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Evolutionary theories suggest biological ageing is caused by accumulated stochastic damage in somatic cells. There is evidence suggesting that accumulated damage to DNA might be an important cause of biological ageing. Firstly, DNA lesions are observed to accumulate with age, moreover there is a positive correlation between lifespan of species and there DNA repair rate, yet most convincing evidence comes from defects in DNA metabolism pathways which give rise to accelerated ageing phenotype in organisms.
What is ageing
Evolutionary theories of ageing.
DNA damage theory of ageing.
DNA repair rate and maximum lifespan of species
DNA lesions increase with age.
DNA repair defects and accelerated ageing syndromes
Not all DNA repair defects accelerate ageing
Delaying ageing up increasing DNA repair
What is aging?
Initially, ageing appears to be a familiar concept, but upon detailed examination, ageing means many things. It has social, behavioural, physiological and cellular aspects. Central to the biology of ageing is the premise that ageing is characterised by failure to maintain stability of the internal environment (homeostasis) under conditions of physiological stress, and this failure to maintain homeostasis is associated with decrease in viability and an increase in vulnerability of the individual over time (Comfort, 1979).
In general, ageing can be defined as a series of time dependent processes occurring in the adult organism that ultimately leads to its death. It represents a progressive decline in vitality and health with an increasing mortality risk common to nearly all metazoans. Ageing phenotype is said to be one of the most complex phenotypes, defined by generalised biological dysfunction.
The limited numbers of cell division of human cells suggest the existence of an intrinsic biological clock. Later studies established telomere shorting as the intrinsic clock counter that eventually triggers cellular senescence.
What are mortality curves?
There is a mathematical model of mortality that is correlated with rate of aging. This helps suggest potential methods for determination the rate of ageing.
What are the bio makers of Ageing?
What's biological ageing vs. chronological aging?
Life span is a continuum. At one end is development and growth, and at the other end is deterioration of functions or senescence and in between the reproductive phase or adulthood.
Evolutionary theories of ageing
The cause of ageing can be examined at many levels, on the broadest evolutionary level, to proximate (mechanistic) cellular level.
One of the earliest explanations for the evolution of ageing was that senescence is programmed in order to limit population or increase the turnover over of generations. This was proposed to be adaptive for organisms living in changing environments, allowing for faster adaptation. The fundamental flaw of this early explanation of ageing was there is little evidence that senescence is a significant cause of mortality in wild populations (Kirkwood, 2000). In contrast to protected populations, natural mortality is mostly due to extrinsic hazards such as predation, starvation and cold.
Three contemporary complementary theories attempt to provide an explanation for the ultimate cause of ageing.
Mutation Accumulation theory of ageing proposes that as a result of extrinsic mortality, there is a progressive weakening in the force of selection with increasing age, therefore mutations that affect health at older ages are not selected against. By an age when wild survivorship has declined to very low levels, the force of selection is too weak to oppose the accumulation of germ-line mutations with late acting deleterious effects (Kirkwood, 2000).
This explanation of aging is similar to the evolutionary explanation of vision deterioration of cave animals: if some function cannot be used to provide reproductive advantage, it will not be supported by selection pressure and maintained in future generations. The probability of an individual reproducing depends on age. It is zero at birth and reaches a peak in young adults. Then it decreases due to the increased probability of death linked to various external (predators, illnesses, accidents) and internal (senescence) causes. In such conditions, deleterious mutations expressed at a young age are severely selected against due to their high negative impact on fitness (number of offspring produced). On the other hand, deleterious mutations expressed only later in life are relatively neutral to selection because their bearers have already transmitted their genes to the next generation. (Leonid A. Gavrilov, 2002)
Antagonistic Pleiotropy theory of aging proposes that genes beneficial at younger age despite being deleterious at older age will be favoured by selection and actively accumulated in populations.
This theory of is based on two assumptions. Firstly, it is assumed that a particular gene may have an effect not on one trait only but on several traits of an organism (Pleiotropy). The second assumption is that these pleiotropic effects may affect individual fitness in opposite (antagonistic) ways.
According to George Williams who first proposed this theory, this conflict arises from "pleiotropic genes that have opposite effects on fitnesses at different ages. Selection of a gene that confers an advantage at one age and a disadvantage at another will depend not only on the magnitudes of the effects themselves, but also on the times of the effects. An advantage during the period of maximum reproductive probability would increase the total reproductive probability more than a proportionately similar disadvantage later on would decrease it. So natural selection will frequently maximize vigor in youth at the expense of vigor later on and thereby produce a declining vigor (aging) during adult life".
This point can be illustrated by a hypothetical gene that increases the fixation of calcium in bones. Such a gene may have positive effects early in life because the risk of bone fracture and subsequent death is decreased, but such a gene may have negative effects later in life because of increased risk of osteoarthritis due to excessive calcification. In the wild, such a gene has no actual negative effect because most animals die long before its negative effects can be observed. There is then a trade-off between an actual positive effect at a young age and a potential negative one at old age; this negative effect may become effective only if animals live in protected environments such as zoos or laboratories. (Leonid A. Gavrilov, 2002)
The antagonistic Pleiotropy theory explains why reproduction may come with a cost for species longevity and may even induce death. Any mutations favouring more intensive reproduction (more offspring produced) will be propagated in future generations even if these mutations have some deleterious effects in later life. For example, mutations causing overproduction of sex hormones may increase the sex drive, libido, reproductive efforts, and success, and therefore they may be favoured by selection despite causing prostate cancer in males and ovarian cancer in females later in life. Thus, the idea of reproductive cost, or more generally of trade-offs, between different traits of the organism follows directly from antagonistic pleiotropy theory. (Leonid A. Gavrilov, 2002).
The final theory of the Disposable Soma is a special case of the antagonistic Pleiotropy theory. The theory proposes that somatic cells are only maintained to ensure continued reproductive success; following reproduction the soma is disposable. This theory postulates gene mutations with antagonistic pleiotropic effects. These hypothetical mutations save energy for reproduction by partially disabling molecular proofreading and other accuracy promoting devices in somatic cells. The authors of the disposable soma theory argued that "it may be selectively advantageous for higher organisms to adopt an energy saving strategy of reduced accuracy in somatic cells to accelerate development and reproduction, but the consequence will be eventual deterioration and death" (Leonid A. Gavrilov, 2002).
The DNA damage theory of ageing proposes that ageing is a result of accumulated nuclear DNA damage in the cells of an organism. Nuclear DNA is subject to constant damage, mostly hydrolysis, oxidation and alkylation. In humans, DNA damage is very frequent (approximately 800 DNA lesions occur per hour in each cell) andÂ several DNA repairÂ processes have evolved to compensate. These DNA repair processes and cell cycle checkpoint enzymes protect nuclear DNA by either repairing the damage or forcing cells into apoptosis or senescence.
Some DNA damage remains in cells despite the action of the DNA repair processes and is accumulated over time. The accumulation of unrepaired DNA damage is more prevalent in certain types of cells, particularly in non-replicating or slowly replicating cells, which cannot rely on DNA repair mechanisms associated with DNA replication such as those in the brain, skeletal and cardiac muscle. This accumulated DNA damage can contribute to ageing directly by increasing cell dysfunction or indirectly by increasing apoptosis and cellular senescence.
DNA is the only biologic molecule that relies solely on repair of existing molecules, without any remanufacture; accumulates damage over a lifetime. DNA is subject to continuous modification from both endogenous reactive chemicals, such as reactive oxygen species, and exogenous environmental factors, such as food agents, industrial genotoxins, and ultraviolet and ionizing radiation (Lindahl, 1993).
DNA is intrinsically unstable, experiencing spontaneous reactions. There are two major forms of spontaneous DNA damage (i) deamination of adenine, cytosine, and guanine, and (ii) depurination resulting from cleavage of the bond between the purine bases and deoxyribose, leaving an apurinic site in DNA.
The DNA is also damaged by products of cell's metabolism essential to the viability of the cell. Metabolism generates reactive oxygen and nitrogen species, lipid peroxidation products, endogenous alkylating agents, estrogen and cholesterol metabolites, and reactive carbonyl species, all of which damage DNA. Reactive oxygen and nitrogen species alone generate several kinds of single-strand breaks and more than 70 oxidative base and sugar products in DNA (Hoeijmakers, 2009).
DNA is also damaged by exogenous physical and chemical agents. UV light induces the formation of pyrimidine dimers, in which two adjacent pyrimidines e.g., thymines are joined by a cyclobutane ring structure. Alkylation can be caused by exogenous chemicals in which there is am addition of methyl or ethyl groups to various positions on the DNA bases. Many carcinogens react with DNA bases, resulting in the addition of large bulky chemical groups to the DNA molecule.Â
SpontaneousÂ DNA damageÂ is thought to be induced continually through endogenous mechanisms and from the environment, processed by the variousÂ DNA replication, repair and recombination systems to ultimately generate mutations as a molecular end point.
Mutation accumulation may lead toÂ cell death, cell transformation and cellÂ senescence, which in turn could underlie the various symptoms of organismal aging, including organ dysfunction, tissue degeneration and a variety of pathological lesions.
What are Mutations as opposed to DNA Damage
As discussed above, DNA damages include a variety of chemical irregularities in the poly-nucleotide structure of the double helix, such as pyrimidine dimers, apurinic sites, cross-links and both large and small chemical additions, called adducts. By contrast, mutations involve changes in the polynucleotide sequence in which the standard A:T or G:C base pairs are substituted, added, deleted, or rearranged. Even when mutations represent large changes, such as extended deletions, the DNA retains its characteristic regularity and consists of an uninterrupted sequence of standard nucleotide pairs.
The introduction of DNA damage usually has different immediate consequences from those occurring after the introduction of mutations. Thus, DNA damages are often recognized and removed by repair enzymes. Those damages which are not immediately removed can interfere with the progress of DNA replication or transcription. Damages are not reproduced upon replication, but sometimes may persist in one strand through successive cycles of replication. Mutations can remain in the DNA indefinitely, affecting gene expression over succeeding generations. They are not thought to be removed by enzyme action.
DNA damage tends to interfere with gene expression by preventing transcription of RNA from DNA, whereas mutation usually results in transcription that usually produces proteins with diminished or altered functionality. Mutations that are not lethal to a cell are more likely to be perpetuated in dividing cells. Both damages and mutations can be removed, but by different mechanisms. Damages can be removed from individual organisms by repair enzymes; they can also be removed from a population of cells by causing death of the individual cells containing them. Mutations can be removed from a population of cells or multi-cellular organisms through the consequences of their expression on the fitness of individual members of the population. Also, mutations may be removed infrequently by chance reverse mutation, or have their phenotypic effects moderated by suppressor mutations.
There is also an important direct connection between DNA damage and mutation. During the processing of damages by repair enzymes, mutations occasionally arise. Moreover, replication past unrepaired damages may result in mistakes leading to mutation. (Bernstein, 1981)
DNA repair capacity correlates with life span?
Studies comparing DNA repair capacity in different mammalian species have shown that repair capacity correlates with lifespan. This correlation provides indirect evidence that DNA repair capacity is important factor in ageing.
Hart and Setlow in 1974 demonstrated that the ability of skin fibroblasts of seven mammalian species to carry out nucleotide excision repair after exposure to ultraviolet light correlated with the lifespan of the species.
(Hart & Setlow, 1974)
Figure 1 shows the rate of synthesis of DNA during non-S periods of the cell (measure of the excision-repair rate) for the different species.
Figure 3 shows the average lifespan of the mammalian species on a log scale is plotted against the extent of unscheduled DNA synthesis. The order of increase in unscheduled synthesis is the same as that of life-span, suggesting Nucleotide excision repair capacity is positively correlated with lifespan.
(Hart & Setlow, 1974)
Further studies involving wide range of mammalian species confirmed this relationship between lifespan and DNA repair capacity.
More evidence came from studies of the enzyme Poly-ADP-ribose polymerase (PARP) which catalyses the post translational modification of proteins and plays a very important role in DNA repair acting as a regulator of base excision repair (BER) and also has a role in non-homologous end-joining (NHEJ) as well as in telomere maintenance.
Burkle et al compared the PARP activities in mononuclear leukocytes from mammalian species of different maximal lifespans and showed a positive correlation between PARP activity and longevity of 13 mammalian species, with human cells showing about five times the PARP activity of rat cells. (Grube, K, and Burkle, A. 1992)
A roughly linear correlation between maximum lifespan of mammals and DNA repair activity was found by a review of five comparative studies which were re-analysed by standardising to an internal repair standard; promoting the suggestion that DNA repair rate is relevant to maximum lifespan. (Cortopassi, G. A., and Wang, E. 1996.)
However, recent analysis by Promislow suggests that positive correlation between mammalian lifespan and DNA repair rates may be an artefact of long-lived species being on average larger in body size.
In mammals there is a strong correlation between adult body size and maximum lifespan. (Promislow, D. E,1994). Promislow put forward a model that attempts to explain why there could be a correlation between body size and DNA repair rates. If we consider individuals from two mammalian species of very different body size but similar lifespan. If there is an assumption that DNA damage accumulation in a cell would eventually lead to fatal cancer, and a further assumption that both individuals want to maintain the same low probability of incurring that threshold level of DNA damage in a given period of time. The larger individual, having more cells, will need to maintain a higher rate of DNA repair to ensure that it has the same probability as the smaller individual in maintaining all its cells in a healthy state. (Promislow, D. E.,1994)
It is possible that large body size could have selected for high DNA repair rates and longer lifespan, giving rise to the observed correlations.
DNA Alterations Increase with Age.
Further evidence for supporting the idea that accumulated DNA damage is responsible for ageing is the observation that DNA lesions and chromosomal abnormalities increase with age in healthy mammals.
Martin et al in 1985 were able to demonstrate that there is a six-fold increase in frequency chromosomal aberrations in metaphase cells from old mice (40 months old) in comparisons to young mice (8 months old) (Martin et al 1985).
Curtis and Crowley study provides further evidence for an increasing level of mutations during aging. They looked at mouseÂ liverÂ parenchymal cellÂ metaphase platesÂ after partial hepatectomy and found considerably higher numbers of cells with abnormal chromosomes in old as compared to young animals (i.e., from about 10% of the cells in 4- to 5-month-oldÂ miceÂ to 75% inÂ miceÂ older than 12 months). Later, such large structural changes in DNA, i.e.,Â aneuploidy, translocations, dicentrics, were observed to increase with donor age in white blood cellsÂ ofÂ humanÂ individuals, i.e., from about 2%-4% of the cells being chromosomally aberrated in young individuals to about six times higher in the elderly.Â (Vijg J, somatic mutations and ageing: a re-evaluation)
Sedelnikova et al were able to show that -foci, which reveal DNA double-strand breaks accumulate in human cell cultures and ageing mice. These cryptogenicÂ -foci remained after repair of radiation-inducedÂ -foci, suggesting that they may represent DNA lesions with unrepairable DSB. The study concluded that the accumulation of unrepairable double strand breaks may have a causal role in mammalian ageing. (Sedelnikova, O. A. et al, 2004)
DNA lesion accumulation may not reflect only the ongoing DNA damage induction but also a declining DNA-repair capacity over time. OnceÂ DNA damageÂ has been generated, it is the role of cellular repair systems to prevent its accumulation. However, studies have shown that aging is associated with a general reduction inÂ DNA repairÂ capacity.
Wissman's team characterised several base excision repair enzymes in mammalian mitochondria. Base excision repair is the main pathway for the removal of small DNA base modification, such as alkylation and deamination. They compared the nuclear and mitochondrial base excision repair capacities between different mouse tissues by detecting and measuring the activities of three major DNA Glycosylases in the mouse brain. They observed a significant age-dependent decrease inÂ uracil,Â 8-oxoGÂ and 5-OH-C incision activities in the mitochondria of allÂ brainÂ regions, while variable patterns of changes were seen in nuclei. (Weissman. L et al, 2007)
Other studies have supported this observation of decline in repair activities with age: Rao et al 2000 and Intano et al 2003.
Despite these studies, the fundamental questions of causation remains; how can one distinguish if this decline in DNA repair and an increase in DNA damage is a caused by ageing rather than an effect of ageing?
Defects in DNA Repair
The most compelling evidence in support of the DNA damage theory of ageing are a set of rare genetic disorders that appear to accelerate ageing. These disorders originate from mutations in the DNA damage repair and response genes.
Werner's syndrome (WS) is thought to be caused by mutations ofÂ RecQÂ type DNA helicase. The syndrome is recognized as a top ranking 'segmental' progeroid syndrome.
Patients withÂ WSÂ show a wide variety of clinical and biological markers similar to normal ageing at an early stage of their life. WS patients develop premature greying, cataract, loss of subcutaneous fat, skin atrophy, osteoporosis, diabetes, atherosclerosis, and malignancies, followed by death at an average age of 46. WS cells senesce prematurely in culture.
WS originates in a recessive mutation in a gene WRN encoding the RecQ helicase. WRN is unique among its family in possessing an exonuclease activity (Huang et al, 1998) which indicates it may be involved in DNA repair. WRN plays a role in the maintenance of overall genomic stability, and may be involved in multiple DNA repair pathways (Bachrati and Hickson, 2003).
One importantÂ in vivoÂ function of WRN is inÂ homology-dependent recombinationÂ repair (HDR). HDR can be used to repair DNA damage while suppressing gene loss or rearrangement. WRN seems to have a role late in HDR when recombinant molecules are topologically disentangled for segregation to daughter cells. In mammalian cells, WRN seems to repair DNA-strand breaks that arise from replication arrest, and therefore functions to limit genetic instability and cell death.
WRN is proposed to function on DNA substrates that are generated during HDR repair, the stabilization or repair of replication forks, or from telomere replication, repair or remodelling. Successful resolution of these substrates suppresses genomic instability and maintains telomere length and structure to ensure high cell viability. In the absence of WRN function, cells accumulate potentially toxic DNA intermediates or critically short telomeres that can trigger genetic instability, DNA damage and apoptotic response pathways. In this model, WS pathogenesis is driven by defective DNA metabolism that leads to genetic instability and mutagenesis. These consequences, together with mutation accumulation and cell loss, might drive the development of cell type, cell lineage or tissue-specific defects. Compromised tissue or organ structure and function then leads to two seemingly divergent outcomes: senescence and mutation-dependent neoplastic proliferation.
(Brian A. Kudlow, 2007)
Cells taken from patients with Ws have increased genome instability (Fukuchi et al, 1989). WS are also sensitive to topoisomerase inhibitors, the enzymes involved in the regulation of supercoling in DNA. As such WS is an indicator that alterations in the DNA over time play a role in ageing.
How exactly does this proves accumulated DNA damage might be causing the accelerated ageing? Or does it prove that dna repair is needed to slow down ageing?
The protein whose mutation is responsible for the Hutchinson-Gilford's syndromes is also a nuclear protein. (Eriksson, M.Â et al. 2003).
HGPS is the most severe of the progeroid syndromes. Individuals with HGPS have a mean lifespan of 13 years. HGPS patients appear normal at birth, but prematurely develop several features that are associated with ageing, including alopecia, atherosclerosis, rapid loss of joint mobility, osteolysis, severe lipodystrophy, scleroderma and varied skin hyperpigmentation.Â (Brian A. Kudlow, 2007)
The genetic basis of HGPS was uncovered in 2003, when it was found that most cases of the disease are associated with a single-nucleotide substitution that leads to aberrant splicing ofÂ LMNA, the gene that encodes the A-type nuclear lamins.(Eriksson, M.Â et al. 2003).
A-type lamins belong to the family of intermediate-filament proteins that, along with the B-type lamins, are the main constituents of theÂ nuclear laminain. In addition to maintaining the integrity and shape of the nuclear envelope, lamin A/C has been implicated in the regulation of transcription, DNA replication, cell-cycle control and cellular differentiation. (Brian A. Kudlow, 2007)
Cells derived from patients with HGPS and HGPS mouse models display several indicators of an activated DNA-damage response, including enhanced phosphorylation of histone H2AX and markedly increased transcription of p53 target genes. Cells from patients with HPGS might also display aneuploidy and chromosome instability.Â
(Brian A. Kudlow, 2007)
Data suggests the DNA machinery is impaired in HGPS, suggesting that changes in the DNA are important in this disease and maybe in normal ageing.
Add a table here from one of the papers that list the different defects in DNA repair system and what the causes of them are.
2i) Counter Evidence A- Mice deficient in DNA repair proteins do not appear to age faster.
Mice deficient in a DNA repair protein known as Pms2 have elevated levels of mutations in multiple tissues and yet did not appear to age faster than controls. (Narayanan et al, 1997).
The Pms2 gene is one of several mammalian homologs of E coli MutL DNA mismatch repair gene. Mismatch repair gene homologs participate in various cellular functions, including transcription coupled repair and recombination; consequently a deficiency in one of these homologs may disrupt genome stability in multiple ways.
Narayanan et al were able to construct transgenic mice that carried disruptions at Pms2 loci, in order to determine the effect of pms2 inactivation on genome integrity in vivo. The transgenic mice defective in Pms2 showed 100-fold elevation in mutation frequency in all tissues examined compared to wild type mice.
According to DNA damage theory of ageing, it could be predicted that such profound genetic instability would cause accelerated ageing, but this was not demonstrated in mice. The results showed that high rates of mutagenesis in multiple tissues are not necessarily associated with accelerated ageing.
DNAÂ mismatch repairÂ deficiency appears to be associated with small insertions/deletions in small mononucleotide repeat runs. It could be entirely possible that smaller mutations, i.e., point mutations and small deletions or insertions, are less important in creating a general spectrum of age-related degeneration than large rearrangement mutations. (J.Vijg, 2000)
There is no consensus on what type of DNA changes are crucial to ageing.
Embryos of mice and flies irradiated with x-rays did not age faster as one would expect. (Strehler 1999.)
Moreover, in 1993 Cosgrove et al was able to demonstrate that X-irradiation of male germ line had no clear cut effect on the lifespan of subsequent offspring. Initially, this finding of no significant effect of a large dose of X-irradiation of mice on the lifespan of their progeny was a surprise. Subsequent X-ray studies by other investigators all support the finding showing no significant difference in length of life between offspring of control and irradiated fathers.
It could be argued that x-rays could not mimic the natural background of DNA damaging agents; more studies should be conducted on type of DNA damage rather the causative agent of such damage to ascertain what type of DNA damage is responsible for the accelerated ageing phenotype.
(What type of DNA damage does X-rays cause? In future more search needed on what type of damage causes accelerated ageing).
2ii) Counter Evidence B- Cannot delay ageing by up regulating
If the DNA damage theory is correct it should be possible to delay ageing by enhancing DNA repair mechanism. In spite of several attempts the evidence is still lacking. (De Magalhaes, 2005a).
Another enzyme that repairs oxidative damage isÂ 8-oxo-dGTPase, which repairs 8-oxo-7,8-dihydroguanine, an abundant and mutagenic form of oxidativeÂ DNA damage. Again in disagreement with the free radical theory of ageing, knocking out the gene responsible forÂ 8-oxo-dGTPaseÂ in mice, although resulting in an increased cancer incidence, did not alter the ageing phenotype (Tsuzuki et al., 2001).Â
in addition, there are no cases of delayed ageing in mammalian models that could be related to increasedÂ DNA repair. For example, mice overexpressing p48, which is important in repairingÂ DNA damageÂ deriving from UV radiation, have improvedÂ DNA repairÂ mechanisms and still do not live longer or age slower (Tang et al., 2000).Â
Over expressing a DNA repair gene called MGMT, lowered the incidence of cancer but did not slow ageing. (Zhou et al 2001)
Mice with extra copies of two DNA repair genes lived 16% longer than controls but it was not established is the ageing was delayed.
In addition, there are no cases of delayed ageing in mammalian models that could be related to increasedÂ DNA repair. For example, mice overexpressing p48, which is important in repairingÂ DNA damageÂ deriving from UV radiation, have improvedÂ DNA repairÂ mechanisms and still do not live longer or age slower (Tang et al., 2000). Consequently, the DNA damage theory of ageing has so far failed to explain why different species age at different rates. The idea that DNA damage derived from ROS is involved in ageing is also debatable. As mentioned earlier, overexpression of catalase in the nucleus did not prevent oxidative damage to DNA (Schriner et al., 2000) and knocking out the gene responsible for 8-oxo-dGTPase failed to accelerate ageing (Tsuzuki et al., 2001). These results hint that the free radical and the DNA damage theories of ageing are not complementary.
Â If progeroid syndromes represent a phenotype of accelerated ageing then changes in DNA over time most likely play an important role in ageing. Nevertheless, the essence of those changes remains to be determined. Since many genetic perturbations affecting DNA repair do not influence ageing (Table 4), is doubtful overall DNA repair is related to ageing. Understanding which aspects, if any, of DNA biology play a role in ageing remains a great challenge in gerontology.
Â Intriguingly, bothÂ DrosophilaÂ andÂ C. elegansÂ are mostly composed of post-mitotic cells, which can explain why results from theseÂ invertebratesÂ are much more supportive of the free radical theory of ageing than results from mice or observations inÂ humansÂ (Table 3).
Future lines of research
A second line of research that could be fruitful in testing the contribution of mutation accumulation to aging is to directly analyze human populations with varying aging rates for polymorphic variation in genes controlling genome stability. This molecular epidemiologic approach is greatly facilitated by the wealth of sequence information that is now becoming available through the human genome program. Unlike the situation in the known syndromes with symptoms of accelerated aging, normal aging is unlikely to be due to one or few completely inactivated genes. Instead, in normal human populations one would expect allelic variation at the many loci harboring genome stability genes to contribute to the observed variation in life span among individuals. As a step in providing a basis for molecular epidemiology studies to relate genetic variation at specific DNA repair genes to reduced DNA repair capacity and cancer susceptibility, Mohrenweiser and Jones  re-sequenced exons of nine genes involved in different repair pathways. They identified 15 different amino acid substitution variants in these nine genes and determined their frequencies in a small sample of individuals. Familial aggregation of cancer has been extensively described and there is increased interest in weakly predisposing alleles as risk factors . Thus far, there has been no attempt to investigate subtle differences in genes specifying genome stability pathways between human populations differing greatly in aging rate. It is conceivable that polymorphic variation at multiple loci could have given rise to genotypes conferring extreme longevity, for example, by optimization of genome stability and other cellular defense systems. We have obtained evidence for a strong familial component of longevity in centenarians  and are currently comparing allelic variants of genes involved in recombinational repair in such long-lived individuals and control groups. Differences in frequency of the variant alleles would indicate a potential functional advantage of those alleles that are more frequent in the disease-resistant long-lived individuals. Protein bioinformatics methods can then help analyze the sequence and structural variation and provide insight into function.