Chromatin Structure and DNA Double-Strand Break Responses in Cancer Progression and Therapy
Total Page：16
File Type：pdf, Size：1020Kb
Oncogene (2007) 26, 7765–7772 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc REVIEW Chromatin structure and DNA double-strand break responses in cancer progression and therapy JA Downs MRC Genome Damage and Stability Centre, University of Sussex, Falmer, Brighton, UK Defects in the detection and repair of DNA double-strand recentwork proposes thatchromosomal instabilityis breaks (DSBs) have been causatively linked to tumourigen- likely to be a driving force in the process of tumourigen- esis. Moreover, inhibition of DNA damage responses esis (Michor, 2005). This suggests that the ability of cells (DDR) can increase the efficacy of cancer therapies that to detect and, appropriately and efficiently, respond to rely on generation of damaged DNA. DDR must occur DNA lesions is critical in preventing the onset of within the context of chromatin, and there have been tumourigenesis. significant advances in recent years in understanding how One of the most dangerous DNA lesions a cell can the modulation and manipulation of chromatin contribute sustain is a DNA double-strand break (DSB). If left to this activity. One particular covalent modification of a unrepaired, a single DSB can be lethal, and can also histone variant—the phosphorylation of H2AX—has been result in chromosomal translocations, deletions and loss investigated in great detail and has been shown to have of genetic information. In eukaryotes, there are two important roles in DNA DSB responses and in preventing main pathways for repairing DSBs; homologous re- tumourigenesis. These studies are reviewed here in the combination (HR) and nonhomologous end joining. In context of their relevance to cancer therapy and diagnos- addition to the repair activities, cells have pathways tics. In addition, there is emerging evidence for contribu- dedicated to the detection and signalling of these tions by proteins involved in mediating higher order dangerous lesions in order to orchestrate the appro- structure to DNA DSB responses. The contributions of a priate response, including cell cycle checkpoints and subset of these proteins—linker histones and high-mobility transcriptional upregulation. group box (HMGB) proteins—to DDR and their potential Not surprisingly, it has been known for some time that significance in tumourigenesis are discussed. defects in the DNA damage response (DDR) pathway, Oncogene (2007) 26, 7765–7772; doi:10.1038/sj.onc.1210874 such as the Mre11, Rad50, Nbs1 (MRN) complex (for review, see Williams et al., 2007) and in the kinases Keywords: chromatin; DNA repair; genome stability; ataxia telangiectasia mutated (ATM) and ATM and linker histone; H2A; H2AX; HMGB RAD3-related (ATR) (for review, see Jackson, 2002; Lavin et al., 2006), are associated with cancer predis- position syndromes in humans, and disruption of DNA repair systems in mouse models leads to an increased risk of cancer (Jackson, 2002; Lavin et al., 2006; Williams Introduction et al., 2007). Obviously, one critical function of these pathways is to respond to exogenously induced DNA Genomes of tumour cells exhibit multiple alterations damage. However, programmed DSBs are created in from the parent cells, and it is now well established that lymphocytes undergoing class switch recombination or tumourigenesis is a multistep process of genetic altera- V(D)J recombination. Importantly, components of the tions. These alterations allow the cell to acquire DDR program are essential for protecting the genome characteristics that are universal among tumours and from chromosome translocations in lymphocytes (for have been referred to as the ‘hallmarks of cancer’ example, Vogel et al., 1999). Unless the breaks created (Hanahan and Weinberg, 2000). It is estimated that this during these processes are efficiently resolved, they can is achieved in 4–7 rate-limiting stochastic events (Hana- serve as substrates for chromosome translocation, and han and Weinberg, 2000). Consequently, genomic deficiency in ATM promotes these aberrant joining instability has been termed as an ‘enabling characteristic’ events (Reina-San-Martin et al., 2003; Bredemeyer et al., of cancer, since the loss of systems to ensure genomic 2006; Franco et al., 2006; Ramiro et al., 2006). integrity allow the cell to more easily acquire these genetic Interestingly, recent work from two labs has demon- changes (Hanahan and Weinberg, 2000). In fact, more strated that the DDR are activated early during tumour- igenesis (Bartkova et al., 2005; Gorgoulis et al., 2005). This was thought to be due to DNA damage caused by Correspondence: Dr JA Downs, MRC Genome Damage and Stability Centre, University of Sussex, Science Park Road, Falmer, Brighton increased replicative stress in rapidly dividing pre-neoplas- BN1 9RQ, UK. tic lesions. These findings demonstrate the central E-mail: [email protected] importance of the DDR pathway, which acts as an Chromatin structure and DSB responses JA Downs 7766 inducible barrier againsttumourprogression and genetic mediating higher order chromatin structure, the linker instability. Once overcome, the cells are subsequently histones and HMG proteins, in DNA DSB responses is vulnerable to accumulating additional genetic changes. emerging. Second, recent studies have examined the Clearly, therefore, understanding the mechanisms of DDR ability to exploit one particular DNA damage-depen- is of central importance to cancer biology. dent chromatin alteration—the phosphorylation of H2AX—for clinical applications. Chromatin and the DNA damage response The linker histone and DNA damage responses Clearly, these DDR must occur within the context of chromatin. At its simplest level, chromatin is made up of Linker histones are found throughout eukaryotic DNA wrapped around an octamer of histone proteins evolution (Ausio, 2000). In lower eukaryotes, they do (two each of H2A, H2B, H3 and H4) to form a not appear to be essential for viability (Ausio, 2000; nucleosome. The histones have histone fold domains, Harvey and Downs, 2004). For example, in budding which are important for the formation of the central yeast, there is a single gene encoding a linker histone, structure of the nucleosome, and flexible tail domains, termed HHO1, and cells lacking this gene show very which protrude from the nucleosome core particle (Luger little phenotypic alterations (Harvey and Downs, 2004). et al., 1997). The packaging of DNA in this manner is In higher eukaryotes, multiple isoforms of linker generally inhibitory to processes such as transcription histones exist. In mice, disruption any one of the eight and replication, and cells have numerous mechanisms by linker histone genes does not result in lethality, and it which this structure can be manipulated to make it more appears that there is upregulation of the remaining amenable to these activities. Broadly, they can be placed genes in order to compensate for the loss (Alami et al., into two groups; the addition of covalent modifications to 2003). However, once three genes are disrupted and the the histone proteins, and the activity of ATP-dependent total amount of linker histone falls significantly, this is chromatin remodelling activities that use the energy an embryonic lethal event (Fan et al., 2005). This derived from the hydrolysis of ATP to alter the contacts suggests that for the function required in higher between the DNA and histone proteins. eukaryotes to sustain viability, the different linker Chromatin has the ability to form compact structures histone genes are functionally redundant and it is the well beyond the initial level of nucleosomes. There are a overall level of linker histone protein in the cell that is number of factors that influence the formation of these critical. Similarly, in chicken DT40 cells, while there are higher order structures, including linker histones and six genes encoding linker histones, the presence of a high-mobility group (HMG) proteins. These are abun- single linker histone gene is sufficient for viability dant architectural chromatin proteins found throughout (Hashimoto et al., 2007). eukaryotic evolution. Not surprisingly, these proteins In yeast, it has been demonstrated that the loss of the are also subjected to complex control by numerous linker histone results in hyper-resistance to methyl covalent modifications that affect their chromatin- methane sulfonate (MMS). By using a combination of binding activities and protein–protein associations. genetic approaches, a model in which the linker histone Chromatin is, therefore, a highly complex polymer acts to inhibit the HR pathway was proposed (Downs capable of extraordinary variability in both structure et al., 2003). While the behaviour of the linker histone and composition. after treatment of cells with MMS was not directly There has been a great deal of research into the examined, it would be reasonable to hypothesize that changes in chromatin structure that occur in response to there may be some regulation of linker histones in DNA damage. In particular, there are many covalent response to DNA damage. In particular, the dissocia- modifications of histones that are now known to be tion of Hho1 from chromatin would be predicted to important for DDR, and there is likely to be a pattern of facilitate the repair of the DNA lesions by HR. modifications on histones in the proximity of DNA Interestingly in this regard, in mammalian cells, a DSBs that contributes to DDR. It is also now apparent population of all five of the linker histone isoforms that that multiple ATP-dependent chromatin remodelling are ubiquitously expressed in somatic cells re-localize complexes contribute to cellular survival after DNA from the nucleus to the cytosol in response to ionizing damage and play a role directly at the sites of DNA radiation (Figure 1; Konishi et al., 2003). However, the damage. These events have been extensively reviewed effect of this re-localization on HR activity was not elsewhere (Peterson and Cote, 2004; Foster and Downs, examined. Rather, in this study, the authors had set out 2005; Morrison and Shen, 2005; van Attikum and to identify pro-apoptotic cellular factors that induce Gasser, 2005; Costelloe et al., 2006; Downs et al., 2007).