Delayed kinetics of DNA double-strand break processing in normal and pathological aging

Human APOBEC3A isoforms translocate to the nucleus and induce DNA double strand breaks leading to cell stress and death

Human APOBEC3 enzymes deaminate single stranded DNA. At least five can deaminate mitochondrial DNA in the cytoplasm, while three can deaminate viral DNA in the nucleus. However, only one, APOBEC3A, can hypermutate genomic DNA. We analysed the distribution and function of the two APOBEC3A isoforms p1 and p2 in transfected cell lines. Both can translocate to the nucleus and hypermutate CMYC DNA and induce DNA double strand breaks as visualized by the detection of ©H2AX or Chk2. APOBEC3A induced G1 phase cell cycle arrest and triggered several members of the intrinsic apoptosis pathway. Activation of purified human CD4+ T lymphocytes with PHA, IL2 and interferon α resulted in C->T hypermutation of genomic DNA and double stranded breaks suggesting a role for APOBEC3A in pro-inflammatory conditions. As chronic inflammation underlies many diseases including numerous cancers, it is possible that APOBEC3A induction may generate many of the lesions typical of a cancer genome.

Statistical analysis of kinetics, distribution and co-localisation of DNA repair foci in irradiated cells: cell cycle effect and implications for prediction of radiosensitivity

Detection of γ-H2AX foci as a measure of DNA double strand break induction and repair provides the basis of a rapid approach to establish individual radiosensitivity. However, the assignment of criteria to define increased radiosensitivity is not straightforward. Experimental end points, analytical methods and proliferative status of the cells sampled for analysis are important. All these issues are addressed in the present study, which was prompted by a clinical request to assess the radiosensitivity status of an SCID paediatric patient being considered for bone marrow transplantation. We investigated the kinetics of repair of radiation-induced γ-H2AX foci in proliferating and confluent cultures of skin fibroblasts obtained from the patient, and from normal and radiosensitive (Artemis-deficient) controls. As well as the standard approach of averaging foci per cell over the entire population ("standard average"), we also examined foci per cell frequency distributions and calculated average foci per cell values in the major Poisson-distributed subpopulation ("principal average"). This approach allowed to avoid distortions such as that due to the S/G2 population in proliferating cells, with focus numbers approaching twice the normal, and to detect subpopulations of cells with defects in focus formation and repair. From the "standard average" analysis and co-localisation of γ-H2AX foci with 53BP1 we assigned the patient's repair status as close-to-normal. However, analysis of "principal average", foci per cell frequency distributions and survival curves challenged this initial conclusion. These studies indicate new dimensions of the γ-H2AX assay that, with further elaboration and exemplification, have the potential to augment its power to predict radiosensitivity.

Increased mammogram-induced DNA damage in mammary epithelial cells aged in vitro

Concerned about the risks of mammography screening in the adult population, we analyzed the ability of human mammary epithelial cells to cope with mammogram-induced DNA damage. Our study shows that an X-ray dose of 20 mGy, which is the standard dose received by the breast surface per two-view mammogram X-ray exploration, induces increased frequencies of DNA double-strand breaks to in vitro aged–but not to young–human mammary epithelial cells. We provide evidence that aged epithelial breast cells are more radiosensitive than younger ones. Our studies point to an inefficient damage response of aged cells to low-dose radiation, this being due to both delayed and incomplete mobilization of repair proteins to DNA strand breaks. This inefficient damage response is translated into an important delay in double-strand break disappearance and consequent accumulation of unrepaired DNA breaks. The result of this is a significant increase in micronuclei frequency in the in vitro aged mammary epithelial cells exposed to doses equivalent to a single mammogram X-ray exploration. Since our experiments were carried out in primary epithelial cell cultures in which cells age at the same time as they undergo replication-dependent telomere shortening, we needed to determine the contribution of these two factors to their phenotype. In this paper, we report that the exogenous expression of human telomerase retrotranscriptase in late population doubling epithelial cells does not rescue its delayed repair phenotype. Therefore, retarded DNA break repair is a direct consequence of cellular aging itself, rather than a consequence of the presence of dysfunctional telomeres. Our findings of long-lasting double strand breaks and incomplete DNA break repair in the in vitro aged epithelial cells are in line with the increased carcinogenic risks of radiation exposures at older ages revealed by epidemiologic studies.

Radiosensitivity in breast cancer assessed by the histone γ-H2AX and 53BP1 foci

BACKGROUND

High expression of constitutive histone γ-H2AX, a sensitive marker of DNA damage, might be indicative of defective DNA repair pathway or genomic instability. 53BP1 (p53-binding protein 1) is a conserved checkpoint protein with properties of a DNA double-strand breaks sensor. This study explores the relationship between the clinical radiosensitivity of tumor patients and the expression/induction of γ-H2AX and 53BP1 in vitro.

METHODS

Using immunostaining, we assessed spontaneous and radiation-induced foci of γ-H2AX and 53 BP1 in peripheral blood mononuclear cells derived from unselected breast cancer (BC) patients (n=57) undergoing radiotherapy (RT). Cells from apparently healthy donors (n=12) served as references.

RESULTS

Non-irradiated cells from controls and unselected BC patients exhibited similar baseline levels of DNA damage assessed by γ-H2AX and 53BP1 foci. At the same time, the γ-H2AX assay of in vitro irradiated cells revealed significant differences between the control group and the group of unselected BC patients with respect to the initial (0.5 Gy, 30 min) and residual (2 Gy, 24 h post-radiation) DNA damage. The numbers of 53BP1 foci analyzed in 35 BC patients were significantly higher than in controls only in case of residual DNA damage. A weak correlation was found between residual foci of both proteins tested. In addition, cells from cancer patients with an adverse acute skin reaction (grade 3) to RT showed significantly increased radiation-induced γ-H2AX foci and their protracted disappearance compared to the group of BC patients with normal skin reaction (grade 0-1). The mean number of γ-H2AX foci after 5 clinical fractions was significantly higher than that before RT, especially in clinically radiosensitive patients.

CONCLUSIONS

The γ-H2AX assay may have potential for screening individual radiosensitivity of breast cancer patients.

TRIAL REGISTRATION

http://www.krebshilfe.de/wir-foerdern.html.

DNA damage in stem cells activates p21, inhibits p53, and induces symmetric self-renewing divisions

DNA damage leads to a halt in proliferation owing to apoptosis or senescence, which prevents transmission of DNA alterations. This cellular response depends on the tumor suppressor p53 and functions as a powerful barrier to tumor development. Adult stem cells are resistant to DNA damage-induced apoptosis or senescence, however, and how they execute this response and suppress tumorigenesis is unknown. We show that irradiation of hematopoietic and mammary stem cells up-regulates the cell cycle inhibitor p21, a known target of p53, which prevents p53 activation and inhibits p53 basal activity, impeding apoptosis and leading to cell cycle entry and symmetric self-renewing divisions. p21 also activates DNA repair, limiting DNA damage accumulation and self-renewal exhaustion. Stem cells with moderate DNA damage and diminished self-renewal persist after irradiation, however. These findings suggest that stem cells have evolved a unique, p21-dependent response to DNA damage that leads to their immediate expansion and limits their long-term survival.

γH2AX as a marker of DNA double strand breaks and genomic instability in human population studies

DNA double strand breaks (DSB) are the gravest form of DNA damage in eukaryotic cells. Failure to detect DSB and activate appropriate DNA damage responses can cause genomic instability, leading to tumorigenesis and possibly accelerated aging. Phosphorylated histone H2AX (γH2AX) is used as a biomarker of cellular response to DSB and its potential for monitoring DNA damage and repair in human populations has been explored in this review. A systematic search was conducted in PubMed for articles, in English, on human studies reporting γH2AX as a biomarker of either DNA repair or DNA damage. A total of 68 publications were identified. Thirty-four studies (50.0%) evaluated the effect of medical procedures or treatments on γH2AX levels; 20 (29.4%) monitored γH2AX in specific pathological conditions with a case/control or case/case design; 5 studies (7.4%) evaluated the effect of environmental genotoxic exposures, and 9 (13.2%) were descriptive studies on cancer and aging. Peripheral blood lymphocytes (44.6%) or biopsies/tissue specimens (24.3%) were the most commonly used samples. γH2AX was scored by optical microscopy as immunostained foci (78%), or by flow cytometry (16%). Critical features affecting the reliability of the assay, including protocols heterogeneity, specimen, cell cycle, kinetics, study design, and statistical analysis, are hereby discussed. Because of its sensitivity, efficiency and mechanistic relevance, the γH2AX assay has great potential as a DNA damage biomarker; however, the technical and epidemiological heterogeneity highlighted in this review infer a necessity for experimental standardization of the assay.

Telomerase reverse transcriptase synergizes with calorie restriction to increase health span and extend mouse longevity

Caloric restriction (CR), a reduction of food intake while avoiding malnutrition, can delay the onset of cancer and age-related diseases in several species, including mice. In addition, depending of the genetic background, CR can also increase or decrease mouse longevity. This has highlighted the importance of identifying the molecular pathways that interplay with CR in modulating longevity. Significant lifespan extension in mice has been recently achieved through over-expression of the catalytic subunit of mouse telomerase (mTERT) in a cancer protective background. Given the CR cancer-protective effects in rodents, we set to address here whether CR impacts on telomere length and synergizes with mTERT to extend mouse longevity. CR significantly decreased tumor incidence in TERT transgenic (TgTERT) mice and extended their lifespan compared to wild-type (WT) controls under the same diet, indicating a synergy between TgTERT and CR in increasing mouse longevity. In addition, longitudinal telomere length measurements in peripheral blood leukocytes from individual mice showed that CR resulted in maintenance and/or elongation telomeres in a percentage of WT mice, a situation that mimics telomere dynamics in TgTERT cohorts. These results demonstrate that CR attenuates telomere erosion associated to aging and that synergizes with TERT over-expression in increasing "health span" and extending mouse longevity.

DNA damage response in CD133 + stem/progenitor cells from umbilical cord blood: low level of endogenous foci and high recruitment of 53BP1

Abstract Purpose: Human hematopoietic stem cells (HSC) are thought to be a major target of radiation-induced leukemogenesis and also provide a relevant cellular model for assessing cancer risk. Cluster of designation 133+ (CD133+) is a marker found in human progenitor and hematopoietic stem cells. Our study examined the repair of radiation-induced DNA double-strand breaks (DSB) in CD133 + umbilical cord blood cells (UCBC).

MATERIALS AND METHODS

After γ-irradiation, endogenous and induced DSB were evaluated in CD133 + UCBC, CD133 - UCBC and peripheral blood lymphocytes (PBL) in terms of phosphorylated histone 2A family member X (γH2AX) and tumor suppressor p53 binding protein 1 (53BP1) foci.

RESULTS

We found that repair signaling in CD133 + UCBC is different from CD133 - UCBC and PBL. These differences include lower endogenous DSB levels and higher 53BP1 recruitment.

CONCLUSIONS

Our data, together with a recent report on radiation-induced γH2AX and 53BP1 foci in CD34 + cells, indicate enhanced DNA repair capacity in HSC as compared to mature lymphocytes.

Ionizing radiation-induced γ-H2AX activity in whole blood culture and the risk of lung cancer

BACKGROUND

Phenotypic biomarkers of DNA damage repair may enhance cancer risk prediction. The γ-H2AX formed at the sites of double-strand break (DSB) after ionizing radiation is a specific marker of DNA damage.

METHODS

In an ongoing case-control study, the baseline and ionizing radiation-induced γ-H2AX levels in peripheral blood lymphocytes (PBL) from frequency-matched 306 untreated patients with lung cancer and 306 controls were measured by a laser scanning cytometer-based immunocytochemical method. The ratio of ionizing radiation-induced γ-H2AX level to the baseline was used to evaluate interindividual variation of DSB damage response and to assess the risk of lung cancer by using unconditional multivariable logistic regression with adjustment of age, sex, ethnicity, smoking status, family history of lung cancer, dust exposure, and emphysema.

RESULTS

The mean γ-H2AX ratio was significantly higher in cases than controls (1.46 ± 0.14 vs. 1.41 ± 0.12, P < 0.001). Dichotomized at the median in controls, high γ-H2AX ratio was significantly associated with increased risk of lung cancer [OR = 2.43; 95% confidence interval (CI): 1.66-3.56]. There was also a significant dose-response relationship between γ-H2AX ratio and lung cancer risk in quartile analysis. Analysis of joint effects with other epidemiologic risk factors revealed elevated risk with increasing number of risk factors.

CONCLUSION

γ-H2AX activity as shown by measuring DSB damage in ionizing radiation-irradiated PBLs may be a novel phenotypic marker of lung cancer risk.

IMPACT

γ-H2AX assay is a robust and quantifiable image-based cytometer method that measures mutagen-induced DSB response in PBLs as a potential biomarker in lung cancer risk assessment.

Mangiferin aglycone attenuates radiation-induced damage on human intestinal epithelial cells

Recent studies suggest that mangiferin aglycone (norathyriol) has great potential as a novel radioprotector without any known toxic side effects. In this study, we assessed the protective effects of mangiferin aglycone against radiation-induced injuries on normal human intestinal epithelial cells (HIECs), while using mangiferin as a reference compound. The in vitro experiments showed that pretreatment of either mangiferin aglycone or mangiferin could inhibit cytotoxic effects of ionizing irradiation (IR) on HIECs. Cellular changes were estimated by measuring cell viability, clonogenic surviving rate, and apoptotic rate. Compared to mangiferin, we found mangiferin aglycone had greater radioprotective effects of mangiferin aglycone on HIECs. It has been demonstrated that the cytotoxicity of ionizing radiation relates to its capacity to induce DNA damage. In view of this, we monitored DNA double-strand breaks (DSBs) using γH2AX foci formation to test whether mangiferin aglycone and mangiferin could modulate genotoxic effects of radiation. It shows that mangiferin aglycone could eliminate 46.8% of the total DSBs of the cells exposed to 2 Gy IR, which is significantly better than mangiferin. Complementing earlier results from our group, it appears possible to conclude that mangiferin aglycone presents potential useful effects on IR-induced damage and may be a better radioprotective agent than mangiferin therapeutically.

Age-related disease association of endogenous γ-H2AX foci in mononuclear cells derived from leukapheresis

The phosphorylated form of histone H2AX (γ-H2AX) forms immunohistochemically detectable foci at DNA double strand breaks. In peripheral blood mononuclear cells (PBMCs) derived from leukapheresis from patients enrolled in the Baltimore Longitudinal Study of Aging, γ-H2AX foci increased in a linear fashion with regards to age, peaking at ∼57 years. The relationship between the frequency of γ-H2AX foci and age-related pathologies was assessed. We found a statistically significant (p = 0.023) 50% increase in foci in PBMCs derived from patients with a known history of vitamin D deficiency. In addition, there were trends toward increased γ-H2AX foci in patients with cataracts (34% increase, p<0.10) and in sleep apnea patients (44%, p<0.10). Among patients ≥57 y/o, we found a significant (p = 0.037) 36% increase in the number of γ-H2AX foci/cell for patients with hypertension compared to non-hypertensive patients. Our results support a role for increased DNA damage in the morbidity of age-related diseases. γ -H2AX may be a biomarker for human morbidity in age-related diseases.

Susceptibility to bystander DNA damage is influenced by replication and transcriptional activity

Direct cellular DNA damage may lead to genome destabilization in unexposed, bystander, cells sharing the same milieu with directly damaged cells by means of the bystander effect. One proposed mechanism involves double strand break (DSB) formation in S phase cells at sites of single strand lesions in the DNA of replication complexes, which has a more open structure compared with neighboring DNA. The DNA in transcription complexes also has a more open structure, and hence may be susceptible to bystander DSB formation from single strand lesions. To examine whether transcription predisposes non-replicating cells to bystander effect-induced DNA DSBs, we examined two types of primary cells that exhibit high levels of transcription in the absence of replication, rat neurons and human lymphocytes. We found that non-replicating bystander cells with high transcription rates exhibited substantial levels of DNA DSBs, as monitored by γ-H2AX foci formation. Additionally, as reported in proliferating cells, TGF-β and NO were found to mimic bystander effects in cell populations lacking DNA synthesis. These results indicate that cell vulnerability to bystander DSB damage may result from transcription as well as replication. The findings offer insights into which tissues may be vulnerable to bystander genomic destabilization in vivo.

DNA damage, chromatin, and transcription: the trinity of aging

Aging brings about numerous cellular defects. Amongst the most prominent are elevated levels of persistent DNA damage, changes to chromatin structure and epigenetic modifications, and alterations of global transcription programs. These are not independent events and recent work begins to shed light on the intricate interplay between these aging-related defects.

Sirtuin 6 (SIRT6) rescues the decline of homologous recombination repair during replicative senescence

Genomic instability is a hallmark of aging tissues. Genomic instability may arise from the inefficient or aberrant function of DNA double-stranded break (DSB) repair. DSBs are repaired by homologous recombination (HR) and nonhomologous DNA end joining (NHEJ). HR is a precise pathway, whereas NHEJ frequently leads to deletions or insertions at the repair site. Here, we used normal human fibroblasts with a chromosomally integrated HR reporter cassette to examine the changes in HR efficiency as cells progress to replicative senescence. We show that HR declines sharply with increasing replicative age, with an up to 38-fold decrease in efficiency in presenescent cells relative to young cells. This decline is not explained by a reduction of the number of cells in S/G(2)/M stage as presenescent cells are actively dividing. Expression of proteins involved in HR such as Rad51, Rad51C, Rad52, NBS1, and Sirtuin 6 (SIRT6) diminished with cellular senescence. Supplementation of Rad51, Rad51C, Rad52, and NBS1 proteins, either individually or in combination, did not rescue the senescence-related decline of HR. However, overexpression of SIRT6 in "middle-aged" and presenescent cells strongly stimulated HR repair, and this effect was dependent on mono-ADP ribosylation activity of poly(ADP-ribose) polymerase (PARP1). These results suggest that in aging cells, the precise HR pathway becomes repressed giving way to a more error-prone NHEJ pathway. These changes in the processing of DSBs may contribute to age-related genomic instability and a higher incidence of cancer with age. SIRT6 activation provides a potential therapeutic strategy to prevent the decline in genome maintenance.

The canonical NF-κB pathway differentially protects normal and human tumor cells from ROS-induced DNA damage

DNA damage responses (DDR) invoke senescence or apoptosis depending on stimulus intensity and the degree of activation of the p53-p21(Cip1/Waf1) axis; but the functional impact of NF-κB signaling on these different outcomes in normal vs. human cancer cells remains poorly understood. We investigated the NF-κB-dependent effects and mechanism underlying reactive oxygen species (ROS)-mediated DDR outcomes of normal human lung fibroblasts (HDFs) and A549 human lung cancer epithelial cells. To activate DDR, ROS accumulation was induced by different doses of H(2)O(2). The effect of ROS induction caused a G2 or G2-M phase cell cycle arrest of both human cell types. However, ROS-mediated DDR eventually culminated in different end points with HDFs undergoing premature senescence and A549 cancer cells succumbing to apoptosis. NF-κB p65/RelA nuclear translocation and Ser536 phosphorylation were induced in response to H(2)O(2)-mediated ROS accumulation. Importantly, blocking the activities of canonical NF-κB subunits with an IκBα super-repressor or suppressing canonical NF-κB signaling by IKKβ knock-down accelerated HDF premature senescence by up-regulating the p53-p21(Cip1/Waf1) axis; but inhibiting the canonical NF-κB pathway exacerbated H(2)O(2)-induced A549 cell apoptosis. HDF premature aging occurred in conjunction with γ-H2AX chromatin deposition, senescence-associated heterochromatic foci and beta-galactosidase staining. p53 knock-down abrogated H(2)O(2)-induced premature senescence of vector control- and IκBαSR-expressing HDFs functionally linking canonical NF-κB-dependent control of p53 levels to ROS-induced HDF senescence. We conclude that IKKβ-driven canonical NF-κB signaling has different functional roles for the outcome of ROS responses in the contexts of normal vs. human tumor cells by respectively protecting them against DDR-dependent premature senescence and apoptosis.

Assessing cell and organ senescence biomarkers

A major goal in cancer and aging research is to discriminate the biochemical modifications that happen locally that could account for the healthiness or malignancy of tissues. Senescence is one general antiproliferative cellular process that acts as a strong barrier for cancer progression, playing a crucial role in aging. Here, we focus on the current methods to assess cellular senescence, discriminating the advantages and disadvantages of several senescence biomarkers.

Accumulation of annexin A5 at the nuclear envelope is a biomarker of cellular aging

Cellular senescence is a permanent cell cycle arrest induced by short telomeres or oncogenic stress in vitro and in vivo. Because no single of the established biomarkers can reliably identify senescent cells, the application of new ones may aid the diagnosis of aged cells. Here we show that annexin A5 accumulates at the nuclear envelope during replicative and drug-induced cellular senescence in primary human fibroblasts. This new cellular aging phenotype that we have termed SA-ANX5 (senescence-associated accumulation at the nuclear envelope of annexin A5) is as efficient and quantitative as the well-established senescence-associated β-galactosidase activity assay and p21 immunoreactivity. SA-ANX5 is also observed in aged human skin where is exclusively detected in DNA damage foci-positive/Ki-67-negative cells. We also observed that depletion of annexin A5 by siRNA in human fibroblasts accelerates premature senescence through the p38MAP kinase pathway. These observations establish SA-ANX5 as a new biomarker for cellular aging and implicate a functional role for annexin A5 in cellular senescence.

Enhanced intrinsic radiosensitivity after treatment with stereotactic radiosurgery for an acoustic neuroma

Enhanced radiosensitivity is an uncommon phenomenon attributable to deficient DNA repair after radiotherapy which can be assessed with the γ-H2AX assay. Reports of radiosensitivity after stereotactic radiosurgery (SRS) are uncommon. We describe a case where the clinical, radiological and laboratory findings suggest enhanced radiosensitivity after SRS for an acoustic neuroma.

'Relax and Repair' to restrain aging

The maintenance of genomic integrity requires the precise identification and repair of DNA damage. Since DNA is packaged and condensed into higher order chromatin, the events associated with DNA damage recognition and repair are orchestrated within the layers of chromatin. Very similar to transcription, during DNA repair, chromatin remodelling events and histone modifications act in concert to 'open' and relax chromatin structure so that repair proteins can gain access to DNA damage sites. One such histone mark critical for maintaining chromatin structure is acetylated lysine 16 of histone H4 (AcH4K16), a modification that can disrupt higher order chromatin organization and convert it into a more 'relaxed' configuration. We have recently shown that impaired H4K16 acetylation delays the accumulation of repair proteins to double strand break (DSB) sites which results in defective genome maintenance and accelerated aging in a laminopathy-based premature aging mouse model. These results support the idea that epigenetic factors may directly contribute to genomic instability and aging by regulating the efficiency of DSB repair. In this article, the interplay between epigenetic misregulation, defective DNA repair and aging is discussed.

Radiation dose detection by imaging response in biological targets

Imaging was one of the earliest techniques to quantify radiation dose. While films and active fluorescent detectors are still commonly used in physical dosimetry, biological imaging is emerging as a new method to visualize and quantify radiation dose in biological targets. Methods for biological imaging are normally based on molecular fluorescent probes, labeling chromatin-conjugated molecules or specific repair proteins. Examples are chromatin-binding coumarin compounds, which become fluorescent under irradiation, or the H2AX histone, which is rapidly phosphorylated at sites of DNA double-strand breaks and can be visualized by immunostaining. Many other DNA repair proteins can be expressed with fluorescent targets, such as green fluorescent protein, thus becoming visible for dose estimation in vivo. The possibility to visualize radiation damage in living biological targets is particularly important for repair kinetic studies, for estimating individual radiation response, and for remote control of living samples exposed to radiation, for instance in robotic space missions. In vivo dose monitoring in particle therapy exploits the production of positron emitters by nuclear interaction of the incident beam in the patient's body. Positron emission tomography (PET) can then be used to visualize and quantify the particle dose in the patient, and it can in principle also be used for radiotherapy with high-energy X rays. Alternatively, prompt γ rays or scattered secondary particles are under study for in vivo dosimetry of ion beams in therapy.

DNA-damage accumulation and replicative arrest in Hutchinson-Gilford progeria syndrome

A common feature of progeria syndromes is a premature aging phenotype and an enhanced accumulation of DNA damage arising from a compromised repair system. HGPS (Hutchinson-Gilford progeria syndrome) is a severe form of progeria in which patients accumulate progerin, a mutant lamin A protein derived from a splicing variant of the lamin A/C gene (LMNA). Progerin causes chromatin perturbations which result in the formation of DSBs (double-strand breaks) and abnormal DDR (DNA-damage response). In the present article, we review recent findings which resolve some mechanistic details of how progerin may disrupt DDR pathways in HGPS cells. We propose that progerin accumulation results in disruption of functions of some replication and repair factors, causing the mislocalization of XPA (xeroderma pigmentosum group A) protein to the replication forks, replication fork stalling and, subsequently, DNA DSBs. The binding of XPA to the stalled forks excludes normal binding by repair proteins, leading to DSB accumulation, which activates ATM (ataxia telangiectasia mutated) and ATR (ATM- and Rad3-related) checkpoints, and arresting cell-cycle progression.

Telomere shortening alters the kinetics of the DNA damage response after ionizing radiation in human cells

Studies of telomerase-deficient mice and human cell lines have showed that telomere shortening enhances sensitivity to ionizing radiation (IR). The molecular basis for this observation remains unclear. To better understand the connection between telomere shortening and radiation sensitivity, we evaluated components of the DNA damage response pathway in normal human fibroblasts with short and long telomeres. Late-passage cells with short telomeres showed enhanced sensitivity to IR compared with early-passage cells with longer telomeres. Compared with early-passage cells, late-passage cells had a higher baseline level of phosphorylated H2AX protein (γH2AX) before IR but diminished peak levels of H2AX phosphorylation after treatment with IR. Both the appearance and disappearance of γH2AX foci were delayed in late-passage cells, indicative of delayed DNA repair. In contrast to the situation with H2AX, ATM and p53 phosphorylation kinetics were similar in early- and late-passage cells, but phosphorylation of the chromatin-bound ATM targets SMC1 and NBS1 was delayed in late-passage cells. Because impaired phosphorylation associated with short telomeres was restricted to chromatin-bound ATM targets, chromatin structure was assessed. DNA from cells with short telomeres was more resistant to digestion with micrococcal nuclease, indicative of compacted chromatin. Moreover, cells with short telomeres showed histone acetylation and methylation profiles consistent with heterochromatin. Together our data suggest a model in which short telomeres induce chromatin structure changes that limit access of activated ATM to its downstream targets on the chromatin, thereby providing a potential explanation for the increased radiation sensitivity seen with telomere shortening.

Histone H4 lysine 16 hypoacetylation is associated with defective DNA repair and premature senescence in Zmpste24-deficient mice

Specific point mutations in lamin A gene have been shown to accelerate aging in humans and mice. Particularly, a de novo mutation at G608G position impairs lamin A processing to produce the mutant protein progerin, which causes the Hutchinson Gilford progeria syndrome. The premature aging phenotype of Hutchinson Gilford progeria syndrome is largely recapitulated in mice deficient for the lamin A-processing enzyme, Zmpste24. We have previously reported that Zmpste24 deficiency results in genomic instability and early cellular senescence due to the delayed recruitment of repair proteins to sites of DNA damage. Here, we further investigate the molecular mechanism underlying delayed DNA damage response and identify a histone acetylation defect in Zmpste24(-/-) mice. Specifically, histone H4 was hypoacetylated at a lysine 16 residue (H4K16), and this defect was attributed to the reduced association of a histone acetyltransferase, Mof, to the nuclear matrix. Given the reversible nature of epigenetic changes, rescue experiments performed either by Mof overexpression or by histone deacetylase inhibition promoted repair protein recruitment to DNA damage sites and substantially ameliorated aging-associated phenotypes, both in vitro and in vivo. The life span of Zmpste24(-/-) mice was also extended with the supplementation of a histone deacetylase inhibitor, sodium butyrate, to drinking water. Consistent with recent data showing age-dependent buildup of unprocessable lamin A in physiological aging, aged wild-type mice also showed hypoacetylation of H4K16. The above results shed light on how chromatin modifications regulate the DNA damage response and suggest that the reversal of epigenetic marks could make an attractive therapeutic target against laminopathy-based progeroid pathologies.

H2AX phosphorylation at the sites of DNA double-strand breaks in cultivated mammalian cells and tissues

A sequence variant of histone H2A called H2AX is one of the key components of chromatin involved in DNA damage response induced by different genotoxic stresses. Phosphorylated H2AX (γH2AX) is rapidly concentrated in chromatin domains around DNA double-strand breaks (DSBs) after the action of ionizing radiation or chemical agents and at stalled replication forks during replication stress. γH2AX foci could be easily detected in cell nuclei using immunofluorescence microscopy that allows to use γH2AX as a quantitative marker of DSBs in various applications. H2AX is phosphorylated in situ by ATM, ATR, and DNA-PK kinases that have distinct roles in different pathways of DSB repair. The γH2AX serves as a docking site for the accumulation of DNA repair proteins, and after rejoining of DSBs, it is released from chromatin. The molecular mechanism of γH2AX dephosphorylation is not clear. It is complicated and requires the activity of different proteins including phosphatases and chromatin-remodeling complexes. In this review, we summarize recently published data concerning the mechanisms and kinetics of γH2AX loss in normal cells and tissues as well as in those deficient in ATM, DNA-PK, and DSB repair proteins activity. The results of the latest scientific research of the low-dose irradiation phenomenon are presented including the bystander effect and the adaptive response estimated by γH2AX detection in cells and tissues.

γH2AX foci as a measure of DNA damage: a computational approach to automatic analysis

The γH2AX focus assay represents a fast and sensitive approach for the detection of one of the critical types of DNA damage - double-strand breaks (DSB) induced by various cytotoxic agents including ionising radiation. Apart from research applications, the assay has a potential in clinical medicine/pathology, such as assessment of individual radiosensitivity, response to cancer therapies, as well as in biodosimetry. Given that generally there is a direct relationship between numbers of microscopically visualised γH2AX foci and DNA DSB in a cell, the number of foci per nucleus represents the most efficient and informative parameter of the assay. Although computational approaches have been developed for automatic focus counting, the tedious and time consuming manual focus counting still remains the most reliable way due to limitations of computational approaches. We suggest a computational approach and associated software for automatic focus counting that minimises these limitations. Our approach, while using standard image processing algorithms, maximises the automation of identification of nuclei/cells in complex images, offers an efficient way to optimise parameters used in the image analysis and counting procedures, optionally invokes additional procedures to deal with variations in intensity of the signal and background in individual images, and provides automatic batch processing of a series of images. We report results of validation studies that demonstrated correlation of manual focus counting with results obtained using our computational algorithm for mouse jejunum touch prints, mouse tongue sections and human blood lymphocytes as well as radiation dose response of γH2AX focus induction for these biological specimens.

Age-associated increase in heterochromatic marks in murine and primate tissues

Chromatin is highly dynamic and subject to extensive remodeling under many physiologic conditions. Changes in chromatin that occur during the aging process are poorly documented and understood in higher organisms, such as mammals. We developed an immunofluorescence assay to quantitatively detect, at the single cell level, changes in the nuclear content of chromatin-associated proteins. We found increased levels of the heterochromatin-associated proteins histone macro H2A (mH2A) and heterochromatin protein 1 beta (HP1β) in human fibroblasts during replicative senescence in culture, and for the first time, an age-associated increase in these heterochromatin marks in several tissues of mice and primates. Mouse lung was characterized by monophasic mH2A expression histograms at both ages, and an increase in mean staining intensity at old age. In the mouse liver, we observed increased age-associated localization of mH2A to regions of pericentromeric heterochromatin. In the skeletal muscle, we found two populations of cells with either low or high mH2A levels. This pattern of expression was similar in mouse and baboon, and showed a clear increase in the proportion of nuclei with high mH2A levels in older animals. The frequencies of cells displaying evidence of increased heterochromatinization are too high to be readily accounted for by replicative or oncogene-induced cellular senescence, and are prominently found in terminally differentiated, postmitotic tissues that are not conventionally thought to be susceptible to senescence. Our findings distinguish specific chromatin states in individual cells of mammalian tissues, and provide a foundation to investigate further the progressive epigenetic changes that occur during aging.

Accumulation of DNA damage in hematopoietic stem and progenitor cells during human aging

Background

Accumulation of DNA damage leading to adult stem cell exhaustion has been proposed to be a principal mechanism of aging. Here we tested this hypothesis in healthy individuals of different ages by examining unrepaired DNA double-strand breaks (DSBs) in hematopoietic stem/progenitor cells matured in their physiological microenvironment.

Methodology/Principal Findings

To asses DNA damage accumulation and repair capacities, γH2AX-foci were examined before and after exposure to ionizing irradiation. Analyzing CD34+ and CD34− stem/progenitor cells we observed an increase of endogenous γH2AX-foci levels with advancing donor age, associated with an age-related decline in telomere length. Using combined immunofluorescence and telomere-fluorescence in-situ hybridization we show that γH2AX-foci co-localize consistently with other repair factors such as pATM, MDC1 and 53BP1, but not significantly with telomeres, strongly supporting the telomere-independent origin for the majority of foci. The highest inter-individual variations for non-telomeric DNA damage were observed in middle-aged donors, whereas the individual DSB repair capacity appears to determine the extent of DNA damage accrual. However, analyzing different stem/progenitor subpopulations obtained from healthy elderly (>70 years), we observed an only modest increase in DNA damage accrual, most pronounced in the primitive CD34+CD38−-enriched subfraction, but sustained DNA repair efficiencies, suggesting that healthy lifestyle may slow down the natural aging process.

Conclusions/Significance

Based on these findings we conclude that age-related non-telomeric DNA damage accrual accompanies physiological stem cell aging in humans. Moreover, aging may alter the functional capacity of human stem cells to repair DSBs, thereby deteriorating an important genome protection mechanism leading to exceeding DNA damage accumulation. However, the great inter-individual variations in middle-aged individuals suggest that additional cell-intrinsic mechanisms and/or extrinsic factors contribute to the age-associated DNA damage accumulation.

Persistence of γ-H2AX and 53BP1 foci in proliferating and non-proliferating human mammary epithelial cells after exposure to γ-rays or iron ions

PURPOSE

To investigate γ-H2AX (phosphorylated histone H2AX) and 53BP1 (tumour protein 53 binding protein No. 1) foci formation and removal in proliferating and non-proliferating human mammary epithelial cells (HMEC) after exposure to sparsely and densely ionising radiation under different cell culture conditions.

MATERIAL AND METHODS

HMEC cells were grown either as monolayers (2D) or in extracellular matrix to allow the formation of acinar structures in vitro (3D). Foci numbers were quantified by image analysis at various time points after exposure.

RESULTS

Our results reveal that in non-proliferating cells under 2D and 3D cell culture conditions, iron-ion induced γ-H2AX foci were still present at 72 h after exposure, although 53BP1 foci returned to control levels at 48 h. In contrast in proliferating HMEC, both γ-H2AX and 53BP1 foci decreased to control levels during the 24-48 h time interval after irradiation under 2D conditions. Foci numbers decreased faster after γ-ray irradiation and returned to control levels by 12 h regardless of marker, cell proliferation status, and cell culture condition.

CONCLUSIONS

The disappearance of radiation-induced γ-H2AX and 53BP1 foci in HMEC has different dynamics that depend on radiation quality and proliferation status. Notably, the general patterns do not depend on the cell culture condition (2D versus 3D). We speculate that the persistent γ-H2AX foci in iron-ion irradiated non-proliferating cells could be due to limited availability of double-strand break (DSB) repair pathways in G0/G1-phase, or that repair of complex DSB requires replication or chromatin remodelling.

Detailed analysis of DNA repair and senescence marker kinetics over the life span of a human fibroblast cell line

We examined phosphorylation of H2AX, a marker for DNA double-strand breaks over the life of a human fibroblast cell line. This marker was compared with a number of other cellular senescence and DNA repair endpoints. An increase in γH2AX foci number was observed after 24 hours of repair time following DNA damage over the course of fibroblast passaging. Progressive and relatively constant changes in growth retardation, doubling time, and telomere length were also observed. The fraction of cells expressing β-gal, a marker of cellular senescence, increased considerably around the 40th passage as did some other cell morphology endpoints. The detectable γH2AX foci at 24 hours after ionizing radiation were far fewer than the number detected at 1 hour across all passage numbers. We conclude that although residual DNA damage level increases with passage number, it is unlikely to be the result of less efficient DNA repair in the aged fibroblast since most DNA damage is repaired, even at late passages.

Tumors induce complex DNA damage in distant proliferative tissues in vivo

That tumors cause changes in surrounding tissues is well documented, but whether they also affect distant tissues is uncertain. Such knowledge may be important in understanding the relationship between cancer and overall patient health. To address this question, we examined tissues distant to sites of implanted tumors for genomic damage using cohorts of C57BL/6 and BALB/c mice with early-stage subcutaneous syngeneic grafts, specifically, B16 melanoma, MO5076 sarcoma, and COLON26 carcinoma. Here we report that levels of two serious types of DNA damage, double-strand breaks (DSBs) measured by γ-H2AX focus formation and oxidatively induced non-DSB clustered DNA lesions (OCDLs), were elevated in tissues distant from the tumor site in tumor-bearing mice compared with their age- and sex-matched controls. Most affected were crypts in the gastrointestinal tract organs and skin, both highly proliferative tissues. Further investigation revealed that, compared with controls, tumor-bearing mice contained elevated amounts of activated macrophages in the distant gastrointestinal tissues, as well as elevated serum levels of several cytokines. One of these cytokines, CCL2/MCP-1, has been linked to several inflammation-related conditions and macrophage recruitment, and strikingly, CCL2-deficient mice lacked increased levels of DSBs and OCDLs in tissues distant from implanted tumors. Thus, this study is unique in being a direct demonstration that the presence of a tumor may induce a chronic inflammatory response in vivo, leading to increased systemic levels of DNA damage. Importantly, these findings suggest that tumors may have more profound effects on their hosts than heretofore expected.

Hyper telomere recombination accelerates replicative senescence and may promote premature aging

Werner syndrome and Bloom syndrome result from defects in the RecQ helicases Werner (WRN) and Bloom (BLM), respectively, and display premature aging phenotypes. Similarly, XFE progeroid syndrome results from defects in the ERCC1-XPF DNA repair endonuclease. To gain insight into the origin of cellular senescence and human aging, we analyzed the dependence of sister chromatid exchange (SCE) frequencies on location [i.e., genomic (G-SCE) vs. telomeric (T-SCE) DNA] in primary human fibroblasts deficient in WRN, BLM, or ERCC1-XPF. Consistent with our other studies, we found evidence of elevated T-SCE in telomerase-negative but not telomerase-positive backgrounds. In telomerase-negative WRN-deficient cells, T-SCE-but not G-SCE-frequencies were significantly increased compared with controls. In contrast, SCE frequencies were significantly elevated in BLM-deficient cells irrespective of genome location. In ERCC1-XPF-deficient cells, neither T- nor G-SCE frequencies differed from controls. A theoretical model was developed that allowed an in silico investigation into the cellular consequences of increased T-SCE frequency. The model predicts that in cells with increased T-SCE, the onset of replicative senescence is dramatically accelerated even though the average rate of telomere loss has not changed. Premature cellular senescence may act as a powerful tumor-suppressor mechanism in telomerase-deficient cells with mutations that cause T-SCE levels to rise. Furthermore, T-SCE-driven premature cellular senescence may be a factor contributing to accelerated aging in Werner and Bloom syndromes, but not XFE progeroid syndrome.

Lifestyle impacts on the aging-associated expression of biomarkers of DNA damage and telomere dysfunction in human blood

Cellular aging is characterized by telomere shortening, which can lead to uncapping of chromosome ends (telomere dysfunction) and activation of DNA damage responses. There is some evidence that DNA damage accumulates during human aging and that lifestyle factors contribute to the accumulation of DNA damage. Recent studies have identified a set of serum markers that are induced by telomere dysfunction and DNA damage, and these markers showed an increased expression in blood during human aging. Here, we investigated the influence of lifestyle factors (such as exercise, smoking, body mass) on the aging-associated expression of serum markers of DNA damage (CRAMP, EF-1alpha, stathmin, n-acetyl-glucosaminidase and chitinase) in comparison with other described markers of cellular aging (p16(INK4a) upregulation and telomere shortening) in human peripheral blood. The study shows that lifestyle factors have an age-independent impact on the expression level of biomarkers of DNA damage. Smoking and increased body mass indices were associated with elevated levels of biomarkers of DNA damage independent of the age of the individuals. In contrast, exercise was associated with an age-independent reduction in the expression of biomarkers of DNA damage in human blood. The expression of biomarkers of DNA damage correlated positively with p16(INK4a) expression and negatively with telomere length in peripheral blood T-lymphocytes. Together, these data provide experimental evidence that both aging and lifestyle impact on the accumulation of DNA damage during human aging.

Oxidation in the nucleotide pool, the DNA damage response and cellular senescence: Defective bricks build a defective house

Activation of persistent DNA damage response (DDR) signaling is associated with the induction of a permanent proliferative arrest known as cellular senescence, a phenomenon intrinsically linked to both tissue aging as well as tumor suppression. The DNA damage observed in senescent cells has been attributed to elevated levels of reactive oxygen species (ROS), failing DNA damage repair processes, and/or oncogenic activation. It is not clear how labile molecules such as ROS are able to damage chromatin-bound DNA to a sufficient extent to invoke persistent DNA damage and DDR signaling. Recent evidence suggests that the nucleotide pool is a significant target for oxidants and that oxidized nucleotides, once incorporated into genomic DNA, can lead to the induction of a DNA strand break-associated DDR that triggers senescence in normal cells and in cells sustaining oncogene activation. Evasion of this DDR and resulting senescence is a key step in tumor progression. This review will explore the role of oxidation in the nucleotide pool as a major effector of oxidative stress-induced genotoxic damage and DDR in the context of cellular senescence and tumorigenic transformation.

Genomic instability and DNA damage responses in progeria arising from defective maturation of prelamin A

Progeria syndromes have in common a premature aging phenotype and increased genome instability. The susceptibility to DNA damage arises from a compromised repair system, either in the repair proteins themselves or in the DNA damage response pathways. The most severe progerias stem from mutations affecting lamin A production, a filamentous protein of the nuclear lamina. Hutchinson-Gilford progeria syndrome (HGPS) patients are heterozygous for aLMNA gene mutation while Restrictive Dermopathy (RD) individuals have a homozygous deficiency in the processing protease Zmpste24. These mutations generate the mutant lamin A proteins progerin and FC-lamina A, respectively, which cause nuclear deformations and chromatin perturbations. Genome instability is observed even though genome maintenance and repair genes appear normal. The unresolved question is what features of the DNA damage response pathways are deficient in HGPS and RD cells. Here we review and discuss recent findings which resolve some mechanistic details of how the accumulation of progerin/FC-lamin A proteins may disrupt DNA damage response pathways in HGPS and RD cells. As the mutant lamin proteins accumulate they sequester replication and repair factors, leading to stalled replication forks which collapse into DNA double-strand beaks (DSBs). In a reaction unique to HGPS and RD cells these accessible DSB termini bind Xeroderma pigmentosum group A (XPA) protein which excludes normal binding by DNA DSB repair proteins. The bound XPA also signals activation of ATM and ATR, arresting cell cycle progression, leading to arrested growth. In addition, the effective sequestration of XPA at these DSB damage sites makes HGPS and RD cells more sensitive to ultraviolet light and other mutagens normally repaired by the nucleotide excision repair pathway of which XPA is a necessary and specific component.

Defective DSB repair correlates with abnormal nuclear morphology and is improved with FTI treatment in Hutchinson-Gilford progeria syndrome fibroblasts

Impaired DSB repair has been implicated as a molecular mechanism contributing to the accelerating aging phenotype in Hutchinson-Gilford progeria syndrome (HGPS), but neither the extent nor the cause of the repair deficiency has been fully elucidated. Here we perform a quantitative analysis of the steady-state number of DSBs and the repair kinetics of ionizing radiation (IR)-induced DSBs in HGPS cells. We report an elevated steady-state number of DSBs and impaired repair of IR-induced DSBs, both of which correlated strongly with abnormal nuclear morphology. We recreated the HGPS cellular phenotype in human coronary artery endothelial cells for the first time by lentiviral transduction of GFP-progerin, which also resulted in impaired repair of IR-induced DSBs, and which correlated with abnormal nuclear morphology. Farnesyl transferase inhibitor (FTI) treatment improved the repair of IR-induced DSBs, but only in HGPS cells whose nuclear morphology was also normalized. Interestingly, FTI treatment did not result in a statistically significant reduction in the higher steady-state number of DSBs. We also report a delay in localization of phospho-NBS1 and MRE11, MRN complex repair factors necessary for homologous recombination (HR) repair, to DSBs in HGPS cells. Our results demonstrate a correlation between nuclear structural abnormalities and the DSB repair defect, suggesting a mechanistic link that may involve delayed repair factor localization to DNA damage. Further, our results show that similar to other HGPS phenotypes, FTI treatment has a beneficial effect on DSB repair.

The involvement of human RECQL4 in DNA double-strand break repair

Rothmund-Thomson syndrome (RTS) is an autosomal recessive hereditary disorder associated with mutation in RECQL4 gene, a member of the human RecQ helicases. The disease is characterized by genomic instability, skeletal abnormalities and predisposition to malignant tumors, especially osteosarcomas. The precise role of RECQL4 in cellular pathways is largely unknown; however, recent evidence suggests its involvement in multiple DNA metabolic pathways. This study investigates the roles of RECQL4 in DNA double-strand break (DSB) repair. The results show that RECQL4-deficient fibroblasts are moderately sensitive to gamma-irradiation and accumulate more gammaH2AX and 53BP1 foci than control fibroblasts. This is suggestive of defects in efficient repair of DSB's in the RECQL4-deficient fibroblasts. Real time imaging of live cells using laser confocal microscopy shows that RECQL4 is recruited early to laser-induced DSBs and remains for a shorter duration than WRN and BLM, indicating its distinct role in repair of DSBs. Endogenous RECQL4 also colocalizes with gammaH2AX at the site of DSBs. The RECQL4 domain responsible for its DNA damage localization has been mapped to the unique N-terminus domain between amino acids 363-492, which shares no homology to recruitment domains of WRN and BLM to the DSBs. Further, the recruitment of RECQL4 to laser-induced DNA damage is independent of functional WRN, BLM or ATM proteins. These results suggest distinct cellular dynamics for RECQL4 protein at the site of laser-induced DSB and that it might play important roles in efficient repair of DSB's.

Ionizing radiation-induced long-term expression of senescence markers in mice is independent of p53 and immune status

Exposure to IR has been shown to induce the formation of senescence markers, a phenotype that coincides with lifelong delayed repair and regeneration of irradiated tissues. We hypothesized that IR-induced senescence markers could persist long-term in vivo, possibly contributing to the permanent reduction in tissue functionality. Here, we show that mouse tissues exposed to a sublethal dose of IR display persistent (up to 45 weeks, the maximum time analyzed) DNA damage foci and increased p16(INK4a) expression, two hallmarks of cellular senescence and aging. BrdU-labeling experiments revealed that IR-induced damaged cells are preferentially eliminated, at least partially, in a tissue-dependent manner. Unexpectedly, the accumulation of damaged cells was found to occur independent from the DNA damage response modulator p53, and from an intact immune system, as their levels were similar in wild-type and Rag2(-/-) gammaC(-/-) mice, the latter being deficient in T, B, and NK cells. Together, our results provide compelling evidence that exposure to IR induces long-term expression of senescence markers in vivo, an effect that may contribute to the reduced tissue functionality observed in cancer survivors.

microRNA expression patterns reveal differential expression of target genes with age

Recent evidence supports a role for microRNAs (miRNAs) in regulating the life span of model organisms. However, little is known about how these small RNAs contribute to human aging. Here, we profiled the expression of over 800 miRNAs in peripheral blood mononuclear cells from young and old individuals by real-time RT-PCR analysis. This genome-wide assessment of miRNA expression revealed that the majority of miRNAs studied decreased in abundance with age. We identified nine miRNAs (miR-103, miR-107, miR-128, miR-130a, miR-155, miR-24, miR-221, miR-496, miR-1538) that were significantly lower in older individuals. Among them, five have been implicated in cancer pathogenesis. Predicted targets of several of these miRNAs, including PI3 kinase (PI3K), c-Kit and H2AX, were found to be elevated with advancing age, supporting a possible role for them in the aging process. Furthermore, we found that decreasing the levels of miR-221 was sufficient to cause a corresponding increase in the expression of the predicted target, PI3K. Taken together, these findings demonstrate that changes in miRNA expression occur with human aging and suggest that miRNAs and their predicted targets have the potential to be diagnostic indicators of age or age-related diseases.

GEMIN2 promotes accumulation of RAD51 at double-strand breaks in homologous recombination

RAD51 is a key factor in homologous recombination (HR) and plays an essential role in cellular proliferation by repairing DNA damage during replication. The assembly of RAD51 at DNA damage is strictly controlled by RAD51 mediators, including BRCA1 and BRCA2. We found that human RAD51 directly binds GEMIN2/SIP1, a protein involved in spliceosome biogenesis. Biochemical analyses indicated that GEMIN2 enhances the RAD51–DNA complex formation by inhibiting RAD51 dissociation from DNA, and thereby stimulates RAD51-mediated homologous pairing. GEMIN2 also enhanced the RAD51-mediated strand exchange, when RPA was pre-bound to ssDNA before the addition of RAD51. To analyze the function of GEMIN2, we depleted GEMIN2 in the chicken DT40 line and in human cells. The loss of GEMIN2 reduced HR efficiency and resulted in a significant decrease in the number of RAD51 subnuclear foci, as observed in cells deficient in BRCA1 and BRCA2. These observations and our biochemical analyses reveal that GEMIN2 regulates HR as a novel RAD51 mediator.

Role of oxidatively induced DNA lesions in human pathogenesis

Genome stability is essential for maintaining cellular and organismal homeostasis, but it is subject to many threats. One ubiquitous threat is from a class of compounds known as reactive oxygen species (ROS), which can indiscriminately react with many cellular biomolecules including proteins, lipids, and DNA to produce a variety of oxidative lesions. These DNA oxidation products are a direct risk to genome stability, and of particular importance are oxidative clustered DNA lesions (OCDLs), defined as two or more oxidative lesions present within 10 bp of each other. ROS can be produced by exposure of cells to exogenous environmental agents including ionizing radiation, light, chemicals, and metals. In addition, they are produced by cellular metabolism including mitochondrial ATP generation. However, ROS also serve a variety of critical cellular functions and optimal ROS levels are maintained by multiple cellular antioxidant defenses. Oxidative DNA lesions can be efficiently repaired by base excision repair or nucleotide excision repair. If ROS levels increase beyond the capacity of its antioxidant defenses, the cell's DNA repair capacity can become overwhelmed, leading to the accumulation of oxidative DNA damage products including OCDLs, which are more difficult to repair than individual isolated DNA damage products. Here we focus on the induction and repair of OCDLs and other oxidatively induced DNA lesions. If unrepaired, these lesions can lead to the formation of mutations, DNA DSBs, and chromosome abnormalities. We discuss the roles of these lesions in human pathologies including aging and cancer, and in bystander effects.

Microwaves from Mobile Phones Inhibit 53BP1 Focus Formation in Human Stem Cells More Strongly Than in Differentiated Cells: Possible Mechanistic Link to Cancer Risk

BACKGROUND

It is widely accepted that DNA double-strand breaks (DSBs) and their misrepair in stem cells are critical events in the multistage origination of various leukemias and tumors, including gliomas.

OBJECTIVES

We studied whether microwaves from mobile telephones of the Global System for Mobile Communication (GSM) and the Universal Global Telecommunications System (UMTS) induce DSBs or affect DSB repair in stem cells.

METHODS

We analyzed tumor suppressor TP53 binding protein 1 (53BP1) foci that are typically formed at the sites of DSB location (referred to as DNA repair foci) by laser confocal microscopy.

RESULTS

Microwaves from mobile phones inhibited formation of 53BP1 foci in human primary fibroblasts and mesenchymal stem cells. These data parallel our previous findings for human lymphocytes. Importantly, the same GSM carrier frequency (915 MHz) and UMTS frequency band (1947.4 MHz) were effective for all cell types. Exposure at 905 MHz did not inhibit 53BP1 foci in differentiated cells, either fibroblasts or lymphocytes, whereas some effects were seen in stem cells at 905 MHz. Contrary to fibroblasts, stem cells did not adapt to chronic exposure during 2 weeks.

CONCLUSIONS

The strongest microwave effects were always observed in stem cells. This result may suggest both significant misbalance in DSB repair and severe stress response. Our findings that stem cells are most sensitive to microwave exposure and react to more frequencies than do differentiated cells may be important for cancer risk assessment and indicate that stem cells are the most relevant cellular model for validating safe mobile communication signals.

The RABIT: a rapid automated biodosimetry tool for radiological triage

In response to the recognized need for high throughput biodosimetry methods for use after large-scale radiological events, a logical approach is complete automation of standard biodosimetric assays that are currently performed manually. The authors describe progress to date on the RABIT (Rapid Automated BIodosimetry Tool), designed to score micronuclei or gamma-H2AX fluorescence in lymphocytes derived from a single drop of blood from a fingerstick. The RABIT system is designed to be completely automated, from the input of the capillary blood sample into the machine to the output of a dose estimate. Improvements in throughput are achieved through use of a single drop of blood, optimization of the biological protocols for in situ analysis in multi-well plates, implementation of robotic-plate and liquid handling, and new developments in high-speed imaging. Automating well-established bioassays represents a promising approach to high-throughput radiation biodosimetry, both because high throughputs can be achieved, but also because the time to deployment is potentially much shorter than for a new biological assay. Here the authors describe the development of each of the individual modules of the RABIT system and show preliminary data from key modules. System integration is ongoing, followed by calibration and validation.

Radiation-induced DNA repair foci: spatio-temporal aspects of formation, application for assessment of radiosensitivity and biological dosimetry

Several proteins involved in DNA repair and DNA damage signaling have been shown to produce discrete foci in response to ionizing radiation. These foci are believed to co-localize to DSB and referred to as ionizing radiation-induced foci (IRIF) or DNA repair foci. Recent studies have revealed that some residual IRIF remain in cells for a relatively long time after irradiation, and have indicated a possible correlation between radiosensitivity of cells and residual IRIF. Remarkably, residual foci are significantly larger in size than the initial foci. Increase in the size of IRIF with time upon irradiation has been found in various cell types and has partially been correlated with dynamics and fusion of initial foci. Although it is admitted that the number of IRIF reflect that of DSB, several studies report a lack of correlation between kinetics for IRIF and DSB and a lack of co-localization between DSB repair proteins. These studies suggest that some proportion of residual IRIF that depend on cell type, dose, and post-irradiation time may represent alternations in chromatin structure after DSB have been repaired or misrepaired. While precise functions of residual foci are presently unknown, their possible link to remaining chromatin alternations, nuclear matrix, apoptosis, delayed repair and misrejoining of DSB, activity of several kinases, phosphatases, and checkpoint signaling has been suggested. Another intriguing possibility is that some of DNA repair foci may mark break-points at chromosomal aberrations (CA). While this possibility has not been confirmed substantially, the residual foci seem to be useful for biological dosimetry and estimation of individual radiosensitivity in radiotherapy of cancer.

Naive CD4 T cells from aged mice show enhanced death upon primary activation

Poor T cell immunity is one of the many defects seen in elderly humans and aged (Ad) mice. We report that naive CD4 T cells from aged mice (ANCD4 cells) showed greater apoptosis upon primary activation than those from young (Yg) mice, with loss of mitochondrial membrane potential, poor activation of Rel family transcription factors and increased DNA damage. Their ability to enhance glycolysis, produce lactate and induce autophagy following activation was also compromised. ANCD4 cells remained susceptible to death beyond first cell division. Activated ANCD4 cells also showed poor transition to a 'central memory' (CM) CD44(high), CD62L(high) phenotype in vitro. This correlated with low proportions of CM cells in Ad mice in vivo. Functionally, too, IFN-gamma responses recalled from T cells of immunized Ad mice, poor to begin with, worsened with time as compared with Yg mice. Thus, ANCD4 cells handle activation-associated stress very poorly due to multiple defects, possibly contributing to poor formation of long-lasting memory.

H2AX: functional roles and potential applications

Upon DNA double-strand break (DSB) induction in mammals, the histone H2A variant, H2AX, becomes rapidly phosphorylated at serine 139. This modified form, termed gamma-H2AX, is easily identified with antibodies and serves as a sensitive indicator of DNA DSB formation. This review focuses on the potential clinical applications of gamma-H2AX detection in cancer and in response to other cellular stresses. In addition, the role of H2AX in homeostasis and disease will be discussed. Recent work indicates that gamma-H2AX detection may become a powerful tool for monitoring genotoxic events associated with cancer development and tumor progression.