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A Brief Overview of the DNA damage response
The inability to repair damaged DNA correctly results in genomic instability and can lead to various disorders and increased rates of tumour development. Organisms respond to such DNA damage by activating a composite damage response pathway. This pathway can ultimately regulate crucial responses including cell cycle arrest (termed checkpoints), apoptosis and senescence and can also activate additional processes including DNA repair mechanisms, transcription and replication in an effort to maintain genomic integrity.
Survival of the species depends on the accurate transmission of genetic information from a cell to its two daughter cells. This transmission requires not only extreme accuracy and precision in DNA replication but also the ability to survive DNA damage from endogenous and environmental agents and keep to minimum the number of heritable mutations. During the process of DNA replication errors resulting in double stand breaks (DSBs) or single strand breaks (SSB) will call on the DNA damage response. Such DNA aberrations arise during normal physiological processes such as DNA mismatches during DNA replication and DNA damage in response to reactive oxygen compounds occurring as by-products of oxidative respiration in the cell.
To combat threats posed by DNA damage, cells have acquired surveillance mechanisms that can react to DNA damage and the resultant changes in chromatin structure with a response pathway that begins with cell cycle arrest referred to as a DNA damage checkpoint. The primary role of the DNA damage checkpoint is to control the cells ability to arrest the cell cycle in response to DNA damage and in doing so allowing time for DNA repair. In addition to this, DNA checkpoints can influence the composition of chromatin, the activation of DNA repair pathways, transcription of “repair” proteins and cell death by apoptosis. This checkpoint is only one step of a much larger DNA damage response pathway that regulates diverse responses in the cell (Fig. 1).
The DNA damage response (DDR) is essentially a biochemical signalling network composed of sensors, transducers and effectors that can recognise abnormalities in the cell and induce a coordinated cellular response. Cells defective in one or more of these components display heightened sensitivity to DNA damaging agents and increased susceptibility to genome instability and tumours. The DDR is highly conserved from yeast to humans and many of the important components in this network, the sensors, mediators transducers and effectors have been identified in both higher and lower eukaryotes.
Sensors of the DDR are ill-understood components of this complex network that recognise abnormal lesions in the DNA. The nature of detecting of these DNA lesions is not clear but in mammalian cells it does involve the MRN [MRE11 (meiotic recombination 11)-Rad 50- NBS (Nijmegan breakage syndrome 1)] complex . Another key sensor and signalling components of the DDR is the phospho-inositide-kinase (PIK) - related proteins which include ATM and ATM-Rad3-related (ATR). The best known ATM/ ATR substrates are the two families of checkpoint kinase (CHK) 1 and 2 which act in combination with the ATM/ ATR sensors to reduced cyclin dependent kinase mechanisms, including those mediated by p53. In doing so these proteins act together to slow down or arrest cell cycle progression at G1-S, intra-S and G2-M cell cycle checkpoints .
Control of cell cycle transition was first seen in SOS response in e-coli and in mammals in A1 cells.
53BP1 has been classified as a mediator of this damage response network as it acts as a signalling adaptor between sensors and transducers. Other such sensor molecules include MDC1 (mediator of DNA-damage checkpoint 1) and BRCA1 (breast cancer 1 early onset). These proteins have no known enzymatic activity but possibly act as recruitment platforms for other DDR proteins and in doing so enhance the DNA damage signal. These mediator proteins are orthologs of S. Cerevisiae Rad9 and S. Pombe Crb2 having similar functional elements and parallel properties despite low overall sequence similarity. In this way, the DDR mechanism of yeast is instructive in our understanding of mammalian response to DNA damage and 53BP1 function. Activated transducers of the DDR then proceed to activate by phosphorykation a variety of downstream effector molecules including p53.
As already mention 53BP1 shares several similar functional elements to its ortholog yeast counterparts and these include domain organisation at the C-terminus where a tandem BRCT domain, first described at the carboxyl terminal of the BRCA1 protein, is located. Upstream to the BRCT domain lies a tandem Tudor domain which have been shown previously to interact with methylated histone residues. ***The 53BP1 orthologs Rad9 and Crb2 have indispensible roles in DNA damage response and signalling and cell cycle arrest in lower eukaryotes. This is not the case with higher cells where it is likely that a number of molecules including 53BP1, BRCA1 and MDC1 are required to fulfil functions which can be performed by the one protein in yeast.
53BP1 recruitment to sites of DNA damage
Following Ionising radiation (IR) which induces double stranded DNA breaks in DNA, 53BP1 undergoes nuclear relocalization to focal structures in order to facilitate checkpoint and repair functions. This nuclear relocalization is dependent on a number of upstream factors including modification of H2AX by phosphorylation at Ser139 by the PIKKs forming γ-H2AX. γ-H2AX then recruits MDC1 which binds it via BRCT domains. MDC1 is then phosphorylated by ATM in this signal transduction cascade of phosphorylation. Once phosphorylated, MDC1 then can recruit RNF8 to the damaged DNA site where it can further catalyse components of this intricate network. It is believed that the recruitment of RNF8 by γ-H2AX-MDC1 molecules is required for subsequent focal recruitment of 53BP1 and binding of 53BP1 to methyated histone residues.
The exact location of 53BP1 required for its' focal recruitment has been mapped to amino acids 1220-1711, the tandem Tudor domain. In yeast orthologs S. Cerevisae Rad9 is recruited to DNA DSBs via binding of Tudor domains to methylated histone 3 on lysine 79 (H3K79me) whereas S. Pombe Crb2 is recruited via methylated histone 4 on lysine 20 (H3K20me). Both of these residues have been reported to be necessary for 53BP1 focal recruitment to DSBs.
1. Uziel T, Lerenthal Y, Moyal L, Andegeko Y, Mittelman L, Shiloh Y: Requirement of the MRN complex for ATM activation by DNA damage. EMBO J 2003, 22:5612-5621.
2. Zhou B-BS, Elledge SJ: The DNA damage response: putting checkpoints in perspective. Nature 2000, 408:433-439.