BMJ No 7115 Volume 315
Education and debate Saturday 25 October 1997
Molecular biology's impact on our understanding of aging
David M A Mann
| Summary points |
| Aging and the disorders of later life are separate entities |
| Both are under genetic control |
| Aging involves defects in mitochondrial DNA which promote oxidative stress mediated cell damage |
| Age related disorders such as Alzheimer's disease are due to the effect of inherited genetic risk factors |
| Life span is determined by the effects of genetic risks associated with age and disease |
Powerful molecular biological tools have begun to open up the very fabric of life - the human genome - and have allowed us to glimpse inside this Pandora's box. We now see that many common disorders of later life - for example, cancer, dementia, and vascular disease - are related to genetic variations that dictate an individual's likelihood of developing illnesses like these. These genetic variations differ from those that determine longevity, though both act synergistically to dictate how long and how well we might live. Control of gene expression will be needed to counteract the adverse actions of these to promote a healthy and productive old age.
Aging and disease - separate entities or continuum of change?
Common disorders such as cardiovascular disease, cancer, stroke, dementia, and diabetes become increasingly prevalent in later life. It is tempting to ascribe these to the body "wearing out|mK - a viewpoint supported by the frequent finding in many old, but otherwise healthy, people of low levels of the same kind of tissue changes generally associated with certain diseases when present in higher amounts. These diseases have been popularly equated with "normal aging," and the idea that in diseased individuals this normal process of aging may have become "exaggerated" or have "accelerated" out of control has often been put forward.
Health and disease might in this way be thought of as occupying a sliding scale of age determined tissue damage, with the one merging into the other at some point in life. Yet the argument is fallacious. Many disorders - for example, dementia - clearly become more common in later life; their incidence peaks in people in their eighties but then declines.(1) Furthermore, like humans, other animals (including the higher primates) age and die, yet they do not spontaneously develop these common disorders of humans. Hence, although growing older is a biological certitude, disease in old age may represent an additional burden of tissue damage superimposed on other ongoing alterations common perhaps to all cell types in the body and applicable, to a greater or lesser extent, to all individuals. These basic changes set the stage on which the disorders of old age can be played out. So what is aging, and what is disease? How important is each of these in determining how each of us will fare in later life? What causes aging and disease, and to what extent are they interdependent?
Molecular revolution
The recent molecular biological revolution has begun to make startling inroads into these areas of uncertainty, particularly in molecular genetics. Variations in the structure or stability of the human genome are now seen to be responsible for an ever growing list of common and not so common disorders by determining not only the probability of whether sufficient tissue damage causing disease will occur but also at what time in life this can be expected and for how long it will last. Moreover, these techniques are now providing the quantitative data necessary to clarify ideas on aging and disease that have been around for many years, and the boundaries are thus being established between which tissue damage might be thought of as being caused by the passage of time alone and which is a reflection of degenerative disease in later life.
| This revolution was sparked by the advent of the polymerase chain reaction(2) - a means of replicating "in the test tube" a small number of copies, or even a single copy, of a segment of DNA (gene) to produce multiple, identical versions whose nucleotide sequence can be determined. This forms the base of many complex molecular analyses that, for example, assign biological characteristics, such as disease trait, to chromosomes or parts thereof (by linkage analysis) or permit the "narrowing down" of genes within that region to the one causing disease (by positional cloning). Once identified, screening techniques based on polymerase chain reaction can quickly and efficiently determine how many individuals with the disease share that particular genetic change, and DNA sequencing can show whether other disease-causing base changes or rearrangements of the DNA are present (fig 1). |  | |
| Fig 1: Molecular approaches to gene identification |
In this way different types of mutation have been identified in all kinds of tissues and in many clinical situations (including aging), ranging from "simple" exchanges of nucleotide bases that alter the coding sequence for one or more amino acids (missense mutations) through to complex additions or expansions (insertions) or subtractions (deletions) of genetic material. Such changes, when transcribed into RNA and subsequently translated, produce a protein of abnormal sequence which creates structural or functional changes that render it defective at its normal task (dysfunctional) or confer a new function (gain of dysfunction) away from normality. These altered proteins may compromise the structural integrity or the metabolic or replicative capabilities of the cell in such a way as to trigger cascades that can eventually be recognised under the microscope as a tissue abnormality.
The nervous system as ideal model
The nervous system has been a traditional target for studies on aging as its principal cells, the neurones, do not divide nor are replaced routinely during adult life, as happens with cells of other tissues. They must thus endure a lifelong catalogue of insults to secure continued survival of both themselves and the individual. The nervous system therefore makes an ideal model in which to study aging and to differentiate from this the changes that relate to the common diseases of old age. Perhaps the best illustration of this conflict is Alzheimer's disease. For years the argument has raged as to whether Alzheimer's disease is a disease or represents the ultimate, or untimely, exaggerated effects of the process of aging.
Genetics of Alzheimer's disease
In the past decade the perception of Alzheimer's disease as a single entity with a characteristic clinical and pathological profile has changed into a realisation that this represents a broad neurodegenerative cascade to which entry can be gained through variousf routes (fig 2).
 |
| Fig 2: Pathogenesis of Alzheimer's disease. Mutations in amyloid precursor protein (APP) gene and presenilin genes increase production of soluble amyloid ß protein. This deposits as insoluble plaques when concentration of amyloid in extracellular fluid is high. Apolipoprotein E E4 protein favours deposition by lowering the concentration threshold to plaque formation. Plaques are infiltrated by microglial cells, which secrete interleukin-1, increasing production of amyloid ß protein and soluble amyloid and stimulating astrocytosis. Reaction oxygen species from microglia and astrocytes damage nerve endings, causing neuritic changes in plaques and the formation of neurofibrillary tangles in nerve cell cytoplasm. Damaged neurones die, causing clinical disability - dementia. Loss of normal secretion of APP (due to breakdown into amyloid) may also stimulate neuritic changes and hasten nerve cell death |
Some cases of Alzheimer's disease involve the inheritance of mutations in particular genes (amyloid precursor protein gene(3) and the presenilin genes(4) in a classic mendelian manner; these mutations are so lethal that disease will virtually always occur at some time, usually well before old age. Yet such mutational events account for only about 70% of all instances where inherited Alzheimer's disease occurs before the age of 60 and contribute nothing towards that greater mass of cases where onset is after this age or to those cases in earlier life where no clear pattern of inheritance is seen. Hence, other and more common genetic events known as polymorphisms or "natural" genetic variations, particularly the possession of the e4 allele of the apolipoprotein E gene, have been associated with Alzheimer's disease.(5) These polymorphic variations do not absolutely dictate whether disease will occur but cause a metabolic deficiency that increases the likelihood that the tissue damage of Alzheimer's disease will occur and thereby raise the level of susceptibility to clinical disease. Possession of one gene copy of the e4 allele doubles the risk of developing Alzheimer's disease; two copies increase the risk eight times (to about 90% certainty).(5) These same genetic factors are, however, associated with other disorders that share many of the tissue characteristics of Alzheimer's disease yet produce quite different and distinctive clinical syndromes. For example, in Down's syndrome the pathology is that of Alzheimer's disease,(6) but it is an extra (normal) copy of the amyloid precursor protein gene on the duplicated chromosome 21 that is causative.(7) In hereditary cerebral haemorrhage with amyloidosis other mutations in this gene are to blame.(8) Cortical Lewy body disease shares the same change in the apolipoprotein E e4 allele as Alzheimer's disease.(9) Hence, genetic changes like these confer metabolic alterations that in turn prescribe a particular pathology - ß amyloid deposition in this instance.(10) It is the extent and distribution of this pathology that will dictate the clinical profile that will ultimately emerge. Tissue changes associated with particular disorders are likely to be the biological products of adverse genetic variations. Modelling these genetic changes of Alzheimer's disease in animals (transgenic mice)(11-13) or in cell lines(13-16) has produced the same cellular metabolic changes and tissue alterations that characterise the human condition.
Thus Alzheimer's disease is a complex, multifactorial process that is essentially under genetic control and represents an interplay between various adverse genetic changes that have an impact on the pathological process in different ways. The nature and balance of these factors determine whether disease will occur, and if so, at what time of life and how rapidly progression will be. Most people will possess one or more of these factors and can thus expect to accumulate some of the pathology of Alzheimer's disease(17-19) in their brains given time - this risk is unlikely to be sufficient in most individuals to ensure progress into clinical disease, but it is none the less probable for many. Other pathologies seen in the brain in old age, such as Lewy bodies, can be equated with Parkinson's disease (clinical or subclinical)(20); indeed the entire spectrum of neurodegenerative disorders is likely to be genetically determined. Pathologies such as plaques, tangles, and Lewy bodies should be taken as indicating nervous system disease whenever or wherever they might occur and are quite distinct from the changes of "normal aging."
Genetic changes in normal aging
So what is normal aging? Does this represent the good face of genes whose obverse side promotes pathology and disease in later life? For example, in contrast to the e4 allele, the apolipoprotein E e2 allele promotes longevity in both normal individuals(21) and those with Down's syndrome(22); it delays the age of onset of Alzheimer's disease.(23-24) Although this is possible in some specific instances it is in general terms unlikely. Health promoting genes will fulfil housekeeping roles, maintaining the faithful production of proteins that sustain cell viability and resistance to damage; they may form part of a genetic cluster that specifies longevity in a programmed manner. Gradual failures in these cell maintenance systems may act as a springboard on which adverse genetic changes promoting pathology might gain momentum and take their toll in later life. But what are these basic changes that potentially affect all of us, whose actions progressively build up, thereby rendering the system so vulnerable in old age?
| The core physiological process supporting the life of all cells is the
oxidative metabolism of glucose in the mitochondrion to provide
transducible energy. Unfortunately, undesirable byproducts (oxidants or
reactive oxygen species) continuously result from this process, and
these damage biomolecules (DNA, lipid, protein, carbohydrate) and
impair their function (fig 3).(25) Not surprisingly
therefore cells are equipped with antioxidant defences, which prevent
or at least restrict these untoward effects.(26) It is a
widely held view of cellular aging that it is this unchecked damage by
reactive oxygen species that leads to an acquired decline in cellular
function with time. Of particular importance is damage caused to the nuclear genes responsible for producing proteins vital for these defences and the mitochondrial genes critical for bioenergy maintenance. Mitochondrial DNA is at particular risk as it is located on the inner membrane, next to the sites of cellular respiration where reactive oxygen species are produced. (27) |  | |
| Fig 3: Reactive oxygen species damage nuclear and
mitochondrial DNA. Unrepaired damage leads to faults in transcription
and translation of proteins, compromising cell metabolism. Failures in
DNA repair mechanisms permit a continuing cycle of oxidant damage,
gradually weakening the function of the cell. Induction of cell death
pathways (apoptosis) leads to loss of nerve cells. |
| Furthermore, it lacks the protective histone coat of nuclear DNA and is deficient in those repair enzymes that correct much of the nuclear damage. Moreover, this nuclear and mitochondrial genetic damage cannot be diluted out by cell division and selection in post-mitotically stable tissues, thereby leaving the cell poorly guarded to deal with further oxidative change. Changes in cytokines, growth factors, or hormonal regulators, which influence gene expression, may compound the situation at transcriptional level. A downward spiral thus sets in, leading eventually to metabolic collapse and apoptotic (programmed) cell death, particularly in times of physical, chemical, or biological stress (fig 4). Individuals who inherit a robust (mitochondrial) genotype may sustain adequate energy capacity into old age, despite these damages. However, other less genetically well blessed individuals may find their energy capacity eroded over time to produce a bioenergetic weakening that in some individuals may act synergistically with inherited gene defects to produce, at an early age, overt clinical disorders such as Parkinson's disease, type II diabetes, and mitochondrial myopathies.(27) |  | |
| Fig 4: Relation between aging and dementia
(Alzheimer's disease). Changes associated with aging (reactive oxygen
species) increase as we get older. Genetic factors leading to the
pathology of Alzheimer's disease can act at any time after age 40 but
have a less powerful effect later in life (the larger the arrow the
more powerful the effect). At age 50 dementia is due nearly entirely to
the genetic influences of Alzheimer's disease. At 100 the development
of dementia is uncommon - when it occurs it is mostly due to
pathological changes of aging |
Hence, paradoxically, the very giver of life is also that which limits life span. The replicative potential (of dividing cells) may become progressively reduced through other DNA damage (telomere loss), and the stable daughter cells produced may, in the absence of their replacement by genetically more favoured cells, promulgate the chromosomal damage and dysfunction borne by their progenitors.(28)
Oxidative or other stochastic damage to DNA may also underpin certain cancers, resulting in the loss of function of tumour suppressor genes and the activation of tumour promoting genes (oncogenes), with subsequent malignancy.(29) Oxidative damage to low density lipoproteins may contribute significantly to atherogenesis and cardiovascular disease.(30)
Conclusion
The recent molecular revolution argues that aging and the common diseases associated with age are fundamentally determined by an individual's own genetic make up, this being partly a function of the inherited genome and the modifications to this that occur over a lifetime. How well and how long a person lives depends on the net balance of this "genetic miasma|mK - what the Victorians used to call "constitution." None the less, lifestyle can have an important role.
Clearly, diet and hygiene, excessive alcohol consumption, cigarette smoking, drug misuse, sexual promiscuity, and occupational hazards can damage even the most perfect of cells and compromise life expectancy. At greater risk are cells and tissues already weakened by aging or disease. Avoiding these risks may increase an individual's likelihood of reaching old age but not necessarily lead to a high quality of life. The goal of future research will be to find how the effects of these adverse genetic changes can be minimised, by for example, gene therapy,(31) and the functioning of health promoting genes maximised, while social risks to health are avoided or controlled along the way.
This present review can only hint at the power that molecular biology has to explain why we age or become ill in later life. How we should respond to its future insights in terms of health care and preventive medicine in later life will be a major challenge for research and society alike.
Department of Pathological
Sciences,
University of Manchester,
Manchester M13
9PT
David M A
Mann,
reader
References
1 Evans D A, Funkenstein H, Albert M S, Scherr P A, Cook N R,
Chown M J, et al. Prevalence of Alzheimer's disease in a community
population of older persons. JAMA 1989;262:2551-6.
2 Saiki R K, Gelfand D H, Stoffel S, Scharf S J, Higuchi R, Horn
G T, et al. Primer-directed enzymatic amplification of DNA with a
thermostable DNA polymerase. Science 1988;239:487-91.
3 Goate A, Chartier-Harlin M C, Mullan M, Brown J, Crawford F,
Fidani L, et al. Segregation of a missense mutation in the amyloid
precursor gene with familial Alzheimer's disease.
Nature 1991;349:704-6.
4 Tanzi R E, Kovacs D M, Kim T-W, Moir R D, Guenette S W, Wasco W.
The gene defects responsible for familial Alzheimer's disease.
Neurobiol Dis 1996;3:159-68.
5 Roses A D. Apolipoprotein E alleles as risk factors in
Alzheimer's disease. Rev Med 1996;47:387-400.
6 Mann D M A. The pathological association between Down syndrome
and Alzheimer disease. Mech Ageing Dev 1988;43:99-136.
7 Neve R L, Finch E A, Dawes L R. Expression of the Alzheimer
amyloid precursor gene transcripts in the human brain.
Neuron 1988;1:669-77.
8 Levy E, Carman M D, Fernandez-Madrid I J, Power M D, Lieberburg
I, van Duinne S G, et al. Mutation of the Alzheimer's disease amyloid
gene in hereditary cerebral haemorrhage, Dutch type.
Science 1990;248:1124-6.
9 Pickering-Brown S, Mann D M A, Bourke J P, Roberts D, Balderson
D, Burns A, et al. Apolipoprotein E4 and Alzheimer's disease pathology
in Lewy body disease and in other ß-amyloid forming diseases.
Lancet 1994;343:1155.
10 Hardy J, Allsop D. Amyloid deposition as the central event in
the aetiology of Alzheimer's disease. Trends Pharmacol
Sci 1991;12:383-8.
11 Games D, Adams D, Alessandrini R, Barbour R, Berthelette P,
Blackwell C, et al. Alzheimer-type neuropathology in transgenic mice
overexpressing V717F ß amyloid precursor protein.
Nature 1995;373:523-7.
12 Duff K, Eckman C, Zehr C, Yu X, Prada C-M, Perez-Tur J, et al..
Increased amyloid-ß42(43) in brains of mice expressing mutant
presenilin 1. Nature 1996;383:710-3.
13 Suzuki N, Cheung T T, Cai X-D, Odaka A, Otvos L, Eckman C, et
al. An increased percentage of long amyloid ß protein secreted by
familial amyloid ß protein precursor (ßAPP717) mutants.
Science 1994;264:1336-40.
14 Borchelt D R, Thinakaran G, Eckman C B, Lee M K, Davenport F,
Ratovitsky T, et al. Familial Alzheimer's disease-linked presenilin 1
variants elevate Aß1-42/1-40 ratio in vitro and in vivo.
Neuron 1996;17:1005-13.
15 Scheuner D, Eckman C, Jensen M, Song X, Citron M, Suzuki N, et
al. The amyloid ß protein deposited in the senile plaques of
Alzheimer's disease is increased in vivo by the
presenilin 1 and 2 and APP mutations linked to familial
Alzheimer's disease. Nature Medicine 1996;2:864-70.
16 Citron M, Westaway D, Xia W, Carlson G, Diehl T, Levesque G, et
al. Mutant presenilins of Alzheimer's disease increase production of
42-residue amyloid ß-protein in both transfected cells and transgenic
mice. Nature Med 1997;3:67-71.
17 Mann D M A, Brown A M T, Prinja D, Jones D, Davies C A. A
morphological analysis of senile plaques in the brains of non-demented
persons of different ages using silver, immunocytochemical and lectin
histochemical staining techniques. Neuropath Appl
Neurobiol 1990;16:17-25.
18 Morris J C, McKeel J D W, Storandt M, Rubin E H, Price J L, Grant
EA, et al. Very mild Alzheimer's disease: informant-based clinical,
psychometric, and pathologic distinction from normal aging.
Neurology 1991;41:469-78.
19 Price J L, Davis P B, Morris J C, White D L. The distribution of
plaques, tangles and related immunohistochemical markers in healthy
ageing and Alzheimer's disease. Neurobiol Ageing
1991;12:295-312.
20 Gibb W R G. Idiopathic Parkinson's disease and the Lewy body
disorders. Neuropath Appl Neurobiol 1986;12:223-34.
21 Schachter F, Faure-Delanef L, Guenot F, Rouger H, Froguel P,
Lesueur-Ginot L, et al. Genetic associations with human longevity at
the APOE and ACE loci. Nature Genet 1994;6:29-31.
22 Royston M C, Mann D M A, Pickering-Brown S, Perry R H, Raghavan R,
Khin-Nu C, et al. ApoE2 allele promotes longevity and protects patients
with Down's syndrome from the development of dementia.
NeuroReport 1994;5:2583-5.
23 Talbot C, Lendon C, Craddock N, Shears S, Morris JC, Goate A.
Protection against Alzheimer's disease with apoE e2.
Lancet 1994;343:1432-3.
24 West H L, Rebeck G W, Hyman B T. Frequency of the apolipoprotein
E, E2 allele is diminished in sporadic Alzheimer disease.
Neurosci Lett 1994;175:46-8.
25 Cohen G, Werner P. Free radicals, oxidative stress, and
neurodegeneration. In: Calne D B, ed. Neurodegenerative
diseases. Philadelphia: W B Saunders, 1994.
26 Cohen G. Catalase, glutathione peroxidase, superoxide dismutase
and cytochrome P-450 in the nervous system. In: Lajtha A, ed.
Handbook of neurochemistry. New York: Plenum Publishing,
1983.
27 Wallace D C. Mitochondrial DNA mutations in human disease and
aging. In: Esser K, Martin G M, eds. Molecular aspects of
aging. Chichester: Wiley, 1995.
28 Osiewacz H D. Aging and genetic instabilities. In: Esser K,
Martin GM, Eds. Molecular aspects of aging. Chichester:
Wiley, 1995.
29 Miller R A. Gerontology: The study of aging as the study of
cancer. In: Esser K, Martin GM, eds. Molecular aspects of
aging. Chichester: Wiley, 1995.
30 Heinecke J W, Stocker R. Lipoprotein oxidation and
atherosclerosis. In: Esser K, Martin GM, eds. Molecular aspects
of aging. Chichester: Wiley, 1995.
31 Lowenstein P R. The use of adenovirus vectors to transfer genes
to identified target brain cells in vitro. In: Lowenstein PR, Enquist
LW, eds. Protocols for gene transfer in neuroscience: towards
gene therapy of neurological disorders. Chichester: Wiley,
1996.
Home | Current issue | Past issues | Classified ads | Career Focus | Feedback
Collections | About this site | About the BMJ | BMA | Medline