The prime objective for every life form is to deliver its genetic material, intact and unchanged, to the next generation. This must be achieved despite constant assaults by endogenous and environmental agents on the DNA. To counter this threat, life has evolved several systems to detect DNA damage, signal its presence and mediate its repair. Such responses, which have an impact on a wide range of cellular events, are biologically significant because they prevent diverse human diseases. Our improving understanding of DNA-damage responses is providing new avenues for disease management.

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The DNA-damage response in human biology and disease

Poly(ADP-ribose) polymerase (PARP1) facilitates DNA repair by binding to DNA breaks and attracting DNA repair proteins to the site of damage. Nevertheless, PARP1-/- mice are viable, fertile and do not develop early onset tumours. Here, we show that PARP inhibitors trigger gamma-H2AX and RAD51 foci formation. We propose that, in the absence of PARP1, spontaneous single-strand breaks collapse replication forks and trigger homologous recombination for repair. Furthermore, we show that BRCA2-deficient cells, as a result of their deficiency in homologous recombination, are acutely sensitive to PARP inhibitors, presumably because resultant collapsed replication forks are no longer repaired. Thus, PARP1 activity is essential in homologous recombination-deficient BRCA2 mutant cells. We exploit this requirement in order to kill BRCA2-deficient tumours by PARP inhibition alone. Treatment with PARP inhibitors is likely to be highly tumour specific, because only the tumours (which are BRCA2-/-) in BRCA2+/- patients are defective in homologous recombination. The use of an inhibitor of a DNA repair enzyme alone to selectively kill a tumour, in the absence of an exogenous DNA-damaging agent, represents a new concept in cancer treatment.

Specific killing of BRCA2-deficient tumours with inhibitors of poly(ADP-ribose) polymerase

The early notion that cancer is caused by mutations in genes critical for the control of cell growth implied that genome stability is important for preventing oncogenesis. During the past decade, knowledge about the mechanisms by which genes erode and the molecular machinery designed to counteract this time-dependent genetic degeneration has increased markedly. At the same time, it has become apparent that inherited or acquired deficiencies in genome maintenance systems contribute significantly to the onset of cancer. This review summarizes the main DNA caretaking systems and their impact on genome stability and carcinogenesis.

Genome maintenance mechanisms for preventing cancer

The DNA Damage Response: Making It Safe to Play with Knives

https://link.springer.com/content/pdf/10.1097%2FWOX.0b013e3182439613.pdf

Oxidative Stress and Antioxidant Defense

Hereditary defects in the repair of DNA damage are implicated in a variety of diseases, many of which are typified by neurological dysfunction and/or increased genetic instability and cancer. Of the different types of DNA damage that arise in cells, single-strand breaks (SSBs) are the most common, arising at a frequency of tens of thousands per cell per day from direct attack by intracellular metabolites and from spontaneous DNA decay. Here, the molecular mechanisms and organization of the DNA-repair pathways that remove SSBs are reviewed and the connection between defects in these pathways and hereditary neurodegenerative disease are discussed.

Single-strand break repair and genetic disease

The DNA repair proteins XRCC1 and DNA ligase III are physically associated in human cells and directly interact in vitro and in vivo. Here, we demonstrate that XRCC1 is additionally associated with DNA polymerase-beta in human cells and that these polypeptides also directly interact. We also present data suggesting that poly (ADP-ribose) polymerase can interact with XRCC1. Finally, we demonstrate that DNA ligase III shares with poly (ADP-ribose) polymerase the novel function of a molecular DNA nick-sensor, and that the DNA ligase can inhibit activity of the latter polypeptide in vitro. Taken together, these data suggest that the activity of the four polypeptides described above may be co-ordinated in human cells within a single multiprotein complex.

XRCC1 Polypeptide Interacts with DNA Polymerase β and Possibly Poly (ADP-Ribose) Polymerase, and DNA Ligase III Is a Novel Molecular ‘Nick-Sensor’ In Vitro

The molecular role of poly (ADP-ribose) polymerase-1 in DNA repair is unclear. Here, we show that the single-strand break repair protein XRCC1 is rapidly assembled into discrete nuclear foci after oxidative DNA damage at sites of poly (ADP-ribose) synthesis. Poly (ADP-ribose) synthesis peaks during a 10 min treatment with H2O2 and the appearance of XRCC1 foci peaks shortly afterwards. Both sites of poly (ADP-ribose) and XRCC1 foci decrease to background levels during subsequent incubation in drug-free medium, consistent with the rapidity of the single-strand break repair process. The formation of XRCC1 foci at sites of poly (ADP-ribose) was greatly reduced by mutation of the XRCC1 BRCT I domain that physically interacts with PARP-1. Moreover, we failed to detect XRCC1 foci in Adprt1?/? MEFs after treatment with H2O2. These data demonstrate that PARP-1 is required for the assembly or stability of XRCC1 nuclear foci after oxidative DNA damage and suggest that the formation of these foci is mediated via interaction with poly (ADP-ribose). These results support a model in which the rapid activation of PARP-1 at sites of DNA strand breakage facilitates DNA repair by recruiting the molecular scaffold protein, XRCC1.

A requirement for PARP‐1 for the assembly or stability of XRCC1 nuclear foci at sites of oxidative DNA damage

XRCC1 and DNA strand break repair.

https://www.cell.com/cell/pdf/S0092-8674(01)00195-7.pdf

Keith W. Caldecott

Papers

Single-strand break repair and genetic disease

XRCC1 Polypeptide Interacts with DNA Polymerase β and Possibly Poly (ADP-Ribose) Polymerase, and DNA Ligase III Is a Novel Molecular ‘Nick-Sensor’ In Vitro

A requirement for PARP‐1 for the assembly or stability of XRCC1 nuclear foci at sites of oxidative DNA damage

XRCC1 and DNA strand break repair.

XRCC1 Stimulates Human Polynucleotide Kinase Activity at Damaged DNA Termini and Accelerates DNA Single-Strand Break Repair