Correlation between senescence and DNA repair in cells from young and old individuals and in premature aging syndromes

Developing transcriptomic signatures as a biomarker of cellular senescence

Cellular senescence is a cell fate driven by different types of stress, where damaged cells exit from the cell cycle and, in many cases, develop an inflammatory senescence-associated secretory phenotype (SASP). Senescence has often been linked to driving aging and the onset of multiple diseases conferred by the harmful SASP, which disrupts tissue homeostasis and impairs the regular function of many tissues. This phenomenon was first observed in vitro when fibroblasts halted replication after approximately 50 population doublings. In addition to replication-induced senescence, factors such as DNA damage and oncogene activation can induce cellular senescence both in culture and in vivo. Despite their contribution to aging and disease, identifying senescent cells in vivo has been challenging due to their heterogeneity. Although senescent cells can express the cell cycle inhibitors p16Ink4a and/or p21Cip1 and exhibit SA-ß-gal activity and evidence of a DNA damage response, there is no universal biomarker for these cells, regardless of inducer or cell type. Recent studies have analyzed the transcriptomic characteristics of these cells, leading to the identification of signature gene sets like CellAge, SeneQuest, and SenMayo. Advancements in single-cell and spatial RNA sequencing now allow for analyzing senescent cell heterogeneity within the same tissue and the development of machine learning algorithms, e.g., SenPred, SenSig, and SenCID, to discover cellular senescence using RNA sequencing data. Such insights not only deepen our understanding of the genetic pathways driving cellular senescence, but also promote the development of its quantifiable biomarkers. This review summarizes the current knowledge of transcriptomic signatures of cellular senescence and their potential as in vivo biomarkers.

Signatures of Extreme Longevity: A Perspective from Bivalve Molecular Evolution

Among Metazoa, bivalves have the highest lifespan disparity, ranging from 1 to 500+ years, making them an exceptional testing ground to understand mechanisms underlying aging and the evolution of extended longevity. Nevertheless, comparative molecular evolution has been an overlooked approach in this instance. Here, we leveraged transcriptomic resources spanning 30 bivalve species to unravel the signatures of convergent molecular evolution in four long-lived species: Margaritifera margaritifera, Elliptio complanata, Lampsilis siliquoidea, and Arctica islandica (the latter represents the longest-lived noncolonial metazoan known so far). We applied a comprehensive approach-which included inference of convergent dN/dS, convergent positive selection, and convergent amino acid substitution-with a strong focus on the reduction of false positives. Genes with convergent evolution in long-lived bivalves show more physical and functional interactions to each other than expected, suggesting that they are biologically connected; this interaction network is enriched in genes for which a role in longevity has been experimentally supported in other species. This suggests that genes in the network are involved in extended longevity in bivalves and, consequently, that the mechanisms underlying extended longevity are-at least partially-shared across Metazoa. Although we believe that an integration of different genes and pathways is required for the extended longevity phenotype, we highlight the potential central roles of genes involved in cell proliferation control, translational machinery, and response to hypoxia, in lifespan extension.

Neurodevelopmental disorders and microcephaly: how apoptosis, the cell cycle, tau and amyloid-β precursor protein APPly

Recent studies promote new interest in the intersectionality between autism spectrum disorder (ASD) and Alzheimer's Disease. We have reported high levels of Amyloid-β Precursor Protein (APP) and secreted APP-alpha (sAPPa ) and low levels of amyloid-beta (Aβ) peptides 1-40 and 1-42 (Aβ40, Aβ42) in plasma and brain tissue from children with ASD. A higher incidence of microcephaly (head circumference less than the 3rd percentile) associates with ASD compared to head size in individuals with typical development. The role of Aβ peptides as contributors to acquired microcephaly in ASD is proposed. Aβ may lead to microcephaly via disruption of neurogenesis, elongation of the G1/S cell cycle, and arrested cell cycle promoting apoptosis. As the APP gene exists on Chromosome 21, excess Aβ peptides occur in Trisomy 21-T21 (Down's Syndrome). Microcephaly and some forms of ASD associate with T21, and therefore potential mechanisms underlying these associations will be examined in this review. Aβ peptides' role in other neurodevelopmental disorders that feature ASD and acquired microcephaly are reviewed, including dup 15q11.2-q13, Angelman and Rett syndrome.

Cellular senescence and neurodegeneration

Advancing age is a major risk factor of Alzheimer's disease (AD). The worldwide prevalence of AD is approximately 50 million people, and this number is projected to increase substantially. The molecular mechanisms underlying the aging-associated susceptibility to cognitive impairment in AD are largely unknown. As a hallmark of aging, cellular senescence is a significant contributor to aging and age-related diseases including AD. Senescent neurons and glial cells have been detected to accumulate in the brains of AD patients and mouse models. Importantly, selective elimination of senescent cells ameliorates amyloid beta and tau pathologies and improves cognition in AD mouse models, indicating a critical role of cellular senescence in AD pathogenesis. Nonetheless, the mechanisms underlying when and how cellular senescence contributes to AD pathogenesis remain unclear. This review provides an overview of cellular senescence and discusses recent advances in the understanding of the impact of cellular senescence on AD pathogenesis, with brief discussions of the possible role of cellular senescence in other neurodegenerative diseases including Down syndrome, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis.

Fragile sites, chromosomal lesions, tandem repeats, and disease

Expanded tandem repeat DNAs are associated with various unusual chromosomal lesions, despiralizations, multi-branched inter-chromosomal associations, and fragile sites. Fragile sites cytogenetically manifest as localized gaps or discontinuities in chromosome structure and are an important genetic, biological, and health-related phenomena. Common fragile sites (∼230), present in most individuals, are induced by aphidicolin and can be associated with cancer; of the 27 molecularly-mapped common sites, none are associated with a particular DNA sequence motif. Rare fragile sites ( ≳ 40 known), ≤ 5% of the population (may be as few as a single individual), can be associated with neurodevelopmental disease. All 10 molecularly-mapped folate-sensitive fragile sites, the largest category of rare fragile sites, are caused by gene-specific CGG/CCG tandem repeat expansions that are aberrantly CpG methylated and include FRAXA, FRAXE, FRAXF, FRA2A, FRA7A, FRA10A, FRA11A, FRA11B, FRA12A, and FRA16A. The minisatellite-associated rare fragile sites, FRA10B, FRA16B, can be induced by AT-rich DNA-ligands or nucleotide analogs. Despiralized lesions and multi-branched inter-chromosomal associations at the heterochromatic satellite repeats of chromosomes 1, 9, 16 are inducible by de-methylating agents like 5-azadeoxycytidine and can spontaneously arise in patients with ICF syndrome (Immunodeficiency Centromeric instability and Facial anomalies) with mutations in genes regulating DNA methylation. ICF individuals have hypomethylated satellites I-III, alpha-satellites, and subtelomeric repeats. Ribosomal repeats and subtelomeric D4Z4 megasatellites/macrosatellites, are associated with chromosome location, fragility, and disease. Telomere repeats can also assume fragile sites. Dietary deficiencies of folate or vitamin B12, or drug insults are associated with megaloblastic and/or pernicious anemia, that display chromosomes with fragile sites. The recent discovery of many new tandem repeat expansion loci, with varied repeat motifs, where motif lengths can range from mono-nucleotides to megabase units, could be the molecular cause of new fragile sites, or other chromosomal lesions. This review focuses on repeat-associated fragility, covering their induction, cytogenetics, epigenetics, cell type specificity, genetic instability (repeat instability, micronuclei, deletions/rearrangements, and sister chromatid exchange), unusual heritability, disease association, and penetrance. Understanding tandem repeat-associated chromosomal fragile sites provides insight to chromosome structure, genome packaging, genetic instability, and disease.

Aneuploidy, inflammation and diseases

This review discusses how numerical aneuploidy may trigger inflammation in somatic cells and its consequences. Therefore we: i) summarized current knowledge on the cellular and molecular pathological effects of aneuploidy; ii) considered which of these aspects are able to trigger inflammation; iii) determined the genetic and environmental factors which may modulate the link between aneuploidy and inflammation; iv) explored the rôle of diet in prevention of aneuploidy and inflammation; v) examined whether aneuploidy and inflammation are causes and/or consequences of diseases; vi) identified the knowledge gaps and research needed to translate these observations into improved health care and disease prevention. The relationships between aneuploidy, inflammation and diseases are complex, because they depend on which chromosomes are involved, the proportion of cells affected and which organs are aneuploid in the case of mosaic aneuploidy. Therefore, a systemic approach is recommended to understand the emergence of aneuploidy-driven diseases and to take preventive measures to protect individuals from exposure to aneugenic conditions.

"Micronuclei and Disease" special issue: Aims, scope, and synthesis of outcomes

The purpose of the "Micronuclei and Disease" special issue (SI) is to: (i) Determine the level of evidence for association of micronuclei (MN), a biomarker of numerical and structural chromosomal aberrations, with risk of specific diseases in humans; (ii) Define plausible mechanisms that explain association of MN with each disease; (iii) Identify knowledge gaps and research needed to translate MN assays into clinical practice. The "MN and Disease" SI includes 14 papers. The first is a review of mechanisms of MN formation and their consequences in humans. 11 papers are systematic reviews and/or meta-analyses of the association of MN with reproduction, child health, inflammation, auto-immune disease, glycation, metabolic diseases, chronic kidney disease, cardiovascular disease, eleven common cancers, ageing and frailty. The penultimate paper focuses on effect of interventions on MN frequency in the elderly. A road map for translation of MN data into clinical practice is the topic of the final paper. The majority of reviewed studies were case-control studies in which the ratio of mean MN frequency in disease cases relative to controls, i.e. the mean ratio (MR), was calculated. The mean of these MR values, estimated by meta-analyses, for lymphocyte and buccal cell MN in non-cancer diseases were 2.3 and 3.6 respectively, and for cancers they were 1.7 and 2.6 respectively. The highest MR values were observed in studies of cancer cases in which MN were measured in the same tissue as the tumour (MR = 4.9-10.8). This special issue is an important milestone in the evidence supporting MN as a reliable genomic biomarker of developmental and degenerative disease risk. These advances, together with results from prospective cohort studies, are helping to identify diseases in which MN assays can be practically employed in the clinical setting to better identify high risk patients and to prioritise them for preventive therapy.

Cellular senescence and Alzheimer disease: the egg and the chicken scenario

Globally, 50 million people live with dementia, with Alzheimer disease (AD) being responsible for two-thirds of the total cases. As ageing is the main risk factor for dementia-related neurodegeneration, changes in the timing or nature of the cellular hallmarks of normal ageing might be key to understanding the events that convert normal ageing into neurodegeneration. Cellular senescence is a candidate mechanism that might be important for this conversion. Under persistent stress, as occurs in ageing, both postmitotic cells - including neurons - and proliferative cells - such as astrocytes and microglia, among others - can engender a state of chronic cellular senescence that is characterized by the secretion of pro-inflammatory molecules that promote the functional decline of tissues and organs. Ablation of senescent cells has been postulated as a promising therapeutic venue to target the ageing phenotype and, thus, prevent or mitigate ageing-related diseases. However, owing to a lack of evidence, it is not possible to label cellular senescence as a cause or a consequence of neurodegeneration. This Review examines cellular senescence in the context of ageing and AD, and discusses which of the processes - cellular senescence or AD - might come first.

Cellular Senescence in Neurodegenerative Diseases

Cellular senescence is a homeostatic biological process characterized by a permanent state of cell cycle arrest that can contribute to the decline of the regenerative potential and function of tissues. The increased presence of senescent cells in different neurodegenerative diseases suggests the contribution of senescence in the pathophysiology of these disorders. Although several factors can induce senescence, DNA damage, oxidative stress, neuroinflammation, and altered proteostasis have been shown to play a role in its onset. Oxidative stress contributes to accelerated aging and cognitive dysfunction stages affecting neurogenesis, neuronal differentiation, connectivity, and survival. During later life stages, it is implicated in the progression of cognitive decline, synapse loss, and neuronal degeneration. Also, neuroinflammation exacerbates oxidative stress, synaptic dysfunction, and neuronal death through the harmful effects of pro-inflammatory cytokines on cell proliferation and maturation. Both oxidative stress and neuroinflammation can induce DNA damage and alterations in DNA repair that, in turn, can exacerbate them. Another important feature associated with senescence is altered proteostasis. Because of the disruption in the function and balance of the proteome, senescence can modify the proper synthesis, folding, quality control, and degradation rate of proteins producing, in some diseases, misfolded proteins or aggregation of abnormal proteins. There is an extensive body of literature that associates cellular senescence with several neurodegenerative disorders including Alzheimer’s disease (AD), Down syndrome (DS), and Parkinson’s disease (PD). This review summarizes the evidence of the shared neuropathological events in these neurodegenerative diseases and the implication of cellular senescence in their onset or aggravation. Understanding the role that cellular senescence plays in them could help to develop new therapeutic strategies.

Micronuclei and Their Association with Infertility, Pregnancy Complications, Developmental Defects, Anaemias, Inflammation, Diabetes, Chronic Kidney Disease, Obesity, Cardiovascular Disease, Neurodegenerative Diseases and Cancer

Micronuclei (MN) are a strong cytogenetic indicator of a catastrophic change in the genetic structure and stability of a cell because they originate from either chromosome breaks or whole chromosomes that have been lost from the main nucleus during cell division. The resulting genetic abnormalities can to lead to cellular malfunction, altered gene expression and impaired regenerative capacity. Furthermore, MN are increased as a consequence of genetic defects in DNA repair, deficiency in micronutrients required for DNA replication and repair and exposure to genotoxic chemicals and ultraviolet or ionising radiation. For all of these reasons, the measurement of MN has become one of the best-established methods to measure DNA damage in humans at the cytogenetic level. This chapter is a narrative review of the current evidence for the association of increased MN frequency with developmental and degenerative diseases. In addition, important knowledge gaps are identified, and recommendations for future studies required to consolidate the evidence are provided. The great majority of published studies show a significant association of increased MN in lymphocytes and/or buccal cells with infertility, pregnancy complications, developmental defects, anaemias, inflammation, diabetes, cardiovascular disease, kidney disease, neurodegenerative diseases and cancer. However, the strongest evidence is from prospective studies showing that MN frequency in lymphocytes predicts cancer risk and cardiovascular disease mortality.

Cellular senescence in brain aging and neurodegenerative diseases: evidence and perspectives

Along with a general decline in overall health, most chronic degenerative human diseases are inherently associated with increasing age. Age-associated cognitive impairments and neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, are potentially debilitating conditions that lack viable options for treatment, resulting in a tremendous economic and societal cost. Most high-profile clinical trials for neurodegenerative diseases have led to inefficacious results, suggesting that novel approaches to treating these pathologies are needed. Numerous recent studies have demonstrated that senescent cells, which are characterized by sustained cell cycle arrest and production of a distinct senescence-associated secretory phenotype, accumulate with age and at sites of age-related diseases throughout the body, where they actively promote tissue deterioration. Cells with features of senescence have been detected in the context of brain aging and neurodegenerative disease, suggesting that they may also promote dysfunction. Here, we discuss the evidence implicating senescent cells in neurodegenerative diseases, the mechanistic contribution of these cells that may actively drive neurodegeneration, and how these cells or their effects may be targeted therapeutically.

Trans-acting epigenetic effects of chromosomal aneuploidies: lessons from Down syndrome and mouse models

An important line of postgenomic research seeks to understand how genetic factors can influence epigenetic patterning. Here we review epigenetic effects of chromosomal aneuploidies, focusing on findings in Down syndrome (DS, trisomy 21). Recent work in human DS and mouse models has shown that the extra chromosome 21 acts in trans to produce epigenetic changes, including differential CpG methylation (DS-DM), in specific sets of downstream target genes, mostly on other chromosomes. Mechanistic hypotheses emerging from these data include roles of chromosome 21-linked methylation pathway genes (DNMT3L and others) and transcription factor genes (RUNX1, OLIG2, GABPA, ERG and ETS2) in shaping the patterns of DS-DM. The findings may have broader implications for trans-acting epigenetic effects of chromosomal and subchromosomal aneuploidies in other human developmental and neuropsychiatric disorders, and in cancers.

Micronucleus frequency in peripheral blood lymphocytes and frailty status in elderly. A lack of association with clinical features

Frailty is a condition of vulnerability that carries an increased risk of poor outcome in elder adults. Frail individuals show fatigue, weight loss, muscle weakness, and a reduced physical function, and are known to frequently experience disability, social isolation, and institutionalization. Identifying frail people is a critical step for geriatricians to provide timely geriatric care and, eventually, to improve the quality of life in elderly. The aim of the present study is to investigate the association between frailty status and micronucleus (MN) frequency, a known marker of genomic instability, in a sample of elder adults. Several clinical features were evaluated and their possible association with MN frequency was tested. Criteria proposed by Fried were used to identify frail subjects. Overall, 180 elder adults entered the study, 93 of them (51.7%) frail. No association between MN frequency and frailty status was found under the specific conditions tested in this study (mean ratio=1.06; 95% CI 0.96-1.18). The inclusion of MN frequency in the Fried's frailty scale minimally improved the classification of study subjects according to the multidimensional prognostic index (MPI). The presence of genomic instability in the ageing process and in most chronic diseases, demands further investigation on this issue.

GIT2 Acts as a Systems-Level Coordinator of Neurometabolic Activity and Pathophysiological Aging

Aging represents one of the most complicated and highly integrated somatic processes. Healthy aging is suggested to rely upon the coherent regulation of hormonal and neuronal communication between the central nervous system and peripheral tissues. The hypothalamus is one of the main structures in the body responsible for sustaining an efficient interaction between energy balance and neurological activity and therefore likely coordinates multiple systems in the aging process. We previously identified, in hypothalamic and peripheral tissues, the G protein-coupled receptor kinase interacting protein 2 (GIT2) as a stress response and aging regulator. As metabolic status profoundly affects aging trajectories, we investigated the role of GIT2 in regulating metabolic activity. We found that genomic deletion of GIT2 alters hypothalamic transcriptomic signatures related to diabetes and metabolic pathways. Deletion of GIT2 reduced whole animal respiratory exchange ratios away from those related to primary glucose usage for energy homeostasis. GIT2 knockout (GIT2KO) mice demonstrated lower insulin secretion levels, disruption of pancreatic islet beta cell mass, elevated plasma glucose, and insulin resistance. High-dimensionality transcriptomic signatures from islets isolated from GIT2KO mice indicated a disruption of beta cell development. Additionally, GIT2 expression was prematurely elevated in pancreatic and hypothalamic tissues from diabetic-state mice (db/db), compared to age-matched wild type (WT) controls, further supporting the role of GIT2 in metabolic regulation and aging. We also found that the physical interaction of pancreatic GIT2 with the insulin receptor and insulin receptor substrate 2 was diminished in db/db mice compared to WT mice. Therefore, GIT2 appears to exert a multidimensional "keystone" role in regulating the aging process by coordinating somatic responses to energy deficits.

Micronuclei as a biological endpoint for genotoxicity: A minireview

The in vitro micronucleus assay has now been applied in many laboratories. This endpoint is useful in biomonitoring or ecotoxicology, as a sensitivity measure of human cells in cancer treatment and also to replace or supplement other in vitro genotoxicity assays. Learning more about the mechanisms of micronucleus formation allows conclusions about its biological significance. It has been demonstrated that disturbance of the mitotic apparatus (spindle, kinetochores) as well as impaired function of topoisomerase II can be involved in micronucleus formation. In addition, the roles of changes in DNA-conformation that are induced by alterations in the status of cytosine-methylation and of the cellular DNA repair capacity have been shown. The fate of micronucleus-containing cells is not known: the cells may theoretically be cytostatic and micronucleus-formation may therefore be a way of the organism to eliminate genetic damage or the cells may survive the loss of that chromosomal material and develop into transformed cells. Published data and ideas of selected areas within this field are reviewed.

Association of micronucleus frequency with neurodegenerative diseases

Micronuclei (MNi) can originate either from chromosome breakage or chromosome malsegregation events and are therefore ideal biomarkers to investigate genomic instability. Studies in peripheral lymphocytes of patients with neurodegenerative diseases, mainly Alzheimer's disease (AD) and Parkinson's disease (PD), revealed an increased micronucleus (MN) frequency in both disorders but originating mainly from chromosome malsegregation events in AD and from chromosome breakage events in PD. Studies in other neurodegenerative diseases are largely missing, and some data in premature ageing disorders characterised by neurodegeneration and/or neurological complications, such as Ataxia telangiectasia, Werner's syndrome, Down's syndrome (DS) and Cockayne's syndrome, indicate that MNi increase with ageing in cultured cells. An increased frequency of aneuploidy characterises several tissues of AD patients, as well as of individuals at increased risk to develop AD, such as mothers of DS individuals and DS subjects themselves. The use of the buccal MN cytome assay in AD and DS subjects allowed finding significant changes in the MN frequency as well as other cellular modifications reflecting reduced regenerative capacity compared to age- and gender-matched controls. These changes in buccal cytome ratios may prove useful as potential future diagnostics to identify individuals of increased risk for these disorders.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

Relation between replicative senescence of human fibroblasts and life history characteristics

Replicative ageing of fibroblasts in vitro has often been used as a model for organismal ageing. The general assumption that the ageing process is mirrored by cellular senescence in vitro is based on lower replicative capacity of human fibroblasts from patients with accelerated ageing syndromes, patients with age related diseases such as diabetes mellitus, and donors of higher chronological age, but these inverse relations have not been reported unequivocally. Therefore, we have performed a formal review on the replicative capacity of fibroblasts from patients suffering from accelerated ageing syndromes, age related diseases and donor age. Some 13 studies including 79 patients with accelerated ageing syndromes showed replicative capacity of fibroblasts to be consistently lower when compared to fibroblasts obtained from age-matched controls. Some 12 studies reported on a total of 160 patients with various age related diseases, but compared to age-matched controls no consistent difference in replicative capacity was reported. Finally, in the period from 1964 to 2006 a total of 23 studies, including some 1115 individuals, reported on the relation between chronological age and replicative capacity of human fibroblasts. Earlier studies preferentially described an inverse relation between replicative capacity and chronological age that was absent in studies including higher numbers of subjects and were published more recently. There was marked heterogeneity between the studies (Egger test: p = 0.018) indicating that publication bias is at play. We conclude that, except for premature ageing syndromes, replicative capacity of fibroblasts in vitro does not mirror key characteristics of human life histories.

Genetic epidemiology in aging research

Over the last two decades, aging research has expanded to include not only age-related disease models, and conversely, longevity and disease-free models, but also focuses on biological mechanisms related to the aging process. By viewing aging on multiple research frontiers, we are rapidly expanding knowledge as a whole and mapping connections between biological processes and particular age-related diseases that emerge. This is perhaps most true in the field of genetics, where variation across individuals has improved our understanding of aging mechanisms, etiology of age-related disease, and prediction of therapeutic responses. A close partnership between gerontologists, epidemiologists, and geneticists is needed to take full advantage of emerging genome information and technology and bring about a new age for biological aging research. Here we review current genetic findings for aging across both disease-specific and aging process domains. We then highlight the limitations of most work to date in terms of study design, genomic information, and trait modeling and focus on emerging technology and future directions that can partner genetic epidemiology and aging research fields to best take advantage of the rapid discoveries in each.

Genotoxicity profiles of common alkyl halides and esters with alkylating activity

Drug synthesis and/or formulation can generate genotoxic impurities. For instance, strong acid/alcohol interactions during the process of drug salt formation produce alkylating agents such as alkyl halides and alkyl esters of alkyl sulfonic acids. The genotoxicity of a few classic alkylating agents such as methyl and ethyl methanesulfonate have been previously well characterized, whereas the majority of compounds from this class have only been tested in the Salmonella reversion assay. Therefore, the goal of this study was to investigate clastogenicity and DEL recombination profiles of 22 halogenated alkanes and alkylesters of sulfuric and alkane-, aryl-sulfonic acids using a battery of cellular and molecular assays. The in-vitro micronucleus assay in CHO cells was used to measure clastogenicity and the deletion recombination (DEL) assay in S. cerevisiae provided a measure of DNA deletions. We also examined the compounds' reactivity towards 4-(p-nitrobenzyl)pyridine (NBP), a surrogate molecule for biological ring nitrogens. Methylating agents were most potent in all three assays and the alkyl chlorides evaluated in our study were negative in all three assays. Also, a strong correlation was found between the MN, DEL and NBP assays. In summary, this study contributes to a better understanding of the genotoxic properties of common alkyl halides and alkyl esters with alkylating activity and might provide guidance for managing risk of genotoxic process-related impurities of drug substances and products.

The buccal cytome and micronucleus frequency is substantially altered in Down's syndrome and normal ageing compared to young healthy controls

The buccal micronucleus cytome assay was used to investigate biomarkers for DNA damage, cell death and basal cell frequency in buccal cells of healthy young, healthy old and young Down's syndrome cohorts. With normal ageing a significant increase in cells with micronuclei (P<0.05, average increase +366%), karyorrhectic cells (P<0.001, average increase +439%), condensed chromatin cells (P<0.01, average increase +45.8%) and basal cells (P<0.001, average increase +233%) is reported relative to young controls. In Down's syndrome we report a significant increase in cells with micronuclei (P<0.001, average increase +733%) and binucleated cells (P<0.001, average increase +84.5%) and a significant decrease in condensed chromatin cells (P<0.01, average decrease -52%), karyolytic cells (P<0.001, average decrease -51.8%) and pyknotic cells (P<0.001, average decrease -75.0%) relative to young controls. These changes show distinct differences between the cytome profile of normal ageing relative to that for a premature ageing syndrome, and highlight the diagnostic value of the cytome approach for measuring the profile of cells with DNA damage, cell death and proportion of cells with proliferative potential (i.e., basal cells). Significant correlations amongst cell death biomarkers observed in this study were used to propose a new model of the inter-relationship of cell types scored within the buccal micronucleus cytome assay. This study validates the use of a cytome approach to investigate DNA damage, cell death and cell proliferation in buccal cells with ageing.

Effects of calorie restriction on chromosomal stability in rhesus monkeys (Macaca mulatta)

The basic tenet of several theories on aging is increasing genomic instability resulting from interactions with the environment. Chromosomal aberrations have been used as classic examples of increasing genomic instability since they demonstrate an increase in numerical and structural abnormalities with age in many species including humans. This accumulating damage may augment many aging processes and initiate age-related diseases, such as neoplasias. Calorie restriction (CR) is one of the most robust interventions for reducing the frequency of age-related diseases and for extending life span in many short-lived organisms. However, the mechanisms for the anti-aging effects of CR are not yet well understood. A study of rhesus monkeys was begun in 1987 to determine if CR is also effective in reducing the frequency of age-related diseases and retarding aging in a long-lived mammal. Male monkeys were begun on the diet in 1987, and females were added in 1992 to examine a possible difference in response to CR by sex. The CR monkeys have been maintained for over 10 years on a low-fat nutritional diet that provides a 30% calorie reduction compared to a control (CON) group. Because of the greater similarity of nonhuman primates to humans in life span and environmental responses to diet compared with those of rodents, the rhesus monkey provides an excellent model for the effects of CR in humans. This study examined the effects of CR on chromosomal instability with aging. Significant age effects were found in both CR and CON groups for the number of cells with aneuploidy: old animals had a higher loss and a higher gain than young animals. However, there was no effect of age on chromosomal breakage or structural aberrations in either diet group. Diet had only one significant effect: the CR group had a higher frequency of chromatid gaps than did the CON group. CR, implemented in adult rhesus monkeys, does not have a major effect on the reduction of numerical or structural aberrations related to aging.

Methionine restriction inhibits colon carcinogenesis

Previously, we demonstrated that life-long methionine restriction (MR) in rats increases life span and inhibits aging-related disease processes. The present study examines the effects of MR on the formation of preneoplastic aberrant crypt foci (ACF) in the colon of azoxymethane (AOM)-treated rats. Six-week-old male F344 rats were placed on essential amino acid-defined diets containing either 0.86% Met (control diet) or 0.17% Met (MR diet) and 1 wk later were given AOM (15 mg/kg/wk, s.c.) for 2 consecutive wk. Ten weeks after the final AOM treatment, ACF formation was markedly reduced in rats fed the MR diet with ACF containing > or = 4 crypts/focus being reduced by over 80% compared to controls (P < 0.001). A similar 83% reduction in ACF containing > or = 4 crypts/focus was observed in rats fed the MR diet only during the post-initiation period (after the final dose of AOM; P < 0.001). Five weeks after AOM administration, a 12% reduction in colonic cell proliferation was observed in MR rats compared to controls (P < 0.05). These results show that MR inhibits colonic tumor development in the rat, an effect that occurs primarily during post-initiation phases of carcinogenesis and may be due, in part, to an inhibition of colonic cell proliferation.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.

Cell type-specific over-expression of chromosome 21 genes in fibroblasts and fetal hearts with trisomy 21

Background

Down syndrome (DS) is caused by trisomy 21 (+21), but the aberrations in gene expression resulting from this chromosomal aneuploidy are not yet completely understood.

Methods

We used oligonucleotide microarrays to survey mRNA expression in early- and late-passage control and +21 fibroblasts and mid-gestation fetal hearts. We supplemented this analysis with northern blotting, western blotting, real-time RT-PCR, and immunohistochemistry.

Results

We found chromosome 21 genes consistently over-represented among the genes over-expressed in the +21 samples. However, these sets of over-expressed genes differed across the three cell/tissue types. The chromosome 21 gene MX1 was strongly over-expressed (mean 16-fold) in senescent +21 fibroblasts, a result verified by northern and western blotting. MX1 is an interferon target gene, and its mRNA was induced by interferons present in +21 fibroblast conditioned medium, suggesting an autocrine loop for its over-expression. By immunohistochemistry the p78^MX1 protein was induced in lesional tissue of alopecia areata, an autoimmune disorder associated with DS. We found strong over-expression of the purine biosynthesis gene GART (mean 3-fold) in fetal hearts with +21 and verified this result by northern blotting and real-time RT-PCR.

Conclusion

Different subsets of chromosome 21 genes are over-expressed in different cell types with +21, and for some genes this over-expression is non-linear (>1.5X). Hyperactive interferon signaling is a candidate pathway for cell senescence and autoimmune disorders in DS, and abnormal purine metabolism should be investigated for a potential role in cardiac defects.

Energy, quiescence and the cellular basis of animal life spans

Animals are routinely faced with harsh environmental conditions in which insufficient energy is available to grow and reproduce. Many animals adapt to this challenge by entering a dormant, or quiescent state. In some animals, such as the nematode Caenorhabditis elegans, quiescence is coincident with increased stress resistance and longevity. Here we review evidence that the rules of life span extension established in C. elegans may be generally true of most animals. That is, that the rate of animal aging correlates inversely with cellular resistance to physiological stress, particularly oxidative stress, and that stress resistance is co-regulated with the quiescence adaptation (where the latter occurs). We discuss evidence for highly conserved intracellular signalling pathways involved in energy sensing that are sensitive to aspects of mitochondrial energy transduction and can be modulated in response to energetic flux. We provide a broad overview of the current knowledge of the relationships between energy, metabolism and life span.

Gene expression responses to DNA damage are altered in human aging and in Werner Syndrome

The accumulation of DNA damage and mutations is considered a major cause of cancer and aging. While it is known that DNA damage can affect changes in gene expression, transcriptional regulation after DNA damage is poorly understood. We characterized the expression of 6912 genes in human primary fibroblasts after exposure to three different kinds of cellular stress that introduces DNA damage: 4-nitroquinoline-1-oxide (4NQO), gamma-irradiation, or UV-irradiation. Each type of stress elicited damage specific gene expression changes of up to 10-fold. A total of 85 genes had similar changes in expression of 3-40-fold after all three kinds of stress. We examined transcription in cells from young and old individuals and from patients with Werner syndrome (WS), a segmental progeroid condition with a high incidence of cancer, and found various age-associated transcriptional changes depending upon the type of cellular stress. Compared to young individuals, both WS and old individuals had similarly aberrant transcriptional responses to gamma- and UV-irradiation, suggesting a role for Werner protein in stress-induced gene expression. Our results suggest that aberrant DNA damage-induced gene regulation may contribute to the aging process and the premature aging in WS.

Reproduce and die! Why aging? Part II

Whilst in part I of this diptych on aging the question why aging exists at all is discussed; this part deals with the question which mechanisms underly aging and, ultimately, dying. It appears that aging is not just an active process as such--although all kinds of internal (e.g., oxigen-free radicals) and external (e.g., UV radiation; disease) actively damage the organism--but more a passive one: it is mainly the result of a diminishing capacity to resist damaging internal and external influences, notably the capacity to repair the ensuing damage of DNA, until, indeed, the genome is entirely beyond repair and all kinds of vital functions detoriate with as a result that, in the end, the body collapses due to some final internal (e.g., a neoplasm or a CVA) or external (e.g., some infection, accident or attack) push. The time-course with which the capacity to repair DNA diminishes, however, is genetically fixed, and is associated with (even determined by) the reproductive strategy of the species in question: once the phase of reproduction is over, the reins are loosened and all kinds of genetic and physiological errors accumulate, giving rise to a large variety of pathology which ultimately carries the pertinent individual to the grave.

Differential gene expression of early and late passage retinal pigment epithelial cells

We examined the gene expression profiles of retinal pigment epithelial (RPE) cells which were aged in vitro by repeated passage. RPE cells from human eyes were cultured to passage 3-5 (early passage) or 19-21 (late passage) and used to study gene expression profiles by cDNA microarray. Results from microarray analysis were further confirmed by real-time PCR. Microarray analysis showed gene expression changes among 588 known genes. The expression levels of 15 genes (2.6%) increased in late passage RPE cells, while 43 genes (7.3%) decreased using a two-fold criterion. These differentially expressed genes encompassed many functional classes. A small number of stress genes, such as clusterin, replication protein A and Ku80, were up-regulated. The down-regulated genes included many enzymes of energy and biomolecule metabolism as well as cell cycle proteins and cell adhesion proteins. Results from real-time PCR were generally consistent with microarray findings. The expression levels of the examined angiogenic factors were either unchanged or down-regulated. Comparing early (p=3-5) and late (p=9-12) passage RPE cells, several categories of differentially expressed genes were identified. However, there was no enhanced expression of known angiogenic factors.

An investigation of DNA excision repair capacity in human CD4+ T cell clones as a function of age in vitro

DNA damage has been shown to increase with age in lymphocytes of healthy humans and in human CD4+ T cell clones. Such genetic damage, if unrepaired, may have a detrimental effect on lymphocyte-mediated immune responses. This study investigated DNA excision repair capacity of human CD4+ T cell clones as a function of age in vitro. Cultures of T cell clones were treated with a range of DNA damaging agents; hydrogen peroxide, N-methyl-N'-nitro-N-nitrosoguanidine or 254 nm ultraviolet irradiation. Following treatment, the amount of DNA damage in the clones was determined over a time course using modified comet assays. The results obtained revealed a decline related to in vitro age in the DNA repair capacity of clones derived from a 26 and a 45 year old donor. This decline may represent at least a partial explanation for the age related increase in DNA damage in these clones when cultured in vitro. In contrast, there was no evidence for a decline related to in vitro age in repair capacity in the clones derived from an 80 year old SENIEUR donor. An alternative mechanism must underlie the age related increased in DNA damage in these clones when cultured in vitro.

Physical and functional interaction of the Werner syndrome protein with poly-ADP ribosyl transferase

Werner's syndrome is a rare disease of premature ageing. The WRN gene product defective in this disorder belongs to the RecQ helicase family and is thought to be involved in DNA metabolism. Another protein, which plays an important role in both DNA replication and repair, is the poly-ADP ribosyl transferase. Here we demonstrate an interaction of these two proteins resulting in ADP-ribosylation of the WRN protein. These results imply that WRN is involved in DNA replication and in DNA repair.

Central role for the Werner syndrome protein/poly(ADP-ribose) polymerase 1 complex in the poly(ADP-ribosyl)ation pathway after DNA damage

A defect in the Werner syndrome protein (WRN) leads to the premature aging disease Werner syndrome (WS). Hallmark features of cells derived from WS patients include genomic instability and hypersensitivity to certain DNA-damaging agents. WRN contains a highly conserved region, the RecQ conserved domain, that plays a central role in protein interactions. We searched for proteins that bound to this region, and the most prominent direct interaction was with poly(ADP-ribose) polymerase 1 (PARP-1), a nuclear enzyme that protects the genome by responding to DNA damage and facilitating DNA repair. In pursuit of a functional interaction between WRN and PARP-1, we found that WS cells are deficient in the poly(ADP-ribosyl)ation pathway after they are treated with the DNA-damaging agents H2O2 and methyl methanesulfonate. After cellular stress, PARP-1 itself becomes activated, but the poly(ADP-ribosyl)ation of other cellular proteins is severely impaired in WS cells. Overexpression of the PARP-1 binding domain of WRN strongly inhibits the poly(ADP-ribosyl)ation activity in H2O2-treated control cell lines. These results indicate that the WRN/PARP-1 complex plays a key role in the cellular response to oxidative stress and alkylating agents, suggesting a role for these proteins in the base excision DNA repair pathway.

Effect of DNA Repair on Aging of Transgenic Drosophila melanogaster: I. mei-41 Locus

Aging appears to be increased by diminished DNA repair. To study this relationship between aging and DNA repair, we measured the life span of Drosophila melanogaster males in the absence of mei-41 excision repair and transgenic flies with 1 or 2 extra copies of the mei-41 wild-type gene. Life span was significantly reduced in the absence of repair and was significantly increased by an extra dose of excision repair. However, these changes in life span with alterations in DNA repair were not large.

Loss of chromosome 16 from renal epithelial cells in humans

This work explores the notion that low-frequency, acquired aneuploidy may play a role in complex genetic traits such as essential hypertension. To this end, renal epithelial cells in urinary sediments and in renal cysts were examined by fluorescent in situ hybridization with DNA probes specific for the heterochromatic and centromere regions of chromosomes 16 and 1. Chromosome 16 was probed because it harbors variant genes causing monogenic hypertension. These genes have also been investigated for their role in essential hypertension. Chromosome 1 was also probed as an internal control. Higher proportions of renal epithelial cells in the urinary sediments showed monosomy of chromosome 16 than monosomy of chromosome 1 (P<0.001). We also observed in epithelial cells of renal cysts a preponderance of monosomy for chromosome 16 over monosomy for chromosome 1 (P<0.024). Low-frequency loss of heterozygosity that results from acquired monosomy of chromosome 16 and perhaps other chromosomes may contribute to expression of complex genetic traits such as essential hypertension, in which the diverse phenotypic manifestations are poorly understood.

The relationship between the cancer cell and the oocyte

What causes cancer? To date, this question is left with no answer. The scientific community keeps defining cancer as a proliferation of cells in an uncontrolled and uncontrollable manner. The hypothesis developed here through an understanding of the cancer cell's behaviour led to its re-definition, providing answers to the questions arising from cancer. Through striking similarities in cell behaviour, I have concluded that the cancer cell is a reprogrammed cell with the launch of the egg cell's genetic program. The unique cell to express this program is the oocyte, therefore this oocyte cell should become the subject of significant study to understand the genesis of cancer. Above all, it will enable us to eradicate in a specific manner the cancer cell.

Enhancement of lipopolysaccharide-stimulated cyclooxygenase-2 mRNA expression and prostaglandin E2 production in gingival fibroblasts from individuals with Down syndrome

It is well known that Down syndrome (DS) is a premature ageing syndrome. Periodontal disease in individuals with DS develops rapidly and extensively in a relatively younger age bracket compared with that in healthy controls. The mechanisms involved in the periodontal inflammatory processes in DS patients are not fully understood. In the present study, the non-inflamed gingival fibroblasts isolated from seven patients with DS (DGF) and seven healthy controls (NDGF) were stimulated with lipopolysaccharide (LPS) derived from Actinobacillus actinomycetemcomitans (A. a.). We measured the level of prostaglandin E2 (PGE2) production by DGF and NDGF by radioimmunoassay, and also measured the mRNA expression of cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) by using the real-time PCR method. We found the higher levels of LPS-stimulated COX-2 mRNA expression and PGE2 production in DGF when compared with those in NDGF. This study may indicate that overexpression of LPS-stimulated COX-2 induced a greater ability of DGF to produce PGE2, and that these phenomena may be responsible for the severer periodontal disease in DS patients.

Accelerated telomere shortening in Fanconi anemia fibroblasts--a longitudinal study

Fanconi anemia (FA) is a fatal inherited disease displaying chromosomal instability, disturbances in oxygen metabolism and a high burden of intracellular radical oxygen species. Oxygen radicals can damage DNA including telomeric regions. Insufficient repair results in single strand breaks that can induce accelerated telomere shortening. In a longitudinal study we demonstrate that telomeric DNA is continuously lost at a higher rate in FA fibroblasts compared to healthy controls. Furthermore, we show that this loss is caused rather by an increased shortening per cell division in regularly replicating cells than by apoptosis.

Garbage catastrophe theory of aging: imperfect removal of oxidative damage?

Increasing evidence suggests an important role of oxidant-induced damage in the progress of senescent changes, providing support for the free radical theory of aging proposed by Harman in 1956. However, considering that biological organisms continuously renew their structures, it is not clear why oxidative damage should accumulate with age. No strong evidence has been provided in favor of the concept of aging as an accumulation of synthetic errors (e.g. Orgel's 'error-catastrophe' theory and the somatic mutation theory). Rather, we believe that the process of aging may derive from imperfect clearance of oxidatively damaged, relatively indigestible material, the accumulation of which further hinders cellular catabolic and anabolic functions. From this perspective, it might be predicted that: (i) suppression of oxidative damage would enhance longevity; (ii) accumulation of incompletely digested material (e.g. lipofuscin pigment) would interfere with cellular functions and increase probability of death; (iii) rejuvenation during reproduction is mainly provided by dilution of undigested material associated with intensive growth of the developing organism; and (iv) age-related damage starts to accumulate substantially when development is complete, and mainly affects postmitotic, cells and extracellular matrix, not proliferating cells. There is abundant support for all these predictions.

An investigation of the age-dependency of chromosome abnormalities in human populations exposed to low-dose ionising radiation

Among various cytogenetic changes stable chromosome aberrations (SCHA) seem to be the most significant for ageing and carcinogenesis. Being nonlethal they can persist through cell divisions and accumulate in time. We studied the age response of SCHA (translocations and insertions) in normal and radiation exposed human populations. Two cohorts of people at the age range of 3--72 years were studied: control (43 persons) and exposed to low doses of accidental irradiation due to Chernobyl accident and atomic bomb testing in Semipalatinsk (67 persons). FISH method was used for visualisation of chromosome aberrations. Metaphases from cultured lymphocytes were hybridised with biotinilated whole chromosome specific DNA probes for 1, 4 and 12 chromosomes, and with pancentromeric probe labelled with digoxigenin. The frequency of SCHA in lymphocytes increased as a quadratic function of donor age in both populations studied, being higher in exposed cohort as compared with control one. No age dependence for dicentrics was observed. The frequency of SCHA is a reliable biomarker of ageing in humans. Quadratic model of their age-response gives reasons to suggest that their increase is due to lower level of DNA repair or/and the genomic instability in older people. The exposure of people to low doses of ionising radiation accelerates the age-related increase of SCHA frequency.

Stress, DNA damage and ageing -- an integrative approach

Ageing is highly complex, involving multiple mechanisms at different levels. Nevertheless, recent evidence suggests that several of the most important mechanisms are linked via endogenous stress-induced DNA damage caused by reactive oxygen species (ROS). Understanding how such damage contributes to age-related changes requires that we explain how these different mechanisms relate to each other and potentially interact. In this article, we review the contributions of stress-induced damage to cellular DNA through (i) the role of damage to nuclear DNA and its repair mediated via the actions of poly(ADP-ribose) polymerase-1, (ii) the role of damage to telomeric DNA and its contribution to telomere-driven cell senescence, and (iii) the role of damage to and the accumulation of mutations in mitochondrial DNA. We describe how an integrative approach to studying these mechanisms, coupled with computational modelling, may be of considerable importance in resolving some of the complexity of cellular ageing.

Extended exposure of adult and fetal mice to 50 Hz magnetic field does not increase the incidence of micronuclei in erythrocytes

The flow cytometer-based micronucleus assay was used to study the effects on chromosomes in erythroid cells of CBA/Ca mice after extended exposure to 50 Hz magnetic field (MF), 14 microT, peak-to-peak (p-p). The study included two different experiments: (a) mice exposed in utero during 18 days of their prenatal stage, and (b) adult mice exposed for 18 days. In experiment (a) 35 days after exposure was terminated, peripheral blood was drawn from the mice exposed in utero to determine whether the exposure had a genotoxic effect on the pluripotent erythroid stem cells. About 200000 polychromatic erythrocytes (PCE) and 200000 normochromatic erythrocytes (NCE) were analysed from each of 20 exposed mice. The EMF exposure did not significantly change the frequency of micronucleated PCE or NCE in comparison with 20 sham-irradiated mice. There was no difference in the proportion of PCE between exposed and unexposed animals. Similarly, in experiment (b) no differences were seen between EMF exposed and unexposed adult mice when samples of peripheral blood were taken at the end of exposure and analyzed for micronuclei in PCE and NCE. The proportion of PCE was the same in both groups. The results indicate that exposure to EMF does not induce direct or indirect effects on chromosomes in erythroid cells expressed as increased levels of micronucleated erythrocytes of mice. No indications of delayed genetic effects were found.

Genomic instability in Down syndrome and Fanconi anemia assessed by micronucleus analysis and single-cell gel electrophoresis

Cytokinesis-block micronucleus (CB-MN) assay and single-cell gel electrophoresis (SCGE) were employed to analyze leukocytes from 14 Fanconi anemia (FA) patients, 30 Down syndrome (DS) patients, and 30 control individuals, to examine the sensitivity of these techniques to detect genomic instability in these 2 diseases. The DS patients presented increased DNA damage as measured by SCGE in relation to controls. The frequencies of micronuclei and dicentric bridges were similar to those of controls. Micronucleus frequency, dicentric bridge frequencies, and DNA damage were higher in FA patients than in controls. The high frequency of micronuclei observed in FA patients seems to be due to clastogenic events, because an increase in the frequency of dicentric bridges was also observed. Micronuclei are expressed mutations and need cell division to appear. The damage detected by SCGE is repairable, and does not require cell division. Under alkaline conditions, SCGE assesses double- and single-strand breaks and alkali-labile sites. The 2 methods are efficient for monitoring mutagenic events in exposed populations or in individuals with genetic instability. While the damage measured by micronucleus analysis is accumulated over a long period of time, DNA damage measured by SCGE reflects recent, unrepaired events.

Telomerase and human tumorigenesis

Human cancer cells, unlike their normal counterparts, have shed the molecular restraints to limited cell growth and are immortal. Exactly how cancer cells manage this at the molecular level is beginning to be understood. Human cells must overcome two barriers to cellular proliferation. The first barrier, referred to as senescence, minimally involves the p53 and Rb tumor-suppressor pathways. Inactivation of these pathways results in some extension of lifespan. However, inactivation of these pathways is insufficient for immortalization. As normal cells undergo repeated rounds of DNA replication, their telomeres shorten due to the inability of traditional DNA polymerases to completely replicate the end of the chromosomal DNA. This shortening continues until the cells reach a second proliferative block referred to as crisis, which is characterized by chromosomal instability, end-to-end fusions, and cell death. Stabilization of the telomeric DNA through either telomerase activation or the activation of the alternative mechanism of telomere maintenance (ALT) is essential if the cells are to survive and proliferate indefinitely. Conversely, loss of telomere stabilization by an already-immortalized cell results in loss of immortality and cell death. Together this indicates that telomere maintenance is a critical component of immortality. In this review we attempt to describe our current understanding of the role of telomere maintenance in senescence, crisis, and tumorigenesis.

WRN mutations in Werner syndrome

Werner syndrome (WS) is one of a group of human genetic diseases that have recently been linked to deficits in cellular helicase function. We review the spectrum of WS-associated WRN mutations, the organization and potential functions of the WRN protein, and potential mechanistic links between the loss of WRN function and pathogenesis of the WS clinical and cellular phenotypes.

The Werner syndrome. A model for the study of human aging

Human aging is a complex process that leads to the gradual deterioration of body functions with time. Various models to approach the study of aging have been launched over the years such as the genetic analysis of life span in the yeast S. cerevisiae, the worm C. elegans, the fruitfly, and mouse, among others. In human models, there have been extensive efforts using replicative senescence, the study of centenerians, comparisons of young versus old at the organismal, cellular, and molecular levels, and the study of premature aging syndromes to understand the mechanisms leading to aging. One good model for studying human aging is a rare autosomal recessive disorder known as the Werner syndrome (WS), which is characterized by accelerated aging in vivo and in vitro. A genetic defect implicated in WS was mapped to the WRN locus. Mutations in this gene are believed to be associated, early in adulthood, with clinical symptoms normally found in old individuals. WRN functions as a DNA helicase, and recent evidence, summarized in this review, suggests specific biochemical roles for this multifaceted protein. The interaction of WRN protein with RPA (replication protein A) and p53 will undoubtedly direct efforts to further dissect the genetic pathway(s) in which WRN protein functions in DNA metabolism and will help to unravel its contribution to the human aging process.

Ku complex interacts with and stimulates the Werner protein

Werner syndrome (WS) is the hallmark premature aging disorder in which affected humans appear older than their chronological age. The protein WRNp, defective in WS, has helicase function, DNA-dependent ATPase, and exonuclease activity. Although WRNp functions in nucleic acid metabolism, there is little or no information about the pathways or protein interactions in which it participates. Here we identify Ku70 and Ku86 as proteins that interact with WRNp. Although Ku proteins had no effect on ATPase or helicase activity, they strongly stimulated specific exonuclease activity. These results suggest that WRNp and the Ku complex participate in a common DNA metabolic pathway.