BRCA1 and BRCA2 are important for DNA double-strand break repair by homologous recombination, and mutations in these genes predispose to breast and other cancers. Poly(ADP-ribose) polymerase (PARP) is an enzyme involved in base excision repair, a key pathway in the repair of DNA single-strand breaks. We show here that BRCA1 or BRCA2 dysfunction unexpectedly and profoundly sensitizes cells to the inhibition of PARP enzymatic activity, resulting in chromosomal instability, cell cycle arrest and subsequent apoptosis. This seems to be because the inhibition of PARP leads to the persistence of DNA lesions normally repaired by homologous recombination. These results illustrate how different pathways cooperate to repair damage, and suggest that the targeted inhibition of particular DNA repair pathways may allow the design of specific and less toxic therapies for cancer.

Targeting the DNA repair defect in BRCA mutant cells as a therapeutic strategy

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.

/pdf/the-dna-damage-response-in-human-biology-and-disease-2gmucsko30.pdf

The DNA-damage response in human biology and disease

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

All life on earth must cope with constant exposure to DNA-damaging agents such as the Sun's radiation. Highly conserved DNA-repair and cell-cycle checkpoint pathways allow cells to deal with both endogenous and exogenous sources of DNA damage. How much an individual is exposed to these agents and how their cells respond to DNA damage are critical determinants of whether that individual will develop cancer. These cellular responses are also important for determining toxicities and responses to current cancer therapies, most of which target the DNA.

Cell-cycle checkpoints and cancer

https://www.cell.com/molecular-cell/pdf/S1097-2765(08)00292-X.pdf

Oxygen Sensing by Metazoans: The Central Role of the HIF Hydroxylase Pathway

Higher order chromatin structure presents a barrier to the recognition and repair of DNA damage. Double-strand breaks (DSBs) induce histone H2AX phosphorylation, which is associated with the recruitment of repair factors to damaged DNA. To help clarify the physiological role of H2AX, we targeted H2AX in mice. Although H2AX is not essential for irradiation-induced cell-cycle checkpoints, H2AX −/− mice were radiation sensitive, growth retarded, and immune deficient, and mutant males were infertile. These pleiotropic phenotypes were associated with chromosomal instability, repair defects, and impaired recruitment of Nbs1, 53bp1, and Brca1, but not Rad51, to irradiation-induced foci. Thus, H2AX is critical for facilitating the assembly of specific DNA-repair complexes on damaged DNA.

/pdf/genomic-instability-in-mice-lacking-histone-h2ax-2pa4mqrh0u.pdf

Genomic instability in mice lacking histone H2AX.

Histone H2AX phosphorylation on a serine four residues from the carboxyl terminus (producing gammaH2AX) is a sensitive marker for DNA double-strand breaks (DSBs). DSBs may lead to cancer but, paradoxically, are also used to kill cancer cells. Using gammaH2AX detection to determine the extent of DSB induction may help to detect precancerous cells, to stage cancers, to monitor the effectiveness of cancer therapies and to develop novel anticancer drugs.

γH2AX and cancer

Histone H2A variants H2AX and H2AZ.

Humans and animals undergo ageing, and although their primary cells undergo cellular senescence in culture, the relationship between these two processes is unclear. Here we show that gamma-H2AX foci (gamma-foci), which reveal DNA double-strand breaks (DSBs), accumulate in senescing human cell cultures and in ageing mice. They colocalize with DSB repair factors, but not significantly with telomeres. These cryptogenic gamma-foci remain after repair of radiation-induced gamma-foci, suggesting that they may represent DNA lesions with unrepairable DSBs. Thus, we conclude that accumulation of unrepairable DSBs may have a causal role in mammalian ageing.

Senescing human cells and ageing mice accumulate DNA lesions with unrepairable double-strand breaks

Sedelnikova, O. A., Rogakou, E. P., Panyutin, I. G. and Bonner, W. M. Quantitative Detection of 125IdU-Induced DNA Double-Strand Breaks with γ-H2AX Antibody. Radiat. Res. 158, 486–492 (2002). When mammalian cells are exposed to ionizing radiation and other agents that introduce DSBs into DNA, histone H2AX molecules in megabase chromatin regions adjacent to the breaks become phosphorylated within minutes on a specific serine residue. An antibody to this phosphoserine motif of human H2AX (γ-H2AX) demonstrates that γ-H2AX molecules appear in discrete nuclear foci. To establish the quantitative relationship between the number of these foci and the number of DSBs, we took advantage of the ability of 125I, when incorporated into DNA, to generate one DNA DSB per radioactive disintegration. SF-268 and HT-1080 cell cultures were grown in the presence of 125IdU and processed immunocytochemically to determine the number of γ-H2AX foci. The numbers of 125IdU disintegrations per cell were measured by exposing...

https://bioone.org/journals/Radiation-Research/volume-158/issue-4/0033-7587(2002)158[0486:QDOIID]2.0.CO;2/Quantitative-Detection-of-125IdU-Induced-DNA-Double-Strand-Breaks-with/10.1667/0033-7587(2002)158[0486:QDOIID]2.0.CO;2.pdf

Olga A. Sedelnikova

Papers

Genomic instability in mice lacking histone H2AX.

γH2AX and cancer

Histone H2A variants H2AX and H2AZ.

Senescing human cells and ageing mice accumulate DNA lesions with unrepairable double-strand breaks

Quantitative Detection of 125IdU-Induced DNA Double-Strand Breaks with γ-H2AX Antibody