Somatic mutations and aging: a re-evaluation

Increased genomic instability is not a prerequisite for shortened lifespan in DNA repair deficient mice

Genetic defects in nucleotide excision repair (NER) are associated with premature aging, including cancer, in both humans and mice. To investigate the possible role of increased somatic mutation accumulation in the accelerated appearance of symptoms of aging as a consequence of NER deficiency, we crossed four different mouse mutants, Xpa-/-, Ercc6(Csb)-/-, Ercc2(Xpd)m/m and Ercc1-/m, with mice harboring lacZ-reporter genes to assess mutant frequencies and spectra in different organs during aging. The results indicate an accelerated accumulation of mutations in both liver and kidney of Xpa defective mice, which correlated with a trend towards a decreased lifespan. Until 52 weeks, Xpa deficiency resulted mainly in 1-bp deletions. At old age (104 weeks), the spectrum had undergone a shift, in both organs, to G:C-->T:A transversions, a signature mutation of oxidative DNA damage. Ercc1-/m mice, with their short lifespan of 6 months and severe symptoms of premature aging, especially in liver and kidney, displayed an even faster lacZ-mutant accumulation in liver. In this case, the excess mutations were mostly genome rearrangements. Csb-/- mice, with mild premature aging features and no reduction in lifespan, and Xpdm/m mice, exhibiting prominent premature aging features and about 20% reduction in lifespan, did not have elevated lacZ-mutant frequencies. It is concluded that while increased genomic instability could play a causal role in the mildly accelerated aging phenotype in the Xpa-null mice or in the severe progeroid symptoms of the Ercc1-mutant mice, shortened lifespan in mice with defects in transcription-related repair do not depend upon increased mutation accumulation.

Cancer as a programmed death of an organism

The hypothesis introduces the idea that there is a critical level of mutagenesis that triggers a program of organism death by means of proliferation of killer cells. Similarly to apoptosis, which is an altruistic suicidal act of a faulty cell threatening the stability of a multicellular organism, a malignant tumor is an altruistic suicide of an individual carrier of harmful alleles threatening genetic stability of the population.

Microsatellite instability (MSI) increases with age in normal somatic cells

Small pool PCR (SP-PCR) is a sensitive method for the detection and quantification of microsatellite instability (MSI) in somatic cells. Here we propose that mutant microsatellite fragments accumulate with age in normal somatic cells and that this increase in MSI can be quantified by SP-PCR. MSI at 6 microsatellite loci was determined by SP-PCR in PBL DNA from 17 "normal" blood bank donors. These individuals varied in age from 20 to 67 y/o. MSI phenotypes were plotted against age in a regression analyses. A positive slope indicated a correlation between age and MSI phenotype (p=0.0006). The mean weighted average mutant frequencies across all loci for all individuals in the age groups (0.009 for 20-30 y/o; 0.019 for 35-50 y/o; 0.034 for 60-70 y/o) were also significantly different from each other (p<0.01). A baseline for increases of MSI with age in human somatic cells was therefore begun and the effectiveness of SP-PCR to evaluate low, but significant, levels of MSI, established.

Telomere dysfunction: a new player in radiation sensitivity

Human individuals often exhibit important differences in their sensitivity to ionising radiation. Extensive literature links radiation sensitivity with impaired DNA repair which is due to a lack of correct functioning in many proteins involved in DNA-repair pathways and/or in DNA-damage checkpoint responses. Given that ionising radiation is an important and widespread diagnostic and therapeutic tool, it is important to investigate further those factors and mechanisms that underlie individual radiosensitivity. Recently, evidence is accumulating that telomere function may well be involved in cellular and organism responses to ionising radiation, broadening still further the currently complex and challenging scenario.

Understanding and modulating ageing

Ageing is characterized by a progressive accumulation of molecular damage in nucleic acids, proteins and lipids. The inefficiency and failure of maintenance, repair and turnover pathways is the main cause of age-related accumulation of damage. Research in molecular gerontology is aimed at understanding the genetic and epigenetic regulation of survival and maintenance mechanisms at the levels of transcription, post-transcriptional processing, post-translational modifications, and interactions among various gene products. Concurrently, several approaches are being tried and tested to modulate ageing in a wide variety of organisms. The ultimate aim of such studies is to improve the quality of human life in old age and prolong the health-span. Various gerontomodulatory approaches include gene therapy, hormonal supplementation, nutritional modulation and intervention by free radical scavengers and other molecules. A recent approach is that of applying hormesis in ageing research and therapy, which is based on the principle of stimulation of maintenance and repair pathways by repeated exposure to mild stress. A combination of molecular, physiological and psychological modulatory approaches can realize "healthy ageing" as an achievable goal in the not-so-distant future.

Mismatch repair system and aging: microsatellite instability in peripheral blood cells from differently aged participants

Age-related alterations of DNA repair could be involved in the accumulation of genetic damage with age. Few data suggest a possible alteration with age of the mismatch repair system, evidenced by the acquisition of microsatellite instability. We aimed to point out a possible implication of this repair system in the accumulation of genetic damage with age. Peripheral blood cell DNA from 226 participants, 110 young (25-35 years), 58 old (85-97 years), and 58 centenarian was analyzed at five polymorphic microsatellite loci (CD4, p53, VWA31, TPOX, and FES/FPS) to point out age-related instabilities or modifications in allele frequencies. FES/FPS microsatellite was the most instable, showing both the appearance of trizygosis in DNA from old participants and differences in allele patterns among age groups, thus indicating an association between increased microsatellite instability and aging, one of the possible causes of which being an impairment of mismatch repair system capacity with age.

Time-course of mitochondrial gene expressions in mice brains: implications for mitochondrial dysfunction, oxidative damage, and cytochrome c in aging

The study of aging is critical for a better understanding of many age-related diseases. The free radical theory of aging, one of the prominent aging hypotheses, holds that during aging, increasing reactive oxygen species in mitochondria causes mutations in the mitochondrial DNA and damages mitochondrial components, resulting in senescence. Understanding a mitochondrial gene expression profile and its relationship to mitochondrial function becomes an important step in understanding aging. The objective of the present study was to determine mRNA expression of mitochondrial-encoded genes in brain slices from C57BL6 mice at four ages (2, 12, 18, and 24 months) and to determine how these altered mitochondrial genes influence age-related changes, including oxidative damage and cytochrome c in apoptosis. Using northern blot analysis, in situ hybridization, and immunofluorescence analyses, we analyzed changes in the expression of mitochondrial RNA encoding the mitochondrial genes, oxidative damage marker, 8-hydroxyguanosine (8-OHG), and cytochrome c in brain slices from the cortex of C57BL6 mice at each of the four ages. Our northern blot analysis revealed an increased expression of mitochondrial-encoded genes in complexes I, III, IV, and V of the respiratory chain in 12- and 18-month-old C57BL6 mice compared to 2-month-old mice, suggesting a compensatory mechanism that allows the production of proteins involved in the electron transport chain. In contrast to the up-regulation of mitochondrial genes in 12- and 18-month-old C57BL6 mice, mRNA expression in 24-month-old C57BL6 mice was decreased, suggesting that compensation maintained by the up-regulated genes cannot be sustained and that the down-regulation of expression results in the later stage of aging. Our in situ hybridization analyses of mitochondrial genes from the hippocampus and the cortex revealed that mitochondrial genes were over-expressed, suggesting that these brain areas are critical for mitochondrial functions. Our immunofluorescence analysis of 8-OHG and cytochrome c revealed increased 8-OHG and cytochrome c in 12-month-old C57BL6 mice, suggesting that age-related mitochondrial oxidative damage and apoptosis are associated with mitochondrial dysfunction. Our double-labeling analysis of in situ hybridization of ATPase 6 and our immunofluorescence analysis of 8-OHG suggest that specific neuronal populations undergo oxidative damage. Further, double-labeling analysis of in situ hybridization of ATPase 6 and immunofluorescence analysis of cytochrome c suggest cytochrome c release is related to mitochondrial dysfunction in the aging C57BL6 mouse brain. This study also suggests that these mitochondrial gene expression changes may relate to the role of mitochondrial dysfunction, oxidative damage, and cytochrome c in aging and in age-related diseases such as Alzheimer's disease and Parkinson's disease.

DNA repair, genome stability, and aging

Aging can be defined as progressive functional decline and increasing mortality over time. Here, we review evidence linking aging to nuclear DNA lesions: DNA damage accumulates with age, and DNA repair defects can cause phenotypes resembling premature aging. We discuss how cellular DNA damage responses may contribute to manifestations of aging. We review Sir2, a factor linking genomic stability, metabolism, and aging. We conclude with a general discussion of the role of mutant mice in aging research and avenues for future investigation.

Making ends meet in old age: DSB repair and aging

Accumulation of somatic mutations has long been considered as a major cause of aging and age-related diseases such as cancer. Genomic rearrangements, which arise from aberrant repair of DNA breaks, are the most characteristic component of the mutation spectra in aging cells and tissues. The studies conducted in the past few years provide further support for the role of DNA double-strand break (DSB) repair in aging and cellular senescence. Evidence was obtained that in addition to accumulation of mutations the efficiency and fidelity of repair declines with age. We propose that DNA damage and age-related decline of DNA repair form a vicious cycle leading to amplification of damage and progression of aging, and discuss a hypothesis on how the interplay between the two pathways of DSB repair, homologous recombination and nonhomologous end joining, may contribute to the aging process.

Age-related alteration in hepatic acyl-CoA: cholesterol acyltransferase and its relation to LDL receptor and MAPK

The aim of this study was to evaluate changes in the regulation of lipid metabolism and mitogen-activated protein kinases (MAPK) in the liver of C57BL/6 mice as they age. This was done by assessing the status of total cholesterol content and its enzyme, acyl-CoA: cholesterol acyltransferase (ACAT), in liver microsomal preparations and the low-density lipoprotein receptor (LDLr) mRNA expression in the livers of 4-24-month-old C57B/6 mice, without exogenous cholesterol feeding. With aging, there was an increase in cholesterol content and ACAT activity in liver microsomes. Northern blot analysis and real-time quantitative polymerase chain reaction data showed that ACAT-2 mRNA increased with age as well. LDLr expression decreased significantly in an age-dependent manner. In addition, we studied the basal and activated forms of MAPK, e.g. extracellular regulatory kinase (ERK-1/2), c-jun NH2-terminal kinase (JNK-1/2) and p38 MAPK. During aging, there was a considerable decrease in phosphorylated ERK-1/2 level while JNK-1/2 and p38 MAPK levels increased with age. Our studies showed an altered LDLr expression and altered phosphorylated MAPK in the liver of C57BL/6 mice during aging. These alterations might contribute to the development of atherosclerosis, hypercholesterolemia and other cholesterol-related conditions.

On evolutionary origin of cancer

BACKGROUND: The necessary and sufficient capabilities of cancer cell have been identified. Strikingly, this list does not include one that would seem to be a key property, namely the ability of cancer cells to kill their "host". This is believed to be a self-evident consequence of the other capabilities (e.g., metastasis), although the available evidence suggests a distinct killer function. Taking into account this unlisted property can significantly affect the current paradigm of carcinogenesis. PRESENTATION OF THE HYPOTHESIS: On the assumption that killer function is a key capability of the cancer cell, it is suggested that cancer has evolved as a mechanism of negative selection of mutant alleles of vitally important genes present in population. Similarly to apoptosis, which is an altruistic suicidal act of a damaged cell, cancer is an altruistic suicidal act of an individual who carries dangerous alleles and presents a hazard for genetic stability of the population. From this point of view, apoptosis is not a protection means against cancer as generally believed, but rather they are the first and second lines of defense against genome instability, respectively. TESTING THE HYPOTHESIS: The modern DNA array technology is capable of revealing gene expression profiles responsible for killer function of cancer cell as well as those specific targets in the body that are most strongly affected by the tumor growth. IMPLICATIONS OF THE HYPOTHESIS: This hypothesis suggests new avenues of cancer research as well as principally new therapeutic strategies.

Cancer in the oldest old

Our previous work revealed that 88% of centenarians delay or escape the age-related lethal diseases cardiac disease, stroke and diabetes. In the cases of those having a history of cancer we have observed anecdotes of centenarians presenting with large primary tumors that would have otherwise been expected to have metastasized and to have been lethal. However, these tumors were removed without consequence. To better understand the relationship between cancer and exceptional longevity, we quantified age of cancer diagnoses, life-time clinically evident cancer prevalence, tobacco use and family histories through medical record review and interviews. One thousand one hundred and forty-three subjects were studied revealing 20% (N=152) of female and 22% (N=80) of male centenarians with a history of non-skin cancer. The most common cancers were prostate (11.7% of males), breast (8.2% of females), and colon (5.7%). The average age of diagnosis was 80.5 years compared to 63.2 years in the general population according to National Cancer Institute SEER data. Similar delays were noted when age of onset was examined according to specific type of cancer. In conclusion, the age of diagnosis of cancer is relatively delayed in those who live to 100 years. Some cancers are very rare among these individuals suggesting that there are certain cancers that may be incompatible with survival to extreme old age.

Pharmacologic manipulation of the ataxia–telangiectasia mutated gene product as an intervention in age-related disease

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by progressive ataxia, elevated cancer incidence, and premature aging. A-T cells, Atm-deficient mice, and individuals with A-T show increased oxidant sensitivity, genomic instability, altered IGF-1 and p53 signaling, and rapid telomere shortening compared to normal controls. The gene mutated in A-T, ATM, regulates DNA repair, IGF-1 and p53 signaling, age pigment removal, antioxidant capacity, and telomere maintenance - pathways involved in and often attenuated with aging. Interestingly, flavonoids with chemopreventative effects, such as quercetin, genistein, and epigallocatechin gallate activate ATM. Since ATM activates pathways which increase genomic stability, oxidant resistance, and/or telomere stability, and since many diseases of old age (i.e., cancer, cardiovascular and neurodegenerative disease), result from attenuation of these pathways, pharmacologic manipulation of ATM activity via flavonoid intake may prove useful in slowing the appearance of age-associated disease.

Open-minded scepticism: inferring the causal mechanisms of human ageing from genetic perturbations

Given the myriad of age-related changes and the many proposed mechanistic theories of ageing, a major problem in gerontology is distinguishing causes from effects. This review aims to identify and evaluate those mechanisms which have gathered experimental support in favour of seeing them as a cause rather than an effect of ageing. Recent results related to energy metabolism and ageing, the free radical and the DNA damage theories of ageing are reviewed and their predictions evaluated through a systems biology rationale. Crucial in this approach are genetic manipulations in animal models that enable researchers to discriminate causes from effects of ageing and focus on the causal structure of human ageing. Based on a system-level interpretation, the GH/IGF-1 axis appears the most likely explanation for caloric restriction and a possible causal mechanism of human ageing. Although much work remains to fully understand the human ageing process, there is little evidence that free radicals are a causal factor in mammalian ageing, though they may be involved in signalling pathways related to ageing. On the other hand, studying how the DNA machinery affects ageing appears a promising avenue for disclosing the human ageing process.

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.

Spontaneous mutations in digestive tract of old mice show tissue-specific patterns of genomic instability

In an attempt to evaluate the possible role of mutations in the age-dependent increase of tumor incidence, we studied the mutational burden that accumulates in the aging process in different parts of the digestive tract in mice. The mutations were monitored in lacZ genes integrated in the mouse genome. The digestive tract was divided into the esophagus, stomach, proximal, medial, and distal part of the small intestine, and the colon. Epithelial tissues were separated from these tissues with the exception of the esophagus, in which case the whole tissue was examined. At a young age, the mutant frequencies as well as the molecular nature of the mutations were similar among the tissues examined. In old age, on the other hand, mutant frequencies were elevated to different degrees among the tissues; they were high in the small intestine and colon, intermediate in the stomach, and low in the esophagus. The molecular characteristics of the mutations also revealed distinct tissue-specificity; there were elevated rates of a small deletion mutation in the esophagus, G:C to T:A transversion in the proximal small intestine, and multiple mutations in the distal small intestine and colon. The results indicate that different parts of the digestive tract suffer from different kinds of mutational stress in the aging process. The nature of the multiple mutations suggests the presence of a mutator phenotype based on an imbalance in deoxyribonucleotide pools.

DNA repair in human fibroblasts, as reflected by host-cell reactivation of a transfected UV-irradiated luciferase gene, is not related to donor age

The effect of donor age on the ability of mammalian cells to repair ultraviolet (UV)-induced DNA damage has been studied using several approaches, most recently via assays that measure the host-cell reactivation (HCR) of UV-irradiated reporter gene-containing plasmid vectors following their transfection into cells. Plasmid HCR assays indirectly quantify a cell line's ability to perform nucleotide excision repair (NER) by measuring the enzyme activity of the repaired reporter gene, e.g., chloramphenical acetyltransferase (cat) or luciferase (luc), and are useful in studies investigating whether increasing age may be a risk factor for the deficient repair of potentially cancer-causing, sunlight-induced, DNA lesions in skin cells. In our study, we quantified the DNA repair ability of cultured, nontransformed, human skin fibroblast lines through their HCR of a transfected UV-C-irradiated plasmid containing luc. HCR was measured at various times after transfection in five lines from normal donors of ages 21-96 years, and from one donor who had xeroderma pigmentosum (XP). The normal lines displayed increasing HCR at successive post-transfection time points and showed no significant correlation between HCR and donor age. The XP-A line, known to be markedly deficient in NER of UV-induced DNA damage, showed minimal evidence of HCR compared to the normal lines. To further assess potential variation in HCR with donor age, fibroblast lines from five old donors, ages 84-94 years, were compared with lines from five young donors, ages 17-26 years. While significant differences in HCR were found between some lines, no significant difference was found between the young and old age groups (P = 0.44). Our study provides no indication that the higher incidence of skin cancer observed with increasing age is due to an age-related decrease in the ability to repair UV-induced DNA damage.

Non-neutral role of replicative senescence in tissue homeostasis and tumorigenesis

Normal somatic cells divide only a limited number of times reaching a state known as replicative senescence. This restraint in reproductive potential has been proposed as a mechanism evolved in higher eukaryotes to protect the organism from developing cancer. However, despite this protection there is a positive correlation between tumor incidence and organism aging when cells are potentially closer to their replication limit. We use simple mathematical models derived from quasispecies theory to analyse the role of senescence in various scenarios with different cell types according to their replicative capacity. The models predict that a situation with cells launching more often the senescence response plays against tissue homeostasis favoring tumor initiation. It is also shown that cancer cells arising early in organism life are more sensitive to genetic instabilities progressing less often toward tissue invasion. The passage of cells through crisis emerges as a mechanism to maintain tissue homeostasis that is weakened in aged individuals. The models introduced, though simple, help to integrate experimental information relating tumorigenesis with cellular and organism aging phenomena.

Behavior of metastatic and nonmetastatic breast tumors in old mice

Breast cancer incidence and mortality increase with age. A better understanding of the biological behavior of metastatic and nonmetastatic breast tumors in older subjects may help to develop improved breast cancer therapies. In this study, we used syngeneic metastatic (4TO7cg) and nonmetastatic (64pT) mouse breast tumor models at three age levels to evaluate various characteristics that are considered to be important for effective anti-breast cancer immunotherapy. These included tumor size and growth, metastases, vascularization, gene expression levels of the tumor-associated antigen (TAA) Mage-b (homologous to human MAGE-B) in primary breast tumors and metastases, and the presence of CD4(+) and CD8(+) T cells in the inguinal lymph nodes at the site of the tumor. The primary breast tumors and metastases were generated by injection of mouse mammary tumor cell lines 4TO7cg or 64pT into a mammary fat pad of normal 3-, 9-, or 21/24-month old BALB/c mice. In the nonmetastatic breast tumor model, significantly smaller tumors were observed in old compared with young mice. This was associated with a significant increase in the percentage of CD8(+) T cells in inguinal lymph nodes and significantly higher Mage-b expression levels in the primary tumors at old age. In the metastatic (4TO7cg) breast tumor model, a less pronounced, not statistically significant, smaller tumor size was found in the old mice, without a difference in the percentage of CD8(+) T cells or Mage-b expression levels. However, in this mouse model almost all metastases showed high levels of Mage-b expression (2- to 3-fold higher than the primary tumors in the same animals) regardless of age. These results indicate that the metastatic and nonmetastatic breast tumor models could be useful model systems to analyze how breast cancer vaccines for humans can be tailored to old age.

Microsatellite instability in in vitro ageing of T lymphocyte clones

Repair of mismatches in mammalian cell DNA is mediated by a complex of proteins that constitute the so-called mismatch repair system (MMR), the main post-replicative pathway for the correction of replication errors. Loss of MMR (as exemplified by germline mutations in some MMR genes, leading to hereditary non-polyposis colorectal cancer) results in increased mutation rates at both coding sequences and in non-coding regions such as microsatellites. In order to evaluate possible functional alterations of this repair system during ageing that could affect immune system efficiency, we studied microsatellite instability at five different loci interspersed in the genome (CD4, VWA31, Tpox, Fes/FPS and p53) in total DNA from T lymphocyte clones derived from hematopoietic stem cells, or peripheral T cells of young or elderly subjects. In addition, these clones had been maintained for different periods in vitro to represent a culture model of ageing. We observed increasing instability accumulating with increasing passages in culture, particularly in CD34+cell-derived clones, but no clear donor age relationship.

Effects of exogenous melatonin--a review

The results of studies on the effect of pineal indole hormone melatonin on the life span of mice, rats, fruit flies, and worms are critically reviewed. In mice, long-term administration of melatonin was followed by an increase in their life span in 12 experiments and had no effect in 8 of 20 different experiments. In D. melanogaster, the supplementation of melatonin to the nutrient medium during developmental stages gave contradictory results, but when melatonin was added to food throughout the life span, an increase in the longevity of fruit flies has been observed. Melatonin decreased the survival of C. elegans but increased the clonal life span of planaria Paramecium tertaurelia. Available data suggest antioxidant and atherogenic effects of melatonin. Melatonin alone turned out to be neither toxic nor mutagenic in the Ames test and revealed clastogenic activity in high concentration in the COMET assay. Melatonin inhibits mutagenesis induced by irradiation and by indirect chemical mutagens and inhibits the development of spontaneous and chemical-induced tumors in mice and rats. Further studies and clinical trials are needed to verify that melatonin is both safe and has geroprotector efficacy for humans.

Accelerating aging by mouse reverse genetics: a rational approach to understanding longevity

Investigating the molecular basis of aging has been difficult, primarily owing to the pleiotropic and segmental nature of the aging phenotype. There are many often interacting symptoms of aging, some of which are obvious and appear to be common to every aged individual, whereas others affect only a subset of the elderly population. Although at first sight this would suggest multiple molecular mechanisms of aging, there now appears to be almost universal consensus that aging is ultimately the result of the accumulation of somatic damage in cellular macromolecules, with reactive oxygen species likely to be the main damage-inducing agent. What remains significant is unravelling how such damage can give rise to the large variety of aging symptoms and how these can be controlled. Although humans, with over a century of clinical observations, remain the obvious target of study, the mouse, with a relatively short lifespan, easy genetic accessibility and close relatedness to humans, is the tool par excellence to model aging-related phenotypes and test strategies of intervention. Here we present the argument that mouse models with engineered defects in genome maintenance systems are especially important because they often exhibit a premature appearance of aging symptoms. Confirming studies on human segmental progeroid syndromes, most of which are based on heritable mutations in genes involved in genome maintenance, the results thus far obtained with mouse models strongly suggest that lifespan and onset of aging are directly related to the quality of DNA metabolism. This may be in keeping with the recent discovery of a possible 'universal survival' pathway that improves antioxidant defence and genome maintenance and simultaneously extends lifespan in the mouse and several invertebrate species.

Spontaneous mutation in Big Blue mice from fetus to old age: tissue-specific time courses of mutation frequency but similar mutation types

Transgenic mouse mutation detection systems permit rapid determination of the frequency and type of mutations allowing direct examination of mutational markers for aging, neurodegeneration, and cancer. The Big Blue transgenic mouse mutation detection system was used to determine the frequency and nature of spontaneous mutations versus age in multiple tissue types. Nuclear DNA was extracted from whole fetus at 13.5 days postcoitus (dpc) and from six tissues postbirth (cerebellum, forebrain, thymus, liver, adipose tissue, and male germline) of Big Blue transgenic mice at four ages: 10 days and at 3, 10, and 25 months postbirth. Forty million total plaque-forming units (pfu) were screened. The time course of mutation frequency with age had a significantly different shape in different tissues (P < 10(-6)). By 13.5 dpc, the whole fetus mutation frequency had already started increasing from the theoretical zero at conception to a value that was about one-half the mid-adulthood (3-10 months) average. From 10 days to 3 months, mutation frequency increased significantly in liver (P = 0.007) and showed an increasing trend in cerebellum, forebrain, and thymus. From 3 to 10 months, there was no significant change in mutation frequency in any tissue examined. From 10 to 25 months, the mutation frequency increased significantly in liver (P < 10(-6)) and adipose tissue (P = 0.002), but not in the other tissues examined (cerebellum, forebrain, and male germline). It is of interest that the mutation frequency in the male germline is consistently the lowest, remaining essentially unchanged in old age. The spectrum of mutation types was unaltered with age, tissue type and gender, although, as previously reported, tandem GG-->TT mutations are tissue specific and show significant increases with age and certain hotspots (Buettner VL et al. [1999]: Environ Mol Mutagen 33:320-324; Hill KA et al. [2003]: Mutat Res 534:173-186). The spectrum of mutation types was generally the same for all tissue types, despite the tissue-specific increases in mutation frequency with age. These data provide a useful reference for future studies of endogenous and exogenous mutagenesis.

DNA enrichment by allele-specific hybridization (DEASH): a novel method for haplotyping and for detecting low-frequency base substitutional variants and recombinant DNA molecules

Detecting rare sequence variants in genomic DNA is central to the analysis of de novo mutation and recombination events and the detection of rare pathological mutations in mixed cell populations. Current PCR techniques suffer from noise that limits detection to variants present at a frequency of at least 10(-4)-10(-5) per cell. We now describe an alternative approach that recovers genomic DNA molecules containing a known single-nucleotide variant by hybridization selection using a biotinylated allele-specific oligonucleotide, followed by hybrid capture on streptavidin-coated paramagnetic beads and subsequent analysis by PCR. This technique of DNA enrichment by allele-specific hybridization (DEASH) is fast, effective for all tested single-nucleotide polymorphisms (SNPs), and can recover large (>10 kb) single-stranded molecules. A single round of DEASH is effective in separating haplotypes from genomic DNA and can not only readily detect and validate DNA molecules containing a single base change at a frequency of 10(-5) per cell, but can also place these changes within the context of an extended haplotype. This technique offers a new approach to the analysis of mutation and recombination, and has the potential to detect very rare de novo base substitutions.

Oxygen accelerates the accumulation of mutations during the senescence and immortalization of murine cells in culture

Oxidative damage is a causal factor in aging and cancer, but it is still not clear how DNA damage, the cellular responses to such damage and its conversion to mutations by misrepair or misreplication contribute to these processes. Using transgenic mice carrying a lacZ mutation reporter, we have previously shown that mutations increase with age in most organs and tissues in vivo. It has also been previously shown that mouse cells respond to oxidative stress, typical of standard culture conditions, by undergoing cellular senescence. To understand better the consequences of oxidative stress, we cultured mouse embryo fibroblasts (MEFs) from lacZ mice under physiological oxygen tension (3%) or the high oxygen tension (20%) associated with standard culture, and determined the frequency and spectrum of mutations. Upon primary culture, the mutation frequency was found to increase approximately three-fold relative to the embryo. The majority of mutations were genome rearrangements. Subsequent culture in 20% oxygen resulted in senescence, followed by spontaneous immortalization. Immortalization was accompanied by an additional three-fold increase in mutations, most of which were G:C to T:A transversions, a signature mutation of oxidative DNA damage. In 3% oxygen, by contrast, MEFs did not senesce and the mutation frequency and spectrum remained similar to primary cultures. These findings demonstrate for the first time the impact of oxidative stress on the genomic integrity of murine cells during senescence and immortalization.

The role of somatic and germline mutations in aging and a mutation interaction model of aging

Mutations with a deleterious effect that is expressed after the average reproductive period are not effectively selected against and can accumulate in the germline. A conservative estimate is that at least 1-2% of new deleterious mutations affect some aspect of DNA replication, repair, or chromosome segregation. Since deleterious mutations can have an effect even as heterozygotes, this mutation accumulation can create an inherited background of late-acting mutations that themselves enhance mutation rate. This can have an interactive effect, in that it may increase the rate of somatic mutation during an individual's lifetime. The aging individual therefore becomes increasingly mosaic for somatic mutations, which in turn could potentially contribute to the gradual deterioration of biological processes and influence what we experience as senescence. Interventions that reduce somatic and germ cell mutations should, therefore, reduce the aging process in present and future generations.

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.

Effect of delta-sleep inducing peptide-containing preparation Deltaran on biomarkers of aging, life span and spontaneous tumor incidence in female SHR mice

From the age of 3 months until their natural deaths, female Swiss-derived SHR mice were subcutaneously injected 5 consecutive days every month with 0.1 ml of normal saline (control) or with 2.5 microg/mouse (approximately 100 microg/kg) of delta-sleep inducing peptide (DSIP, Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu) as the preparation Deltaran solved in 0.1 ml of saline. There were 54 mice in each group. The results of this study show that the treatment with Deltaran did not influence food consumption, but decreased the body weight of mice; it slowed down the age-related switching-off of estrous function; it decreased by 22.6% the frequency of chromosome aberrations in bone marrow cells; it did not influence mean life span; and it increased by 17.1% life span of the last 10% of the survivors and by 24.1% maximum life span in comparison with the control group. We also found that treatment with Deltaran significantly decreased total spontaneous tumor incidence (by 2.6-fold), mainly mammary carcinomas and leukemias in mice as compared with the control group. This is the first report on geroprotector and anticarcinogenic effect of DSIP-containing preparation Deltaran.

The relationship between aging and carcinogenesis: a critical appraisal

The incidence of cancer increases with age in humans and in laboratory animals alike. There are different patterns of age-related distribution of tumors in different organs and tissues. Aging may increase or decrease the susceptibility of various tissues to initiation of carcinogenesis and usually facilitates promotion and progression of carcinogenesis. Aging may predispose to cancer by several mechanisms: (1) tissue accumulation of cells in late stages of carcinogenesis; (2) alterations in homeostasis, in particular, alterations in immune and endocrine system and (3) telomere instability linking aging and increased cancer risk. Increased susceptibility to the effects of tumor promoters is found both in aged animals and aged humans, as predicted by the multistage model of carcinogenesis. Available evidence supporting the relevance of replicative senescence of human cells and telomere biology to human cancer seems quite strong, however, the evidence linking cellular senescence to human aging is controversial and required additional studies. Data on the acceleration of aging by carcinogenic agents as well as on increased cancer risk in patients with premature aging are critically discussed. In genetically modified mouse models (transgenic, knockout or mutant) characterized by the aging delay, the incidence of tumors usually similar to those in controls, whereas the latent period of tumor development is increased. Practically all models of accelerated of aging in genetically modified animals show the increase in the incidence and the reduction in the latency of tumors. Strategies for cancer prevention must include not only measures to minimize exposure to exogenous carcinogenic agents, but also measures to normalize the age-related alterations in internal milieu. Life-span prolonging drugs (geroprotectors) may either postpone population aging and increase of tumor latency or decrease the mortality in long-living individuals in populations and inhibit carcinogenesis. At least some geroprotectors may increase the survival of a short-living individuals in populations but increase the incidence of malignancy.

Somatic mutations and ageing in silico

Considerable evidence points to an accumulation of somatic mutations in older cells and organisms but the causal role of mutations in the ageing process is still unclear. In addition to demonstrating that mutations accumulate, it is important to address the question of whether they do so at a sufficient rate and with a dynamic profile that is consistent with them playing a causative role. We describe the development of in silico models that can be used to explore the role of somatic mutations in ageing and which form a part of a growing effort to build predictive mathematical and computer models that can help unravel the complexity of the functional genomics of ageing. Our models address, in particular, how mutations affect populations of dividing cells like human fibroblasts, in which the challenge to the somatic mutation theory is greatest, since selection at the cellular level will tend to suppress the accumulation of mutations.

Assessment of age-associated cognitive deficits in rats: a tricky business

Every living organism is affected by changes as a consequence of aging. Perhaps the most appropriate concept to describe age-related changes is that of 'functional age'. Laboratory rodents are especially suited as models of cognitive aging in humans, because they have a relatively short life-span and because many tests have been developed to investigate their cognitive performance. Examples from studies using the Morris water escape task were chosen to describe and discuss factors which must be considered before drawing conclusions about age-related cognitive deficits. In particular, the roles of rearing and housing conditions, of sensorimotor impairments, and of motivational differences between young and old rats are discussed. Conclusions are drawn about how aging studies should be conceived and performed.

Mutation theory of aging, assessed in transgenic mice and knockout mice

A vital question in the mutation theory of aging is whether mutation accumulates with age. If it does, what are the causes and consequences of the accumulation of mutation? The recent development of transgenic mice has made it possible to study mutation in different kinds of tissues and at a molecular level. An application of these mice to the study of age-dependent alteration has revealed that mutation does accumulate in the aging process. Studies have also revealed several important characteristics of mutation associated with aging. (1) The rate of age-dependent increase of mutant frequency varies among different types of tissue. (2) The rate is not in parallel with the cell proliferation rate of the tissue. (3) Some types of mutation are unique to specific tissues, suggesting the presence of a mechanism of mutation relative to tissue type. On the other hand, several kinds of knockout mice defective in DNA repair have been shown to exhibit tissue lesions and shortened life span. These characteristics provide a new view on the relationship between aging and the genome maintenance system. Here we review the current status of research on the correlation between mutation and aging undertaken by the use of transgenic and knockout mice.

Genome dynamics in aging mice

Random spontaneous genome rearrangements are difficult to detect in vivo, especially in postmitotic tissues. Using a lacZ-plasmid reporter mouse model, we have previously presented evidence for the accumulation of large genome rearrangements in various tissues, including postmitotic tissues, during aging. These rearrangements, which were found to be organ-specific and to increase with age, have one breakpoint in the lacZ-reporter locus and the second elsewhere in the mouse genome. In this present work, we have used a mouse genome sequence database to physically characterize a total of 49 genome rearrangements in the brain, heart, and liver from young and old mice at two lacZ-plasmid reporter loci. Half of all breakpoints in the mouse genome occurred in chromosomes 3 and 4, each carrying a lacZ-reporter cluster, at distances varying from <100 kb to 66 Mb, indicating intrachromosomal deletions or inversions. The other half of the breakpoints in the mouse genome was found randomly on any of the other chromosomes, indicating translocations. Alternatively, part of the intra- and extrachromosomal events could involve transpositions. Regions of extended homology were not found at the breakpoints. These results lead us to postulate potential mechanisms for the origin of large genome rearrangements in mouse tissues and to predict their possible impact as a potential cause of aging.

Mechanisms and consequences of somatic mosaicism in humans

Somatic mosaicism -- the presence of genetically distinct populations of somatic cells in a given organism -- is frequently masked, but it can also result in major phenotypic changes and reveal the expression of otherwise lethal genetic mutations. Mosaicism can be caused by DNA mutations, epigenetic alterations of DNA, chromosomal abnormalities and the spontaneous reversion of inherited mutations. In this review, we discuss the human disorders that result from somatic mosaicism, as well as the molecular genetic mechanisms by which they arise. Specifically, we emphasize the role of selection in the phenotypic manifestations of mosaicism.

Does p53 affect organismal aging?

The p53 protein plays a critical role in the prevention of cancer. It responds to a variety of cellular stresses to induce either apoptosis, a transient cell cycle arrest, or a terminal cell cycle arrest called senescence. Senescence in cultured cells is associated with augmented p53 activity and abrogation of p53 activity may delay in vitro senescence. Increasing evidence suggests that p53 may also influence aspects of organismal aging. Several mutant mouse models that display alterations in longevity and aging-related phenotypes have defects in genes that alter p53 signaling. Recently, my laboratory has developed and characterized a p53 mutant mouse line that appears to have an enhanced p53 response. These p53 mutants exhibit increased cancer resistance, yet have a shortened longevity and display a number of early aging-associated phenotypes, suggesting a role for p53 in the aging process. The nature of the aging phenotypes observed in this p53 mutant line is consistent with a model in which aging is driven in part by a gradual depletion of stem cell functional capacity.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Premature aging in mice deficient in DNA repair and transcription

One of the factors postulated to drive the aging process is the accumulation of DNA damage. Here, we provide strong support for this hypothesis by describing studies of mice with a mutation in XPD, a gene encoding a DNA helicase that functions in both repair and transcription and that is mutated in the human disorder trichothiodystrophy (TTD). TTD mice were found to exhibit many symptoms of premature aging, including osteoporosis and kyphosis, osteosclerosis, early greying, cachexia, infertility, and reduced life-span. TTD mice carrying an additional mutation in XPA, which enhances the DNA repair defect, showed a greatly accelerated aging phenotype, which correlated with an increased cellular sensitivity to oxidative DNA damage. We hypothesize that aging in TTD mice is caused by unrepaired DNA damage that compromises transcription, leading to functional inactivation of critical genes and enhanced apoptosis.

Large genome rearrangements as a primary cause of aging

In his introductory chapter of the Mutation Research special issue on 'Genetic Instability and Aging', the late Bernard Strehler provided some historical perspectives on the long-standing hypothesis that aging is primarily caused by changes in the genome of somatic cells (Strehler, 1995, Mutat. Res. 338 (1995) 3). Based on his own findings of a loss of ribosomal RNA gene copies in postmitotic tissues of dogs as well as humans during aging, his main conclusion was that deletional mutations are more likely than point mutations to be a main causal factor in aging. To directly assess the levels of different types of spontaneous mutations in organs and tissues during aging, we have used a mouse model harboring a chromosomally integrated cluster of lacZ-containing plasmids that can be recovered and analyzed in Escherichia coli. Our results indicate the accumulation of mutations in some but not all organs of the mouse with significant differences in mutational spectra. In addition to point mutations, genome rearrangements involving up to 66 Mb of genomic DNA appeared to be a major component of the mutational spectra. Physical characterization of the breakpoints of such rearrangements indicated their possible origin by erroneous, non-homologous DNA double-strand break repair. Based on their increased occurrence during aging in some tissues and their often very large size, we have designed a model for an aging tissue in terms of a cellular mosaic with a gradual increase in genome rearrangements that leads to functional senescence, neoplastic transformation or death of individual cells by disrupting nuclear architecture and patterns of gene regulation.

Cell signaling in aging and apoptosis

Alterations in apoptotic potential, due to perturbations in cell signaling cascades, could underlie age-related organ-specific cellular degeneration and death. While increased apoptosis could lead to cell loss, as in neuronal degeneration, loss of apoptosis competence might well result in the loss of phenotypic fidelity of somatic cells, which could explain to some extent, the age-related increase in cancer incidence. Results from our laboratory indicate that after subjecting young and old rats to genotoxic stress in the form of methyl methanesulfonate (MMS), an apoptotic response is quickly mounted in the liver of the young animals but virtually absent in the same organ of old animals (Nature Med. 8 (2002) 3). To address the possible molecular signaling defect(s) responsible for the age-related dysfunction of apoptosis in response to MMS, mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal kinases (JNKs) and p38 MAPKs, were evaluated in the liver of young and old rats after MMS treatment. The results demonstrated distinct age-specific patterns of MMS-induced MAPKs activation, suggesting that the balance between cell survival and apoptosis after genotoxic stress may be impaired during aging. These results are discussed in terms of the relative importance in aging of biological redundancy, a concept put forward by the late Bernard Strehler, and cellular fidelity.

mtLOH (mitochondrial loss of heteroplasmy), aging, and 'surrogate self'

In tribute to Dr Strehler, an attempt is made to use a style of reasoning found in some of his later papers as an outline of this article. First, general arguments in favor of the involvement of somatic mutations in mtDNA in the aging process are presented. Second, evidence is provided in support of a general tendency of mitochondrial genomes to reach homoplasmic state at the cellular level, for which we propose the term mitochondrial loss of heteroplasmy (mtLOH). This process is likely to facilitate the involvement of mtDNA mutations in the aging process by streamlining the phenotypic expression of the mutant genotype. Third, preliminary evidence of the very high incidence of clonal deletions in pigmented neurons of substantia nigra is reported. This observation highlights the possibility that accumulation of mtDNA mutations specific in certain cell types of a complex tissue may account for the involvement of mtDNA mutations in the aging process despite the relatively low average incidence of these mutations in the tissue as a whole. High incidence of mtDNA deletions in pigmented neurons evokes Strehler's idea that efforts to delay aging may not be the most cost-efficient way of preserving 'self awareness and a joyful sense of life', as he put it. A potential alternative suggested by Strehler, i.e. creation of a 'surrogate self' by computer simulation may deserve more attention than it currently enjoys.

Islet expression of the DNA repair enzyme 8-oxoguanosine DNA glycosylase (Ogg1) in human type 2 diabetes

BACKGROUND: It has become increasingly clear that beta-cell failure plays a critical role in the pathogenesis of type 2 diabetes. Free-radical mediated beta-cell damage has been intensively studied in type 1 diabetes, but not in human type 2 diabetes. Therefore, we studied the protein expression of the DNA repair enzyme Ogg1 in pancreases from type 2 diabetics. Ogg1 was studied because it is the major enzyme involved in repairing 7,8-dihydro-8-oxoguanosine DNA adducts, a lesion previously observed in a rat model of type 2 diabetes. Moreover, in a gene expression screen, Ogg1 was over-expressed in islets from a human type 2 diabetic. METHODS: Immunofluorescent staining of Ogg1 was performed on pancreatic specimens from healthy controls and patients with diabetes for 2-23 years. The intensity and islet area stained for Ogg1 was evaluated by semi-quantitative scoring. RESULTS: Both the intensity and the area of islet Ogg1 staining were significantly increased in islets from the type 2 diabetic subjects compared to the healthy controls. A correlation between increased Ogg1 fluorescent staining intensity and duration of diabetes was also found. Most of the staining observed was cytoplasmic, suggesting that mitochondrial Ogg1 accounts primarily for the increased Ogg1 expression. CONCLUSION: We conclude that oxidative stress related DNA damage may be a novel important factor in the pathogenesis of human type 2 diabetes. An increase of Ogg1 in islet cell mitochondria is consistent with a model in which hyperglycemia and consequent increased beta-cell oxidative metabolism lead to DNA damage and the induction of Ogg1 expression.

Moderate G6PD deficiency increases mutation rates in the brain of mice

Mice that harbored the x-ray-induced low efficiency allele of the major X-linked isozyme of glucose-6-phospate dehydrogenase (G6PD), Gpdx(a-m2Neu), and, in addition, harbored the transgenic shuttle vector for the determination of mutagenesis in vivo, pUR288, were employed to further our understanding of the interdependence of general metabolism, oxidative stress control, and somatic mutagenesis. The Gpdx(a-m2Neu) mutation conferred moderate G6PD deficiency in hemizygous males (Gpdx(a-m2Neu/y)) displaying residual enzyme activities of 27% in red blood cells and 13% in brain (compared to wild-type controls, Gpdx(a/y) males). In spite of this mild phenotype, the brains of G6PD-deficient males exhibited a significant distortion of redox control ( approximately 3-fold decrease in the ratio of reduced glutathione to oxidized glutathione), a considerable accumulation of promutagenic etheno DNA adducts ( approximately 13-fold increase in ethenodeoxyadenosine and approximately 5-fold increase in ethenodeoxycytidine), and a substantial elevation of somatic mutation rates ( approximately 3-fold increase in mutant frequencies in lacZ, the target and reporter gene of mutagenesis in the shuttle vector, pUR288). The mutation pattern in the brain was dominated by illegitimate genetic recombinations, a presumed hallmark of oxidative mutagenesis. These findings suggested a critical function for G6PD in limiting oxidative mutagenesis in the mouse brain.

On key lesions and all that: a tribute to Paul Lohman

This paper is a tribute to Paul Lohman at the occasion of his retirement from the position of Professor in the Medical Faculty at the Leiden University in The Netherlands and as Director of its Department of Radiation Genetics and Chemical Mutagenesis. Paul's contributions to the science of genetic toxicology are discussed in the context of more recent insights as to how mammalian cells process DNA damage, and how this may lead to cancer and, possibly, aging. Starting with his work on the characterization of UV-induced DNA repair in cultured cells from xeroderma pigmentosum patients and the development of methodology for monitoring the removal of UV-induced lesions in human cells, the concept of the key lesion is introduced. Among the myriad of DNA lesions that can be induced in DNA as a consequence of exposure to a range of natural or synthetic mutagens, key lesions are the ones responsible for subsequent adverse effects, for example, because they give rise to mutation. The development of methods using immunofluorescence microscopy to detect and identify such key lesions and quantitate them at the single cell level, is one of the highlights of Paul's career. Based on the perceived need to evaluate mutational end points in vivo in relation to specific lesions identified by his immunofluorescence methods, Paul subsequently made crucial contributions to the development of the first transgenic mouse model to measure mutations in chromosomally integrated reporter genes. In parallel to his experimental work, Paul greatly contributed to genetic toxicology at the theoretical level by his work on the development and evaluation of methods for assessment or prediction of risks of exposure to environmental mutagens. Finally, Paul has served the discipline of genetic toxicology in a more administrative role in various ways, both locally as one of the founders of the Medical Genetics Center South-West Netherlands and internationally by playing a prominent role in organizations such as ICPEMC. Here, his numerous contributions to the journal Mutation Research, both as author on many papers and as Executive Managing Editor should not go unmentioned.

Mutational fingerprints of aging

Using a lacZ plasmid transgenic mouse model, spectra of spontaneous point mutations were determined in brain, heart, liver, spleen and small intestine in young and old mice. While similar at a young age, the mutation spectra among these organs were significantly different in old age. In brain and heart G:C-->A:T transitions at CpG sites were the predominant mutation, suggesting that oxidative damage is not a major mutagenic event in these tissues. Other base changes, especially those affecting A:T base pairs, positively correlated with increasing proliferative activity of the different tissues. A relatively high percentage of base changes at A:T base pairs and compound mutants were found in both spleen and spontaneous lymphoma, suggesting a possible role of the hypermutation process in splenocytes in carcinogenesis. The similar mutant spectra observed at a young age may reflect a common mutation mechanism for all tissues that could be driven by the rapid cell division that takes place during development. However, the spectra of the young tissues did not resemble that of the most proliferative aged tissue, implying that replicative history per se is not the underlying causal factor of age-related organ-specific differences in mutation spectra. Rather, differences in organ function, possibly in association with replicative history, may explain the divergence in mutation spectra during aging.