Changes in the level and distribution of Ku proteins during cellular senescence

Colchicine prevents oxidative stress-induced endothelial cell senescence via blocking NF-κB and MAPKs: implications in vascular diseases

BACKGROUND

Smoking, alcohol abuse, and hypertension are - among others, potential risk factors for cardiovascular diseases. These risk factors generate oxidative stress and cause oxidative stress-induced DNA damage, resulting in cellular senescence and senescence-associated secretory phenotype (SASP). The SASP factors in feed-forward response exacerbate inflammation and cause tissue remodeling, resulting in atherosclerotic plaque formation and rupture.

RESULTS

Colchicine inhibited ROS generation and mitigated oxidative stress-induced DNA damage. It dampened oxidative stress-induced endothelial cell senescence and improved the expression of DNA repair protein KU80 and aging marker Lamin B1. The drug attenuated the expression of senescence marker P21 at mRNA and protein levels. The pathway analysis showed that colchicine inhibited NF-κB and MAPKs pathways and subdued mTOR activation. Colchicine also attenuated mRNA expression of interleukin (IL)-1β, IL-6, IL-8, MCP-1, ICAM-1, and E-selectin. Furthermore, colchicine reduced the mRNA and protein expression of matrix metalloproteinase (MMP-2).

CONCLUSION

In summary, colchicine blocked oxidative stress-induced senescence and SASP by inhibiting the activation of NF-κB and MAPKs pathways.

Colchicine Protects against Ethanol-Induced Senescence and Senescence-Associated Secretory Phenotype in Endothelial Cells

Inflammaging is a potential risk factor for cardiovascular diseases. It results in the development of thrombosis and atherosclerosis. The accumulation of senescent cells in vessels causes vascular inflammaging and contributes to plaque formation and rupture. In addition to being an acquired risk factor for cardiovascular diseases, ethanol can induce inflammation and senescence, both of which have been implicated in cardiovascular diseases. In the current study, we used colchicine to abate the cellular damaging effects of ethanol on endothelial cells. Colchicine prevented senescence and averted oxidative stress in endothelial cells exposed to ethanol. It lowered the relative protein expression of aging and senescence marker P21 and restored expression of the DNA repair proteins KU70/KU80. Colchicine inhibited the activation of nuclear factor kappa B (NFκ-B) and mitogen activated protein kinases (MAPKs) in ethanol-treated endothelial cells. It reduced ethanol-induced senescence-associated secretory phenotype. In summary, we show that colchicine ameliorated the ethanol-caused molecular events, resulting in attenuated senescence and senescence-associated secretory phenotype in endothelial cells.

Molecular mechanisms of alveolar epithelial cell senescence and idiopathic pulmonary fibrosis: a narrative review

Background and Objective

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive interstitial pneumonia of unknown etiology. An increasing number of studies have reported that the incidence of IPF increases with age. Simultaneously, the number of senescent cells increased in IPF. Epithelial cell senescence, an important component of epithelial cell dysfunction, plays a key role in IPF pathogenesis. This article summarizes the molecular mechanisms associated with alveolar epithelial cell senescence and recent advances in the applications of drugs targeting pulmonary epithelial cell senescence to explore novel therapeutic approaches for the treatment of pulmonary fibrosis.

Methods

All literature published in English on PubMed, Web of Science, and Google Scholar were electronically searched online using the following keyword combinations: aging, alveolar epithelial cell, cell senescence, idiopathic pulmonary fibrosis, WNT/β-catenin, phosphatidylinositol-3-kinase/protein kinase B (PI3K/Akt), mammalian target of rapamycin (mTOR), and nuclear factor kappa B (NF-κB).

Key Content and Findings

We focused on signaling pathways associated with alveolar epithelial cell senescence in IPF, including WNT/β-catenin, PI3K/Akt, NF-κB, and mTOR signaling pathways. Some of these signaling pathways are involved in alveolar epithelial cell senescence by affecting cell cycle arrest and secretion of senescence-associated secretory phenotype-associated markers. We also found that changes in lipid metabolism in alveolar epithelial cells can be induced by mitochondrial dysfunction, both of which contribute to cellular senescence and development of IPF.

Conclusions

Decreasing senescent alveolar epithelial cells may be a promising strategy for the treatment of IPF. Therefore, further investigations into new treatments of IPF by applying inhibitors of relevant signaling pathways, as well as senolytic drugs, are warranted.

Doxycycline Attenuated Ethanol-Induced Inflammaging in Endothelial Cells: Implications in Alcohol-Mediated Vascular Diseases

Excess alcohol consumption is a potential risk factor for cardiovascular diseases and is linked to accelerated aging. Drug discovery to reduce toxic cellular events of alcohol is required. Here, we investigated the effects of ethanol on human umbilical vein endothelial cells (HUVECs) and explored if doxycycline attenuates ethanol-mediated molecular events in endothelial cells. Initially, a drug screening using a panel of 170 drugs was performed, and doxycycline was selected for further experiments. HUVECs were treated with different concentrations (300 mM and 400 mM) of ethanol with or without doxycycline (10 µg/mL). Telomere length was quantified as telomere to single-copy gene (T/S) ratio. Telomere length and the mRNA expression were quantified by qRT-PCR, and protein level was analyzed by Western blot (WB). Ethanol treatment accelerated cellular aging, and doxycycline treatment recovered telomere length. Pathway analysis showed that doxycycline inhibited mTOR and NFκ-B activation. Doxycycline restored the expression of aging-associated proteins, including lamin b1 and DNA repair proteins KU70 and KU80. Doxycycline reduced senescence and senescence-associated secretory phenotype (SASP) in ethanol-treated HUVECs. In conclusion, we report that ethanol-induced inflammation and aging in HUVECs were ameliorated by doxycycline.

Changes in DNA double-strand break repair during aging correlate with an increase in genomic mutations

A double -strand break (DSB) is one of the most deleterious forms of DNA damage. In eukaryotic cells, two main repair pathways have evolved to repair DSBs, homologous recombination (HR) and non-homologous end-joining (NHEJ). HR is the predominant pathway of repair in the unicellular eukaryotic organism, S. cerevisiae. However, during replicative aging the relative use of HR and NHEJ shifts in favor of end-joining repair. By monitoring repair events in the HO-DSB system, we find that early in replicative aging there is a decrease in the association of long-range resection factors, Dna2-Sgs1 and Exo1 at the break site and a decrease in DNA resection. Subsequently, as aging progressed, the recovery of Ku70 at DSBs decreased and the break site associated with the nuclear pore complex at the nuclear periphery, which is the location where DSB repair occurs through alternative pathways that are more mutagenic. End-bridging remained intact as HR and NHEJ declined, but eventually it too became disrupted in cells at advanced replicative age. In all, our work provides insight into the molecular changes in DSB repair pathway during replicative aging. HR first declined, resulting in a transient increase in the NHEJ. However, with increased cellular divisions, Ku70 recovery at DSBs and NHEJ subsequently declined. In wild type cells of advanced replicative age, there was a high frequency of repair products with genomic deletions and microhomologies at the break junction, events not observed in young cells which repaired primarily by HR.

WNT/beta-catenin signalling interrupts a senescence-induction cascade in human mesenchymal stem cells that restricts their expansion

Senescence, the irreversible cell cycle arrest of damaged cells, is accompanied by a deleterious pro-inflammatory senescence-associated secretory phenotype (SASP). Senescence and the SASP are major factors in aging, cancer, and degenerative diseases, and interfere with the expansion of adult cells in vitro, yet little is known about how to counteract their induction and deleterious effects. Paracrine signals are increasingly recognized as important senescence triggers and understanding their regulation and mode of action may provide novel opportunities to reduce senescence-induced inflammation and improve cell-based therapies. Here, we show that the signalling protein WNT3A counteracts the induction of paracrine senescence in cultured human adult mesenchymal stem cells (MSCs). We find that entry into senescence in a small subpopulation of MSCs triggers a secretome that causes a feed-forward signalling cascade that with increasing speed induces healthy cells into senescence. WNT signals interrupt this cascade by repressing cytokines that mediate this induction of senescence. Inhibition of those mediators by interference with NF-κB or interleukin 6 signalling reduced paracrine senescence in absence of WNT3A and promoted the expansion of MSCs. Our work reveals how WNT signals can antagonize senescence and has relevance not only for expansion of adult cells but can also provide new insights into senescence-associated inflammatory and degenerative diseases.

Autophagy and PTEN in DNA damage-induced senescence

The use of DNA-damaging agents such as radiotherapy and chemotherapy has been a mainstay treatment protocol for many cancers, including lung and prostate. Recently, FDA approval of inhibitors of DNA repair, and targeting innate immunity to enhance the efficacy of DNA-damaging agents have gained much attention. Yet, inherent or acquired resistance against DNA-damaging therapies persists as a fundamental drawback. While cancer eradication by causing cancer cell death through induction of apoptosis is the ultimate goal of anti-cancer treatments, autophagy and senescence are two major cellular responses induced by clinically tolerable doses of DNA-damaging therapies. Unlike apoptosis, autophagy and senescence can act as both pro-tumorigenic as well as tumor suppressive mechanisms. DNA damage-induced senescence is associated with a pro-inflammatory secretory phenotype, which contributes to reshaping the tumor- immune microenvironment. Moreover, PTEN (phosphatase and tensin homolog) is a tumor supressor deleted in many tumors, and has been implicated in both senescence and autophagy. This review presents an overview of the literature on the regulation and consequences of DNA damage- induced senescence in cancer cells, with a specific focus on autophagy and PTEN. Both autophagy and senescence occur concurrently in the same cells in response to DNA damaging agents. However, a deterministic relationship between these fundamental processes has been controversial. We present experimental evidence obtained with tumor cells, with a prime focus on two models of cancer, prostate and lung. A better understanding of mechanisms associated with DNA damage-induced cellular senescence is central to fully exploit the potential of DNA-damaging agents against cancer.

The multifaceted roles of DNA repair and replication proteins in aging and obesity

Efficient mechanisms for genomic maintenance (i.e., DNA repair and DNA replication) are crucial for cell survival. Aging and obesity can lead to the dysregulation of genomic maintenance proteins/pathways and are significant risk factors for the development of cancer, metabolic disorders, and other genetic diseases. Mutations in genes that code for proteins involved in DNA repair and DNA replication can also exacerbate aging- and obesity-related disorders and lead to the development of progeroid diseases. In this review, we will discuss the roles of various DNA repair and replication proteins in aging and obesity as well as investigate the possible mechanisms by which aging and obesity can lead to the dysregulation of these proteins and pathways.

Age-associated deficient recruitment of 53BP1 in G1 cells directs DNA double-strand break repair to BRCA1/CtIP-mediated DNA-end resection

DNA repair mechanisms play a crucial role in maintaining genome integrity. However, the increased frequency of DNA double-strand breaks (DSBs) and genome rearrangements in aged individuals suggests an age-associated DNA repair deficiency. Previous work from our group revealed a delayed firing of the DNA damage response in human mammary epithelial cells (HMECs) from aged donors. We now report a decreased activity of the main DSB repair pathways, the canonical non-homologous end-joining (c-NHEJ) and the homologous recombination (HR) in these HMECs from older individuals. We describe here a deficient recruitment of 53BP1 to DSB sites in G1 cells, probably influenced by an altered epigenetic regulation. 53BP1 absence at some DSBs is responsible for the age-associated DNA repair defect, as it permits the ectopic formation of BRCA1 foci while still in the G1 phase. CtIP and RPA foci are also formed in G1 cells from aged donors, but RAD51 is not recruited, thus indicating that extensive DNA-end resection occurs in these breaks although HR is not triggered. These results suggest an age-associated switch of DSB repair from canonical to highly mutagenic alternative mechanisms that promote the formation of genome rearrangements, a source of genome instability that might contribute to the aging process.

Systemic Delivery of Tumor-Targeting siRNA Nanoparticles against an Oncogenic LncRNA Facilitates Effective Triple-Negative Breast Cancer Therapy

Long noncoding RNAs (lncRNAs), by virtue of their versatility and multilevel gene regulation, have emerged as attractive pharmacological targets for treating heterogeneous and complex malignancies like triple-negative breast cancer (TNBC). Despite multiple studies on lncRNA functions in tumor pathology, systemic targeting of these "undruggable" macromolecules with conventional approaches remains a challenge. Here, we demonstrate effective TNBC therapy by nanoparticle-mediated RNAi of the oncogenic lncRNA DANCR, which is significantly overexpressed in TNBC. Tumor-targeting RGD-PEG-ECO/siDANCR nanoparticles were formulated via self-assembly of multifunctional amino lipid ECO, cyclic RGD peptide-PEG, and siDANCR for systemic delivery. MDA-MB-231 and BT549 cells treated with the therapeutic RGD-PEG-ECO/siDANCR nanoparticles exhibited 80-90% knockdown in the expression of DANCR for up to 7 days, indicating efficient intracellular siRNA delivery and sustained target silencing. The RGD-PEG-ECO/siDANCR nanoparticles mediated excellent in vitro therapeutic efficacy, reflected by significant reduction in the invasion, migration, survival, tumor spheroid formation, and proliferation of the TNBC cell lines. At the molecular level, functional ablation of DANCR dynamically impacted the oncogenic nexus by downregulating PRC2-mediated H3K27-trimethylation and Wnt/EMT signaling, and altering the phosphorylation profiles of several kinases in the TNBC cells. Furthermore, systemic administration of the RGD-PEG-ECO/siDANCR nanoparticles at a dose of 1 mg/kg siRNA in nude mice bearing TNBC xenografts resulted in robust suppression of TNBC progression with no overt toxic side-effects, underscoring the efficacy and safety of the nanoparticle therapy. These results demonstrate that nanoparticle-mediated modulation of onco-lncRNAs and their molecular targets is a promising approach for developing curative therapies for TNBC and other cancers.

Nuclear rupture at sites of high curvature compromises retention of DNA repair factors

The nucleus is physically linked to the cytoskeleton, adhesions, and extracellular matrix-all of which sustain forces, but their relationships to DNA damage are obscure. We show that nuclear rupture with cytoplasmic mislocalization of multiple DNA repair factors correlates with high nuclear curvature imposed by an external probe or by cell attachment to either aligned collagen fibers or stiff matrix. Mislocalization is greatly enhanced by lamin A depletion, requires hours for nuclear reentry, and correlates with an increase in pan-nucleoplasmic foci of the DNA damage marker γH2AX. Excess DNA damage is rescued in ruptured nuclei by cooverexpression of multiple DNA repair factors as well as by soft matrix or inhibition of actomyosin tension. Increased contractility has the opposite effect, and stiff tumors with low lamin A indeed exhibit increased nuclear curvature, more frequent nuclear rupture, and excess DNA damage. Additional stresses likely play a role, but the data suggest high curvature promotes nuclear rupture, which compromises retention of DNA repair factors and favors sustained damage.

SIPS as a model to study age-related changes in proteolysis and aggregate formation

Aging is accompanied by the accumulation of cellular damage over time in response to stress, lifestyle and environmental factors ultimately leading to age-related diseases and death. Additionally, the number of senescent cells increases with age. Senescence is most likely not a static endpoint, it represents a series of hallmarks including morphological changes, alterations in protein turnover and accumulation of protein aggregates. The importance of protein oxidation and aggregate accumulation in the progression of aging is not yet fully understood and research to what extent the accumulation of oxidized proteins has an effect on senescence and the aging process is still ongoing. To study the mechanisms of aging, the impact of senescence and the role of protein aggregates on the aging process, cell culture models are useful tools. Most notably stress induced premature senescence (SIPS) models have contributed to the identification of mechanisms involved in the aging process and helped unravel the age-related changes in proteolysis and the importance of protein aggregation. Here we review characteristics of replicative and premature senescence, how to induce most frequently used senescence models and gained knowledge on age-related changes in the major proteolytic systems.

DNA repair and aging: the impact of the p53 family

Cells are constantly exposed to endogenous and exogenous factors that threaten the integrity of their DNA. The maintenance of genome stability is of paramount importance in the prevention of both cancer and aging processes. To deal with DNA damage, cells put into operation a sophisticated and coordinated mechanism, collectively known as DNA damage response (DDR). The DDR orchestrates different cellular processes, such as DNA repair, senescence and apoptosis. Among the key factors of the DDR, the related proteins p53, p63 and p73, all belonging to the same family of transcription factors, play multiple relevant roles. Indeed, the members of this family are directly involved in the induction of cell cycle arrest that is necessary to allow the cells to repair. Alternatively, they can promote cell death in case of prolonged or irreparable DNA damage. They also take part in a more direct task by modulating the expression of core factors involved in the process of DNA repair or by directly interacting with them. In this review we will analyze the fundamental roles of the p53 family in the aging process through their multifaceted function in DDR.

Non-homologous end joining: advances and frontiers

DNA double-strand breaks (DSBs) are the most serious form of DNA damage. In human cells, non-homologous end joining (NHEJ) is the major pathway for the repair of DSBs. Different types of DSBs result in different subsets of NHEJ repair strategies. These variations in NHEJ repair strategies depend on numerous elements, such as the flexible recruitment of NHEJ-related proteins, the complexity of the DSB ends, and the spatial- and temporal-ordered formation of the multi-protein complex. On the one hand, current studies of DNA DSBs repair focus on the repair pathway choices between homologous recombination and classic or alternative NHEJ. On the other hand, increasing researches have also deepened the significance and dug into the cross-links between the NHEJ pathway and the area of genome organization and aging. Although remarkable progress has been made in elucidating the underlying principles during the past decades, the detailed mechanism of action in response to different types of DSBs remains largely unknown and needs further evaluation in the future study.

Programmed Cell Death Genes Are Linked to Elevated Creatine Kinase Levels in Unhealthy Male Nonagenarians

Declining health in the oldest-old takes an energy toll for the simple maintenance of body functions. The underlying mechanisms, however, differ in males and females. In females, the declines are explained by loss of muscle mass; but this is not the case in males, in whom they are associated with increased levels of circulating creatine kinase. This relationship raises the possibility that muscle damage rather than muscle loss is the cause of the increased energy demands of unhealthy aging in males. We have now examined factors that contribute to the increase in creatine kinase. Much of it (60%) can be explained by a history of cardiac problems and lower kidney function, while being mitigated by moderate physical activity, reinforcing the notion that tissue damage is a likely source. In a search for genetic risk factors associated with elevated creatine kinase, the Ku70 gene XRCC6 and the ceramide synthase gene LASS1 were investigated because of their roles in telomere length and longevity and healthy aging, respectively. Single nucleotide polymorphisms in these two genes were independently associated with creatine kinase levels. The XRCC6 variant was epistatic to one of the LASS1 variants but not to the other. These gene variants have potential regulatory activity. Ku70 is an inhibitor of the proapoptotic Bax, while the product of Lass1, ceramide, operates in both caspase-dependent and -independent pathways of programmed cell death, providing a potential cellular mechanism for the effects of these genes on tissue damage and circulating creatine kinase.

DNA double strand break repair, aging and the chromatin connection

Are DNA damage and mutations possible causes or consequences of aging? This question has been hotly debated by biogerontologists for decades. The importance of DNA damage as a possible driver of the aging process went from being widely recognized to then forgotten, and is now slowly making a comeback. DNA double strand breaks (DSBs) are particularly relevant to aging because of their toxicity, increased frequency with age and the association of defects in their repair with premature aging. Recent studies expand the potential impact of DNA damage and mutations on aging by linking DNA DSB repair and age-related chromatin changes. There is overwhelming evidence that increased DNA damage and mutations accelerate aging. However, an ultimate proof of causality would be to show that enhanced genome and epigenome stability delays aging. This is not an easy task, as improving such complex biological processes is infinitely more difficult than disabling it. We will discuss the possibility that animal models with enhanced DNA repair and epigenome maintenance will be generated in the near future.

In Vitro Expansion of Bone Marrow Derived Mesenchymal Stem Cells Alters DNA Double Strand Break Repair of Etoposide Induced DNA Damage

Mesenchymal stem cells (MSCs) are of interest for use in diverse cellular therapies. Ex vivo expansion of MSCs intended for transplantation must result in generation of cells that maintain fidelity of critical functions. Previous investigations have identified genetic and phenotypic alterations of MSCs with in vitro passage, but little is known regarding how culturing influences the ability of MSCs to repair double strand DNA breaks (DSBs), the most severe of DNA lesions. To investigate the response to DSB stress with passage in vitro, primary human MSCs were exposed to etoposide (VP16) at various passages with subsequent evaluation of cellular damage responses and DNA repair. Passage number did not affect susceptibility to VP16 or the incidence and repair kinetics of DSBs. Nonhomologous end joining (NHEJ) transcripts showed little alteration with VP16 exposure or passage; however, homologous recombination (HR) transcripts were reduced following VP16 exposure with this decrease amplified as MSCs were passaged in vitro. Functional evaluations of NHEJ and HR showed that MSCs were unable to activate NHEJ repair following VP16 stress in cells after successive passage. These results indicate that ex vivo expansion of MSCs alters their ability to perform DSB repair, a necessary function for cells intended for transplantation.

DNA double-strand break repair is impaired in presenescent Syrian hamster fibroblasts

Background

Studies of DNA damage response are critical for the comprehensive understanding of age-related changes in cells, tissues and organisms. Syrian hamster cells halt proliferation and become presenescent after several passages in standard conditions of cultivation due to what is known as «culture stress». Using proliferating young and non-dividing presenescent cells in primary cultures of Syrian hamster fibroblasts, we defined their response to the action of radiomimetic drug bleomycin (BL) that induces DNA double-strand breaks (DSBs).

Results

The effect of the drug was estimated by immunoblotting and immunofluorescence microscopy using the antibody to phosphorylated histone H2AX (gH2AX), which is generally accepted as a DSB marker. At all stages of the cell cycle, both presenescent and young cells demonstrated variability of the number of gH2AX foci per nucleus. gH2AX focus induction was found to be independent from BL-hydrolase expression. Some differences in DSB repair process between BL-treated young and presenescent Syrian hamster cells were observed: (1) the kinetics of gH2AX focus loss in G0 fibroblasts of young culture was faster than in cells that prematurely stopped dividing; (2) presenescent cells were characterized by a slower recruitment of DSB repair proteins 53BP1, phospho-DNA-PK and phospho-ATM to gH2AX focal sites, while the rate of phosphorylated ATM/ATR substrate accumulation was the same as that in young cells.

Conclusions

Our results demonstrate an impairment of DSB repair in prematurely aged Syrian hamster fibroblasts in comparison with young fibroblasts, suggesting age-related differences in response to BL therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-015-0046-4) contains supplementary material, which is available to authorized users.

Integrated Stochastic Model of DNA Damage Repair by Non-homologous End Joining and p53/p21-Mediated Early Senescence Signalling

Unrepaired or inaccurately repaired DNA damage can lead to a range of cell fates, such as apoptosis, cellular senescence or cancer, depending on the efficiency and accuracy of DNA damage repair and on the downstream DNA damage signalling. DNA damage repair and signalling have been studied and modelled in detail separately, but it is not yet clear how they integrate with one another to control cell fate. In this study, we have created an integrated stochastic model of DNA damage repair by non-homologous end joining and of gamma irradiation-induced cellular senescence in human cells that are not apoptosis-prone. The integrated model successfully explains the changes that occur in the dynamics of DNA damage repair after irradiation. Simulations of p53/p21 dynamics after irradiation agree well with previously published experimental studies, further validating the model. Additionally, the model predicts, and we offer some experimental support, that low-dose fractionated irradiation of cells leads to temporal patterns in p53/p21 that lead to significant cellular senescence. The integrated model is valuable for studying the processes of DNA damage induced cell fate and predicting the effectiveness of DNA damage related medical interventions at the cellular level.

Aging and radiation: bad companions

Aging involves a deterioration of cell functions and changes that may predispose the cell to undergo an oncogenic transformation. The carcinogenic risks following radiation exposure rise with age among adults. Increasing inflammatory response, loss of oxidant/antioxidant equilibrium, ongoing telomere attrition, decline in the DNA damage response efficiency, and deleterious nuclear organization are age-related cellular changes that trigger a serious threat to genomic integrity. In this review, we discuss the mechanistic interplay between all these factors, providing an integrated view of how they contribute to the observed age-related increase in radiation sensitivity. As life expectancy increases and so it does the medical intervention, it is important to highlight the benefits of radiation protection in the elderly. Thus, a deep understanding of the mechanistic processes confining the threat of aging-related radiosensitivity is currently of foremost relevance.

Age-dependent changes in DNA repair in radiation-exposed mice

Ionizing radiation (IR) is a well-known human carcinogen. Young and adult individuals are known to respond to radiation in a different manner. In this study, we analyzed changes in the spleen of juvenile (two-week-old), adult (two-month-old) and old (18-month-old) C57BL/6 male mice subjected to a whole-body exposure to 1 Gy of X rays. We measured the number of γ-H2AX foci and ATM protein levels as a reflection of the level of DNA double-strand breaks (DSBs), and found that old animals had a high frequency of occurrence of noninduced DSBs. Exposure to X rays resulted in a rapid increase in the number of DSBs in juvenile and adult animals at 6 h postirradiation followed by a return to preirradiated DSB values at 96 h postirradiation. No changes were observed in old animals. The analysis of the levels of proteins involved in DNA damage base excision and mismatch repair pathways, including KU70, RAD51, POL β, POL δ, POL ε, APE1 and MSH2 showed substantial age-dependent radiation-induced differences. Finally, we demonstrated that old animals had a higher background level of cell apoptosis compared to younger animals, but in contrast to younger animals, old animals were not able to commit spleen cells to apoptosis after being irradiated. Thus, spleen cells of old mice have a high level of spontaneous DNA damage, but they are not able to deal with additional radiation-induced damage as efficiently as younger animals, substantiating age-depending differences in radiation-induced DNA damage and repair response and its outcomes.

Methylation of the nonhomologous end joining repair pathway genes does not explain the increase of translocations with aging

Chromosome translocations are especially frequent in human lymphomas and leukemias but are insufficient to drive carcinogenesis. Indeed, several of the so-called tumor specific translocations have been detected in peripheral blood of healthy individuals, finding a higher frequency of some of them with aging. The inappropriate repair of DNA double strand breaks by the nonhomologous end joining (NHEJ) pathway is one of the reasons for a translocation to occur. Moreover, fidelity of this pathway has been shown to decline with age. Although the mechanism underlying this inefficacy is unknown, other repair pathways are inactivated by methylation with aging. In this study, we analyzed the implication of NHEJ genes methylation in the increase of translocations with the age. To this aim, we determined the relationship between translocations and aging in 565 Spanish healthy individuals and correlated these data with the methylation status of 11 NHEJ genes. We found higher frequency of BCL2-JH and BCR-ABL (major) translocations with aging. In addition, we detected that two NHEJ genes (LIG4 and XRCC6) presented age-dependent promoter methylation changes. However, we did not observe a correlation between the increase of translocations and methylation, indicating that other molecular mechanisms are involved in the loss of NHEJ fidelity with aging.

Knock-in reporter mice demonstrate that DNA repair by non-homologous end joining declines with age

Accumulation of genome rearrangements is a characteristic of aged tissues. Since genome rearrangements result from faulty repair of DNA double strand breaks (DSBs), we hypothesized that DNA DSB repair becomes less efficient with age. The Non-Homologous End Joining (NHEJ) pathway repairs a majority of DSBs in vertebrates. To examine age-associated changes in NHEJ, we have generated an R26NHEJ mouse model in which a GFP-based NHEJ reporter cassette is knocked-in to the ROSA26 locus. In this model, NHEJ repair of DSBs generated by the site-specific endonuclease, I-SceI, reconstitutes a functional GFP gene. In this system NHEJ efficiency can be compared across tissues of the same mouse and in mice of different age. Using R26NHEJ mice, we found that NHEJ efficiency was higher in the skin, lung, and kidney fibroblasts, and lower in the heart fibroblasts and brain astrocytes. Furthermore, we observed that NHEJ efficiency declined with age. In the 24-month old animals compared to the 5-month old animals, NHEJ efficiency declined 1.8 to 3.8-fold, depending on the tissue, with the strongest decline observed in the skin fibroblasts. The sequence analysis of 300 independent NHEJ repair events showed that, regardless of age, mice utilize microhomology sequences at a significantly higher frequency than expected by chance. Furthermore, the frequency of microhomology-mediated end joining (MMEJ) events increased in the heart and lung fibroblasts of old mice, suggesting that NHEJ becomes more mutagenic with age. In summary, our study provides a versatile mouse model for the analysis of NHEJ in a wide range of tissues and demonstrates that DNA repair by NHEJ declines with age in mice, which could provide a mechanism for age-related genomic instability and increased cancer incidence with age.

DNA damage disrupting both DNA strands, termed double strand breaks (DSBs), poses a threat to cell survival. If repaired inappropriately, such DNA breaks lead to genomic rearrangements, mutations, and ultimately cancer. Nonhomologous end joining (NHEJ) is the major pathway for repairing double-stranded breaks in mammals. Errors associated with NHEJ have been implicated in the aging process because mice with mutations in NHEJ genes exhibit premature aging. It remains unknown, however, whether NHEJ becomes impaired during normal aging. Studies of age-related changes in NHEJ have been hampered by the lack of a mouse model that would allow detection and quantification of NHEJ events. Here we report generation of NHEJ reporter mice containing a GFP-based NHEJ cassette knocked-into the ROSA26 locus. Using this mouse model, we were able to compare NHEJ across different tissues and demonstrate that NHEJ becomes less efficient and more error-prone with age. Our results provide a mechanism for age-related genomic instability and increased cancer incidence with age. The NHEJ reporter mice will be useful for a broad range of studies in the fields of aging and DNA repair.

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.

The role of DNA damage and repair in aging through the prism of Koch-like criteria

Since the first publication on Somatic Mutation Theory of Aging (Szilárd, 1959), a great volume of knowledge in the field has been accumulated. Here we attempted to organize the evidence "for" and "against" the hypothesized causal role of DNA damage and mutation accumulation in aging in light of four Koch-like criteria. They are based on the assumption that some quantitative relationship between the levels of DNA damage/mutations and aging rate should exist, so that (i) the longer-lived individuals or species would have a lower rate of damage than the shorter-lived, and (ii) the interventions that modulate the level of DNA damage and repair capacity should also modulate the rate of aging and longevity and vice versa. The analysis of how the existing data meets the proposed criteria showed that many gaps should still be filled in order to reach a clear-cut conclusion. As a perspective, it seems that the main emphasis in future studies should be put on the role of DNA damage in stem cell aging.

Castration therapy results in decreased Ku70 levels in prostate cancer

PURPOSE

Neoadjuvant castration improves response to radiotherapy of prostate cancer. Here, we determine whether castration therapy impairs nonhomologous end-joining (NHEJ) repair of DNA double-strand breaks (DSB) by downregulating Ku70 protein expression.

EXPERIMENTAL DESIGN

Twenty patients with locally advanced prostate cancer were enrolled, and 6 to 12 needle core biopsy specimens were taken from the prostate of each patient before treatment. Bilateral orchidectomy was conducted in eight patients and 12 patients were treated with a GnRH agonist. After castration, two to four similar biopsies were obtained, and the levels of Ku70 and γ-H2AX foci were determined by immunofluorescence in verified cancer tissues.

RESULTS

We observed that the androgen receptor binds directly to Ku70 in prostate tissue. We also found a reduction of the Ku70 protein levels in the cell nuclei in 12 of 14 patients (P < 0.001) after castration. The reduction in Ku70 expression correlated significantly with decreased serum prostate-specific antigen (PSA) levels after castration, suggesting that androgen receptor activity regulates Ku70 protein levels in prostate cancer tissue. Furthermore, a significant correlation between the reductions of Ku70 after castration versus changes induced of castration of γ-H2AX foci could be seen implicating a functional linkage of decreased Ku70 levels and impaired DNA repair.

CONCLUSIONS

Castration therapy results in decreased levels of the Ku70 protein in prostate cancer cells. Because the Ku70 protein is essential for the NHEJ repair of DSBs and its downregulation impairs DNA repair, this offers a possible explanation for the increased radiosensitivity of prostate cancer cells following castration.

Dynamic compartmentalization of DNA repair proteins within spiral ganglion neurons in response to noise stress

ABSTRACT In response to stress, spiral ganglion neurons may remodel intracellular pools of DNA repair proteins. This hypothesis was addressed by determining the intracellular location of three classic DNA excision repair proteins (XPA, CSA, and XPC) within the neurons under normal conditions, one day after noise stress (105 dB/4 hr) and following DNA repair adjuvant therapy with carboxy alkyl esters (CAEs; 160 mg/kg/28 days). Under normal conditions, three intracellular compartments were enriched with at least one repair protein. These intracellular compartments were designated nuclear, cytoplasmic, and perinuclear. After the noise stress each repair protein aggregated in the cytoplasm. After CAE therapy each intracellular compartment was enriched with the three DNA repair proteins. Combining noise stress with CAE therapy resulted in the enrichment of at least two repair proteins in each intracellular compartment. The combined results suggest that in response to noise stress and/or otoprotective therapy, spiral ganglion neurons may selectively remodel compartmentalized DNA repair proteins.

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.

Agrobacterium tumefaciens T-DNA Integration and Gene Targeting in Arabidopsis thaliana Non-Homologous End-Joining Mutants

In order to study the role of AtKu70 and AtKu80 in Agrobacterium-mediated transformation and gene targeting, plant lines with a T-DNA insertion in AtKu80 or AtKu70 genes were functionally characterized. Such plant lines lacked both subunits, indicating that heterodimer formation between AtKu70 and AtKu80 is needed for the stability of the proteins. Homozygous mutants were phenotypically indistinguishable from wild-type plants and were fertile. However, they were hypersensitive to the genotoxic agent bleomycin, resulting in more DSBs as quantified in comet assays. They had lower end-joining efficiency, suggesting that NHEJ is a critical pathway for DSB repair in plants. Both Atku mutants and a previously isolated Atmre11 mutant were impaired in Agrobacterium T-DNA integration via floral dip transformation, indicating that AtKu70, AtKu80, and AtMre11 play an important role in T-DNA integration in Arabidopsis. The frequency of gene targeting was not significantly increased in the Atku80 and Atku70 mutants, but it was increased at least 10-fold in the Atmre11 mutant compared with the wild type.

Autophagy-dependent senescence in response to DNA damage and chronic apoptotic stress

Autophagy regulates cell survival and cell death upon various cellular stresses, yet the molecular signaling events involved are not well defined. Here, we established the function of a proteolytic Cyclin E fragment (p18-CycE) in DNA damage-induced autophagy, apoptosis, and senescence. p18-CycE was identified in hematopoietic cells undergoing DNA damage-induced apoptosis. In epithelial cells exposed to DNA damage, chronic but not transient expression of p18-CycE leads to higher turnover of LC3 I/II and increased emergence of autophagosomes and autolysosomes. Levels of p18-CycE, which was generated by proteolytic cleavage of endogenous Cyclin E, were greatly increased by chloroquine and correlated with LC 3II conversion. Preventing p18-CycE genesis blocked conversion of LC3 I to LC3 II. Upon DNA damage, cytoplasmic ataxia-telangiectasia-mutated (ATM) was phosphorylated in p18-CycE-expressing cells resulting in sustained activation of the adenosine-mono-phosphate-dependent kinase (AMPK). These lead to sustained activation of mammalian autophagy-initiating kinase ULK1, which was abrogated upon inhibiting ATM and AMPK phosphorylation. Moreover, p18-CycE was degraded via autophagy followed by induction of senescence. Both autophagy and senescence were prevented by inhibiting autophagy, which leads to increased apoptosis in p18-CycE-expressing cells by stabilizing p18-CycE expression. Senescence was further associated with cytoplasmic co-localization and degradation of p18-CycE and Ku70. In brief, chronic p18-CycE expression-induced autophagy leads to clearance of p18-CycE following DNA damage and induction of senescence. Autophagy inhibition stabilized the cytoplasmic p18-CycE-Ku70 complex leading to apoptosis. Thus, our findings define how chronic apoptotic stress and DNA damage initiate autophagy and regulate cell survival through senescence and/or apoptosis.

Pim-1 knockdown potentiates paclitaxel-induced apoptosis in human hormone-refractory prostate cancers through inhibition of NHEJ DNA repair

The knockdown of Pim-1 or inhibition of Pim-1 activity significantly increased γ-H2A.X expression. The effect was correlated to apoptosis and was attributed to the inhibition of nonhomologous DNA-end-joining (NHEJ) repair activity supported by the following observations: (1) inhibition of ATM and DNA-PKcs activities, (2) down-regulation of Ku expression and nuclear localization and (3) decrease of DNA end-binding of both Ku70 and Ku80. The data suggest that Pim-1 plays a crucial role in the regulation of NHEJ repair. In the absence of Pim-1, the ability of DNA repair significantly decreases when exposed to paclitaxel, leading to severe DNA damage and apoptosis.

Cellular stress response pathways and ageing: intricate molecular relationships

Ageing is driven by the inexorable and stochastic accumulation of damage in biomolecules vital for proper cellular function. Although this process is fundamentally haphazard and uncontrollable, senescent decline and ageing is broadly influenced by genetic and extrinsic factors. Numerous gene mutations and treatments have been shown to extend the lifespan of diverse organisms ranging from the unicellular Saccharomyces cerevisiae to primates. It is becoming increasingly apparent that most such interventions ultimately interface with cellular stress response mechanisms, suggesting that longevity is intimately related to the ability of the organism to effectively cope with both intrinsic and extrinsic stress. Here, we survey the molecular mechanisms that link ageing to main stress response pathways, and mediate age-related changes in the effectiveness of the response to stress. We also discuss how each pathway contributes to modulate the ageing process. A better understanding of the dynamics and reciprocal interplay between stress responses and ageing is critical for the development of novel therapeutic strategies that exploit endogenous stress combat pathways against age-associated pathologies.

Loss of p21(Sdi1) expression in senescent cells after DNA damage accompanied with increase of miR-93 expression and reduced p53 interaction with p21(Sdi1) gene promoter

To answer what is a critical event for higher incidence of tumor development in old than young individuals, primary culture of human diploid fibroblasts were employed and DNA damage was induced by doxorubicin or X-ray irradiation. Response to the damage was different between young and old cells; loss of p21(sdi1) expression in spite of p53(S¹⁵) activation in old cells along with [³H]thymidine and BrdU incorporation, but not in young cells. The phenomenon was confirmed by other tissue fibroblasts obtained from different donor ages. Induction of miR-93 expression and reduced p53 binding to p21 gene promoter account for loss of p21(sdi1) expression in senescent cells after DNA damage, suggesting a mechanism of in vivo carcinogenesis in aged tissue without repair arrest.

Restoration of senescent human diploid fibroblasts by modulation of the extracellular matrix

Human diploid fibroblasts have the capacity to complete a finite number of cell divisions before entering a state of replicative senescence characterized by growth arrest, changes in morphology, and altered gene expression. Herein, we report that interaction with extracellular matrix (ECM) from young cells is sufficient to restore aged, senescent cells to an apparently youthful state. The identity of the restored cells as having been derived from senescent cells has been confirmed by a variety of methods, including time lapse live cell imaging and DNA finger print analysis. In addition to cell morphology, phenotypic restoration was assessed by resumption of proliferative potential, growth factor responsiveness, reduction of intracellular reactive oxygen species levels, recovery of mitochondrial membrane potential, and increased telomere length. Mechanistically, we find that both Ku and SIRT1 are induced during restoration and are required for senescent cells to return to a youthful phenotype. These observations demonstrate that human cellular senescence is profoundly influenced by cues from the ECM, and that senescent cell plasticity is much greater than that was previously believed to be the case.

A data mining approach for classifying DNA repair genes into ageing-related or non-ageing-related

Background

The ageing of the worldwide population means there is a growing need for research on the biology of ageing. DNA damage is likely a key contributor to the ageing process and elucidating the role of different DNA repair systems in ageing is of great interest. In this paper we propose a data mining approach, based on classification methods (decision trees and Naive Bayes), for analysing data about human DNA repair genes. The goal is to build classification models that allow us to discriminate between ageing-related and non-ageing-related DNA repair genes, in order to better understand their different properties.

Results

The main patterns discovered by the classification methods are as follows: (a) the number of protein-protein interactions was a predictor of DNA repair proteins being ageing-related; (b) the use of predictor attributes based on protein-protein interactions considerably increased predictive accuracy of attributes based on Gene Ontology (GO) annotations; (c) GO terms related to "response to stimulus" seem reasonably good predictors of ageing-relatedness for DNA repair genes; (d) interaction with the XRCC5 (Ku80) protein is a strong predictor of ageing-relatedness for DNA repair genes; and (e) DNA repair genes with a high expression in T lymphocytes are more likely to be ageing-related.

Conclusions

The above patterns are broadly integrated in an analysis discussing relations between Ku, the non-homologous end joining DNA repair pathway, ageing and lymphocyte development. These patterns and their analysis support non-homologous end joining double strand break repair as central to the ageing-relatedness of DNA repair genes. Our work also showcases the use of protein interaction partners to improve accuracy in data mining methods and our approach could be applied to other ageing-related pathways.

Microsatellite instability in Arabidopsis increases with plant development

Plant development consists of the initial phase of intensive cell division followed by continuous genome endoreduplication, cell growth, and elongation. The maintenance of genome stability under these conditions is the main task performed by DNA repair and genome surveillance mechanisms. Our previous work showed that the rate of homologous recombination repair in older plants decreases. We hypothesized that this age-dependent decrease in the recombination rate is paralleled with other changes in DNA repair capacity. Here, we analyzed microsatellite stability using transgenic Arabidopsis (Arabidopsis thaliana) plants that carry the nonfunctional β-glucuronidase gene disrupted by microsatellite repeats. We found that microsatellite instability increased dramatically with plant age. We analyzed the contribution of various mechanisms to microsatellite instability, including replication errors and mistakes of DNA repair mechanisms such as mismatch repair, excision repair, and strand break repair. Analysis of total DNA polymerase activity using partially purified protein extracts showed an age-dependent decrease in activity and an increase in fidelity. Analysis of the steady-state RNA level of DNA replicative polymerases α, δ, Pol I-like A, and Pol I-like B and the expression of mutS homolog 2 (Msh2) and Msh6 showed an age-dependent decrease. An in vitro repair assay showed lower efficiency of nonhomologous end joining in older plants, paralleled by an increase in Ku70 gene expression. Thus, we assume that the more frequent involvement of nonhomologous end joining in strand break repair and the less efficient end-joining repair together with lower levels of mismatch repair activities may be the main contributors to the observed age-dependent increase in microsatellite instability.

The DNA-PK catalytic subunit regulates Bax-mediated excitotoxic cell death by Ku70 phosphorylation

DNA repair deficiency results in neurodegenerative disease and increased susceptibility to excitotoxic cell death, suggesting a critical but undefined role for DNA damage in neurodegeneration. We compared DNA damage, Ku70-Bax interaction, and Bax-dependent excitotoxic cell death in kainic acid-treated primary cortical neurons derived from both wild-type mice and mice deficient in the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) encoded by the Prkdc gene. In both wild-type and Prkdc(-/-) neurons, kainic acid treatment resulted in rapid induction of DNA damage (53BP1 foci formation) followed by nuclear pyknosis. Bax deficiency, by either Bax shRNA-mediated knockdown or gene deletion, protected wild-type and heterozygous but not Prkdc(-/-) neurons from kainate-induced excitotoxicity. Cotransfection of DNA-PKcs with Bax shRNA restored Bax shRNA-mediated neuroprotection in Prkdc(-/-) neurons, suggesting that DNA-PKcs is required for kainate-induced activation of the pro-apoptotic Bax pathway. Immunoprecipitation studies revealed that the DNA-PKcs-nonphosphorylatable Ku70 (S6A/S51A) bound 3- to 4-fold greater Bax than wild-type Ku70, suggesting that DNA-PKcs-mediated Ku70 phosphorylation causes release of Bax from Ku70. In support of this, kainic acid induced translocation of a Bax-EGFP fusion protein to the mitochondria in the presence of a cotransfected wild-type, but not mutant Ku70 (S6A/S51A) gene when examined at 4 and 8 h following kainate addition. We conclude that DNA-PKcs links DNA damage to Bax-dependent excitotoxic cell death, by phosphorylating Ku70 on serines 6 and/or 51, to initiate Bax translocation to the mitochondria and directly activate a pro-apoptotic Bax-dependent death cascade.

Increase in double-stranded DNA break-related foci in early-stage thymocytes of aged mice

Cellular and molecular mechanisms involved in aging are notoriously complex. Aging-related immune decline of T lymphocyte function is partly caused by attrition of thymic T cell development, which involves programmed creation and repair of DNA breaks for generating T cell receptors. Aging also leads to significant alterations in the cellular DNA repair ability. We show that higher levels of gamma-phosphorylated H2AX (pH2AX), which marks DNA double-stranded breaks (DSBs), were detectable in early thymocyte subsets of aged as compared to young mice. Also, while only 1-2 foci of nuclear accumulation of pH2AX were detectable in these early thymocytes from young mice, cells from aged mice showed higher numbers of pH2AX foci. In CD4-CD8- double-negative (DN) thymocytes of aged mice, which showed the highest levels of DSBs, there was a modest increase in levels of the DNA repair protein MRE11, but not of either Ku70, another DNA repair protein, or the cell cycle checkpoint protein p53. Thus, immature thymocytes in aged mice show a marked increase in DNA DSBs with only a modest enhancement of repair processes, and the resultant cell cycle block could contribute to aging-related defects of T cell development.

New anti-Ku80 monoclonal antibody F10H2.B3 as a useful marker for dividing cells in culture

We report on the development of a mouse monoclonal antibody (named F10H2.B3) using the native cellular fragments of human fetal neural stem cells as immunogens. Molecular analysis has shown that the target antigen of F10H2.B3 is Ku80 (ATP-dependent DNA helicase 2 subunit 2 [EC 3.6.1.-]). We suggest this antibody could be used in certain conditions as a proliferation marker for cells of different origin.

DNA double-strand breaks: a potential causative factor for mammalian aging?

Aging is a pleiotropic and stochastic process influenced by both genetics and environment. As a result the fundamental underlying causes of aging are controversial and likely diverse. Genome maintenance and in particular the repair of DNA damage is critical to ensure longevity needed for reproduction and as a consequence imperfections or defects in maintaining the genome may contribute to aging. There are many forms of DNA damage with double-strand breaks (DSBs) being the most toxic. Here we discuss DNA DSBs as a potential causative factor for aging including factors that generate DNA DSBs, pathways that repair DNA DSBs, consequences of faulty or failed DSB repair and how these consequences may lead to age-dependent decline in fitness. At the end we compare mouse models of premature aging that are defective for repairing either DSBs or UV light-induced lesions. Based on these comparisons we believe the basic mechanisms responsible for their aging phenotypes are fundamentally different demonstrating the complex and pleiotropic nature of this process.

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

Accumulation of DNA damage may play an essential role in both cellular senescence and organismal aging. The ability of cells to sense and repair DNA damage declines with age. However, the underlying molecular mechanism for this age-dependent decline is still elusive. To understand quantitative and qualitative changes in the DNA damage response during human aging, DNA damage-induced foci of phosphorylated histone H2AX (gamma-H2AX), which occurs specifically at sites of DNA double-strand breaks (DSBs) and eroded telomeres, were examined in human young and senescing fibroblasts, and in lymphocytes of peripheral blood. Here, we show that the incidence of endogenous gamma-H2AX foci increases with age. Fibroblasts taken from patients with Werner syndrome, a disorder associated with premature aging, genomic instability and increased incidence of cancer, exhibited considerably higher incidence of gamma-H2AX foci than those taken from normal donors of comparable age. Further increases in gamma-H2AX focal incidence occurred in culture as both normal and Werner syndrome fibroblasts progressed toward senescence. The rates of recruitment of DSB repair proteins to gamma-H2AX foci correlated inversely with age for both normal and Werner syndrome donors, perhaps due in part to the slower growth of gamma-H2AX foci in older donors. Because genomic stability may depend on the efficient processing of DSBs, and hence the rapid formation of gamma-H2AX foci and the rapid accumulation of DSB repair proteins on these foci at sites of nascent DSBs, our findings suggest that decreasing efficiency in these processes may contribute to genome instability associated with normal and pathological aging.

Deletion of Ku70, Ku80, or both causes early aging without substantially increased cancer

Ku70 forms a heterodimer with Ku80, called Ku, that is critical for repairing DNA double-stand breaks by nonhomologous end joining and for maintaining telomeres. Mice with either gene mutated exhibit similar phenotypes that include increased sensitivity to ionizing radiation and severe combined immunodeficiency. However, there are also differences in the reported phenotypes. For example, only Ku70 mutants are reported to exhibit a high incidence of thymic lymphomas while only Ku80 mutants are reported to exhibit early aging with very low cancer levels. There are two explanations for these differences. First, either Ku70 or Ku80 functions outside the Ku heterodimer such that deletion of one is not identical to deletion of the other. Second, divergent genetic backgrounds or environments influence the phenotype. To distinguish between these possibilities, the Ku70 and Ku80 mutations were crossed together to generate Ku70, Ku80, and double-mutant mice in the same genetic background raised in the same environment. We show that these three cohorts have similar phenotypes that most resemble the previous report for Ku80 mutant mice, i.e., early aging without substantially increased cancer levels. Thus, our observations suggest that the Ku heterodimer is important for longevity assurance in mice since divergent genetic backgrounds and/or environments likely account for these previously reported differences.

Changes in DNA repair during aging

DNA is a precious molecule. It encodes vital information about cellular content and function. There are only two copies of each chromosome in the cell, and once the sequence is lost no replacement is possible. The irreplaceable nature of the DNA sets it apart from other cellular molecules, and makes it a critical target for age-related deterioration. To prevent DNA damage cells have evolved elaborate DNA repair machinery. Paradoxically, DNA repair can itself be subject to age-related changes and deterioration. In this review we will discuss the changes in efficiency of mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER) and double-strand break (DSB) repair systems during aging, and potential changes in DSB repair pathway usage that occur with age. Mutations in DNA repair genes and premature aging phenotypes they cause have been reviewed extensively elsewhere, therefore the focus of this review is on the comparison of DNA repair mechanisms in young versus old.