An investigation of mutation as a function of age in humans

Age-associated remodeling of T cell immunity and metabolism

Aging results in remodeling of T cell immunity and is associated with poor clinical outcomes in age-related diseases such as cancer. Among the hallmarks of aging, changes in host and cellular metabolism critically affect the development, maintenance, and function of T cells. Although metabolic perturbations impact anti-tumor T cell responses, the link between age-associated metabolic dysfunction and anti-tumor immunity remains unclear. In this review, we summarize recent advances in our understanding of aged T cell metabolism, with a focus on the bioenergetic and immunologic features of T cell subsets unique to the aging process. We also survey insights into mechanisms of metabolic T cell dysfunction in aging and discuss the impacts of aging on the efficacy of cancer immunotherapy. As the average life expectancy continues to increase, understanding the interplay between age-related metabolic reprogramming and maladaptive T cell immunity will be instrumental for the development of therapeutic strategies for older patients.

Early age decline in DNA repair capacity in the liver: in depth profile of differential gene expression

Aging is associated with progressive decline in cell function and with increased damage to macromolecular components. DNA damage, in the form of double-strand breaks (DSBs), increases with age and in turn, contributes to the aging process and age-related diseases. DNA strand breaks triggers a set of highly orchestrated signaling events known as the DNA damage response (DDR), which coordinates DNA repair. However, whether the accumulation of DNA damage with age is a result of decreased repair capacity, remains to be determined. In our study we showed that with age there is a decline in the resolution of foci containing γH2AX and pKAP-1 in diethylnitrosamine (DEN)-treated mouse livers, already evident at a remarkably early age of 6-months. Considerable age-dependent differences in global gene expression profiles in mice livers after exposure to DEN, further affirmed these age related differences in the response to DNA damage. Functional analysis identified p53 as the most overrepresented pathway that is specifically enhanced and prolonged in 6-month-old mice. Collectively, our results demonstrated an early decline in DNA damage repair that precedes ‘old age’, suggesting this may be a driving force contributing to the aging process rather than a phenotypic consequence of old age.

High Level of Tobacco Carcinogen-Derived DNA Damage in Oral Cells Is an Independent Predictor of Oral/Head and Neck Cancer Risk in Smokers

Exposure to tobacco-specific nitrosamines (TSNA) and polycyclic aromatic hydrocarbons (PAH) is recognized to play an important role in the development of oral/head and neck squamous cell cancer (HNSCC). We recently reported higher levels of TSNA-associated DNA adducts in the oral cells of smokers with HNSCC as compared with cancer-free smokers. In this study, we further investigated the tobacco constituent exposures in the same smokers to better understand the potential causes for the elevated oral DNA damage in smokers with HNSCC. Subjects included cigarette smokers with HNSCC (cases, n = 30) and cancer-free smokers (controls, n = 35). At recruitment, tobacco/alcohol use questionnaires were completed, and urine and oral cell samples were obtained. Analysis of urinary 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and N'-Nitrosonornicotine (NNN; TSNA biomarkers), 1-hydroxypyrene (1-HOP, a PAH), cotinine, 3'-hydroxycotinine, and the nicotine metabolite ratio (NMR) were performed. Cases and controls differed in mean age, male preponderance, and frequency of alcohol consumption (but not total alcoholic drinks). Univariate analysis revealed similar levels of NNN, 1-HOP, and cotinine between groups but, as reported previously, significantly higher DNA adduct formation in the cases. Multiple regression adjusting for potential confounders showed persistent significant difference in DNA adduct levels between cases and controls [ratio of geometric means, 20.0; 95% CI, 2.7-148.6). Our cohort of smokers with HNSCC demonstrates higher levels of TSNA-derived oral DNA damage in the setting of similar exposure to nicotine and tobacco carcinogens. Among smokers, DNA adduct formation may act as a predictor of eventual development of HNSCC that is independent of carcinogen exposure indicators. Cancer Prev Res; 10(9); 507-13. ©2017 AACRSee related editorial by Johnson and Bauman, p. 489.

The protective effect of the flavonoids on food-mutagen-induced DNA damage in peripheral blood lymphocytes from colon cancer patients

The food mutagens IQ (2-amino-3-methylimidazo[4,5-f]quinoline) and PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) are heterocyclic amines (HCA), generated when heating proteinaceous food. This study investigates the protective potential of the flavonoids quercetin (Q) and rutin (R) against oxidative stress induced in vitro by IQ and PhIP in lymphocytes from healthy individuals and untreated, newly diagnosed colon cancer patients using the Comet assay. In the presence of up to 500μM Q and R, the DNA damage resulting from a high dose of PhIP (75μM) or IQ (150μM) was significantly reduced (P<0.001) to levels comparable to six times lower IQ or 7.5 times lower PhIP doses. Lymphocytes from colon cancer patients had greater baseline DNA damage than those from healthy individuals (P<0.01) and this higher level of damage was also observed throughout in vitro treatment. Except for the >50years of age group and male gender, confounding factors such as smoking, drinking and/or dietary habits were not found to be significant. In conclusion, flavonoids reduced oxidative stress caused by food mutagens in vitro in lymphocytes of healthy individuals and colon cancer patients. Thus, dietary supplementation with flavonoid-rich vegetables and fruits may prove very effective in protecting against oxidative stress.

Role of telomere dysfunction in genetic intratumor diversity

Most solid tumors are unable to maintain the stability of their genomes at the chromosome level. Indeed, cancer cells display highly rearranged karyotypes containing translocations, amplifications, deletions, and gains and losses of whole chromosomes, which reshuffle steadily. This chromosomal instability most likely occurs early in the development of cancer, and may represent an important step in promoting the multiple genetic changes required for the initiation and/or progression of the disease. Different mechanisms may underlie chromosome instability in cancer cells, but a prominent role for telomeres, the tip of linear chromosomes, has been determined. Telomeres are ribonucleoprotein structures that prevent natural chromosome ends being recognized as DNA double-strand breaks, by adopting a loop structure. Loss of telomere function appears from either alteration on telomere-binding proteins or from the progressive telomere shortening that normally occurs under physiological conditions in the majority of cells in tissues. Importantly, unmasked telomeres may either trigger the senescent phenotype that has been linked to the aging process or may initiate the chromosome instability needed for cancer development, depending on the integrity of the DNA damage checkpoint responses. Telomere dysfunction contributes to chromosome instability through end-to-end chromosome fusions entering breakage-fusion-bridge (BFB) cycles. Resolution of chromatin bridge intermediates is likely to contribute greatly to the generation of segmental chromosome amplification events, unbalanced chromosome rearrangements, and whole chromosome aneuploidy. Noteworthy is the fact that telomere length heterogeneity among individuals may directly influence the scrambling of the genome at tumor initiation. However, reiterated BFB cycles would randomly reorganize the cell karyotype, thus increasing the genetic diversity that characterizes tumor cells. Even though a direct link is still lacking, multiple evidence lead one to believe that telomere dysfunction directly contributes to cancer development in humans. The expansion of highly unstable cells due to telomere dysfunction enhances the genetic diversity needed to fuel specific mutations that may promote cell immortalization and the acquisition of a tumor phenotype.

An investigation of the effects of MitoQ on human peripheral mononuclear cells

MitoQ is a ubiquinone derivative targeted to mitochondria which is known to have both antioxidant and anti-apoptotic properties within mammalian cells. Previous research has suggested that the age-related increase in oxidative DNA damage in T lymphocytes might contribute to their functional decline with age. This paper describes the impact of mitoQ on unchallenged or oxidatively challenged ex vivo human peripheral blood mononuclear cells from healthy 25-30 or 55-60 year old volunteers. When cells were challenged with hydrogen peroxide (H(2)O(2)), following mitoQ treatment (0.1-1.0 μM), the ratio of reduced to oxidized forms of glutathione increased, the levels of oxidative DNA damage decreased and there was an increase in the mitochondrial membrane potential. Low levels of mitoQ (0.1 or 0.25 μM) had no impact on endogenous DNA damage, whilst higher levels (0.5 and 1.0 μM) of mitoQ significantly reduced endogenous levels of DNA damage. The results of this investigation suggest that mitoQ may have anti-immunosenescent potential.

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.

Senescence-dependent MutS alpha dysfunction attenuates mismatch repair

DNA damage and mutations in the genome increase with age. To determine the potential mechanisms of senescence-dependent increases in genomic instability, we analyzed DNA mismatch repair (MMR) efficiency in young and senescent human colonic fibroblast and human embryonic lung fibroblast. It was found that MMR activity is significantly reduced in senescent cells. Western blot and immunohistochemistry analysis revealed that hMSH2 and MSH6 protein (MutS alpha complex), which is a known key component in the MMR pathway, is markedly down-regulated in senescent cells. Moreover, the addition of purified MutS alpha to extracts from senescent cells led to the restoration of MMR activity. Semiquantitative reverse transcription-PCR analysis exhibited that MSH2 mRNA level is reduced in senescent cells. In addition, a decrease in E2F transcriptional activity in senescent cells was found to be crucial for MSH2 suppression. E2F1 small interfering RNA expression reduced hMSH2 expression and MMR activity in young human primary fibroblast cells. Importantly, expression of E2F1 in quiescent cells restored the MSH2 expression as well as MMR activity, whereas E2F1-infected senescent cells exhibited no restoration of MSH2 expression and MMR activity. These results indicate that the suppression of E2F1 transcriptional activity in senescent cells lead to stable repression of MSH2, followed by a induction of MutS alpha dysfunction, which results in a reduced cellular MMR capacity in senescent cells.

An adjustment factor for mode-of-action uncertainty with dual-mode carcinogens: the case of naphthalene-induced nasal tumors in rats

The U.S. Environmental Protection Agency (USEPA) guidelines for cancer risk assessment recognize that some chemical carcinogens may have a site-specific mode of action (MOA) involving mutation and cell-killing-induced hyperplasia. The guidelines recommend that for such dual MOA (DMOA) carcinogens, judgment should be used to compare and assess results using separate "linear" (genotoxic) versus "nonlinear" (nongenotoxic) approaches to low-level risk extrapolation. Because the guidelines allow this only when evidence supports reliable risk extrapolation using a validated mechanistic model, they effectively prevent addressing MOA uncertainty when data do not fully validate such a model but otherwise clearly support a DMOA. An adjustment-factor approach is proposed to address this gap, analogous to reference-dose procedures used for classic toxicity endpoints. By this method, even when a "nonlinear" toxicokinetic model cannot be fully validated, the effect of DMOA uncertainty on low-dose risk can be addressed. Application of the proposed approach was illustrated for the case of risk extrapolation from bioassay data on rat nasal tumors induced by chronic lifetime exposure to naphthalene. Bioassay data, toxicokinetic data, and pharmacokinetic analyses were determined to indicate that naphthalene is almost certainly a DMOA carcinogen. Plausibility bounds on rat-tumor-type-specific DMOA-related uncertainty were obtained using a mechanistic two-stage cancer risk model adapted to reflect the empirical link between genotoxic and cytotoxic effects of the most potent identified genotoxic naphthalene metabolites, 1,2- and 1,4-naphthoquinone. Bound-specific adjustment factors were then used to reduce naphthalene risk estimated by linear extrapolation (under the default genotoxic MOA assumption), to account for the DMOA exhibited by this compound.

Mutational load distribution analysis yields metrics reflecting genetic instability during pancreatic carcinogenesis

Considering carcinogenesis as a microevolutionary process, best described in the context of metapopulation dynamics, provides the basis for theoretical and empirical studies that indicate it is possible to estimate the relative contribution of genetic instability and selection to the process of tumor formation. We show that mutational load distribution analysis (MLDA) of DNA found in pancreatic fluids yields biometrics that reflect the interplay of instability, selection, accident, and gene function that determines the eventual emergence of a tumor. An in silico simulation of carcinogenesis indicates that MLDA may be a suitable tool for early detection of pancreatic cancer. We also present evidence indicating that, when performed serially in individuals harboring a p16 germ-line mutation bestowing a high risk for pancreatic cancer, MLDA may be an effective tool for the longitudinal assessment of risk and early detection of pancreatic cancer.

Photoprotection in human skin--a multifaceted SOS response

Human skin has developed elaborate defense mechanisms for combating a wide variety of potentially damaging environmental factors; principal among these is UV light. Despite these defenses, short-term damage may include painful sunburn and long-term UV damage results in both accelerated skin aging and skin cancers such as basal cell carcinoma, squamous cell carcinoma and even malignant melanoma. While UV radiation damages many cellular constituents, its most lasting effects involve DNA alteration. The following sections briefly review UV-inducible protective responses in bacteria and in skin, thymidine dinucleotides (pTT) as a powerful probe of DNA damage responses, and potential means of harnessing these inducible responses therapeutically to reduce the now enormous burden of cutaneous photodamage in our society.

The L84F polymorphism in the O6-Methylguanine-DNA-Methyltransferase (MGMT) gene is associated with increased hypoxanthine phosphoribosyltransferase (HPRT) mutant frequency in lymphocytes of tobacco smokers

OBJECTIVES

O-methylguanine-DNA-methyltransferase (MGMT) is a crucial DNA repair protein that removes DNA adducts formed by alkylating mutagens. Several coding single nucleotide polymorphisms (cSNPs) in the MGMT gene have been reported. Their biological significance, however, is not known.

METHODS

We used a newly modified cloning HPRT mutant lymphocyte assay to test the hypothesis that inheritance of the L84F and I143V coding single nucleotide polymorphism in the MGMT gene is associated with increases in HPRT mutant frequency in lymphocytes of individuals exposed to alkylating agents. In addition, we expanded and sequenced 109 mutant clones to test the hypothesis that the mutation spectrum would shift to a larger percentage of base substitutions and G-->A transition mutations in cells with L84F and I143 V coding single nucleotide polymorphisms.

RESULTS

We observed no significant effect for the I143 V coding single nucleotide polymorphism on mutant frequency. In contrast, we observed a significant increase in mutant frequency (P<0.01) in lymphocytes from smokers with the 84F coding single nucleotide polymorphism compared with smokers homozygous for the referent L84 wild-type allele. A multiple regression analysis indicated that the mutant frequency increased significantly as a function of the 84F coding single nucleotide polymorphism and smoking, according to the model; mutant frequency (x10)=0.90+0.618 (84F polymorphism)+0.46 (smoking) with R=0.22. Mutation spectra analysis revealed an apparent increase, which was short of statistical significance (P=0.08), in base substitutions in cells with the 84F polymorphism.

CONCLUSIONS

These new data suggest that the 84F coding single nucleotide polymorphism may alter the phenotype of the MGMT protein, resulting in suboptimal repair of O-methylguanine lesions after exposure to alkylating agents.

Genetic and epigenetic alterations as biomarkers for cancer detection, diagnosis and prognosis

The development of cancer is driven by the accumulation of scores of alterations affecting the structure and function of the genome. Equally important in this process are genetic alterations and epigenetic changes. Whereas the former disrupt normal patterns of gene expression, sometimes leading to the expression of abnormal, constitutively active proteins, the latter deregulate the mechanisms such as transcriptional control leading to the inappropriate silencing or activation of cancer-associated genes. Both types of changes are inheritable at the cellular level, thus contributing to the clonal expansion of cancer cells. In this review, we summarize current knowledge on how genetic alterations in oncogenes or tumour suppressor genes, as well as epigenetic changes, can be exploited in the clinics as biomarkers for cancer detection, diagnosis and prognosis. We propose a rationale for identifying alterations that may have a functional impact within a background of "passenger" alterations that may occur solely as the consequence of deregulated genetic and epigenetic stability. Such functional alterations may represent candidates for targeted therapeutic approaches.

Environmental and nutritional effects on the epigenetic regulation of genes

Major efforts have been directed towards the identification of genetic mutations, their use as biomarkers, and the understanding of their consequences on human health and well-being. There is an emerging interest, however, in the possibility that environmentally-induced changes at levels other than the genetic information could have long-lasting consequences as well. This review summarises our current knowledge of how the environment, nutrition, and ageing affect the way mammalian genes are organised and transcribed, without changes in the underlying DNA sequence. Admittedly, the link between environment and epigenetics remains largely to be explored. However, recent studies indicate that environmental factors and diet can perturb the way genes are controlled by DNA methylation and covalent histone modifications. Unexpectedly, and not unlike genetic mutations, aberrant epigenetic alterations and their phenotypic effects can sometimes be passed on to the next generation.

Genotype profiles of loci encoding DNA repair enzymes in newborn and elderly populations: no evidence of association with longevity

The comparison of genotype frequencies between neonates and elderly populations can aid in the identification of loci, and polymorphisms within those loci, that affect longevity. Here we have compared genotype frequencies of seven polymorphisms at four loci involved in DNA repair between a cohort of newborns (n = 290) and a retired population (average age at sampling 70.02 years; n = 430) who have suffered a lifetime of DNA damage from normal, metabolic processes, and on whom selection on DNA repair gene variants may be expected to have acted. No differences in genotype frequencies at the four SNP loci were seen, indicating that there is no evidence of association with longevity in this population. Significant differences in frequency of certain repeat sizes at three microsatellite loci were detected. However, since there is no known functional consequence of these repeat lengths, the action of selection cannot yet be ascribed.

Accumulation of mutations and somatic selection in aging neural stem/progenitor cells

Genomic instability within somatic stem cells may lead to the accumulation of mutations and contribute to cancer or other age-related phenotypes. However, determining the frequency of mutations that differ among individual stem cells is difficult from whole tissue samples because each event is diluted in the total population of both stem cells and differentiated tissue. Here the ability to expand neural stem/progenitor cells clonally permitted measurement of genomic alterations derived from a single initial cell. C57Bl/6 x DBA/2 hybrid mice were used and PCR analysis with strain-specific primers was performed to detect loss of heterozygosity on nine different chromosomes for each neurosphere. The frequency with which changes occurred in neurospheres derived from 2-month- and 2-year-old mice was compared. In 15 neurospheres derived from young animals both parental chromosomes were present for all nine chromosome pairs. In contrast, 16/17 neurospheres from old animals demonstrated loss of heterozygosity (LOH) on one or more chromosomes and seven exhibited a complete deletion of at least one chromosomal region. For chromosomes 9 and 19 there is a significant bias in the allele that is lost where in each case the C57Bl/6 allele is retained in 6/6 neurospheres exhibiting LOH. These data suggest that aging leads to a substantial mutational load within the neural stem cell compartment which can be expected to affect the normal function of these cells. Furthermore, the retention of specific alleles for chromosomes 9 and 19 suggests that a subset of mutational events lead to an allele-specific survival advantage within the neural stem cell compartment.

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.

Effects of a reduced oxygen tension culture system on human T cell clones as a function of in vitro age

Oxidative DNA damage has been suggested to contribute to the decline in T cell clone (TCC) function with increased age in vitro. To test this hypothesis the effect of a reduced oxygen tension culture system (6% O(2)) on TCCs was examined. Specifically, the effects of the altered culture conditions on DNA damage levels, in vitro lifespan and proliferative capacity were assessed in five independently derived human CD4+ TCCs. DNA damage levels over the entire lifespan were significantly lowered by reducing oxygen tension. Lifespan (total population doublings (PDs) achieved) and proliferative capacity (PDs/week) were reduced for all clones under reduced oxygen tension when compared to standard culture conditions. This observed tendency warrants further investigation using a greater number of clones from donors of all age groups before definitive conclusions regarding the effect of low oxygen tension on the lifespan and proliferative capacity of TCC can be made. However, these results may suggest that the reduced oxygen tension culture system has interfered with some aspect of T cell biology not yet examined within the remit of this study.

Age-Associated Activation of Epigenetically Repressed Genes in the Mouse

Epigenetic control of gene expression is a consistent feature of differentiated mammalian cell types. Epigenetic expression patterns are mitotically heritable and are stably maintained in adult cells. However, unlike somatic DNA mutation, little is known about the occurrence of epigenetic change, or epimutation, during normal adult life. We have monitored the age-associated maintenance of two epigenetic systems—X inactivation and genomic imprinting—using the genes Atp7a and Igf2, respectively. Quantitative measurements of RNA transcripts from the inactive and active alleles were performed in mice from 2 to 24 months of age. For both genes, older animal cohorts showed reproducible increases in transcripts expressed from the silenced alleles. Loss of X chromosome silencing showed cohort mean increases of up to 2.2%, while imprinted-gene activation increased up to 6.7%. The results support the hypothesis that epigenetic loss of gene repression occurs in normal tissues and may be a contributing factor in progressive physiological dysfunction seen during mammalian aging. Quantitatively, the loss of epigenetic control may be one to two orders of magnitude greater than previously determined somatic DNA mutation.

Daily grape juice consumption reduces oxidative DNA damage and plasma free radical levels in healthy Koreans

Grape contains flavonoids with antioxidant properties which are believed to be protective against various types of cancer. This antioxidative protection is possibly provided by the effective scavenging of reactive oxygen species (ROS), thus defending cellular DNA from oxidative damage and potential mutations. This study of healthy adults tested whether a daily regimen of grape juice supplementation could reduce cellular DNA damage in peripheral lymphocytes and reduce the amount of free radicals released. Sixty-seven healthy volunteers (16 women and 51 men) aged 19-57 years were given 480 ml of grape juice daily for 8 weeks in addition to their normal diet, and blood samples were drawn before and after the intervention. The DNA damage was determined by using the single cell gel (comet) assay with alkaline electrophoresis and was quantified by measuring tail length (TL). Levels of free radicals were determined by reading the lucigenin-perborate ROS generating source, using the Ultra-Weak Chemiluminescence Analyzer System. Grape juice consumption resulted in a significant decrease in lymphocyte DNA damage expressed by TL (before supplementation: 88.75 +/- 1.55 microm versus after supplementation: 70.25 +/- 1.31 microm; P=0.000 by paired t-test). Additionally, grape juice consumption for 8 weeks reduced the ROS/photon count by 15%, compared to the beginning of the study. The preventive effect of grape juice against DNA damage was simultaneously shown in both sexes. These results indicate that the consumption of grape juice may increase plasma antioxidant capacity, resulting in reduced DNA damage in peripheral lymphocytes achieved at least partially by a reduced release of ROS. Our findings support the hypothesis that polyphenolic compounds contained in grape juice exert cancer-protective effects on lymphocytes, limiting oxidative DNA damage possibly via a decrease in free radical levels.

Age-related base excision repair activity in mouse brain and liver nuclear extracts

To assess DNA repair activity relative to age, in vitro base excision repair assays were performed using brain and liver nuclear extracts prepared from mice of various ages. An 85% decline in repair activity was observed in brain nuclear extracts and a 50% decrease in liver nuclear extracts prepared from old mice compared with 6-day-old mice. Brain nuclear extracts prepared from old mice showed a decreased abundance of DNA polymerase-beta, but the addition of purified protein did not restore base excision repair activity. Abundances of other tested base excision repair proteins did not change relative to age. The conclusion is that, during aging, a decline in DNA repair could contribute to increased levels of DNA damage and mutagenesis.

Mitochondrial DNA damage in lymphocytes: a role in immunosenescence?

An age-related increase of DNA damage/mutation has been previously reported in human lymphocytes. The high copy number and mutation rate make the mtDNA genome an ideal candidate for assessing damage and to act as a potential biomarker of ageing. In the present study, two assays were developed to evaluate the level of mtDNA(4977) and the accumulation of point mutations with age. A competitive polymerase chain reaction (PCR) methodology incorporating three primers was used to detect and quantify the levels of mtDNA(4977) and a novel heteroduplex reference strand conformational analysis (RSCA) technique was used to analyse the accumulation of point mutations. The assays were applied to an in vitro model of T cell ageing and ex vivo DNA samples from an elderly cohort of subjects and a younger control group. The mtDNA(4977) was detected in all the DNA samples examined but only a very low concentration was observed and no age-related increase or accumulation was observed. No accumulation of point mutations was identified using RSCA within the T cell clones as they were aged or the ex vivo lymphocytes from the elderly cohort. A higher level of variation was observed within the ex vivo DNA samples, verifying the high resolution of RSCA and its ability to identify different mtDNA species, although no correlation with age was observed. The low level of mtDNA damage observed with respect to the ex vivo lymphocyte DNA samples within this study may be due in part to the high turnover of blood cells/mtDNA, which may inhibit the accumulation of genetically abnormal mtDNA that may play a role in immunosenescence. A similar explanation may also apply to the in vitro model of T cell ageing if the vast majority of the cells are replicating rather than entering senescence.

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.

Age-related accumulation of oxidative DNA damage and alterations in levels of p16(INK4a/CDKN2a), p21(WAF1/CIP1/SDI1) and p27(KIP1) in human CD4+ T cell clones in vitro

T cells in vivo have been shown to accumulate DNA damage with age. To investigate the effects of DNA damage on T cell biology we have utilised an in vitro human CD4+ T cell clone model. Levels and types of DNA damage were determined in 11 independent T cell clones as a function of their in vitro lifespan. Increased levels of reactive oxygen species (ROS) induced DNA damage with increasing age were found in all clones analysed using a modified alkaline comet assay. T cell clones underwent apoptosis at the end of their lifespans. There were no consistent changes in the mRNA levels for the cyclin-dependent kinase inhibitors (CKI) p16, p21, and p27 during the clones' lifespans. It appears that the increased levels of ROS induced DNA damage in the T cells is not the major trigger of apoptosis, via the p53/p21 pathway. In addition, at the end of their lifespans, the T cell clones did not display the CKI phenotype reported for senescent cells (an increase in p16 and p21 levels). Thus, while the T cell clones appear sensitive to ROS-induced DNA damage, the molecular mechanisms through which this influences T cell dysfunction with age remains to be elucidated.

Human cancer: etiologic agents/dose responses/DNA repair/cellular and animal models

The observation of cancer in an individual does not identify the causative agent(s). However, epidemiological data on populations do indicate that a large fraction of human cancers are associated with lifestyle/diet. Such studies may also help identify the etiologic agents but unless there are good dose-response data for humans and/or animal models, the probability of identifying the agent is not high. Cancers may result from endogenous reactions, such as oxidations or from exogenous agents, such as tobacco smoke (lung cancer), sunlight exposure (skin cancer), aflatoxin (liver cancer), and relatively high doses of ionizing radiations (many types of cancers). Many carcinogenic chemicals have been identified in the workplace but, they usually do not affect the overall population. Most cancer causing agents affect cellular DNA and change its coding specificity and act as cancer initiators. The repair of DNA damage ameliorates most of these endogenous and exogenous changes. The important role of DNA repair in controlling the induction of human cancer came from the observation that individuals with the skin cancer-susceptible, human disease xeroderma pigmentosum (XP) were defective in nucleotide excision repair. Endogenous DNA damages are usually damages to individual bases and are usually repaired by systems of glycosylases and endonucleases. It should be useful to investigate the rates of appearance of tumors in normal mice and in mice knocked out for specific repair enzymes because such mice could be used to test the roles of diet and caloric input in affecting particular types of endogenous damages.

Total antioxidant levels, gender, and age as risk factors for DNA damage in lymphocytes of the elderly

During past years, the association of oxidative stress with DNA damage and its possible clinical translation into chronic degenerative illnesses, such as atherosclerosis, cancer, diabetes mellitus and Alzheimer's disease, has been demonstrated. In addition, it has been pointed out that age and gender are factors that influence the generation of DNA damage; however, this is still controversial. We have previously reported the results of a study of 88 subjects older than 60 years of age in whom DNA damage is related with serum levels of total antioxidants. The results of this study demonstrate a greater frequency of DNA damage in elderly persons with normal levels of antioxidants, in addition to males, and in the younger group of subjects, i.e., 60-69 years. In this work, we enlarged our study sample to 160 elderly subjects; in this way, we were able to evaluate the consistency of the influence of total antioxidants, age, and gender on the magnitude and grade of DNA damage in lymphocytes of the elderly. The results demonstrated that 45% of the subjects showed DNA damage, measured by an alkaline unicellular electrophoresis technique (comet assay). Similarly, 62% of the subjects presented low levels of total antioxidant levels measured by a colorimetric method (Randox Kit). A greater percentage of DNA damage was observed in subjects with normal levels of antioxidants (48%) compared with subjects with low levels (43%), although the difference was not statistically significant. The group of subjects 70 years of age or older showed a greater percentage of DNA damage (50%) than the group of subjects of 60-69 years of age (41%). However, the difference was again not statistically significant (P>0.05). With respect to gender, 64% of males and 38% of females had DNA damage with an odds ratio (OR) of 2.86 and a 95% confidence interval (CI) of 1.31-6.32 (P<0.05). In the logistic regression analysis, the interaction of the male sex variables with low antioxidants had an OR of 2.5 (CI 95%, 1.33-4.68; P<0.01). We conclude that the interaction of male sex factors-low levels of antioxidants would justify the indication of antioxidant dietetic supplements.

Health span and life span in transgenic mice with modulated DNA repair

One way to better understand the contribution of DNA repair, DNA damage, and mutagenesis in aging would be to enhance DNA repair activity, lower DNA damage, and lower mutagenesis. Because the repair protein O6-methylguanine-DNA methyltransferase (MGMT) acts alone and stoichiometrically, the human MGMT (hMGMT) cDNA was selected to test the feasibility of enhancing DNA repair activity in transgenic mice. MGMT activity is largely responsible for ameliorating the deleterious effects of O6-methylguanine (O6mG) lesions in DNA in a direct reversal mechanism. A transgene was constructed consisting of a portion of the human transferrin (TF) promoter and hMGMT cDNA such that hMGMT is expressed in transgenic mouse brain and liver. Expression of hMGMT was associated with a significant reduction in the occurrence of an age-related hepatocellular carcinoma in male mice at 15 months of age. Longitudinal and cross-sectional studies were initiated to determine whether the reduced incidence of hepatocellular carcinoma would impact median or maximum life span. The cross-sectional study performed on 15-month-old male animals confirmed the reduced occurrence of spontaneous hepatocellular carcinoma. At 30 months of age, however, the occurrence of hepatocellular carcinoma in at least one transgenic line was similar to that for nontransgenic animals. The longitudinal study is ongoing; however, at present no significant differences in life span have been detected. Tissues expressing the MGMT transgene also displayed greater resistance to alkylation-induced tumor formation. These results suggest that transgenes can be used to direct enhanced DNA repair gene expression and that enhanced expression can protect animals from certain spontaneous and induced tumors.

The effects of carnosine on oxidative DNA damage levels and in vitro lifespan in human peripheral blood derived CD4+T cell clones

Carnosine (beta-alanyl-L-histidine), an abundant naturally-occurring dipeptide has been shown to exhibit anti-ageing properties towards cultured cells, possibly due in part to its antioxidant/free radical scavenging abilities. In this paper the results of an investigation on the effects of carnosine, at the physiological concentration of 20 mM, on oxidative DNA damage levels and in vitro lifespan in peripheral blood derived human CD4+ T cell clones are reported. Under the culture conditions used (20% O(2)) long term culture with carnosine resulted in a significant increase in the lifespan of a clone derived from a healthy young subject. No such extension was observed when a T cell clone from a healthy old SENIEUR donor was similarly cultured. Culture with carnosine from the midpoint of each clone's lifespan did not have any effect on longevity, independent of donor age. Oxidative DNA damage levels were measured in the clones at various points in their lifespans. Carnosine acted as a weak antioxidant, with levels of oxidative DNA damage being lower in T cells grown long term in the presence of carnosine. The possibility that carnosine might confer anti-ageing effects to T cells under physiological oxygen tensions would appear to be worthy of further investigation.

The effect of vitamin C or vitamin E supplementation on basal and H2O2-induced DNA damage in human lymphocytes

There is a wealth of epidemiological information on antioxidants and their possible prevention of disease progression but very little of the research on antioxidants has involved intervention studies. In this study, the potential protective effect of vitamin C or E supplementation in vivo against endogenous and H2O2-induced DNA damage levels in lymphocytes was assessed. The supplementation involved fourteen healthy male and female non-smokers mean age 25-53 (SD 1.82) years, who were asked to supplement an otherwise unchanged diet with 1000 mg vitamin C daily for 42 d or 800 mg vitamin E daily for 42 d. DNA damage in H2O2-treated peripheral blood lymphocytes (PBL) and untreated PBL before and after supplementation, and during a 6-week washout period was assessed using an ELISA. At each sampling time-point, the red cell concentrate activities of superoxide dismutase, catalase and glutathione peroxidase were also determined. Supplementation with vitamin C or vitamin E decreased significantly H2O2-induced DNA damage in PBL, but had no effect on endogenous levels of DNA damage. The activities of the antioxidant enzymes superoxide dismutase and glutathione peroxidase were suppressed during the supplementation period. These supplementation regimens may be used to limit the possible adverse effects of reactive oxygen species (including those produced during the course of an immune response) on lymphocytes in vivo, and so help to maintain their functional capacity.

Effect of dietary intake and lifestyle factors on in vivo mutant frequency at the HPRT gene locus in healthy human subjects

This paper describes the results of a study designed to assess the effects of a variety of dietary and lifestyle factors on background levels of mutant frequency (MF) at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene locus in humans. Eighty-three healthy and free-living subjects (aged 20-80 yr; 61 males and 22 females; mean age of 63.07 +/- 14.71 yr) were recruited. Background levels of MF were determined for each subject using a cloning assay. The mean MF/10(6) clonable cells (MF) for the study subjects was 4.63 +/- 2.20. An interview-administered questionnaire was completed by each study subject in order to assess details of dietary history, physical activity, health and potential genotoxin exposure history. A 7-day estimated dietary record method with a food frequency questionnaire was used to determine average intakes of energy and macronutrients (including alcohol), and a range of micronutrients (including vitamin and mineral supplement usage). The relationships between individual dietary and lifestyle factors and HPRT MF were investigated by univariate and multivariate analysis (data was adjusted for age, lymphocyte plating efficiency [PE] and energy intake [EI]). Univariate analysis revealed a significant positive correlation between EI and MF and multivariate analysis revealed significant positive correlations between, body mass index (BMI), % energy intake from total carbohydrate, starch, fat and MF. These findings suggest that a reduction in EI may be a useful preventative measure against the onset of carcinogenesis in humans. No correlations were found between alcohol intake and MF or between estimated antioxidant intake and MF. Thus, estimated intakes of antioxidants may not reflect their bioavailability and functional capacity in vivo and it may be more useful to examine actual plasma/cell levels vs. MF to establish if any significant relationship exists.

Validation of the human T-lymphocyte cloning assay--ring test report from the EU concerted action on HPRT mutation (EUCAHM)

The T-cell cloning assay, which enables the enumeration and molecular analysis of 6-thioguanine resistant (HPRT-negative) mutant T-cells, has been extensively used for studying human somatic gene mutation in vivo. However, large inter-laboratory variations in the HPRT mutant frequency (MF) call for further investigation of inter-laboratory differences in the experimental methodology, and development of an optimal but easy uniform cloning protocol. As part of the EU Concerted Action on HPRT Mutation (EUCAHM), we have carried out two Ring tests for the T-cell cloning assay. For each test, duplicate and coded samples from three buffy coats were distributed to five laboratories for determination of MF using six different protocols. The results indicated a good agreement between split samples within each laboratory. However, both the cloning efficiencies (CEs) and MFs measured for the same blood donors showed substantial inter-laboratory variations. Also, different medium compositions used in one and the same laboratory resulted in a remarkable difference in the level of MF. A uniform operating protocol (UOP) was proposed and compared with the traditional protocols in the second Ring test. The UOP (preincubation) increased the CE in laboratories traditionally using preincubation, but decreased the CE in laboratories traditionally using priming. Adjusted for donor, use of different protocols contributed significantly to the overall variation in lnCE (P = 0.0004) and lnMF (P = 0.03), but there was no significant laboratory effect on the lnCE (P = 0.38) or lnMF (P = 0.14) produced by the UOP alone. Finally, a simplified version of the UOP using the serum-free medium X-Vivo 10 and PMA was tested in one laboratory, and found to produce a considerable increase in CE. This modified UOP needs to be further evaluated in order to be used for future databases on HPRT MFs in various populations.

Biomarkers of DNA damage in marine mammals

Certain environmental contaminants found in marine mammals have been shown to cause DNA damage and cancer. The micronuclei (MN), sister chromatid exchange (SCE) and/or chromosome aberration (CA) assays were used to assess baseline (spontaneous) levels of DNA damage in blood lymphocytes of individuals of the relatively healthy and lightly contaminated Arctic beluga whale (Delphinapterus leucas), Sarasota Bay, FL, bottlenose dolphin (Tursiops truncatus) and Northwestern Atlantic grey (Halichoerus grypus) and harp (Phoca groenlandicus) seal populations. MN cell (MNC) frequencies ranged between 2 and 14/1000 binucleated (BN) cells and were statistically similar between species. In bottlenose dolphins, MNC frequency was correlated with age and was significantly higher in females than in males. No intraspecific variation in MNC frequency was found in beluga whales. Intraspecific variation was not tested in seals due to the small sample size. Frequencies of SCEs and total CAs, excluding gaps, ranged, respectively, between 1 and 15 SCE(s)/per cell and 4-6 CAs/100 cells in beluga whales. SCE and CA frequencies did not vary with age or sex in beluga whales. The MN, SCE and CA assays were found to be practical tools for the detection of DNA damage in marine mammals and could be used in the future to compare DNA damage between relatively lightly and highly contaminated populations.

The aging paradox: free radical theory of aging

There are more than 300 theories to explain the aging phenomenon. Many of them originate from the study of changes that accumulate with time. Among all the theories, the free radical theory of aging, postulated first by Harman, is the most popular and widely tested, and is based on the chemical nature and ubiquitous presence of free radicals. This review aims to recapitulate various studies on the role of free radicals in DNA damage-both nuclear as well as mitochondrial-the oxidative stress they impose on cells, the role of antioxidants, the presence of autoantibodies, and their overall impact on the aging process.

T cell immunosenescence in vitro and in vivo

The term "immunosenescence" refers to an age-associated dysregulation of immune function which contributes to the increased susceptibility of the elderly to infectious disease. Although there are age-associated changes measurable in the innate immune system (Pawelec et al., 1998c), it is the adaptive, particularly T cell, system which is most susceptible to the deleterious effects of aging. In this minireview, characteristics of aging in long-term human T cell cultures will be summarized, and the parallels between the in vitro model and in vivo immunosenescence will be documented. The use of culture models to screen for ways of manipulating immunosenescence in vitro may provide a basis for intervention to ameliorate immunosenescence in vivo.

Age-associated decreases in human DNA repair capacity: Implications for the skin

Multiple pathways are involved in accurate synthesis and distribution of DNA during replication, repair and maintenance of genomic integrity. An increased error rate, abovethe spontaneous mutation baseline, has been implicated in carcinogenesis and aging. Moreover, cytogenetic abnormalities are increased in Down's, Edwards', Patau's, and Klinefelter's syndromes with increasing maternal age, and in Marfan's and Apert's syndromes with paternal age. In response to DNA damage, multiple overlapping systems of DNA repair have evolved, preferentially repairing the transcribed strand within transcriptionally-active regions of the genome. These include direct reversal of dimers and specific adducts and pathways for base excision, nucleotide excision, and mismatch repair. A consensus has emerged that some DNA repair capacities decline with organism age, contradictory reports notwithstanding. As is the case for inborn defects in humans, knockout mice lacking components of nucleotide excision repair or DNA-damage checkpoint arrest have increased frequencies of skin and internal cancers, whereas mice overexpressing DNA repair genes have fewer spontaneous cancers. Oxidative stress and resultant free radicals can damage genomic and mitochondrial DNA; damage increases with age but decreases with caloric restriction. We review recent studies of long-lived C. elegans mutants which appear to involve metabolic attenuation, the role of telomere shortening and telomerase in cellular senescence, and the genetic bases of progeroid syndromes in humans. Finally, we discuss roles of extrinsic and intrinsic factors in skin aging, and their association with DNA damage, emphasizing preventive and protective measures and prospects for intervention by modulating DNA repair pathways in the skin.

Binding of circulating antibodies to reactive oxygen species modified-DNA and detecting DNA damage by a monoclonal antibody probe

Circulating antibodies in the sera of normal, healthy humans of different age groups to native DNA (nDNA) and reactive oxygen species modified DNA (ROS-DNA) was studied by competition ELISA. Sera from the young population (< 50 years of age) showed negligible levels of anti-DNA antibodies. In contrast, anti-DNA antibodies were found in three sera from the moderately aged group (50-59 years) with a high binding to ROS-DNA (43-47%). In the aged group (> 60 years), four sera showed higher recognition of ROS-DNA (> 60% inhibition) over nDNA (55-60%). Oxidative lesions in human genomic DNA were immunochemically detected using the monoclonal anti-ROS-DNA antibody as a probe. The antibody has a high specificity for ROS-DNA and preferentially recognizes ROS-modified epitopes on nucleic acids. The study indicates low recognition of DNA isolated from the younger population, while in the age group 50-59 years, one DNA isolate showed a high inhibition (57%) in monoclonal antibody binding. Four DNA isolates from the aged group showed substantial inhibition in antibody activity to the extent of 49, 53, 64 and 69%. The results demonstrate an age-related increase in the levels of anti-DNA antibodies, with a higher recognition and binding to ROS-DNA. A high reactivity of DNA isolated from aged individuals by the monoclonal antibody indicates increased oxidative stress leading to DNA damage. It is suggested that free radical damage to DNA in vivo, particularly in the aged, alters its antigenicity and stimulates an immune response against modified DNA. These antibodies are cross-reactive to nDNA.

DNA damage and mutation: contributors to the age-related alterations in T cell-mediated immune responses?

The genetic material of our cells is susceptible to damage by a wide variety of extrinsic and intrinsic entities. The amount of genetic damage accumulated in vivo will depend upon an individual's ability to defend against and/or repair DNA damage. T cells in vivo have been shown to accumulate DNA damage and mutations over time. The accumulation of such genetic damage will occur in T cells possessing a 'naive' or a 'memory' phenotype. Since T cells are required to undergo extensive clonal expansion upon antigenic stimulation, DNA damage and mutations may result in: a failure of T cells to proliferate, because of DNA damage-mediated cell cycle arrest; decreased rates of proliferation, as a consequence of selection in vivo against cells carrying certain mutations and/or apoptosis, triggered by critical levels of DNA damage. Thus, when T cells, containing critical levels of genetic damage, are required to undergo rapid clonal expansion in the presence of antigen, insufficient numbers of T cells may be produced and so the immune response would be sub-optimal. In this paper the possible contribution of DNA damage and/or mutation to the age-related alterations in T cell-mediated immune responses will be discussed.

Genomic instability in the type II TGF-beta1 receptor gene in atherosclerotic and restenotic vascular cells

Cells proliferating from human atherosclerotic lesions are resistant to the antiproliferative effect of TGF-beta1, a key factor in wound repair. DNA from human atherosclerotic and restenotic lesions was used to test the hypothesis that microsatellite instability leads to specific loss of the Type II receptor for TGF-beta1 (TbetaR-II), causing acquired resistance to TGF-beta1. High fidelity PCR and restriction analysis was adapted to analyze deletions in an A10 microsatellite within TbetaR-II. DNA from lesions, and cells grown from lesions, showed acquired 1 and 2 bp deletions in TbetaR-II, while microsatellites in the hMSH3 and hMSH6 genes, and hypermutable regions of p53 were unaffected. Sequencing confirmed that these deletions occurred principally in the replication error-prone A10 microsatellite region, though nonmicrosatellite mutations were observed. The mutations could be identified within specific patches of the lesion, while the surrounding tissue, or unaffected arteries, exhibited the wild-type genotype. This microsatellite deletion causes frameshift loss of receptor function, and thus, resistance to the antiproliferative and apoptotic effects of TGF-beta1. We propose that microsatellite instability in TbetaR-II disables growth inhibitory pathways, allowing monoclonal selection of a disease-prone cell type within some vascular lesions.

The T cell in the ageing individual

Aged persons frequently manifest declining parameters of those immune functions which protect the young against disease. Longitudinal studies are beginning to show that number, type and function of T cells may be associated with longevity, morbidity and mortality in free-living elderly humans. Multi-faceted alterations in the ability of T cells from old donors to respond to stimulation are being dissected, and pathways which are compromized in the elderly compared to the young are being defined. Successful immune responses depend upon waves of rapid and extensive clonal expansion to combat primary infection, followed by death of most T cells, survival of memory cells and their later reactivation and further expansion. This implies that the finite replicative potential of T cells might impose a critical limiting factor on the maintenance of immune responses in the face of thymic involution and drastically reduced capacity to generate new naive T cells. This type of 'clonal exhaustion' can readily be studied in vitro using human T cell clones and the findings can be applied to the in vivo situation. Understanding the processes of replicative senescence in such in vitro models may shed light not only on some of the underlying mechanisms of immunosenescence but also in situations of chronic antigenic stimulation in vivo. Moreover, it might begin to indicate how the system could be manipulated on the one hand to prevent or reverse T cell senescence without nullifying the control mechanisms of tumor suppression, and on the other hand, to reconstitute possibly faulty suppression, for example in autoimmune disease.

Unstable and stable chromosomal aberrations in lymphocytes of people exposed to Chernobyl fallout in Bryansk, Russia

Analyses of unstable and stable chromosomal aberrations in peripheral blood lymphocytes were used in the assessment of radiation exposure of residents of a village situated in the Chernobyl fallout-contamination zone of Bryansk, Russia. Blood samples were taken from subjects residing in villages with high (> 1100 kBq/m2 137 Cs; Mirnyi) and very low (< 37 kBq/m2 137 Cs; Krasnyi Rog) contamination, 7 years after the Chernobyl accident. The groups were matched by age, sex, smoking habits and previous medical radiological exposures. A total of 200 people (100 exposed, 100 controls) were analysed for the presence of unstable aberrations from Giemsa-stained slides. To study stable aberrations, chromosome painting analyses were performed on 100 subjects (50 exposed, 50 controls), using painting probes for chromosomes 1, 2 and 4 and a pancentromeric probe. People living in the contaminated area showed significantly higher rates of unstable chromosome-type aberrations but not chromatid-type aberrations in their lymphocytes, indicating radiation exposure as a causative factor for the observed difference. No significant differences were found in the aberration rates between the two areas by the chromosome painting method. The levels of chromosome exchanges were low in both populations, but consistently higher in Mirnyi compared with the control area. The magnitude of radiation exposure resulting from Chernobyl fallout was estimated on the basis of excess stable chromosomal aberrations in the lymphocytes of the Mirnyi population compared with the controls.

The effect of donor age on the processing of UV-damaged DNA by cultured human cells: reduced DNA repair capacity and increased DNA mutability

Aging in humans carries an increased risk of skin cancer, a disorder linked to somatic mutations in sun damaged skin. DNA repair plays a major role in protection against sun damage. We found an age-related decline in post-UV DNA repair capacity (measured by the ability to repair a UV-treated plasmid (pCMVcat)) of-0.6% per year (p = 0.0001) in cultured primary skin fibroblasts from normal donors from the first to the tenth decade of life. There was a corresponding age-related increase in post-UV mutability (measured as mutations introduced into a transfected, UV-treated plasmid (pSP189)) of +0.6% per year (p = 0.001) in lymphoblastoid cell lines from normal donors of the same age range. This study indicates that aging in humans is associated with decreasing ability to process new UV-induced DNA damage and this age-related reduction in DNA repair capacity and increase in DNA mutability is reflected in cultured skin and blood cells.

Chromosome aberrations in humans in relation to site of residence

Baseline frequencies of chromosome aberrations (CAs) were assessed in three samples of healthy individuals, 60 living in a rural area (Po Delta), 134 in Pisa downtown and 116 in Cascina, a small town near Pisa, Italy. The three groups were similar for average age, sex ratio, smoking, drinking habit, and occupation. Multifactor ANOVA showed that CA frequencies increased significantly with age (p < 0.0001 excluding and including gaps), and with smoking habit (p = 0.0045 including gaps; p = 0.04 excluding gaps). Gender, drinking habit and occupation exerted no statistically significant effects. Multifactor ANOVA showed also a significant effect of the site of residence on the frequency of the CA, including gaps (p = 0.0003) and excluding gaps (p = 0.03). The CA frequency of the Pisa samples was statistically significantly higher than that of the Po Delta samples. Air pollution was considered to be a possible factor in determining the observed differences among the sites of residence, as levels of air pollutants (SO2 and TSP, total suspended articles) were more elevated in Pisa and Cascina than in the Po Delta. In addition, respiratory symptoms used as indirect indicators of air pollution at individual level were significantly more frequent in the Pisa population than in Cascina or in the Po Delta. These findings might support the hypothesis that air-pollution levels, even within E.E.C. (European Economic Community) air-quality standards, may influence baseline CA frequencies.

Chromosomal aberration analysis in 85 control individuals

Chromosomal aberrations (CA) were studied in the peripheral blood lymphocytes of 85 healthy male volunteers from Barcelona (Catalonia, Spain). The effect that factors such as age, life style, work exposure and medical treatment had on the cytological endpoints was studied by means of a Poisson regression model. The results obtained indicate a significant positive relationship between the age of the subjects analyzed and the total of chromosome-type aberrations. With respect to the other variables analyzed, a positive association was found between the frequent consumption of analgesics and the incidence of chromatid-type aberrations.

Use of transgenic mouse models for studying somatic mutations in aging

Theories on the causes of aging, based on the accumulation of somatic mutations in tissues of an organism, were formulated decades ago, but remain insufficiently tested. Transgenic animals, equipped with integrated bacterial reporter genes that can be efficiently rescued from total genomic DNA of all tissues and organs, represent ideal tools for investigating the types and frequencies of spontaneous mutants accumulating during aging. The first of such systems, based on the transgenic integration of bacteriophage lambda shuttle vectors that contain the bacterial lacZ gene as mutational target, was constructed in our laboratory and is now routinely used. Results obtained with this and the related LacI system that are relevant for the somatic mutation theory of aging will be discussed. One conclusion is that, due to the nature of the transgene, lambda-based systems have the disadvantage that deletion type mutations are underrepresented in comparison to point mutations. To overcome those limitations, we constructed a new transgenic mouse model carrying a pUR288 plasmid shuttle vector with the lacZ reporter gene. Some preliminary data obtained with this model serve to illustrate its potential use to extensively test the somatic mutation theory of aging.