cycle and bring about a halt in the cell cycle to enable the fault to be repaired or to lead the cell to programmed cell death. These control mechanisms are also known as checkpoints. These are biochemical pathways that are activated when a fault occurs and can influence other critical steps of the cell cycle. Of particular importance for the cell cycle is the DNA damage checkpoint. Another important checkpoint is the spindle assembly checkpoint, which is not well biochemically characterized, however. 

As a reaction to DNA damage or a replication block, the cell induces a series of different physiological responses that enable DNA repair. These responses include  

The signaling pathways that lead from the appearance of DNA damage to a halt in the cell cycle include a number of components of which the function has only partially been characterized. So far, it has been estabhshed that the DNA damage checkpoints of different organisms have a common homologous component, namely a protein kinase, which belongs to the superfamUy of P13-kinases (see 6.6). 

For the fission yeast S. pombe, it has been possible to put together a very plausible model that leads to a halt in the cell cycle at the G2/M transition on damage of DNA and prevents entry into mitosis (Fig. 13.16). 

In mammals, there are similar signaling pathways that lead to a halt in the cell cycle when DNA damage is present or when DNA rephcation is incomplete (Zeng et al, 1998). Important components of these signaling pathways are the ATM kinase and the p53 protein (see 14.4.4.5). 

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Cell Cycle Control. Figure 3 The DNA damage checkpoint. In response to DNA damage cells activate p53 dependent and independent checkpoint pathways leading to cell cycle arrest at G1/S and G2/M allowing DNA repair. If the cellular damage cannot be repaired, cells can initiate apoptosis. 

In contrast to the DNA damage checkpoint, the mitotic spindle checkpoint is essential for cell viability. Dierefore, targeting kinases of the spindle checkpoint including Bubl, BubRl, and Mpsl might be a valid strategy for anticancer treatment. 

In addition to their inherent self-sustaining properties, brain tumor stem cells may be more resistant to chemotherapy and radiation therapy than other tumor cells. Bao et al. (2006) found glioma stem cells (CD133+) were relatively radioresistant compared to CD133- tumor cells and preferentially activated the DNA damage checkpoint response. This relative resistance to standard treatment approaches of tumor stem cells compared to the majority of other cells within a tumor may underlie our current inability to cure patients with aggressive brain tumors such as glioblastoma. 

Sanchez Y, Bachant J, Wang H et al 1999 Control of the DNA damage checkpoint by chkl and rad53 protein kinases through distinct mechanisms. Science 286 1166-1171... 

Many of the control mechanisms of the cell cycle are of intrinsic nature and are constitutive, i.e., they are operational in every cell cycle and ensure the ordering of the individual steps. However, other control mechanisms exist that are not active in every cell cycle these are only induced when defects are detected in central cell cycle events. These control mechanisms are known as checkpoints. An example of a checkpoint that is only activated when required is the DNA damage checkpoint (see 13.7). This is a biochemical pathway that detects DNA damage and creates a signal that arrests cells in the Gl, S or G2 phase of the cell cycle. 

The p53 protein has been identified as a component of the DNA damage checkpoint in animals and humans, from which it is assumed to be homologous to the DNA damage checkpoint in the yeast S. cerevisiae (see 13.7). 

N. C. Vihlworth and R Bernards. Rad-dependent response of the chkl-encoded protein kinase at the DNA damage checkpoint [see comments]. Science, 271 (5247), 353-356, 1996. 

Also ]. C. Ford, F. al Khodairy, E. Fotou, K. S. Sheldrick, D. J. Griffiths, and A. M. Carr. 14-3-3 protein homologs required for the DNA damage checkpoint in fission yeast. Science, 265 (5171), 533-535, 1994. Also N. Rhind, B. Furnari, and P. Russell. Cdc2 tyrosine phosphorylation is required for the DNA damage checkpoint in fission yeast. Genes Devel, 11 (4), 504-511, 1997. 

Sancar A, Lindsey-Boltz LA, Unsal-Kacmaz K, Linn S. Molecular mechanisms of mammalian DNA repair and the DNA damage checkpoints. Annu. Rev. Biochem. 2004 73 39-85. 

Smits VA, Klompmaker R, Arnaud L, Rijksen G, Nigg EA, Medema RH. Polo-like kinase-1 is a target of the DNA damage checkpoint. Nat. Cell. Biol. 2000 2 672-676. 

Wang B, Matsuoka S, Carpenter PB, Elledge SJ. 53BP1, a mediator of the DNA damage checkpoint. Science 2002 298(5597) 28. 

Majka J, Burgers PM. Yeast Radl7/Mec3/Ddcl a sliding clamp for the DNA damage checkpoint. Proc. Natl. Acad. Sci. U.S.A. 2003 100(5) 2249-2254. 

Melo J, Toczyski D. A unified view of the DNA-damage checkpoint. 

Non-genotoxic stresses like ribonucleotide depletion or under- or oversupply with oxygen can activate the p53 response without participation of the DNA damage checkpoints. The link between these stresses and p53 appears to be provided by a subspecies of the Jun-N-terminal kinase (JNK2), which is activated by various stresses and enhances p53 stability by phosphorylation on Thr81. Overall, however, this pathway is only poorly characterized. 

Cells that fail to replicate all their chromosomes do not enter mitosis. Operation of the unrepIicated-DNA checkpoint control involves the recognition of unreplicated DNA and inhibition of MPF activation (see Figure 21-32, [T]). Recent genetic studies in 5. pombe and biochemical studies with Xenopus egg extracts suggest that the ATR and Chkl protein kinases, which also function in the DNA-damage checkpoint, inhibit entry into mitosis by cells that have not completed DNA synthesis. 

The DNA-damage checkpoint blocks progression through the cell cycle until the damage is repaired. Damage to DNA can result from chemical agents and from irradiation with ultraviolet (UV) light or y-rays. 

As we discuss in detail in Chapter 23, inactivation of tumor-suppressor genes contributes to the development of cancer. The proteins encoded by several tumor-suppressor genes, including ATM and Chk2, normally function in the DNA-damage checkpoint. Patients with mutations in both copies of ATM or Chk2 develop cancers far more frequently than normal. Both of these genes encode protein kinases. 

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