Rapid accumulation of genome rearrangements in liver but not in brain of old mice

Investigating the effects of acute and chronic stress on DNA damage

A hallmark of the vertebrate stress response is a rapid increase in glucocorticoids and catecholamines; however, this does not mean that these mediators are the best, or should be the only, metric measured when studying stress. Instead, it is becoming increasingly clear that assaying a suite of downstream metrics is necessary in stress physiology. One component of this suite could be assessing double-stranded DNA damage (dsDNA damage), which has recently been shown to increase in blood with both acute and chronic stress in house sparrows (Passer domesticus). To further understand the relationship between stress and dsDNA damage, we designed two experiments to address the following questions: (1) how does dsDNA damage with chronic stress vary across tissues? (2) does the increase in dsDNA damage during acute stress come from one arm of the stress response or both? We found that (1) dsDNA damage affects tissues differently during chronic stress and (2) the hypothalamic-pituitary-adrenal axis influences dsDNA damage with acute stress, but the sympathetic-adreno-medullary system does not. Surprisingly, our data are not explained by studies on changes in hormone receptor levels with chronic stress, so the underlying mechanism remains unclear.

Genetic Characterization of Rat Hepatic Stellate Cell Line PAV-1

The rat hepatic stellate cell line PAV-1 was established two decades ago and proposed as a cellular model to study aspects of hepatic retinoic acid metabolism. This cell line exhibits a myofibroblast-like phenotype but also has the ability to store retinyl esters and synthesize retinoic acid from its precursor retinol. Importantly, when cultured with palmitic acid alone or in combination with retinol, the cells switch to a deactivated phenotype in which the proliferation and expression of profibrogenic marker genes are suppressed. Despite these interesting characteristics, the cell line has somehow fallen into oblivion. However, based on the fact that working with in vivo models is becoming increasingly complicated, genetically characterized established cell lines that mimic aspects of hepatic stellate cell biology are of fundamental value for biomedical research. To genetically characterize PAV-1 cells, we performed karyotype analysis using conventional chromosome analysis and multicolor spectral karyotyping (SKY), which allowed us to identify numerical and specific chromosomal alteration in PAV-1 cells. In addition, we used a panel of 31 species-specific allelic variant sites to define a unique short tandem repeat (STR) profile for this cell line and performed bulk mRNA-sequencing, showing that PAV-1 cells express an abundance of genes specific for the proposed myofibroblastic phenotype. Finally, we used Rhodamine-Phalloidin staining and electron microscopy analysis, which showed that PAV-1 cells contain a robust intracellular network of filamentous actin and process typical ultrastructural features of hepatic stellate cells.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

The Role of NAD+ in Regenerative Medicine

The understanding of the molecular and cellular basis of aging has grown exponentially over recent years, and it is now accepted within the scientific community that aging is a malleable process; just as it can be accelerated, it can also be slowed and even reversed. This has far-reaching implications for our attitude and approach toward aging, presenting the opportunity to enter a new era of cellular regenerative medicine to not only manage the external signs of aging but also to develop therapies that support the body to repair and restore itself back to a state of internal well-being. A wealth of evidence now demonstrates that a decline in cellular nicotinamide adenine dinucleotide (NAD+) is a feature of aging and may play a role in the process. NAD+ plays a pivotal role in cellular metabolism and is a co-substrate for enzymes that play key roles in pathways that modify aging. Thus, interventions that increase NAD+ may slow aspects of the aging trajectory, and there is great interest in methods for cellular NAD+ restoration. Given these recent advancements in understanding the cellular aging process, it is important that there is an integration between the basic scientists who are investigating the underlying mechanisms of cellular aging and the surgeons and aesthetic practitioners who are providing antiaging therapies. This will allow the effective translation of this vastly complex area of biology into clinical practice so that people can continue to not only stay looking younger for longer but also experience improved health and wellness.

Age-related somatic mutation burden in human tissues

The genome of multicellular organisms carries the hereditary information necessary for the development of all organs and tissues and to maintain function in adulthood. To ensure the genetic stability of the species, genomes are protected against changes in sequence information. However, genomes are not static. De novo mutations in germline cells are passed on to offspring and generate the variation needed in evolution. Moreover, postzygotic mutations occur in all somatic cells during development and aging. These somatic mutations remain limited to the individual, generating tissues that are genome mosaics. Insight into such mutations and their consequences has been limited due to their extremely low abundance, with most mutations unique for each cell. Recent advances in sequencing, including whole genome sequencing at the single-cell level, have now led to the first insights into somatic mutation burdens in human tissues. Here, we will first briefly describe the latest methodology for somatic mutation analysis, then review our current knowledge of somatic mutation burden in human tissues and, finally, briefly discuss the possible functional impact of somatic mutations on the aging process and age-related diseases, including cancer and diseases other than cancer.

A Mechanistic Theory of Development-Aging Continuity in Humans and Other Mammals

There is consensus among biogerontologists that aging occurs either as the result of a purposeful genome-based, evolved program or due to spontaneous, randomly occurring, maladaptive events. Neither concept has yet identified a specific mechanism to explain aging’s emergence and acceleration during mid-life and beyond. Presented herein is a novel, unifying mechanism with empirical evidence that describes how aging becomes continuous with development. It assumes that aging emerges from deterioration of a regulatory process that directs morphogenesis and morphostasis. The regulatory system consists of a genome-wide “backbone” within which its specific genes are differentially expressed by the local epigenetic landscapes of cells and tissues within which they reside, thereby explaining its holistic nature. Morphostasis evolved in humans to ensure the nurturing of dependent offspring during the first decade of young adulthood when peak parental vitality prevails in the absence of aging. The strict redundancy of each morphostasis regulatory cycle requires sensitive dependence upon initial conditions to avoid initiating deterministic chaos behavior. However, when natural selection declines as midlife approaches, persistent, progressive, and specific DNA damage and misrepair changes the initial conditions of the regulatory process, thereby compromising morphostasis regulatory redundancy, instigating chaos, initiating senescence, and accelerating aging thereafter.

Local non-pituitary growth hormone is induced with aging and facilitates epithelial damage

Microenvironmental factors modulating age-related DNA damage are unclear. Non-pituitary growth hormone (npGH) is induced in human colon, non-transformed human colon cells, and fibroblasts, and in 3-dimensional intestinal organoids with age-associated DNA damage. Autocrine/paracrine npGH suppresses p53 and attenuates DNA damage response (DDR) by inducing TRIM29 and reducing ATM phosphorylation, leading to reduced DNA repair and DNA damage accumulation. Organoids cultured up to 4 months exhibit aging markers, p16, and SA-β-galactosidase and decreased telomere length, as well as DNA damage accumulation, with increased npGH, suppressed p53, and attenuated DDR. Suppressing GH in aged organoids increases p53 and decreases DNA damage. WT mice exhibit age-dependent colon DNA damage accumulation, while in aged mice devoid of colon GH signaling, DNA damage remains low, with elevated p53. As age-associated npGH induction enables a pro-proliferative microenvironment, abrogating npGH signaling could be targeted as anti-aging therapy by impeding DNA damage and age-related pathologies.

Chesnokova et al. show that non-pituitary growth hormone (npGH) is induced in aging DNA-damaged colon epithelium and suppresses DNA damage response by attenuating the phosphorylation of DNA repair proteins. npGH induction promotes DNA damage accumulation, resulting in age-associated colon microenvironment changes. Accordingly, disrupted GH signaling in aging mice prevents accumulated DNA damage.

From DNA damage to mutations: All roads lead to aging

Damage to the repository of genetic information in cells has plagued life since its very beginning 3-4 billion years ago. Initially, in the absence of an ozone layer, especially damage from solar UV radiation must have been frequent, with other sources, most notably endogenous sources related to cell metabolism, gaining in importance over time. To cope with this high frequency of damage to the increasingly long DNA molecules that came to encode the growing complexity of cellular functions in cells, DNA repair evolved as one of the earliest genetic traits. Then as now, errors during the repair of DNA damage generated mutations, which provide the substrate for evolution by natural selection. With the emergence of multicellular organisms also the soma became a target of DNA damage and mutations. In somatic cells selection against the adverse effects of DNA damage is greatly diminished, especially in postmitotic cells after the age of first reproduction. Based on an abundance of evidence, DNA damage is now considered as the single most important driver of the degenerative processes that collectively cause aging. Here I will first briefly review the evidence for DNA damage as a cause of aging since the beginning of life. Then, after discussing the possible direct adverse effects of DNA damage and its cellular responses, I will provide an overview of the considerable progress that has recently been made in analyzing a major consequence of DNA damage in humans and other complex organisms: somatic mutations and the resulting genome mosaicism. Recent advances in studying somatic mutagenesis and genome mosaicism in different human and animal tissues will be discussed with a focus on the possible mechanisms through which loss of DNA sequence integrity could cause age-related functional decline and disease.

The 28 + 28 day design is an effective sampling time for analyzing mutant frequencies in rapidly proliferating tissues of MutaMouse animals

The Organisation for Economic Co-Operation and Development Test Guideline 488 (TG 488) uses transgenic rodent models to generate in vivo mutagenesis data for regulatory submission. The recommended design in TG 488, 28 consecutive daily exposures with tissue sampling three days later (28 + 3d), is optimized for rapidly proliferating tissues such as bone marrow (BM). A sampling time of 28 days (28 + 28d) is considered more appropriate for slowly proliferating tissues (e.g., liver) and male germ cells. We evaluated the impact of the sampling time on mutant frequencies (MF) in the BM of MutaMouse males exposed for 28 days to benzo[a]pyrene (BaP), procarbazine (PRC), isopropyl methanesulfonate (iPMS), or triethylenemelamine (TEM) in dose-response studies. BM samples were collected + 3d, + 28d, + 42d or + 70d post exposure and MF quantified using the lacZ assay. All chemicals significantly increased MF with maximum fold increases at 28 + 3d of 162.9, 6.6, 4.7 and 2.8 for BaP, PRC, iPMS and TEM, respectively. MF were relatively stable over the time period investigated, although they were significantly increased only at 28 + 3d and 28 + 28d for TEM. Benchmark dose (BMD) modelling generated overlapping BMD confidence intervals among the four sampling times for each chemical. These results demonstrate that the sampling time does not affect the detection of mutations for strong mutagens. However, for mutagens that produce small increases in MF, sampling times greater than 28 days may produce false-negative results. Thus, the 28 + 28d protocol represents a unifying protocol for simultaneously assessing mutations in rapidly and slowly proliferating somatic tissues and male germ cells.

Aging and Alzheimer's disease connection: Nuclear Tau and lamin A

Age-related pathologies like Alzheimer`s disease (AD) imply cellular responses directed towards repairing DNA damage. Postmitotic neurons show progressive accumulation of oxidized DNA during decades of brain aging, which is especially remarkable in AD brains. The characteristic cytoskeletal pathology of AD neurons is brought about by the progressive changes that neurons undergo throughout aging, and their irreversible nuclear transformation initiates the disease. This review focusses on critical molecular events leading to the loss of plasticity that underlies cognitive deficits in AD. During healthy neuronal aging, nuclear Tau participates in the regulation of the structure and function of the chromatin. The aberrant cell cycle reentry initiated for DNA repair triggers a cascade of events leading to the dysfunctional AD neuron, whereby Tau protein exits the nucleus leading to chromatin disorganization. Lamin A, which is not typically expressed in neurons, appears at the transformation from senile to AD neurons and contributes to halting the consequences of cell cycle reentry and nuclear Tau exit, allowing the survival of the neuron. Nevertheless, this irreversible nuclear transformation alters the nucleic acid and protein synthesis machinery as well as the nuclear lamina and cytoskeleton structures, leading to neurofibrillary tangles formation and final neurodegeneration.

Pathogenic Mechanisms of Somatic Mutation and Genome Mosaicism in Aging

Age-related accumulation of postzygotic DNA mutations results in tissue genetic heterogeneity known as somatic mosaicism. Although implicated in aging as early as the 1950s, somatic mutations in normal tissue have been difficult to study because of their low allele fractions. With the recent emergence of cost-effective high-throughput sequencing down to the single-cell level, enormous progress has been made in our capability to quantitatively analyze somatic mutations in human tissue in relation to aging and disease. Here we first review how recent technological progress has opened up this field, providing the first broad sets of quantitative information on somatic mutations in vivo necessary to gain insight into their possible causal role in human aging and disease. We then propose three major mechanisms that can lead from accumulated de novo mutations across tissues to cell functional loss and human disease.

Aging-associated distinctive DNA methylation changes of LINE-1 retrotransposons in pure cell-free DNA from human blood

LINE-1 hypomethylation of cell-free DNA has been described as an epigenetic biomarker of human aging. However, in the past, insufficient differentiation between cellular and cell-free DNA may have confounded analyses of genome-wide methylation levels in aging cells. Here we present a new methodological strategy to properly and unambiguously extract DNA methylation patterns of repetitive, as well as single genetic loci from pure cell-free DNA from peripheral blood. Since this nucleic acid fraction originates mainly in apoptotic, senescent and cancerous cells, this approach allows efficient analysis of aged and cancerous cell-specific DNA methylation patterns for diagnostic and prognostic purposes. Using this methodology, we observe a significant age-associated erosion of LINE-1 methylation in cfDNA suggesting that the threshold of hypomethylation sufficient for relevant LINE-1 activation and consequential harmful retrotransposition might be reached at higher age. We speculate that this process might contribute to making aging the main risk factor for many cancers.

FOXO3a acts to suppress DNA double-strand break-induced mutations

Genomic instability is one of the hallmarks of aging, and both DNA damage and mutations have been found to accumulate with age in different species. Certain gene families, such as sirtuins and the FoxO family of transcription factors, have been shown to play a role in lifespan extension. However, the mechanism(s) underlying the increased longevity associated with these genes remains largely unknown and may involve the regulation of responses to cellular stressors, such as DNA damage. Here, we report that FOXO3a reduces genomic instability in cultured mouse embryonic fibroblasts (MEFs) treated with agents that induce DNA double-strand breaks (DSBs), that is, clastogens. We show that DSB treatment of both primary human and mouse fibroblasts upregulates FOXO3a expression. FOXO3a ablation in MEFs harboring the mutational reporter gene lacZ resulted in an increase in genome rearrangements after bleomycin treatment; conversely, overexpression of human FOXO3a was found to suppress mutation accumulation in response to bleomycin. We also show that overexpression of FOXO3a in human primary fibroblasts decreases DSB-induced γH2AX foci. Knocking out FOXO3a in mES cells increased the frequency of homologous recombination and non-homologous end-joining events. These results provide the first direct evidence that FOXO3a plays a role in suppressing genome instability, possibly by suppressing genome rearrangements.

Contributions of DNA Damage to Alzheimer's Disease

Alzheimer’s disease (AD) is the most common type of neurodegenerative disease. Its typical pathology consists of extracellular amyloid-β (Aβ) plaques and intracellular tau neurofibrillary tangles. Mutations in the APP, PSEN1, and PSEN2 genes increase Aβ production and aggregation, and thus cause early onset or familial AD. Even with this strong genetic evidence, recent studies support AD to result from complex etiological alterations. Among them, aging is the strongest risk factor for the vast majority of AD cases: Sporadic late onset AD (LOAD). Accumulation of DNA damage is a well-established aging factor. In this regard, a large amount of evidence reveals DNA damage as a critical pathological cause of AD. Clinically, DNA damage is accumulated in brains of AD patients. Genetically, defects in DNA damage repair resulted from mutations in the BRAC1 and other DNA damage repair genes occur in AD brain and facilitate the pathogenesis. Abnormalities in DNA damage repair can be used as diagnostic biomarkers for AD. In this review, we discuss the association, the causative potential, and the biomarker values of DNA damage in AD pathogenesis.

Protein synthesis and quality control in aging

Aging is characterized by the accumulation of damage and other deleterious changes, leading to the loss of functionality and fitness. Age-related changes occur at most levels of organization of a living organism (molecular, organellar, cellular, tissue and organ). However, protein synthesis is a major biological process, and thus understanding how it changes with age is of paramount importance. Here, we discuss the relationships between lifespan, aging, protein synthesis and translational control, and expand this analysis to the various aspects of proteome behavior in organisms with age. Characterizing the consequences of changes in protein synthesis and translation fidelity, and determining whether altered translation is pathological or adaptive is necessary for understanding the aging process, as well as for developing approaches to target dysfunction in translation as a strategy for extending lifespan.

Single-cell whole-genome sequencing reveals the functional landscape of somatic mutations in B lymphocytes across the human lifespan

Accumulation of mutations in somatic cells has been implicated as a cause of aging since the 1950s. However, attempts to establish a causal relationship between somatic mutations and aging have been constrained by the lack of methods to directly identify mutational events in primary human tissues. Here we provide genome-wide mutation frequencies and spectra of human B lymphocytes from healthy individuals across the entire human lifespan using a highly accurate single-cell whole-genome sequencing method. We found that the number of somatic mutations increases from <500 per cell in newborns to >3,000 per cell in centenarians. We discovered mutational hotspot regions, some of which, as expected, were located at Ig genes associated with somatic hypermutation (SHM). B cell-specific mutation signatures associated with development, aging, or SHM were found. The SHM signature strongly correlated with the signature found in human B cell tumors, indicating that potential cancer-causing events are already present even in B cells of healthy individuals. We also identified multiple mutations in sequence features relevant to cellular function (i.e., transcribed genes and gene regulatory regions). Such mutations increased significantly during aging, but only at approximately one-half the rate of the genome average, indicating selection against mutations that impact B cell function. This full characterization of the landscape of somatic mutations in human B lymphocytes indicates that spontaneous somatic mutations accumulating with age can be deleterious and may contribute to both the increased risk for leukemia and the functional decline of B lymphocytes in the elderly.

Frequent homozygous deletions of the CDKN2A locus in somatic cancer tissues

Here we present and describe data on homozygous deletions (HD) of human CDKN2 A and neighboring regions on the p arm of Chromosome 9 from cancer genome sequences deposited on the online Catalogue of Somatic Mutations in Cancer (COSMIC) database. Although CDKN2 A HDs have been previously described in many cancers, this is a pan-cancer report of these aberrations with the aim to map the distribution of the breakpoints. We find that HDs of this locus have a median range of 1,255,650bps. When the deletion breakpoints were mapped on both the telomere and centromere proximal sides of CDKN2A, most of the telomere proximal breakpoints concentrate to a narrow region of the chromosome which includes the gene MTAP.. The centromere proximal breakpoints of the deletions are distributed over a wider chromosomal region. Furthermore, gene expression analysis shows that the deletions that include the CDKN2A region also include the MTAP region and this observation is tissue independent. We propose a model that may explain the origin of the telomere proximal CDKN2A breakpoints Finally, we find that HD distributions for at least three other loci, RB1, SMAD4 and PTEN are also not random.

Burden of unique and low prevalence somatic mutations correlates with cancer survival

Tumor mutational burden correlates with improved survival and immunotherapy response in some malignancies, and with tumor aggressiveness in others. To study the link between mutational burden and survival, we analyzed survival effects of tumor exonic missense mutation burden (TEMMB) across 6947 specimens spanning 31 cancers which have undergone whole exome sequencing as part of TCGA. We adjusted TEMMB for age, sex, stage, and recruitment center, and computed Cox-proportional models of TEMMB survival effects. We assigned a recurrence score (RS) to each cohort, defining RS as the burden of recurrent mutations exceeding 1% population prevalence. High TEMMB was associated with improved survival in cutaneous melanoma: hazard ratio (HR) = 0.71 [0.60–0.85], p = 0.0002, urothelial bladder carcinoma: HR = 0.74 [0.59–0.93], p = 0.01, and ovarian carcinoma: HR = 0.80 [0.70–0.93], p = 0.003. High TEMMB was associated with decreased survival in colorectal adenocarcinoma: HR = 1.32 [1.00–1.74], p < 0.05. We identified that TEMMB survival effects were governed by the balance of recurrent and non-recurrent mutations. In cancers with a low RS, high TEMMB was correlated with better survival outcomes (r = 0.49, p = 0.02). In conclusion, TEMMB effects on survival depend on recurrent mutation enrichment; tumor types that are highly enriched in passenger mutations show a survival benefit in the setting of high tumor mutational burden.

An integrative theory for cancer (Review)

In the integrative theory, chronic irritations induce tumors with genetic alterations and rapid proliferative ability. Tumor cells reprogram the metabolism and employ aerobic glycolysis to sustain rapid growth. The host provides both the nutrients and exhaust system to support tumor growth via the tumor microenvironment. Under certain conditions, such as aging, diabetes, obesity and a high‑fat diet, the exhaust system is impaired, triggering a metabolic imbalance between the tumor and host. This is similar to a problematic car with an advanced motor with an out‑of‑date exhaust system. The metabolic imbalance causes a metabolic catastrophe, making tumor cells reside in a dismal environment and forcing them to invade, metastasize and undergo necrosis. Tumor necrosis, particularly in metastases, leads to non‑specific systemic inflammation, which is the major cause of cancer‑related mortality. On the whole, the integrative theory views cancer in an integrative manner and proposes that both genetic alterations and tumor‑host interaction as regards metabolism and immunology determine the destiny of the tumor and host. Although cancer is a genetic disease, tumor biology is basically the nature of the host.

Somatic mutation load and spectra: A record of DNA damage and repair in healthy human cells

Somatic genome instability is a hallmark of cancer genomes and has been linked to aging and a variety of other pathologies. Large-scale cancer genome and exome sequencing have revealed that mutation load and spectra in cancers can be influenced by environmental exposures, the anatomical site of exposures, and tissue type. There is now an abundance of data favoring the hypothesis that a substantial portion of the mutations in cancers originate prior to carcinogenesis in stem cells of the healthy individual. Rapid advances in sequencing of noncancer cells from healthy humans have shown that their mutation loads and spectra resemble cancer data. Similar to cancer genomes, mutation profiles of healthy cells show marked intra-individual variation, thus providing a metric of the various factors-environmental and endogenous-involved in mutagenesis in these individuals. This review focuses on the current methodologies to measure mutation loads and to determine mutation signatures for evaluating the environmental and endogenous sources of DNA damage in human somatic cells. We anticipate that in future, such large-scale studies aimed at exploring the landscapes of somatic mutations across different cell types in healthy people would provide a valuable resource for designing personalized preventative strategies against diseases associated with somatic genome instability. Environ. Mol. Mutagen. 59:672-686, 2018. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Somatic Mutagenesis in Mammals and Its Implications for Human Disease and Aging

DNA mutations as a consequence of errors during DNA damage repair, replication, or mitosis are the substrate for evolution. In multicellular organisms, mutations can occur in the germline and also in somatic tissues, where they are associated with cancer and other chronic diseases and possibly with aging. Recent advances in high-throughput sequencing have made it relatively easy to study germline de novo mutations, but in somatic cells, the vast majority of mutations are low-abundant and can be detected only in clonal lineages, such as tumors, or single cells. Here we review recent results on somatic mutations in normal human and animal tissues with a focus on their possible functional consequences.

The cytokinesis-blocked micronucleus assay as a novel biomarker for selection of lung cancer screening participants

Background

Despite the promising results of the National Lung Screening Trial in reducing lung cancer mortality among high risk smokers, several challenges remain to be addressed. These include the high false positive rates and the large number of smokers screened in order to prevent one lung cancer death. In addition, host genetic susceptibility has not been integrated into selection of who should be screened. These challenges highlight the need to develop robust ways to identify susceptible smokers for appropriate screening.

Methods

We used the cytokinesis block micronucleus (CBMN) assay to assess smoking induced genetic instability among NLST participants. Blood cultures were prepared at time of entry into the screening study and DNA damage was recorded as the frequency of binucleated nucleoplasmic bridges and micronuclei. Low dose CT (LDCT) and chest X-ray (CXR) image findings were available upon unblinding of the NLST study and imaging data were merged with blood marker data for statistical analysis.

Results

A total of 641 participants were included in this study. The frequency of the CBMN endpoints at time of entry into the study was significantly higher among study participants who had a positive finding during the 3-year screening or reported lung cancer at the end of the follow-up period as compared to participants who were negative. Growth curve models were used to compare trajectories of change in CBMN endpoints between entry into the study and end-of-screening period. A statistically significant increase was predicted for CBMN endpoints among the study participants who were positive versus those who remained negative at the end-of-screening period (P<0.001).

Conclusions

Genetic instability biomarkers have the potential of facilitating the identification of genetically susceptible high-risk smokers who would benefit from targeted lung screening programs.

PIWI-piRNA pathway: Setting the pace of aging by reducing DNA damage

Transposable elements (TEs) are powerful drivers of genome evolutionary dynamics but are principally deleterious to the host organism by compromising the integrity and function of the genome. The transposition of TEs may result in mutations and DNA damage. DNA double-strand breaks (DSBs), which may be caused by the transposition, are one of the processes directly linked to aging. TEs may thus be considered to constitute an internal source of aging and the frequency of transposition may, in turn, be considered to affect the pace of aging. The PIWI-piRNA pathway is a widespread strategy used by most animals to effectively suppress transposition. Interestingly, the PIWI-piRNA pathway is expressed predominantly in the animal germline, a more or less continuous immortal lineage set aside after the first few cell divisions of a developing embryo. Recent findings further imply that the PIWI-piRNA pathway and TE suppression constitute an important mechanism regulating aging. This article discusses the proposed role of the PIWI-piRNA pathway in setting the pace of aging as well as the possible mechanisms underlying this process.

Molecular Mechanisms Determining Lifespan in Short- and Long-Lived Species

Aging is a global decline of physiological functions, leading to an increased susceptibility to diseases and ultimately death. Maximum lifespans differ up to 200-fold between mammalian species. Although considerable progress has been achieved in identifying conserved pathways that regulate individual lifespan within model organisms, whether the same pathways are responsible for the interspecies differences in longevity remains to be determined. Recent cross-species studies have begun to identify pathways responsible for interspecies differences in lifespan. Here, we review the evidence supporting the role of anticancer mechanisms, DNA repair machinery, insulin/insulin-like growth factor 1 signaling, and proteostasis in defining species lifespans. Understanding the mechanisms responsible for the dramatic differences in lifespan between species will have a transformative effect on developing interventions to improve human health and longevity.

Genome instability: a conserved mechanism of ageing?

DNA is the carrier of genetic information and the primary template from which all cellular information is ultimately derived. Changes in the DNA information content through mutation generate diversity for evolution through natural selection but are also a source of deleterious effects. It has since long been hypothesized that mutation accumulation in somatic cells of multicellular organisms could causally contribute to age-related cellular degeneration and death. Assays to detect different types of mutations, from base substitutions to large chromosomal aberrations, have been developed and show unequivocally that mutations accumulate in different tissues and cell types of ageing humans and animals. More recently, next-generation sequencing-based methods have been developed to accurately determine the complete landscape of base substitution mutations in single cells. The first results show that the somatic mutation rate is much higher than the germline mutation rate and that base substitution loads in somatic cells are high enough to potentially affect cellular function.

A high-fidelity method for genomic sequencing of single somatic cells reveals a very high mutational burden

Postzygotic mutations in somatic cells lead to genome mosaicism and can be the cause of cancer, possibly other human diseases and aging. Somatic mutations are difficult to detect in bulk tissue samples. Here, we review the available assays for measuring somatic mutations, with a focus on recent single-cell, whole genome sequencing methods. Impact statement Somatic mutations cause cancer, possibly other diseases and aging. Yet, very little is known about the frequency of such mutations in vivo, their distribution across the genome, and their possible functional consequences other than cancer. Even in cancer, we do not know the heterogeneity of mutations within a tumor and if seemingly normal cells in its surroundings already have elevated mutation frequencies. Here, we review a new, whole genome amplification system that allows accurate quantification and characterization of single-cell mutational landscapes in human cells and tissues in relation to disease.

Molecular and phenotypic biomarkers of aging

Individuals of the same age may not age at the same rate. Quantitative biomarkers of aging are valuable tools to measure physiological age, assess the extent of ‘healthy aging’, and potentially predict health span and life span for an individual. Given the complex nature of the aging process, the biomarkers of aging are multilayered and multifaceted. Here, we review the phenotypic and molecular biomarkers of aging. Identifying and using biomarkers of aging to improve human health, prevent age-associated diseases, and extend healthy life span are now facilitated by the fast-growing capacity of multilevel cross-sectional and longitudinal data acquisition, storage, and analysis, particularly for data related to general human populations. Combined with artificial intelligence and machine learning techniques, reliable panels of biomarkers of aging will have tremendous potential to improve human health in aging societies.

Expression of progerin does not result in an increased mutation rate

In the premature ageing disease Hutchinson-Gilford progeria syndrome (HGPS), the underlying genetic defect in the lamin A gene leads to accumulation at the nuclear lamina of progerin—a mutant form of lamin A that cannot be correctly processed. This has been reported to result in defects in the DNA damage response and in DNA repair, leading to the hypothesis that, as in normal ageing and in other progeroid syndromes caused by mutation of genes of the DNA repair and DNA damage response pathways, increased DNA damage may be responsible for the premature ageing phenotypes in HGPS patients. However, this hypothesis is based upon the study of markers of the DNA damage response, rather than measurement of DNA damage per se or the consequences of unrepaired DNA damage—mutation. Here, using a mutation reporter cell line, we directly compared the inherent and induced mutation rates in cells expressing wild-type lamin A or progerin. We find no evidence for an elevated mutation rate in progerin-expressing cells. We conclude that the cellular defect in HGPS cells does not lie in the repair of DNA damage per se.

Aging impairs double-strand break repair by homologous recombination in Drosophila germ cells

Aging is characterized by genome instability, which contributes to cancer formation and cell lethality leading to organismal decline. The high levels of DNA double-strand breaks (DSBs) observed in old cells and premature aging syndromes are likely a primary source of genome instability, but the underlying cause of their formation is still unclear. DSBs might result from higher levels of damage or repair defects emerging with advancing age, but repair pathways in old organisms are still poorly understood. Here, we show that premeiotic germline cells of young and old flies have distinct differences in their ability to repair DSBs by the error-free pathway homologous recombination (HR). Repair of DSBs induced by either ionizing radiation (IR) or the endonuclease I-SceI is markedly defective in older flies. This correlates with a remarkable reduction in HR repair measured with the DR-white DSB repair reporter assay. Strikingly, most of this repair defect is already present at 8 days of age. Finally, HR defects correlate with increased expression of early HR components and increased recruitment of Rad51 to damage in older organisms. Thus, we propose that the defect in the HR pathway for germ cells in older flies occurs following Rad51 recruitment. These data reveal that DSB repair defects arise early in the aging process and suggest that HR deficiencies are a leading cause of genome instability in germ cells of older animals.

Cancer Immunotherapy: Whence and Whither

The current concepts and practice of cancer immunotherapy evolved from classical experiments that distinguished "self" from "non-self" and the finding that humoral immunity is complemented by cellular immunity. Elucidation of the biology underlying immune checkpoints and interactions between ligands and ligand receptors that govern the immune system's ability to recognize tumor cells as foreign has led to the emergence of new strategies that mobilize the immune system to reverse this apparent tolerance. Some of these approaches have led to new therapies such as the use of mAbs to interfere with the immune checkpoint. Others have exploited molecular technologies to reengineer a subset of T cells to directly engage and kill tumor cells, particularly those of B-cell malignancies. However, before immunotherapy can become a more effective method of cancer care, there are many challenges that remain to be addressed and hurdles to overcome. Included are manipulation of tumor microenvironment (TME) to enhance T effector cell infiltration and access to the tumor, augmentation of tumor MHC expression for adequate presentation of tumor associated antigens, regulation of cytokines and their potential adverse effects, and reduced risk of secondary malignancies as a consequence of mutations generated by the various forms of genetic engineering of immune cells. Despite these challenges, the future of immunotherapy as a standard anticancer therapy is encouraging. Mol Cancer Res; 15(6); 635-50. ©2017 AACR.

Evaluation of the LacZ reporter assay in cryopreserved primary hepatocytes for In vitro genotoxicity testing

Assessment of genotoxic potential is an important step in the safety evaluation of chemical substances. Under most regulatory jurisdictions, the first tier of testing comprises a standard battery of in vitro genotoxicity tests in bacterial and mammalian cells. However, the mammalian cell tests commonly used exhibit a relatively high rate of misleading positive results, which may lead to unnecessary in vivo testing. We previously established a proof-of-concept for the LacZ reporter assay in proliferating primary hepatocytes as a promising alternative genotoxicity test. Here, cryopreserved instead of freshly isolated hepatocytes were used and the assay was evaluated in more detail. We examined the effect of cryopreservation on phenotype and metabolic capacity of the LacZ hepatocytes, and assessed the predictive performance of the assay by testing a set of substances comprising true positive, true negative, and misleading positive substances. Additionally, a historical negative control database was created and the type of mutations induced was analyzed for two of the substances tested. Our findings indicate that proliferating cryopreserved primary hepatocytes derived from LacZ plasmid mice retain their hepatocyte-specific characteristics and metabolic competence. Furthermore, we demonstrate that both gene mutations and genome rearrangements due to large deletions can be detected with the LacZ reporter assay. The assay seems to have a lower rate of misleading positive test results compared to the assays currently used. Together, our findings strongly support the use of the LacZ reporter assay in cryopreserved primary hepatocytes as follow-up to the standard in vitro test battery for genotoxicity testing. Environ. Mol. Mutagen. 57:643-655, 2016. © 2016 Wiley Periodicals, Inc.

DNA repair in species with extreme lifespan differences

Differences in DNA repair capacity have been hypothesized to underlie the great range of maximum lifespans among mammals. However, measurements of individual DNA repair activities in cells and animals have not substantiated such a relationship because utilization of repair pathways among animals—depending on habitats, anatomical characteristics, and life styles—varies greatly between mammalian species. Recent advances in high-throughput genomics, in combination with increased knowledge of the genetic pathways involved in genome maintenance, now enable a comprehensive comparison of DNA repair transcriptomes in animal species with extreme lifespan differences. Here we compare transcriptomes of liver, an organ with high oxidative metabolism and abundant spontaneous DNA damage, from humans, naked mole rats, and mice, with maximum lifespans of ∼120, 30, and 3 years, respectively, with a focus on genes involved in DNA repair. The results show that the longer-lived species, human and naked mole rat, share higher expression of DNA repair genes, including core genes in several DNA repair pathways. A more systematic approach of signaling pathway analysis indicates statistically significant upregulation of several DNA repair signaling pathways in human and naked mole rat compared with mouse. The results of this present work indicate, for the first time, that DNA repair is upregulated in a major metabolic organ in long-lived humans and naked mole rats compared with short-lived mice. These results strongly suggest that DNA repair can be considered a genuine longevity assurance system.

Growth conditions that increase or decrease lifespan in Saccharomyces cerevisiae lead to corresponding decreases or increases in rates of interstitial deletions and non-reciprocal translocations

BACKGROUND

Accumulation of DNA damage, mutations, and chromosomal abnormalities is associated with aging in many organisms. How directly various forms of genomic instability contribute to lifespan in different aging contexts is still under active investigation. Testing whether treatments that alter lifespan change mutation rates early during lifespan could provide support for genomic instability being at least partly responsible for changes in the rates of aging.

RESULTS

Rates of mutations, direct repeat recombination, or retrotransposition were measured in young cell populations from two strain backgrounds of Saccharomyces cerevisiae exposed to several growth conditions that shortened or extended yeast chronological lifespan. In most cases, rates of genomic instability did not consistently increase in young cells exposed to lifespan-shortening conditions or decrease in young cells exposed to lifespan-extending conditions. The mutation rate for a copy of the CAN1 gene integrated onto the right arm of chromosome VIII did show expected increases or decreases in young cells in the lifespan-altering growth conditions. These mutations were determined to frequently result from non-allelic recombination events, including non-reciprocal translocations, and were more strongly stimulated by using hydroxyurea to induce DNA replication stress than by the general DNA-damaging agent methyl methanesulfonate.

CONCLUSIONS

The results are not consistent with changes in mutation rates in general mediating the influence of alternative growth conditions on yeast lifespan. The strong correlation between non-allelic recombination events and the effects of the alternative growth conditions on lifespan indicates that genomic instability due to changes in recombination rates may directly contribute to the rate of aging or that lifespan-altering treatments may consistently increase or decrease DNA replication stress. These results further support the connection between DNA replication stress and aging observed in multiple organisms. Chromosomal abnormalities that likely arise from recombination events are more prevalent in multiple human tissues with increasing age, and further work in yeast could help to define mechanisms responsible for this observation and the impact of chromosomal abnormalities on aging.

Do DNA Double-Strand Breaks Drive Aging?

DNA double-strand breaks (DSBs) are rare, but highly toxic, lesions requiring orchestrated and conserved machinery to prevent adverse consequences, such as cell death and cancer-causing genome structural mutations. DSBs trigger the DNA damage response (DDR) that directs a cell to repair the break, undergo apoptosis, or become senescent. There is increasing evidence that the various endpoints of DSB processing by different cells and tissues are part of the aging phenotype, with each stage of the DDR associated with specific aging pathologies. In this Perspective, we discuss the possibility that DSBs are major drivers of intrinsic aging, highlighting the dynamics of spontaneous DSBs in relation to aging, the distinct age-related pathologies induced by DSBs, and the segmental progeroid phenotypes in humans and mice with genetic defects in DSB repair. A model is presented as to how DSBs could drive some of the basic mechanisms underlying age-related functional decline and death.

Age-related somatic mutations in the cancer genome

Aging is associated with an increased risk of cancer, possibly in part because of an age-related increase in mutations in normal tissues. Due to their extremely low abundance, somatic mutations in normal tissues frequently escape detection. Tumors, as clonal expansions of single cells, can provide information about the somatic mutations present in these cells prior to tumorigenesis.

Here, we used data from The Cancer Genome Atlas (TCGA), to systematically study the frequency and spectrum of somatic mutations in a total of 6,969 patients and 34 different tumor types as a function of the age of the patient. After using linear modeling to control for the age structure of different tumor types, we found that the number of identified somatic mutations increases exponentially with age. Using additional data from the literature, we found that accumulation of somatic mutations is associated with cell division rate, cancer risk and cigarette smoking, with the latter also associated with a distinct spectrum of mutations.

Our results confirm that aging is associated with the accumulation of somatic mutations, and strongly suggest that the level of genome instability of normal cells, modified by both endogenous and environmental factors, is the main risk factor for cancer.

Impaired DNA double-strand break repair contributes to the age-associated rise of genomic instability in humans

Failing to repair DNA double-strand breaks by either nonhomologous end joining (NHEJ) or homologous recombination (HR) poses a threat to genome integrity, and may have roles in the onset of aging and age-related diseases. Recent work indicates an age-related decrease of NHEJ efficiency in mouse models, but whether NHEJ and HR change with age in humans and the underlying mechanisms of such a change remain uncharacterized. Here, using 50 eyelid fibroblast cell lines isolated from healthy donors at the age of 16-75 years, we demonstrate that the efficiency and fidelity of NHEJ, and the efficiency of HR decline with age, leading to increased IR sensitivity in cells isolated from old donors. Mechanistic analysis suggests that decreased expression of XRCC4, Lig4 and Lig3 drives the observed, age-associated decline of NHEJ efficiency and fidelity. Restoration of XRCC4 and Lig4 significantly promotes the fidelity and efficiency of NHEJ in aged fibroblasts. In contrast, essential HR-related factors, such as Rad51, do not change in expression level with age, but Rad51 exhibits a slow kinetics of recruitment to DNA damage sites in aged fibroblasts. Further rescue experiments indicate that restoration of XRCC4 and Lig4 may suppress the onset of stress-induced premature cellular senescence, suggesting that improving NHEJ efficiency and fidelity by targeting the NHEJ pathway holds great potential to delay aging and mitigate aging-related pathologies.

Reprint of "DNA, the central molecule of aging"

Understanding the molecular mechanism of aging could have enormous medical implications. Despite a century of research, however, there is no universally accepted theory regarding the molecular basis of aging. On the other hand, there is plentiful evidence suggesting that DNA constitutes the central molecule in this process. Here, we review the roles of chromatin structure, DNA damage, and shortening of telomeres in aging and propose a hypothesis for how their interplay leads to aging phenotypes.

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.

DNA, the central molecule of aging

Understanding the molecular mechanism of aging could have enormous medical implications. Despite a century of research, however, there is no universally accepted theory regarding the molecular basis of aging. On the other hand, there is plentiful evidence suggesting that DNA constitutes the central molecule in this process. Here, we review the roles of chromatin structure, DNA damage, and shortening of telomeres in aging and propose a hypothesis for how their interplay leads to aging phenotypes.

Stem Cell-Specific Mechanisms Ensure Genomic Fidelity within HSCs and upon Aging of HSCs

Whether aged hematopoietic stem and progenitor cells (HSPCs) have impaired DNA damage repair is controversial. Using a combination of DNA mutation indicator assays, we observe a 2- to 3-fold increase in the number of DNA mutations in the hematopoietic system upon aging. Young and aged hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) do not show an increase in mutation upon irradiation-induced DNA damage repair, and young and aged HSPCs respond very similarly to DNA damage with respect to cell-cycle checkpoint activation and apoptosis. Both young and aged HSPCs show impaired activation of the DNA-damage-induced G1-S checkpoint. Induction of chronic DNA double-strand breaks by zinc-finger nucleases suggests that HSPCs undergo apoptosis rather than faulty repair. These data reveal a protective mechanism in both the young and aged hematopoietic system against accumulation of mutations in response to DNA damage.

Toward an evolutionary model of cancer: Considering the mechanisms that govern the fate of somatic mutations

Our understanding of cancer has greatly advanced since Nordling [Nordling CO (1953) Br J Cancer 7(1):68-72] and Armitage and Doll [Armitage P, Doll R (1954) Br J Cancer 8(1):1-12] put forth the multistage model of carcinogenesis. However, a number of observations remain poorly understood from the standpoint of this paradigm in its contemporary state. These observations include the similar age-dependent exponential rise in incidence of cancers originating from stem/progenitor pools differing drastically in size, age-dependent cell division profiles, and compartmentalization. This common incidence pattern is characteristic of cancers requiring different numbers of oncogenic mutations, and it scales to very divergent life spans of mammalian species. Also, bigger mammals with larger underlying stem cell pools are not proportionally more prone to cancer, an observation known as Peto's paradox. Here, we present a number of factors beyond the occurrence of oncogenic mutations that are unaccounted for in the current model of cancer development but should have significant impacts on cancer incidence. Furthermore, we propose a revision of the current understanding for how oncogenic and other functional somatic mutations affect cellular fitness. We present evidence, substantiated by evolutionary theory, demonstrating that fitness is a dynamic environment-dependent property of a phenotype and that oncogenic mutations should have vastly different fitness effects on somatic cells dependent on the tissue microenvironment in an age-dependent manner. Combined, this evidence provides a firm basis for understanding the age-dependent incidence of cancers as driven by age-altered systemic processes regulated above the cell level.

Controlled induction of DNA double-strand breaks in the mouse liver induces features of tissue ageing

DNA damage has been implicated in ageing, but direct evidence for a causal relationship is lacking, owing to the difficulty of inducing defined DNA lesions in cells and tissues without simultaneously damaging other biomolecules and cellular structures. Here we directly test whether highly toxic DNA double-strand breaks (DSBs) alone can drive an ageing phenotype using an adenovirus-based system based on tetracycline-controlled expression of the SacI restriction enzyme. We deliver the adenovirus to mice and compare molecular and cellular end points in the liver with normally aged animals. Treated, 3-month-old mice display many, but not all signs of normal liver ageing as early as 1 month after treatment, including ageing pathologies, markers of senescence, fused mitochondria and alterations in gene expression profiles. These results, showing that DSBs alone can cause distinct ageing phenotypes in mouse liver, provide new insights in the role of DNA damage as a driver of tissue ageing.

Accumulation of DNA damage is a hallmark of cellular ageing but cause and effect are unclear. Here White et al. induce clean DNA double-strand breaks in the liver of mice using a modified restriction enzyme and demonstrate that DNA damage alone is sufficient to recapitulate some aspects of tissue ageing.

The impact of base excision DNA repair in age-related neurodegenerative diseases

The aging process and several age-related neurodegenerative disorders have been linked to elevated levels of DNA damage induced by ROS and deficiency in DNA repair mechanisms. DNA damage induced by ROS is a byproduct of cellular respiration and accumulation of damage over time, is a fundamental aspect of a main theory of aging. Mitochondria have a pivotal role in generating cellular oxidative stress, and mitochondrial dysfunction has been associated with several diseases. DNA base excision repair is considered the major pathway for repair of oxidized bases in DNA both in the nuclei and in mitochondria, and in neurons this mechanism is particularly important because non-diving cells have limited back-up DNA repair mechanisms. An association between elevated oxidative stress and a decrease in BER is strongly related to the aging process and has special relevance in age-related neurodegenerative diseases. Here, we review the role of DNA repair in aging, focusing on the implications of the DNA base excision repair pathways and how alterations in expression of these DNA repair proteins are related to the aging process and to age-related neurodegenerative diseases.

Somatic mutations, genome mosaicism, cancer and aging

Genomes are inherently unstable due to the need for DNA sequence variation in the germ line to fuel evolution through natural selection. In somatic tissues mutations accumulate during development and aging, generating genome mosaics. There is little information about the possible causal role of increased somatic mutation loads in late-life disease and aging, with the exception of cancer. Characterizing somatic mutations and their functional consequences in normal tissues remains a formidable challenge due to their low, individual abundance. Here, I will briefly review our current knowledge of somatic mutations in animals and humans in relation to aging, how they arise and lead to genome mosaicism, the technology to study somatic mutations and how they possibly could cause non-clonal disease.

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.

DNA damage and its links to neurodegeneration

The integrity of our genetic material is under constant attack from numerous endogenous and exogenous agents. The consequences of a defective DNA damage response are well studied in proliferating cells, especially with regards to the development of cancer, yet its precise roles in the nervous system are relatively poorly understood. Here we attempt to provide a comprehensive overview of the consequences of genomic instability in the nervous system. We highlight the neuropathology of congenital syndromes that result from mutations in DNA repair factors and underscore the importance of the DNA damage response in neural development. In addition, we describe the findings of recent studies, which reveal that a robust DNA damage response is also intimately connected to aging and the manifestation of age-related neurodegenerative disorders such as Alzheimer's disease and amyotrophic lateral sclerosis.