Why Study Aging?
Biologists have long suspected that the mechanisms of aging would never be understood fully until a better understanding of genetics was obtained. As genetic information has exploded, a number of theories of aging have emerged. Each of these theories has focused on a different aspect of the genetic changes observed in aging cells and organisms. Animal models, from simple organisms such as Tetrahymena (a single-celled, ciliated protozoan) and Caenorhabditis (a nematode worm) to more complex organisms like Drosophila (fruit fly) and mice, have been used extensively in efforts to understand the genetics of aging. The study of mammalian cells in culture and the genetic analysis of human progeroid syndromes (that is, premature aging syndromes) such as Werner syndrome and diseases of old age such as Alzheimer’s disease have also improved the understanding of aging. From these data, several theories of aging have been proposed.
Genetic Changes Observed in Aging Cells
Most of the changes thus far observed represent some kind of degeneration or loss of function. Many comparisons between cells from younger and older individuals have shown that more mutations
are consistently present in older cells. In fact, older cells seem to show greater genetic instability in general, leading to chromosome deletions, inversions, and other defects. As these errors accumulate, the cell cycle slows down, decreasing the ability of cells to proliferate rapidly. These genetic problems are partly a result of a gradual accumulation of mutations, but the appearance of new mutations seems to accelerate with age due to an apparent reduced effectiveness of DNA repair mechanisms.
Cells that are artificially cultured have been shown to undergo a predictable number of cell divisions
before finally becoming senescent, a state where the cells simply persist and cease dividing. This phenomenon was first established by Leonard Hayflick in the early 1960s when he found that human fibroblast cells would divide up to about fifty times and no more. This phenomenon is now called the Hayflick limit. The number of divisions possible varies depending on the type of cell, the original age of the cell, and the species of organism from which the original cell was derived. It is particularly relevant that a fibroblast cell from a fetus will easily approach the fifty-division limit, whereas a fibroblast cell from an adult over age fifty may be capable of only a few divisions before reaching senescence.
The underlying genetic explanation for the Hayflick limit appears to involve regions near the ends of chromosomes called telomeres. Telomeres are composed of thousands of copies of a repetitive DNA sequence and are a required part of the ends of chromosomes due to certain limitations in the process of DNA replication. Each time a cell divides, it must replicate all of the chromosomes. The process of replication inevitably leads to loss of a portion of each telomere, so that with each new cell division the telomeres get shorter. When the telomeres get to a certain critical length, DNA replication seems to no longer be possible, and the cell enters senescence. Although the process discussed above is fairly consistent with most studies, the mechanism whereby a cell knows it has reached the limit is unknown.
A result of these genetic changes in aging humans is that illnesses of all kinds are more common, partly because the immune system seems to function more slowly and less efficiently with age. Other diseases, like cancer, are a direct result of the relentless accumulation of mutations. Cancers generally develop after a series of mutations or chromosomal rearrangements have occurred that cause the mutation of or inappropriate expression of proto-oncogenes. Proto-oncogenes are normal genes that are involved in regulating the cell cycle and often are responsible for moving the cell forward toward mitosis (cell division). Mutations in proto-oncogenes transform them into oncogenes (cancer genes), which results in uncontrolled cell division, along with the other traits displayed by cancer cells.
Progeroid Syndromes as Models of Aging
Several progeroid syndromes have been studied closely in the hope of finding clues to the underlying genetic mechanisms of aging. Although such studies are useful, they are limited in the sense that they display only some of the characteristics of aging. Also, because they are typically due to a single mutant gene, they represent a gross simplification of the aging process. Several genetic analyses have identified the specific genetic defects for some of the progeroid syndromes, but often this has only led to more questions.
Down syndrome
is the most common progeroid syndrome and is usually caused by possession of an extra copy of chromosome 21 (also called trisomy 21). Affected individuals display rapid aging for a number of traits such as atherosclerosis and cataracts, although the severity of the effects varies greatly. The most notable progeroid symptom is the development of Alzheimer’s disease–like changes in the brain such as senile plaques and neurofibrillary tangles. One of the genes sometimes involved in Alzheimer’s disease is located on chromosome 21, possibly accounting for the common symptoms.
Werner syndrome is a very rare autosomal recessive disease. The primary symptoms are severe atherosclerosis and a high incidence of cancer, including some unusual sarcomas and connective tissue cancers. Other degenerative changes include premature graying, muscle atrophy, osteoporosis, cataracts, and calcification of heart valves and soft tissues. Death, usually by atherosclerosis, often occurs by fifty or sixty years of age. The gene responsible for Werner syndrome has been isolated and encodes a DNA helicase (called WRN DNA helicase), an enzyme that is involved in helping DNA strands to separate during the process of replication. The faulty enzyme is believed to cause the process of replication to stall at the replication fork, the place where DNA replication is actively taking place, which leads to a higher-than-normal mutation rate in the DNA,
although more work is needed to be sure of its mechanism.
Hutchinson-Gilford progeria shows even more rapid and pronounced premature aging. Effects begin even in early childhood with balding, loss of subcutaneous fat, and skin wrinkling, especially noticeable in the facial features. Later, bone loss and atherosclerosis appear, and most affected individuals die before the age of twenty-five. The genetic inheritance pattern for Hutchinson-Gilford progeria is still debated, but evidence suggests it may be due to a very rare autosomal dominant gene, which may represent a defect in a DNA repair system.
Cockayne syndrome, another very rare autosomal recessive defect, displays loss of subcutaneous fat, skin photosensitivity (especially to ultraviolet, or UV, light), and neurodegeneration. Age of death can vary but seems to center around forty years of age. The specific genetic defect is known and involves the action of a few different proteins. At the molecular level, the major problems all relate to some aspect of transcription, the making of messenger RNA (mRNA) from the DNA template, which can also affect some aspects of DNA repair.
Another, somewhat less rare, autosomal recessive defect is ataxia telangiectasia. It displays a whole suite of premature aging symptoms, including neurodegeneration, immunodeficiency, graying, skin wrinkling, and cancers, especially leukemias and lymphomas. Death usually occurs between forty and fifty years of age. The specific defect is known to be loss of a protein kinase, an enzyme that normally adds phosphate groups to other proteins. In this case, the kinase appears to be involved in regulating the cell cycle, and its loss causes shortening of telomeres and defects in the repair of double-stranded breaks in DNA. One of the proteins it appears to normally phosphorylate is p53, a tumor-suppressor gene whose loss is often associated with various forms of cancer.
Although the genes involved in the various progeroid syndromes are varied, they do seem to fall into some common functional types. Most have something to do with DNA replication, transcription, or repair. Other genes are involved in control of some part of the cell cycle. Although many other genes remain to be discovered, they will likely also be involved with DNA or the cell cycle in some way. Based on many of the common symptoms of aging, these findings are not too surprising.
Genetic Models of Aging
The increasing understanding of molecular genetics has prompted biologists to propose a number of models of aging. Each of the models is consistent with some aspect of cellular genetics, but none of the models, as yet, is consistent with all evidence. Some biologists have suggested that a combination of several models may be required to adequately explain the process of aging. In many ways, understanding of the genetic causes of aging is in its infancy, and geneticists are still unable to agree on even the probable number of genes involved in aging. Even the extent to which genes control aging at all has been debated. Early studies based on correlations between time of death of parents and offspring or on the age of death of twins suggested that genes accounted for 40 to 70 percent of the heritability of longevity. Later research on twins has suggested that genes may only account for 35 percent or less of the observed variability in longevity, and for twins reared apart, the genetic effects appear to be even less.
Genetic theories of aging can be classified as either genome-based or mutation-based. Genome-based theories include the classic idea that longevity is programmed, as well as some evolution-based theories such as antagonistic pleiotropy, first proposed by George C. Williams, and the disposable soma theory. Mutation-based theories are based on the simple concept that genetic systems gradually fall apart from “wear and tear.” The differences among mutation-based theories generally involve the causes of the mutations and the particular genetic systems involved. Even though genome-based and mutation-based theories seem to be distinct, there is actually some overlap. For example, the antagonistic pleiotropy theory (a genome-based theory) predicts that selection will “weed out” lethal mutations whose effects are felt during the reproductive years, but that later in life lethal mutations will accumulate (a mutation-based theory) because selection has no effect after the reproductive years.
Genome-Based Theories of Aging
The oldest genome-based theory of aging, sometimes called programmed senescence, suggested that life span is genetically determined. In other words, cells (and by extrapolation, the entire organism) live for a genetically predetermined length of time. The passing of time is measured by some kind of cellular clock and when the predetermined time is reached, cells go into a self-destruct sequence that eventually causes the death of the organism. Evidence for this model comes from the discovery that animal cells, when grown in culture, are only able to divide a limited number of times, the so-called Hayflick limit discussed above, and then they senesce and eventually die. Further evidence comes from developmental studies where it has been discovered that some cells die spontaneously in a process called apoptosis. A process similar to apoptosis could be responsible for cell death at old age. The existence of a cellular clock is consistent with the discovery that telomeres shorten as cells age.
In spite of the consistency of the experimental evidence, this model fails on theoretical grounds. Programmed senescence, like any complex biological process, would be required to have evolved by natural selection, but natural selection can only act on traits that are expressed during the reproductive years. Because senescence happens after the reproductive years, it cannot have developed by natural selection. In addition, even if natural selection could have been involved, what advantage would programmed senescence have for a species?
Because of the hurdles presented by natural selection, the preferred alternative genome-based theory is called antagonistic pleiotropy. Genes that increase the chances of survival before and during the reproductive years are detrimental in the postreproductive years. Because natural selection has no effect on genes after reproduction, these detrimental effects are not “weeded” out of the population. There is some physiological support for this in that sex hormones, which are required for reproduction earlier in life, cause negative effects later in life, such as osteoporosis in women and increased cancer risks in both sexes.
The disposable soma theory is similar but is based on a broader physiological base. It has been noted that there is a strong negative correlation among a broad range of species between metabolic rate and longevity. In general, the higher the average metabolic rate, the shorter-lived the species. In addition, the need to reproduce usually results in a higher metabolic rate during the reproductive years than in later years. The price for this high early metabolic rate is that systems burn out sooner. This theory is not entirely genome-based, but also has a mutation-based component. Data on mutation rates seem to show a high correlation between high metabolic rate and high mutation rates.
One of the by-products of metabolism is the production of free oxygen radicals, single oxygen atoms with an unpaired electron. These free radicals are highly reactive and not only cause destruction of proteins and other molecules, but also cause mutations in DNA. The high metabolic rate during the reproductive years causes a high incidence of damaging DNA mutations that lead to many of the diseases of old age. After reproduction, natural selection no longer has use for the body, so it gradually falls apart as the mutations build up. Unfortunately, all attempts so far to assay the extent of the mutations produced have led to the conclusion that not enough mutations exist to be the sole cause of the changes observed in aging.
Mutation-Based Theories of Aging
The basic premise of all the mutation-based theories of aging is that the buildup of mutations eventually leads to senescence and death, the ultimate cause being cancer or the breakdown of a critical system. The major support for these kinds of theories comes from a number of studies that have found a larger number of genetic mutations in elderly individuals than in younger individuals, the same pattern being observed even when the same individual is assayed at different ages. The differences among the various mutation-based theories have to do with what causes the mutations and what kinds of DNA are primarily affected. As mentioned above, the disposable soma theory also relies, in part, on mutation-based theories.
The most general mutation-based theory is the somatic mutation/DNA damage theory, which relies on background radiation and other mutagens in the environment as the cause of mutations. Over time, the buildup of these mutations begins to cause failure of critical biochemical pathways and eventually causes death. This theory is consistent with experimental evidence from the irradiation of laboratory animals. Irradiation causes DNA damage, which, if not repaired, leads to mutations. The higher the dose of radiation, the more mutations result. It has also been noted that there is some correlation between the efficiency of DNA repair
and life span. Further support comes from observations of individuals with more serious DNA repair deficiencies, such as those affected by xeroderma pigmentosum. Individuals with xeroderma pigmentosum have almost no ability to repair the type of DNA damage caused by exposure to UV light, and as a result they develop skin cancer very easily, which typically leads to death.
The major flaw in this theory is that it predicts that senescence should be a random process, which it is not. A related theory called error catastrophe also predicts that mutations will build up over time, eventually leading to death, but it suffers from the same flaw. Elderly individuals do seem to possess greater amounts of abnormal proteins, but that does not mean that these must be the ultimate cause of death.
The free radical theory of aging is more promising and is probably one of the most familiar theories to the general public. This theory has also received much more attention from researchers. The primary culprit in this theory is free oxygen radicals, which are highly reactive and cause damage to proteins, DNA, and RNA. Free radicals are a natural by-product of many cellular reactions and most specifically of the reactions involved in respiration. In fact, the higher the metabolic rate, the more free radicals will likely be produced. Although this theory also involves a random process, it is a more consistent and predictable process, and through time it can potentially build on itself, causing accelerated DNA damage with greater age.
Significant attention has focused on mitochondrial DNA (mtDNA). Because free radicals are produced in greater abundance in respiration, which takes place primarily in the mitochondria, mtDNA should show more mutations than nuclear DNA. In addition, as DNA damage occurs, the biochemical pathways involved in respiration should become less efficient, which would theoretically lead to even greater numbers of free radicals being produced, which would, in turn, cause more damage. This kind of positive feedback cycle would eventually reach a point where the cells could not produce enough energy to meet their needs and they would senesce. Assays of mtDNA have shown a greater number of mutations in the elderly, and it is a well-known phenomenon that mitochondria are less efficient in the elderly. Muscle weakness is one of the symptoms of these changes.
The free radical theory has some appeal, in the sense that ingestion of increased amounts of antioxidants in the diet would be expected to reduce the number of free radicals and thus potentially delay aging. Although antioxidants have been used in this way for some time, no significant increase in life span has been observed, although it does appear that cancer incidence may be reduced.
From Theory to Practice
Many of the genetic theories of aging are intriguing and even seem to be consistent with experimental evidence from many sources, but none of them adequately addresses longevity at the organismal level. Although telomeres shorten with age in individual cells, cells continue to divide into old age, and humans do not seem to die because all, or most, of their cells are no longer able to divide. Cells from older individuals do have more mutations than cells from younger individuals, but the number of mutations observed does not seem adequate to account for the large suite of problems present in old age. Mitochondria, on average, do function more poorly in older individuals and their mtDNA does display a larger number of mutations, but many mitochondria remain high functioning and appear to be adequate to sustain life.
Essentially, geneticists have opened a crack in the door to a better understanding of the causes of aging, and the theories presented here are probably correct in part, but much more research is needed to sharpen the understanding of this process. The hope of geneticists, and of society in general, is to learn how to increase longevity. Presently, it seems all that is possible is to help a larger number of people approach the practical limit of 120 years through lifestyle modification and medical intervention. Going significantly beyond 120 years is probably a genetic problem that will not be solved for some time.
Key Terms
antioxidant
:
a molecule that preferentially reacts with free radicals, thus keeping them from reacting with other molecules that might cause cellular damage
free radical
:
a highly reactive form of oxygen in which a single oxygen atom has a free, unpaired electron; free radicals are common by-products of chemical reactions
mitochondrial DNA (mtDNA)
:
the genome of the mitochondria, which contain many of the genes required for mitochondrial function
pleiotropy
:
a form of genetic expression in which a gene has multiple effects; for example, the mutant gene responsible for cystic fibrosis causes clogging of the lungs, sterility, and excessive salt in perspiration, among other symptoms
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